We have found a new way to do astronomyby / February 12, 2016 / Leave a comment
I’m not even trying to suppress a little glow of delight that I was the one who got to ask Stephen Hawking, a month before the announcement that gravitational waves (GWs) have been detected, when or if that would happen. That isn’t to claim any great prescience; it was an obvious thing to ask, not least because the most sophisticated GW detectors on the planet, the Advanced LIGO facilities in Washington and Louisiana, had just switched back on after both being upgraded. And the fact that Hawking replied, during the Q&A of his BBC Reith Lectures at the Royal Institution in London, that GWs would probably be seen in the next five years, implies that he was as unaware as I was that one had already been detected.
It still staggers me that these ripples in spacetime, caused by incredibly violent astrophysical events such as a merger of two black holes, were just waiting to be seen the moment Advanced LIGO (the Laser Interferometer Gravitational-Wave Observatory) turned its two detectors back on last September. They were only doing a test run, for goodness’ sake, and there it was: a 0.2-second chirp in the data, which at first the scientists couldn’t believe was the genuine item. When I spoke to LIGO’s executive director David Reitze at the beginning of last year, he said that he expected that it would take 2-3 years after completing the upgrade to bring Advanced LIGO to its design specification. I don’t think even he had any real expectation of such an amazing result so soon.
And after all, it is beyond imagining. Over a billion years ago, well before any multicellular creatures existed on Earth, two black holes crushed together, and the ripples that squeeze and stretch space by less than a trillionth of an inch have been steadily crossing the cosmos ever since, occasionally passing through stars as if they weren’t there. In the meantime we evolved, Einstein devised the theory of general relativity, we built these multi-kilometre-sized instruments to sense such minute changes in the travel distance of light beams, and then we flipped the switch just as the wave arrived. Of course, the implication is that they must be happening all the time, all over the universe. Still, I think Reitze’s alleged comment on hearing the news—“Holy shit, what is this?”—rather understates the matter.
I’ve explained GWs elsewhere in Prospect, and Frank Close has splendidly put the new discovery in context. As I mentioned before, there was really little doubt that GWs must exist. We already had indirect evidence for them from observations of so-called binary pulsars, paired neutron stars whose orbits very slowly decay because of the energy that their GWs carry off. We’ve also had several false sightings, starting with a claim made in 1969 by a physicist named Joseph Weber at the University of Maryland, who tried to make a detector out of an aluminium cylinder that would vibrate when a GW passed it. And in 2014 a team of astronomers claimed to see the imprint of “primordial” GWs triggered in the Big Bang on the microwave radiation in which that event has bathed the cosmos—but the signal turned out to come from the obscuring effect of dust in our galaxy. This time we can trust the claim, though. The results have been scrutinized in a peer-reviewed paper published by the premier physics journal Physical Review Letters, and the signal matches what Einstein’s theory predicts for colliding black holes, as deduced by painstaking calculations on supercomputers.
Those calculations even tell us the likely masses of the black holes: 36 and 29 times that of the sun, which merged into one 62 times the solar mass. The difference in mass was radiated away as energy in the GWs. These figures illustrate why, beyond confirming the dramatic predictions of general relativity, detection of GWs is such a big deal. It’s a new way to do astronomy.
Black holes themselves can be indirectly seen from all the high-energy radiation (such as X-rays) emitted from the matter surrounding them, which becomes extremely hot as it is squeezed and sucked in by the intense gravitational field. So once we had telescopes to look at X-rays, such as NASA’s Chandra X-Ray Observatory launched in 1999, we have been able to “see” evidence for black holes. (Chandra was named after the Indian astrophysicist Subrahmanyan Chandrasekhar, one of the pioneers in the use of general relativity to understand the collapse of stars into neutron stars and black holes.)
In the same way, GW detectors give us a new window on the universe, allowing us to survey it not now at a new wavelength of electromagnetic radiation (like light, radio waves and X-rays) but using an entirely new kind of radiation. Merging black holes don’t produce any significant signature in the electromagnetic spectrum, so we wouldn’t see them if it weren’t for their GWs.
This was always the intention for LIGO, as well as for the other GW detectors already operating or in development. We currently have also the Virgo detector near Pisa in Italy, and a GW observatory called Kagra is under construction in Japan. It will be housed underground at the Kamioka mine in Gifu Prefecture that already hosts neutrino detectors; the rock shields the instruments from false signals coming from influences such as cosmic rays. Kagra is expected to start operating in 2018-19, and it will be more advanced and sensitive than Advanced LIGO.
These other detectors will be crucial for GW astronomy. With only one or two observatories, as Advanced LIGO has, it is hard if not impossible to get an accurate fix on the location of the GW source, just as you can’t properly judge distances with just one eye. With detectors in other places, you can narrow down this location by triangulation. The more there are, the better the accuracy.
