The Kidnapping of Kip Thorne

A couple of weeks ago I helped kidnap Physics Nobel Prize winner Kip Thorne. He was at one of those glitterati Hollywood banquets that he goes to from time to time. There were 45 of us. We broke into the banquet room, surrounded the guests, and — after singing to him — hustled Kip out the door and off to Pasadena, where we gave him a meal instead in an Indian restaurant.

Well, it was almost like that anyway. It all started when some of Kip’s former PhD students conceived the brilliant idea of throwing a surprise party for Kip, to celebrate with him his sharing in the Nobel Prize in Physics in December. The team was led by Richard Price, editor of the American Journal of Physics. Carolee Weinstein, Kip’s wife, was on the team, as were Carlton Caves and Sándor Kovács. It was a  simple plan: we all turn up in Pasadena on Saturday January 13, Carolee delivers Kip, we shout “Surprise!”, and we all have a great time.

At first the planning went perfectly. The word circulated discreetly, and something like 45 people signed on, a few even coming from Asia, South America, and Europe.  And best of all, word of it never reached Kip’s ears. The event known as “Kip’s Spawn Reunion” was looking good!

But — it was such a well-kept secret that Kip got himself double-booked. That same weekend, Caltech was hosting a Physics Summit. The Summit series has traditionally attracted top thinkers in physics and related fields, and what could be more appropriate than to invite the two new local Nobel Physics Laureates, Kip and Barry Barish, to the Gala on Saturday January 13? And to invite local members of the LIGO project too, in order to recognize their contribution to the huge success of the gravitational wave detection enterprise. Kip, among the founding members of the series, happily accepted the invitation to deliver remarks at the Gala, despite having just returned from India the day before. And why not — there was nothing else in his calendar.

But unfortunately there was something else in all of Kip’s spawn’s calendars! And changing the date wasn’t an option: air tickets had been booked, hotels had been arranged. Happily, what to some people might be a disaster, to Richard Price and his team was an opportunity. They enlisted the help of the Caltech professor of physics in charge of the series, and through her they got the cooperation of the Gala venue staff. Together they evolved an audacious plan that required secrecy, complex coordination, the devious cooperation of many people, and luck!

Something unlike anything that had happened to a Nobel Prize winner before was going to happen to Kip on the 13th of January. He was going to be abducted by some of his closest colleagues.

If if worked, then something unlike anything that had happened to a Nobel Prize winner before was going to happen to Kip on the 13th of January. He was going to be abducted by some of his closest colleagues.

Late Saturday afternoon, about 45 people rendezvoused at the New Delhi Palace restaurant in Pasadena and then piled into a bus and headed for Hollywood. With military precision timing — or at least as close to that as Los Angeles traffic would allow — the bus arrived at the venue. We waited a block away until we were sure that Kip and the others had gone inside, and then we advanced. The venue staff, grinning conspiratorially, guided us to the service elevator that took us to the rooftop level, where the banquet room was. Four elevator-loads later, we were clustered near the swimming pool, out of sight of the event guests, admiring a pretty spectacular nighttime view of the Hollywood hills. Then word came by text from inside the dining room: Kip was speaking.

Single-file, with serious demeanor, we made our way through the kitchen and into the dining room, winding our way around the walls to encircle the guests. Kip looked astonished, remarked “What is going on?” — but, trooper that he is, he carried on, not missing a word of his speech paying tribute to his LIGO colleagues. His audience, of course, was more than a little distracted, but when Kip asked the LIGO team to stand, the rest of the guests pitched in with plenty of applause. Kip was followed at the lectern by J. Nolan, the acclaimed screenwriter, producer, and author, who paid a very warm tribute to Kip, with whom he had worked on the film Interstellar. Then, on cue from the professor who had become our co-conspirator, we took over.

First, four of us took turns loudly scolding Kip for not responding to our emails and phone calls because he was too busy with travel, poetry, Hollywood, … .

Richard then tried to reassure Kip by reminding us all that we had always found him to be a stable genius, and kind of, like, you know, really smart.

Richard then tried to reassure Kip by reminding us all that we had always found him to be a truly stable genius, and, like, you know, really smart.

The proceedings finally reached their nadir as all 45 of us burst into a rendition of that great Bernie and The Gravitones hit, Wise Old Advisor From Pasadena (which had been created on the occasion of Kip’s 60th birthday), singing along to the original, which the venue was playing over the their audio system.

By the end of the second verse we had already long overstayed our welcome, so it was time for our final move: Richard and Carolee got Kip to stand up and lead us, single-file again, out of the room and into the elevators. Down we went and onto the bus and back to Pasadena. We returned to the New Delhi Palace, where an excellent buffet awaited us, and where Kip, beaming all the time, wandered the room, making sure he caught up with each of his former students.

Well, that is the true story of how we kidnapped Kip Thorne. And I think he enjoyed it!

__________

[For those of you who might want an official record of the proceedings, here is our script, with many thanks to Richard Price —

The voices of the Former Students are shouted by four Spawn in diverse locations around the room.

Student #1 (Cliff Will): Kip, I was your student. Kip, I’ve written the first draft of the paper we talked about. I’ve been trying to get in touch with you.  I tried email. I tried calling. When I called I was told you are away, in Stockholm or India. They weren’t sure.

Student #2 (Bill Press): Kip, I was your student. Kip I would like you to be on a panel I am organizing, but I haven’t been able to get in touch with you. I tried email. When I called I think that they said that you were at a poetry reading, but I probably heard wrong.

Student #3: (Bernie Schutz) Kip, I was your student. Kip, I need to submit my grant proposal, and I was hoping for a letter of support from you. I’ve sent you email. I tried calling but I was told that you were on a movie set.

Student #4: (Saul Teukolsky) Kip, I was your student. Kip, I need a letter of recommendation for my tenure decision. I tried sending email. Then I tried calling. They had no idea where you were.

Richard (to Kip):

Kip we are your academic spawn. Kip.. Why hast thou forsaken us?  You are our mentor; we are your mentees. We are the products; you are the producer.  In unstable times, and in our unstable lives you have been a stable genius.  And kind of, like, you know, really smart. But now… now you are a cinema celebrity, a poet of some renown, Kip-

We would hate to embarrass you; the last thing we would want to do is to embarrass you, but we are reclaiming you because to us, you will always be our wise old advisor from Pasadena. (Music blares.)]

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Another wave, and a smile

Last week the international gravitational wave collaboration* announced its third very secure detection. Our new acquaintance GW170104 — named for its arrival date — passed through our two LIGO detectors and whispered to them about the coalescence of a pair of black holes in a binary system that had taken place almost three billion years ago. And then it raced on, hardly affected at all by its encounter with us and our planet. In that respect it was much like its two predecessors, GW150914 and GW151226. But every detection is special, and this one is very special indeed.

One reason is simple: it brought a big smile to all of us in the collaboration, and allowed us a big sigh of relief — our first two detections were not just a lucky fluke, nor a super-mysterious instrumental malfunction. The dates of the detections tell the story: the first two in 2015, this one at the start of 2017. The big gap is there because the LIGO detectors had been shut down for most of 2016 in order to improve their sensitivity. Once we started up again, with significantly modified detectors, Nature provided us with helpful reassurance: the waves keep coming, and they look just the same as they did in the previous version of the detectors. (Goodness knows what we missed during 2016!)

The second important thing about GW170104 is that it has started our transition from mainly doing fundamental gravitational physics to primarily doing gravitational-wave astronomy. The first detection GW150914 was the science event of the decade because it showed how right Einstein was with general relativity: his amazing fundamental theory of gravity had predicted gravitational waves and black holes, and in just one event, lasting only about two tenths of a second, both predictions were spectacularly confirmed. But now, as we continue to observe more such events, we become more and more focussed on what they tell us about the Universe. We will of course use all new data to further test our confidence in Einstein’s general relativity. But the new detections are going to explore more and more of what Kip Thorne, one of the founders of LIGO, famously called the Dark Side of the Universe.

