Can the Universe Expand Faster Than Light?

[Jakecp]

I have been thinking about light and i have read that the universe expanded faster than speed of light. So , then why people say that the speed of light is the fastest thing known if people know that the universe expanded faster than speed of light . Also , if the universe could have expanded faster than the speed of light , can we make something that can travel faster than speed of light using expansion ?

 

[Isaac0427]

Well, let me correct you on one thing- we know almost nothing about how the universe expanded, or even about how it expands now. It is all theoretical. If it is true, however, the speed of light still would be the fastest speed, because the speed of the universe really doesn't count. We will never achieve a speed greater than the speed of light. It is against the laws of physics. I am not sure, but I believe it is addressed in the FAQs why that is. I can kind of explain why, but I am going to let a more experienced person give you a more detailed explanation.

 

[davenn]

hi there

have a look at this wiki article on metric expansion, it should answer you Q's

https://en.wikipedia.org/wiki/Metric_expansion_of_spacecheers
Dave

 

[Isaac0427]

I just want to add, I checked, and there is nothing on the FAQs. (Mentors, I would suggest adding an FAQ about the speed of light, I have noticed a lot of similar questions on here).

 

[davenn]

I just want to add, I checked, and there is nothing on the FAQs. (Mentors, I would suggest adding an FAQ about the speed of light, I have noticed a lot of similar questions on here).

this thread isn't to do with the speed of light as such, rather the expansion speed of the universe
which is what the link I provided give lots of info on

there is a FAQ thread on the speed of light ...

https://www.physicsforums.com/threads/why-is-the-speed-of-light-the-same-in-all-frames-of-reference.534862/

Dave

 

[phinds]

Well, let me correct you on one thing- we know almost nothing about how the universe expanded, or even about how it expands now. It is all theoretical.

Nonsense. We know quite a lot about the current expansion. For example, we know that the recession of galaxies at the edge of the observable universe is about 3c. There is nothing theoretical about our knowledge of the current expansion even to the point of knowing that the rate of acceleration is decreasing slightly over time. Things that are totally supported by experiments/empirical evidence are way beyond just theoretical.

If it is true, however, the speed of light still would be the fastest speed, because the speed of the universe really doesn't count.

True. It is not proper motion, just recession.

We will never achieve a speed greater than the speed of light. It is against the laws of physics.

also true although I hesitate to use the word "never" in such contexts even though I think it applies in this case.

 

[Jakecp]

I don't agree , it should count as motion because we are moving space while it expands because when the universe expand it can't be filled with nothing . It must be filled with dark matter or something so it should count as motion and then there is a speed faster than light even though it does not count as motion and we may be able to make some kind of expansion with 3c speed or more . Also , who wrote the rules of the universe? Why c is the fastest speed? Why it should be? It has no Mass right? it should also not count as motion because it is just a wave or nothing as the space expansion. Why space expansion does not count and light does? Light is not motion because nothing at all is moving because light has no mass.

 

[davenn]

I don't agree , it should count as motion because we are moving space while it expands because when the universe expand it can't be filled with nothing . It must be filled with dark matter or something so it should count as motion and then there is a speed faster than light even though it does not count as motion and we may be able to make some kind of expansion with 3c speed or more . Also , who wrote the rules of the universe? Why c is the fastest speed? Why it should be?

So you have a couple of PHD's hanging on your wall that you can make such unsubstantiated claims ?
it doesn't really matter if your agree or not, studies have been done on the subject by people well above your and my pay-rates and intelligence

Did you even bother to read through the link I gave ? did you try and understand any of it ?

I suggest strongly that you take a few steps back and do some learning before arguing against some well established theories

Dave

 

[phinds]

I don't agree ...

Irrelevant. It's not up for a vote. As Dave said, you should learn some basics before spouting nonsense. This stuff is not intuitively obvious but thinking that your opinion trumps facts is not science it's voodoo.

 

[Isaac0427]

Nonsense. We know quite a lot about the current expansion. For example, we know that the recession of galaxies at the edge of the observable universe is about 3c. There is nothing theoretical about our knowledge of the current expansion even to the point of knowing that the rate of acceleration is decreasing slightly over time. Things that are totally supported by experiments/empirical evidence are way beyond just theoretical.

Yes, I shouldn't have added the part about the current expansion. I was just trying to make the point that what happened in the past is impossible to know. I have read many theories about what could have happened. We don't know anything about it for sure.

