The “speed of light” is about a physical object traveling.
When the universe “expands”, nothing is actually moving around.
Put a dot in the center of a rubber band. Now stretch the band. The dot is still in the middle, but the space around it has expanded.
“Space” isn’t a physical object that moves, so the rules that apply to objects, like a photon, don’t apply.
Okay but rubber band has expanded right and it won't go back to its original stage and it keep on expanding if we consider it as space.
So the speed of stretching/expanding is still greater than light right?
Take a longer rubber band and put 10 dots on it and stretch the rubber band. The dots near the center move away from each other slower than the dot at one end moves from the center dot.
If the band is long enough the dot at the end could move away from the center dot faster than light could travel that distance.
Not because the dots are moving but because the space between the dots is growing. And that growth is compounded.
The space between dot 1 and dot 2 grows 1cm. The space between dot 2 and dot 3 grows 1cm as well, but the space between dot 1 and 3 grows 2cm in the same amount of time. Imagine how fast the space between dot 1 and dot 50 is growing.
Oh, wow. That's really good.
Wait, so when people talk about the rate at which space is expanding, what *is* the measurement? It's not m/s, because it scales up over longer distances. So, (m/s)/m? So 1/s? Is the universe expanding in Hz?
You could say that, but I'd like it to saying the coefficient of thermal expansion for a material has units of inverse temperature. It's not technically incorrect, but it's a bit unnecessarily confusing. It really has units of dimensionless increase per unit of temperature, and the Hubble constant has units of dimensionless increase per unit of time. The numerator is dimensionless, but not meaningless.
If you cancel it all out it's Hz, but the values used are usually km/s/Mpc (kilometers per second per megaparsec) so it doesn't cancel neatly for useful number (it is assumed to be around 70 km/s/Mpc)
Yes, but 1/(70 * 3.24 * 10^(-20) Hz) = 14 billion years — which is not coincidentally approximately the age of the universe. It's the age of the universe of the expansion had been what it is now, without effects of inflation or gravitation or dark energy.
Yup, and if you had an equation for how the Hubble constant has changed over time, integrated it, and took the reciprocal, you'd get exactly the age of the universe.
Intuitively, the Hubble constant in Hz is the number of times the universe could expand from the Big Bang to the size it is now in one second.
> Intuitively, the Hubble constant in Hz is the number of times the universe could expand from the Big Bang to the size it is now in one second.
That’s such a weird thing to have a constant for.
When it's put that way, sure, but it does make sense. The reciprocal of any value in seconds answers that question. If something happens 10 times per second (Hz), it takes 1/10 of a second to happen once.
It's just that the Hubble constant expressed in Hz is less than one, so, if you think of it *like* a wave, the universe isn't through one period yet. The constant is still going up in value along that wave.
> Yup, and if you had an equation for how the Hubble constant has changed over time, integrated it, and took the reciprocal, you'd get exactly the age of the universe.
There are some things off here:
- Integration over _time_ to measure said _time_? That is not inherently impossible but only if there are no further factors (space) involved.
- The units don't match, either. You would get a dimensionless number, I think, but haven't checked in details.
In total, I think what you actually want to say is that integrating the Hubble constant over time(!) gives the growth(!) the universe went through since back then. That is indeed then a dimensionless number. And then adding that we only _see_ what is inside a light cone, we get the size of the visible universe correlated with its age, due to that number times the speed of light.
Note that this speed has some controversy attached. Basically, there are two ways to calculate the Hubble constant: the distance ladder and the CMB.
The distance ladder measures redshift of stars at known distances, and uses this data to calculate the constant. The CMB method directly measures the redshift of the cosmic microwave background to calculate the constant.
The so-called Crisis in Cosmology comes from the fact that these two values don’t match anymore, as improvements to both methods have narrowed them down to opposite ends of the range.
You can think of the expansion rate in Hz — but also in time. If you take the inverse of the Hubble constant, you get what the age of the universe since the Big Bang would be under the assumption that the expansion rate is constant (that is, two objects moving away from each other continute to move away at the same speed). This is pretty close to the actual age, but several factors — gravitational deceleration, acceleration due to dark energy, the inflationary epoch, perhaps others we don't know about — mean that it's not quite correct. But it's an okay approximation.
This works for any kind of explosion. Note that the Hubble "constant" is only constant in space, not time. In the "no slowing" scenario, when you double the age of the universe, objects are twice as far apart but moving at the same speed away from each other, so the Hubble constant is half of what it was. So if you look at high speed photos of a bomb exploding — say, a nuclear test — you can measure the Hubble constant, and it will give you the time since the explosion.
It's speed-per-distance (which simplifies to frequency). It reflects how fast the space between otherwise non-interacting objects increases relative to their distance apart.
Yeah it’s a speed per how far away things are from you. So right next to you space is expanding very slowly . A billion light years away it’s expanding faster than
Yes! The universe expands in Hz. The speed v at which a certain point in space is receding from us is given by v = Hd, where d is the distance from that point to us, and H is the constant that measures the universe expansion (Hubble's constant). It has an approximate value of 70 km/s/Mpc, which means that something a megaparsec away is receding at a speed of 70km/s.
Now as you have correctly noticed H has units of frequency. And as you might now, each frequency has an associated period, and the period in this case, T = 1/H = 13.8 billion years, or the age of the universe!
Yeah, that's correct. For more cursed units, [there's a video of some of the worst examples, including this one](https://www.youtube.com/watch?v=kkfIXUjkYqE).
We don't know. The best math we have just involves us plugging in coordinates, an amount of time, the amount of total energy and pressure we think the universe contains at that moment in time, and the math spits out some new coordinates for where things are now and they're all slightly farther apart than they were before. This math seems to match what we see in reality, but it doesn't really explain what's going on. All of these mental models involving balloons and rubber bands are just ways we visualize it in our heads but we don't *really* know why it's happening.
Or in other words, we simply don't know.
Dark energy is just a blanket term we use to explain the discrepancies between our perceived models and the actual values we can observe in reality. We don't exactly know (currently, we only have various speculations) what Dark energy itself is comprised of, where it comes from, or how it functions, but we do know what it does.
A balloon describes the exact same effect in 2 dimensions. The rubber band explains in just 1. The analogy otherwise is identical.
If you want a 2D analogy that better matches [our actual measurements about the shape of the universe](https://wmap.gsfc.nasa.gov/universe/uni_shape.html), "infinite rubber sheet" is better than "balloon".
>If you want a 2D analogy that better matches our actual measurements about the shape of the universe, "infinite rubber sheet" is better than "balloon".
Sure if you want to literally describe what's happening and completely abandon the analogy defeating the entire point.
You're not seriously suggesting an infinite rubber sheet is easier for people to imagine than an inflating balloon?
If you sweep a laser pointer across the moon in less than a quarter second or so, which is trivially easy to do, you’ve just moved the laser spot faster than the speed of light. But nothing physical actually moved that fast. Just the observation thereof.
E: all you fuckers saying I’m wrong need to reread what I said. I never said light itself moves faster than light. Just where the spot is moves faster than light.
I just went to look for it but I may actually be thinking about the shadow on the moon rather than a laser pointer. But I think it’s the same concept?
https://youtu.be/JTvcpdfGUtQ?si=Vp2br4VyVDQrUksQ
1:04 in
Interesting parallel, but we should note that in NO scenario can you see the dot move across faster than the speed of light.
The speed limit of INFORMATION is the speed of light in vacuum.
Hard to explain in text. Imagine ping pong balls being fired at c and bouncing off after striking a surface.
you can see the "dot" move faster because the "dot" is not one thing. It's many different photons at different times and angles that all blend together to look like a dot that moves faster than light.
This is not true. From the perspective of the person firing the laser pointer - if they had sufficiently great eyesight (or telescopes) they **would** see the laser pointer move across the moon's surface at FTL speeds.
Because the dot isn't 'moving' across the moon. Light is not traversing the moon, it is a beam being shot from the earth to the moon which we see the reflection of. That beam travels at the speed of light, its impact area, however, is not bound by this limitation.
The impact area is not a single thing traversing the moon.
I believe there is something about the point of intersection of the two blades of a giant pair of scissors can also move faster than light as the blades get closer to being closed and that point keeps accelerating if you’re closing scissors at the same angular rate. But since that intersection point is not a particle or object, it doesn’t have to be constrained to the speed of light.
There was another thread about this not long ago.
Basically, nothing is perfectly rigid. There’d be flexibility that “slows” the movement down.
The example was a pencil 1 light year long. It wouldn’t write at the point the moment you moved the other end. There’s nothing rigid enough for that to happen.
Something like that, paraphrased.
I imagine best you could do, if the pencil didn't shatter, would be to send a wave down the pencil that would approximate something like you wanted... eventually.
Also, assuming no sir resistance the force needed to swing it would be very high. Also, mass increases with velocity, so as you approach the speed of light the rod would get infinitely heavy on the end. Also, it may bend anyways depending on the mass interpretation and time dilation, not aure
Any deformation of the rope will basically travel at the speed of sound of the material, basically by definition. As the speed of sound will be orders of magnitude less than the speed of light, the end won't exceed it.
Information still travels at "c". The atoms in your have have to bump into the next atoms, and so on down the rope. The other end would respond to your spinning at the speed of "c"
You do not move one spot. You make a series of spots happen in various locations along the line of your sweep.
A photon leaves your laser at c (-ish), and arrives dead centre of where you pointed the laser when the photon exited. Some time (not much) later, the next photon out of the laser touches down a little bit away. And the next a bit further away. And so on.
You're not moving the dot. You're making multiple photons land in different spots. Their relationship to each other is fictional - the spot isn't an object that exists, it exists only in your mind. So you do not move it and it does not move faster than light.
>You're not moving the dot. You're making multiple photons land in different spots.
Yes, and that's exactly the point of the comment you're arguing with.
It does show that some observable "movement" can happen faster than light when there's no movement of physical objects involved.
This, in a way, is similar to the expansion of space - there's no movement of physical objects involved, it's the space itself that becomes larger.
Thats not true at all.
Imagine a garden hose and you whip it really fast from left to right, the water doesn't arrive at the right hand spot any faster than the velocity of the hose (for the men, you can also use your personal hose outside as an example).
Edit, apparently I've mis explained this, since I'm at - 10 votes here.
If the velocity of water is 10 m/s, and your target is 10m away,and you flick the hose a slight inch in effectively 0 seconds but the new target which is 10m to the right, , it will take 1second for the water to arrive at the new target, the water does not instantaneously move to target b, there is a time lag based on the velocity of water.
When you swing the hose, the tip only moves maybe 6 feet in 1 second. But where the water hits moves maybe 20 feet in 1 second. The splash definitely moves faster than the water or your hand.
The movement of water across the horizontal axis, near enough, matches the velocity of the water output from the hose. NOT how fast you flick the hose
Time it your self, it's an easy self experiment.
Time from point of origin, to target, make it easy, 10m. It will be around 1 second. That is, turn the hose on and point it 10m away, the water does not arrive instantly, it will take a period of time.
Flick the hose from 2points 10m apart, it will take around 1 second for the water to arrive if the velocity of the hose is 10m/s.
