# I Is the speed of light constant?

#### Akwinder Singh Mander

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 travelling faster than the galaxy itself, which is in turn travelling faster than c. And if that's the case then the speed of light isn't really constant, is it?

Related Special and General Relativity News on Phys.org

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
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.

#### Akwinder Singh Mander

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)

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
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.

#### Akwinder Singh Mander

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.

#### Ibix

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 travelling 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.

#### Akwinder Singh Mander

If you fly there in a conventional rocket, no. You'll always be travelling 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

#### phinds

Gold Member
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.

#### Akwinder Singh Mander

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

#### phinds

Gold Member
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.

#### Ibix

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.

#### Akwinder Singh Mander

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?

#### Ibix

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.

#### timmdeeg

Gold Member
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?

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
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.

#### timmdeeg

Gold Member
Yes. The underlying assumption is the assumption of using hypersurfaces of constant cosmological time to make your space-time foliation.
Thanks!

#### Chris Miller

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.,

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
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.

#### Chris Miller

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.

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
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.

#### timmdeeg

Gold Member
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:

"Is the speed of light constant?"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving