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.

 

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!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! 🍾🎉