Monday, 12 August 2013

Black holes aren't dangerous

Black holes have a bad press.  They are seen as cosmic vacuum cleaners, sucking up everything in space, as the end of everything, as places where physical laws break down.

This is all wrong, all of it, in interesting ways.

Black holes don't suck up everything.  Black hole gravity is no different from any other kind of gravity.  If the Sun were changed instantly into a black hole of the same mass, the orbits of the planets would not shift at all, not even an inch.  If the Moon were made into a black hole the nights would be darker, but that's all that would happen.  The tides would be the same.

Things do fall into black holes, but it's really quite hard to do, as they are so very small on a cosmic scale.  If you fall towards a black hole, by far the most likely thing to happen is that you will miss it and either be flung away or end up in orbit.

Black holes are dangerous if you get too close, but then just about anything in the skies is dangerous too.  You would be killed by tidal forces close to a typical black hole, but you would be destroyed by heat long before you got to the equivalent distance of the centre of a star.

Physical laws don't break down in black holes - we just don't understand what is going on.  To say physical laws break down is like saying that the laws of landscape break down at edge of a map.  The real world doesn't care about the shape of our maps.

Even tiny black holes that might be made in a particle accelerator are harmless - they would disappear extremely rapidly in a tiny burst of radiation, but even if they didn't, it wouldn't matter - such black holes are so incredibly small that there is no chance an atom could randomly fall into one as they are so much smaller than atoms.  The chance of even a single particle ending up in the hole is remote.

Black holes aren't at all like science fiction usually portrays them - they are amazing and fascinating and strange, but almost always harmless.


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