Speed of light and the universe

[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.

 

[Orodruin]

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

The first thing to realize is that there is no such thing as "being in motion". Unless you are accelerating there exists an inertial frame in which you are at rest. You cannot objectively say something is moving without specifying what it is moving relative to. This is not something peculiar to relativity, it is true also in classical mechanics.

 

[Isaac0427]

Well thanks.

I'm sorry but we are not going to babyfeed the information to you. Research time dilation and then ask questions on the relativity forum. When you put in the effort to research and ask questions, we will put in the effort to answer those questions.