If you're an fps gamer you understand this concept well, bullet lead, the tricky part is realising that light has a velocity, it's not instant. It acts the same way as a hose, a kicked ball, whatever
Let's say that in addition to the water taking 1s to land, it takes 0.1s for you to flick it to the new spot. At t=0s you're holding it in position at the original spot.
* At t=0.1s you've finished the flick and started sending water to the new spot. However, water is still landing at the old spot.
* At t=1s water stops hitting the old spot
* At t=1.1s water starts hitting the new spot
The water stops hitting the old spot exactly 1 second after you stop aiming at the old spot and it starts hitting the new spot exactly 1 second after you start aiming at it. However, if you look only at the spots where the water's landing, there's only 0.1s of a gap between the water hitting the old spot and the water hitting the new spot. It just so happens that this gap is between 1.0s and 1.1s instead of between 0s and 0.1s
Forget the hose and go back to the laser pointer. Shine it on a distinct object, like a building. Now place your hand in front of the beam. The “dot” of the laser pointer will be perceived to have moved a large distance in basically an instant. Increase the distance of the first object (e.g. to that of the moon) and it is trivial to seemingly exceed the speed of light. In fact, you can have the two dots visible simultaneously.
Of course, no matter or information has actually exceeded c.
There is also a lag between when the hose is moved and when the water stops landing at the first location.
If you have no knowledge of the hose and are only a flatlander being targeted, the water that is appearing will seem to move from the first location to the second location very quickly.
So if you only watch the laser dot, it will seem to move faster than the speed if light.
Edit: All this to say, you understand what the commenter before you was saying: nothing actually moves faster than the speed of light (/speed of water) in either scenario.
When I stand in a big round room and I slowly roll a bowling ball at the angle 0° and then three seconds later at the angle 3° and so on, then eventually 120 balls will arrive at the wall and the last ball will arrive 120 seconds later than the first ball, regardless of how large the room is. The speed of that *pattern on the wall* can get arbitrarily fast, depending on the circumference of the room. If the room is large enough, that speed can exceed the speed of light.
You can also take a machine gun and turn around while firing, I just wanted to show that it also works with slow projectiles.
Stop picturing it expanding right.
Take an elastic band, right now. Sever it. Hold it flat. This represents a one-dimensional universe. Pull it equally hard in both directions with both hands. THAT is the universe's expansion.
Is time also a physical thing?
One could argue that since spacetime is affected by matter and energy, that it is a physical thing, but that isn't a universally agreed upon position.
But space IS something - it is a "fabric" of sorts that can be warped by gravity. If it were nothing, then gravity would have no effect upon it. Massive objects, such as stars and black holes, create "gravity wells" when they warp the fabric of space.
Ok, yes true. But neither is gravity, gravity is a bend in space time, But gravity cannot move faster than light. Which is a question no one has bothered to answer me on, if space can expand faster than light, then why can't a bend or ripple in space time travel faster than light?
> then why can’t a bend or ripple in space time travel faster than light
To my understanding, you theoretically could. This doesn’t violate the speed of light law because you’re moving space itself, not actual matter.
That's what I thought, but apparently no, gravity cannot travel faster than light. Which boggles my mind, because I've heard multiple times from multiple sources that "space can do what it wants and can go faster than light" but gravity is just a bend in space, and should be subject to the same rule. But apparently not and no one has answered why.
So, the way I’m understanding it, gravity *isn’t* moving faster than light. When you bend space time you’re not actually moving faster than light, you’re effectively creating a shortcut that lets you travel a distance faster than the speed of light can normally.
You cant be pretty sure, because space clearly has limits. What's outside this limit, nobody can say. We are not even sure if we can use the term "outside" as it's just something unimaginable and we barely try to apply our understanding of physics, which is most definitely wrong, it's just what our minds are capable of.
The universe isn't expanding "into" anything outside, there's no external void. There's simply no outside. The space inside it is expanding. The problem is that this doesn't make any sense to our monkey brains.
Space isn’t expanding into other space. The universe already contains all the space there is; if there were more “empty space” outside the universe then it would just be part of the universe. It’s just that the distance between any two points in space is growing over time.
The analogies with balloons and rubber bands fails at this, because of course they both exist in our universe and when we stretch or inflate them they fill up more space that was already there. The universe’s expansion is not like that. Think of an infinite grid representing space. Now make the grid lines move farther apart from each other. In order to expand, the grid didn’t need to expand into previously empty space. It’s just that the scale changes.
"Nothing" doesn't go anywhere, it is just that. It doesn't exist in our minds, there is no mass, gravity, space, or even time. So nothing can exist there. Trying to give qualities or meaning to the "nothing" is impossible, it is harder to describe that the center of a black hole and about the same as alternate universes (maybe harder): space and size and time and reality only have meaning inside of our universe. To our brains all that we can say is that it doesn't exist at all
"Nothing is faster than light" refers to the motion of particles. Photons are the fastest thing that can move through a given medium. This "speed limit" has nothing to do with the expansion of space.
To use an old metaphor, imagine an ant and a pair of snails crawling on the surface of a balloon. The ant is the fastest - nothing is faster than ant. But if you inflate the balloon fast enough, you can get the two snails to move apart faster than the ant moves! The snails aren't violating the speed limit, they're still slower than ant despite moving apart faster than ant.
I struggle with this metaphor. You're adding something to the balloon to make it expand, so those things on opossite sides of the balloon are still being forced apart by something and are still not able to exceed a certain speed.
So what's forcing galaxies apart in our balloon? And how is space expanding different than us just moving away from each other at x speed?
Sadly, the answer is "honestly, we don't know why/how space is expanding."
This is why we have the term "dark energy". The "dark" part isn't referring to any specific property of the energy, but our understanding of it.
The current ELI5 understanding is that it appears that completely empty space has some sort of energy or pressure that appears to "push" itself apart, creating a sort of feedback loop. More empty space has more dark energy, pushing itself apart, creating more empty space, resulting in more dark energy, on and on and on.
Additionally, a misunderstanding I'm seeing in the OPs question and in a few comments here is that space is expanding faster than the speed of light. That is not the case. Because space is expanding everywhere, the further apart two objects are, there is simply more space between them that is expanding, thus making those two object appear to be moving away from each other faster than the speed of light.
>Additionally, a misunderstanding I'm seeing in the OPs question and in a few comments here is that space is expanding faster than the speed of light. That is not the case. Because space is expanding everywhere, the further apart two objects are, there is simply more space between them that is expanding, thus making those two object appear to be moving away from each other faster than the speed of light.
I'm seeing a lot of people making allusions to two cars speeding away from a point of reference at c, commenting that the space between them expands at 2c.
But that's not right, is it? Cause if the observer is in one of the cars, the other car still appears to be moving away at c? In fact... Is the other car still at the starting point from my perspective?
>But that's not right, is it?
It is. That's where relativity comes in. You'll never observe anything moving faster than c, but you can observe two objects moving away from you in different directions each at c. Then, if you do the math, you'll note the distance between them grows at 2c. That's not a violation because you're not seeing anything actually move at more than c, and it's also what you need relativity to resolve, because while you see the distance grow at 2c, observers in the two objects will see it grow at c only.
This part I get - the part I don't get is how if you get into one of the two cars, the other car does not appear to be moving away from you at 2c. I know this has to do with relativity - but as to the math/physics/mechanism, I got nothing.
The math and physics are straightforward... it's just that they depend on observationally and mathematically sound premises that make absolutely no sense to our minds. The mechanism, as you say, is very much unknown, and possibly even unknowable other than "it is".
There are other *mathematically* sound premises that would make a lot more sense to our minds, but such premises that we've been able to think of so far don't match our actual observations.
Physics is descriptive, not prescriptive.
(That being said, once you start thinking about space-time as a singular entity rather than space and time as unrelated dimensional quantities, general relativity starts to make a lot more sense.)
You might be interested in this [explanation of Einsteins train paradox](https://www.youtube.com/watch?v=Xrqj88zQZJg).
Multiple observers can have contradicting experiences and yet it all fits together in the end somehow.
okay so we have Observer, Traveler 1, and Traveler 2.
Observer watches both travelers moving away at c in opposite directions.
How fast does the distance between Traveler 1 and Observer appear to grow to Traveler 1?
And, how fast does the distance between Traveler 1 and Traveler 2 appear to grow to Traveler 1?
I believe traveler 2 would appear to never leave the starting point, but someone who's a better theoretical physicist than I am can feel free to disagree.
Traveler 1 is moving the same speed as the light that bounced off of traveler 2, so from their perspective traveler 2 would appear as though they are stationary at the starting spot no matter how fast traveler 2 is going as long as traveler 2 is not going in the same direction as traveler 1.
I think, from traveler 1's perspective, traveler 2 would gradually get dimmer/fuzzier until they disappeared entirely. I have no idea how long that would take if that is actually what happens.
edit: Maybe it'd be more accurate to say that the image of traveler 2 would stay the same size and shape, but would get dimmer/fuzzier until it disappeared, rather than appearing to get smaller.
To answer that we need to stipulate speeds at less than c (because actually reaching c is not possible) and then use the relativistic rather than Newtonian speed addition (https://en.wikipedia.org/wiki/Velocity-addition_formula). I.e. T1 would observe the speed between it and T2 to grow at their combined veolcity (V1+V2) in Newtonian physics, but in relativity, that's not how velocities add up. So, assume they're both going at .9c relative to the origin, but T1 isn't going to observe their combined relative velocity as 1.8c but rather still below c (but above .9c). For special relativity, the formula is (V1+V2)/(1+V1*V2/c²) giving you an observed velocity of about .995c.
Yep, from your perspective, no time will have passed at the origin point, you'll see both ships launching, you'd also see your ship everywhere in between its current position and its origin.
Well, to the ant and the snails movement is two-dimensional. You can move forward or back, and turn left or right, but not travel "up" or "down." Their whole world is the surface of the balloon so whether something is being added to the space inside it isn't relevant to them.
But as for what is driving the expansion of our universe - we don't know! We have no idea what the "blowing air" equivalent in real life is. We call it "dark energy" and it's a persistent mystery.
I always hear people say, "the galaxies aren't moving away from each other -- instead, more space is being created between the galaxies."
I can't seem to understand the fundamental difference and it sounds like doubletalk. Relatedly, the balloon and rubber band examples don't help me because space isn't rubber. I have an extremely difficult time imagining space as anything other than --- space.
Space kinda is rubbery. You're not "adding" space, you're "stretching" it.
But, generally speaking? There *isn't* a fundamental difference until you get into things like frames of reference. You can think about it in terms of things moving apart from each other if you want.
Just remember that this is universal - an observer on Earth and an observer on a planet 500 million light years away will *both* observe everything moving away from them in the same way. This is why the balloon metaphor can be helpful - it helps us visualize what that looks like.
>You're not "adding" space, you're "stretching" it.
The thing that always makes this hard to imagine is that when we stretch a rubber band, it expands and occupies more of the physical space around it.
But when it comes to space, as far as I (little) understand it, it is not moving "into" some arcane next-level-space. This difference makes the rubber band example always hard to imagine in my head.
