Hay-Schild controversy

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Hay-Schild controversy was a dispute that arose in the physics community in 1960 over the correct way to interpret the application of the equivalence principle to the case of centrifuged clocks. A research group at Harwell Atomic Energy Research Establishment in the UK presented two gravity-shift experiments, one using the Earth’s gravitational field, and the other using a Coriolis field generated inside a centrifuge. The Harwell group argued that in the second experiment, the measured redshift could be calculated either by using the equivalence principle and calculating the effect as the result of acceleration (i.e. gravitational time dilation), or by applying special relativity to the problem and treating the effect as the result of speed-dependent time dilation.

Alfred Schild quickly published a rebuttal, pointing out that the “flat-spacetime” and “curved-spacetime” explanations are not dual, since the two geometrical descriptions are incompatible, suggesting that at least one of the two approaches to modelling the experiment must be physically wrong. Since special relativity could not be wrong (argued Schild), it must instead be that the equivalence principle does not apply in cases where it might otherwise invalidate special relativity. This is now the accepted mainstream view.

Contents 1 Harwell group, 1960 2 Schild, 1960 3 Modern view 4 References

[edit] Harwell group, 1960

The Harwell group’s "conventional" gravity-shift experiment and their "centrifuge" version were published on consecutive pages of Physical Review Letters. According to the second paper,

"Einstein's principle of equivalence states that a gravitational field is locally indistinguishable from an accelerated system. ... therefore ... "

: " The expected shift can be calculated in two ways. One can treat the acceleration as an effective gravitational field and calculate the difference in potential between the source and absorber, or one can obtain the same answer using the time dilatation of special relativity. "

[edit] Schild, 1960

Alfred Schild's paper "Equivalence Principle and red-shift measurements" (American Journal of Physics [28] 778-780 (1960) ) rejects the “Coriolis field” calculation:

" The group at Harwell [Hay, Schiffer, etc] has also measured the red-shift produced in rotating disks. The question arises whether, by virtue of the equivalence principle, such effects in accelerated systems are to be regarded as verifications of general relativity.
There seems to be some confusion on this point and even some lack of unanimity among theoretical physicists. It is one of the purposes of this note to clear up this question. The confusion is unnecessary, because within the framework of the theory of relativity the answer is simple and definite. It is "no!" "

Schild argues that since the "flat" and "curved" calculations both predict the same experimental outcome, but are geometrically incompatible, they must be regarded as competing explanations. If special relativity cannot be wrong, and the equivalence principle should not be wrong, it must instead be that the equivalence principle is being applied inappropriately.

" If ad hoc assumptions are ruled out, the equivalence principle leads in a natural manner to a curved spacetime. This is inconsistent with special relativity, which deals with flat Minkowski spacetime, and with Newtonian gravitation, which is itself inconsistent with special relativity. Thus, in the sense of mathematical logic, it is meaningless (or uninteresting) to ask for consequences of a self-contradictory theoretical system consisting of the equivalence principle, special relativity and gravitation. "

In the context of Einstein's general theory of relativity, which incorporates both special relativity and the equivalence principle, the two components should not be in conflict by definition. Where the two approaches seem to be incompatible, special relativity is assumed to have a greater claim to priority. The correct role of general relativity (says Schild) is only to predict the things that are not already dealt with adequately by SR.

[edit] Modern view

The modern textbook treatment of centrifuge-class experiments (including experiments using circular particle accelerator storage rings) is to adopt Schild’s "priority of SR” arguments. Accelerational redshift effects are now said to be known to be insignificant in these situations, in adoption of the clock hypothesis of special relativity.

[edit] References

• Cranshaw, Schiffer and Whitehead, "Measurement of the Gravitational Red Shift Using the Mössbauer Effect in Fe⁵⁷"

Phys. Rev. Lett. [4], 163-164 (1960) [1]

• Hay, Schiffer, Cranshaw and Egelstaff, "Measurement of the Red Shift in an Accelerated System Using the Mössbauer Effect in Fe⁵⁷"

Phys. Rev. Lett. [4], 165-166 (1960) [2]

• Alfred Schild, "Equivalence Principle and red-shift measurements"

Am. J. Phys. [28] 778-780 (1960)