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LISA will fly and listen for black holes eating each other
Today, some bittersweet spacecraft news: The LISA Pathfinder mission is shutting down. That’s always a bit sad, but in this case, in sum, it’s actually good news: That’s because it accomplished all its goals. And even better, it means that a bigger, beefier mission will take its place! That mission, called LISA, was recently approved by the European Space Agency to continue its planning phase, aiming for a launch in 2034.
Why am I happy about this? Because LISA is the Laser Interferometer Space Antenna, and it will use what is essentially Star Trek technology to detect merging black holes all across the Universe.
So, yeah. How awesome is that? And, for a while, I feared it would never get off the ground. It hasn’t yet, but the odds are looking much better now.
OK, you probably want a modicum of background here. I’ll be glad to help.
Maybe you’ve read reports about LIGO, the Laser Interferometer Gravitational-Wave Observatory, which recently detected black holes merging for the third time. I wrote about that event and gave a lot of background a couple of years ago when LIGO bagged its first black hole coalescence.
In a nutshell, one of the predictions of Einstein’s Theory of Relativity is that when matter is accelerated it creates ripples in the fabric of spacetime, much as shaking a bedsheet up and down causes ripples in the fabric. These ripples are stronger if the objects are very massive, very dense and accelerated very rapidly.
You don’t get more massive, more dense and more accelerated things in this Universe than two black holes at the very moment they eat each other.
There are a few ways this can happen. Probably the most common is from black holes that form when massive stars explode. If those stars are orbiting each other in a binary system, then, eventually, after both stars blow up, you get two black holes orbiting each other. As they emit gravitational waves — those Einsteinian spacetime ripples — they spiral in toward one another. Over a long time (usually billions of years), as the distance between them closes, they orbit faster and faster. Then, finally, accelerating each other to very nearly the speed of light, they merge into a single bigger black hole, emitting a fierce, sharp blast of gravitational waves.
These ripples in spacetime then move across the Universe at the speed of light. When they wash over our planet, they physically compress and expand space itself. The effect is incredibly tiny by the time these waves reach us: A typical ruler would only shrink or expand by a tiny fraction of the size of a proton! But these effects can be measured because we are very clever apes, we humans.
LIGO was built to find these ripples, and after decades of trying, it works! It can now feel the Universe shake as black holes collide.
But LIGO, as amazing as it is, isn’t nearly as sensitive as what’s possible. Enter LISA.
LISA is similar to LIGO, but it’ll be in space. There are lots of advantages to this. For example, LIGO is so sensitive it has to worry about individual oxygen atoms hitting its mirrors, distorting the signal. In space there’s no air, so that’s an improvement.
Also, this stretching of spacetime is easier to measure if you have a longer baseline. If your detector is short it only stretches and contracts a little bit, but if it’s 10 times longer the effect is 10 times bigger. LIGO has mirrors spaced a few kilometers apart, making it highly sensitive. Because LISA is in space, its detectors can be much farther apart. In fact, the plan right now is for the components to be separated by about 2.5 million kilometers!
If you want to think of it as sound (which it isn’t, but the analogy isn’t bad), LIGO can hear the loudest black hole mergers. LISA will hear the whispers. In fact, it should also be able to detect mergers between neutron stars and even white dwarfs, which are far “quieter” than their denser black hole brethren.
So, how does it work? LISA is actually three disc-shaped spacecraft, launched together on one rocket. They each have an onboard propulsion system that will move them to their final separation of several million kilometers, forming an equilateral triangle in roughly the same orbit as Earth, but 20 or so million kilometers away from us.
Like LIGO, LISA will use lasers. Each spacecraft will have onboard two lasers, each of which will fire at one of the other two spacecraft. Using a technique called interferometry, the distances between the spacecraft can be measured with utter precision:
But there’s a problem with this. The spacecraft need to be able to measure their relative positions with incredible accuracy, so that the teeny tiny effects of a passing gravitational wave can be measured. But there are lots of forces in space that would totally wash that out. Tides from the Earth, Moon, and Sun, cosmic rays, solar wind and more would all be far stronger, moving the spacecraft around and ruining the measurements.
To overcome this, inside each laser assembly is a small, exquisitely crafted cube made of gold and platinum (yes, seriously; they’re very stable and that makes them useful). Each cube, called a test mass, is about 4.5 or so centimeters on a side and has a mass of about 2 kilograms. They are totally disconnected from the LISA spacecraft, untouched by it in any way, allowed to float completely freely. The tolerance is extreme: No force on the cube is allowed more than about that exerted by the weight of a bacterium.
See what I mean by Star Trek technology?
In this way, the cubes are freely floating in orbits around the Sun, and the spacecraft keep position around them. Using extremely sensitive sensors, each spacecraft keeps itself precisely aligned with the cube inside it, measuring their exact location at all times.
The cubes act as benchmarks for the spacecraft around them. As long as the cubes are allowed to move freely, then a gravitational wave passing through them would change their relative separation, allowing it to be detected. The spacecraft act like shields, preventing outside forces from affecting them … really, these forces affect the spacecraft, which then use incredibly low-thrust engines to maintain their strictly controlled positions. If there’s a force on the spacecraft, say the solar wind, then the thrusters counteract that to make sure the spacecraft stays perfectly centered around the cubes. And I do mean weak: It would take a thousand of these thrusters to generate the same weight as a piece of paper in your hand!
I like to think of all this using an odd analogy: curling. That’s a sport played on an ice lane where a player throws a heavy mass (called a stone) and tries to place it in a target area downrange. Other players, called sweepers, have brooms and they rapidly sweep the ice ahead of the stone, decreasing the friction and making sure the stone’s trajectory is true.
For LISA, the test masses are the stone, and the sweepers are the spacecraft. They never touch the stone, but they make sure its path is true.
Now, if a gravitational wave passes through the LISA spacecraft, the pattern of light created by the laser changes, and this can be measured with ridiculous accuracy. Even though they will be separated from each other by a distance several times greater than the distance of the Moon from Earth, they will measure their relative positions to an accuracy of a few trillionths of a meter. Yes, trillionths. For those who love words as much as I do, a trillionth of a meter is a picometer. Feel free to work that into your next conversation.
And, again, this exemplifies the idea of how astonishingly advanced this tech is.
This brings us back to LISA Pathfinder. We know all this technology needed for LISA will work because the European Space Agency successfully tested it using Pathfinder. It launched in late 2015 and was equipped with lasers, cubes and other bits of tech LISA will utilize to measure the whisper from colliding hyperdense cosmic objects. It was amazingly successful and completed its mission on June 30. Today it will be shut down, having paved the way for LISA to continue.
I’m glad this is happening. Many years ago, NASA was partnered with the European Space Agency to help build LISA. I actually worked a bit on the Education and Public Outreach for the mission, writing up descriptions of how it worked and what it would do. But shortsighted budgetary decisions meant NASA had to pull out of the development, which upset me greatly at the time.
However, over time and with a lot of cajoling by scientists, the U.S. has rejoined the mission as a senior partner, with the ESA leading the way. I’m very glad to see this. Now that LIGO has shown we can detect gravitational waves, and LISA Pathfinder has shown the advanced technology is possible, LISA itself will open the floodgates of data. It took a huge effort for LIGO to allow us to dip our toes in the water. Hopefully LISA will let us dive in.
My thanks to NASA LISA Study Scientist Dr. Ira Thorpe for talking to me about how the spacecraft measure their distances and clearing up a misconception I had about the test masses!