Gravitational waves from colliding black holes were first observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO) instruments in 2015. Building on these efforts on the ground, an international group of scientists is working to develop a space-based gravitational wave observatory called the Laser Interferometer Space Antenna (LISA). Dr. Thorpe developed instrumentation used on LISA.
Tech Briefs: Could you tell me about the LISA Pathfinder project.
(James) Ira Thorpe: It is basically a technology demonstrator for a future mission we’d like to do. However, it’s important to note that there are two different missions, one of which is happening now and is nearly complete; the other is just getting started.
Tech Briefs: When did the project begin and what was its goal?
Thorpe: For quite a long time, people have been thinking about building a gravitational wave detector in space. It paralleled the development of gravitational detectors on the ground, which led to the Laser Interferometer Gravitational-Wave Observatory (LIGO). Those instruments made the historic first discovery of gravitational waves back in 2015. On June 1 of this year, they announced their third observation of gravitational waves generated by a merger of two black holes.
The concept of gravitational waves dates back to Einstein, nearly a hundred years ago. The idea of building an instrument to measure them didn’t come along until sometime in the 1950s or early 1960s however. By the mid-1970s people started thinking about sending a detector into space. In the 80s and 90s, both in Europe and the U.S., there were serious studies as to what kind of spacecraft would be needed. In a short time, it was recognized that the scientific promise of such a project was enormous. By going into space, you can access parts of the gravitational wave spectrum, the millihertz frequency band, which can’t be accessed from the ground. The challenge is that a detector in space has to be really big and far from earth’s noise.
That was the motivation for doing it. People recognized that the science in that band was compelling, but the technology was going to be pretty difficult. In the early 2000s, NASA and the European Space Agency (ESA) initiated a joint project — the NASA project office was set up at Goddard in about 2001. They decided early on, that rather than do it all in one step, they’d do a technology demonstrator mission where they would build a single spacecraft that would validate some of the key unproven technologies before building the full three-spacecraft constellation. That single spacecraft became LISA Pathfinder, and was led by the ESA with NASA partnering.
For various reasons, it took longer than people were hoping for, but when LISA Pathfinder eventually flew in 2015 — it’s been operating since Dec 2015 — it’s worked fantastically. It has put us in a good position technically to build a space-based detector. LIGO making its first detections on the ground in September of that year after decades of effort, amped up interest from the public as well as the scientific community. So we’re in a really fantastic position for pushing this mission forward.
Tech Briefs: What has been validated with this first step?
Thorpe: There are two levels of validation that have been accomplished. One, which is a key technique for gravitational wave research, is called “drag free flight.” This has to do with the basics of detecting a gravitational wave. There are two things you need to do. One is to make some sort of reference object that feels only the forces of gravity but is not affected by non-gravitational forces, which can mask the subtle effects of the gravitational wave. The way we do this on the ground is to build incredibly complex mechanisms to suspend mirrors by thin glass wires. You put in a vacuum system and then have the heroic engineering job of isolating the test mass from vibrations on the ground, while still supporting it.
In space, we have the advantage of being in zero gravity — we can just let the test mass go and feel only the gravity forces due to the gravitational wave. But if we just left it in open space, it would still be subject to forces such as solar radiation pressure and micro-meteorites. To solve this problem, we put the test masses inside, but not touching, the spacecraft. Once we get to operational orbit, we release a test mass inside a small cavity within the spacecraft. The test mass is a gold-platinum alloy cube that measures about 4 cm on each side and weighs about two kilograms. Since the object resides within an electrostatic housing, but is not touching its walls, it is in perfect free flight. The spacecraft literally flies around the cube — so if there’s an external force like a solar wind or a micrometeorite, the spacecraft senses a drift toward the cube and fires a corrective thruster. If that is done properly, the spacecraft will not touch the cube, and will remain only under the influence of gravity.
Although that was the primary technology developed for LISA Pathfinder, the mission also validated several other technological requirements. The lasers that were used for the metrology, for example, while not major innovations, were tested at the proof-of-concept component level.
Tech Briefs: Could you explain how the lasers are used.
Thorpe: As I mentioned, the first thing we need is the isolated test mass. We also need a second test mass at a great distance from the first. The only way to detect a gravitational wave is by looking at differences. In the final configuration, there will be test masses in two spacecraft separated by millions of kilometers. There will be an interferometric laser link between the two in order to measure the relative distance between the masses. This will be the means for observing a passing gravitational wave. For this technology demonstration, we essentially shrank that large constellation down into a single spacecraft. We used two test masses and a small interferometer to measure their relative displacement, on the level of femtometers. The goal of this experiment was to measure the limits within which we could isolate the non-gravitational forces. We were not only able to measure the level of performance, but to also understand the limiting factors so when we design the next phase we will understand what environmental noise factors we will have to control and how any changes we make to the spacecraft will affect the performance of the final instruments.
Tech Briefs: Were you able to determine those factors?
