Is the speed of light constant?

In summary: There has been a lot of recent discussion of this topic online. Again, as has already been explained to you they are NOT "moving faster than light", the are RECEEDING faster than light. There is no proper motion involved. Please pay attention to what is being explained to you.
  • #1
Akwinder Singh Mander
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It is said that due to the expansion of the universe, there are some distant galaxies that are moving away from us faster than the speed of light. They can't actually move faster than speed of light itself, because the law of physics over there are supposed to be the same as the law of physics over here --- that means that if light itself is being emitted from such a faraway galaxy, away from us, that light will be traveling faster than the galaxy itself, which is in turn traveling faster than c. And if that's the case then the speed of light isn't really constant, is it?
Any explanation please?
 
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  • #2
Akwinder Singh Mander said:
there are some distant galaxies that are moving away from us faster than the speed of light.
They are not moving faster than the speed of light. The distance to them is growing faster than the speed of light (this rests on the assumption of a particular choice of simultaneity, but it is a rather intuitive one). You can only measure relative speeds locally, i.e., if you have two objects so close that they are covered by a patch of space-time in which curvature effects are negligible.
 
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  • #3
Orodruin said:
They are not moving faster than the speed of light. The distance to them is growing faster than the speed of light (this rests on the assumption of a particular choice of simultaneity, but it is a rather intuitive one). You can only measure relative speeds locally, i.e., if you have two objects so close that they are covered by a patch of space-time in which curvature effects are negligible.
So, it means somehow if we develop a technology, can we approach those galaxies with speed more than speed of light (with respect to those galaxies in which curvature effects are not negligible)
 
  • #4
You must then define what you mean with "speed relative to". As I said, this is only well defined if the objects are close enough.

You may want to check out solutions like the Alcubierre drive. Note that making it real would essentially require negative energy density, which is not at all clear that it could exist.
 
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  • #5
Orodruin said:
You must then define what you mean with "speed relative to". As I said, this is only well defined if the objects are close enough.

You may want to check out solutions like the Alcubierre drive. Note that making it real would essentially require negative energy density, which is not at all clear that it could exist.
ok. Thank u for your comments. I'll check Alcubierre drive.
 
  • #6
Akwinder Singh Mander said:
So, it means somehow if we develop a technology, can we approach those galaxies with speed more than speed of light (with respect to those galaxies in which curvature effects are not negligible)
If you fly there in a conventional rocket, no. You'll always be traveling slower than light with respect to nearby galaxies, even if your idea of which galaxies are nearby is changing.

I'm not sure that it's actually possible to reach high-redshift galaxies without a faster-than-light drive. The rate of distance increase is greater than the rate at which even light can close the distance.
 
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  • #7
Ibix said:
If you fly there in a conventional rocket, no. You'll always be traveling slower than light with respect to nearby galaxies, even if your idea of which galaxies are nearby is changing.

I'm not sure that it's actually possible to reach high-redshift galaxies without a faster-than-light drive. The rate of distance increase is greater than the rate at which even light can close the distance.
If it is the question that whether speed of light is constant and the answer is to be in Yes or No, what will be the answer. and then please explain too. I will be thankful to you
 
  • #8
Akwinder Singh Mander said:
If it is the question that whether speed of light is constant and the answer is to be in Yes or No, what will be the answer. and then please explain too. I will be thankful to you
The speed of light in a vacuum is c. Period. There is no "explanation", it's just what we observe it to be.
 
  • #9
phinds said:
The speed of light in a vacuum is c. Period. There is no "explanation", it's just what we observe it to be.
But due to the expansion of the universe, there are some distant galaxies that are moving away from us faster than the speed of light. Any comments
 
  • #10
Akwinder Singh Mander said:
But due to the expansion of the universe, there are some distant galaxies that are moving away from us faster than the speed of light. Any comments
Again, as has already been explained to you they are NOT "moving faster than light", the are RECEEDING faster than light. There is no proper motion involved. Please pay attention to what is being explained to you.

Google "metric expansion"
 
  • #11
Akwinder Singh Mander said:
If it is the question that whether speed of light is constant and the answer is to be in Yes or No, what will be the answer. and then please explain too. I will be thankful to you
There isn't an answer in those terms.

Light will always pass you at c in a vacuum, as @phinds says. But the reason that distant galaxies are getting further away is not that they are moving fast; rather the natural definition of distance on those scales is changing. So (we expect) an alien near those distant galaxies will see light passing it at c just as we do. It's just that the distance between us is growing.
 
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  • #12
Ibix said:
There isn't an answer in those terms.