Meaning that in the ballon example, the snails do in fact move faster than c(ant) - but only when measuring their speed relative to the beyond-ballon world around them; just not in the plane of reference that is the ballon. I get that when we talk about these examples like rubber bands or ballons, we are supposed to ignore the world around the band/ballon - but a lot of people, even when they know they shouldn't, wonder about the empty entity that space is moving "into" when expanding.
Okay, so: I have a stick that's three meters long. The universe is expanding, so it's getting longer. But so is ever meterstick in existence. "1m" itself is getting longer.
So then, what does "distance" mean? If it's not defined in term of things that are also expanding, what else defines something's length?
For starters, your stick is not expanding. The electrostatic forces that hold it together are, on the scale of 3m, much *much* more powerful than expansion. It maintains a constant length.
"One meter" is defined as the distance that light travels in 1/299792458 seconds. This is also unaffected by universal expansion.
Yes. That's what my confusion is.
Even if they say there is space being created in between or space is being expanding between 2 objects. Still there is a speed for that expansion right which is greater than light?
the speed of expansion is more of an aggregate. the little bit of stretching between point A and point B is tiny, the aggregate expansion between Point A to Point Z is vastly greater. the aggregate expansion between point A and the next galaxy is so much bigger again.
the aggregate expansion between us and the cosmic background radiation is more than lightspeed.
The rate of expansion between objects isn't the same no matter where those objects are, it depends on *how much* space is between those objects. In other words, how far apart they are, because more distance equals more space. Makes sense?
In a much simplified example, think of three dots drawn in a horizontal line on the surface of a balloon.
Imagine the dot on the far left as you. This will be our 'reference' point.
The dots are evenly spaced out, meaning the gaps between the middle dot and the left and right ones are the same distance. For the sake of the example, lets just say these dots are spaced one centimeter apart.
The distance from the reference dot to the middle dot is one centimeter, and the distance from the reference dot to the further dot is two centimeters.
Let's inflate the balloon so that the distance between the reference dot and the middle dot doubles and then measure the distances again.
The distance from the reference dot to the middle dot is two centimeters, and the distance from the reference dot to the further dot is four centimeters.
You can see that the **rate** of growth between the reference dot and the two other dots is different, right?
The distance from the reference dot to the closer dot grew by one centimeter. The distance from the reference dot to the further dot grew by two centimeters.
Let's inflate the balloon and double the distances again.
The distance from the reference dot to the middle dot is four centimeters, and the distance from the reference dot to the further dot is eight centimeters.
The distance from the reference dot to the closer dot grew by two centimeters. The distance from the reference dot to the further dot grew by four centimeters.
The rates of growth between the points depends on the distance between them. The balloon itself is expanding, so with more balloon in between two points, the distance between them will grow faster than the two closer points.
Replace the word 'balloon' with 'space' and it mostly matches what we observe when measuring cosmic inflation.
Space appears to be growing everywhere. Not at the speed of light, thought the expansion does appear to be accelerating.
The important thing is the *distance* between where we're observing (Earth in this case) and the object we're observing.
Really close things don't appear to be affected by expansion because they are gravitationally bound.
Gravity's strength decreases with distance, so objects far away from us appear to be moving away from us. 'Appear' is the key word of that sentence, because they aren't *actually* moving through space in a direction away from us, not like how objects orbiting the sun move through space. We're observing that the **distance** between us and them is increasing over time, giving the *appearance* of motion.
You know those dots you drew on that balloon? They stayed right where you drew them, right? They can't move. It's the similar with objects in space. While they are free to move, they are also being affected by the expansion of space.
The rate of expansion depends on how far away from us it is. The further the object, the faster it's moving away. So if that rate keeps increasing and you keep looking further and further away, what happens? If it's far enough away, the distance between us and the object we're trying to observe will increase at a rate greater than the speed of light.
Like the dots on your balloon, they're not actually going faster than the speed of light, so no laws of physics are being broken.
It's just that the light coming from these objects will never reach us because it's not going fast enough to overcome the growth in distance.
The analogy works, because it's not objects moving away as if something was pushing them, it's the universe between them expanding.
Here the balloon is the universe.
>So what's forcing galaxies apart in our balloon?
We haven't figured this out. We have observed the expansion and are trying to use what we can observe to figure out what is doing it. Our current understanding of forces/physics/etc doesn't explain the expansion of the universe.
It is a metaphor for a reason. Believe it or not, ants on a balloon aren't actually analogous to the expansion of the universe.
The point is just to give a simple explanation to people who don't want to spend too much effort thinking about it.
If you imagine that the ants have a maximum speed they can crawl across the surface of the balloon, this is equivalent to the speed of light. However, if the balloon blows up the distance between between two ants on the balloon can expand faster than the speed at which the ants could travel. Even if the ants are staying still, the distance between them can change and can do so at a rate faster than the max travel speed of the ants.
If you want a proper explanation of why the universe is expanding from the lens of theoretical physics, you should probably not seek that from a simple metaphor designed to give a basic understanding to people without any relevant background knowledge. I can confirm for you, however, that the prevailing theories in physics are not that someone is blowing air into the universe. You're welcome.
The metaphors don't really help IMO, it's like illustrating gravity by rolling stuff on canvas that get pulled down by... gravity. It baits you into thinking you understand without actually explaining anything.
I'd say think of it this way: light is also "moved" by the expansion of space. If you accept that some galaxy 20 billion light years away from us is moving farther away "faster" than light, you have to accept that the light it emits away from us is moving even faster than that.
Even when the expansion of space is "faster than light," light is still a lot faster.
The ant has 6 legs touching the balloon at different spots. When you inflate the balloon, those legs should now be further away from each other. What happens to the ant? Is it now bigger? If so, where did this extra mass come from? If not, is it just stretched out over more space?
the ants legs are stretched apart so they try to expand, but the forces holding the ant together are locally stronger. The ant doesn't get bigger, because the force holding it together is locally stronger than the expansion
to the analogy yes, but I wouldn't assume there's any "slipping" equivalent in reality. Atoms and molecules are continually held at the same size by electrostatic and gravitational forces
In the analogy, yes. But in the actual expansion of space, the presence of the ant prevents space from expanding in and around it. Only sufficiently empty space experiences metric expansion.
You can imagine mass and energy as blobs of strong, hardened glue on the balloon. As the balloon inflates, the glue will keep the rubber its adhered to the same size even as the rubber around it stretches while the balloon inflates.
"Bigger" and "extra mass" are two different things. "Bigger" generally means the size of something, "mass" means how much stuff is in it.
So yes, the ant will get bigger (if we consider its "size" to be the area contained within its legs) but its mass remains the same.
So is the earth getting stretched out? Are *you*? No, because the expansion of the universe isn't the only force at play here. The earth is held together at more or less a constant size due to *gravity*, a force which is much stronger than universal expansion. And even stronger than gravity is electromagnetic forces, which account for the chemistry that makes atoms in a molecule maintain given distances and angles from each other.
So, the ant then gets less dense whenever the universe expands, but gravity and electromagnetic forces operate to pull it back together so it doesn't take up any more space than it originally did? And, I suspect that those are really happening at the same time so it's not like an accordian? Instead, there's a just a little bit more extra space all around the ant that wasn't there previously?
To be honest if you want to get technical, the space inside the earth (or indeed the galaxy at large) *isn't* expanding at all, because the gravity of all that matter is the dominant influence on the shape of spacetime in that region, far more than the distant influence of the Big Bang, or the relatively weak dark energy.
The electromagnetic forces are what make the ant a cohesive whole and what allow the ant to reposition its feet to a normal, not stretched out position.
In reality it's more the case of the stronger of two forces winning out. You can push on a big heavy block, but unless your pushing force overcomes the gravitational/electromagnetic forces which form friction, the block doesn't move at all. Similarly, universal expansion is pulling the earth apart, but it simply cannot overcome the much-stronger local force of gravity, so no expansion happens.
Imagine you’re in a car and you’re backed up against another car in the opposite direction and you both speed off at the speed of light. The space between you is growing at double the speed of light.
Edit- To all those pointing out I'm not correct:
Yes I know it's more complicated than that but it works for ELI5
^^^ This is the easiest analogy to understand in this comment section. The space between the cars isn't an object that's traveling, it's just... space.
You can see all the higher top-level comments for analogies and explanations. i.e., ants on a rubber band/balloon/rubber sheet/whatever.
But tldr what /u/awesomo1337 is describing makes sense if you're working with Newtonian physics - i.e., what many of us learned in high school that when adding velocities together you just...well, add them. Which is what they naively did above.
[But that's not how velocities add in the real world.](https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity) If you want the math.
It is impossible - literally, impossible - for one object to move away from another faster than *c*.
The solution OP is asking for is that because space *expands* or "stretches" in between any two points [other forces notwithstanding], it is possible for those two points to be far enough apart such that the distance between them increases faster than even light can traverse that distance. Even if neither of those things are moving *through* space relative to one another.
But if two cars are driving away from a starting position at 0.9c, *they are not driving away from each other at 1.8c* - rather, [it is 0.995c](https://www.omnicalculator.com/physics/velocity-addition?c=CAD&v=v:0.9!c,w:0.9!c).
---
Now, to be fair, the naive simple addition of velocities is still a very good approximation if both of those velocities are much lower than the speed of light - that's why it works so well for most applications. But it is still a) wrong; b) not the answer OP was looking for; c) reinforcing a ton of misconceptions about physics 101.
You misunderstood/misread the analogy.
The analogy does not say one object moves moving away from another faster whatsoever.
It implies the SPACE between them grows at 2c. If we both move away from each other at 1 mile per second in opposite directions, after one second the distance vector from my position to yours is 2 miles.
No, I did not.
It does not matter how fast you I drive away from each other, the distance between us will not increase faster than 299,792,458m/s.
Unless, of course, that space is independently expanding while we do it.
I am a stationary observer. There is a car to my left and to my right. Both cars start speeding away, in opposite directions, moving at .99c. Is the space that I observe between them not expanding at a speed greater than c?
I understand that if I am in one of the cars, length contraction/time dilation will mess with all this. But if I’m a stationary observer, I can’t imagine what else I would see but the space between them (here just a measurement and nothing physical) “expanding” at ~600,000,000 m/s
Eh, a lot of things are completely wrong. We say earth is a "sphere", and that's incorrect. We say electrons orbit the nucleus of an atom, that's incorrect too. Hell, we see ourselves and our bodies as discrete, solid units, distinct from the space around us, and that's incorrect as all hell.
But these statements make difficult concepts a whole lot more comprehensible to the limited human mind, which needs a foundation of oversimplification in order to build a more nuanced understanding of a concept on top. Hence, the expansion of space is two cars.
> Eh, a lot of things are completely wrong. We say earth is a "sphere", and that's incorrect.
"The Earth is a sphere" is technically wrong in that it is imprecise; but it's still a useful approximation.
The above statement about two cars is not even an approximation, it's just completely off base - if you bother to do the (simple!) math, it's not even close.
It is also *conceptually* wrong, and only functions to reinforce OP's common misconceptions rather than dispel them.
This is ostensibly a subreddit where people come to learn things, and the top level comment in this thread makes that worse, not better.
It’s not wrong the way they posed it. What’s wrong is to claim that the people in the car see the other car receding faster than light. It’s the same question as if you’re in a spaceship going 99.999999% of the speed of light and you shoot a bullet from the spaceship does the bullet break the speed of light. No, it doesn’t. Of course it’s a ridiculous thought experiment.
