Black holes are very interesting things, aren’t they? There’s something fascinating about those things which are so hard to understand. Black holes are one of the most asked about objects at almost any public space talk. Certainly school children seem to be obsessed with them!

So a question I was asked this week (by both John and Margaret) was ‘what is the smallest size of a black hole?’. If a star can collapse into a small space and become a black hole, can a planet or even a proton? Well yes, they can, so long as you squash them down hard enough.

There is no theoretical limit on the sizes of black holes. A black hole is determined by how much mass is being contained within a radius. If you put enough material inside a small enough space, you get an object which is so dense that even light cannot escape: a black hole.

Put another way, if you were to squash down any object far enough you eventually get a black hole. This critical size is called the Schwarzschild radius. For example, the Schwarzschild radius for the Sun would be 3km and for the Earth it would be about 9mm. Just imagine compressing the Earth down to just 18mm across!

The follow-up question from John was a great one:

Newton’s olde cannon fired objects toward the horizon at faster and faster speeds until they reached escape velocity and ‘fell’ into orbit. Fair enough, but this idea was never applied to light because light was just too damn fast and also no one believed it was affected by gravity anyway. Now we know better. So does this mean that if a beam of light approached a black hole at precisely the right glancing angle to match its speed that it would not fall in but form a kind of ‘light in orbit’… like saturns rings but made only of light?

I had never though of this before – which is why I love people sending in questions. So I went to find out from some people who should know, here at the university, whether light could orbit a black hole. The answer surprised me: ‘yes’.

It turns out that light can orbit a black hole, but not for very long. Just beyond the event horizon of a black hole (the distance at which nothing can escape), there is a short distance in which an incident photon is deflected into a circular ‘photon orbit’. It won’t stay very long in this state though. These orbits are highly unstable and soon the photon would either spiral into the black hole’s event horizon or be ejected outward again. I just really like that this can happen, even it happens for a very short time. I wonder what it looks like?

I hope that this is just as interesting for everyone to learn about. If anyone out there knows more on this topic, I’d love to hear from you.

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