 

[Isaac0427]

I don't agree , it should count as motion because we are moving space while it expands because when the universe expand it can't be filled with nothing . It must be filled with dark matter or something so it should count as motion and then there is a speed faster than light even though it does not count as motion and we may be able to make some kind of expansion with 3c speed or more . Also , who wrote the rules of the universe? Why c is the fastest speed? Why it should be? It has no Mass right? it should also not count as motion because it is just a wave or nothing as the space expansion. Why space expansion does not count and light does? Light is not motion because nothing at all is moving because light has no mass.

Ok, then if you want to count it as faster than the speed of light, try recreating the Big Bang. As @phinds said, it is just recession, not proper motion, and light is motion. It just is. It is fact. Many, many expariments have tried to pass the speed of light. None have succeeded. One claimed to have succeeded but it was a bad expariment, and they retracted their results.

 

[Orodruin]

One claimed to have succeeded but it was a bad expariment, and they retracted their results.

If you are thinking about the OPERA experiment it was not (and is not) their physics goal to pass the speed of light. Their neutrino speed measurement was a byproduct, something they could do in addition to their main physics goal (which is measuring neutrino oscillations into tau neutrinos). It is not a bad experiment in itself and it has been successful in its main physics goal. They simply suffered from being a bit too eager to announce their FTL measurement when they probably should have taken a step back and triple-checked everything.

 

[bapowell]

Yes, I shouldn't have added the part about the current expansion. I was just trying to make the point that what happened in the past is impossible to know. I have read many theories about what could have happened. We don't know anything about it for sure.

Yes, it is impossible to know what happened in the past with certainty, but that doesn't mean that all ideas are equally relevant. This is why humans have developed science: we can make observations, develop models, and make inferences about the universe -- both past and present -- that can then be tested.

Many historical observations can be well-understood in the context of modern science: if you came across a crater, how much would you bet it was due to a meteorite and not an abandoned excavation project by ancient earth-visiting aliens? Neither can be verified with certainty, but I suspect I know which explanation you're betting on.

 

[bapowell]

I have been thinking about light and i have read that the universe expanded faster than speed of light. So , then why people say that the speed of light is the fastest thing known if people know that the universe expanded faster than speed of light . Also , if the universe could have expanded faster than the speed of light , can we make something that can travel faster than speed of light using expansion ?

The universe does not expand at a given speed. That's misconception number 1. The speed at which objects recede from one another, v, is a function of the expansion rate, H, and their distance apart, r, via Hubble's Law, v = Hr. The more distant an object, the faster it appears to recede from us. This is true no matter what kind of expansion we're talking about. Notice that as long as H is positive, there is always a distance at which objects will recede with a speed greater than light, r = c/H. There is nothing wrong with this since the objects are at rest locally, their relative velocity due solely to the expansion of space. So misconception number 2, which is regrettably pervasive in the popular science arena, is that the universe underwent some kind of superluminal expansion early on. These are references to primordial inflation, which indeed was a unique phase of expansion in the early universe, notable because length scales increased exponentially.

So, no, space does not -- ever -- expand faster than the speed of light because expansion does not occur at a given speed. And, while objects do indeed recede from one another at certain speeds, and if they are far enough apart they will recede form one another at speeds surpassing that of light, this is true always and is not a unique property of inflation or any other type of expansion.

 

[Isaac0427]

If you are thinking about the OPERA experiment it was not (and is not) their physics goal to pass the speed of light. Their neutrino speed measurement was a byproduct, something they could do in addition to their main physics goal (which is measuring neutrino oscillations into tau neutrinos). It is not a bad experiment in itself and it has been successful in its main physics goal. They simply suffered from being a bit too eager to announce their FTL measurement when they probably should have taken a step back and triple-checked everything.

That's interesting. I was not aware of that, all I knew was the false neutrino speed part.

 

[rootone]

The OPERA result turned out to have a very mundane explanation eventually.
A fiber optic cable was wrongly connected.

 

[Jakecp]

wait , i am not sure of what i am going to write but , galaxies are going apart from each other faster every time right? so you say that they receed at 3c which means that they are moving at 3c further away?

 

[Jakecp]

Also , we know almost nothing of dark energy ( i think) what if dark matter and energy can travel faster than C? did Einstein knew of this in his relativity theory?

 

[phinds]

wait , i am not sure of what i am going to write but , galaxies are going apart from each other faster every time right? so you say that they receed at 3c which means that they are moving at 3c further away?

