It’s almost a hundred years ago that Einstein
developed his General Theory of Relativity, and this led to some predictions that have
already been tested, such as the bending of light when it goes
around massive objects like the Sun, the gravitational red-shift effect which is
used every day in our GPS tracking systems, and black holes. But there’s one prediction that hasn’t been
directly tested yet, and this is gravitational waves.
These are ripples in the fabric of space and time
that move through space at the speed of light, and what they actually do is stretch and shrink
the dimensions of space ever so slightly. This year is the International Year of Light,
150 years since James Clerk Maxwell came up with the Maxwell equations,
this was built upon by a lot of theorists and then finally the culmination was Einstein’s
Theory of General Relativity. So it would be wonderful to detect gravitational
waves this year. I mostly focus on looking at the data that
comes out of very large detectors to pick out the elusive signal of a gravitational
wave. The field has evolved from small groups in
single universities to a large global collaboration. I’m here today in Glasgow to talk with Jim
Hough who has been working in gravitational wave
physics now for 40 years. His expertise is in the design, development
and building of gravitational wave detectors. So, before we go to see the more modern experimental
apparatus, I thought I’d like to show you what I started
out by building here. This is an aluminium bar gravitational wave
detector, and of course the effect of the gravitational waves should be to squeeze the
bar and let it go again, and that should produce voltage from the transducers
which we put into electronics. So you were actually involved in this from
the very beginning? Oh, from 1971.
Many of us wanted to be part of showing that Einstein was in fact correct. Sometimes, as more of a theorist, I forget
how important experimental tests are. The first piece of the puzzle of testing General
Relativity actually clicked into place only a couple of years after Einstein developed
his theory, and that was of course Eddington and Dyson
going to observe the total eclipse, and watch the light bending from the nearby
stars as they pass by the Sun. One of the problems of course with gravitational
waves was that it wasn’t in fact until the late 1950s
that Joseph Weber and John Wheeler began to think that they were detectable. So Joseph Weber actually did set up a number
of these aluminium bars and began to see signals that he claimed were
gravitational waves. A number of us thought it would be a very
good idea to test to see whether Weber was seeing anything.
And we did not see the signals that he was seeing.
And it’s not really surprising of course: with an apparatus like this,
you’re limited by the fact that the two halves are really rather close together. The ideal thing to do is to put them kilometres
apart, and to do that you have to use lasers. That’s what you’re going to see in the clean
room: you’re going to see a prototype for developing the laser interferometry for
such detectors. So welcome to the experimental lab Stephen!
Now, the laser is not on, so we can safely take off our goggles
and speak to each other properly. Excellent, that’s a bit more comfortable. So, over here we’ve got the laser, which drives
the whole apparatus. You first of all stabilise the wavelength
of the laser to the length of this long cavity, then you split it two ways, comparing the
phase of the light that comes out from the two arms. Can you explain how we went from lab-based
prototypes to large-scale international collaborations? How did that come about? Well of course we started with independent
groups, all competing with each other, all hoping
to see gravitational waves first. But then, as the experiments had to get bigger,
to get more sensitive, very few countries really can afford to do
that themselves. So what has happened is there have been a
number of international collaborations set up:
the LIGO collaboration in the US, and the VIRGO collaboration in Italy,
these big interferometers with arm lengths up to 4 kilometres long
do need to be international, and countries do need to work together to jointly fund them. So LIGO and VIRGO of course are at the leading
edge of international collaborations, but international collaboration is not new.
Even back in the late 1700s, there was a collaboration between France and Britain,
to measure the distance between the Greenwich observatory,
and the equivalent observatory in France. General William Roy was commissioned by the
British Government to do this particular measurement,
I think it was one of the most accurate determinations makable at that time. So, some of the earliest examples of collaboration
were trying to map the globe, and you talked about William Roy.
Now what we’re trying to do is map the Solar System, the Galaxy, and the Universe.
one example of this is the Gaia satellite, which is a European collaboration
that is already mapping, in great detail, the positions of millions of stars. And, of course, collaboration is very important
in all fields, I mean CERN is a very good example of that, for particle physics. So it’s probably fair to say that we’re in
the era of big science, and more and more we’re going to see these multinational projects,
both for reasons of multidisciplinary challenges, but also, obviously, financial. It is amazing that when collaborations like
this are formed just how much they can do and how important they are.