The black hole – the blind spot of the universe

Nobel Prize in Physics The black hole – the blind spot of the universe

(This report first appeared in 2019 in P magazine.M. It has now been updated on the occasion of the Nobel Prize announcement.)

Physicists are waiting patiently – for the next tremor in the universe. A brief shock that makes our planet tremble for a fraction of a second. Triggered by a wave that has been traveling for billions of years and originates from the most extreme objects in the universe: black holes.

Researchers are waiting for a signal from LIGO, currently the most complex and important project for the study of black holes. The large-scale facility is located in the USA and has revolutionized our view of the universe in recent years. For it has detected gravitational waves for the first time and made collisions of black holes visible. Now that the Nobel Prize has been won (2017) and the champagne bottles have been drunk, the real work of LIGO, the "Laser Interferometer Gravitational Wave Observatory" has begun: the exploration of the dark part of our universe.

Black holes, as LIGO has shown, are no figments of physicists’ imagination. They exist – and with astonishing frequency. They shape our universe, even our immediate cosmic neighborhood. Above all, the giants show scientists their limits: No one knows what happens inside a black hole. Unraveling this mystery opens the door to the physics of the future.

When this door opened a crack for the first time in September 2015, Frank Ohme was on vacation. "For many years we had prepared the experiment", explains the theoretical physicist. "When the last tests were made, I wanted to take a last time off." But as soon as the two gravitational wave detectors were switched on, both reported a discovery. A mistake? An exercise for emergencies? "At first, I didn’t take the signal seriously at all – it just seemed too perfect. Only when I returned from vacation, I realized after weeks: This is real! We have stumbled upon something really big!"

What has LIGO observed?

The task of Ohme and other theorists was to find out what exactly LIGO had observed there. A tough nut to crack, because the measured signal lasted only 0.2 seconds – and was extremely weak: It only caused the detectors to oscillate by a fraction of the diameter of an atomic nucleus. But the experts were able to reconstruct a gigantic event from the data: the final dance of two black holes. 1.4 billion light-years from Earth, they orbited each other, dozens of times per second, at up to 60 percent of the speed of light. They came closer and closer until they were only 350 kilometers apart. Then they crashed into each other, merging into a single large black hole. The merger was such a powerful process that it set space, the entire universe, vibrating – this was detected by LIGO 1.4 billion years later.

A milestone of science. Never before had researchers detected oscillations of space, the gravitational waves. In addition, the measurement provided the long-awaited proof that black holes really exist. This was considered probable before, but since these objects are "black", it is not possible to observe them directly they cannot be observed directly with optical telescopes. With LIGO, mankind was given the opportunity to listen to the colossi, as it were.

With the gravitational wave detectors, researchers can finally investigate the dark part of the universe. And indeed they were surprised right at the first observation: Black holes with this mass were not expected – one was as heavy as 30 suns, the other as 36 suns. The observation was not an isolated case. Between 2015 and 2017 LIGO encountered nine more black hole collisions. The mysterious giants are apparently so frequent in the universe that they do not meet so rarely at all.

The experiment stood still last year, it was improved and made more sensitive. In addition, the two facilities in the USA were joined by a third near Pisa in Italy. The three started to a one-year – their so far longest – measuring time. "We expect three to four times more discoveries than before", said Ohme. Observation of black holes becomes the norm.

Technology from Germany

"Much of the technology was developed in Germany, such as the lasers, but also the most important analytical equipment and models." Ohme and his team at the Albert Einstein Institute in Hannover, Germany, simulate what the signals of possible collisions look like to better find them in the jumble of data. Although they use supercomputers, the calculations take up to eight weeks – for a single simulation. What the team is able to glean from the data, however, is astounding – for example, the history of black holes can be reconstructed: If the angular momentum of both colossi points in the same direction, they probably formed together, otherwise they met in space too late.

The basis of Ome’s calculations are the equations of general relativity, the magnum opus of Albert Einstein, with which he designed a new blueprint of the universe more than 100 years ago. In November 1915 Einstein presented his ideas in Berlin. Within weeks they spread – even to the battlefields of the First World War. Karl Schwarzschild was stationed on the eastern front in Russia at the time. The director of the Potsdam Observatory had voluntarily joined the army in 1914, and as a lieutenant Schwarzschild calculated trajectories of field artillery – and in his free time the consequences of Einstein’s theory. He came across that phenomenon which later became known as a black hole. In January 1916, he published his calculations; only four months later, he died of an autoimmune disease, possibly aggravated by the frontline service.

Schwarzschild did not live to see the success of his ideas. But what had he actually discovered? According to Einstein’s theory, space can be stretched like a rubber band; any body with mass deforms it. This deformation affects the orbits of other bodies; these move as if attracted by the mass – this is how Einstein explains the phenomenon of gravitation.

