Preferred frame
A preferred frame — also called a privileged frame or absolute frame — is a frame of reference singled out from all others, in which the laws of physics take their simplest form and against which motion can be called absolute rather than merely relative.
Whether such a frame exists is one of the oldest and sharpest divisions in physics, and it is the question on which a large part of the literature catalogued on this wiki turns.
The mainstream view
Modern physics holds that there is no preferred frame.
The position rests on the principle of relativity: the laws of physics are the same in all inertial frames, so no experiment performed inside a uniformly moving laboratory can reveal its motion. Special relativity builds this in as a postulate, together with the invariance of the speed of light, and derives the Lorentz transformation from the two. General relativity extends the equivalence to accelerated frames.
The historical turning point is taken to be the Michelson–Morley experiment of 1887, whose failure to detect the Earth's motion through a stationary aether is read as evidence that no such medium — and so no frame attached to it — exists. Lorentz's aether theory preserved a preferred frame but made it undetectable in principle, and was set aside as carrying an unobservable entity to no predictive advantage. On the standard account the two theories agree on every observation, and special relativity is preferred for its economy.
Mainstream physics does allow that some frames are distinguished without being dynamically preferred. Inertial frames are collectively privileged over rotating and accelerating ones, since only the latter require causes outside the system — Newton's rotating bucket and Einstein's spinning spheres make the point. And the cosmic microwave background defines a frame in which its radiation is isotropic, relative to which the Earth's motion is measured at about 370 km/s. This is treated as a contingent fact about the distribution of matter in our universe, not as a violation of relativity: the CMB frame is convenient, not fundamental, and the laws of physics are held to look the same in it as anywhere else.
Lorentz invariance is among the most stringently tested symmetries in physics, and searches for its violation continue to return null results at very high precision.
For the older history of the question — Newton and absolute space, Leibniz's relationism, Mach's principle — see Absolute space and time.
The view on this wiki
The researchers documented here dispute this on several distinct grounds. They do not all agree with one another, and the arguments are worth separating, because they are of different kinds: some are empirical, some are structural, and some concern what a theory is entitled to assume.
The experiments were never null
The most persistent objection is that the founding experiment did not give the result it is credited with. Michelson and Morley measured a fringe shift smaller than predicted but not zero, and when the work was continued for over thirty years by Morley's collaborator Dayton C Miller, it yielded a small, systematic, repeatable signal whose direction varied with sidereal time — the signature of a real cosmic motion rather than an instrumental artefact.
Miller's result was set aside rather than refuted, and attributed after his death to temperature gradients. James DeMeo has examined his raw records directly and defends their integrity in Reanalyzing Dayton C. Miller's Raw Interferometer Data. Hector A Munera has reanalysed the historical experiments for systematic error and argues for their compatibility with absolute space.
Later observers report small non-null effects of their own: Yuri M Galaev on ether-drift velocity and viscosity in the optical band, Eugene I Shtyrkov from geostationary satellites, Ronald R Hatch in In Search of an Ether Drift. See Michelson–Morley experiment for the full literature.
Rotation is detected, but translation is not
The sharpest structural argument is that light-speed anisotropy is routinely measured — just not for uniform translation.
The Michelson–Gale–Pearson experiment of 1925 measured the Earth's rotation optically, and the Sagnac effect is not merely real but technologically indispensable: ring-laser gyroscopes depend on it and GPS must correct for it. Howard C Hayden and Cynthia Kolb Whitney put the question in a title — If Sagnac and Michelson-Gale, Why Not Michelson-Morley?
The contention is that a principled account is owed of why rotational motion through space should produce a measurable optical asymmetry while translational motion produces none.
Ronald R Hatch, a working GPS engineer, argued that the system as actually operated is not relativistic in the way it is popularly described: clock corrections and signal timing are computed in a single common frame centred on the Earth, not frame-by-frame, and the Sagnac correction applied to signals is of exactly the kind an absolute frame would require. Tom Van Flandern, who worked on GPS timing, argued similarly that its operation is more naturally described with a preferred frame than without one.
The CMB dipole is the frame, observed
Reginald T Cahill presses the boldest empirical claim: that the small drift Miller and others measured is not noise but a genuine detection of absolute motion, and that its direction agrees with the motion inferred from the cosmic microwave background dipole. His papers The Detection of Absolute Motion: From 1887-2005 and Michelson-Morley Experiments Revisited and the Cosmic Background Radiation Preferred Frame argue that the aether-drift experiments and modern cosmology have been measuring the same preferred frame by different means, and that only the interpretation differs.
