One of the best New York Times headlines ever (although sexist) referred to the total solar eclipse on May 29, 1919 - the longest solar eclipse in over 500 years. Two teams looked to see if the apparent position of stars near the eclipsed solar disc would be shifted as Einstein had predicted a few years earlier. Sir Arthur Eddington's team, on the island of Principe, had clear weather and was able to confirm Einstein's prediction. Much to his dismay, Einstein became a public superstar.
General Relativity makes predictions that seem strange as we're used to the everyday force of gravity as described by Newton. Black holes are probably the most famous prediction of something that pops out of the physics. They're not massive bodies, but rather a region in space associated with enough mass in a compact region that nothing, including light, that falls in can ever leave. It's the stuff of good and bad science fiction and good science.
A black hole the mass of the Sun would have a six kilometer diameter. A five solar mass black hole would be thirty kilometers across You may object as the volume of a sphere goes as the cube of its radius. It turns out black hole's diameter increases linearly with mass. A massive black hole can be much larger than you might expect.1
Besides the date, the motivation for this post is a video from NASA showing how big some of the supermassive black holes at the centers of galaxies can get. It turns out there's a limit ( it's a bit technical) and the largest black hole shown here is close. But just wow!
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1 Blame General Relativity - at least that's the usual approach. You can see it by working out the Schwartzschild radius, but that requires a bit of differential geometry and, trust me, I'm not clever enough to give a quick non-technical explanation... and not for a lack of trying. But there's a simple way to see it. The event horizon is a limit in spacetime rather than an object. You can use a Newtonian analogy and high school physics to think about it. Consider the escape speed some radius out from a spherically symmetric mass - it's a spherical surface where the sum of its kinetic and potential energy is zero. Fix the velocity and you get
r = 2GM/v2
So no surprises. You get a similar scaling in General Relativity and you then realize you're not describing an object.
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