Nico Cliff, you’ve been working in general relativity since like a million years before I was even born. You must have had some pretty exciting moments during your career. What was number one?
Cliff It has to be learning that gravitational waves had been detected by LIGO. We were all pretty confident that the advanced interferometers would eventually detect waves, but we had no idea when. As you know, during the fall of 2015 there were many rumors going around that LIGO had something, but since I was not part of the LIGO–Virgo collaboration, I had no inside knowledge. Finally, in late January 2016, about two weeks before the actual announcement, various science journalists started calling me to ask if I knew anything. All I could say was that I had heard the same rumors as everybody else. Finally, I emailed some colleagues in my department at the University of Florida, who were members of the collaboration, if I should begin thinking about preparing some remarks for journalists. One of them answered “you should always be prepared for whatever life sends your way.” This was not very helpful! But a few days later, another colleague came into my office, closed the door and made me swear not to reveal what he was going to show me, which was the draft of the discovery paper. Needless to say I was blown away! I’m normally a very calm person, but when I saw my wife that evening, she immediately said, “What’s wrong?” I had to tell her! (I knew she could keep the secret.) A week later came the big press conference. In many ways, my career began in 1969 with Weber’s flawed claims of detection of gravitational waves, so I’d been waiting almost fifty years for that moment.
I have a feeling that your most exciting moment might be the same, but how did you learn about it and what was your reaction?
Nico I was in the same boat as you. Back when I was a graduate student at Penn State, I was a member of the LIGO collaboration. But then, after graduate school, I decided I was more interested in theoretical physics, so I stepped away, although my research has always remained very close to the science that one could do with LIGO observations. By the fall of 2015 I was already at Montana State, and although colleagues of mine were members of the collaboration, I wasn’t, so I was not privy to any “secret” information. But I was hearing the same rumors as everybody else. By the time of the announcement, everybody in our institute essentially knew what was happening, so we gathered in the conference room and bought a cake to celebrate when those five magical words were uttered: “We have detected gravitational waves.”
I was a bit surprised when I heard that one of my colleagues told his ten-year-old son about the discovery essentially the day it was made, back in September 2015. But the boy kept the secret! Now, we get alerts on our mobile phones. I’m happy we are finally done with the secrecy. I’ll bet this is nothing like what the field was like back when you were young, you know, back in the hippie era.
Cliff Indeed, my hair was appropriately long, particularly compared to today. That reminds me of another exciting moment from “back in the day.” In the fall of 1974 I was a brand new assistant professor at Stanford University, working with the astrophysicist Robert V. Wagoner. In late September, Bob burst into my office waving an “IAU telegram.”
Nico A telegram! Really?
Cliff In those pre-internet days, new discoveries in astronomy were announced by a service of the International Astronomical Union, which would send a telegram to observatories and universities worldwide outlining the basic facts of the discovery. Bob, who is one of the most enthusiastic physicists I know, shouted “Cliff, they’ve just
discovered a pulsar in a binary system! Whatever you are working on, drop it. We have to get to work on this new system.” Of course, this was the Hulse–Taylor binary pulsar, which we discussed in
Chapter 5. Within a few weeks, Bob wrote a key paper on how it would be possible to measure the change in the orbital period of the binary and thus prove the existence of gravitational waves, and I wrote a paper on the implications of measuring the advance of the pericenter of the orbit. For the next eight or so years, the science related to the binary pulsar occupied almost half of my research life. And when Joe Taylor announced at the 1978 Texas Symposium his team’s measurement of the inspiral of the orbit in agreement with Einstein’s prediction for gravitational wave energy loss (page 128), it was a fantastic moment. Of course, the icing on the cake came in October 1993, when I received a fat envelope from the Nobel Foundation containing an invitation for me and my wife to attend the Nobel Prize ceremonies in Stockholm honoring Joe and Russell Hulse.
Nico Wow! Obviously, there have been many moments during your career. Another big moment for me was when Frans Pretorius completed the first computer simulation of the merger of two black holes in full general relativity. When my physics life started at Washington University in Saint Louis, in your gravity group, if you recall, nobody could simulate such a collision. It was really hard even to use a computer to simulate a single black hole just sitting there doing nothing (even though we had the Schwarzschild solution, which you can write down in one line on a piece of paper). One of the problems was the apparent singularity at the event horizon of the black hole. Even though some mathematical functions become infinite there, we know how to handle that and we know that nothing physically bad happens there.
