Do black holes have "hair"?

Do black holes obliterate what falls into them, or do they leave a faint ghost of the structure of what they have digested?
25 January 2016

Interview with 

Dr Andrew Pontzen, University College London


Black holes are so far removed from what we can see and test, that they are the playground of mathematicians and theoretical physicists. This month Stephen Hawking, the king of black hole physics, published a paper stating that black holes have "hairs". This challenges the notion that blackholes destroy everything that enters them completely, leaving a shadow of what might have entered at the event horizon. Andrew Pontzen, a cosmologist from University College London, explains this idea to Kat Arney...

Andrew - I do occasionally wish that some of my physicists colleagues would be better at naming things.  This is possibly the least useful name anyone's ever come up with, but the point is supposed to be that if you look at Einstein's theory of general relativity, which is what underlies everything we've been talking about today, about black holes.  It makes a very firm prediction that everything about a single black hole, you're looking at a single black hole, it is completely summarised by just three numbers and those three numbers are: it's mass, so how much stuff is in there; it's charge, so if you throw something in that's got static electricity on it it can keep that, and also it's spin, how much it's spinning.  And according to general relativity that's all there is to know about a black hole, absolutely nothing else to know about it, and when we talk about hair, what we're saying is, actually maybe there's complications.  Every theoretical physicist knows having hair is a real nuisance, it's difficult to make it look any good and so, when we say a black hole has hair we just mean it has extra things that you need to worry about, as well as just those three numbers.

Kat - Okay.  So they're not actually like a hairy hole at the middle of the Universe?

Andrew - No, sadly not.

Kat - Okay.  Well that's shot down my idea anyway.  So how does Stephen Hawking come into this?  

Andrew - The whole question was originally posed by Stephen Hawking in a way, because I think earlier on Matt Middleton was mentioning Hawking radiation and that's the one mechanisms we know of by which you can get rid of a black hole.  That very slowly a black hole will throw off bits and pieces and then, eventually, it loses all its mass and it actually disappears if you wait a very, very, very long time and that was really one of Stephen Hawking's great discoveries in theoretical physics.  But he immediately realised that it poses a kind of strange question, which is that in physics, we normally think that information can't be lost.  So, say I chuck something into my black hole so, I mean. I think earlier on Georgia was threatening to throw herself into a black hole so, let's suppose she does that and let's suppose she weighs 60 kg or something - something like that, then the black hole has grown by 60 kg.  

Now equally well, we could get it to do that by throwing in sack of potatoes that weighed 60 kg and really then the black hole wouldn't then know any different because, as I was saying earlier on, it's just got these three numbers so all that's happened is it's matters gone up a bit.  Then, according the Stephen Hawking's 'Hawking radiation' idea, the black hole disappears and, somehow, all the complexity about Georgia that makes her different from a sack of potatoes, has been completely lost.  That sits very uncomfortably with our normal idea that in physics that you can't lose information.  If you have a complicated thing sitting there you can't just get rid of all that complexity.  So, this poses the question, how can it be that you lose information?

Kat - Sort of what things were, what things were like?

Andrew - Exactly.  And where's it gone and so for a long time Stephen Hawking said "well you know, tough, deal with it.  The informations lost, it's a new law of physics." But...

Kat - I love physicists...

Andrew - Theoretical physicists...

Kat - We'll just make up another equation just to explain it...

Andrew - Well, you know, when you're stuck you sort of have to.  But, he changed his mind.  He said "well maybe, looking at the way things are going, looking at new ideas people have about how black holes really work. Maybe somehow or other that information isn't completely lost,"  and now what he's saying is the information is somehow stored.  And it's stored in a very subtle way so that you couldn't really look at a black hole and tell that Georgia had fallen into it but, in a very, very subtle way, on the event horizon itself, it's stored a faint imprint of the fact that it was Georgia and not a sack of potatoes that fell into that black hole.

Kat - And so, presumably, this is the metaphorical hair, the sort of fringe round the edge that has all this information in it.

Andrew - Exactly.  You've turned Georgia into hair.

Kat -   But if black holes do actually record this, what does this mean for our understanding of black holes, of the Universe, of the theory of physics that we have?

Andrew - If you can make sense of the way that a black hole behaves without losing information, then it really points us towards the next level of understanding black holes and, in particular, what we really want to bring in is what we call a 'quantum theory of gravity.'  That's combining Einstein's ideas about warping space and time with the other great development in the 20th century, which was this idea of quantum mechanics. Historically it's been very hard to fit those things together and, perhaps, as we begin to understand these very extreme objects, we'll slowly begin to understand, actually, how should these things fit and together and what does a better theory of physics actually look like.

Kat - And sort of very briefly, obviously, the theoreticians come up with their theories and all of that kind of stuff but, to find out if that is how the universe is working, you need to test it.  So, could you explain how could we actually test whether this information is stored?

Andrew - Oh dear, yes, that's pretty hard. I mean there's a reason why theoretical physicists are called 'theoretical physicists.'  You know they really try and push the boundaries of theory for, but don't necessarily come up with the tests so, typically, tests are a long way behind.  But, if you look at what's happened with black holes, it was thought that the whole idea of black holes was untestable but now we're thinking about things like the Event Horizon Telescope, so maybe one day.


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