The recent LIGO result seems likely to lead to a lot of rewriting of textbooks. Some topics that occur to me:(adsbygoogle = window.adsbygoogle || []).push({});

(1) Obviously it needs to be discussed in any GR textbook's discussion of gravitational waves.

(2) The fact that the waveform matches so well with GR calculations seems to be extremely strong evidence that black holes do exist and that they have the properties described in quantitative detail by GR. Previously, I don't think we had any observations that really probed radii close to the Schwarzschild radius. Now, I don't think there's much room left for theories in which runaway gravitational collapse stops short of the Schwarzschild radius. Also, this is a spectacular test of the Einstein field equations for very strong gravitational fields.

(3) For many decades, the better textbook treatments of SR have been trying to convince students that c is not to be interpreted as the speed of light. We've been able to say that c is a space-to-time conversion factor, or a universal speed limit, or the speed at which massless things travel. But until now, we didn't have a really good example, other than light, of something massless. Gluons aren't observed as free particles, and neutrinos turn out not to be massless. The aLIGO event makes gravitational waves into an excellent concrete example, and it also does support the prediction that gravitational waves travel at c. (The time delay is on the right order of magnitude, it has a statistically likely value, and it was not greater than the maximum allowed by GR. Also, the paper gives very tight limits on dispersion.)

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# LIGO: rewriting the textbooks

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