# Was Einstein Right?

It’s about time I started talking a bit about the science of this week’s announcement. One of the more exciting aspects of detecting gravitational waves was that they would confirm the last major prediction of the theory of General Relativity—Einstein’s revolutionising theory of how gravity works.

Well, they did that. We saw gravitational waves, and they seem to
travel at the speed of light, so so-far it’s looking pretty positive
for Mr Einstein. But what if General Relativity (let’s call it GR from
now on, I’ll misspell relativity if I need to type it too frequently)
isn’t the whole story? What if some other theory makes almost all the
same predictions as GR, but some more ones which, for example, let us
combine gravity with quantum mechanics. (This is a *big* problem at
the moment in physics. Theorists are convinced that it should be
possible, but they don’t know how, so they’re very keen to find weaknesses
in the current theories of each which can be used to find a way to
join them up).

Before I go on, I think I need to explain what *General Relativity*
is. At its heart, the ideas behind GR are quite easy. The mathematics
they need to be expressed in, however, require a graduate-level
lecture course (and even then it’s an up-hill struggle for most people
like me to get their head around). So let’s get started. Well, maybe
not. Let’s just go with the easy idea, and hope that’s enough.

The (mathematical) history of gravity starts with Isaac Newton, who
had two major gravity-based break-throughs. First, sitting under apple
trees can lead to an unpleasant concussion, and second, that gravity
is a mutual attraction between objects which have mass. As it turned
out, Newton’s theory asked as many new questions as it answered. By
the 20th Century
James Clerk Maxwell
had produced an elegant
theory of electromagnetism
which concluded that light travelled at a fixed speed, and that
nothing could travel faster than it. We call that speed $c$ when we
put it in equations, from the Latin word *celeritas*, meaning
*swiftness*. This lead Einstein to wonder “what would happen if I was
on a train travelling close to the speed of light? What would I
see?”. He came up with a theory, the *Special* theory of
relativity. It turns out that’s not the type of theory that you buy a
card and chocolates for on 14 February, but it’s a theory which only
describes a specific set of situations: in this case, situations where
the train never accelerates. Much like trains in Scotland. Einstein
was curious about gravity. Newton’s theory suggested that if you moved
one mass every other mass would immediately feel the effect of that
move. That means, in the technical language of physics, that it had an
infinite speed of propagation.

Physicists *hate* infinity.

Einstein set out to work out what speed gravity really travels at. It took him ten years to get his head around a whole range of very new branches of mathematics, but eventually he was ready to show the world his new theory. On 25 November 1915 he distorted the very fabric of spacetime, and made a revelation of great gravity.

Einstein’s big idea was that whenever you put something heavy somewhere in space it causes space to bend around it. A bit like when you set a cup of tea down on the sofa and sit next to it, you bend the sofa. The Einstein said that that bending of space causes other heavy things to move. A bit like the way your cup of tea moves along the sofa and spills itself on your leg. This was a big deal. From that day onward there was no longer any excuse for spilling tea on yourself when you sit down on the sofa. Perhaps more importantly it lead to two predictions which were really rather important for my work, and for the science story which has been boosting a large number of people’s twitter profile lately.

It predicted the existence of places in space which were so strongly curved that anything passing them would spiral into them, and never escape. It also predicted that when heavy objects move in orbit around each other they radiate gravitational waves. I’ll bet this sounds familiar. As it happens we were pretty confident black holes existed before we found gravitational waves from them, but the evidence was slightly indirect—we’re able to see stars orbiting something at the centre of our Galaxy, for example, but we can’t see anything there. Something dark and heavy is attracting everything. Which is exactly what we expect black holes to ‘look’ like. Gravitational waves were perhaps the last major prediction of the theory to be verified observationally, but did that observation actually prove Einstein right?

The LSC (LIGO Scientific Collaboration) was able to perform some tests on the data from GW150914, the first directly detected gravitational wave, in order to verify GR. The paper whcih describes the tests is available here.

GW150914 was the first time we’d been able to see inside an extreme
gravitational system, what we call a *strong*-field situation, with
the two black holes travelling at around half the speed of light. So
it allowed a number of new tests to be done which hadn’t been possible
by measuring other binary systems (like pulsars). Those tests found no
violations of General Relativity from the evidence of GW150914, but
the configuration of the detectors, they weren’t able to test for the
two different *polarisation states* which GR predicts—this is
important because some alternative theories of gravity predict that
there could be up to two more of these states, and so that’s an
investigation for the future.

So it seems, in the end, that yes: Einstein was right, and he’s safe until we have more, and better detections.

I’ll leave you with this photo of the Glasgow group (the Institute for Gravitational Research) taken at our local press conference for the announcement last week, with yours truly looking over-dressed.