# Is there really an absolute reference frame?

1. Nov 3, 2011

### pche8701

Is there really an absolute reference frame??

According to mechanics as we understand today, all reference frames are equally valid in analysing in the universe, whether measurements are taken relative to the earth/sun.....

However if all reference frames are equal, why can't we take measurements relative to a photon of light? And if we did would this not be an absolute reference frame? since the only thing constant in he universe is the speed of light. On another note, if we did take measurements relative to a photon of light, then how would we see the universe? Because according to Einstein's theories, once we travel at the speed of light, length becomes infinitely small. Does that mean relative to light the universe is a tiny object no more than a few cm across? If so why does light take ~ 8min to reach us (since relative to it, it sees the universe being drastically length contracted)?

2. Nov 3, 2011

3. Nov 4, 2011

### harrylin

Re: Is there really an absolute reference frame??

Welcome to Physicsforums.
Despite general relativity theory, inertial reference frames work better than frames that are in rest with respect to objects in arbitrary motion.
No instruments for measurements of length and time can be set up that move at the speed of light, as you indicated yourself: a tiny object is not infinitely small. Thus such a frame cannot exist.

However, as measured with a reference frame that moves very fast relative to the solar system, the solar system is strongly contracted and sunlight takes very little time to reach the Earth.

4. Nov 4, 2011

### Phrak

Re: Is there really an absolute reference frame??

For your argument to mean anything you should have to define inertial.

The problem, as I see it, is that 1/0 is ambiguous. It could be a positive or negative value.

5. Nov 4, 2011

### Staff: Mentor

Re: Is there really an absolute reference frame??

I use the standard GR definition. An inertial object is one whose proper acceleration is 0, and an inertial frame is one where any object at rest wrt the frame is inertial.

6. Nov 5, 2011

### Phrak

Re: Is there really an absolute reference frame??

Nicely put. Perhaps more formally-

For $x_i$ spatial coordinates, define $v_i=\frac{dx_i}{d \tau}$, if $|v|<c$ and $\frac{dv_i}{d \tau}=0$, then $x_i$ is an inertial frame.

I suppose one would also have to specifiy that it's in general true only at one coordinate.

Last edited: Nov 5, 2011
7. Nov 5, 2011

### pervect

Staff Emeritus
Re: Is there really an absolute reference frame??

Consider the worldline of a normal object. You can mark off intervals along it, which represent proper time. This feature is important for defining the time coordinate in any "reference frame" you're setting up.

Now consider the worldlie of a photon. If you mark off intervals along it, the invariant interval, the Lorentz interval, which in the previous case of the worldline of a normal object represented proper time, is zero.

This is the fundamental obstacle in setting up a "reference frame" for a photon. The traidional idea of a reference frames has a time cooridnate and three space coordinates.

What passes for a reference frame for a photon doesn't have any such time component, because the Lorentz interval along the photon's worldline is always zero.

There are some things you can do, but none of these things is going to be the classical "time + space" reference frame you're used to.

8. Nov 6, 2011

### kmarinas86

Re: Is there really an absolute reference frame??

What determines whether or not a frame is "non-rotating". With respect to what "non-rotating" grid does rotation actually occur within? What establishes it?

9. Nov 6, 2011

### D H

Staff Emeritus
Re: Is there really an absolute reference frame??

That rotation and acceleration are absolute is axiomatic in general relativity. In short, "because we said so." Physicists don't like "because we said so," so they test and test such statements. The bad thing about experiments is that they cannot explain why. The good thing about experiments is that they can check whether reality (or reality as best we measure it) is consistent/inconsistent with theory -- and so far those measurements agree with theory.

Experiments can also point the way to devices that measure rotation. Ring laser gyros (expensive) and MIMS gyros (cheap) measure rotation. That they do so as a purely local experiment says something very profound about the universe.

10. Nov 8, 2011

### lukesfn

Re: Is there really an absolute reference frame??

If you assume light has a reference frame, the thing that prevents it from being a universal reference frame is that light moves in a certain direction, so the contraction of the universe is only in the direction that the light is moving in. There would be a different reference frame for every direction.