That is a very garbled sentence so I'm not sure what you are asking. There are objects at the edge of the observable universe that are receding from us at 3c. There are objects that are closer to use that are receding at 2c. There are objects that are closer still that are receding at 1c. Pick a number. You can find an object at the appropriate distance that is receding at the number times c.

This is NOT the "rate of expansion of the universe", its just the recession of objects compared to Earth.

 

[phinds]

Also , we know almost nothing of dark energy ( i think) what if dark matter and energy can travel faster than C? did Einstein knew of this in his relativity theory?

That makes no sense at all.

Dark energy is not a thing that can have a speed and dark energy and dark matter are totally unrelated to each other. Einstein did not know of either one, and that makes no difference to anything at all.

 

[Ironmanlet]

I am sure it has been covered already, but the finite speed of light limits particles speed.

space is not a particle or matter. Matter can not move at the speed of light. Space is not matter.

 

[Jakecp]

Well , if he didn't took that in consideration , why can't dark matter or energy travel faster than c? I know that is the rule einstein once said but why? i have not read the relativity theory yet and i know that c is that fast because it has 0 mass , but how did he knew that , that was the fastest speed ? I think light can be slowed down but don't know how. The fact , is there any chance of something beeing faster than light?

 

[Jakecp]

If space is not matter , then what is it? How should i imagine the limits of space? What limits the space ? A wall? Nothing? If nothing limits the space , how can it have a limit if the limit is nothing ?

 

[Jakecp]

So , another question , the universe is expanding from everywhere? or just from the edges?

 

[Drakkith]

Well , if he didn't took that in consideration , why can't dark matter or energy travel faster than c? I know that is the rule einstein once said but why? i have not read the relativity theory yet and i know that c is that fast because it has 0 mass , but how did he knew that , that was the fastest speed ? I think light can be slowed down but don't know how. The fact , is there any chance of something beeing faster than light?

Not as far as we know. There are problems involving causality if something could travel faster than c. Note that c is the speed of light in a vacuum. In a medium, such as glass or water, light slows down and we can make things travel faster through these mediums than light does, leading to this effect: https://en.wikipedia.org/wiki/Cherenkov_radiation

If space is not matter , then what is it? How should i imagine the limits of space? What limits the space ? A wall? Nothing? If nothing limits the space , how can it have a limit if the limit is nothing ?

Saying that space has no limit does not mean that space has a limit and that that limit 'is nothing'. It means the limit does not exist.

As for what space is, space is, well, space. Literal space. The space between objects. The framework made up of three dimensions in which we can move freely in any of them.

So , another question , the universe is expanding from everywhere? or just from the edges?

There is no known edge to the universe. Universal expansion means that the distance between all unbound objects increases over time. It does not necessarily mean that there is an expanding edge to the universe, though that is a possibility.

 

[rootone]

... just from the edges?

We know from observation that the observable universe is expanding and all of it expanding except gravitationaly bound matter, (galaxies etc).
So galaxies everywhere are getting more distant from other galaxies, but the galaxies themselves don't expand.
This is what leads to the conclusion of there being a 'big bang', a starting point in time for it.
We don't know if there is more universe beyond what is observable, (or what logically follows from what is observable.)
There could be infinite space beyond what we will ever be able to observe, or maybe the observable universe is all there is.
One possibility is that the universe as a whole is infinite, in which case it always was infinite, yet it is expanding too.

You might say 'wait a minute', how can something infinite get bigger?
Well 'infinite' can do things which finite can't.
https://en.wikipedia.org/wiki/Hilbert's_paradox_of_the_Grand_Hotel

 

[phinds]

There could be infinite space beyond what we will ever be able to observe, or maybe the observable universe is all there is.

The problem with this point of view is how could you possibly explain the boundary at the edge of the observable universe and how much hubris do you think it takes for us to assume that we are the center of everything? For those reasons, my opinion of the point of view that the OU is all there is is not printable within the forum guidelines

 

[rootone]

I am not saying that this is my view, only that we don't know what there may be beyond the observable.
Hence we can't positively rule anything in as being most likely or anything out as being impossible.
I meant that space outside of the observable universe could be any possibility ranging from nothing to infinite.

 

[Isaac0427]

Well , if he didn't took that in consideration , why can't dark matter or energy travel faster than c?