Light will always pass you at c in a vacuum, as @phinds says. But the reason that distant galaxies are getting further away is not that they are moving fast; rather the natural definition of distance on those scales is changing. So (we expect) an alien near those distant galaxies will see light passing it at c just as we do. It's just that the distance between us is growing.
So it means that distance between two things (very far away in universe) can grow faster than light?
 
  • #13
Akwinder Singh Mander said:
So it means that distance between two things (very far away in universe) can grow faster than light?
Yes. But not because either of them is moving in any naive sense.
 
  • #14
Ibix said:
Yes. But not because either of them is moving in any naive sense.
Isn't the growing of distances "faster than light" not due to the chosen FRW-coordinates and thus not really invariant?
 
  • #15
timmdeeg said:
Isn't the growing of distances "faster than light" not due to the chosen FRW-coordinates and thus not really invariant?
Yes. The underlying assumption is the assumption of using hypersurfaces of constant cosmological time to make your space-time foliation.
 
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  • #16
Orodruin said:
Yes. The underlying assumption is the assumption of using hypersurfaces of constant cosmological time to make your space-time foliation.
Thanks!
 
  • #17
Akwinder Singh Mander said:
But due to the expansion of the universe, there are some distant galaxies that are moving away from us faster than the speed of light. Any comments
I've been given to understand that the relative velocity of even local objects may exceed c in non-inertial frames. E.g., A and B start at the same place and travel away from each other at .85c. After an hour on each's clock each turns around and returns at .85c. Just before A reverses course, B is only halfway out. Just after, B is halfway back, having traveled 1 light hour in the say minute or so it took A to reverse course. And vice versa.,
 
  • #18
Chris Miller said:
I've been given to understand that the relative velocity of even local objects may exceed c in non-inertial frames. E.g., A and B start at the same place and travel away from each other at .85c. After an hour on each's clock each turns around and returns at .85c. Just before A reverses course, B is only halfway out. Just after, B is halfway back, having traveled 1 light hour in the say minute or so it took A to reverse course. And vice versa.,
This has little to do with the speed of light being a local speed limit. It is the result of using different simultaneity conventions in different inertial frames.
 
  • #19
Orodruin said:
This has little to do with the speed of light being a local speed limit. It is the result of using different simultaneity conventions in different inertial frames.
Absolutely, just as two distant galaxies separating via Hubble expansion faster than c has nothing to do with the speed of light being a local constant/limit.
 
  • #20
Chris Miller said:
Absolutely, just as two distant galaxies separating via Hubble expansion faster than c has nothing to do with the speed of light being a local constant/limit.
No, it is not the same thing at all.
 
  • #21
Chris Miller said:
I've been given to understand that the relative velocity of even local objects may exceed c in non-inertial frames.
I'm not sure what you mean saying "local objects". If an object is located here and another object located somewhere else then their "relative velocity" ##v##, isn't well defined in curved spacetime. Whereas locally measured it is (because you can assume flat spacetime locally), whereby ##v<c## if both objects have nonzero restmass. Which requires the worldliness of the two objects have to intersect.
 
Last edited:

1. What is the speed of light and how is it measured?

The speed of light, denoted as c, is a physical constant that represents the maximum speed at which all matter and information in the universe can travel. It is measured in a vacuum and is approximately 299,792,458 meters per second.

2. Is the speed of light truly constant, or does it vary?

According to Einstein's theory of relativity, the speed of light is a fundamental constant that does not change regardless of the observer's frame of reference. This has been confirmed through numerous experiments and is widely accepted in the scientific community.

3. How was it determined that the speed of light is constant?

One of the key experiments that led to the understanding of the constancy of the speed of light was the Michelson-Morley experiment in 1887. This experiment aimed to measure the speed of Earth's movement through the hypothetical "aether," which was thought to be the medium through which light travels. However, the results showed that the speed of light was the same in all directions, regardless of Earth's movement, thus supporting the idea of its constant nature.

4. Does the speed of light ever change in extreme conditions, such as in black holes?

While the speed of light is constant in a vacuum, it can be affected by the medium through which it travels. For example, light travels slower in water or air than it does in a vacuum. In extreme conditions, such as near a black hole, the gravity can also affect the speed of light. However, these changes are still within a very small range and do not alter the fundamental constant nature of the speed of light.

5. How does the constancy of the speed of light relate to the theory of relativity?

The constancy of the speed of light is a key component of Einstein's theory of relativity. This theory states that the laws of physics are the same for all observers in uniform motion, and the speed of light is the same for all observers regardless of their frame of reference. This has significant implications for our understanding of space, time, and the universe as a whole.

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