Schwarzschild’s equations

Schwarzschild now calculated an extreme case: If the density of a body is infinitely large, then it deforms the space around it so strongly that nothing can escape from this environment. Even light – which holds the speed record in the universe – cannot escape from such a large mass. No one can imagine such a structure, it is extremely complex – and four-dimensional. For the sake of illustration, it is sometimes pretended that our universe is flat like the jumping cloth of a trampoline. An infinitely small and infinitely dense object would dent the cloth like a funnel. Marbles, deposited at the edge of the funnel, roll into it. However, if you give them an extra impulse, they can roll out of the funnel and escape its spell. But the deeper objects get into the funnel with its ever steeper walls, the smaller their chance of escape becomes. At some point, even the fastest objects can no longer resist the slope.

Schwarzschild’s greatest triumph was that he could calculate the size of a black hole. This does not mean the expansion of the matter contained in the funnel (which, according to theory, is infinitely small and dense), but rather the area around the matter from which no more information leaks out. So Schwarzschild realized that the black hole has an outer boundary: that place where the curvature of space (or: the slope of the funnel) becomes so strong that even light can’t overcome it anymore.

This boundary is called "event horizon, because an external observer can no longer look into this area to see what is happening in it. Schwarzschild already recognized that the event horizon depends solely on the mass of the black hole, it grows proportionally with it. If a black hole could be observed in the universe, then one would see no funnel and also no hole, but a black ball, whose diameter corresponds to the event horizon.

Black holes are bizarre objects: Parts of the universe that are completely cut off from the rest, from which nothing penetrates to the outside, in which space is curved in an extreme way. For a long time, they were regarded merely as a mathematical curiosity of a theory that was difficult to understand. That they actually exist seemed unthinkable. But then, starting in the 1970s, astronomers encountered more and more phenomena that they could only explain by such an extremely compact and heavy object. Nowadays it is clear: there are lots of black holes out there. Even in our vicinity. In the middle of our Milky Way, 26.500 light-years from Earth, gigantic amounts of stars and gas swirl around an invisible object, banned by a tremendous force. If stars come too close to the object, they will be wiped out. The object at the center cannot be observed, but its properties can be determined from the chaos orbiting it: On its small size, just a few dozen suns, it combines the mass of 4.1 million suns. According to today’s theories, such an extremely dense object can only be a supermassive black hole. Its official name is Sagittarius A* (the "*") belongs to the name!)

A black hole sits in the middle

The Milky Way is no special case: In the center of probably every galaxy sits a black hole. It forms an anchor, so to speak, it holds the cluster of stars together. Without its gravity, the galaxies could possibly not exist at all.

Historic day for astronomy "Important moment for us all" – How researchers captured the first image of a black hole

And Sagittarius A* is not alone in the Milky Way: About 60 other, smaller black holes have already been discovered in our home galaxy. In 2018, a team led by Chuck Hailey of New York’s Columbia University extrapolated that there may be as many as 10,000 black holes in the Milky Way. But this is not a reason to worry: They would still be many light years away from us, otherwise we would have noticed them long ago. Moreover, black holes are by no means monsters that devour everything in their vicinity: Their gravity is no stronger than that of stars of the same mass. To be threatened by a black hole, one must come very close to it.

How all these black holes were formed? In principle, a body must be compressed below the size of its event horizon to become a black hole. For example, the mass of the sun would have to be squeezed to the size of a sphere six kilometers in diameter, the earth to a diameter less than 1.7 centimeters. Under normal circumstances, this is impossible: Electromagnetic forces between the atoms prevent matter from condensing to such an extent. But under extreme conditions, as in the big bang, this is different.

The black holes known so far are the remains of former stars. When huge stars, heavier than 40 suns, have burned up their energy at the end of their lifetime, their matter collapses under its own weight, it condenses into a smaller and smaller lump until it falls below the size corresponding to the event horizon of the star. From the outside only the event horizon is visible, but inside the collapse continues: The remains of the star condense further and further into an infinitely small, infinitely dense object, a "singularity", which bends the space infinitely strongly. So much for the theory. "But physicists do not believe that something as absurd as a singularity really exists. In a finite world, there can be nothing infinite", explains Claus Kiefer, theoretical physicist at the University of Cologne. "At this point Einstein’s theory breaks down."

So even though general relativity correctly predicts that black holes exist, the theory can’t fully explain the inside of black holes. Relativity shows how the world works on a large scale: how the universe came to be, how gravity works, and how galaxies and stars move. "But in the black hole and in the big bang, processes take place in the smallest of spaces.", Kiefer explains. "This world of the small is described by another theory: the quantum mechanics." It is therefore not surprising that the theory of relativity leads to nonsensical infinite results exactly there, where it penetrates into the domain of the quantum theory. This was created to explain how atoms work, but not collapsing stars. "Inside the black hole, relativity and quantum mechanics meet – we just don’t know yet how."