So you have 2 problems here:
1. Dark mater and dark energy are not the same thing, nor are they dependent on each other. They are completely different. I believe some scientists don't choose to call it dark energy, to avoid this confusion.
2. Scientists are in no position to make any claims about the speed of dark mater or dark energy. As a revision to number 1, they have a large similarity: we bearly know any of the properties of either.

 

[timmdeeg]

Also , we know almost nothing of dark energy ( i think) what if dark matter and energy can travel faster than C? did Einstein knew of this in his relativity theory?

The assumptions of dark energy and dark matter fit to the data we have. From this the dark energy is a vacuum energy which fills the universe homogeneous. So, its density is the same everywhere.

Think of a gas in the corner of a box. The gas (its molecules) will travel until its density is the same everywhere within the box. Once having reached this state of equilibrium the gas will not travel anymore, though the molecules it consists of are still able to move.

In contrast to the gas the dark energy is perfectly homogenous. It has this state from the very beginning, hence it can't travel at any time.

 

[Jakecp]

i am doing a proyect of this but i don't understand something. Why does the expansion of the universe is not motion and light is? In both nothing is moving , is there?

 

[Jakecp]

another question .

There is something moving when the universe expand , space and time because without the universe there is no space and time out of it so we can't go out of it because there is nothing , nor space nor time to travel but but when the universe is expanding at 3c+ we have new space matter moving , dark matter filling the gaps into that space at 3c. In light , is there something really moving? If it has 0 mass is it really something? Why? If light is just a wave with no mass which means it can't be touched , smelled or that. It has no mass and for something to exist it has to have mass otherwise it is nothing right?

 

[Chronos]

We know with great certainty about what happened in the past, we need only gaze at distant galaxies to see the universe as it was billions of years ago. Reconciling those ancient views with our observations of nearby galaxies is a part of modern cosmology.

 

[Drakkith]

It has no mass and for something to exist it has to have mass otherwise it is nothing right?

No. There are several things without mass which exist. Light is just one of them. Another example is the gluon, a massless subatomic particle.

 

[Jakecp]

and why space does not exist then?

 

[rootone]

Space is not a particle or any form of substance.
Space can have stuff in it, but is not itself made of anything.
You question is similar to asking why a kilometer does't exist., yet you do know what a kilometer IS.
Can you pick up one meter and see what it's made of ?, no of course not.
A meter does exist though, it's just not a material object.

 

[phinds]

... when the universe is expanding at 3c+

The universe is NOT expanding at 3c or any other speed. It is expanding in a uniform way such that things farther away from each other recede from each other faster than things close together. Things at the edge of our observable universe are receding from us at about 3c but this this in not way can reasonably be interpreted to mean "the universe is expanding at 3c" .

... dark matter filling the gaps into that space at 3c ...

No, nothing is "filling the gaps", things are just getting farther apart.

 

[Warp]

Let me try to explain it in another way:

Nothing can travel from point A to point B faster than c. However, due to the metric expansion of space, the distance between A and B can grow faster than c. This does not break general relativity, but it's in fact predicted by and completely conformant with it.

It might not be immediately apparent what the difference between the two things is, but they are not the same thing. Even if the distance between A and B is growing faster than c, there is nothing traveling between them faster than c.

(The metric expansion of the universe is not the only situation where GR predicts this to happen. Another example happens close to a rotating black hole. The distance between points within the ergosphere and outside of it can grow faster than c. Again, this doesn't break GR, but is predicted by it. Although it could ostensibly produce interesting physical paradoxes, but that's another topic.)

Curiously, the metric expansion of space can cause light to travel "faster than c" in a sense. If points A and B are receding from each other faster than c due to the metric expansion of space, it is still possible for a photon to reach B from A, if this rate of expansion is below a certain threshold. The photon, however, never locally breaks the speed limit of c at any point.

Why this happens can be better understood with an analogy: Suppose that you have a 1 meter long infinitely stretchable rubber band, and a snail at one end. The snail travels 1 cm / minute. However, the rubber band is stretched at a rate of 1.1 cm / minute (in other words, it becomes 1.1 cm longer every minute.) Obviously the other end of the rubber band will be receding faster from the snail than its traveling speed of 1 cm / minute. Yet eventually the snail will reach the other end (even though the snail never locally breached the 1 cm / minute speed). Essentially, the rubber band "transports" the snail in the direction of movement as it stretches.

In the same way the metric expansion of space "transports" light along its direction of movement, making it, in a sense, travel "faster than c" (even though c is never breached locally.)

