Distant stars and length contraction

In summary: The frame where CMB is at rest might yield other interesting symmetries or signatures.The frame where CMB is at rest might yield other interesting symmetries or signatures. This is something that should be clarified.
  • #1
Ookke
172
0
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?
stars.png
 
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  • #2
Ookke said:
Assuming that stars are somewhat uniformly spread around us,
You are also assuming a finite amount of stars, and nothing beyond them.

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

Ookke said:
Can we use this method to tell is Earth moving relative to rest of the universe?
Relative to some stars in the universe.
 
  • #3
Ookke said:
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?

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
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
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
Vanadium 50 said:
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.

The frame where CMB is at rest might yield other interesting symmetries or signatures.
 
  • #7
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
1977ub said:
The frame where CMB is at rest might yield other interesting symmetries or signatures.

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
Vanadium 50 said:
1977ub said:
The frame where CMB is at rest might yield other interesting symmetries or signatures.
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?

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
bcrowell said:
Examples of symmetrie

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
pervect said:
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
Vanadium 50 said:
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.

Maybe we had different understandings of what the OP was talking about. I didn't interpret this thread as being about overthrowing SR.
 
  • #13
bcrowell said:
I didn't interpret this thread as being about overthrowing SR.
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
Ookke said:
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,
Isotropy of the distribution of matter is a boundary condition, not a law of nature. Thread closed.
 

FAQ: Distant stars and length contraction

What are distant stars?

Distant stars are celestial objects that are located far away from Earth. They are typically millions or billions of light years away and can be observed through telescopes. These stars emit light and other forms of electromagnetic radiation, which allows us to study them.

What is length contraction?

Length contraction is a phenomenon predicted by Einstein's theory of relativity. According to this theory, objects that are moving at high speeds will appear to be shorter in the direction of motion when observed by an outside observer. This effect becomes more pronounced as the speed of the object approaches the speed of light.

How are distant stars related to length contraction?

Distant stars are related to length contraction because the light emitted by these stars must travel a great distance to reach us on Earth. As this light travels through space, it may encounter objects or gravitational fields that cause it to change direction or speed. This can result in the observed length of the light's path being altered, leading to the phenomenon of length contraction.

Can length contraction be observed in the study of distant stars?

Yes, length contraction can be observed in the study of distant stars. This is because the light emitted by these stars has to travel a great distance to reach us, and during its journey, it may experience changes in speed or direction. These changes can result in the observed length of the light's path being altered, which can be measured by scientists.

Why is understanding length contraction important in the study of distant stars?

Understanding length contraction is important in the study of distant stars because it helps us to accurately interpret the observations and measurements that we make. By considering the effects of length contraction, scientists can account for any discrepancies or distortions in the data collected from distant stars, allowing for a more accurate understanding of these celestial objects and the universe as a whole.

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