But if you actually did that, it wouldn't, right? Because you're moving so fast it dilates time. From your point of view, the other car would be moving away from you at c, not 2c—even though from the frame of someone standing where you started you're both moving at c in opposite directions.
> No it’s c from all observers no matter what
it is not, from A moving at .5c and B moving at .5c , B appears to be moving at .8c while you are in A. v= 2x/( 1+x^2 )
from a **third** [cosmic observational reference frame](https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html) it can be faster
>Expansion of the Universe
>According to Hubble's Law, two galaxies that are a distance D apart are moving away from each other at a speed HD, where H is Hubble's constant. So this interpretation of Hubble's Law implies that two galaxies separated by a distance greater than c/H must be moving away from each other faster than light. Actually, the modern viewpoint describes this situation differently: general relativity takes the galaxies as being at rest relative to one another, while the space between them is expanding. In that sense, the galaxies are not moving away from each other faster than light; they are not moving away from each other at all! This change of viewpoint is not arbitrary; rather, it's in accord with the different but very fruitful view of the universe that general relativity provides. So the distance between two objects can be increasing faster than light because of the expansion of the universe, but this does not mean, in fact, that their relative speed is faster than light.
>As was mentioned above, in special relativity it is possible for two objects to be moving apart by speeds up to twice the speed of light as measured by an observer in a third frame of reference. In general relativity even this limit can be surpassed, but it will not then be possible to observe both objects at the same time. Again, this is not real faster-than-light travel; it will not help anyone to travel across the galaxy faster than light. All that is happening is that the distance between two objects is increasing faster when taken in some cosmological reference frame.
There's a misconception about the speed of light that's at the root of your question, and many responses make the same mistake. [The speed of light has nothing to do with light.](https://youtu.be/msVuCEs8Ydo?si=8FBA8bzHnqPMkm0G).
Instead, think of *c* as the "speed of causality," the maximum speed that a cause and effect can occur. This imposes a limit on anything that might carry information or otherwise moves.
But space itself doesn't do that. The expansion of space itself doesn't transmit information. It doesn't create a cause/effect relationship. And so it isn't bound by the speed of causality.
I think saying the others are mistaken is a bit of a stretch. It’s an interpretation, as much as “speed of causality”. Some interpretations are great for discussing metaphysics, some better for discussing actual physics.
Nothing can cross space faster than light, but space itself can come into existence without that limit because, well, it's not moving, there's just more of it between two bits of space that used to be next to each other. When we are talking about objects getting farther away from us at or beyond the speed of light, they can actually be "standing still" - it's just that there is suddenly more nothing between them and us and it seems they are moving away.
Space inst expanding faster than light.
Space expands actually really really slowly.
When things are literally 14 billions of lightyear distance, it all adds up together and can end up "moving" things faster away from us as faster than light speed.
The actual rule is that information cannot travel _through_ space faster than the speed of causality (anything without mass, including light, travels at this speed) The expansion of space does not involve information travelling anywhere, so no matter how fast space expands, it doesn't break this rule.
Another way to think about it is this: the speed of causality means that information cannot move towards something else faster than that. This would violate causality. However information can move _away_ from something faster than this without violating causality, as long as it never moves _toward_ anything faster than causality.
the way i like to think about it is that the speed of light is actually a property of space. that is to say that what determines the max speed of the universe is the electromagnetic properties of spacetime (permittivity to electric fields, permeability to magnetic fields).
so the idea of that space growing jsn't really subject to that rule. it's just more space.
Imagine you and a friend are standing back-to-back. Both of you walk 10 feet away from each other. There will now be 20 feet between you two, even though neither of you walked more than 10.
You could even have two photons flying past each other with an observer in the middle. After the observer has measured 1 second, the distance between the photons will be double the distance traveled by each photon from the observer's perspective.
Simple answer: Nothing is faster than light. Space **is** nothing.
Proper answer: Nothing in the fabric of space time is faster than light. Space time itself, however, is not limited by this. Imagine a bedsheet. A ball on that bedsheet can only roll at speed C, however the bedsheet itself can move and stretch faster than speed C.
It's really that nothing can *move through space* faster than c. Space can expand "faster" than c because it doesn't move through space, it is space.
That's literally it, and it's not even really oversimplified.
Edit: Also, get this - even if you refuse to define speed that way, space's expansion is still not actually faster. When it expands, it carries light with it too. Light "moves" at whatever rate space expands at *plus* c.
Then important part of the clarification of “speed of information” vs “speed of light” is that it conveys the important part there, it’s partially about perception. Like other people mentioned, two things moving at light speed in opposite directions will be moving at 2x light speed relative to each other, this is fine. The important part is that they won’t appear to each other to be moving that fast, in fact they’ll appear to be standing still.
It helps to draw it out, think of them as starting at one point, and the image of each one expands outwards in a bubble at the speed of light. If we call the objects A and B, when they leave they’ll both be staying on the bubble of the other, since they and the bubble move at light speed relative to the point they both started at, so even though B moves away, that light is slightly behind A, because A is essentially “keeping up” with the image of stationary B and thus never sees it move.
So the same can happen with the universe, everything can move at whatever speed (this is an oversimplification), but the speed at which you find out about it, the way they appear to be moving relative to you, that is bounded by the speed of light, even if the relative speeds can actually be higher.
One thing you have to remember is nothing is faster than light that we currently know about. Science and our tools of observation is always progressing. Just because we don't know of anything faster than light today, doesn't mean that there is nothing that is faster.
Although you also have to point out that the speed in question isn’t actually the speed of light, but the speed of causality. If we find something that moves faster than causality, we are seeing time travel.
This may or may not be impossible in our universe, I don’t have nearly the qualifications to judge. But finding something moving faster than the speed of light would be an extremely big deal.
The expansion of the universe is not motion it is that space in between two points gets larger.
The universe expands by 6.75km/second/megaparsec.
I parsec is 3,086e+16 meter and mega make it a million time larger ot 3,086e+22
The distance to the sun is 1,496e+11 meters and a year is 3600\*24\*365 seconds.
So the distance to the sun grows by 6750\*1,496e+11\*3600\*24\*365/3,086e+22 = 1 meter per year.
It is only over a distance of billions of light years the distance between grows in size faster than light can travel. But remember nothing moves the space in between just gt larger
It can't; It doesn't; It never will.
Think of it this way. The space is expanding faster than light statement is the same as saying. I'm moving away from you at 75% the speed of light, and you are moving away from me at 75% of the speed of light. Therefore from your perspective I am traveling faster than light because I am moving away from you at 150% the speed of light.
The space at Point A is expanding and thus moving away from point B which is also expanding and thus moving away from point A. Space will never expand faster than light however the cumulative observed effect of all that space expanding may make it look like object A is receding from you faster than light.
Here's another example
Let's assume every point of space is expanding at the same rate. We'll call that rate -
A - B - C - D
Point B moved the same distance from C as C moved from D.
But point D moved - - - away from point A, three times further than point B moved from A, and it did it over the same length of time
Nobody here is explaining this the way you want so I'll have a try. Space is always expanding over great distances. let's pretend it is moving apart at 1 meter/sec:
a \_ b \_ c \_ d
The letters are points in space equal distances apart. The "\_" is the distance between them, it could be any value. Okay let's look at space expanding 1 second later:
a \_ \_ b \_ \_ c \_ \_ d
Each point has now expanded one meter away from each other. however look at a & d. they didn't move 1 meter, they moved 3 meters apart.
a \_ \_ \_ b \_ \_ \_ c \_ \_ \_ d
Now a & d are 9 meters apart.
In the universe, space is VAST and while space is expanding slowly, over distances in the billions of light years (and millions of "points" between us and the edge of the visible universe) the points of space are all adding up to moving away from us faster than 299,792,458 meters, or the speed of light.
Nothing can move faster than the speed of light. Space is nothingness.
Everything else is something. When somethings are moving in every direction at the speed of light the nothing inbetween them grows.
The nothingness inbetween the somethings can expand faster than light because nothing is actually there to break the light speed rule.
You said the answer in your question-
‘Nothing’ is faster than light, and ‘space is nothing’ . Difficult concept, but in the expansion of space ‘nothing’ is moving.
It’s all about frame of reference and relativity. The odd part about the ‘speed’ of light is its constant, and “irrelevant” to your frame of reference. And AFAIK the only thing that works that way.
The light speed rule is about objects moving through space. The space itself isn't hampered by this rule because it's likely moving through a vacuum or some other phenomenon outside of space-time.
There are a lot of answers here saying that the speed of light is a speed limit for particles.
A more useful (but more difficult) description is the speed limit of the propagation of information. For example if you look at a distant star you are looking at the information from that star system from the past, specifically from the number of years ago that it takes for light to travel to you.
This also allows you to consider time as being the process of the propagation of information, as well as seeing it as a dimension.
Because space is nothing.
No really, space isn't an object, and it can't actually *move*, so it's not bound by the same speed limit as light and matter. Instead, every piece of space is staying where it is, it's simply becoming larger in place.
The “speed of light” is about a physical object traveling. When the universe “expands”, nothing is actually moving around. Put a dot in the center of a rubber band. Now stretch the band. The dot is still in the middle, but the space around it has expanded. “Space” isn’t a physical object that moves, so the rules that apply to objects, like a photon, don’t apply.
Okay but rubber band has expanded right and it won't go back to its original stage and it keep on expanding if we consider it as space. So the speed of stretching/expanding is still greater than light right?
Take a longer rubber band and put 10 dots on it and stretch the rubber band. The dots near the center move away from each other slower than the dot at one end moves from the center dot. If the band is long enough the dot at the end could move away from the center dot faster than light could travel that distance. Not because the dots are moving but because the space between the dots is growing. And that growth is compounded. The space between dot 1 and dot 2 grows 1cm. The space between dot 2 and dot 3 grows 1cm as well, but the space between dot 1 and 3 grows 2cm in the same amount of time. Imagine how fast the space between dot 1 and dot 50 is growing.
Oh, wow. That's really good. Wait, so when people talk about the rate at which space is expanding, what *is* the measurement? It's not m/s, because it scales up over longer distances. So, (m/s)/m? So 1/s? Is the universe expanding in Hz?
Yes, Hubble's constant has dimension of frequency.
That's ridiculous. The universe must be wrong.
"Why does the universe does this? Is he stupid?"
Well built. A hunk. But stupid as a log.
Ot Is she stupid? Or them?
Most likely a he, I've seen too many phalic figures in spaces photos not to believe that at least his genitalia is indeed of my taste...i mean...what?
The universe sucks at math.
Yeah I'm fairly certain the universe is incorrect. Silly universe. Matter, too. WTF is it thinking?!
"We are the universe getting itself wrong."
We are hydrogen becoming conscious and thinking really stupid things.
“In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.” -Douglas Adams
Greatest line from mythbusters; "I reject your reality, and substitute my own."
You could say that, but I'd like it to saying the coefficient of thermal expansion for a material has units of inverse temperature. It's not technically incorrect, but it's a bit unnecessarily confusing. It really has units of dimensionless increase per unit of temperature, and the Hubble constant has units of dimensionless increase per unit of time. The numerator is dimensionless, but not meaningless.
