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