A black hole’s event horizon is a one-way bridge to nowhere, a gateway to a netherworld cut off from the rest of the cosmos.

Understanding what happens at that pivotal boundary could reveal the hidden influences that have molded the universe from the instant of the Big Bang.

Today some of the best minds in physics are fixated on the event horizon, pondering what would happen to hypothetical astronauts and subatomic particles upon reaching the precipice of a black hole. At stake is the nearly 100-year quest to unify the well-tested theories of general relativity and quantum mechanics into a supertheory of quantum gravity.

But the event horizon is more than just a thought experiment or a tool to merge physics theories. It is a very real feature of the universe, a pivotal piece of cosmic architecture that has shaped the evolution of stars and galaxies. As soon as next year, a telescope the size of Earth may allow us to spot the edge of the shadowy abyss for the first time (see sidebar).

Discovered in 1964, Cygnus X-1 (seen here in X-rays) became the first astronomical object to be classified as a black hole. CXC/SAO/NASA

By studying the event horizon through both theory and observation, scientists could soon figure out how the universe began, how it evolved and even predict its ultimate fate. They’d also be able to answer a crucial question: Would a person falling into a black hole be stretched and flattened like a noodle, dying by spaghettification, or be incinerated?

Gravitational gusto

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Scientists thought about the possibility of black holes and event horizons long before either term existed. In 1783, British geologist and astronomer John Michell considered Newton’s work on gravity and light and found that, in theory, a star with 125 million times the mass of the sun would have enough gravitational oomph to pull in any object trying to escape — even one traveling at light speed.

Although stars can never attain that much mass, Albert Einstein’s 1916 general theory of relativity put Michell’s hunch about supermassive objects onto solid theoretical ground. Later that year, German astronomer Karl Schwarzschild used general relativity to show that some stars could collapse under their own gravity and create a deep pit in the fabric of space-time. Anything, including light, that came within a certain distance of the collapsed star’s center of mass could never come out. That point of no return became known as the event horizon.

Confirmation for the existence of black holes came decades later. In 1974, scientists detected a heavy dose of radio waves emitted from the center of the Milky Way, about 26,000 light-years away. They eventually concluded that there must be a black hole there. Today, astronomers know that virtually every galaxy harbors a giant black hole at its center, shaping the formation of millions of stars and even neighboring galaxies with its immense gravitational influence. Galaxies also contain millions of small- and medium-sized black holes, each with an event horizon past which light is never seen again.

STRETCHING SPACETIME According to general relativity, the sun’s mass makes an imprint on the fabric of spacetime that keeps the planets in orbit. A neutron star leaves a greater mark. But a black hole is so dense that it creates a pit deep enough to prevent light from escaping. James Provost

But the repercussions of black holes’ extreme gravity eventually led to conflicts with one of the keystones of 20th century physics: quantum mechanics. The trouble began in the mid-1970s, when University of Cambridge physicist Stephen Hawking proposed that black holes are not eternal. In the far, far future, when black holes have devoured almost all the matter in the universe, leaving little else to consume, energy should slowly leak out from their event horizons. That energy, now known as Hawking radiation, should continue seeping out until each black hole evaporates completely.

Hawking quickly realized the drastic consequences of his proposal. In a chaos-inducing 1976 paper, he explained that if a black hole eventually disappears, then so should all the information about all the particles that ever fell into it. That violates a central tenet of quantum mechanics: Information cannot be destroyed. Physicists could accept that all the properties of all the particles within a black hole were locked up, forever inaccessible to those outside a black hole’s event horizon. But they were not OK with that safe vanishing without a trace. “It violated everything I knew about quantum mechanics,” says Stanford theoretical physicist Leonard Susskind, who heard Hawking’s ideas at a conference in 1981. “It couldn’t be right.”

Violating theories

Susskind dug into this black hole information paradox, and by the turn of the century he thought he had resolved it with a proposal called complementarity. In essence, he argued that information can simultaneously cross the event horizon and never cross the event horizon, so long as no single observer can see it in both places.

If a particle were to fall into a black hole, an astronaut falling alongside it would see nothing special happen as both coasted across the event horizon and into the black hole’s interior. But another astronaut watching from outside would never see his friend or the particle pass the event horizon; from his point of view, the particle would get perilously close to the horizon but never quite cross it. Eventually, as the black hole evaporated perhaps a trillion trillion trillion trillion years later (astronauts in thought experiments have remarkable longevity), the astronaut outside the black hole would see the Hawking radiation associated with the infalling particle.

