#FaveAstroPlot : Binary pulsars and general relativity

Another interesting astronomical plot is one that ended up providing an indirect evidence of a prediction of general relativity, gravitational waves. General relativity, proposed by Albert Einstein, predicts that gravitational masses can radiate energy in the form of gravitational waves but in reality, these waves are so weak (because gravity itself is weakly interacting) that it is hard to detect them directly. There are experiments to detect them directly (eg : LIGO and LISA) and indirectly (Pulsar Timing Array) but it was indirectly proved by Hulse & Taylor in 1974 when they discovered a double pulsar PSR 1913+16. This peculiar double pulsar had the interesting property that it's period was decaying, instead of remaining constant. As you can see from this plot on twitter, the horizontal line is what is expected if we assume that there is zero orbital decay and the data points are observed time periods over the course of a couple of decades. What's happening behind the scenes is that the pulsars are losing energy in the form of outgoing gravitational waves and because their overall energy is decreasing, their orbital periods decrease.

There are another interesting predictions of general relativity that also has observational evidence but I haven't come across any tweet mentioning it. It's that light can be bent by gravity. This stems from the fact that spacetime itself is curved and the curvature of spacetime depends on the mass of the object in it's vicinity. What we perceive as curved lines are infact straight lines from the point of view of the photon (light ray) but explaining that gets a bit involved and needs a bit of general relativity so I shall not go there this time. Observationally, this was first proved during a solar eclipse when an astronomer showed that our Sun bends the light of stars visibly behind it by a small but not insignificant amount (more about which you can read here and here).

Personally, I don't like theory as much as experimental or simulation work but a course on general relativity was enlightening and it gave me pretty interesting insight into cosmology. So, if you're a student and if there's a course on general relativity being offered in your college, you should take it!


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