Cliff Indeed, we discussed this in
Chapter 6. But while our minds can fathom infinity and even make peace with it, computers can’t; they simply stop and emit an error message whenever they encounter a number divided by zero. And this was only one problem with computer solutions of Einstein’s equations.
Nico But it all changed in 2005. I was a graduate student at Penn State at the time, and I recall being in my office, because I was too young to attend a conference in Banff, Canada, that more senior students were attending to present their work. Suddenly, a graduate student or postdoc, I can’t remember, barged into my office and excitedly told me about the computer simulation that Frans Pretorius had presented in Banff. My jaw dropped! He had done it! It would now be possible to predict what the gravitational waves produced in the final part of a black hole merger looked like. It was the biggest breakthrough I had ever experienced in relativity, and I was super excited. Soon, many other groups around the world were able to merge black holes on the computer, using techniques quite different from Frans’, but getting the same result. The black hole merger problem was basically solved.
Cliff That breakthrough came at just the right time, because LIGO was laying the groundwork for advanced LIGO, and this gave confidence that we would have good theoretical predictions for the waves by the time real waves were detected.
Nico Without that 2005 breakthrough we would not have been able to interpret the signals shown in
Figure 8.2 so clearly and confidently as coming from a binary black hole merger, and we would not have been able to measure the masses and the distance so accurately.
Cliff By the way, 2005 was quite a year. It was also the “World Year of Physics,” when physicists worldwide celebrated the centenary of Einstein’s “miracle year” of 1905, when he wrote five papers that transformed physics. Two were on special relativity, one on the photoelectric effect (for which he won the Nobel Prize in 1921), one on the quantum nature of light, and one on Brownian motion. During that year I even went on a four-week, twenty-one-city speaking tour of Canada sponsored by the Canadian Association of Physicists, giving a public lecture entitled (what else?) “Was Einstein Right?” For part of the tour I was accompanied by the Borealis String Quartet, a renowned ensemble based in Vancouver, who performed as a warm-up for my lecture. They played some Mozart (Einstein’s favorite composer) and a piece called “From Water to Ice” composed specially for the tour by a University of Alberta physics and music student.
Nico Awesome! I always think it is cool to combine the physics of Einstein with the arts, and we did some of that in 2015 at Montana State for the celebration of the centenary of general relativity. We created an immersive art installation about a black hole with an accretion disk, with some cool spacey music, and also an original interactive movie about Einstein and his theory, in which at the end of the movie the public could generate their own “gravitational waves” by waving a hollow plastic tube (about two feet long) over their heads. Fortunately, people didn’t whack each other on the head with the “gravitational wave generators” we gave them! Since then, we’ve also created a pretty cool spoken word event that combines poetry and physics, as well as a planetarium show about gravitational waves.
Getting back to things we’ve experienced (and I’m not rubbing it in, or anything), when you were starting out, general relativity as a field was just emerging from the doldrums. How have things changed?
Cliff An obvious change is the growth of the field. Back then there were only a dozen or so relativity “groups” around the world. Every few months we used to get (by snail mail, of course) a list of newly published papers in the field sent from the headquarters of the general relativity society in Bern, Switzerland. The list contained maybe thirty titles. Today, the online archive for general relativity where we post our papers has thirty papers per day!
Nico And that number continues to grow. Also, many papers having to do with general relativity are sent directly to the high-energy theory archive or the astrophysics archive, because the authors feel that the papers are of interest to those communities. The overlaps and interactions between general relativity and other fields of physics are really amazing. We mentioned the first Texas Symposium in 1963 (
Chapter 6) as an awkward first step to bring relativists and astrophysicists together, but man, are they together now! The neutron star merger paper had 3,000 relativity and astrophysics authors, ten times the number who attended that Texas Symposium. And topics like quantum gravity, string theory and dark energy attract researchers equally from general relativity and particle physics.
Cliff Another big change in general relativity is the role of experiment.
Nico I recall from page 146 the phrase that was used to describe the early days of the field: a “theorist’s paradise and an experimentalist’s purgatory.” Without experimental data, scientists don’t take you seriously. Is this how you felt?
Cliff I was lucky enough to begin my career right when this was all changing. What made it exciting was that you could play with lofty spacetime concepts and at the same time “get your hands dirty” with actual data. For instance, my notes from my Caltech graduate student days include mathematical calculations on the motions of bodies predicted by general relativity as well as by other theories of gravity. But from the same period, I also have notes from discussions I had with scientists from the nearby Jet Propulsion Laboratory on what radar tracking accuracy NASA would provide on the proposed Viking mission to Mars, so that they could test the Shapiro time delay effect. As we saw in
Chapter 3, that mission was a great success. As time went on, new experimental tests, the development of gravitational wave detectors, and observations in astronomy and cosmology made experiment an increasingly important aspect of general relativity. Today, there is a real symbiosis between theory and experiment, and that makes the field exciting and vibrant, a far cry from the “low water mark” days.
