Exploring Historical Context of GR Predicted Gravitational Redshift

[stevenb]

I was just reading the old document by H.A. Lorentz " The Einstein Theory of Relativity" which is freely available in ebook and text format. I find it interesting to read some of the comments from those times, and Lorentz provides a nice summary of the thoughts of the days soon after a second prediction of GR was confirmed (May 29, 1919); namely the Eddingtion comfirmation of the GR predicted bending of light near a star. Previous to this, it seems that the explanation of the perhelion of Mercury was the only experimental evidence for GR.

My question relates to a comment that is made about the failure of GR with regards to gravitational redshift. At the time of this article, there had been no spectroscopic evidence for gravitational redshift. This in itself does not surprise me since it was another 10 years before Hubble discovered the expansion of the universe. However, what surprises me is that the comment seems to indicate that the spectroscopic experts of the time felt that the experimental evidence actually contradicted GR.

Can anyone provide a historical context for this? Did the experimentalists actually have data sufficient to hold this opinion? Apparently, they didn't catch the redshift from the expanding galaxies, but maybe they were only looking locally. However, what made them think that they had the data which was not consistent with GR? Obviously, GR has withstood the test of time on a cosmological scale, but I'd like to understand the history and technical details behind this, if possible. Did the experimentalists have bad data, bad interpretations or were they just blowing smoke?

Can anyone shed some historical light on this? For reference, I provide the a quote of the relavent paragraph, but the entire article is readily available if you search " Lorentz The Einstein Theory of Relativity". It's only a few pages long.

"(3) In the excitement of this sensational verification, there has
been a tendency to overlook the third experimental test to which
Einstein's theory was to be subjected. If his theory is correct as it
stands, there ought, in a gravitational field, to be a displacement
of the lines of the spectrum towards the red. No such effect has
been discovered. Spectroscopists maintain that, so far as can be
seen at present, there is no way of accounting for this failure if
Einstein's theory in its present form is assumed. They admit that some
compensating cause may be discovered to explain the discrepancy, but
they think it far more probable that Einstein's theory requires some
essential modification. Meanwhile, a certain suspense of judgment
is called for. The new law has been so amazingly successful in two
of the three tests that there must be some thing valid about it,
even if it is not exactly right as yet."

 

[bcrowell]

At the time of this article, there had been no spectroscopic evidence for gravitational redshift. This in itself does not surprise me since it was another 10 years before Hubble discovered the expansion of the universe. However, what surprises me is that the comment seems to indicate that the spectroscopic experts of the time felt that the experimental evidence actually contradicted GR.

In GR, you can't really classify redshifts as gravitational or Doppler. I'm pretty sure that astronomical redshifts had been discovered at the time, but they could be interpreted without GR, or even SR. Someone who was a skeptic about GR could simply interpret them as Doppler shifts. It wasn't until about 1922 that it was proved that the universe extended beyond the Milky Way: http://en.wikipedia.org/wiki/Edwin_Hubble#The_Universe_goes_beyond_the_Milky_Way_galaxy I think the final step in Hubble's discovery of cosmic expansion was the establishment of a distance scale, not the observation of Doppler shifts.

The Lorentz quote surprises me. I'd thought that the history was something like the following. First, W.S. Adams tested GR's prediction of gravitational redshifts for spectra of white dwarfs in 1924. The results were consistent with GR, but were not a firm confirmation of GR because the quality of the data was very poor. It wasn't until 1959 that Pound and Rebka did a really convincing demonstration. Presumably the Lorentz quote is an example of the typical haze of confusion that gets swept under the rug in textbook accounts of the history of science. It would be interesting to know what measurement he had in mind that he thought contradicted GR.

 

[Russell E]

The Lorentz quote surprises me. I'd thought that the history was something like the following. First, W.S. Adams tested GR's prediction of gravitational redshifts for spectra of white dwarfs in 1924. The results were consistent with GR, but were not a firm confirmation of GR because the quality of the data was very poor. It wasn't until 1959 that Pound and Rebka did a really convincing demonstration. Presumably the Lorentz quote is an example of the typical haze of confusion that gets swept under the rug in textbook accounts of the history of science. It would be interesting to know what measurement he had in mind that he thought contradicted GR.

The first attempts to check for the redshift effect were based on solar observations. The effect for the sun is very small. By 1920, some astronomers (e.g., Grebe and Bachem) had made observations that seemed to confirm the relativistic prediction, whereas others (e.g., St John) had reported observations that were not consistent with the relativistic prediction. So it was very much in doubt (from an experimentalist's point of view) for several years.

 

[stevenb]

Thanks for both of your inputs on this. This insight helps me understand the situation better. I'm still a little surprised by Lorentz's comment. It is what I might have expected from others, but not from him. But, maybe he was just being overly cautious as a dispassionate scientist, or maybe even playing a reverse psychology strategy to not offend the opponents. Standing beside your adversaries and walking them over to your side is more effective than a direct confrontation, perhaps?