# Distant stars and length contraction

1. Jun 17, 2015

### Ookke

Please see the picture. Red dots are stars, lines are the path that light takes, our observer is at center. Assuming that stars are somewhat uniformly spread around us, I suppose that stationary observer (left picture) sees about as many stars in every direction, but an observer moving in y-direction (right picture) sees stars concentrated on the sides due to length contraction and fewer stars up and down. Can we use this method to tell is earth moving relative to rest of the universe? If space looks same in every direction, we are not moving?

2. Jun 17, 2015

### A.T.

You are also assuming a finite amount of stars, and nothing beyond them.

Not quite:
https://en.wikipedia.org/wiki/Aberration_of_light

Relative to some stars in the universe.

3. Jun 17, 2015

### bcrowell

Staff Emeritus
This is one way of defining the Hubble flow. If the distribution of nearby galaxies looks isotropic and homogeneous, you're at rest relative to the average motion of the nearby galaxies. We then say that you're at rest relative to the Hubble flow.

An equivalent method is to look for the frame in which the CMB looks isotropic.

4. Jun 17, 2015

### Staff: Mentor

In addition to the aberration mentioned by AT there is also Doppler shift. This is maybe not so relevant for stars as for the CMB.

5. Jun 17, 2015

Staff Emeritus
I don't think your question is really answered. What you describe is a way to determine your motion with respect to a bunch of stars. There's no problem with this. But the other part of your argument, that there is something special about the frame where these stars at rest, is not established. Indeed, in SR, there's nothing special about it, the frame where the CMB is at rest, or the frame where Lincoln was shot. These frames may be interesting to us, but they are not physically special.

6. Jun 17, 2015

### 1977ub

The frame where CMB is at rest might yield other interesting symmetries or signatures.

7. Jun 17, 2015

### pervect

Staff Emeritus
Something that should be clarified. As AT mentioned, the effect of stellar aberration means that what you'd visually see through a telescope would be different than your diagram, you'd "see" the stars concentrated ahead of you. There's some pictures at http://www.exo.net/~pauld/stars/PD_images_relativ.html, these were probably taken from the paper "In search of the starbow", http://scitation.aip.org/content/aapt/journal/ajp/47/4/10.1119/1.11834. This paper used to be online but I don't see it anywhere anymore.

The image you drew can be described as what you'd compute from your photographs, using the concept of "now" applicable to your frame of reference and compensating for travel time.

It is true that there is a special frame of reference in which the universe, as a whole, is isotropic (the same in all directions). This is usually called the cosmic microwave background frame or CMB frame, people replace the distribution of stars (in your picture) with measurements of the background microwave radiation from the big big bang, as in the WMAP experimenmts http://map.gsfc.nasa.gov/.

However, it would be confusing at the minimum (and generally regarded as wrong) to assume that people were using this particlaur frame in any given instance. There's nothing that forces us to use this frame, and many times it's inconvenient. So when you want people to know you're using that frame, you can specify "the CMB frame", and people will known what you're talking about (well, physicists and astronomers at least.

In short, the existence of the CMB frame doesn't really excuse sloppiness in descriptions of motion. It's OK to say "not moving with respect to the CMB frame", it's ambiguous at the minimum to say "not moving" without the necessary qualifiers as "compared to what".

8. Jun 17, 2015

Staff Emeritus
Why? How is this different from the frame where Lincoln was shot? "One sounds plausible and the other sounds crazy" is not a good answer - how physically do they differ and what is the mechanism for this to be a preferred frame?

9. Jun 18, 2015

### bcrowell

Staff Emeritus
Examples of symmetries: (1) matter is homogeneous and isotropic, (2) CMB is isotropic on the average (dipole moment vanishes).

Examples of conveniences: (1) stress-energy tensor has a simple form, (2) observations made on earth can be discussed without boosting, (3) energies of cosmic rays correspond to the energy scale of the processes that created them.

10. Jun 18, 2015

Staff Emeritus
But these are symmetries of the environment one is in. Not the sort of thing you would think would overthrow SR - which is what this would encounter.

11. Jun 18, 2015

### Ookke

This is nice. Not exactly what I thought, but there is some concentration of stars and the rear view gets empty. At least we can rule out the possibility that earth is moving very fast relative to the stars we see?

CMB frame that was brought up sounds like preferred frame to me, at least for aesthetic reasons. SR states that laws of nature must be the same in every inertial frame. If we consider "space is isotropic" as law of nature, then CMB frame would be the only valid one, but this would be very strict position. It sounds plausible that CMB frame could have some difference in gravitational or even inertial effects compared to other frames, but "sounds plausible" of course has no value by itself.

12. Jun 18, 2015

### bcrowell

Staff Emeritus
Maybe we had different understandings of what the OP was talking about. I didn't interpret this thread as being about overthrowing SR.

13. Jun 18, 2015

### Ookke

And I didn't mean it that way. It's always about limits of applicability too, but overthrowing SR altogether is not reasonable to even try. CMB frame is interesting, though.

14. Jun 18, 2015

### Staff: Mentor

Isotropy of the distribution of matter is a boundary condition, not a law of nature. Thread closed.