Nico And I guess that’s how physics advances. We speculate, and we then build a theory to cement this speculation. But it is not until experiments are done and data is analyzed that we can really trust our theory. And if the predictions of the theory don’t match the data, we then discard it, until we find a theory that works. But it’s got to work for all the experiments ever done, for all of the data, and then it must survive future tests. It’s just amazing to me how a single theory, Einstein’s, has managed to do this and remain right for over a century. And it’s not as if physicists haven’t tried to come up with other theories that could supersede Einstein’s and maybe explain some of the anomalies we discussed in
Chapter 1. Yet these theories, at least the ones that are able to make predictions, tend to disagree with one observation or another. Experiment seems to really like Einstein’s theory, in spite of how crazy and wacky it seems. This is maybe what I find most amazing about general relativity.
Cliff I agree. Any physicist of the 1920s or 1930s would recognize that the general relativity theory we have discussed in this book is exactly the same as the general relativity that Einstein presented in November 1915. But those same physicists would be astonished and maybe mystified by today’s picture of quarks, leptons, gluons, photons, neutrinos and Higgs bosons that are the fundamental building blocks of matter, and the fundamental theories, quantum electrodynamics, quantum chromodynamics and electroweak theory, that govern them. It is so radically different from the model of their day in which the world was made of electrons, protons and neutrons, all governed by electromagnetism and simple quantum mechanics, that they might think they had been dropped into an alternate universe. These radical changes in the theory of the fundamental particles were driven mostly by experimental results that revealed anomalies or new phenomena that had to be addressed by new or updated theories.
Nico And if we finally detect the particle that makes up dark matter, even more dramatic changes in those theories may be necessary.
Cliff Absolutely. Yet from another point of view, I think maybe the longevity of general relativity is not so surprising. Newton’s theory of gravity held sway for over 230 years until general relativity came along. So by that standard, Einstein’s theory is still a youngster, like you, Nico. The problem is that gravity is so darn weak. The gravitational force between two protons is more than a trillion trillion trillion times smaller than the electric force between them. So you have to work much, much harder to find the tiny deviations in gravity that might announce a modification of general relativity. So, while the timescale for seeing changes in elementary particle theory may be measured in decades—during my own time as a physicist I’ve been witness to many big changes in that field—perhaps the timescale for seeing changes in our theory of gravity is measured in centuries.
Nico Well, that’s disappointing! I feel like you just dumped a bucket of cold reality on my head! But I suppose you are right.
Cliff There’s a legend in our field that I’ve never been wrong.
Nico True. I even lost a bet with you and had to fork over an expensive bottle of Malbec wine. But we digress. It may well take a very long time to see a big change in our understanding of gravity, to see another scientific revolution à la Einstein. It’s disappointing because what got me interested in relativity in the first place was Star Trek! Not so much all the action with the phasers and photon torpedos, but rather the possibility that new technology would allow us to find loopholes in the current laws of physics and let us explore new worlds and new civilizations. Our current understanding of Einstein’s theory suggests that there are no such loopholes, and without them it’ll be impossible to witness a black hole from up close, as in Interstellar, or to visit those cool new worlds!
Cliff But isn’t the world we’ve got, with its whirling neutron stars, crashing black holes and runaway cosmic expansion, exciting enough for you?
Nico Of course it is! Especially now that we have gravitational wave data! But it’s one thing to see the action on the battlefield from up on the hills, like an imperious general on an old horse, and another thing to be up close on the battlefield itself, swinging your sword left and right. It’s like the difference between reading about an amazing basketball game and being there with court-side seating! Sure, the event is the same, but the information and the experience are totally different.
Cliff Fair point … but I still prefer to be on Earth rather than close to a black hole. On the other hand, if I spent enough time there, when I returned to Earth I could be younger than you! By the way, I’m now curious: are you talking about the original Star Trek with Captain Kirk? That show is way before your time!
Nico Yes, I have seen the original series, but I was referring to Star Trek, The Next Generation. Back in Argentina, where I grew up, they would show reruns of this show every week on cable television. So by the time I moved to the United States to study physics in 1999, I already knew that black holes were the thing I wanted to study the most. It took me about a year to realize that to study black holes what I really needed to study was Einstein’s theory!