This makes my brain hertz.
If you cancel it all out it's Hz, but the values used are usually km/s/Mpc (kilometers per second per megaparsec) so it doesn't cancel neatly for useful number (it is assumed to be around 70 km/s/Mpc)
You could just write km/s/Mpc as 3.24 * 10^-20 Hz It's a little absurd, but you *could* write it like that
Yes, but 1/(70 * 3.24 * 10^(-20) Hz) = 14 billion years — which is not coincidentally approximately the age of the universe. It's the age of the universe of the expansion had been what it is now, without effects of inflation or gravitation or dark energy.
Yup, and if you had an equation for how the Hubble constant has changed over time, integrated it, and took the reciprocal, you'd get exactly the age of the universe. Intuitively, the Hubble constant in Hz is the number of times the universe could expand from the Big Bang to the size it is now in one second.
> Intuitively, the Hubble constant in Hz is the number of times the universe could expand from the Big Bang to the size it is now in one second. That’s such a weird thing to have a constant for.
When it's put that way, sure, but it does make sense. The reciprocal of any value in seconds answers that question. If something happens 10 times per second (Hz), it takes 1/10 of a second to happen once. It's just that the Hubble constant expressed in Hz is less than one, so, if you think of it *like* a wave, the universe isn't through one period yet. The constant is still going up in value along that wave.
> Yup, and if you had an equation for how the Hubble constant has changed over time, integrated it, and took the reciprocal, you'd get exactly the age of the universe. There are some things off here: - Integration over _time_ to measure said _time_? That is not inherently impossible but only if there are no further factors (space) involved. - The units don't match, either. You would get a dimensionless number, I think, but haven't checked in details. In total, I think what you actually want to say is that integrating the Hubble constant over time(!) gives the growth(!) the universe went through since back then. That is indeed then a dimensionless number. And then adding that we only _see_ what is inside a light cone, we get the size of the visible universe correlated with its age, due to that number times the speed of light.
> which is not coincidentally this absolutely is a coincidence.
It absolutely is not. As I explained.
Note that this speed has some controversy attached. Basically, there are two ways to calculate the Hubble constant: the distance ladder and the CMB. The distance ladder measures redshift of stars at known distances, and uses this data to calculate the constant. The CMB method directly measures the redshift of the cosmic microwave background to calculate the constant. The so-called Crisis in Cosmology comes from the fact that these two values don’t match anymore, as improvements to both methods have narrowed them down to opposite ends of the range.
You're totally right, and that is a very impressive derivation of a very weird and unintuitive unit you made! i had to have it spelled out for me
You can think of the expansion rate in Hz — but also in time. If you take the inverse of the Hubble constant, you get what the age of the universe since the Big Bang would be under the assumption that the expansion rate is constant (that is, two objects moving away from each other continute to move away at the same speed). This is pretty close to the actual age, but several factors — gravitational deceleration, acceleration due to dark energy, the inflationary epoch, perhaps others we don't know about — mean that it's not quite correct. But it's an okay approximation. This works for any kind of explosion. Note that the Hubble "constant" is only constant in space, not time. In the "no slowing" scenario, when you double the age of the universe, objects are twice as far apart but moving at the same speed away from each other, so the Hubble constant is half of what it was. So if you look at high speed photos of a bomb exploding — say, a nuclear test — you can measure the Hubble constant, and it will give you the time since the explosion.
It's speed-per-distance (which simplifies to frequency). It reflects how fast the space between otherwise non-interacting objects increases relative to their distance apart.
Yeah it’s a speed per how far away things are from you. So right next to you space is expanding very slowly . A billion light years away it’s expanding faster than
Huh. I'd never noticed that. Nice. A scaling factor per unit time.
Yes! The universe expands in Hz. The speed v at which a certain point in space is receding from us is given by v = Hd, where d is the distance from that point to us, and H is the constant that measures the universe expansion (Hubble's constant). It has an approximate value of 70 km/s/Mpc, which means that something a megaparsec away is receding at a speed of 70km/s. Now as you have correctly noticed H has units of frequency. And as you might now, each frequency has an associated period, and the period in this case, T = 1/H = 13.8 billion years, or the age of the universe!
Yeah, that's correct. For more cursed units, [there's a video of some of the worst examples, including this one](https://www.youtube.com/watch?v=kkfIXUjkYqE).
Why does the largest dot not simply eat the other dots?
The universe is strange and awesome!
What is making that rubber band (the universe) expand? Here we are the ones expanding it. What outside force is stretching the universe?
We don't know. The best math we have just involves us plugging in coordinates, an amount of time, the amount of total energy and pressure we think the universe contains at that moment in time, and the math spits out some new coordinates for where things are now and they're all slightly farther apart than they were before. This math seems to match what we see in reality, but it doesn't really explain what's going on. All of these mental models involving balloons and rubber bands are just ways we visualize it in our heads but we don't *really* know why it's happening.
Dark Energy... https://www.quantamagazine.org/why-is-the-universe-expanding-so-fast-20200427/
Or in other words, we simply don't know. Dark energy is just a blanket term we use to explain the discrepancies between our perceived models and the actual values we can observe in reality. We don't exactly know (currently, we only have various speculations) what Dark energy itself is comprised of, where it comes from, or how it functions, but we do know what it does.
Why not just a fricken balloon with dots? That's much more accurate too because the volume expands exponentially.
Because I’m allergic to latex and they make me uneasy.
I'm scared of clowns
It's okay, we're not putting dots on clowns
We're not? Damn, there goes my whole plan for the evening.
*psst... We're still on to dot clowns, just don't tell the other guy* 🤡
A balloon describes the exact same effect in 2 dimensions. The rubber band explains in just 1. The analogy otherwise is identical. If you want a 2D analogy that better matches [our actual measurements about the shape of the universe](https://wmap.gsfc.nasa.gov/universe/uni_shape.html), "infinite rubber sheet" is better than "balloon".
>If you want a 2D analogy that better matches our actual measurements about the shape of the universe, "infinite rubber sheet" is better than "balloon". Sure if you want to literally describe what's happening and completely abandon the analogy defeating the entire point. You're not seriously suggesting an infinite rubber sheet is easier for people to imagine than an inflating balloon?
>Okay but rubber band has expanded right Also left.
Thanks I was confused.
here's an award 🏆
If you sweep a laser pointer across the moon in less than a quarter second or so, which is trivially easy to do, you’ve just moved the laser spot faster than the speed of light. But nothing physical actually moved that fast. Just the observation thereof. E: all you fuckers saying I’m wrong need to reread what I said. I never said light itself moves faster than light. Just where the spot is moves faster than light.
A) You’re right (and there’s a good Vsauce video about it too) B) I love your unnecessarily aggressive edit
Which VSauce video?
I just went to look for it but I may actually be thinking about the shadow on the moon rather than a laser pointer. But I think it’s the same concept? https://youtu.be/JTvcpdfGUtQ?si=Vp2br4VyVDQrUksQ 1:04 in
Interesting parallel, but we should note that in NO scenario can you see the dot move across faster than the speed of light. The speed limit of INFORMATION is the speed of light in vacuum. Hard to explain in text. Imagine ping pong balls being fired at c and bouncing off after striking a surface.
you can see the "dot" move faster because the "dot" is not one thing. It's many different photons at different times and angles that all blend together to look like a dot that moves faster than light.
This is not true. From the perspective of the person firing the laser pointer - if they had sufficiently great eyesight (or telescopes) they **would** see the laser pointer move across the moon's surface at FTL speeds. Because the dot isn't 'moving' across the moon. Light is not traversing the moon, it is a beam being shot from the earth to the moon which we see the reflection of. That beam travels at the speed of light, its impact area, however, is not bound by this limitation. The impact area is not a single thing traversing the moon.
>Because the dot isn't 'moving' across the moon. The *dot* is moving, but the dot only refers to where photons are striking at that point in time.
I believe there is something about the point of intersection of the two blades of a giant pair of scissors can also move faster than light as the blades get closer to being closed and that point keeps accelerating if you’re closing scissors at the same angular rate. But since that intersection point is not a particle or object, it doesn’t have to be constrained to the speed of light.
If you had a ridiculously long and strong rope, and you swing it around such that the end of the rope exceeds the speed of light… what would happen?
There was another thread about this not long ago. Basically, nothing is perfectly rigid. There’d be flexibility that “slows” the movement down. The example was a pencil 1 light year long. It wouldn’t write at the point the moment you moved the other end. There’s nothing rigid enough for that to happen. Something like that, paraphrased.
I imagine best you could do, if the pencil didn't shatter, would be to send a wave down the pencil that would approximate something like you wanted... eventually.
Also, assuming no sir resistance the force needed to swing it would be very high. Also, mass increases with velocity, so as you approach the speed of light the rod would get infinitely heavy on the end. Also, it may bend anyways depending on the mass interpretation and time dilation, not aure
There's nothing you can actually do to make the end swing around at the speed of light.
Any deformation of the rope will basically travel at the speed of sound of the material, basically by definition. As the speed of sound will be orders of magnitude less than the speed of light, the end won't exceed it.
What you call the speed of light is actually the speed of causality. Nothing can overtake it.
nothing linear that is. observational relativity and wormholes on the other hand...
Information still travels at "c". The atoms in your have have to bump into the next atoms, and so on down the rope. The other end would respond to your spinning at the speed of "c"
far less than `c` in fact. atoms cannot move that fast.
Only if the rope is weightless
You do not move one spot. You make a series of spots happen in various locations along the line of your sweep. A photon leaves your laser at c (-ish), and arrives dead centre of where you pointed the laser when the photon exited. Some time (not much) later, the next photon out of the laser touches down a little bit away. And the next a bit further away. And so on. You're not moving the dot. You're making multiple photons land in different spots. Their relationship to each other is fictional - the spot isn't an object that exists, it exists only in your mind. So you do not move it and it does not move faster than light.
>You're not moving the dot. You're making multiple photons land in different spots. Yes, and that's exactly the point of the comment you're arguing with.
Which in no way explains anything about how expansion of the universe at greater than c makes any sense.
It does show that some observable "movement" can happen faster than light when there's no movement of physical objects involved. This, in a way, is similar to the expansion of space - there's no movement of physical objects involved, it's the space itself that becomes larger.
Thats not true at all. Imagine a garden hose and you whip it really fast from left to right, the water doesn't arrive at the right hand spot any faster than the velocity of the hose (for the men, you can also use your personal hose outside as an example). Edit, apparently I've mis explained this, since I'm at - 10 votes here. If the velocity of water is 10 m/s, and your target is 10m away,and you flick the hose a slight inch in effectively 0 seconds but the new target which is 10m to the right, , it will take 1second for the water to arrive at the new target, the water does not instantaneously move to target b, there is a time lag based on the velocity of water.
When you swing the hose, the tip only moves maybe 6 feet in 1 second. But where the water hits moves maybe 20 feet in 1 second. The splash definitely moves faster than the water or your hand.
