What if everything that goes up doesn’t come down
By Pawan Naidu
A long time ago, in a galaxy far far away, two black holes collide. How do we know? More than a billion years later, on Sept. 14, 2015, we felt it. In the world’s most sensitive measuring device, laser beams shifted just enough so the ripples in space-time that washed over Earth could be recorded.
This first detection of gravitational waves was the culmination of an epic scientific quest that resulted in proving Albert Einstein’s landmark theory of gravity. Since then, detectors have seen them multiple times. However, this is only the beginning. Although everything we have learned from the first waves is consistent with Einstein’s masterpiece, the coming discoveries from the sightings could tear it apart.
Gravitational waves take us into the unknown, where gravity is so intense that the best theories on gravitational waves struggle to give definite.
“If there is something wrong with general relativity, we are going to find it here,” said Salvatore Vitale, a researcher of gravitational waves at the Massachusetts Institute of Technology.
Engineers are recalibrating lasers to make detectors even more sensitive, and physicists are giddy with the possibility about what they might reveal. Theorists are now considering cosmic outliners that could transform our understanding of black holes, gravity, and space-time itself. Black holes are notoriously hard to find and their anomalous signals also won’t be easy to discover, because in some cases we barely know what we’re looking for.
Our modern understanding of gravity dates back to 1915 when Einstein finalized his Theory of General Relativity. Our previous knowledge of gravity was theorized by Sir Isaac Newton in 1687. Newton’s theory works on the small scale, but it breaks down once you consider objects like stars and planets. In Einstein’s new theory, gravity was the result of massive objects warping the fabric of the Universe to create depressions that pull in anything nearby.
The one idea we couldn’t scrutinize until now was that massive bodies approaching each other would squeeze and stretch space-time so much that ripples would be spread out in every direction, like a raindrop hitting a pond. The problem was that although the events thought to produce these gravitational waves were extremely powerful, the waves themselves would be incredibly gentle. Space-time is stiff, it doesn’t vibrate or bend easily. To stand a chance of seeing gravitational waves that have traveled billions of light years across the Cosmos, you need to detect wrinkles as small as we are compared to the Milky Way, or as a grain of sand is compared to the Earth.
Now you understand the challenge for the scientists who dreamed up the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the late 1960s. Today, it consists of two L-shaped detectors – one in Livingston, Louisiana, the other in Hanford, Washington. Each uses mirrors to bounce synchronized laser beams up and down the four-kilometer long vacuum chamber. When a gravitational wave passes through distorting space, the arm lengths vary an infinitesimally – and yet the beams shift measurably out of sync.
That was one heck of an accomplishment but achieving this sensitivity was only half the battle. It also took a king’s effort to figure out what the signals coming out of LIGO should look like, and how we could differentiate them from all the other vibrations passing through the detectors, such as earthquakes. Astonishingly, the nine hundred scientists of the LIGO Scientific Collaboration pulled it off. They can not only spot a gravitational wave from across the Universe but even understand the information these outliers carry. For instance, that’s how we know the masses of the black holes producing these waves, or that the collisions of ultra-dense neutron stars can seed the Cosmos with heavy elements.
Even so, the era of gravitational wave astronomy is just beginning. Since LIGO’s second observation run ended in August 2017, the detectors have been receiving upgrades that will significantly improve their sensitivity. We have also welcomed a third detector, Virgo, to the family near Pisa in Italy. The result is that, when the third observational run starts later this year, we should be picking up gravitational waves at least once a week, and the signals will be clearer than anything so far. That will ramp up the chances of seeing something that doesn’t fit with general relativity.
“This is not to denigrate Einstein,” said Richard O’Shaughnessy, assistant professor of astrophysics at the Rochester Institute of Technology, New York. “But there are good reasons to think his theory breaks down on some scales.”
The best-known difficulties are two cosmological theories thrown in to complicate general relativity’s predictions: dark matter, which is unseen stuff dreamed up to explain why galaxies seem to rotate faster than their measured mass allows; and dark energy, a mysterious influence pulling the Universe apart at an ever-increasing rate that has already surpassed the speed of light.
We also can’t forget that general relativity is incompatible with quantum mechanics, the theory of all the fundamental particles and forces, excluding gravity. For the most part, quantum theory reserves its predictions for the world of the very small. But when it comes to black holes, it has something important to say.
According to Einstein, the event horizon of a black hole represents the surface beyond which nothing can escape once it is pulled in. For all its importance as a physical border, the horizon itself is insubstantial, because there is nothing there. But quantum mechanics suggests there should be a firewall, a ring of high-energy particles that would incinerate anything that passes through.
Scientist never had a way to test the idea. But soon after LIGO’s first detection in 2015, two independent teams proposed that a firewall, or anything of substance at the event horizon, would reflect gravitational waves from colliding black holes.
