Black Holes

Scientists Are Using Quantum Particles to Peek Inside Black Holes

Black holes are like the Hotel California: you can never leave. Anything that enters a black hole might as well have been snuffed out from existence — and that's a problem for quantum mechanics, which says that information can never be destroyed. Now, a team of physicists thinks they've figured out how to reconstruct the information inside a black hole, which might solve that problem for good. To do it, they're using a bizarre phenomenon known as quantum entanglement.

Black Hole Blues

A black hole is produced when a massive object, like a star's core, collapses in on itself ever smaller until all of that mass is condensed into a very, very small point. Because Einstein's theories say that the more massive the object, the more it warps spacetime — the relativity version of saying it exerts a larger gravitational pull — a spot that small and that massive would warp spacetime so much that to escape beyond the safe zone known as the event horizon, you'd need to go faster than the speed of light. And that's impossible, so nothing can escape a black hole.

This is a schematic of the black hole information paradox. Alice drops a qubit into a black hole and asks whether Bob can reconstruct the qubit using only the outgoing Hawking radiation.

Once something falls into a black hole, it's scrambled beyond recognition. What were once molecules, atoms, and elementary particles are now a jumble of nameless, characterless stuff, like a pile of airline luggage that's lost its baggage tags. That is, the information about the matter inside a black hole is destroyed. And as we said, information can't be destroyed. It's against the rules of the universe. This dilemma is what's known as the black hole information paradox.

In 1975, Stephen Hawking discovered something new about black holes: They emit radiation. Specifically, quantum fluctuations at the event horizon lead to pairs of particles popping into existence, then going their separate ways: one falls into the black hole, the other grabs some extra energy to propel itself outward. As more and more particles grab more and more energy, the black hole slowly evaporates over a ridiculously long timescale — like, longer than the age of the universe.

Some say that's a solution to the black hole information paradox: wait for the black hole to evaporate, and the information can be retrieved. But a faster solution, according to the authors of a paper published earlier this month, may lie in those paired particles. The one that left might tell us something about the one that fell in.

Related Video: The Black Hole Information Paradox

Two Bits Enter, One Bit Leaves

Two quantum bits — whether they're photons on a black hole's event horizon or the qbits that form the basic unit of data in a quantum computer — can share a peculiar relationship known as entanglement, where measuring the quantum state of one immediately tells you the quantum state of the other, regardless of its distance. This phenomenon is one way that quantum computers can perform such powerful calculations, and scientists are also toying with entanglement to "teleport" information from one location to another. This team thinks they can do the same thing to uncover information about what's inside a black hole.

Of course, nobody has a black hole lying around the lab (the state of science funding, am I right?), so the team used a quantum computer instead, one that performed calculations using entangled qbits. Then, they entangled three atoms with each other (their makeshift "black hole") and dropped an entangled qbit in (their incoming particle of Hawking radiation). The result? They successfully measured when and how much the atoms in their black hole had scrambled.

We won't be probing the contents of black holes any time soon, of course — but the researchers think others can use this technique to diagnose issues with quantum computers.

"Regardless of whether real black holes are very good scramblers," said Chris Monroe, a co-author of the study, "studying quantum scrambling in the lab could provide useful insights for the future development of quantum computing or quantum simulation."

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For more fascinating black hole science, check out "A Brief History of Time" by Stephen Hawking. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer April 10, 2019

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