Gravitational waves: the bigger picture

There's been a lot of excitement surrounding the discovery but how does this fit within our wider ideas about physics?
26 February 2016

Interview with 

Dr Andrew Pontzen, UCL


This week, we're discussing the recent discovery of gravitational waves. We'veequations had a lot of questions come into our web forum and by email so Kat Arney put them to UCL cosmologist Andrew Pontzen. First up, Kat asked whether the wave was definitely produced from teo colliding black holes, as new evidence from Harvard seems to suggest it could have come from a star...

Andrew - Oh gosh, well - welcome to the world of theoretical physics where any new piece of data gets people tremendously excited and people start pouring over it in a lot of detail.  One of the most striking things about the wave that was detected is just how closely it matches what people expected to see if two black holes merged.  So it would be very challenging to think it's something substantially different.

Kat - And do we know exactly which black holes they were and where they were in space?

Andrew - Not exactly, no.  Certainly we don't have any sense of - oh we've seen these black holes before and now we know they're merging.  This is a totally unknown object  until the moment at which it's gravitational wave arrived.  Now the plan is, in the long term, to be able to accurately work out exactly what direction did a gravitational wave come from. But the trouble is, at the moment, we're not quite able to do that and the reason is that, at the moment, there are two detectors running and the whole idea of being able to pinpoint it on the sky requires a kind of triangulation...

Kat - So you need another one, basically?

Andrew - ... you need another one.  You need at least three if you want to be able to find a pinpoint location on the sky to point a telescope and say, yes it came from that direction.

Kat - Do we know anything more from these results about what gravitational waves are actually made of.  Because we know for light beams that they're made of photons and sometimes people might have heard this word gravitons; the idea that gravitational waves might be made of particles as well.  Does this move us any more forward with this kind of idea?

Andrew - Well, unfortunately this doesn't really help us with that particular puzzle.  In the same way that experiments were done on light long before it was realised that, actually, you could imagine making light dimmer, and dimmer, and dimmer until, eventually, rather than it behaving like a wave you're just left with single particles coming out of your light bulb, which is what we mean when we say a photon.  You could imagine that a similar process might be possible with gravitational waves and somehow you make their intensity smaller, and smaller, and smaller until you're left with a single graviton, but the intensity of these waves is far too high for us to be able to say anything about that at all.  So, at the moment, this discovery really doesn't shed any light on that particular direction.

Kat - There's been so much talk over the past couple of weeks about this discovery.  It really has been incredibly exciting, certainly on social media, the national media, the global media.  Do we actually believe it? Do you reckon it really, really is real this time because there's been false alarms in the past - we're going to hear about one of those later - but is this the real deal? What do you reckon?

Andrew - Well, I mean as a scientist you always have to have a little tiny bit of doubt left.  However, for what it's worth I, and more importantly, an awful lot of the world's experts on gravitational waves are extremely convinced by this particular detection and the reason is twofold.  First of all you can simply go and look at the data and compare it to the theoretical prediction for what those waves should look like and you can see what an exquisite fit the data produces.  But, on top of that, if you look in that paper you'll also find a lengthy discussion of what are the statistical chances you could be fooled; that something could turn up in your detector that looks very much like this but actually isn't, and they put the chances extremely low.  And so, my money's on this being absolutely real.


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