A hint of spin

But the really special aspect of the new detection is the hint it gives us about the way the black holes may have been spinning. Black holes spin, as do all astronomical bodies. And the normal expectation is that the spins in a binary system of black holes will be in a consistent direction, just like the spins of planets in the solar system. The Sun and the planets all formed together out of a rotating cloud of gas, so it is natural that they inherited their spins from that cloud, so that these spins are all roughly in the same sense, which is also the same as the sense of the orbital rotation of the planets around the Sun, and indeed of the spin of the Sun itself. In our previous detection, GW151226, we found evidence that the more massive of the two holes was spinning, and the evidence was that its spin was probably aligned with the sense of the orbital rotation. But in GW170104, there is just a hint that the black holes’ spins were either anti-aligned with each other, or they were anti-aligned with the sense of their orbital motion about one another before they merged.

if this was the case, then it would suggest that the two black holes had not been formed together from a single primeval cloud of gas. Instead, they were probably unrelated single black holes, which just by chance linked up into a binary system. This ought to sound bizarre to you, wildly improbable. After all, black holes, even the rather massive ones in this system, are only something like 100 km across, maybe twice the size of London. How could two such small objects ever encounter one another if they were wandering through the vast emptiness of space between the stars in their galaxy? And you would be right: that just wouldn’t happen. But it would be much more probable if it happened inside the big dense clusters of stars that we call globular clusters.

These globular clusters are roughly spherical micro-galaxies, and in them heavy things like black holes sink, over millions of years, down toward their centers, where there can be thousands of stars in a volume that would contain just one star in neighbourhood of our Sun. In these conditions, near encounters between black holes do sometimes happen. What’s more, if a third object, even a normal star, is nearby, then sometimes the third object can be a kind of catalyst, helping the black holes to form a binary system by stealing and speeding off with some of their energy. So the hint that the spins in GW170104 don’t match up is a hint that this system might have been formed in this haphazard way.

The magnetic side of gravity

But here’s the amazing part: how did we get this hint in the first place, about how things spin in a system that is totally in the Dark Side? What did we read in the signal we call GW170104 that tells us about the spins? The answer lies again in the fundamentals of Einstein’s theory. Einstein predicted that spin creates a different form of gravity, a magnetic form. Think of a spinning charge, or of the electric current winding round and round through the coil in an electromagnet: this creates magnetism, which we know is the partner of the electric force in the theory of electromagnetism. Magnets exert forces on other magnets and on moving charges. In a closely analogous way, a spinning mass creates a form of gravity that we call gravitomagnetism, through which spinning masses exert gravitational forces on other spinning masses and on moving masses. So our spinning black holes are gravitomagnets, which interact with one another and with their orbital motion, and this slightly changes the orbital periods of the last few orbits before the holes merge together. We always look in our data for these changes, and in this case we found a hint not only that the spins were significant, but that they might be anti-aligned.

But it is only a hint. The odds are only 4-1 in favor of the spins’ being anti-aligned rather than their being consistently aligned. It could be simply a distortion in the signal caused by a slightly improbable level of noise in our detectors. But since we will never get any more information about this particular system, we will never be able to say for sure whether this was how GW170104 in fact formed. So GW170104 is not only special, it is also frustrating!

What sort of physics are we doing?

This frustration has pushed the global gravitational wave collaboration to confront the question of whether we are still primarily doing fundamental physics or whether we have started the transition to astronomy. One of the differences between these two disciplines is their expectation of how much uncertainty they can tolerate in a measurement. The observation of gravitomagnetism is of course very fundamental, even though this prediction of Einstein has already been verified by an experiment in orbit around the earth [1] and by observations of the orbits of satellites themselves [2]. A fundamental physicist wants to pin down the laws of physics, and that needs a great deal of certainty. As a measurement of gravitomagnetism, our hint of spin doesn’t do as well as the previous experiments have done, and is certainly not up to our standard for deciding we have detected something in the first place. While the actual detection of GW170104 has very high significance (odds of better than 70,000 to 1 that no chance noise-generated event this strong would have happened in coincidence between the two detectors in a whole year’s worth of data), the information we can glean about spin from the slight spin-induced nuance in the orbital motion has only 4-1 odds of being real.  So a number of physicists in the collaboration did not want to see the spin result highlighted strongly in the announcement of our observation.

