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A century of experiments in general relativity

Today we discuss experiments in general relativity with Dr. Daniel Kennefick, PhD in physics from the California Institute of Technology, Professor of Physics at the University of Arkansas, and author of "No Shadow of a Doubt: The 1919 Eclipse That Confirmed Einstein's Theory of Relativity".

David: Experiment to test a theory is a simple idea. In practice it is subtle. Do you see an evolution in that subtlety over time in astrophysics and cosmology?

Dan: I think there has been a great evolution in the subtlety of experiment, most particularly as we move farther away from direct experience and rely more in deduction. For instance we cannot see events in the early Universe and must rely on calculations, informed by experimental laboratory work to deduce how primordial nucleosynthesis must have gone and then compare those calculations with observations of element abundances in the Universe today. That is certainly much more subtle than trying to estimate the distance to the Sun, which was already quite hard.

David: You describe the bet by Kip Thorne that LIGO would detect a gravitational wave from merging black holes before the waveform could be generated from simulation. How did Thorne expect the LIGO team to recognize the wave from data without a template?

Dan: Kip's bet essentially had two sides to it. One, which came to pass, was that the event might be large enough to be sure it was there even without a template. The other is that it would later turn out the event was there earlier, but went unfound for lack of a template. In either case, I suspect, Kip envisaged winning the bet.

David: Is it legitimate to consider LIGO detections as deeper experiments of general relativity than the 1919 eclipse data?

Dan: I think it is fair to say this is a deeper test of GR than the eclipse experiment. It involved testing GR in a strong gravity regime, finding evidence for a phenomenon that no one envisaged prior to 1900.

David: From a theory perspective, is understanding the existence of energy-carrying gravitational waves trickier than identifying a cosmological constant term?

Dan: I think GWs are trickier than the cosmological constant, though both are subtle and tricky concepts. To elucidate gravitational waves in the theory was much harder to do and was much harder to interpret than lambda.

David: In a parallel universe where the 1914 Crimean eclipse expeditions succeeded, does relativity still become a cornerstone of modern physics?

Dan: I think GR would still have succeeded though there would have been controversy over the fact that Einstein would have subsequently changed his "prediction". I think people would still have recognized how important it was that the equivalence principle was confirmed and that he was on the right track. As an example, when Einstein received his famous telegraph from Lorentz in 1919 and was so excited, Lorentz was only saying that Eddington had confirmed deflection, not necessarily the full GR prediction. It’s important to remember that any deflection was viewed as Einstein’s prediction and was viewed by him as important confirmation of the truth of the equivalence principle.

David: Thank you Professor!

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