Neutron stars are really cool!
A conversation with physicist Lawrence Kidder, senior research associate at Cornell University and co-leader of the Cornell-founded Simulation of eXtreme Spacetimes (SXS) collaboration group.
On October 3rd, The Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics 2017 to physicists Rainer Weiss, Barry C. Barish, and Kip S. Thorne, for “decisive contributions to the LIGO detector and the observation of gravitational waves.”
The announcement was celebrated, even if it was predictable, because astronomers are super-excited about this new type of telescope, “a new window in astronomy”, that guarantees scientific surprises. But this was also a special prize, because it’s actually shared not only by the three (huge) names on the official “ticket”, but by a team of over a thousand scientists and engineers.
An important role in validating the detection of gravitational waves was also played by a team of astronomers and physicists working in the Simulation of eXtreme Spacetimes (SXS) collaboration group, calculating and completing a full catalog of theoretical solutions since 2000, when supercomputers first became capable of the task. So Lawrence Kidder, senior research associate and a co-leader of the SXS collaboration, was a good expert to tell us more about why astronomers are so excited about gravitational wave astronomy.
First of all, congratulations! There were only three people on the Nobel prize, but we can say it was shared by a huge, international team of over a thousand people, and you and your team were among them.
Lawrence Kidder: Yes, it was very exciting for everyone in the field.
There is a lot of joy because this was a project spanned over decades, involving a lot of tenacity. What made you say you want to be a part of this, that this is the future of astronomy?
As a graduate student I learned about gravitational waves and the dream of detecting them and it sounded like a very interesting area to work in, so I got involved, just because it would be a different way to look at the universe. Gravitational waves can let you learn things about the structure of black holes and neutron stars, that you can’t learn from electromagnetic observations, just because they can propagate out information that light just can’t give us.
You are the co-leader of the Simulation of the Extreme Spacetimes (SXS) group. That’s a cool name, so tell us more about your work.
The SXS collaboration is a group of about eight universities now which share a common computer code, called the Spectral Einstein Code (SpEC), which we use to model two black holes or a black hole and a neutron star or two neutron stars orbiting around each other, merging into a single body, and computing the gravitational waves that should be emitted by such a system. We started on this code in about 2000, so we’ve been working on it for about 17 years. It took us probably the first ten years to get any success of modelling the two black holes merging into one another, but over the last seven years we’ve been slowly putting together a catalog of what the waves look like when you vary the masses and how fast the black holes are rotating, so the gravitational waves signature changes depending on the ratio of the two masses and how fast the two black holes are spinning. And we can use our computer code to predict what the differences are and then this can be compared with what the detector sees and you can see if the predictions of Einstein’s theory agree with what is observed.
And so far everything works?
Yes, so far, the observations and the theory are consistent. The initial measurements by LIGO and Virgo, now that the latter is online, can give you some bounce on how far the theory is tested. As detectors improve, they’ll be able to provide stronger tests of general relativity and so in the next few decades, if we have more and more observations and better detectors, we’ll improve our comparisons and see if Einstein’s theory is really correct. So far, everything is good.
How much does Virgo help?
The third observatory helps in localising where the source is on the sky. For two detectors, you get these large, banana-shaped regions on the sky typically, and then the third detector can help narrow down the region where you would look on the sky. So one of the primary benefits of having more detectors is the localisation on the sky. Also, the more detectors you have the more you can also test properties of the gravitational waves — in general relativity, gravitational waves have what’s called two different polarisations. A general theory of gravity can have up to six different polarisations and so the more detectors you have the better you’re able to put constrains on it.
How many would you like?
How many do you have? (laughs) How many polarisations there are, of course. I would like at least six now.
Also, the more detectors you have it also increases the overall sensitivity of the network of detectors, so you could see fainter sources with more detectors.
The signal from first detection of gravitational waves, back in 2015, came from far, far away —the black hole merger that produced those waves took place 1.3 billion years ago. What about all the other “gravitational wave” noise in between?
The frequency of the gravitational waves depends on how close the two bodies are and their orbits. Nearby star systems do produce weak gravitational waves, however the orbital frequency of these systems is in the millihertz range, to the minus 3 hertz, having an orbital period of thousands of seconds, where the black holes we’ve seen so far have different orbital frequencies — the detectors are sensitive to between about 20 hertz and higher, so the number of nearby sources that have an orbital frequency that high are very few… There just aren’t any. There’s going to be a space based gravitational wave detector, launched by the European Space Agency in the 2030s, called LISA, from Laser Interferometer Space Antenna, and that would be sensitive to these binaries that are in our own galaxy, the stellar binaries, which will be a noise source in that detector. It’s just that the frequency of the gravitational waves they emit is not in LIGO’s frequency band.
