Speed of gravity

From Free net encyclopedia

Template:Expert

In mathematical physics, particularly in the context of classical theories of gravitation, the speed of gravity refers to the speed at which gravitational radiation propagates. This is the speed at which news or field updating information is propagated as a wave.

If no other theory is specified, it is generally understood that the theory in question is our current gold standard theory of gravitation, namely general relativity.

Contents 1 Newtonian gravitation 2 General relativity 3 Experimental measurement? 4 External links 5 References

Newtonian gravitation

Isaac Newton's formulation of a gravitational force law requires that each object `knows' the instantaneous position of all other gravitating objects through their gravitational attraction. In modern terms, Newtonian gravitation is described by the Poisson equation, according to which, when one somehow moves a gravitating object, the field is everywhere immediately updated.

This aspect of his theory troubled Newton, who felt that such updating information should propagate at some finite speed. He experimented with the effect of introducing such a finite updating speed, and found that this would destroy the remarkable agreement of his original theory with all astronomical observations available in his own time. (Discrepancies between Newtonian gravitation and observation were not widely recognized until the 19th century, long after Newton's death.)

General relativity

In general relativity, the gravitational potential is identified with the metric tensor and the gravitational force with the Christoffel symbols of the space-time manifold. Tidal gravitational field is assocaited with the curvature of space-time. General relativity predicts that gravitational radiation should exist and propagate as a wave at the speed of light. To avoid confusion, we should point out that a slowly evolving source for a weak gravitational field will produce, according to general relativity, similar effects to those we might expect from Newtonian gravitation. In particular, a slowly evolving Coulomb component of a gravitational field should not be confused with a possible additional radiation component; see Petrov classification. Nonetheless, any of the Petrov-type gravitational field obeys the principle of causality, so that the slowly evolving "Coulomb component" of the gravitational field can not transfer information about position of the source of the gravitational field with the speed faster than the speed of light.

This finite speed of gravitational interaction in general relativity may at first seem to lead to exactly the same sorts of problems with aberration of gravity that Newton was originally concerned with. Although the calculations are considerably more complicated, one can show that general relativity does not suffer from these problems just as electromagnetic retarded Lienard-Wiechert potential theory does not.

It is not very easy to construct a self-consistent gravity theory in which gravitational interaction propagates at a speed other than the speed of light, which complicates discussion of this possibility. However, the mainstream view is that there is no known observational or experimental evidence which suggests that the speed of gravity differs from the speed of light. Unfortunately, until recently there existed no experimental measurement of the speed of gravity.

Experimental measurement?

In September 2002, Sergei Kopeikin and Edward Fomalont announced in a conference in Seattle, WA that they had made an indirect measurement of the speed of gravity, using their data from VLBI measurement of the retarded position of Jupiter on its orbit during Jupiter's transit across the line-of-sight of a bright radio source - quasar [[QSO J0842+1835]]. Kopeikin and Fomalont concluded that the speed of gravity is between 0.8 and 1.2 times the speed of light, which would be fully consistent with the theoretical prediction of general relativity that the speed of gravity is exactly the same as the speed of light.

Several physicists, including Clifford M. Will and Steve Carlip, have criticised these claims on the grounds that they have allegedly misinterpreted the results of their measurements. However, Kopeikin and Fomalont continue to vigorously argue their case. (See the citations below for the details of the arguments pro and con.)

It is important to understand that none of the participants in this controversy are claiming that general relativity is "wrong". Rather, the debate concerns whether or not Kopeikin and Fomalont have really provided yet another verification of one of its fundamental predictions.

External links

• Does Gravity Travel at the Speed of Light? in The Physics FAQ

• Hazel Muir, First speed of gravity measurement revealed, a New Scientist article on Kopeikin's original announcement.

• Clifford M. Will, Has the Speed of Gravity Been Measured?.

• Meta Research, Inc., an organization founded by Tom Van Flandern which collects his writings

• Did Einstein Cheat?, by John Farrell, a Salon article which contains unusually frank comments by several physicists on Van Flandern's claims

• [1], a collection of replies by physicist Steve Carlip and others to UseNet posts by Tom Van Flandern.

References

<references/>

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite arXiv

• Template:Cite journal }}

• Template:Cite arXiv

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite arXiv

• Template:Cite arXiv

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite journal }}

• Template:Cite arXiv

Promoting Van Flandern's claims:

• Template:Cite journal

(See Meta Research for further citations.)

Criticizing Van Flandern's claims:

• Template:Cite journal }}