In George Smoot's 2006 Nobel Lecture, having won the Nobel prize for his research on the Cosmic Microwave Background (CMB), he refers to the possibility of the CMB being a special frame stating "modern efforts to find violations of Special Relativity look to this reference frame as the natural frame that would be special".

Smoot's "aether.lbl.gov" website also states of the CMB: "This would seem to violate the postulates of Galilean and Special Relativity but there is a preferred frame".

Are there peer reviewed articles that consider the possibility that the CMB represents a special frame?


2 Answers 2


If you were going to search for a violation of the equivalence of frames, you'd want to test a proposed special frame against some other one (rather than two randomly selected frames). The local rest frame of the CMB is probably the best bet going (just like the heliocentric frame was the best bet for Michelson and Morley at the dawn of the 20th century).

Note that if the CMB frame were the frame of the luminiferous ether, M&M would have found it easily as our velocity relative the CMB is about ten times their sensitivity threshold (not to mention LIGO and similar large scale interferometers...), so the search has got to be for some other kind of specialness.

Late addition: You can do Lorentz violation searches as parasitic science in a lot of contexts. Generally these measurements have low individual resolving power but taken together they start to add up. A recent example which I know about because it happened at a experiment which I am associated with is Double Chooz' First Test of Lorentz Violation with a Reactor-based Antineutrino Experiment (also on arXiv).

  • $\begingroup$ but is there really such a search, are there really mainstream research groups that study such a possibility as would be implied by Smoot's phrase "modern research efforts...look at..." $\endgroup$
    – DavePhD
    Apr 19, 2014 at 23:31
  • $\begingroup$ None that I have heard of, but that doesn't mean much ... there's lots of stuff going on that I don't hear about. $\endgroup$ Apr 19, 2014 at 23:33
  • $\begingroup$ I see A search for directional violations of the Lorentz invariance... form the KLOE Collaboration, where they say "The CMB is a unique rest frame: even if this fact does not imply by itself any anisotropy of the physical laws, the existence of such a natural rest frame provides a rational framework for the interpretation of any asymmetry that might possibly be discovered." $\endgroup$
    – DavePhD
    Apr 20, 2014 at 13:05

There are apparently some extensions of the Standard Model that allow for Lorentz symmetry to be violated, although from what I understand the symmetry is broken by spontaneous symmetry breaking which means the theory would have been symmetric in the era when the forces were unified, and the symmetry was broken by a random decay to different vacuum state. So in this case there wouldn't be any asymmetry in the fundamental laws of physics, just in the particular vacuum state which our region of the universe has, which was fixed by contingent events in the past. The article "Breaking Lorentz Symmetry" from PhysicsWeb discusses such Lorentz-symmetry-violating extensions of the Standard Model in more detail, here's the section explaining the relevance of spontaneous symmetry breaking:

The alignment of a magnet is a classic example of what is called spontaneous symmetry breaking. The interactions of the individual dipoles in a magnet do not depend on any particular direction, and their dynamics are rotationally invariant. For the magnet to form, however, the dipoles must spontaneously align in some direction, which "spontaneously breaks" the rotational symmetry.

In 1989 Kostelecky and Stuart Samuel of the City University of New York showed that string theory allows for Lorentz symmetry to be spontaneously broken in the early universe. If Lorentz symmetry is spontaneously broken, small relic background fields - which are called tensor-valued vacuum expectation values - would permeate the universe and point in spontaneously chosen directions. An elementary particle in the presence of one of these relic fields would then experience interactions that have a preferred direction in space time. In particular, there could be preferred directions in 3D space in any fixed reference frame, such as an Earth-based laboratory.

At a fundamental level, Lorentz symmetry would still hold dynamically, and all interactions would remain invariant under observer Lorentz transformations. However, the presence of the relic fields would break the particle Lorentz invariance, leading to variations in physical interactions as the motion or orientation of a particle changes with respect to the relic fields.

Not sure if there'd be reason to expect the preferred frame of such a relic field to be the same as the CMB frame, though.

By the way, the difference between "observer" Lorentz symmetry and "particle" Lorentz symmetry discussed in that article, with "observer" symmetry being about the fundamental laws but "particle" symmetry being possibly broken by relic fields, is also discussed on pages 275-276 of the book Out of this World: Colliding Universes, Branes, Strings, and Other Wild by Stephen Webb--those pages can be read on google books here.

That book specifically says that it is in the context of M theory, the speculated unification of various string theories, that it's expected that observer Lorentz symmetry would be preserved but particle Lorentz symmetry might be violated. I also found another reference to the assumption that the fundamental laws of M theory are expected (though only as an ansatz or educated guess) to be invariant under the Lorentz transformation, on p. 368 of the book *Duality and Supersymmetric Theories which refers on p. 368 (viewable here) to an argument that "M-Theory ... is a fully Lorentz invariant theory in 11 dimensions." But in other approaches to quantum gravity, I think physicists have less confidence that the theory will incorporate some form of Lorentz invariance. For example, see p. 8 of this paper which discusses possibilities in an approach known as emergent gravity:

While the previous discussion refers to the standard framework (GR plus QFT), different outcomes for the speed of light postulate can be envisaged when departures from GR are taken into account. The search for such departures has been boosted in recent years by rising of the emergent gravity paradigm. Within this framework it is in fact very natural to see also Lorentz invariance as an emergent spacetime symmetry broken at high energy. Indeed, we have nowadays several toy models where a finite speed of propagation can emerge in systems having no fundamental speed limit [34]. For example, this is the case with the speed of sound in Newtonian (non-relativistic) condensed matter systems. Individual particles of the system can move at arbitrarily large speeds; however, collective density disturbances of wavelengths larger than the inter-particle distance, all propagate at the same finite speed, the speed of sound.

If electromagnetic fields were emergent collective excitations of an underlying system with the speed of light playing the role of the speed of sound, then, any particle or excitation moving at speeds large than c would slow down by emitting electromagnetic radiation, much as in the Cˇerenkov effect. The speed of light will appear as insurmountable in practice. This perspective offers an answer to the question with which we started this essay: Because all the physics that we know of, even that in accelerators, is low-energy physics and all the known fields collective variables of a yet unknown underlying system. Maybe it is allowed to travel faster that light, but only for high-energy beings.

The breakdown of Lorentz invariance generally manifest itself via dispersion relations for matter modified at energies close to the Planck scale, about $10^{19}$ GeV.

And this recent paper says:

the fate of relativistic symmetries in the Planckian regime has attracted interest from other angles (see e.g. 6, 7 and 8). Relativistic symmetries may be left unscathed by the new structures at the Planck scale (e.g. 9), but there are at least two other possibilities. Planck-scale effects may break relativistic invariance, introducing a preferred-frame [10], [11], [12], [13] and [14]; or they may deform the relativistic symmetry transformations, preserving the relativity of inertial frames [15], [16], [17], [18], [19] and [20].

Looking at the references that follow the phrase "introducing a preferred-frame" in the paper, it looks as though they all deal with approaches to quantum gravity distinct from M theory, such as loop quantum gravity.


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