Each of the two theories explains for itself a large part of the world. But they do not fit together. Quantum mechanics pretends that gravity does not exist (it is negligible between atoms), while relativity does not even know what a quantum is. In addition, the mathematical framework of both theories is completely different. They just don’t understand each other – as if two people were talking about completely different topics. In different languages. "We want to unite both theories to a new, more fundamental one, a theory of quantum gravity", says Kiefer, who himself conducts research on such theories. "Black holes play a key role in the search for quantum gravity."

But how will researchers discover this new physics if no information escapes the black hole? Apparently the scientists are threatened by the frustrating fate that they stand in front of a black box and will never know what is going on in it.

The Hawking theory

Cosmology Stephen Hawking – the man who understood black holes

This would probably be the end of the story if Stephen Hawking had not published in 1974 the theory that made him world famous. The physicist, who was already suffering from the motor neuron disease ALS, succeeded in applying quantum mechanics to the event horizon. He discovered that every black hole must have a temperature. Seemingly a triviality, after all, every thing in the universe has some temperature. But what has a temperature, emits heat radiation – the black hole thus loses energy! And since, according to Einstein, energy is the same as mass, black holes become lighter and smaller as long as they do not absorb new matter, so they literally evaporate.

"Hawking has shown: Black holes are not really one hundred percent black. They radiate!", says Kiefer. Radiation forms at the event horizon. According to a common thought model, particle pairs are created there – as everywhere in the vacuum of the universe – out of nothing (P.M. 09/2018), which disappear again directly. However, if one of the partners falls into the black hole, the other stays out and cannot dissolve – it flies away as Hawking radiation.

Hawking bridged the gap between relativity and quantum mechanics with his predicted radiation. The black hole turned out to be a great candidate to explore the interplay between relativity and quantum mechanics. And Hawking showed: Black holes are by no means the end of the cosmic food chain – at the end of the universe they dissipate in a sea of light.

However, according to Hawking, the radiation to be expected from the known black holes is so weak that it could not be observed so far – which is why Hawking, who died in 2018, was also denied the Nobel Prize. "Oddly enough, the bigger a black hole is, the colder it is", explains Kiefer. "Large black holes evaporate only very, very slowly. Those like the ones inside our galaxy need 10,84 years to do so." In comparison: The universe is only 10 10 years old. So it is not worth to wait for the evaporation of supermassive black holes.

Simulation of a black hole

That’s why some researchers are looking for Hawking radiation not in space – but in their labs. For example, Israeli physicist Jeff Steinhauer: In his laboratory at the Technion in Haifa, he simulated a black hole in 2016. "I was fascinated by the idea. I worked on the experiment for seven years", tells Steinhauer. It’s based on an analogy: in some ways, a black hole is like a waterfall that a fish is trying to swim up. No matter how hard he tries, if the water falls faster than the fish in the water can move forward, the tides will sweep the fish into the depths. This is also the case for light in a black hole, which is drawn into the black hole by the curvature of space faster than it can spread out.

Instead of fish in water, Steinhauer studied sound waves propagating in a fluid of rubidium atoms. So that no heat disturbed the experiment, it was cooled down to just one billionth of a degree Kelvin, i.e. almost to absolute zero. "Within the fluid, we created a region where it flowed slower than the speed of sound, and an adjacent region where it flowed faster. Here the sound waves did not arrive any more against the stream", explains Steinhauer. "The boundary between the domains resembled the event horizon." And indeed: At this boundary, the researcher observed sound waves that arose of their own accord and matched Hawking’s prediction. "We were able to show that Hawking’s idea and his calculations were completely correct."

A few months ago, Steinhauer was even able to prove that his artificial black hole has exactly the temperature that Hawking calculated. No one has ever come as close as Steinhauer to a black hole. However, he himself says: "It is still not a proof that Hawking radiation also occurs in black holes in the universe."

Claus Kiefer from Cologne finds Steinhauer’s experiment interesting, but does not believe that such analogy experiments will unravel the mystery of cosmic black holes. Rather, he hopes that astronomers will find clues to Hawking radiation after all. If small black holes were formed shortly after the Big Bang, they could have evaporated by now – the echo of their death would reverberate through the universe. New telescopes should look for this in the future.

Quantum gravity could then be investigated by these echoes. "We can’t describe the final phase of evaporation within the framework of current physics", explains Kiefer. "As the black hole shrinks, at some point what is going on inside it becomes significant." It’s as if the black hole lifts its veil for a brief moment at the moment of its death, revealing its secret.

Frank Ohme of the LIGO experiment hopes that gravitational wave detectors will one day find clues to new physics. "The collision of two black holes is a brutal, highly dynamic process. It puts Einstein’s theory to an extreme test." He therefore explores alternatives to relativity in his simulations. "There are countless theories. Some of them predict slight differences in the course of collisions." But Einstein’s theory still delivers the better predictions – even after more than 100 years.

If detectors become more sensitive, Ohme hopes that one day it will be possible to measure fine deviations from Einstein’s theory. Physicists would have to build better and better facilities to capture those signals that cross the universe towards the earth. Because the future of physics is already on its way to us.

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