## Eclipse and relativity

A few hours ago the longest total solar eclipse of the Century swept across Asia. And a few days ago Evalyn Gates provided a wonderful guest post on gravitational lensing. This seems like an opportune time to note that gravitational lensing and total solar eclipses are inextricably linked.

One of the most interesting predictions of Einstein’s new theory of relativity was that gravity would cause light to bend. Imagine you are looking at a distant source of light, for example a star, or a faraway galaxy, or a quasar at the edge of the Universe. And let’s assume that, along the line-of-sight to the distant source there’s a massive object, for example the Sun, or a black hole, or a galaxy, or a cluster of galaxies. The gravity from the massive object will “pull” on the photons as they pass, shifting their paths, and thereby affecting the image that we see in our telescopes. In the simple case of a distant point source of light (e.g., a far away star), and a compact spherically symmetric lens (e.g., a black hole), the bending angle is given by
$\displaystyle \theta=(G/c^2)4M/r$
In this equation M is the mass of the lens, r is the minimum distance between the (unperturbed) line-of-sight to the source and the lens, G is the gravitational constant, and c is the speed of light. This was a crucial prediction of Einstein’s new theory, and one way to test it was to see if the stars on the sky “jump” as the Sun (which is quite massive, and traverses the sky quite briskly) comes nearby on the sky.

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This is a very nice explanation, not only of the importance of eclipses for 20th Century science, but also it provides a nice view of EInstein’s impact. Plus it has an equation that I have not seen since college. It was my inability to really ‘get’ relativity, quantum mechanics, etc. that proved to me that biology was the place for me.

Luckily for me, you do not need to work with quantum mechanics to have a nice carer in biotechnology.

I did always love the irony that eclipses, which had been a source of superstitious behavior for centuries, were used to prove the most non-intuitive theories ever created. Einstein’s work put the final nail in Newtonian mechanics but replaced it with something that, to me, seemed to have as many odd, unusual ramifications, with apparent non-rational conclusions, as any bit of fabulous magic.

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