Susskind’s explanation is unintuitive, but at least it’s elegant. For both observers, information is preserved (SN: 9/25/04, p. 202). Plus, the outside astronaut can potentially piece together everything that fell into the vast black hole interior just by monitoring the event horizon. This idea, proposed by Juan Maldacena at the Institute for Advanced Study in Princeton, N.J., is called the holographic principle: Just as a two-dimensional hologram can depict a three-dimensional object, the surface of a black hole theoretically reveals everything inside of it. (Story continues below graphic)

Pasta or Barbecue? Since the 1970s, physicists have had trouble coming up with a proposal that describes the fate of something, or someone, falling into a black hole that doesn’t violate well-tested theories. Until 2012, complementarity (left side of image) seemed to do the job. It said that an astronaut falling into a black hole won’t notice anything special as he crosses the event horizon. Yet someone outside will never see his friend reach the horizon. Information is preserved for both observers. But complementarity breaks another rule of quantum mechanics (see “Problematic entanglements,” below). Some argue that walls of radiation along event horizons incinerate incoming matter (right side of image). Credit: James Provost

But in 2012, a quartet of physicists including Joseph Polchinski from the University of California, Santa Barbara reignited the black hole information paradox by demonstrating that in solving one problem, Susskind and Maldacena had created another. The issue centers on another facet of quantum mechanics called entanglement, which intertwines the properties of multiple particles regardless of the distance between them. Susskind and Maldacena’s complementarity relies on entanglement to preserve information. As the proposal goes, particles of Hawking radiation are linked to each other so that over time an observer could measure the radiation and piece together what’s inside the black hole.

In yet another thought experiment, Polchinski and his team pondered what would happen if just one of a pair of entangled particles near a black hole’s event horizon fell in, while the other escaped as Hawking radiation. According to complementarity, the escaping particle would also have to be entangled with another Hawking particle. But that’s a no-no in quantum mechanics: Particles entangled with each other outside a black hole cannot also be entangled with particles inside the black hole. Physicists call this forbidden arrangement entanglement polygamy.

Problematic entanglements For information to be preserved, outgoing particles of Hawking radiation have to be entangled (quantum linked) to each other. But for general relativity to be correct, particles inside the black hole have to be entangled with particles outside the black hole. Unfortunately, these two entanglements can’t coexist. Breaking one of the entanglements creates a firewall. Credit: James Provost

To remedy this violation of quantum theory, Polchinski’s team took its thought experiment a step further and tried severing the entanglement spanning the event horizon. The result: An impenetrable wall of energy formed at the event horizon, incinerating and shutting out any object big or small that tried to pass. They called this unforgiving boundary a firewall.

Unfortunately, while the firewall would play by the rules of quantum mechanics, it would violate Einstein’s theory of general relativity. According to Einstein, an astronaut should not notice anything unusual as he crosses the event horizon; in fact, he shouldn’t even know he’s crossed it until later, when he begins getting spaghettified, or stretched like a noodle, from the extreme gravity of the black hole’s interior and realizes that even a light-speed escape attempt would do no good. A firewall, on the other hand, would provide a pretty noticeable hint that the astronaut had reached the event horizon: He would fry instantly. If firewalls exist, then general relativity requires tweaking.

This firewall problem once again pits general relativity against quantum mechanics, and it has sparked new interest in thinking about the strange physics taking place at the event horizon. “I don’t even see a good framework of an idea to solve the problem,” Polchinski says.

Astronomical stakes

These thought experiments may seem academic, but the implications go well beyond the fates of a handful of particles. Event horizons seem to be the best theoretical test bed for combining general relativity and quantum mechanics into a unified theory of quantum gravity. “The last frontier for fundamental physics is quantum gravity,” says Janna Levin, an astrophysicist at Columbia University’s Barnard College. “And this one puzzle is offering us a chance to see the key elements.”

Physicists have had trouble developing a theory of quantum gravity because compared with the universe’s other three forces — strong, weak and electromagnetism — gravity is pathetically feeble. It’s the only force that is negligible at the small scales dominated by quantum physics. The quest for a theory of quantum gravity gained added significance after the recent discovery of ripples in spacetime dating back to a mere 10-36 seconds after the birth of the universe (SN: 4/5/14, p. 6). Understanding the universe so soon after the Big Bang is an amazing achievement, but a lot of interesting stuff happened in that trillionth of a trillionth of a trillionth of a second before those ripples cascaded through the infant cosmos.

If physicists are ever going to reach all the way back to the very beginning of the universe, Levin says, they will have to understand how the universe behaved when it was incredibly small and incredibly massive simultaneously. The best way to figure that out is to formulate a theory of quantum gravity by demystifying another such compact, massive environment: a black hole. “The event horizon is where gravity starts to come into its own,” says Sheperd Doeleman, an astronomer at MIT’s Haystack Observatory. “It rips off the Clark Kent business suit and starts to become as strong as the other forces.”

With so much at stake, many prominent physicists are stepping up and throwing some intriguing ideas into the mix.

The all-star roster includes Hawking. In a brief, cryptic January posting to the physics preprint server arXiv.org, he suggested that event horizons are not the points of no return proposed by Schwarzschild nearly a century ago. If event horizons occasionally allow stuff inside the black hole to escape, Hawking argued, then firewalls need not exist. While Hawking’s comments grabbed headlines — it didn’t hurt that his write-up included the misleading phrase “there are no black holes” — nobody is quite sure what the black hole savant has in mind. “People want to know what Hawking thinks,” says Sabine Hossenfelder, a cosmologist at the Nordic Institute for Theoretical Physics in Stockholm. “But practically, his paper has no use for me.” She wants Hawking to release a comprehensive paper explaining his argument and the reasoning behind it.