Thorpe: Yes, there will be more detailed publications about what we have learned. I would say that we now understand the vast majority of the noise, although not yet all of it. However, I’m confident that we’ll eventually have a good handle on it. But even if we don’t, the performance we observed is sufficient for meeting all of the scientific goals for LISA. Of course, as experimental physicists we won’t be satisfied until we’ve explained every little bump and wiggle in the spectrum and where it comes from.
Tech Briefs: Is there a simple way you can explain gravitational waves?
Thorpe: I can try. It’s essentially a stretching and squeezing of space and time, which is the way that information about gravity propagates around the universe. One of the most well-known concepts from Einstein’s theory of relativity is that nothing can travel faster than the speed of light. What the theory really says, is that information can’t travel faster than the speed of light, because if it could, you could never definitively say that A happened before B. This was what motivated Einstein to develop his theory of gravity. According to Newton’s theory, if Jupiter moved a little bit to the left, you would instantaneously be aware of it because you could sense the change in the gravitational field. But Einstein would say that information would then have traveled faster than the speed of light, which doesn’t make sense. This was the motivation for developing his general theory of relativity. One of the things derived from that, is that the way gravity propagates is that waves in space and time carry gravity information from one place to the other. In a normal everyday universe, the wave aspect of gravity is not important, but when there are extreme astrophysical events like two black holes crashing into one another, the effects that in our own solar system are kind of miniscule, become dominant. Gravitational waves actually cause the black holes to crash into each other and they then propagate out into the universe. Although by the time they get to us they’re pretty small, they’re at a level where we can detect them.
Tech Briefs: You detect them by measuring the changes in distance between the two test masses?
Thorpe: Exactly. It’s a little bit like a tide. The ocean is raised along the line to the moon, so it gets a little squished along the orthogonal line, thereby generating our tides. This is a classic gravitational effect: stretching in one direction and compression in the other. Gravitational waves do the same thing. So, if you lay out a ring of particles – imagine it like a string of pearls – you’ll see that the originally circular ring gets stretched into an oval in the vertical direction and squished in the horizontal and then as the wave continues it gets squished in the vertical direction and stretched in the horizontal and it kind of bounces back and forth. This kind of stretching and squeezing phenomenon comes with the wave. The idea is that the string of pearls is our detector. To continue the analogy, we take, for example, three of those pearls and measure the laser distances between them, and we make them our drag-free gravitational wave sensors. We then look for that stretching and squishing on our detectors. The reason we want to make the separation between the detectors really big, is because it’s space that’s stretching, so the more distance we put between the detectors, the bigger our signal. You can understand this with an analogy to the concept of strain, which comes from materials engineering. Strain is a change in length divided by the original length — if you pull on a metal bar, the amount of stretch you get depends upon the length of the bar. That’s the same measure used for gravitational waves: the amplitude of the wave is measured as the change of separation between two test masses divided by their original separation. However, this amplitude, which is a dimensionless number, is very small, even for incredibly energetic gravitational wave events, a number on the order of 1 part in 1021.
The LIGO detectors are 4 km long and make measurements on the order of 10-19 meters — fractions of a proton. By going into space, we can make the size our detectors millions of kilometers, even bigger than the earth-moon system. We can then detect changes at the picometer (10-12) level.
Tech Briefs: What kinds of information can you get from gravitational waves that you can’t get from other measurement systems like radio or IR?
Thorpe: The first, obvious thing, is that you can detect things that don’t produce any light, in particular, black holes (that’s why we call them black). The only other way we can infer the presence of a black hole is based on gases or other materials falling into them, heating up, and emitting x-rays. We can detect those x-rays and do good work understanding that black hole. But the gravity itself is the more fundamental way of looking at black holes. For example, the three events that LIGO has reported have all been mergers of black holes.
It’s mind boggling that the energy per unit time coming from one of these events is larger than all the stars in the universe combined — and not a single photon is produced —it’s all coming out in gravitational waves.
Tech Briefs: And no electromagnetic radiation of any kind?
Thorpe: For a black hole, the expectation is none. But if there’s material in the vicinity, there may be electromagnetic radiation. People are working very hard to see it — it’s a very hot topic right now — to see if you can identify the source and measure a signal.
We know from general relativity, however, that the amount of energy coming out of the merger of two black holes in gravitational waves alone is the most energetic event since the big bang, but it’s completely dark.
It’s relatively straightforward to measure distances of a black hole system using a gravitational wave detector. On the other hand, it’s very difficult to use it to measure red shift and velocity. The exact opposite is true for electromagnetic astronomy — it can be used to measure velocities very well but have a difficult time measuring distance. So, it would be really powerful to observe the same event with gravitational wave detectors and electromagnetic telescopes. You would then have what is being called multi-messenger astronomy, which is very challenging to implement. One problem is that the present generation of gravitational wave detectors don’t do a very good job of locating the source in the sky, they kind of paint a wide swath, aimed at where they think the source might have originated. The universe is a busy and complicated place, so it’s difficult for the telescopes to identify which funny thing corresponds to the gravitational wave event. So far, they haven’t been able to do that.