LISA is a proposed space-based gravitational wave observatory consisting of a constellation of three spacecraft, linked over millions of kilometers via lasers.
This might angry you, but people are starting to complain on the internet that “not another two black hole merger”…
In the four detections that have been made so far, the black holes were fairly… “boring”, I guess you would say, in that we haven’t learned much about how fast they’re spinning, or so on. As the detectors get better we’ll learn more information about them. But it does give you some knowledge about the populations of these systems in the universe that we could compare how many of them we see with how many we predict from how we think star systems evolve and compare that and learn something about the universe in that way. Also, we don’t think that when two black holes merge there should be any counterpart electromagnetic radiation, but the LIGO team does tell electromagnetic observers, when we see detections, where we think it is and they can go look and see if they see something at the same time and if there ever was something seen at the same time that would be interesting. You never know what you’re going to find.
What are you simulating right now?
We do a variety of simulations, we do these binary black holes, which we have done a lot of, we’ve done over a thousand different sets of runs where we vary the masses and spins. We’re also trying to simulate neutron stars colliding with black holes and two neutron stars merging into either a hypermassive neutron star or a black hole and these systems will also be potential sources of gravitational waves that LIGO and Virgo can see. And these systems are even more exciting. The ones involving neutron stars are more interesting because you can learn things about nuclear matter at high densities and other interesting things. And so we want to be able to study them too. Our simulations with neutron stars are much harder because the techniques we use for our black hole simulations aren’t as accurate when we have neutron stars involved because there is a lot of messy matter that collides together in these scenarios and there’s also more physics that we have to take into account. And so, for a given number of sources it’s much more difficult to do a lot of these simulations and to do them accurately. We’re trying to develop new codes that can model these systems better than we can today and utilize the larger, supercomputers in the world to do these simulations. That’s why a lot of my time is now spent working on developing this new code and it may be a year or two away the simulations have improved that they’ll be useful for comparing with gravitational waves observations.
So it’s not an euphemism, you really need supercomputers for this…
Yes! For the ones for neutron stars, definitely. The black holes ones are easier, let’s say. Our code can run on an modern desktop, to do these simulations, not at the highest accuracy, but for a fairly decent accuracy, it could. But we’re targeting our neutron star simulations for the larger supercomputers.
Black holes and neutron stars are, let’s say, known unknowns. But what could you uncover using gravitational waves astronomy?
Yes, there’s the potential to discover something new, so people do analyse their data for just the arbitrary burst of gravitational radiation and so if one of those were seen and its signature didn’t look like a black hole merger, for example, that would be very exciting as well, and theorists would try and go off and try to figure out how to explain what the potential source of those gravitational waves would be. That would be very exciting. Whenever you turn on a new instrument there’s always the potential to see something that you had no idea about, so everyone would be excited if something like that was seen.
I love your website for SXS (black-holes.org) because it manages to explain, in English, some pretty complicated astrophysics. It’s getting harder and harder to do popular science.
Yes, it’s definitely a problem, so we’re always trying to find good analogies in order to explain things to the general public. I’ve given talks recently to various groups and it’s always a challenge to be able to convey something very complex in terms that you hope that the audience gets some sort of feel for what you’re trying to describe. It’s definitely something that we all need to work on, but some people are better at it. It’s a welcome challenge, because in the end we really need to be able to communicate what we do to the general public so that they continue to fund these nice experiments.
Who else should have won a Nobel prize in astronomy for recent achievements?
I’m always impressed by the better and better results studying nearby planetary systems and learning more information about what the wide variety of different solar systems around nearby stars are like. That’s one area I’ve definitely been following!
SF theory of the week: aliens communicating using gravitational waves…
If an alien civilisation would want to communicate using gravitational waves they would have to be able to manipulate stars. In theory, you could… If you can imagine that there’s a civilisation advanced enough to manipulate black holes and to figure out how to put an electromagnetic charge on them, then they might be able to build some sort of communication device. The problem with this method is that, if you don’t put a charge in your black holes, when the two black holes merge then you just have a single black hole and you don’t have anymore something to produce gravitational waves with again. But if can put electromagnetic charges on them you might be able to have them orbit around each other forever, because you could get a repulsive force between the charged black holes But that’s all highly theoretical and would be a massive feat of engineering. Probably there would be better ways to communicate than that, I would guess.
