GR and the Principle of Reversibility of Light

[studiot]

Does General Relativity imply that the Principle of Reversibility of Light does not hold in our Universe?

[mathematic]

What does "Principle of Reversibility of Light" mean?

[StringJunky]

9 minutes ago, mathematic said:

What does "Principle of Reversibility of Light" mean?

GIYF. 

Quote

The principle that if a beam of light is reflected back on itself, it will traverse the same path or paths as it did before reversal. The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed.

[studiot]

Yes that's right, SJ.

 

The problem is that light travels along geodesics.

Surveyors are used to the fact that if you set up a theodolite at some point A on the Earth's surface, sight on B, and set out a row of pegs leading to B, and then move to B and set out a row of pegs from B to A there will be two rows of pegs, not one.

This due to the fact that the direction of gravity at A and B will point to slightly different centres.

 

I am just trying to get my head round the implications extending this to the universal spacetime manifold which is more irregular than the Earth.

 

So thanks guys for responding.

[Mordred]

I can't see the lightpaths being perfectly reversible. Only as a reasonable approximation will they appear similar but never perfectly identical. The problem is on a time dependent metric spacetime at each infinitisimal coordinate would never have identical conditions. So following the path of least resistance each infinitisimal coordinate will have different conditions causing slight path alterations.

 

Edited April 26, 2018 by Mordred

[StringJunky]

8 minutes ago, Mordred said:

I can't see the lightpaths being perfectly reversible. Only as a reasonable approximation will they appear similar but never perfectly identical

A laser fired at the reflector on the moon returns ok... or is that still within the bounds of an approximation?

[Mordred]

Bounds of a reasonable approximation. Under GR via infinitisimal coordinates the paths will have slight deviations. ie coordinates at each planck length spacing. Keep in mind the beams could still arrive at the same origin point but the path taken can still vary

Edited April 26, 2018 by Mordred

[StringJunky]

3 minutes ago, Mordred said:

Bounds of a reasonable approximation. Under GR via infinitisimal coordinates the paths will have slight deviations. ie coordinates at each planck length spacing

I suppose when you are talking Planck accuracy then it is an approximation.  C'mon man, that's worse than splitting hairs.  

[Mordred]

Well its a truism that the HUP applies to both the field at every coordinate as well as the particle. So it becomes an arbitrary choice as to the accepted degree of accuracy

[StringJunky]

6 minutes ago, Mordred said:

Well its a truism that the HUP applies to both the field at every coordinate as well as the particle. So it becomes an arbitrary choice as to the accepted degree of accuracy

Because a particle is a field excitation.

[Mordred]

Yes but every field coordinate is a function or field value which infliences the particle excitation at every coordinate lol. GR at the QFT level can get rather funky.

[uncool]

Taking GR as a classical (non-quantum) theory:

 

The answer to the question should be: what do you mean by "the same path"?

 

The assumption in that principle is that the "path" doesn't care about time. Which is fine in a classical (non-curved spacetime) world, but breaks down in GR.

[Mordred]

Well even in Classical cosmology the question becomes. Will the conditions at each coordinate be the same on the return path. So for example on the FRW metric this could apply to how much did the universe expand from forward path to return path.

 Example What if the light path is several Mpc in length with an expansion rate of 70 km/Mpc/sec ?

Ie Late integrated Sache Wolfe effect would vary from forward to reverse direction.

 In essence spacetime itself is never static. The Schwartzchild metric and Newton approximation under GR is just that a good approximation. There is still always variations that occur from one time slice to the next.

Edited April 26, 2018 by Mordred

[beecee]

And just to add my 2cents worth, doesn't light when traversing also warp/curve spacetime  itself, albeit ever so slightly. So according to that 2 cents worth, it would be a different path.

1 hour ago, Mordred said:

Bounds of a reasonable approximation. Under GR via infinitisimal coordinates the paths will have slight deviations. ie coordinates at each planck length spacing. Keep in mind the beams could still arrive at the same origin point but the path taken can still vary

and which Mordred seems to be saying here. 

Edited April 26, 2018 by beecee

[studiot]

Thanks to all who replied, particularly Mordred and Uncool (+1 apiece).

I really hadn't considered the time aspect which makes it impossible in space time for the same coordinate points to be traversed, even if you take the scanario I proposed which was simultaneous viewing from both ends.

I suppose we will have to rephrase it so that the timescale of observation is short compared to that of any material changes.

I really don't know the answer to my question which is why I asked it.

This is all part of my attempt to reconcile theoretical and observational / practical Physics.

[Lasse]

On 2018. 04. 25. at 8:34 PM, studiot said:

Does General Relativity imply that the Principle of Reversibility of Light does not hold in our Universe?

Hmmm. From the infinite mass there is light in the Universe. Do you ask is it possible to reverse energy to mass?

Edited April 26, 2018 by Lasse

[studiot]

2 minutes ago, Lasse said:

Hmmm. From the infinite mass there is light in the Universe. Do you ask is it possible to reverse energy to mass?

Interesting question.

That is not what I asked, sorry.

[Mordred]

2 hours ago, studiot said:

Thanks to all who replied, particularly Mordred and Uncool (+1 apiece).

I really hadn't considered the time aspect which makes it impossible in space time for the same coordinate points to be traversed, even if you take the scanario I proposed which was simultaneous viewing from both ends.

I suppose we will have to rephrase it so that the timescale of observation is short compared to that of any material changes.

I really don't know the answer to my question which is why I asked it.

This is all part of my attempt to reconcile theoretical and observational / practical Physics.

 

 Your welcome it was a qood question to ask