Patrick Hayden, a Stanford quantum physicist, has an idea similar to complementarity. He agrees with the arguments laid out by Polchinski’s team but suggests that it would be extremely difficult for a single observer to determine that a particle is engaged in entanglement polygamy. In fact, he says it would take a person so long to experimentally verify it that the black hole would have already evaporated. Once again, it may turn out that a black hole information paradox is allowed to exist for the simple reason that no one could ever detect it.

The most potentially paradigm-shifting idea comes from the dogged duo of Susskind and Maldacena. They address the firewall problem by combining entanglement, a mind-bending facet of quantum mechanics, with the sci-fi–sounding concept of wormholes. Wormholes are shortcuts through spacetime, the rough equivalent of crossing a mountain via tunnel rather than climbing over it. According to Susskind and Maldacena, every pair of entangled particles is connected by a wormhole, drastically shortening the distance between them. (Story continues below timeline)

Evolution of black hole theories Black holes have given physicists headaches since Stephen Hawking proposed his eponymous radiation. A time line of proposals to prevent black holes from destroying information:

Applying this to event horizons, they say that individual particles of Hawking radiation are linked via wormhole to the inside of the black hole. The proposal eliminates the need for firewalls by turning entanglement into a shortcut through spacetime rather than a mysterious long-distance link. In essence, the particles inside and outside the event horizon become one and the same.

Susskind and Maldacena’s proposal, while pretty wild, is stirring cautious optimism. “As physicists, we often rely on our sense of smell in judging scientific ideas,” Caltech theoretical physicist John Preskill wrote on his blog Quantum Frontiers. “At first whiff, [the wormhole proposal] may smell fresh and sweet, but it will have to ripen on the shelf for a while.” If Susskind and Maldacena are right, it would mean that quantum mechanics determines not only the behavior of particles at very small scales but also the large-scale structure of the universe. “Entanglement creates the hooks that hold space together,” Susskind says.

And in Susskind’s mind, that’s the beauty of the event horizon. A firewall proposal that he’s sure is wrong but can’t yet explain why may be the ticket to unraveling the great mysteries of the universe. Perhaps complementarity, wormholes or a mystery mechanism up Stephen Hawking’s sleeve will simultaneously rectify the black hole information paradox and deliver a theory of quantum gravity. “Once in a while, a conflict comes along and completely changes the way we think about things,” Susskind says. “This firewall story may be one of them.”



Picture perfect

With all the talk about hypothetical astronauts and entangled particles, it’s easy to forget that black holes are actual objects in the universe. It may be up for debate whether matter falling in gets stretched or burned, but there’s no doubt that throughout the cosmos incalculable amounts of gas and dust are flowing across the event horizons of black holes.

High-energy jets emanate from the central black hole of Centaurus A, a galaxy 12 million light years away. ESO/WFI (visible); MPIFR/ESO/APEX/A.Weiss et al. (microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray)

Astronomers know this because, despite the fact that no light can escape the event horizon, many black holes are fairly easy to detect. As the supergravity of a black hole reels in gas and dust, a traffic jam emerges near the event horizon. As matter bumps into other matter, it heats up and glows, emitting X-rays and other high-energy radiation. “Black holes are sitting in a luminous soup of billion-degree gas,” MIT’s Sheperd Doeleman says. Sometimes all that searing gas rockets away from the black hole in concentrated jets that can course more than a million light-years.

Astronomers aren’t sure why some galaxies’ black holes are voracious eaters, glowing brightly, while others seem dark and inactive, Doeleman says. The Milky Way’s central black hole, which weighs about 4 million times the mass of the sun, is relatively dormant. Astronomers are holding out hope that they’ll get to see the local black hole light up over the next year as a large gas cloud called G2 swings perilously close to its event horizon (SN: 8/24/13, p. 9).

Doeleman has even greater ambitions. He leads a team that plans to directly image the event horizon of the Milky Way’s central black hole. That’s pretty hard to do: In fact, it requires a telescope the size of Earth.

So next year, Doeleman and his colleagues will unveil what amounts to an Earth-sized telescope.

The Event Horizon Telescope, the first instrument designed specifically for spying the structure of a black hole, combines multiple radio telescopes to achieve a resolution equivalent to that of a single one that is much larger (SN: 10/9/10, p. 22). This year, Doeleman is heading to the Atacama Large Millimeter/submillimeter Array in Chile, the world’s most powerful radio telescope network, to install extraordinarily precise atomic clocks that will allow researchers to combine the Chilean telescopes’ data with those from observatories in Hawaii, Spain and eventually the South Pole.

If all goes well, as early as next year a virtual telescope with the sensitivity of an Earth-sized radio dish will deliver images of a bright ring of hot gas surrounding a circular shadow: the heart of a black hole, bounded by the event horizon. “We’ve been working on this for a decade,” Doeleman says. “It’s exhilarating to be so close.”

Theorists aren’t as excited about the massive scope. After all, an Earth-sized telescope can’t zoom in on a single particle and resolve the information paradox. But perhaps a photograph will provide some inspiration. For the first time they’ll be able to take a good look at the mysterious boundary that has perplexed them for so long. — Andrew Grant