Astronomers, by contrast, have learned to live with uncertainty: you can never do a controlled experiment on a star, so you have to be content with whatever data it sends you. Some of the astronomers in our collaboration felt that the possible implications of this hint of spin for our understanding of the formation history of this system was interesting enough to deserve prominence in our announcement, even at this low significance level. The philosophy is to put the data out there so that other astronomers can judge whether they find it interesting or not. This clash of scientific cultures led to a rather lively discussion inside the collaboration on how to report the tantalising but rather uncertain measurement on the spins of the black holes, and it even delayed the announcement of the detection by some weeks. What you can read in the paper we wrote and the publicity we put out is, of course, our compromise. As you might guess from this post, I find myself on the side of the astronomers.

But it was already 5 months ago…

The detection we announced last week happened 5 months ago. Has anything else interesting been detected since then? Well, by the confidentiality rules of our collaboration, I can’t answer that question. But what I can say is that the increases in sensitivity that we looked forward to a year ago are taking more time than we hoped. These incredibly complicated detectors, with their astonishing sensitivity to such weak gravitational waves, are also immensely sensitive to all kinds of disturbances, from outside the detectors and especially from inside. The two LIGO detectors are indeed more sensitive now than they were before they shut down at the beginning of 2016, but they will need much more work before they reach the ultimate goal that we call Advanced LIGO. The Italian-French detector Virgo, near Pisa, has also had a number of challenges in improving its sensitivity. We are hoping it will start observations soon, finally giving us the three-detector network that is required in order to get accurate information about the location of events on the sky and their distances.

We can expect a further planned shutdown of LIGO, and probably of Virgo, late this year or early next year, and another long wait for data as the sensitivity is further improved. But this kind of wait should be worthwhile. A factor of 2 increase in sensitivity corresponds to a factor of 2 increase in the range of our detectors, in the maximum distance to which we can detect events. This implies an increase in the volume we can survey by a factor of 8, so it would bring us more events in 2 months than we presently get in a year.

Gold from gravitational waves

And higher sensitivity might also get us a new kind of event: mergers of neutron stars from binary orbits. They are less massive than black holes, so their signals are weaker, but they are no less interesting. Whereas black holes merge, well, blackly, neutron stars merge in a burst of color. They should produce a spectacular display of light, radio waves, X-rays, and gamma-rays, easily detectable by telescopes on Earth.

Besides all the physics and astronomy that we will learn from neutron star mergers, here is the really spooky part: when we finally observe such an event, we may be observing a re-run of our own cosmic history. Astronomers believe that most of the common heavy elements on Earth — gold, silver, platinum, mercury, uranium and many more — may have been created in just one such event, the merger of two neutron stars. They were ejected by the explosion triggered by the merger, polluting the nearby gas cloud that would eventually condense into our Sun and solar system. In fact the explosion might even have triggered that cloud to condense, setting off the long chain of events that led to our own existence. The rings on our fingers, and perhaps even our fingers themselves, may have come from a nearby gravitational wave event that happened long long before we were able to build detectors to observe it!

So there is much more to come, with patience.

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* The collaboration consists of the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration. Altogether there are over 1000 scientific members as authors to the papers.

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1. Everitt; et al. (2011). “Gravity Probe B: Final Results of a Space Experiment to Test General Relativity”. Physical Review Letters. 106 (22): 221101.

2. Ciufolini, I.; Pavlis, E. C. (2004). “A confirmation of the general relativistic prediction of the Lense–Thirring effect”. Nature. 431, 958–960. Ciufolini, I., et al (2016): “A Test of General Relativity Using the LARES and LAGEOS Satellites and a GRACE Earth’s Gravity Model”.Eur. Phys. J. C. 76, 120.

 

!Happy Birthday GW150914!

Just a year ago today, after travelling some 1.4 billion years, the gravitational wave chirp we christened GW150914 passed through Earth. It disturbed the two gravitational wave detectors of the LIGO observatory enough for us to notice it, to get excited about it, and to get a large fraction of the general public excited about it! But GW150914 just kept on going and is now one further year along in its journey through the Universe. And it will keep going, spreading out and getting weaker but not otherwise being much disturbed, forever. Literally forever.

And GW150914 hardly noticed us! When we observe the Universe with our telescopes, detecting light or radio waves or gamma rays from the enormous variety of luminous objects out there, we capture the energy that enters our telescopes. The photons from a distant star terminate their journeys in our telescopes, leaving a tiny hole in the ever expanding cloud of photons that we didn’t catch. We simply eat up the ones we catch. But GW150914 transferred an absolutely minuscule amount of its energy into the LIGO detectors. We and the famous chirp enjoyed a brief handshake, and then it was gone.