(However, light reaching the other point requires a rate of expansion below a certain threshold, which can still be larger than c. However, if it's larger than that threshold, the photon will never reach the other point. Same as with the rubber band and snail.)

 

[phinds]

@Warp, good post. I do think however that your statement

The photon, however, never locally breaks the speed limit of c at any point.

while perfectly true could be slightly confusing to a novice and would be much better stated as "the photon always travels locally at c". This comports with the snail always traveling locally at 1cm/minute. [which, come to think of it, you should also express in absolute terms rather than "never exceeds ... "]

This is a very minor quibble to a good post. I do this because it's my job as Nitpicker in Chief.

 

[Jakecp]

Technically , C travels faster than itself because of this principle :

If you are moving , time is faster. If you are not moving , time is slower. It was something Einstein said once.

Now , imagine you are on something that travels at C with a chronometer. C will seem to travel faster if you are on it than if you are out counting. So C travels faster than C ?

 

[phinds]

Technically , C travels faster than itself because of this principle :

If you are moving , time is faster. If you are not moving , time is slower. It was something Einstein said once.

Now , imagine you are on something that travels at C with a chronometer. C will seem to travel faster if you are on it than if you are out counting. So C travels faster than C ?

Sorry but this is utter nonsense. First, time is NOT faster or slower due to your motion (you completely misunderstand time dilation) and second you can't have "something that travels at C with a chronometer", it is literally impossible. I suggest you do some reading on the subject before digging your hole any deeper.

 

[Jakecp]

I said IMAGINE , it is impossible of course. Then , don't just say nonsense , explain why. Make your point of time dilation. I thought that if something was on motion time was faster than if it wasn't . I read about it and einstein discovered it.

 

[guywithdoubts]

I said IMAGINE , it is impossible of course. Then , don't just say nonsense , explain why. Make your point of time dilation. I thought that if something was on motion time was faster than if it wasn't . I read about it and einstein discovered it.

Proper time (your own) always ticks at the same rate, but depending on movement relative to other clocks you will observe them tick differently from yours, and so will they. You can't travel a c, your thought experiment is invalid from the very premise.

 

[Orodruin]

I said IMAGINE , it is impossible of course. Then , don't just say nonsense , explain why. Make your point of time dilation. I thought that if something was on motion time was faster than if it wasn't . I read about it and einstein discovered it.

Please realize that what you have read in popular scientific presentations are not a good starting point for actually learning science. In order to do so you need to study properly. Also realize that there are many people on this forum who are experts in these subjects and that if you say something that is obviously wrong according to the theory you are going to be called on it. This forum is not for hypothesising freely based on what you may have read in popular science. If you do not have the required level of understanding, please do not try to argue against those who do.

 

[Isaac0427]

I said IMAGINE

Now , imagine you are on something that travels at C with a chronometer. C will seem to travel faster if you are on it than if you are out counting. So C travels faster than C ?

Instead of just saying this is practically nonsense, I will tell you why this is mathematically nonsense. A Lorentz transformation deals with how an observer (in the primed frame of reference) would experience something while moving with a constant nonzero velocity relative to the unprimed frame of reference. You are saying that the observer is moving the speed of light in the primed frame of reference relative to a not moving unprimed frame of reference. Multiple of the components in the Lorentz transformation include a denominator of √1-v²/c². If v²=c², the denominator would be √1-1, which would be zero, and a denominator of zero is undefined. Even if only one component of the matrix is undefined, the entire primed coordinate frame is undefined. Nothing can happen, even theoretically, in an undefined coordinate frame, so even in theory, the perspective of something moving at C is nonsense.

 

[Jakecp]

ok but taking in account that observation , that if you are in motion things go faster. Light is faster than what we measure?

 

[Isaac0427]

ok but taking in account that observation , that if you are in motion things go faster. Light is faster than what we measure?

I don't think you quite understand time dialation.

 

[phinds]

ok but taking in account that observation , that if you are in motion things go faster. Light is faster than what we measure?

As I have already pointed out, you clearly do not understand time dilation. AGAIN, I suggest that you read up on this stuff.

 

[Jakecp]

Well thanks.

 

[Drakkith]

ok but taking in account that observation , that if you are in motion things go faster. Light is faster than what we measure?

Consider three observers, A, B and C. A and B are moving in opposite directions away from observer C. If a beam of light passes by all three observers, then all three will measure that beam of light as traveling at speed c. The only difference is that the frequency of the light will be different for each observer.