The Black Hole Paradox—Is There A Way Out?

The Black Hole Paradox concerning information loss in black holes puzzled physicists for decades. Recently, scientists found a promising solution using principles of quantum mechanics.

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By Veronika Duvanova

You’ve probably heard of black holes—the monsters of outer space that you wish would swallow you whole as you flunk your math test. Black holes are regions in space-time with extreme gravitational pulls that even light cannot escape. They result from the catastrophic deaths of large stars that collapse under their own gravity. Black holes have an event horizon that serves as the boundary between the outside world and the calamitous interior, where all matter that enters—including you—gets compressed into an infinitesimal point in space called the singularity. 

Can information escape a black hole? Theoretical physicists weren’t always sure. Renowned physicist Stephen Hawking developed the Black Hole Paradox in 1976. The paradox states that though information cannot be lost, it seems to vanish as it falls into a black hole. Information in this context relates to how efficient it is for someone to tell an object’s initial state from its final state. According to quantum theory, information cannot disappear from the universe. However, our general understanding of black holes violates this principle because once something falls in, all of its particles become scrambled in such complex and random ways. Hawking determined that black holes slowly release their contents as radiation, called Hawking Radiation, that gradually shrinks the black hole until it evaporates. The particles that compose Hawking Radiation are so mixed up that the radiation has almost no informational value. Thus, information seems to be lost as a black hole dies.

Since the Black Hole Paradox indicated a flaw in either theoretical physics or quantum physics, physicists were desperate to find a solution. A major theme in quantum physics is entanglement: when two particles become coupled or entangled, they are still connected even when separated by great distances. For example, when two electrons are entangled, they spin opposite each other. One has an “up” spin and one has a “down” spin, so when they combine into one unit of information, there is spinning in both directions. Even if you were to separate the two electrons by billions of kilometers, they would continue to have opposite spins. No matter where entangled particles are, they always carry one joint unit of information. 

In terms of black holes, quantum entanglement is the key ingredient in producing Hawking Radiation. At the event horizon, as one particle falls into a black hole, its entangled counterpart escapes. All these escapees accumulate into radiation. After a black hole evaporates at the end of its life, all the particles that entered the black hole seemingly disappear. Even though the counterpart particles are outside, the no-information-loss rule is violated because the original entangled partners “disappear,” so the joint units of information are gone.

In 2012, physicists created a thought experiment involving an astronaut inside a black hole. In the experiment, the astronaut analyzed the particles a black hole releases as radiation and determined that some of the emitted particles have become entangled with each other. Next, the astronaut went into the black hole and determined that some of the emitted particles were also entangled with their counterparts inside the black hole. Since there was already a full set of information outside the black hole, there was no need for the entangled counterparts inside the black hole. As a result, they reluctantly concluded that the black hole has no interior and is the end of space-time. This conclusion was not well received because of its implication that physics is inconsistent between the outside and inside of a black hole. Many scientists decided to search for better alternatives to this conclusion.

Some physicists turned their attention towards quantum error correction (QEC) in black holes. QEC is a special algorithm used to find and fix errors that result from noise. Noise refers to any external interference—like electromagnetic signals from WiFi—that degrades information. In a May 2019 paper, physicists determined that a black hole empties out information at some point in its life. Once the black hole is filled to its maximum information capacity, excess information indeed escapes through Hawking Radiation!

This contradicts Hawking’s assertion that black hole radiation cannot have information since the particles inside were randomly scrambled. Because of this contradiction, Hawking most likely made a mistake—Hawking Radiation is not truly random. Information can escape a black hole’s interior, a conclusion that put an end to the Black Hole Paradox.

In 2020, a team of physicists verified this conclusion by proposing that information input in a black hole is encoded and disposed to its exterior. As a black hole receives incoming information, it records the configuration of particles into a database of quantum data. Then, it performs postselection, a method used by information scientists to repeat a random process as many times as necessary to get a desired outcome—think of postselection as rolling a dice as many times as needed to get 20 threes in a row. In a similar manner, black holes weed out unnecessary information about the configurations of particles that are not real and expel them as radiation. This theory is plausible because some arrangements of particles are so complex that it would take too long to be able to observe them. As a result, black holes are able to shrink while still containing as much essential information as possible. Eventually, it's all released. 

It took decades for scientists to realize Hawking’s error in assuming that black hole radiation is random. Hawking Radiation actually contains information that seems scrambled because it goes through the QEC process in black holes. This ensures that information is conserved, complying with quantum physics. This promising solution to the Black Hole Paradox allows scientists to rest assured that there is no major flaw in physics that permits such a paradox. Additionally, scientists were able to link the microcosmic quantum world to gigantic, macrocosmic black holes, strengthening our understanding of both the big and the small.