What was your first encounter with lasers?
As an undergrad in 1964, I had an interview at a little company in Mountain View, California, called Spectra-Physics. Nobody was in the entry room, so I sat and waited for half an hour until I heard people yelling and then I walked through to the back. Earl Bell greeted me and said “Let me show you a laser.” He had just built the first ionised-gas laser. I was fascinated. I became employee 13 and worked there for a year.
How did your laser work lead you to the hunt for gravitational waves?
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In 1988, I was visiting the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, when Pete Bender showed me his plans to detect gravitational waves in space using extremely stable and precise lasers. The plan required three satellites orbiting the sun in a triangular formation. The idea was that light from a laser in one satellite would be split into two beams, then each of these beams would travel a million kilometres to one of the other satellites and bounce back. If gravitational waves – tiny ripples in space-time – passed through the region, we would see signs of them when the returning light beams recombined. I had co-invented a laser of the kind that Pete needed, so I said we could probably make one for him. But that satellite system, now called the Laser Interferometer Space Antenna (LISA), still hasn’t been built.
So how did you get involved with the Laser Interferometer Gravitational-wave Observatory?
Within weeks of that visit, Rainer Weiss at MIT called to ask me about a laser for , a ground-based experiment that and was in development at the time. I persuaded them to use our laser and .
An upgraded LIGO has reported on two gravitational wave detections so far. What did that first detection reveal?
The first signal it detected was caused by the merger of two black holes 1.3 billion light years away. We’ve calculated that one of these black holes had the mass of 29 suns, and the other 36 suns. The collision basically converted three suns’ worth of matter into gravitational wave energy in one-fifth of a second. That’s just mind-boggling. When someone asks how many hydrogen bombs that’s equivalent to, I tell them trillions and trillions. It’s more power for 0.2 seconds than all the stars in the universe are radiating.
What about the long-awaited LISA?
The , launched , is testing technology that will pave the wave for – an updated version of Bender’s plan. Because we can put much larger gravitational antennas in space than on the ground, eLISA would be large enough to observe the very long gravitational waves from supermassive black holes colliding in the centres of distant galaxies.
We’re at the Galileo stage of gravitational wave science. Like Galileo seeing the moons of Jupiter with his telescope, we have detected gravitational waves with our first instrument, albeit an upgraded one. Decades from now, more sensitive instruments will discover much more.
What else can lasers be used for in space?
I dream about building orbiting telescope arrays. You need extremely precise clocks to successfully combine the data from all the instruments in a telescope array. The latest in precision timing is laser-based clocks that are so precise that if they had started running at the birth of the universe, they would only be out by milliseconds today.
A dozen optical telescope satellites could orbit in formation, with optical clocks and laser measurements giving the required precision. The array could be as wide as Earth, and because laser light wavelengths are a million times shorter than radio waves, the resolution would be much finer than the best radio telescope array. It would be sensitive enough to hunt for signs of life on exoplanets.
Could ground-based lasers be used to launch spacecraft too?
Absolutely. A 20-megawatt laser could be used as part of a of, say, 1 cubic metre. Such a laser may cost $3 billion, though, so it would be uneconomical. But in the 1980s we paid $200,000 for a laser that I could buy today for less than $5, so it won’t be too long before people can get serious about laser launches to put things into orbit. I recently heard a proposal for a gigawatt laser to launch a fleet of spacecraft the size of cellphones, equipped with light sails, to visit the nearest star four light years away. The laser would accelerate them to a quarter of the speed of light so they could survey for planets and life. That would be an exciting venture.
What other frontiers can lasers open?
Lasers are coming closer to generating fusion power. The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in California is by far the world’s largest laser, and one of the most complex optical devices ever built. Since 2009, it has been heating and compressing fuel capsules containing isotopes of hydrogen to temperatures and densities higher than at the centre of the sun. That’s enough to fuse the nuclei to create helium and release energy. NIF can generate a 500-trillion-watt pulse lasting a few billionths of a second. The next step would be to build a super-powerful laser that fires 15 shots per second. That could drive a fusion power plant. After that, we can begin thinking about how fusion energy can complement other energy sources.
How much more powerful can these things get?
Well, a project called the Extreme Light Infrastructure is now being built in Romania. It will focus laser pulses lasting less than a trillionth of a second to intensities so high that they can ionise the vacuum, creating positrons and electrons. It takes black holes to do that in nature.
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Robert Byer is a professor of applied physics at Stanford University, co-director of the Stanford Photonics Research Center, and a member of the LIGO collaboration
This article appeared in print under the headline “We’re like Galileo with his new telescope”