Cliff Yes, you obviously got interested early, because I remember that already as a sophomore at Washington University in 2000 you wanted to do research in the topic, first with our colleague Matt Visser, and then with me.
Nico What about you? What got you interested in general relativity research?
Cliff My path was about as different from yours as it could be. At my undergraduate school, McMaster University in Canada, there were no courses in general relativity, and no professors who knew anything about it (this was around 1967). I had read a few popular books and Scientific American articles about Einstein and his theories, but that was it. I arrived at Caltech in the fall of 1968 knowing only that I wanted to do theoretical physics, but I had no idea what kind. Many new Caltech grad students in those days wanted to work on particle physics with Richard Feynman, but didn’t realize that by that time he was not taking any new students. Some fellow Canadian students told me that I should talk to this brand new professor with a funny name, “Kip.” I had never heard of him because he was so new that the brochure that Caltech had sent me describing their graduate physics program didn’t list him! Finally, a month and a half into the semester I approached him. He told me I was probably out of luck, because he was teaching the general relativity course as we spoke, but it would not be taught the following year. So I quickly dropped an astronomy elective course and signed up for Kip’s course, but I had to catch up on almost half a term’s worth of material and homework sets. It was exhausting! But it all worked out, because by the end of that first year Kip invited me to start attending his research group meetings, and the rest is history!
Nico But what about the future of general relativity? Before I moved from Montana State to the University of Illinois I was giving a public talk at the Museum of the Rockies in Bozeman, and an audience member asked me if I “believed” in string theory. I was a bit shocked at first because, as you know, we don’t use the word “believe” in physics. I don’t “believe” in gravity; I just grab a rock and let it go, and it falls down. Gravity just is, whether I “believe” in it or not. As my students say: “Gravity doesn’t care what you think.” So after explaining this as politely as I could, I still had to address the elephant in the room:
what about string theory, or other theories of quantum gravity for that matter?
I’m a bit torn about this because we all understand the limitations of Einstein’s theory. As we discussed in
Chapter 1, we don’t really understand what dark matter is or what dark energy is, and they make up 95 percent of the energy and matter in the universe. We suspect that the singularities in Einstein’s theory, like the one at the very center of a black hole, are a sign of something missing in the theory. And this is intimately tied up with the fact that quantum mechanics and Einstein’s theory are just incompatible at a mathematical level. Some other more “fundamental” theory must replace Einstein’s at the very small scales. This more fundamental theory has got to be compatible with quantum mechanics.
So, sure, something like string theory or maybe loop quantum gravity has got to be right. But which one is the right fundamental theory? What I “believe in” or like or find aesthetically pleasing should not matter one bit. What should matter, what should help me decide, is experiment and observation. If there is a quantum gravity theory that makes predictions and these predictions match what we observe in nature, then that’s it. We are done! But that’s just not where we are at present. None of the quantum gravity theories that are out there are sufficiently complete and devoid of mathematical problems that we can make unique predictions to compare against experiments. But even if they were, the effects of quantum gravity in the regimes where we can make observations today are so minuscule that they seem impossible to measure with today’s (or tomorrow’s) technology. Am I being too pessimistic?
Cliff Nico, you strike me as someone without a molecule of pessimism in your bones! With so much exciting science to be done with gravitational waves, black holes and neutron stars, there’s tons to be optimistic about.
We titled this book Is Einstein Still Right?, and we will leave it to our readers to make up their own minds. But another question is: “Will Einstein always be right?” Given my seniority and the fact that I have nothing to lose, I’m going to go out on a long limb here.
Nico Are you saying you are prepared to wager a bottle of wine?
Cliff Sure, why not? Here goes: It would not surprise me if the solution to the universal acceleration turned out to be simply Einstein’s original cosmological constant, a constant of nature like Planck’s constant or Newton’s constant of gravitation, which we have now been able to measure. And it would not surprise me if general relativity turned out to be absolutely correct according to any future experiment accessible to humankind.
Of course, having made that statement, I am reminded of the story about Yogi Berra, the famous New York Yankees baseball player and manager (one of my childhood heroes), and an individual blessed with the most sublimely illogical mind. At some point after his retirement from baseball, his wife Carmen asked him: “Yogi, you were born in St. Louis, you played baseball in New York and we now live in New Jersey. If you should die first, where would you like me to bury you?” Yogi’s answer: “Surprise me!”