For decades, physicists searched in vain for evidence of gravitational waves, the stretches and squeezes in spacetime that were first predicted by Albert Einstein’s theory of general relativity a century ago. Even Einstein himself was uncertain that they existed. But then, in February and June of this year, scientists detected two events that produced gravitational waves.
Now that gravitational-wave detection is likely becoming a regular occurrence—we’ll probably find evidence of many more in the next few years—physicists are again pondering an obscure detail about gravitational waves that was once also thought virtually impossible to observe—gravitational-wave memory, which involves permanent changes in the distance between two objects.
“For so many years, people were simply concentrating on making that first detection of gravitational waves,” says Paul Lasky, and astrophysicist at Monash University in Australia. “Once that first detection happened, our minds have become focused on the vast potential of this new field.”
Physicists are turning again to the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) project that first discovered gravitational waves in the past year since it might also be able to detect the permanent distortions in spacetime that such waves may leave behind as they zip across the universe.
If scientists can find signs of this gravitational-wave memory, such a discovery could help support research that famed theoretical physicist Stephen Hawking and his colleagues are conducting to solve a major puzzle—whether black holes violate the laws of physics by destroying information or not.
To understand what gravitational-wave memory entails, imagine a binary system in outer space consisting of two black holes. There are two astronauts drifting side by side in orbit around this black hole binary, who “are initially separated from one another by some distance, 10 meters say,” Lasky says. As the black holes spiral towards one another, they will release gravitational waves that will travel at the speed of light and cause the distance between the astronauts and the black holes to fluctuate, and they will also cause the distance between the astronauts to oscillate between a little more and a little less than 10 meters. Once the black holes have collided and merged into a single black hole, these oscillations will stop, and the astronauts will once again be separated by a constant distance. However, “this constant distance is not the original 10 meters, but will be slightly more or slightly less,” Lasky says. The gravitational waves will have permanently stretched or squeezed the fabric of spacetime.
“In other words, spacetime has a permanent memory of the collision of these two black holes,” he says.
This memory effect was first proposed by Russian scientists in 1974, but it has largely remained obscure since. “Although it’s a strange phenomenon that certainly has public appeal, our ability or potential to detect it has always been very remote,” Lasky says. “The effect is so weak that people did not believe we would be able to measure it with LIGO.”
Gravitational waves are generated when anything with mass moves. However, gravitational waves are extraordinarily weak, making them extremely difficult to detect. Only those from the most violent events in the universe—the collisions of black holes or neutron stars—can produce gravitational waves strong enough for LIGO to pick up.
Now, though, Lasky and his colleagues think they have discovered a way for LIGO to detect gravitational-wave memory. They recently detailed their findings in the journal Physical Review Letters.
“This is a very clever way of measuring gravitational-wave memory and exploring it observationally,” says LIGO co-founder Kip Thorne, a physicist at the California Institute of Technology, who did not participate in this study. “I never thought it’d be possible with LIGO.”
LIGO uses a pair of widely separated detectors in the United States—one in Hanford, Washington, the other in Livingston, Louisiana—to sense the warping that gravitational waves cause as they move through matter. Two detectors are used so each could be used to confirm the results of its partner.
Both LIGO detectors are shaped like gigantic Ls, with arms 2.5 miles long. The arms of each detector are normally the same length, so the laser beams that shoot down their arms take the same amount of time to traverse each. However, when gravitational waves pass through the Earth, if they make the arms expand and contract in size by within one-ten-thousandth the diameter of a proton, the detector can detect the split-second difference that it takes for laser beams to travel down one arm versus the other.
That sensitivity was on full display when LIGO gravitational waves were first picked up. They emanated from an event known as GW150914 located nearly 1.3 billion light years away, which likely involved black holes each roughly 29 and 36 times the mass of the Sun slamming into one another near the speed of light. In a fraction of a second, the catastrophic merger converted an amount of mass equal to three times the Sun’s into gravitational-wave energy, with a peak power output about 50 times that of the entire visible universe. Nevertheless, these gravitational waves only caused the LIGO arms to wobble by about one-five-hundredth of a femtometer, Lasky says. To put that into perspective, a femtometer is millionth of a billionth of a meter, and a proton is only about 1.68 femtometers wide.