Tech Briefs: Could you tell me about what you personally have been, and will be, doing on this project.
Thorpe: My background is in the development of instrumentation for LISA, especially interferometry. I’ve been working on the various parts of the system that we will use to measure the distance between the two test masses over this multi-million kilometer baseline. Our mission so far has been to demonstrate the technology. We’re hoping that the actual mission will launch in the late 2020s or early 2030s.
Tech Briefs: Will that be the launch of three spacecraft?
Thorpe: We will do a single launch for the three spacecraft in a stack. Then they each go to their respective orbits. The orbit is essentially a large triangle. It’s in the same orbit as the Earth’s around the Sun, but offset a little — typically behind the earth by about 20°. The configuration of orbits will passively maintain the triangular constellation for a number of years with the distance between the satellites staying relatively constant to a percent or so without our intervention.
Each satellite is its own little drag-free control point, which connects to one of the other satellites and exchanges a laser beam with it. You can’t measure the gravitational wave on any one of the satellites but when you take the data down to the ground and combine it, you get the gravitational wave. It will operate in a very different mode than LIGO, which operates at higher frequencies, where signals happen very quickly. They’ve got their detector running and they have to do rapid online analysis because their signals last for 0.1 sec or maybe up to a second or two, whereas our signals last for months or years. We’ll work more like a particle physics detector, where as we continue to operate, the signals will start to grow out of the noise. We will continue to integrate and catalog the signals with increasingly refined parameters. We expect to identify tens of thousands of individual sources as well as a background of some unresolved sources. In addition to black holes like the ones LIGO has been observing, we’ll be looking at a wide variety of sources, including neutron stars and super-massive black holes and all kinds of interesting things.
Tech Briefs: And you’ll be able to distinguish the different sources from each other?
Thorpe: Yes, we rely on the fact that we can make good models of what the sources are. That is another difference from electromagnetic astronomy. Black holes are comparatively simple compared to, say, stars. And while the equations are complicated, they do a very good job of modeling the system. So, if you can solve the equations, your answers model nature perfectly. Whereas when you’re looking at a star or galaxy, you’ve got a lot of “messy” physics, which is difficult to model. You need to worry about all the chemistry, the nuclear physics, the thermodynamics, the fluid mechanics, etc., that go into a binary interacting star. The nice thing about merging black holes is that it’s just general relativity. Although general relativity is also hard, supercomputer simulations of merging black holes have produced a series of waveforms that can be used to make templates of what the output of the detector should look like. You then take the output of the detector and sort of layer it over the templates to determine the best fit. By doing that, you can figure out which black holes you saw and what are their masses, spins, inclination angles, distances — these are the sorts of things astrophysicists are interested in knowing. You can then start to build catalogs of this information.
LIGO is already doing that. They’ve determined that, of the first three signals they’ve seen, at least two have been of black holes of significantly higher mass than have ever been seen before. So, with just these three detections, the astrophysical theorists are having to rethink their ideas about where black holes come from — the ones they saw were not what was expected.
This is kind of a tradition. You build a new window into the universe: the first x-ray telescope, the first radio telescope, and you may see some things you expected, but the really interesting things are what you didn’t expect. Gravitational waves give us the chance to look at previously unexplored parts of the universe — the discovery potential is off the charts.
Tech Briefs: That’s fascinating.
Thorpe: That’s why I’m doing it even though the launch might take place in 2030, and I’ve already been working on it for 10 or 15 years. I come at it from an engineering and instrument-building perspective. It’s a very challenging instrument to build and the science payoff is just incredible. For someone like me, who likes both the instruments and the science, it’s the perfect marriage.
It’s pretty rare that you get an opportunity to fly a mission like LISA Pathfinder. Typically, space agencies don’t do pure technology demonstrators — it’s usually a little bit of science with a little bit of technology. You develop a new kind of camera and you take it up to demonstrate the technology and at the same time, you do some science. Or you build a new detector and look at the sun or the center of the galaxy — some obvious source that everybody’s looked at a hundred times.
Tech Briefs: Are there Earthbound applications that can be derived from your mission?
Thorpe: Like all scientists, I get that question a lot. I understand the reason for asking it, but I really don’t think it’s a very good question when the subject is basic research. The work that we did to make Pathfinder happen will undoubtedly have impacts in many areas but it is difficult to predict or identify them. It’s unlikely that one of our major subsystems will be directly utilized in an industrial or consumer application. More likely are little lessons in manufacturing, materials, data analysis, controls, etc. that we’ve learned will be applied elsewhere. For example, maybe something we learned about avoiding bubbles in the propellant feed systems of our microthrusters might lead to longer-living or cheaper inkjet printer cartridges. I have no idea whether that will be the case, but the point is it’s impossible to say precisely where these impacts will be. The other major impact basic research like LISA Pathfinder has is in training technical professionals, many of whom move on to industrial jobs. This includes the scientists, engineers, technicians, etc. That is probably the most valuable product we produce.