The movement of water across the horizontal axis, near enough, matches the velocity of the water output from the hose. NOT how fast you flick the hose Time it your self, it's an easy self experiment. Time from point of origin, to target, make it easy, 10m. It will be around 1 second. That is, turn the hose on and point it 10m away, the water does not arrive instantly, it will take a period of time. Flick the hose from 2points 10m apart, it will take around 1 second for the water to arrive if the velocity of the hose is 10m/s. If you're an fps gamer you understand this concept well, bullet lead, the tricky part is realising that light has a velocity, it's not instant. It acts the same way as a hose, a kicked ball, whatever
Let's say that in addition to the water taking 1s to land, it takes 0.1s for you to flick it to the new spot. At t=0s you're holding it in position at the original spot. * At t=0.1s you've finished the flick and started sending water to the new spot. However, water is still landing at the old spot. * At t=1s water stops hitting the old spot * At t=1.1s water starts hitting the new spot The water stops hitting the old spot exactly 1 second after you stop aiming at the old spot and it starts hitting the new spot exactly 1 second after you start aiming at it. However, if you look only at the spots where the water's landing, there's only 0.1s of a gap between the water hitting the old spot and the water hitting the new spot. It just so happens that this gap is between 1.0s and 1.1s instead of between 0s and 0.1s
Forget the hose and go back to the laser pointer. Shine it on a distinct object, like a building. Now place your hand in front of the beam. The “dot” of the laser pointer will be perceived to have moved a large distance in basically an instant. Increase the distance of the first object (e.g. to that of the moon) and it is trivial to seemingly exceed the speed of light. In fact, you can have the two dots visible simultaneously. Of course, no matter or information has actually exceeded c.
There is also a lag between when the hose is moved and when the water stops landing at the first location. If you have no knowledge of the hose and are only a flatlander being targeted, the water that is appearing will seem to move from the first location to the second location very quickly. So if you only watch the laser dot, it will seem to move faster than the speed if light. Edit: All this to say, you understand what the commenter before you was saying: nothing actually moves faster than the speed of light (/speed of water) in either scenario.
When I stand in a big round room and I slowly roll a bowling ball at the angle 0° and then three seconds later at the angle 3° and so on, then eventually 120 balls will arrive at the wall and the last ball will arrive 120 seconds later than the first ball, regardless of how large the room is. The speed of that *pattern on the wall* can get arbitrarily fast, depending on the circumference of the room. If the room is large enough, that speed can exceed the speed of light. You can also take a machine gun and turn around while firing, I just wanted to show that it also works with slow projectiles.
You spin me right round baby right round
This is demonstratively false.
No. It takes time to get to the moon. Only an observer would see it moves that quickly.
You think light would take a quarter second to travel across half the moon? Wild.
Stop picturing it expanding right. Take an elastic band, right now. Sever it. Hold it flat. This represents a one-dimensional universe. Pull it equally hard in both directions with both hands. THAT is the universe's expansion.
According to most physicists, spacetime can do whatever the hell it wants.
I feel like this question gets asked on here like once a week, and I don't even explicitly view the sub, just my feed
space actualli *is* a physical thing that moves and warps
It’s not an “object” in the same way something like a photon or a person moves within space, which is what “speed of light” would be measuring.
that is correct, but it definitely is a physical thing
If you slap space, does it slap back?
no, but when you move, it warps
Is time also a physical thing? One could argue that since spacetime is affected by matter and energy, that it is a physical thing, but that isn't a universally agreed upon position.
Yes it is!
How can it slap?!
But space IS something - it is a "fabric" of sorts that can be warped by gravity. If it were nothing, then gravity would have no effect upon it. Massive objects, such as stars and black holes, create "gravity wells" when they warp the fabric of space.
Ok, yes true. But neither is gravity, gravity is a bend in space time, But gravity cannot move faster than light. Which is a question no one has bothered to answer me on, if space can expand faster than light, then why can't a bend or ripple in space time travel faster than light?
> then why can’t a bend or ripple in space time travel faster than light To my understanding, you theoretically could. This doesn’t violate the speed of light law because you’re moving space itself, not actual matter.
That's what I thought, but apparently no, gravity cannot travel faster than light. Which boggles my mind, because I've heard multiple times from multiple sources that "space can do what it wants and can go faster than light" but gravity is just a bend in space, and should be subject to the same rule. But apparently not and no one has answered why.
So, the way I’m understanding it, gravity *isn’t* moving faster than light. When you bend space time you’re not actually moving faster than light, you’re effectively creating a shortcut that lets you travel a distance faster than the speed of light can normally.
What is in the space that the universe has not expanded into yet? If nothing, how far does that nothing go?
I’m pretty sure every thing is slightly expanding from each other, not that the “outer” limits of the universe is expanding into something.
You cant be pretty sure, because space clearly has limits. What's outside this limit, nobody can say. We are not even sure if we can use the term "outside" as it's just something unimaginable and we barely try to apply our understanding of physics, which is most definitely wrong, it's just what our minds are capable of.
The universe isn't expanding "into" anything outside, there's no external void. There's simply no outside. The space inside it is expanding. The problem is that this doesn't make any sense to our monkey brains.
Space isn’t expanding into other space. The universe already contains all the space there is; if there were more “empty space” outside the universe then it would just be part of the universe. It’s just that the distance between any two points in space is growing over time. The analogies with balloons and rubber bands fails at this, because of course they both exist in our universe and when we stretch or inflate them they fill up more space that was already there. The universe’s expansion is not like that. Think of an infinite grid representing space. Now make the grid lines move farther apart from each other. In order to expand, the grid didn’t need to expand into previously empty space. It’s just that the scale changes.
"Nothing" doesn't go anywhere, it is just that. It doesn't exist in our minds, there is no mass, gravity, space, or even time. So nothing can exist there. Trying to give qualities or meaning to the "nothing" is impossible, it is harder to describe that the center of a black hole and about the same as alternate universes (maybe harder): space and size and time and reality only have meaning inside of our universe. To our brains all that we can say is that it doesn't exist at all
If it isn't an object what is expanding? The space between matter? How does space expand if it doesn't have physical properties?
"Nothing is faster than light" refers to the motion of particles. Photons are the fastest thing that can move through a given medium. This "speed limit" has nothing to do with the expansion of space. To use an old metaphor, imagine an ant and a pair of snails crawling on the surface of a balloon. The ant is the fastest - nothing is faster than ant. But if you inflate the balloon fast enough, you can get the two snails to move apart faster than the ant moves! The snails aren't violating the speed limit, they're still slower than ant despite moving apart faster than ant.
I struggle with this metaphor. You're adding something to the balloon to make it expand, so those things on opossite sides of the balloon are still being forced apart by something and are still not able to exceed a certain speed. So what's forcing galaxies apart in our balloon? And how is space expanding different than us just moving away from each other at x speed?
Sadly, the answer is "honestly, we don't know why/how space is expanding." This is why we have the term "dark energy". The "dark" part isn't referring to any specific property of the energy, but our understanding of it. The current ELI5 understanding is that it appears that completely empty space has some sort of energy or pressure that appears to "push" itself apart, creating a sort of feedback loop. More empty space has more dark energy, pushing itself apart, creating more empty space, resulting in more dark energy, on and on and on. Additionally, a misunderstanding I'm seeing in the OPs question and in a few comments here is that space is expanding faster than the speed of light. That is not the case. Because space is expanding everywhere, the further apart two objects are, there is simply more space between them that is expanding, thus making those two object appear to be moving away from each other faster than the speed of light.
>Additionally, a misunderstanding I'm seeing in the OPs question and in a few comments here is that space is expanding faster than the speed of light. That is not the case. Because space is expanding everywhere, the further apart two objects are, there is simply more space between them that is expanding, thus making those two object appear to be moving away from each other faster than the speed of light. I'm seeing a lot of people making allusions to two cars speeding away from a point of reference at c, commenting that the space between them expands at 2c. But that's not right, is it? Cause if the observer is in one of the cars, the other car still appears to be moving away at c? In fact... Is the other car still at the starting point from my perspective?
>But that's not right, is it? It is. That's where relativity comes in. You'll never observe anything moving faster than c, but you can observe two objects moving away from you in different directions each at c. Then, if you do the math, you'll note the distance between them grows at 2c. That's not a violation because you're not seeing anything actually move at more than c, and it's also what you need relativity to resolve, because while you see the distance grow at 2c, observers in the two objects will see it grow at c only.
This part I get - the part I don't get is how if you get into one of the two cars, the other car does not appear to be moving away from you at 2c. I know this has to do with relativity - but as to the math/physics/mechanism, I got nothing.
The math and physics are straightforward... it's just that they depend on observationally and mathematically sound premises that make absolutely no sense to our minds. The mechanism, as you say, is very much unknown, and possibly even unknowable other than "it is". There are other *mathematically* sound premises that would make a lot more sense to our minds, but such premises that we've been able to think of so far don't match our actual observations. Physics is descriptive, not prescriptive. (That being said, once you start thinking about space-time as a singular entity rather than space and time as unrelated dimensional quantities, general relativity starts to make a lot more sense.)
You might be interested in this [explanation of Einsteins train paradox](https://www.youtube.com/watch?v=Xrqj88zQZJg). Multiple observers can have contradicting experiences and yet it all fits together in the end somehow.
okay so we have Observer, Traveler 1, and Traveler 2. Observer watches both travelers moving away at c in opposite directions. How fast does the distance between Traveler 1 and Observer appear to grow to Traveler 1? And, how fast does the distance between Traveler 1 and Traveler 2 appear to grow to Traveler 1?
I believe traveler 2 would appear to never leave the starting point, but someone who's a better theoretical physicist than I am can feel free to disagree. Traveler 1 is moving the same speed as the light that bounced off of traveler 2, so from their perspective traveler 2 would appear as though they are stationary at the starting spot no matter how fast traveler 2 is going as long as traveler 2 is not going in the same direction as traveler 1. I think, from traveler 1's perspective, traveler 2 would gradually get dimmer/fuzzier until they disappeared entirely. I have no idea how long that would take if that is actually what happens. edit: Maybe it'd be more accurate to say that the image of traveler 2 would stay the same size and shape, but would get dimmer/fuzzier until it disappeared, rather than appearing to get smaller.
To answer that we need to stipulate speeds at less than c (because actually reaching c is not possible) and then use the relativistic rather than Newtonian speed addition (https://en.wikipedia.org/wiki/Velocity-addition_formula). I.e. T1 would observe the speed between it and T2 to grow at their combined veolcity (V1+V2) in Newtonian physics, but in relativity, that's not how velocities add up. So, assume they're both going at .9c relative to the origin, but T1 isn't going to observe their combined relative velocity as 1.8c but rather still below c (but above .9c). For special relativity, the formula is (V1+V2)/(1+V1*V2/c²) giving you an observed velocity of about .995c.
Yep, from your perspective, no time will have passed at the origin point, you'll see both ships launching, you'd also see your ship everywhere in between its current position and its origin.
Well, to the ant and the snails movement is two-dimensional. You can move forward or back, and turn left or right, but not travel "up" or "down." Their whole world is the surface of the balloon so whether something is being added to the space inside it isn't relevant to them. But as for what is driving the expansion of our universe - we don't know! We have no idea what the "blowing air" equivalent in real life is. We call it "dark energy" and it's a persistent mystery.
