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Gravity Be Not Proud

The discovery that black holes emit particles and might eventually evaporate threw theoretical physics into chaos. Here’s why.

Part 4 of a four-part series on exploring black holes. Part 1 is here, followed by Part 2 and Part 3.

In the 1960s, a young physicist named Stephen Hawking had a big problem: He was going to die. He had been diagnosed with amyotrophic lateral sclerosis—ALS, often known as Lou Gehrig’s disease—which is almost invariably fatal within a few years of the onset of symptoms. But he beat the odds, and when his symptoms stabilized after a few years, Hawking decided to focus on an area of physics that wasn’t heavily researched: the intersection of quantum physics and gravity.

Stephen Hawking. Photo: NASA

Hawking ended up being one of the very rare ALS patients to survive the condition, at the eventual cost of being confined to a wheelchair and communicating primarily through a computer. And his work on black holes — along with the work of a small handful of other physicists — opened up a new field of research in quantum gravity.

The most shocking discovery to come out of Hawking’s work: Black holes can emit radiation and can eventually evaporate.

Unfortunately for physicists, the radiation from a real black hole is too faint to be seen, and even a smaller black hole, like the ones seen by LIGO, would take a mind-blowingly long time to evaporate. However, the prediction of this Hawking radiation and death of black holes exposed a major problem in theoretical physics, one that is still unsolved today.

Quantum physics and general relativity are the two fundamental theories in physics describing the world as we observe it, but they are incompatible on a fundamental level. General relativity is the theory of gravity: how mass and energy change the shape of spacetime, and how the shape of spacetime governs the motion of matter and light. Quantum physics describes the other forces of nature on a microscopic scale, but the way it does so fails when we try to apply it to gravity. Most of the time, it’s not important: Gravity is a very weak force acting on large-scale objects like stars and planets, while quantum physics governs the very small. However, black holes are a place where gravity is strongest, and they eat the subatomic particles governed by quantum physics. There’s no way to describe the entire process where a black hole swallows an electron without combining the two theories into one quantum theory of gravity, which we don’t have.

Without that theory, Hawking and his colleagues opted for a hybrid approach. They used quantum physics to describe what particles were doing, while using general relativity’s description of gravity as the background for the particles. Hawking in particular was interested in knowing whether the event horizon — the boundary of black hole — could ever shrink. The naive answer would seem to be no: Nothing can escape from a black hole, so the event horizon should grow or stay the same size.

Isn’t this picture of Hawking radiation pretty? Too bad it doesn’t look anything like the reality would. But let’s face it: We can’t see a black hole up close enough to witness Hawking radiation, so we can enjoy the picture as it is. Image: XMM-Newton/ESA/NASA

However, the combination of quantum theory and general relativity shows that strong gravity makes particles. Quantum physics says the cosmos is full of “virtual” particles, which interact with the normal particles we know and dictate how the forces of nature behave. Those virtual particles act differently under strong acceleration, like inside a particle collider, or when they are subjected to strong gravity, like near a black hole.

Virtual particles are made in pairs to ensure that energy and other important physical quantities are conserved. When a pair of these particles is created near the event horizon, sometimes one falls into the black hole, while the other escapes. Since the black hole’s gravity made the particles in the first place, the newly born particle carries away a tiny bit of the gravitational energy. By the famous equation E=mc², the energy loss is equivalent to the black hole losing a small amount of mass. The event horizon’s size is dictated by the black hole’s mass, so it shrinks. The emitted particles are called Hawking radiation, and given enough time, that radiation would make the black hole shrink and eventually evaporate into nothing. Black holes are not forever.

Of course, for Hawking radiation to have a big effect, the black hole must radiate more energy than it eats in the form of matter and light. The amount of radiation depends on the strength of gravity at the event horizon, which roughly scales like the inverse of the black hole’s mass. In other words, the more massive the black hole, the weaker its gravity at the event horizon and the less Hawking radiation it produces. (As I noted in the first part of this series, the giant star Betelgeuse is both more massive and far bigger than our sun, so the result is far weaker gravity at the surface of the star.)