The neo-Lorentzian position
A substantial group accepts the observational record entirely and disputes only the interpretation. If Lorentz's theory and Einstein's agree on all predictions, then the choice between them is not settled by experiment, and the question of which is true remains open.
Franco Selleri derived his inertial transformations from the homogeneity of space, obtaining a one-parameter family of theories in which special relativity is a single member and absolute simultaneity is preserved in the others. Joseph Levy, Simon J Prokhovnik and Petr Beckmann developed related accounts in which a physical medium or cosmological substratum defines the frame while the observable predictions are unaltered.
The recurring point is that the one-way speed of light has never been measured — only the two-way average — and that its isotropy is therefore a convention adopted in setting clocks, not an experimental result. If so, absolute simultaneity is not excluded by evidence.
Absolute simultaneity
Franco Selleri argued the case directly in Eight Proofs of Absolute Simultaneity, and Thomas E Phipps in Absolute Simultaneity With and Without Light Signals. Mogens True Wegener reaches the same conclusion from the philosophy of cosmology: cosmology in practice cannot do without a cosmic time — the age of the universe, the sequence of epochs — and he holds this is a commitment the subject cannot abandon rather than a convenient fiction.
Quantum non-locality demands it
The most authoritative support comes from an unexpected quarter. John Stewart Bell, whose theorem established the non-locality of quantum mechanics, was openly prepared to restore a preferred frame to accommodate it:
[T]he cheapest resolution is something like going back to relativity as it was before Einstein, when people like Lorentz and Poincaré thought that there was an aether — a preferred frame of reference — but that our measuring instruments were distorted by motion in such a way that we could not detect motion through the aether.
— John Stewart Bell
Bell was not a dissident and this was not his settled view, but the argument is his and it is serious: if influences propagate faster than light, as his theorem's experimental confirmation appears to require, then a frame is needed in which to say what happens first.
Theories built on a preferred frame
Some researchers here do not merely argue for a preferred frame but construct physics from one.
Cláudio Nassif's Symmetrical Special Relativity adds an invariant minimum speed to relativity's invariant maximum, attached to a background field he calls the ultra-referential — a preferred frame that breaks Lorentz symmetry at the lowest energies while reducing to special relativity elsewhere. He derives the cosmological constant from it and has proposed a laboratory test.
The mainstream Einstein-aether theory is worth noting as a point of contact: it is a respectable, actively researched modification of general relativity containing a dynamical preferred frame, which shows the idea is not in itself unphysical.
Experiments to settle it
Rather than argue only from the historical record, several researchers have proposed or performed new tests: Ken H Seto's Proposed Experiments to Detect Absolute Motion, A A Efimov's Experimental Detection of Absolute Space, Martin Ruderfer's Detection of Absolute Motion from Atomic Timekeeping Data, and measurements of the one-way speed of light such as Emil D Gigov's Measurement of the One-Way Speed of Light.
Paul Wesley argued the general case in Evidence for Newtonian Absolute Space and Time, and Hector A Munera extended it to cosmology in Redshift in Absolute Space: Periodicity of Quasars and Other Cosmological Implications and Towards the Reinstatement of Absolute Space, and Some Possible Cosmological Implications.
Where the disagreement actually lies
It is worth being clear about what is and is not in dispute, because the two sides often argue past one another.
Very little of this literature claims that special relativity gives wrong predictions for the experiments it has been tested against. The claims are of three other kinds:
- Empirical — that certain results (Miller's, and the small residuals) are real signals that were dismissed rather than explained, and that the record is therefore not as clean as textbooks report.
- Underdetermination — that Lorentzian and Einsteinian accounts are observationally equivalent, so experiment cannot decide between them, and the standard choice rests on economy rather than evidence.
- Conventionality — that the isotropy of the one-way speed of light is built into the clock-synchronisation procedure, so experiments assuming it cannot be used to prove it.
Against these, the mainstream replies that the small residuals are consistent with systematic error, that an undetectable frame does no work, and that Lorentz invariance now survives tests far more sensitive than any available to Miller.
What would settle it is a reproducible, independently confirmed detection of anisotropy in a modern experiment — which is why the proposals listed above matter more to this debate than any amount of reinterpretation.