Secrets of discovery

Did you expect that gravitational waves would be discovered during your lifetime?

Yes, and I thought it quite likely it would come from two colliding black holes of just the sort that we did see. I wrote a popular book called Black Holes and Time Warps: Einstein’s Outrageous Legacy, published in 1994, and I wrote a prologue to this book during my honeymoon in Chile in 1984. In that prologue, I described the observation of two black holes, both weighing 25 solar masses, spiralling together and merging and producing three solar masses of energy and gravitational waves, and that’s very close to what we’ve seen. So I was already in the 1980s targeting black holes as the most likely kind of source; for me this was not a surprise, it was a great satisfaction that everything came out the way I thought it probably would.

Can you summarise how an instrument such as LIGO could observe such a weak and rare phenomenon?

The primary inventor of this kind of gravitational-wave detector is Ray Weiss at MIT. He not only conceived the idea, in parallel with several other people, but he, unlike anybody else, identified all of the major sources of noise that would have to be dealt with in the initial detector and he invented ways to deal with each of those. He estimated how much noise would remain after the experiment did what he proposed to limit each noise source, and concluded that the sensitivity that could be reached would be good enough. There was a real possibility of seeing the waves that I as a theorist and colleagues were predicting. Weiss wrote a paper in 1972 describing all of this and it is one of the most powerful papers I’ve ever read, perhaps the most powerful experiment-related paper. Before I read it, I had heard about his idea and concluded it was very unlikely to succeed because the required sensitivities were so great. I didn’t have time to really study it in depth, but it turned out I was wrong. I was sceptical until I had discussions with Weiss and others in Moscow. I then became convinced, and decided that I should devote most of the rest of my career to helping them succeed in the detection of gravitational waves.

How will the new tool of “multi-messenger astronomy” impact on our understanding of the universe?

Concerning the colliding black hole that we’ve seen so far, astronomers who rely on electromagnetic signals have not seen anything coming from them. It’s conceivable that in the future something may be seen because disturbances caused when two black holes collide and merge can lead to X-ray or perhaps optical emissions. We also expect to see many other sources of gravitational waves. Neutron stars orbiting each other are expected to collide and merge, which is thought to be a source of gamma-ray bursts that have already been seen. We will see black holes tear apart and destroy a companion neutron star, again producing a very strong electromagnetic emission as well as neutrino emission. So the co-ordinated gravitational and electromagnetic observation and neutrino observations will be very powerful. With all of these working together in “multi-messenger” astronomy, there’s a great richness of information. That really is the future of a large portion of this field. But part of this field will be things like black holes, where we see only gravitational waves.

Do gravitational waves give us a bearing on gravitons?

Although we are quite sure gravitational waves are carried by gravitons, there is no chance to see individual gravitons based on the known laws of physics. Just as we do not see individual photons in a radio wave because there are so many photons working together to produce the radio wave, there are even more gravitons working together to produce gravitational waves. In technical terms, the mean occupation number of the gravitational-wave field that is seen is absolutely enormous, close to 10⁴⁰. With so many gravitons there is no hope, unfortunately, to see individual gravitons.

Will we ever reconcile gravity with the three other forces?

I am quite sure gravity will be reconciled with the other three forces. I think it is quite likely this will be done through some version of string theory or M theory, which many theorists are now working on. When it does happen, the resulting laws of quantum gravity will allow us to address questions related to the nature of the birth of the universe. It would also tell us whether or not it is possible to build time machines to go backward in time, what is the nature of the interior of a black hole, and address many other interesting questions. This is a tremendously important effort, by far the most important research direction in theoretical physics today and recent decades. There’s no way I could contribute very much there.

Regarding future large-scale research infrastructures, such as those proposed within CERN’s Future Circular Collider programme, what are the lessons to be learnt from LIGO?

Maybe the best thing to learn is having superb management of large physics budgets, which is essential to make the project succeed. We’ve had excellent management, particularly with Barry Barish, who transformed LIGO and took over as director when we were just about ready to begin construction (Robbie Waught, who had helped us write a proposal to get the funding from the NSF and Congress, also got two research teams at Caltech and MIT to work together in an effective manner). Barry created the modern LIGO and he is an absolutely fantastic project director. Having him lead us through that transition into the modern LIGO was absolutely essential to our success, plus a very good experiment idea and a superb team, of course.

You were an adviser to the blockbuster film Interstellar. Do you have any more science and arts projects ahead?

I am 76. I was a conventional professor for almost 50 years, and I decided for my next 50 years that I want to do something different. So I have several different collaborations: one on a second film; collaborations in a multimedia concert about sources of gravitational waves with Hans Zimmer and Paul Franckman, who did the music and visual effects for Interstellar; and collaborations with Chapman University art professor Lia Halloran on a book with her paintings and my poetry about the warped side of the universe. I am having great fun entering collaborations between scientists and artists and I think, at this point of my life, if I have a total failure with trying to write poetry, well that’s alright: I’ve had enough success elsewhere.