What is a black hole?

Event horizons, singularities and time dilation: we get to the bottom of the weird physics of black holes.
25 January 2016

Interview with 

Matt Middleton, Cambridge University Institute of Astronomy


Black holes are mysterious regions of space with sufficient gravity to bend the very fabric of space itself - called spacetime - in such a way that nothing, not even light, can escape. Anything unfortunate enough to come into contact with one will be annihilated, becoming part of the black hole itself. But how do these mysterious entities work and how did they form? Georgia Mills met with Cambridge University Institute of Astronomy researcher Matt Middleton for 'Black Holes 101'.

Matt - A black hole in it's simplest possible terms is an object that is incredibly dense, and the speed that it takes to get away from an object is related to how dense it is.  The more dense it is, the faster you've got to go and there's a point to which it so dense that not even light can get away from its surface.

Georgia - Like being the fastest thing in the universe.

Matt - Yes, absolutely,  In terms of a more rigorous understanding you can imagine there's, for instance, those games you used to play in McDonald's where you throw the coin around and it has that funnel shape. If you can imagine being that coin and trying to get out of that machine, not only are you doing charity a massive disservice, but it's also very, very difficult, and you can imagine trying to go at the speed of light and still not being able to climb out of that thing, that's kind of like what a black hole is.  It's this pit that when you go beyond a certain point you can no longer escape.

Georgia - This point of no return is known as 'the event horizon'.  We can't see inside because any light that made it there won't escape.  But somewhere inside this is what's known as 'the singularity'.  This is where all the mass of the black hole is contained in a miniscule area, and to fit all of this mass in such a tiny space, the singularity has to be practically infinitely dense, which pushes our understanding of physics to its limits... So how are black holes created?

Matt - Stars are in constant constant battle against themselves.  They're producing lots of radiation; that radiation provides and outward push against the inward pull of gravity, and eventually stars sadly run out of fuel and then gravity's going to win.  It collapses in on itself; as it collapses in, material gets thrown away in what we call a  supernova explosion.  These are really, really big, powerful events.

Georgia - Most stars collapse down to smaller, stable sizes but, if they were big enough, the gravitational forces involved are so strong that nothing can stabilise them and they can collapse down and down and become a black hole.  These black holes can grow bigger over time, sucking in anything that strays close enough - from planets to stars and even, potentially, other black holes.  What about when you get close to the event horizon; this is when things start to get a little weird.  I'll give you an example, here's an alarm clock - lets push it towards the black hole. The ticking gets slower, and slower, and slower, so what's going on?

Matt - I don't know what it takes for you to hate a clock enough to throw it into a black hole...

Georgia - I don't like getting up in the morning...

Mat - You really, really don't - we've all been there haven't we?  Okay, so, the clock, if it's ticking, it's going to get slower, and slower, and slower because it's approaching the black hole; it's on increasingly curved space-time and there's this thing called 'gravitational time dilation'. And what we know and what we observe is that clocks run slower closer to massive objects. So for the clock, nothing's happening, it's just ticking at a normal happy rate but, for us, it seems to be ticking slower, and slower, and slower, until eventually, it will appear to be stop ticking at all - that's essentially when it's at the event horizon.

Georgia - Well, now my clock's been destroyed I want it back; I'm going to throw myself into the black hole.  From my point of view, what would happen to me?

Matt - Right.  Okay, some seriously bad things are going to happen to you. So, there are these things called 'tidal forces'.  We know tidal forces, they affect the moon-earth systems, tides, etc.  It's why, if you're falling toward a body your feet would feel a greater pull initially than your head so you'd be stretched out.  If you take that to the next level, in a black hole you get thinner, and thinner, and thinner and you become 'spaghettified' which is a - oh it's a nasty word.  You start to do some crazy things like if you looked to your right or left, you might be able to see the back of your head due to photons travelling around a black hole caught in an orbit, so it would probably be quite a crazy trip.  And eventually, of course sadly, you'd be pulled inexorably towards the singularity where your mass would be collapsed down to infinity and you'd contribute to the mass of the black hole.

Georgia - Okay, I've been putting you through your paces with these questions.  I'm going to send you some quick-fire questions now because we get a lot sent in from the audience.  Everyone's very curious about them.  So, first one - can black holes move around?

Matt - Oh absolutely, absolutely.  I mean, in orbits they don't just sort of jiggle around or anything. All you really think about is as an object right, with a mass, so absolutely, that mass can interact with other masses and be perturbed and move around all over the place - sure.

Georgia - How common are they compared to other stellar objects?

Matt - Oh rare.  I think the upper limit for stellar mass black holes in our galaxy is sort of like a 100 million, something like that, but we have billions upon billions upon billions of stars.  There's millions, and millions, and millions of them, possibly but you're not going to look out and go - oh look, a black hole...

Georgia - With your binoculars...

Matt - Yes, yes.  You not gonna say - oh what's that coming towards us.  I can't see it, it's black.  Oh no, not another one.

Georgia - Do black holes every die?

Matt - In principle, yes.  So there's this thing Hawking radiation.  If you imagine that a particle is created on the event horizon and it breaks in half; half of it can go in and half of it can come out.  So, if you have these things called 'imaginary particles', you have like electron-positron pairs, some goes out, some comes in, but that thing that's gone out is carrying some energy and, therefore, mass of the black hole itself, and this is what's called 'hawking radiation'.  Now eventually, if a black holes sat on it's own for long enough, eventually there will be enough loss from this for it to die, but the rate at which it loses, the rate at which it evaporates is proportional to mass of black hole cubed.  So the bigger your black hole, the longer it's going to take and so, yes, eventually they will die but they're going to take sort of the lifetime of the universe to do so.

Georgia - Final question then - will the large hadron collider create black holes?

Matt - No.  Slightly more information?

Georgia - Slightly more information...

Matt - Slightly more information, okay.  The energy required to make a mini black hole, which is what they're worried about, is 10 to the 19 GeV.  The Hadron Collider runs at a few TeV so it can't do it.  You're out by about huge orders of magnitude. What was postulated was that if we have multidimensional spacetime you could lower the threshold to make a mini blackhole to a few TeV, and then there was the possibility that the LHC when it hits these protons together could create these mini black holes. But, Hawking Radiation would remove these black holes almost instantly. We're not to be worried.


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