Is light's speed c constant?

[swansont]

Is that a yes?

 

 

Light in a vacuum travels at c.

[michel123456]

GR explains light going at 1/3rd of C?

Yes or No?

 

Please.

[Strange]

Yes or No?

 

Please.

 

 

There isn't really a yes/no answer.

 

So, in some circumstances one observer may say that the time it takes for light to get from A to B means it was travelling at less than c. But any observer between A and B will measure the speed of that light (locally) as c.

 

But, yes, this is explained by GR.

[swansont]

Yes or No?

 

Please.

 

 

The question is ill-formed. To understand why this is requires learning about relativity. There is no short-cut here. To answer this yes or no would be an oversimplification that gets the physics wrong.

[Bird11dog]

Is the average gravitational field strength for the Universe weaker now than in the early Universe? Yes. Does time speed up in a weaker gravitational field strength? Yes.Then time was slower in the early Universe therefore the speed of light was faster in the early Universe. v = d/t

[swansont]

Is the average gravitational field strength for the Universe weaker now than in the early Universe? Yes. Does time speed up in a weaker gravitational field strength? Yes.Then time was slower in the early Universe therefore the speed of light was faster in the early Universe. v = d/t

 

 

What about length contraction?

[Mordred]

Is the average gravitational field strength for the Universe weaker now than in the early Universe? Yes. Does time speed up in a weaker gravitational field strength? Yes.Then time was slower in the early Universe therefore the speed of light was faster in the early Universe. v = d/t

If you ever want irrefutable proof this is incorrect all you need to do is compare the null geodesic Worldlines between the FLRW metric (Einstein field equation equivalence) and the Schwartzchild metric.

 

In the former there is no spacetime curvature of the Worldline. In the latter there is.

 

( the key is two terms proper time of a metric and coordinate time of a metric.

Edited May 23, 2017 by Mordred

[Strange]

Is the average gravitational field strength for the Universe weaker now than in the early Universe? Yes.

 

 

Do you have a reference for this?

[Bird11dog]

Yes, was the average mass density higher or lower in the early expanding Universe? My post should be self evident in an expanding Universe.

[Strange]

Yes, was the average mass density higher or lower in the early expanding Universe? My post should be self evident in an expanding Universe.

 

 

Not obvious to me.

[Bird11dog]

Does the average strength of the Universe's gravitational field get weaker as it expands? I'm pretty sure that is obvious. If true then the Universal clock must be speeding up.

[Strange]

Does the average strength of the Universe's gravitational field get weaker as it expands? I'm pretty sure that is obvious. If true then the Universal clock must be speeding up.

 

 

It isn't obvious to me. (It is not obviously wrong, either.) And there is no such thing as "the universal clock".

 

Also, remember it is a theory of relativity: you can compare two clocks at different gravitational potential. But you can't compare a clock in the past with one now.

 

Also, even if you are right it has no bearing on the speed of light, as swansont says.

[Mordred]

I was at work so didn't have time (pardon the pun) to give a proper explanation.

 

Lets start with a light beam emitted from say the CMB.. Yes you have a higher density than now. However that doesn't matter. What matters is the spacetime mass distribution at each hyperslice during the lights travel.

 

So the universe at each each moment in time has a uniform and even mass distribution. The lightpath remains straight. Spacetime at each moment is of a uniform distribution. In essence Euclidean flat regardless of the density value.

 

As you track the history of the light beam leading edge this remains true. At no time in the history of the lightbeam leading edge does it encounter a non uniform mass distribution so it never encounters a curvature term to alter its path. Its path always remains straight.

Edited May 23, 2017 by Mordred

[studiot]

I was at work so didn't have time (pardon the pun) to give a proper explanation.

 

Lets start with a light beam emitted from say the CMB.. Yes you have a higher density than now. However that doesn't matter. What matters is the spacetime mass distribution at each hyperslice during the lights travel.

 

So the universe at each each moment in time has a uniform and even mass distribution. The lightpath remains straight. Spacetime at each moment is of a uniform distribution. In essence Euclidean flat regardless of the density value.

 

As you track the history of the light beam leading edge this remains true. At no time in the history of the lightbeam leading edge does it encounter a non uniform mass distribution so it never encounters a curvature term to alter its path. Its path always remains straight.

 

 

Nice explanation. +1

[swansont]

Does the average strength of the Universe's gravitational field get weaker as it expands? I'm pretty sure that is obvious. If true then the Universal clock must be speeding up.

 

 

Time dilation is not dependent upon gravitational field strength, it is dependent on gravitational potential.

[michel123456]

I was at work so didn't have time (pardon the pun) to give a proper explanation.

 

Lets start with a light beam emitted from say the CMB.. Yes you have a higher density than now. However that doesn't matter. What matters is the spacetime mass distribution at each hyperslice during the lights travel.

 

So the universe at each each moment in time has a uniform and even mass distribution. The lightpath remains straight. Spacetime at each moment is of a uniform distribution. In essence Euclidean flat regardless of the density value.

 

As you track the history of the light beam leading edge this remains true. At no time in the history of the lightbeam leading edge does it encounter a non uniform mass distribution so it never encounters a curvature term to alter its path. Its path always remains straight.

For the path to remain straight, doesn't that mean that time expands equally as space does? That the ration d/t remains constant?

[Silvestru]

The universe is expanding symmetrically. Why would light change it's path?

http://www.space.com/25673-universe-expansion-real-time-cosmology.html

[Bird11dog]

 

 

It isn't obvious to me. (It is not obviously wrong, either.) And there is no such thing as "the universal clock".

 

Also, remember it is a theory of relativity: you can compare two clocks at different gravitational potential. But you can't compare a clock in the past with one now.

 

Also, even if you are right it has no bearing on the speed of light, as swansont says.

 

If the average clock rate for the Universe in the early Universe is slower? v = d/t ?

Edited May 24, 2017 by Bird11dog

[Mordred]

no

[michel123456]

no

The lake?

[Mordred]

That was in response Birds post time was not slower in the past Time dilation requires a non uniform mass distribution.

Edited May 24, 2017 by Mordred

[michel123456]

That was in response Birds post time was not slower in the past Time dilation requires a non uniform mass distribution.

That is the question: if Space was squeezed in the past, and Time remained unchanged, then logically SOL would have been different. Or do I miss something?

[swansont]

That is the question: if Space was squeezed in the past, and Time remained unchanged, then logically SOL would have been different. Or do I miss something?

 

 

You miss something. You can't discuss the invariant speed of light under the condition of expansion of space. It doesn't apply.

[michel123456]

 

 

You miss something. You can't discuss the invariant speed of light under the condition of expansion of space. It doesn't apply.

SOL is not invariant under expansion of space. Is that it?

[swansont]

SOL is not invariant under expansion of space. Is that it?

Right.. It would be like making a ruler out of a spring. It's a nonsensical way to do the measurement.