Gravitational redshift
| Scientific Theory | |
|---|---|
| Name | Gravitational redshift |
| Type | Physical effect and its disputed interpretation |
| Author(s) | Effect predicted by Albert Einstein; interpretation disputed — see Alexander F Mayer, Pharis E Williams, Jorge A Guala-Valverde |
| Keywords | gravitational redshift, Einstein shift, Pound–Rebka, time dilation, cosmological redshift, dark energy |
| Year | 1907 onward |
Gravitational redshift, also called the Einstein shift, is the reddening of light that climbs out of a gravitational field: radiation leaving a massive body is observed at a lower frequency than it was emitted. Unlike most subjects in this cluster, the existence of the effect is not in dispute — it has been measured in the laboratory and is applied routinely in engineering. What the critical literature catalogued here disputes is narrower but consequential: what the effect proves, and how much of the cosmic redshift it may account for.
The effect
Light escaping a gravitational well loses energy, and since a photon's energy is proportional to its frequency, its spectral lines shift toward the red. The effect was measured directly by Pound and Rebka in 1959 using gamma rays over a 22-metre tower at Harvard, has been observed in the spectra of white dwarfs, and must be corrected for continuously in the satellite clocks of the Global Positioning System. On this much, mainstream and critical accounts agree.
Does it confirm General Relativity?
Gravitational redshift is routinely presented as one of the classic confirmations of Einstein's general theory of relativity, and as evidence for curved spacetime and gravitational time dilation. Critics on this wiki make the point that the same result follows from far more modest assumptions: applying energy conservation to a photon of mass-equivalent E/c² climbing against a Newtonian potential yields the standard formula without any appeal to curved spacetime.
On this reading the measurement confirms that light loses energy in a gravitational field — a result compatible with several theories — rather than establishing Einstein's particular geometric interpretation. It is therefore treated here as a genuine effect whose evidential weight for General Relativity has been overstated. Related re-examinations include Guala-Valverde's Gravitational Redshift Revisited (1992).
Gravitational redshift and the cosmological redshift
The more far-reaching claim advanced in this literature is that gravitational redshift is not merely a local curiosity but a significant or dominant component of the redshift observed in distant objects — redshift that standard cosmology attributes wholly to the expansion of space.
Pharis E Williams proposed using Hubble Telescope observations to separate a cosmological object's redshift into its gravitational and distance parts, in Using the Hubble Telescope to Determine the Split of a Cosmological Object's Redshift in its Gravitational and Distance Parts (2001). Others have developed cosmologies in which the cosmic redshift is gravitational in origin and no expansion is required; see Gravitational Redshift within a Non-Expanding Universe and Gravitational Cosmic Redshift with Variable Light Speed.
Alexander F Mayer has argued more radically that the canonical cosmological model fails, and that the observations attributed to dark energy — the apparent acceleration of cosmic expansion inferred from distant supernovae — arise instead from a correct treatment of the geometry of time, in On the Geometry of Time in Physics and Cosmology and the Fall of the Canonical Cosmological Model (2011).
If any substantial part of the observed redshift of distant objects is gravitational rather than kinematic, then distances derived from redshift are systematically wrong, and the expansion history inferred from them — including the acceleration attributed to dark energy — requires reconstruction. This connects the topic directly to the wider argument set out at Redshift and Intrinsic redshift.
Mainstream position
Standard astrophysics holds that the gravitational contribution to the redshift of ordinary distant galaxies is negligible: the effect scales with the gravitational potential at the emitting surface, which for a normal galaxy is minute compared with redshifts of order unity or greater. Substantial gravitational redshift is expected only for compact objects such as white dwarfs and neutron stars. Proponents of the alternatives reply that this estimate assumes the very cosmological model in question, and that the split between gravitational and distance contributions is an empirical question that has not been settled observationally.
Criticisms from researchers on this wiki
Researchers catalogued here dispute both the standard evidential claim — that the effect confirms General Relativity — and the standard assumption that its cosmological contribution is negligible:
- Alexander F Mayer — argues that the canonical cosmological model fails outright, and that the observations attributed to dark energy follow from a correct treatment of the geometry of time rather than accelerating expansion, in On the Geometry of Time in Physics and Cosmology and the Fall of the Canonical Cosmological Model (2011).
- Pharis E Williams — proposes an observational programme to separate the gravitational from the distance component of a cosmological object's redshift, in Using the Hubble Telescope to Determine the Split of a Cosmological Object's Redshift in its Gravitational and Distance Parts (2001), treating the split as an empirical question rather than a settled one.
- Jorge A Guala-Valverde — re-examines the derivation and interpretation of the effect in Gravitational Redshift Revisited (1992).
- Gravitational Redshift within a Non-Expanding Universe and Gravitational Cosmic Redshift with Variable Light Speed — develop cosmologies in which the cosmic redshift is gravitational in origin and no expansion is required.
- Gravitational Redshift and Age of the Stars and Redshift of Photons Penetrating a Hot Plasma — examine the effect in stellar and plasma contexts respectively.
The common criticism is that the mainstream estimate of a negligible cosmological contribution assumes the very model in dispute: if redshift is not primarily kinematic, the distances used to argue that galactic potentials are too shallow are themselves unreliable. See Redshift and Intrinsic redshift. Further material is indexed under Category:Cosmology and Category:Relativity.