What Does It Mean to Be at Rest Relative to the CMB?

Click For Summary

Discussion Overview

The discussion revolves around the concept of being at rest relative to the cosmic microwave background (CMB). Participants explore the implications of this state, including the nature of Doppler shifts, the existence of a universal rest frame, and the potential violation of Lorentz invariance in different contexts.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that being at rest relative to the CMB means being in a frame where the frequency is measured uniformly in all directions, avoiding redshift and blueshift.
  • Others argue that the Milky Way's motion relative to the CMB suggests a local frame that is not invariant due to universal expansion.
  • There is a suggestion that defining a universal rest frame could imply a violation of Lorentz invariance, raising questions about momentum conservation across different regions of the universe.
  • Some participants note that the rest frame with respect to the CMB does not imply a preferred frame in the laws of physics, which are said to hold regardless of motion relative to the CMB.
  • Concerns are raised about the implications of Lorentz invariance being violated, particularly at the Planck scale, and how this might affect the definition of physical laws and conservation principles.
  • It is mentioned that preferred frame violations of Lorentz invariance have been excluded by high-precision experiments, such as the Michelson–Morley experiment.

Areas of Agreement / Disagreement

Participants express both agreement and disagreement on various aspects of the discussion. While some points, such as the nature of Doppler shifts and the implications of a universal rest frame, are acknowledged, there remains contention regarding the consequences of Lorentz invariance and the existence of a preferred frame.

Contextual Notes

Participants highlight the dependence on definitions and the potential for different observers to define frames differently due to universal expansion. The discussion also touches on unresolved assumptions regarding Lorentz invariance and its implications for physics.

cbetanco
Messages
133
Reaction score
2
When one says they are at rest relative to the cosmic microwave background, does this mean that they are in a frame where the same frequency is measureed in all directions? Because, I could imagine if I boosted in one direction, then the spectrum behind me would be redshifted, and the spectrum in front of me would be blue shifted. So to be at rest with respect to the CMB I would need to be in the frame where there is no shift in either direction. Is this right? Or do I have it all wrong?
 
Physics news on Phys.org
I think that's right - you adjust until the CMB Doppler shift approaches uniformity in all directions.

"The Milky Way is moving at around 552 km/s with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra)" (Wikipedia)...

I suspect that this frame ends up being local because of the universal expansion, so this frame it is not "invariant", if that's the right word?
 
cbetanco said:
When one says they are at rest relative to the cosmic microwave background, does this mean that they are in a frame where the same frequency is measureed in all directions? Because, I could imagine if I boosted in one direction, then the spectrum behind me would be redshifted, and the spectrum in front of me would be blue shifted. So to be at rest with respect to the CMB I would need to be in the frame where there is no shift in either direction. Is this right? Or do I have it all wrong?

That is correct. The actual data is corrected for the motion of the Earth, Sun, and galaxy.
 
bahamagreen said:
I suspect that this frame ends up being local because of the universal expansion, so this frame it is not "invariant", if that's the right word?

But locally, we can define a universal rest frame, that of the CMB? Does this also mean that if there were such a preferred frame, that Lorentz invariance would be violated locally? But now, if you could define such a frame, and Lorentz invariance was locally violated, if I go somewhere else in the universe, then because of this universal expansion, I would have to define a different frame where I was at rest relative to CMB. Would this imply momentum conservation was violated globally, since the preferred rest frame is not translationally invariant?
 
cbetanco said:
But locally, we can define a universal rest frame, that of the CMB? Does this also mean that if there were such a preferred frame, that Lorentz invariance would be violated locally? But now, if you could define such a frame, and Lorentz invariance was locally violated, if I go somewhere else in the universe, then because of this universal expansion, I would have to define a different frame where I was at rest relative to CMB. Would this imply momentum conservation was violated globally, since the preferred rest frame is not translationally invariant?

The rest frame with respect to the CMB has nothing to do with the non-existent "preferred frame." Experiment shows us that there is no such preferred frame as far as the laws of physics go. The CMB has no bearing on the laws of physics. They work just as well whether something is at rest with respect to the CMB or not.
 
Well, if Lorentz invariance was violated (like at the Planck scale), it would be possible to define a preferred frame. The CMB would seem to be the natural choice. But since this frame is local, I would guess different observers living at different parts of the universe would define this frame differently. Also, if we defined the lagrangian to be the lagrangian where we are at rest with the CMB, then different observers would have to define different lagrangians at differnet points in space, which would mean physics is not translationally invariant, and then momentum conservation would be violated (all assuming Lorentz invariance is violated!).
 
cbetanco said:
Well, if Lorentz invariance was violated (like at the Planck scale), it would be possible to define a preferred frame.

Preferred frame violations of Lorentz invariance are already excluded to high precision by http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment" experiments.
 
Last edited by a moderator:

Similar threads

Undergrad What was the role of frequency in the Michelson-Morley experiment's null result?
  • · Replies 3 ·
Replies
3
Views
2K
High School What Is the Proper Time of the CMB and Its Implications?
  • · Replies 9 ·
Replies
9
Views
2K
Undergrad Galilean relativity in terms of homogeneity
  • · Replies 51 ·
2
Replies
51
Views
4K
Graduate Question about "Rest frame of a photon" FAQ
  • · Replies 19 ·
Replies
19
Views
4K
High School Doubt about the relativity of simultaneity
  • · Replies 33 ·
2
Replies
33
Views
4K
Undergrad The wave equation interpretation of special relativity
  • · Replies 10 ·
Replies
10
Views
1K
Graduate Does the CMB allow special rest frames to be defined?
  • · Replies 7 ·
Replies
7
Views
7K
Undergrad Does Special Relativity Deal with Non-Inertial Frames?
  • · Replies 21 ·
Replies
21
Views
5K
Undergrad The CMB drag on an object moving through space
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Is there a drag on Earth caused by its motion relative to the CMB rest frame?
  • · Replies 5 ·
Replies
5
Views
2K