Not that GW150914 had little energy to give: quite the opposite. At its peak, it was 20% as “bright” as the full moon! For the few milliseconds of its passage, GW150914 outdid any star in the sky. Of course, its energy wasn’t in the form of light, so it wasn’t visible to anyone who by chance happened to be looking straight at it. But the energy was there: the gravitational wave energy going through that lucky stargazer’s pupil was 20% of the light energy that would have gone in, had the stargazer turned to gaze at the full moon. The difference, as I noted above, is that the moon’s light energy would have been deposited in the stargazer’s retina; the gravitational wave energy didn’t stay around but just kept going through, leaving almost nothing behind.

It was the same story with all the other objects that GW150914 had encountered before it reached Earth. And it will be the same in the future, which is why the chirp will keep going, forever.

This seeming lack of engagement on the part of GW150914, its reluctance to share its energy with us, comes basically from the extreme weakness of gravity itself. Light and other forms of electromagnetic radiation connect to electric charges, and the coupling between them is strong because the electric force is strong, much stronger than gravity.

There is a simple way to get a feeling for the big disparity between these two forces. Pick up a tennis ball and you are demonstrating the immense superiority of the electric force over gravity. The weight of the ball is the result of all the atoms in our entire planet pulling back on it with their gravitational attraction. The electric force governs the structure of atoms and molecules, and regulates chemistry and the structure of materials. Your arm muscles’ chemistry easily defeats the total gravitational attraction, even though the muscle mass doing the work is less than one part in 10^24 of Earth’s mass. (That is, Earth has one million million million million times more mass than the muscles of one of your arms!) So when GW150914 passed through you (as it did one year ago), it was too weak to disturb you, so of course almost no energy was transferred to you.

How is this weakness consistent with the fact that it was carrying such a huge amount of energy? Here the best way to understand this apparent contradiction is to go back to Einstein’s basic picture of gravity, that gravity is the warping of space and time. It should be no surprise that it is exceedingly difficult to warp space. Before Einstein, nobody even thought it might be possible. A measure of how hard it is to bend space is that the waves of space that carry this warping, the gravitational waves, travel at the speed of light. Now, think about waves in other materials, and how stiffness of the material is related to the speed of the waves. Sound, for example, travels pretty fast through air but much faster through steel. Water waves travel rather slowly, but a crack in an ice sheet can streak across the sheet in no time flat.

By this measure, space is the stiffest medium we know, because its waves go at the speed of light, the fastest speed possible, a speed that is immensely faster than that of waves in any other material we know. But bending a stiff thing is hard, so bending space is hardest of all. To get GW150914 going required a huge energy input, even for a wave with such a weak effect on us. The chirp, as it started out, carried as much energy in total as one would get by converting the mass of three Suns into pure energy via Einstein’s famous E = m c². That was a blast equivalent to 10^34 Hiroshima-scale nuclear explosions. (That’s ten thousand million million million million million bombs!) All this energy came out in a fraction of a second. If you added up all the energy (in light, mainly) that all the stars and other objects in the entire Universe were putting out during that fraction of a second, you would come to a number that is 10 to 100 times smaller.

So our friend GW150914 was a messenger, giving us notice of an almost unimaginable event that was briefly more luminous in gravitational wave energy than the luminosity in light of the entire rest of the Universe put together. And that brings us to whose birthday today really is: that of the black hole that was formed in that inconceivably large gravitational-wave explosion. It was formed by the merging together of two pretty hefty black holes, one about 35 times as massive as the Sun and the other about 30 times. The black hole that was born on that day 1.4 billion years ago ended up with a mass of 62 solar masses. That is 3 less than the sum of 35 and 30: the deficit is the 3 solar masses that got converted into gravitational wave energy and set out across the Universe.

We know all this about GW150914’s pedigree because we were ready for this kind of message. We already knew how to read the information encoded in the message, encoded by the dynamics of Einstein’s gravity. That is a story for another time, for a future entry in my blog. For today, I and many of my colleagues in the gravitational wave collaboration are just going to raise a glass and wish GW150914 a very happy birthday, and many returns of the day! 🍾🎉