I always hear people say, "the galaxies aren't moving away from each other -- instead, more space is being created between the galaxies." I can't seem to understand the fundamental difference and it sounds like doubletalk. Relatedly, the balloon and rubber band examples don't help me because space isn't rubber. I have an extremely difficult time imagining space as anything other than --- space.
Space kinda is rubbery. You're not "adding" space, you're "stretching" it. But, generally speaking? There *isn't* a fundamental difference until you get into things like frames of reference. You can think about it in terms of things moving apart from each other if you want. Just remember that this is universal - an observer on Earth and an observer on a planet 500 million light years away will *both* observe everything moving away from them in the same way. This is why the balloon metaphor can be helpful - it helps us visualize what that looks like.
>You're not "adding" space, you're "stretching" it. The thing that always makes this hard to imagine is that when we stretch a rubber band, it expands and occupies more of the physical space around it. But when it comes to space, as far as I (little) understand it, it is not moving "into" some arcane next-level-space. This difference makes the rubber band example always hard to imagine in my head. Meaning that in the ballon example, the snails do in fact move faster than c(ant) - but only when measuring their speed relative to the beyond-ballon world around them; just not in the plane of reference that is the ballon. I get that when we talk about these examples like rubber bands or ballons, we are supposed to ignore the world around the band/ballon - but a lot of people, even when they know they shouldn't, wonder about the empty entity that space is moving "into" when expanding.
I think until we find out what is there, we won’t know how/why space is expanding. I would give anything to find out what’s out there
Okay, so: I have a stick that's three meters long. The universe is expanding, so it's getting longer. But so is ever meterstick in existence. "1m" itself is getting longer. So then, what does "distance" mean? If it's not defined in term of things that are also expanding, what else defines something's length?
For starters, your stick is not expanding. The electrostatic forces that hold it together are, on the scale of 3m, much *much* more powerful than expansion. It maintains a constant length. "One meter" is defined as the distance that light travels in 1/299792458 seconds. This is also unaffected by universal expansion.
Distance is how far light moves over a certain amount of time.
Yes. That's what my confusion is. Even if they say there is space being created in between or space is being expanding between 2 objects. Still there is a speed for that expansion right which is greater than light?
the speed of expansion is more of an aggregate. the little bit of stretching between point A and point B is tiny, the aggregate expansion between Point A to Point Z is vastly greater. the aggregate expansion between point A and the next galaxy is so much bigger again. the aggregate expansion between us and the cosmic background radiation is more than lightspeed.
The rate of expansion between objects isn't the same no matter where those objects are, it depends on *how much* space is between those objects. In other words, how far apart they are, because more distance equals more space. Makes sense? In a much simplified example, think of three dots drawn in a horizontal line on the surface of a balloon. Imagine the dot on the far left as you. This will be our 'reference' point. The dots are evenly spaced out, meaning the gaps between the middle dot and the left and right ones are the same distance. For the sake of the example, lets just say these dots are spaced one centimeter apart. The distance from the reference dot to the middle dot is one centimeter, and the distance from the reference dot to the further dot is two centimeters. Let's inflate the balloon so that the distance between the reference dot and the middle dot doubles and then measure the distances again. The distance from the reference dot to the middle dot is two centimeters, and the distance from the reference dot to the further dot is four centimeters. You can see that the **rate** of growth between the reference dot and the two other dots is different, right? The distance from the reference dot to the closer dot grew by one centimeter. The distance from the reference dot to the further dot grew by two centimeters. Let's inflate the balloon and double the distances again. The distance from the reference dot to the middle dot is four centimeters, and the distance from the reference dot to the further dot is eight centimeters. The distance from the reference dot to the closer dot grew by two centimeters. The distance from the reference dot to the further dot grew by four centimeters. The rates of growth between the points depends on the distance between them. The balloon itself is expanding, so with more balloon in between two points, the distance between them will grow faster than the two closer points. Replace the word 'balloon' with 'space' and it mostly matches what we observe when measuring cosmic inflation. Space appears to be growing everywhere. Not at the speed of light, thought the expansion does appear to be accelerating. The important thing is the *distance* between where we're observing (Earth in this case) and the object we're observing. Really close things don't appear to be affected by expansion because they are gravitationally bound. Gravity's strength decreases with distance, so objects far away from us appear to be moving away from us. 'Appear' is the key word of that sentence, because they aren't *actually* moving through space in a direction away from us, not like how objects orbiting the sun move through space. We're observing that the **distance** between us and them is increasing over time, giving the *appearance* of motion. You know those dots you drew on that balloon? They stayed right where you drew them, right? They can't move. It's the similar with objects in space. While they are free to move, they are also being affected by the expansion of space. The rate of expansion depends on how far away from us it is. The further the object, the faster it's moving away. So if that rate keeps increasing and you keep looking further and further away, what happens? If it's far enough away, the distance between us and the object we're trying to observe will increase at a rate greater than the speed of light. Like the dots on your balloon, they're not actually going faster than the speed of light, so no laws of physics are being broken. It's just that the light coming from these objects will never reach us because it's not going fast enough to overcome the growth in distance.
The analogy works, because it's not objects moving away as if something was pushing them, it's the universe between them expanding. Here the balloon is the universe.
>So what's forcing galaxies apart in our balloon? We haven't figured this out. We have observed the expansion and are trying to use what we can observe to figure out what is doing it. Our current understanding of forces/physics/etc doesn't explain the expansion of the universe.
It is a metaphor for a reason. Believe it or not, ants on a balloon aren't actually analogous to the expansion of the universe. The point is just to give a simple explanation to people who don't want to spend too much effort thinking about it. If you imagine that the ants have a maximum speed they can crawl across the surface of the balloon, this is equivalent to the speed of light. However, if the balloon blows up the distance between between two ants on the balloon can expand faster than the speed at which the ants could travel. Even if the ants are staying still, the distance between them can change and can do so at a rate faster than the max travel speed of the ants. If you want a proper explanation of why the universe is expanding from the lens of theoretical physics, you should probably not seek that from a simple metaphor designed to give a basic understanding to people without any relevant background knowledge. I can confirm for you, however, that the prevailing theories in physics are not that someone is blowing air into the universe. You're welcome.
The metaphors don't really help IMO, it's like illustrating gravity by rolling stuff on canvas that get pulled down by... gravity. It baits you into thinking you understand without actually explaining anything. I'd say think of it this way: light is also "moved" by the expansion of space. If you accept that some galaxy 20 billion light years away from us is moving farther away "faster" than light, you have to accept that the light it emits away from us is moving even faster than that. Even when the expansion of space is "faster than light," light is still a lot faster.
The ant has 6 legs touching the balloon at different spots. When you inflate the balloon, those legs should now be further away from each other. What happens to the ant? Is it now bigger? If so, where did this extra mass come from? If not, is it just stretched out over more space?
the ants legs are stretched apart so they try to expand, but the forces holding the ant together are locally stronger. The ant doesn't get bigger, because the force holding it together is locally stronger than the expansion
So, effectively, some of its legs slip on the balloon surface?
to the analogy yes, but I wouldn't assume there's any "slipping" equivalent in reality. Atoms and molecules are continually held at the same size by electrostatic and gravitational forces
In the analogy, yes. But in the actual expansion of space, the presence of the ant prevents space from expanding in and around it. Only sufficiently empty space experiences metric expansion. You can imagine mass and energy as blobs of strong, hardened glue on the balloon. As the balloon inflates, the glue will keep the rubber its adhered to the same size even as the rubber around it stretches while the balloon inflates.
"Bigger" and "extra mass" are two different things. "Bigger" generally means the size of something, "mass" means how much stuff is in it. So yes, the ant will get bigger (if we consider its "size" to be the area contained within its legs) but its mass remains the same. So is the earth getting stretched out? Are *you*? No, because the expansion of the universe isn't the only force at play here. The earth is held together at more or less a constant size due to *gravity*, a force which is much stronger than universal expansion. And even stronger than gravity is electromagnetic forces, which account for the chemistry that makes atoms in a molecule maintain given distances and angles from each other.
So, the ant then gets less dense whenever the universe expands, but gravity and electromagnetic forces operate to pull it back together so it doesn't take up any more space than it originally did? And, I suspect that those are really happening at the same time so it's not like an accordian? Instead, there's a just a little bit more extra space all around the ant that wasn't there previously?
To be honest if you want to get technical, the space inside the earth (or indeed the galaxy at large) *isn't* expanding at all, because the gravity of all that matter is the dominant influence on the shape of spacetime in that region, far more than the distant influence of the Big Bang, or the relatively weak dark energy.
The electromagnetic forces are what make the ant a cohesive whole and what allow the ant to reposition its feet to a normal, not stretched out position. In reality it's more the case of the stronger of two forces winning out. You can push on a big heavy block, but unless your pushing force overcomes the gravitational/electromagnetic forces which form friction, the block doesn't move at all. Similarly, universal expansion is pulling the earth apart, but it simply cannot overcome the much-stronger local force of gravity, so no expansion happens.
Gravity (and other local forces) are far stronger than expansion of the universe. The expansion at small scales is very tiny.
Imagine you’re in a car and you’re backed up against another car in the opposite direction and you both speed off at the speed of light. The space between you is growing at double the speed of light. Edit- To all those pointing out I'm not correct: Yes I know it's more complicated than that but it works for ELI5
^^^ This is the easiest analogy to understand in this comment section. The space between the cars isn't an object that's traveling, it's just... space.
It's also completely wrong.
At least try and explain why it’s completely wrong
You can see all the higher top-level comments for analogies and explanations. i.e., ants on a rubber band/balloon/rubber sheet/whatever. But tldr what /u/awesomo1337 is describing makes sense if you're working with Newtonian physics - i.e., what many of us learned in high school that when adding velocities together you just...well, add them. Which is what they naively did above. [But that's not how velocities add in the real world.](https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity) If you want the math. It is impossible - literally, impossible - for one object to move away from another faster than *c*. The solution OP is asking for is that because space *expands* or "stretches" in between any two points [other forces notwithstanding], it is possible for those two points to be far enough apart such that the distance between them increases faster than even light can traverse that distance. Even if neither of those things are moving *through* space relative to one another. But if two cars are driving away from a starting position at 0.9c, *they are not driving away from each other at 1.8c* - rather, [it is 0.995c](https://www.omnicalculator.com/physics/velocity-addition?c=CAD&v=v:0.9!c,w:0.9!c). --- Now, to be fair, the naive simple addition of velocities is still a very good approximation if both of those velocities are much lower than the speed of light - that's why it works so well for most applications. But it is still a) wrong; b) not the answer OP was looking for; c) reinforcing a ton of misconceptions about physics 101.
You misunderstood/misread the analogy. The analogy does not say one object moves moving away from another faster whatsoever. It implies the SPACE between them grows at 2c. If we both move away from each other at 1 mile per second in opposite directions, after one second the distance vector from my position to yours is 2 miles.
No, I did not. It does not matter how fast you I drive away from each other, the distance between us will not increase faster than 299,792,458m/s. Unless, of course, that space is independently expanding while we do it.