The cosmic microwave background is 2.7 degrees above absolute zero, but it’s still much hotter than the Hawking radiation produced by black holes we see. Image: ESA

As a result, a supermassive black hole like the one at the center of the Milky Way makes so little Hawking radiation that we’d never detect it. The (relatively) low-mass black holes LIGO spotted aren’t much better, even though their gravity is stronger at the event horizon. In fact, all the black holes we’ve ever observed are far too massive to emit much Hawking radiation. Even if they aren’t eating stars or plasma, they emit fewer particles than they absorb from the cosmic microwave background, the radiation left over from when the universe was very young.

If a black hole stops eating long enough for Hawking radiation to take its toll, the evaporation process is still amazingly slow. The Milky Way’s black hole would take a nice 10⁶⁹ years — that’s a one followed by 69 zeroes — to evaporate away. The universe is currently only 14 billion years old, which is in the ballpark of 10¹⁰ years. Black holes will outlive every star and every galaxy.

Yet their evaporation is the source of a big problem for physics.

To paraphrase Leo Tolstoy, electrons are all alike. Unlike baseballs or cars — products that may have slight variations depending on the manufacturer — every electron is precisely identical to every other. The physics of electrons is largely based on that sameness, characterized by a small number of numbers: their mass, electric charge, and just a few others.

Simulated view of a black hole with the Milky Way in the background. Image: Ute Kraus, Physikdidaktik Ute Kraus, Universität Hildesheim

Like electrons, black holes are described by just three numbers: their mass, spin, and electric charge. They aren’t identical in the same way electrons are; they can have a wide range of masses, spins, and charges (though realistic black holes probably don’t even have a measurable electric charge). But they are remarkably simple.

Ironically, this simplicity is the source of a lot of trouble. When a black hole eats a cloud of plasma, absorbs light, or merges with another black hole, the result is a more massive black hole. It “forgets” what made it bigger; there’s nothing in the three numbers describing the black hole to tell a scientist about its history.

To astronomers, that’s a good thing: They don’t have to worry about a black hole’s chemistry or where it is in its life cycle, as they would for a star. Black holes are creatures of pure gravity; they just create complicated astrophysics through their interactions with matter.

But to theoretical physicists, the simplicity of black holes is a problem. An electron can only be created or destroyed following specific rules dictated by quantum physics. When an electron falls into a black hole, however, its identity is forgotten and its properties are merged with the collective. It isn’t precisely destroyed, but there’s no way for outside observers to know later that an electron contributed to the total properties of the black hole that they can measure. But it gets worse when we account for black hole evaporation.

Hawking radiation is mostly in the form of very low-energy photons, even if it’s made up of other particles like electrons. It may eat huge numbers of electrons, but it won’t emit them in the form of Hawking radiation; those electrons are lost and forgotten, forever. By the time a black hole has evaporated, all the particles that went into it have been destroyed by some process that violates the normal rules of quantum physics.

This is the “black hole information paradox”: Either black holes break the rules and completely forget what kinds of particles they munch, or they somehow preserve the information even as they evaporate. Stephen Hawking is in the second camp, providing multiple arguments about how that preservation could occur. Not everyone likes his ideas, though, since they involve speculative physics; one version even does away with event horizons, something other physicists are understandably reluctant to accept.

Unfortunately, Hawking radiation is too weak for us to test these ideas using observations. We’re stuck thinking about them using pure theoretical physics, and it’s possible we won’t solve the problem until we have a full quantum theory of gravity. In the meantime, the Event Horizon Telescope (discussed in Part 3) and gravitational wave experiments could help us figure out if there’s any unexpected phenomena hiding where gravity is strongest.

Black holes are a natural consequence of general relativity, essential in astronomy for understanding the life cycle of stars and the structure of galaxies. They’ve pushed us to accept bizarre concepts in the past; we’ll see if they lead us to entirely new physics in the future. In the meantime, they stand as some of astronomy’s most interesting objects, destructive creators of beauty and wonder.