Still more precision, sensitivity and versatility would come from a space-based GW observatory. Sensing distortions in space relies on light interference. Laser light beams are sent along two perpendicular directions and reflected back by mirrors, and any tiny differences in path length translate into an out-of-step relationship in the peaks and troughs of the light waves, causing interference when they come together again. This design was proposed in 1972 by German-born physicist Rainer Weiss, a co-founder of LIGO, who is sure now to win a Nobel prize for launching the field. The further you can send the light beams before reflecting them, the more precise the detection. LIGO, Virgo and Kagra have “arms” several kilometres long, within which the light paths have to be maintained at ultra-high vacuum to avoid anything scattering and disrupting the beams. But some researchers hope that it will be possible to conduct the same experiment in space, sending light between spacecraft several million kilometres apart, which will offer a huge improvement in detection capability.
Several years ago NASA had planned to collaborate with the European Space Agency (ESA) on such a space-based GW detector called LISA. But in 2011 the Americans pulled out. ESA has now elected to go it alone, and last December it launched a pilot mission, LISA Pathfinder, to establish the feasibility of the idea. The spacecraft, which has only one arm that is a mere 38 cm long, arrived at its destination on 22 January. If it all continues to go well, LISA Pathfinder will pave the way for a European GW space observatory called eLISA (Evolved Laser Interferometer Space Antenna) several years down the line. American GW scientists were already frustrated at NASA’s decision to drop the idea; now that looks more than ever like sheer folly. Chinese scientists have also proposed a space-based GW observatory, called TianQin, although if it ever becomes a reality that’s going to be at least a decade away.
With all these detectors operating, GW astronomy could become a reality, enabling us to study objects and phenomena that no other methods could. It’s not just a case of “more is better.” Just as with ordinary telescopes, different instruments will have different windows of operation: eLISA, for example, would register GWs of a different frequency than Advanced LIGO, and so each could see things the other cannot.
What kind of things? Extraordinary ones, for sure. Already, the new results provide some of the best evidence we have that black holes are real, even if few doubted that. (There are, however, still plenty of debates about the details of black hole physics.) How these objects behave, especially when they come together, should provide one of the best tests of general relativity, and perhaps even disclose its limits.
We’ll also get a new view of neutron stars: burnt-out stars whose collapse is arrested before the black-hole stage to create bodies with extreme densities. In neutron stars all the atoms have been crushed into a sea of fundamental particles called neutrons: the protons and electrons have all merged into these electrically neutral particles. A neutron star packs the mass of a star into the volume of a tiny moon just a few kilometres across, and thimbleful of neutron-star matter would weigh 500 billion kilograms. The intense gravity should make the neutron star almost uncannily spherical, but any slight deviation—“mountains” a few millimetres high—could give the star’s rotation an asymmetrical wobble that excites GWs.
We’ll also be able to search for wrinkles in spacetime called cosmic strings. Not to be confused with the minuscule strings of string theory, these are gigantic filaments, thinner than a proton, thought to thread through space where it became “kinked” during the early stages of the Big Bang. They’re a little like fault lines or cracks, and like the kinks in a sheet of paper wrapped around a football they can never be smoothed out. They result in an extreme curvature of spacetime, and if a cosmic string breaks it should release a burst of GWs.
GWs could also tell us about fundamental physics. While it’s not clear that their detection can help very much in the quest for a unified theory of gravity and quantum mechanics—the aim of string theory—one thing they might disclose is what the hypothesized “quantum particle” of the gravitational force is like. All of the other three fundamental forces of nature—the electromagnetic force and the strong and weak nuclear forces that operate inside the atomic nucleus—are known to have particles associated with them, such that two bodies feel the force when a corresponding “force particle” passes between them. Electrical and magnetic forces, for example, are mediated by the electromagnetic force particle, the photon. It stands to reason, then, that gravity too should have a particle avatar, called the graviton.
No one knows what a graviton is like, but one key question is whether it has a mass (like the force particles of the strong and weak forces) or whether it is massless (like the photon). Only if the graviton is massless can GWs travel at the speed of light, as general relativity predicts. Otherwise this speed must be slightly less, since special relativity forbids any massive object from attaining light speed. Already the latest detection has been used to place the strongest limits so far on the permissible mass of the graviton: it can’t have a mass greater than 10-55 grams—which is about 1022 times smaller than the mass of the “ultra-light” particles called neutrinos. But if the graviton had a mass, however small, the implications would be huge. It would reveal a glimpse of new physics beyond the scope of general relativity, and might even account for the mysterious “dark energy” that is causing the expansion of the universe to accelerate.
For all these reasons, the detection of GWs is not, like the detection of the Higgs boson, the final closing of a chapter in physics. It is the start of a new one. That makes it, in my view, much more exciting.