A lab experiment appears to validate an idea about black holes proposed by none other than theoretical physicist Stephen Hawking. Moreover, the idea — black holes emit energy called Hawking radiation over time and gradually shrink — seems counterintuitive. How can that possibly be true? We've all heard that nothing can escape the gravitational pull of a black hole. Not even light!
Let's back up a second and learn what black holes are not. They are not nothing. For quantum theory to hold true, real nothingness isn't a thing. Like my aunt loves to say, “It's always something.” Though I don't think she was talking about black holes.
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So what is a black hole? It's what's left over after a massive star dies. Stars have enormous mass, meaning they also exert a strong gravitational pull. While a star is active, the fusion reactions in its core balance out with the gravitational pull of its mass and the star holds its shape. But over time, the fuel for fusion reactions starts to get scarce, and gravity begins to win the tug-of-war.
As a result, the star gets smaller and denser. It begins to pull more material inward toward the core. The core heats up as this happens. Eventually, you get enough energy for an explosion — the star goes supernova. The star throws energy and matter outward with enormous force, but the spent core remains, massive and dense.
That spent core warps space-time, sinking into it. It's like putting a heavy bowling ball on a trampoline. The weight of the ball deforms the trampoline, making it dip down. Black holes do the same thing with space-time, only they do it in more than two dimensions.
Around the opening of the hole is the event horizon. Once you go past this line, you belong to the black hole. That applies even to light itself. But if that's true, how could black holes radiate energy? How could Stephen Hawking be right?
Quantum theory tells us that even in a black hole there would be fluctuating energy fields. The fluctuations generate photon pairs. More often than not, the photons destroy one another, like members in a boy band who have finally grown tired of touring.
But sometimes one photon will appear on the inner edge of the event horizon while the other is on the outer edge. The innermost photon is doomed and gets pulled down into the black hole. The newly divorced photon on the outer edge zooms off to outer space. This is Hawking radiation. You can listen to Neil deGrasse Tyson explain Hawking radiation and vanishing black holes in this video:
According to Hawking's hypothesis, the photon that falls into the black hole actually makes it shrink a tiny bit due to having negative energy. And Hawking also proposes that a black hole destroys information, something that flies in the face of the idea that the total amount of information within the universe is a constant.
And now, at last, we get to the experiment. Experimental physicist Jeff Steinhauer simulated a black hole in the lab and observed what appeared to be Hawking radiation emissions. He created the acoustic black hole using ultracold atoms that create virtual particles of sound called phonons. Just as a real black hole creates virtual photons that sometimes become real, the simulated black hole creates packets of sound.
When the simulated black hole creates a real pair of phonons, one becomes captured by a supersonic region and is trapped. This is similar to a photon being swallowed by a real black hole, diminishing the black hole in the process.
The lab experiment isn't conclusive proof that Hawking was right. Some physicists think that equating the artificial black hole with an astronomical black hole is too large of a leap. It may be that what holds true for one isn't the case for the other. And the fluctuations Steinhauer induced may only resemble those found in the vacuum of space, meaning the results could be misleading.
Scientists would need to replicate Steinhauer's experiment to make certain the results are valid. Even then, it might take a while before the scientific community at large is ready to consider the results as support for Stephen Hawking's predictions back in the 1970s. But it's possible we're a step closer to a fuller understanding of the mysterious black hole.
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