Blog Ekkehard Friebe

Liebe Leser! Damit Ihr Interesse nicht erlahmt, bringe ich heute etwa anderes, und zwar eine kritische Arbeit in englischer Sprache (bisher unveröffentlichter Entwurf vom 8. August 2001) von einem deutschen Physiker.
Der Autor ist Dr. Wolfgang Engelhardt:

Zitat:

A Remark on the Constancy of the Velocity of Light
W. Engelhardt

I Introduction
In his famous paper of 1905 Einstein postulated that the velocity of light be constant in all inertial systems. Measurements with increasing accuracy confirmed the justification of this conjecture, so that today we have in fact replaced the normal meter by the cesium second and nine numbers. A physicist measuring the velocity of light, who would come up with a result different from the nine legal numbers, would just have used an illegal system of units.

In his General Theory of Relativity (GRT) Einstein conceded that the velocity of light may depend on the gravitational potential which would explain the deflection of light passing heavy masses. In order to verify any variation of c experimentally, one would need to measure the velocity of light at different gravitational potential. This is, however, no longer possible having abolished the normal meter. It is, therefore, now custom to postulate also in GRT the constancy of c and explain the observed deflection and the longer duration of the passage of light near gravitational centers by a distorted metric of space.

There are, however, experiments carried out on earth which are much easier to interpret when we follow Einstein’s original conjecture, namely that the velocity of light depends on the gravitational potential. Curiously enough, these experiments are commonly taken as a confirmation of GRT, but this is in fact only true when we allow for a variation of the velocity of light. If not, a violation of the energy principle would be the consequence.

In this note we discuss the famous experiment by Pound and Rebka of 1960 who used the Mössbauer effect to measure the “apparent weight of photons”. We compare it with the “Maryland experiment” of Alley proving that atomic clocks run faster with increasing distance from the gravitational center. Both experiments are frequently said to confirm in an “equivalent” way a simple formula derived in GRT, but, as already hinted by Pound and Rebka, this is actually not true. We include in our comparison a simple gedanken experiment which is based on the conservation of energy and we come to the conclusion that the real experiments may be reconciled when we allow for a variation of the velocity of light.

II Two experiments checking on certain predictions of GRT
As soon as the Mössbauer effect offered a relative accuracy of 10^-15 Pound and Rebka used it immediately to check on two predictions of GRT: 1) Clocks run faster with increasing distance from a gravitational center. 2) The energy of photons increases when they “fall” in a gravitational field. The first prediction is, of course, not generally true. If the clock is a pendulum-clock, it swings definitely at a reduced frequency on top of a mountain. If the clock is the rotating earth itself, it has obviously the same frequency on the mountain and in the valley. None of these clocks, however, offers the precision to detect the faint effect predicted by GRT:  

Δν/ν  =  ΔΦ/c²       (1)

 which is accessible by atomic clocks using the equivalence:  

h ν  =  E₂ – E₁       (2)

If one stipulates that the energy difference of two atomic levels depends on the gravitational potential in a way to yield (1), atomic clocks should run faster at higher elevation. On the other hand, if one ascribes an effective mass to a photon according to the relationship:  

h ν  =  m_eff c²         (3)

“falling” photons should gain energy in the gravitational field and the increase in frequency would be precisely that given by (1). In the Pound-Rebka experiment the photon emitting nuclei are at an elevated height and the photons fall to the bottom of the tower where the absorber is placed. One would, therefore, expect to observe twice the effect given by (1). As only the single effect was measured, Pound and Rebka quote two “schools of thought”: School I says that clocks run faster on the top of the tower, but the photons do not change their frequency on the way down. School II claims that the frequency of clocks is independent of their elevation, but the falling photons increase their energy and thereby their frequency. Although the authors tend towards school II by entitling their paper “apparent weight of photons”, they leave the question open, also in the second publication by Pound which confirms (1) with higher accuracy. In modern seminars students are told that the contradicting views of school I and II are “equivalent” which “solves” the problem termino- logically.

Since Alley’s experiment of 1976 it is possible to distinguish between the two schools. By flying atomic clocks at a height of 10 km at low speed to minimize the velocity effect predicted by SRT, it was possible to confirm equation (1) by a direct comparison of the clocks after landing, without dropping photons from the aircraft to the ground station. Taking together the results of Pound and Alley it is now clear that photons do not change their frequency when approaching or escaping from a gravitational center. School I is confirmed by experiment. Pound and Rebka measured the single effect, because the emitting nuclei produced higher energetic photons on the top of the tower.

This conclusion raises, of course, a problem. The deflection of star light passing “Einsteinlenses”, of which the first one known was our sun, seems to favor school II, since photons are apparently attracted by a gravitational center. In the next Section we consider a simple gedanken experiment based on the conservation of energy and find that the experiments of Alley and Pound can be reconciled with Einstein’s original conjecture assuming a variation of the velocity of light.

(Zitatende)

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Beste Grüße Ekkehard Friebe