“As long as there is something there, you should get echoes,” said Niayesh Afshordi, an assistant professor at the Perimeter Institute in Waterloo, Canada, who leads one of the groups to New Scientist. “The question is, how long after the merger to expect them.”
In 2016, Afshordi’s team came up with a simple model for what could be at the horizon and estimated how far apart the resulting echoes would be. Then they looked at the LIGO data available to the public. What they found was astonishing: for each three gravitational wave signals they checked, the black hole mergers were indeed followed by echoes at precisely the intervals predicted.
Exciting, but there is more to the story. Afshordi’s calculations suggest that with no firewall, the chances of obtaining a similar signal from random noise would be one in a hundred, falling significantly short of the one in three million figure required to convince his peers. If Afshordi’s claims do check out, however, it would be the first direct contradiction of general relativity – something could escape the event horizon of a black hole. That would leave two options.
Either Einstein’s theory would need to be rewritten to accommodate the firewall, or we would have to forget the notion of black holes altogether. In the latter case, general relativity could still survive more or less undamaged, so long as some pretender emerged to mimic the behavior of these cosmic beasts.
“Either way, it would be a big revolution,” said Paolo Pani, assistant professor of physics at the Sapienza University of Rome to New Scientist.
Predictably, not everyone is jumping on the bandwagon. When members of the LIGO collaboration completed their own analysis earlier this year, the chances that random noise could have caused a similar signal doubled. The team also pointed out that the strongest evidence for echoes came from the black hole merger with the lowest statistical significance.
In other words, the loudest echoes came from the faintest signals, which gives cause to pause.
“Imagine someone was shouting at you and you hear no echo, but then you hear a whisper and get echoes from that,” said Nicolas Yunes, assistant professor of physics at Montana State University. “Doesn’t that seem a bit weird?”
At the moment, many cosmologists believe that Afshordi’s claims are premature. But that could change as theorists get better at modeling the signals they expect firewalls to produce, getting more data, and as the data analysis improves.
One thing is for sure, the hunt for black hole echoes is well and truly on. The LIGO collaboration has said this is one of their goals.
“Now we have competition,” said Afshordi. “They are a huge force so, on the one hand, it is intimidating. But it is nice that this has been recognized as a worthwhile exercise.”
The gravitational waves washing in over the next couple of years should definitively settle the issue.
“Oh it’s beautiful,” said Luis Lehner, a researcher of strong gravity and numerical relativity, also at the Perimeter Institute. “When LIGO switches on again we’re going to have many more events, so if this thing is real the signal will get stronger, and if not, we move on.”
“That’s what’s so nice about this scenario,” agreed Pani. “This is something that can be verified or ruled out in the next few years.”
Still, others prefer to chase more definitive prospects. Rather than seeking exotic phenomena with dramatic consequences for Einstein’s theory, scientists are tweaking the underlying equations to see what they imply for LIGO observations. In some cases, these modifications could solve cosmological conundrums. In others, they might smooth the way to a viable quantum theory of gravity.
Either way, says Yunes, “with modified theories of gravity, you’re building on solid foundations.”
You don’t have to make up the shape of signals based on ill-defined ideas of what to look for. Instead, you add fresh ingredients to Einstein’s equations and solve them to build a full prediction of what you expect to see in these unique conditions. Then you test it against the data.
The trick is to find weak spots in those foundations. General relativity has several core principles and from those arise specific predictions: that gravitational waves will travel at the speed of light, or that they preferentially corkscrew anticlockwise along their direction of travel.
“You break one of these principles and then you ask what would be the consequences,” said Yunes.
The LIGO detections so far have already disproven several modified theories of gravity that tried to explain dark matter or dark energy, including some that predicted space-time ripples would travel at below the speed of light. But there are plenty more where they came from.
Yunes, for his part, is partial to a theory that connects to gravity in such a way that it would explain why matter was not annihilated by antimatter in the first moments of Universe, and why the Universe isn’t filled only with radiation. Such modifications imply that gravitational waves would be more likely to corkscrew clockwise rather than anticlockwise, meaning we would observe black holes spiraling into one another much faster and more violently than relativity predicts.
Emanuele Berti, associate professor of physics, at the University of Mississippi is among those exploring different theories of gravity.
“We all have our own favourites,” he said to New Scientist. “But the broader point is that these theories are mathematically tractable.”
In other words, we know how black holes should behave if these theories are correct, so we can accurately predict how the gravitational waves they produce should look.
In that regard, firewall echoes have some catching up to do. But with LIGO’s upcoming campaign promising so much quality data, even those who remain unconvinced are happy to admit that we can now entertain them with a straight face.
“The LIGO collaboration has been extremely conservative for good reasons,” said Lehner. “But now we know it can detect gravitational waves, the possibilities embolden riskier strategies. This is just the beginning of this story, and it’s going to be amazing.”