In June, LIGO detected gravitational waves a second time, from an event named GW151226. Suddenly, scientists weren’t just looking at the possibility of detecting one event, but potentially dozens. They’d likely end up with so much data that they could begin to probe other theories involving gravitational waves, including gravitational-wave memory.
Fractionally As Powerful
All kinds of events that emit gravitational waves will also generate this memory effect, “even very weak events,” Lasky says. The memory effect will look like an additional flare of gravitational waves near the end of the event that caused the main burst of gravitational waves, he adds.
However, the memory effect is extremely small. “In general, we expect the size of the memory effect to be between about one-tenth and one-hundredth of that of the gravitational waves,” Lasky says. “For almost all events other than the most catastrophic collisions in spacetime, the effect cannot be measured.”
For instance, while GW150914 caused the LIGO arms to wobble by only about one-five-hundredth of a femtometer, the memory effect from this collision would only be “about one-twentieth of the size of the gravitational waves that were detected by LIGO,” Lasky says, or about one-ten-thousandth of a femtometer.
Whether this memory effect causes the distance between two points to permanently grow or shrink depends on their orientation with respect to the source of the gravitational waves. An event that stretches spacetime in one direction will squeeze it in the direction at right angles to it. For example, consider three astronauts arranged at equal distances around a gravitational-wave source, with two along the equator of the source and the third at one of its poles. If gravitational-wave memory causes the distance between a pair of these astronauts to shrink, it will make distance between the other pair grow, Lasky says.
Previous research suggested that LIGO is not sensitive enough to detect signs of the memory effect from any single black hole merger. While this remains true, scientists have predicted that LIGO should detect the mergers of tens to hundreds of black holes in the next few years, Lasky says.
“Our work has shown that the combination of all these mergers will enable us to measure the memory effect over time,” he says. “The key is being able to stack the signals from all of the events in a clever way.”
In principle, this stacking boosts the detectability of the memory effect. The researchers then compare predictions of how the stacked gravitational waves should look if the memory effect is or is not there.
Lasky and his colleagues predict that LIGO may be able to discern the memory effect after detecting 35 to 90 mergers involving black holes with similar masses and distances from Earth as GW150914. He added that LIGO researchers are striving to make the gravitational-wave observatory even more sensitive, which would mean fewer mergers needed to detect the memory effect.
“I thought it would’ve been very difficult to detect the memory effect using LIGO unless the signal came from very close by, but I didn’t consider this idea of using multiple events to build up evidence for this effect,” says theoretical physicist Marc Favata at Montclair State University in New Jersey, who has done previous work on gravitational-wave memory but did not take part in this research. “It’s very exciting work. It’s always good when one finds there is actually a likelihood of observing something in nature that you’ve been working on for years.”
The detection of gravitational-wave memory would amount to more than just the discovery of an esoteric phenomenon. “Gravitational-wave memory is tremendously important in modern physics when it comes to the matter of whether or not information is stored on the surface of a black hole,” says Thorne, the Caltech physicist.
The conventional view of black holes suggests that their gravitational pull is so strong that nothing can escape from them, not even light, which is why they are called black holes. The border past which there is supposedly no return is known as the event horizon. However, this view suggests that all information about whatever falls past an event horizon is destroyed. This contradicts quantum physics, the best explanation so far for how the universe behaves on a subatomic level, which suggests that information can never be destroyed.
Steven Hawking and his colleagues Andrew Strominger at Harvard University and Malcolm Perry at the University of Cambridge recently proposed a possible answer to this “black hole information paradox” that involves gravitational-wave memory. They suggest that black holes possess “soft hair”—that is, essentially zero-energy forms of electromagnetic and gravitational radiation that release information as black holes evaporate.
“In the last few years, we have discovered that ‘empty’ space, as described by Einstein’s theory of gravity, is not as empty as we once thought,” Strominger says. “It actually has a lot of hidden structure which can store information. The information is stored at the edges of space such as infinity or the horizon of a black hole. We all this structure ‘soft hair.’ ”
“The gravitational-memory effect measures the soft hair,” Strominger says. “It would be most interesting to directly measure it.”
“This is indeed a very exciting prospect,” Lasky says. “It is fantastic that people are thinking about the implications of such a detection, and it is especially wonderful to hear that it could have an influence on such fundamental questions as the black hole information paradox.”