I find it very interesting to try to imagine what a hypothetical point particle of light's reference frame might look like when taking General Relativity into consideration. The shape of the universe must become very strange, and the start point of the journey must be in the same position as the end point, and the particle doesn't actually move anywhere.

11. Nov 8, 2011

### DrGreg

Re: Is there really an absolute reference frame??

FAQ: Rest frame of a photon

12. Nov 8, 2011

### lukesfn

Re: Is there really an absolute reference frame??

However, I believe that weather or not the assumption in my argument was correct or not is irrelevant and I prefer my argument for that reason. From my point of view, and I believe the point of view of the original poster, my argument would be easier to understand.

Back onto your post the meaningfulness of the rest frame of a photon...
I'm afraid I don't understand the mathematics of this well enough, however, intuitively, it would appear to me that time stops, so time becomes zero in all equations, making the time dimension non existent in this frame. Also, the direction that the photon is moving along would be divided by infinity, which I would interpreted as becoming infinitely small and non existent. Therefore the inertial frame of a photo would be in a 2d space rather then a 4d space. Perhaps this is still pretty meaningless, however, I'm interested if it makes sense mathematically? I can't think of an intuitive reason why the maths would break down for this.

13. Nov 8, 2011

### Staff: Mentor

Re: Is there really an absolute reference frame??

The correctness of the assumptions is never irrelevant.

14. Nov 8, 2011

### lukesfn

Re: Is there really an absolute reference frame??

Perhaps I should have been more specific and claimed that the correctness of my assumption was irrelevant to my argument.

15. Nov 9, 2011

### pervect

Staff Emeritus
Re: Is there really an absolute reference frame??

It seems to me that the fact that a photon doesn't have a reference frame invalidates your argument.

If you think you've got some argument when you take into account the new information that photons do not have reference frames, please present your argument again without the false assumption.

16. Nov 9, 2011

### lukesfn

Re: Is there really an absolute reference frame??

It seems to me that it is only a questions of semantics that is the issue. Let me try to modify my argument slightly, although perhaps I will still not use the best language.

If it where possible define a reference frame for a photon, then I would postulate that any definition of a reference frame would be dependent on the direction of travel of the photon, and the direction of travel of a photo is not absolute, so such reference frame would not be absolute?

Is that still problematic? If so, I expect that the argument can be put in such a way that isn't problematic.

Otherwise, perhaps it is possible to make a slight extension to, modification of, or maybe even reinterpretation of SR or GR where a reference frame of a photo does exist and make sense. It seems to me that an argument that avoids the necessity for any such argument so is more fundamental. (Or has somebody already proven that such a thing is not possible?)

Aside from that, I'm still struggling to understand why a reference frame for a photon would be such non-sense, from my position of ignorance, I am struggling why it should make such little sense when following the curves as time reduces towards zero and space reduces towards zero along the direction of movement, and end up with something that isn't mathematical nonsense. I would have thought it would be similar to a singularity in a black hole or something. Maybe the singularity can't exist in reality but it can exist in the theory.

17. Nov 9, 2011

### harrylin

Re: Is there really an absolute reference frame??

Something that can't exist in reality is perhaps not mathematical nonsense, but still it may be considered physical nonsense; and what would you want to do with a theoretically frozen universe of zero volume and infinite mass density?

18. Nov 9, 2011

### lukesfn

Re: Is there really an absolute reference frame??

I'd use it as a conceptual tool to explain an argument.

19. Nov 9, 2011

### harrylin

Re: Is there really an absolute reference frame??

Then it may be useful to present that argument: for it's very easy to present a faulty argument based on infinities.

20. Nov 9, 2011

### lukesfn

Re: Is there really an absolute reference frame??

Sorry, I was being cute, the argument I was referring to was the one I made earlier in the thread, and the point of it was to make infinity irrelevant, but it seems to have backfired. Still, it is the best answer for me.

Anyway, I think I need to leave this place, its too distracting.