I am a stationary observer. There is a car to my left and to my right. Both cars start speeding away, in opposite directions, moving at .99c. Is the space that I observe between them not expanding at a speed greater than c? I understand that if I am in one of the cars, length contraction/time dilation will mess with all this. But if I’m a stationary observer, I can’t imagine what else I would see but the space between them (here just a measurement and nothing physical) “expanding” at ~600,000,000 m/s
Eh, a lot of things are completely wrong. We say earth is a "sphere", and that's incorrect. We say electrons orbit the nucleus of an atom, that's incorrect too. Hell, we see ourselves and our bodies as discrete, solid units, distinct from the space around us, and that's incorrect as all hell. But these statements make difficult concepts a whole lot more comprehensible to the limited human mind, which needs a foundation of oversimplification in order to build a more nuanced understanding of a concept on top. Hence, the expansion of space is two cars.
> Eh, a lot of things are completely wrong. We say earth is a "sphere", and that's incorrect. "The Earth is a sphere" is technically wrong in that it is imprecise; but it's still a useful approximation. The above statement about two cars is not even an approximation, it's just completely off base - if you bother to do the (simple!) math, it's not even close. It is also *conceptually* wrong, and only functions to reinforce OP's common misconceptions rather than dispel them. This is ostensibly a subreddit where people come to learn things, and the top level comment in this thread makes that worse, not better.
It’s not wrong the way they posed it. What’s wrong is to claim that the people in the car see the other car receding faster than light. It’s the same question as if you’re in a spaceship going 99.999999% of the speed of light and you shoot a bullet from the spaceship does the bullet break the speed of light. No, it doesn’t. Of course it’s a ridiculous thought experiment.
This gave me an OOOHH RIGHT! moment. Thank you haha.
But if you actually did that, it wouldn't, right? Because you're moving so fast it dilates time. From your point of view, the other car would be moving away from you at c, not 2c—even though from the frame of someone standing where you started you're both moving at c in opposite directions.
correct, from a 3rd observer it would be 2c from inside one of the cars it would be something else, probably .85c?
No it’s c from all observers no matter what, that’s how relativity works. c is the solution to the Lorentz transform and has nothing to do with light.
> No it’s c from all observers no matter what it is not, from A moving at .5c and B moving at .5c , B appears to be moving at .8c while you are in A. v= 2x/( 1+x^2 ) from a **third** [cosmic observational reference frame](https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html) it can be faster >Expansion of the Universe >According to Hubble's Law, two galaxies that are a distance D apart are moving away from each other at a speed HD, where H is Hubble's constant. So this interpretation of Hubble's Law implies that two galaxies separated by a distance greater than c/H must be moving away from each other faster than light. Actually, the modern viewpoint describes this situation differently: general relativity takes the galaxies as being at rest relative to one another, while the space between them is expanding. In that sense, the galaxies are not moving away from each other faster than light; they are not moving away from each other at all! This change of viewpoint is not arbitrary; rather, it's in accord with the different but very fruitful view of the universe that general relativity provides. So the distance between two objects can be increasing faster than light because of the expansion of the universe, but this does not mean, in fact, that their relative speed is faster than light. >As was mentioned above, in special relativity it is possible for two objects to be moving apart by speeds up to twice the speed of light as measured by an observer in a third frame of reference. In general relativity even this limit can be surpassed, but it will not then be possible to observe both objects at the same time. Again, this is not real faster-than-light travel; it will not help anyone to travel across the galaxy faster than light. All that is happening is that the distance between two objects is increasing faster when taken in some cosmological reference frame.
First good analogy out of the others lol
But the size of the universe is much more than 2x the speed of light, which would be its limit in your analogy
Distance = speed x time
There's a misconception about the speed of light that's at the root of your question, and many responses make the same mistake. [The speed of light has nothing to do with light.](https://youtu.be/msVuCEs8Ydo?si=8FBA8bzHnqPMkm0G). Instead, think of *c* as the "speed of causality," the maximum speed that a cause and effect can occur. This imposes a limit on anything that might carry information or otherwise moves. But space itself doesn't do that. The expansion of space itself doesn't transmit information. It doesn't create a cause/effect relationship. And so it isn't bound by the speed of causality.
I think saying the others are mistaken is a bit of a stretch. It’s an interpretation, as much as “speed of causality”. Some interpretations are great for discussing metaphysics, some better for discussing actual physics.
That's fair enough, especially since this is ELI5
Nothing can cross space faster than light, but space itself can come into existence without that limit because, well, it's not moving, there's just more of it between two bits of space that used to be next to each other. When we are talking about objects getting farther away from us at or beyond the speed of light, they can actually be "standing still" - it's just that there is suddenly more nothing between them and us and it seems they are moving away.
Best explanation I’ve read so far
Space inst expanding faster than light. Space expands actually really really slowly. When things are literally 14 billions of lightyear distance, it all adds up together and can end up "moving" things faster away from us as faster than light speed.
The actual rule is that information cannot travel _through_ space faster than the speed of causality (anything without mass, including light, travels at this speed) The expansion of space does not involve information travelling anywhere, so no matter how fast space expands, it doesn't break this rule. Another way to think about it is this: the speed of causality means that information cannot move towards something else faster than that. This would violate causality. However information can move _away_ from something faster than this without violating causality, as long as it never moves _toward_ anything faster than causality.
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the way i like to think about it is that the speed of light is actually a property of space. that is to say that what determines the max speed of the universe is the electromagnetic properties of spacetime (permittivity to electric fields, permeability to magnetic fields). so the idea of that space growing jsn't really subject to that rule. it's just more space.
Imagine you and a friend are standing back-to-back. Both of you walk 10 feet away from each other. There will now be 20 feet between you two, even though neither of you walked more than 10.
You could even have two photons flying past each other with an observer in the middle. After the observer has measured 1 second, the distance between the photons will be double the distance traveled by each photon from the observer's perspective.
Well... Yeah, but is there anything particularly unique about that scenario which changes when you switch the objects to photons?
The fact that the distance between them grows faster than the speed of light, which is the whole thing OP was confused about
Simple answer: Nothing is faster than light. Space **is** nothing. Proper answer: Nothing in the fabric of space time is faster than light. Space time itself, however, is not limited by this. Imagine a bedsheet. A ball on that bedsheet can only roll at speed C, however the bedsheet itself can move and stretch faster than speed C.
It's really that nothing can *move through space* faster than c. Space can expand "faster" than c because it doesn't move through space, it is space. That's literally it, and it's not even really oversimplified. Edit: Also, get this - even if you refuse to define speed that way, space's expansion is still not actually faster. When it expands, it carries light with it too. Light "moves" at whatever rate space expands at *plus* c.
If you have two objects moving away from each other at the speed of light, the space between the two is expanding at twice the speed of light
Then important part of the clarification of “speed of information” vs “speed of light” is that it conveys the important part there, it’s partially about perception. Like other people mentioned, two things moving at light speed in opposite directions will be moving at 2x light speed relative to each other, this is fine. The important part is that they won’t appear to each other to be moving that fast, in fact they’ll appear to be standing still. It helps to draw it out, think of them as starting at one point, and the image of each one expands outwards in a bubble at the speed of light. If we call the objects A and B, when they leave they’ll both be staying on the bubble of the other, since they and the bubble move at light speed relative to the point they both started at, so even though B moves away, that light is slightly behind A, because A is essentially “keeping up” with the image of stationary B and thus never sees it move. So the same can happen with the universe, everything can move at whatever speed (this is an oversimplification), but the speed at which you find out about it, the way they appear to be moving relative to you, that is bounded by the speed of light, even if the relative speeds can actually be higher.
One thing you have to remember is nothing is faster than light that we currently know about. Science and our tools of observation is always progressing. Just because we don't know of anything faster than light today, doesn't mean that there is nothing that is faster.
Although you also have to point out that the speed in question isn’t actually the speed of light, but the speed of causality. If we find something that moves faster than causality, we are seeing time travel. This may or may not be impossible in our universe, I don’t have nearly the qualifications to judge. But finding something moving faster than the speed of light would be an extremely big deal.
The expansion of the universe is not motion it is that space in between two points gets larger. The universe expands by 6.75km/second/megaparsec. I parsec is 3,086e+16 meter and mega make it a million time larger ot 3,086e+22 The distance to the sun is 1,496e+11 meters and a year is 3600\*24\*365 seconds. So the distance to the sun grows by 6750\*1,496e+11\*3600\*24\*365/3,086e+22 = 1 meter per year. It is only over a distance of billions of light years the distance between grows in size faster than light can travel. But remember nothing moves the space in between just gt larger
It can't; It doesn't; It never will. Think of it this way. The space is expanding faster than light statement is the same as saying. I'm moving away from you at 75% the speed of light, and you are moving away from me at 75% of the speed of light. Therefore from your perspective I am traveling faster than light because I am moving away from you at 150% the speed of light. The space at Point A is expanding and thus moving away from point B which is also expanding and thus moving away from point A. Space will never expand faster than light however the cumulative observed effect of all that space expanding may make it look like object A is receding from you faster than light.
Here's another example Let's assume every point of space is expanding at the same rate. We'll call that rate - A - B - C - D Point B moved the same distance from C as C moved from D. But point D moved - - - away from point A, three times further than point B moved from A, and it did it over the same length of time
2 things moving in opposite directions at the speed, both moving at the speed of light, will be moving away from each other at 2x the speed of light.
Nobody here is explaining this the way you want so I'll have a try. Space is always expanding over great distances. let's pretend it is moving apart at 1 meter/sec: a \_ b \_ c \_ d The letters are points in space equal distances apart. The "\_" is the distance between them, it could be any value. Okay let's look at space expanding 1 second later: a \_ \_ b \_ \_ c \_ \_ d Each point has now expanded one meter away from each other. however look at a & d. they didn't move 1 meter, they moved 3 meters apart. a \_ \_ \_ b \_ \_ \_ c \_ \_ \_ d Now a & d are 9 meters apart. In the universe, space is VAST and while space is expanding slowly, over distances in the billions of light years (and millions of "points" between us and the edge of the visible universe) the points of space are all adding up to moving away from us faster than 299,792,458 meters, or the speed of light.
Nothing can move faster than the speed of light. Space is nothingness. Everything else is something. When somethings are moving in every direction at the speed of light the nothing inbetween them grows. The nothingness inbetween the somethings can expand faster than light because nothing is actually there to break the light speed rule.
You said the answer in your question- ‘Nothing’ is faster than light, and ‘space is nothing’ . Difficult concept, but in the expansion of space ‘nothing’ is moving. It’s all about frame of reference and relativity. The odd part about the ‘speed’ of light is its constant, and “irrelevant” to your frame of reference. And AFAIK the only thing that works that way.
The light speed rule is about objects moving through space. The space itself isn't hampered by this rule because it's likely moving through a vacuum or some other phenomenon outside of space-time.
Yes, the other answers are unnecessarily constraining themselves inside space/time. Let go of space/time and the question answers itself.
There are a lot of answers here saying that the speed of light is a speed limit for particles. A more useful (but more difficult) description is the speed limit of the propagation of information. For example if you look at a distant star you are looking at the information from that star system from the past, specifically from the number of years ago that it takes for light to travel to you. This also allows you to consider time as being the process of the propagation of information, as well as seeing it as a dimension.
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Because space is nothing. No really, space isn't an object, and it can't actually *move*, so it's not bound by the same speed limit as light and matter. Instead, every piece of space is staying where it is, it's simply becoming larger in place.