Image: Hot iron gas rides a wave of space-time around a black hole in this computer image taken from a Rossi X-ray Timing Explorer observation. Credit: NASA
If most people know one thing about black holes, they probably know that nothing can escape from them, not even light.
Yet this most basic tenet about black holes has actually been disproven by the theory of quantum mechanics, explains theoretical physicist Edward Witten of the Institute for Advanced Study in Princeton, NJ, in an essay published online today (Aug. 2) in the journal Science.
Black holes, in the classical picture of physics, are incredibly dense objects where space and time are so warped that nothing can escape from their gravitational grasp. In another essay in the same issue of Science, theoretical physicist Kip Thorne of Caltech describes them as “objects made wholly and solely from curved spacetime.”
Yet this basic picture appears to contradict the laws of quantum mechanics, which govern the universe’s tiniest elements.
“What you get from classical general relativity, and also what everyone understands about a black hole, is that it can absorb anything that comes near, but it can’t emit anything. But quantum mechanics doesn’t allow such an object to exist,” Witten said in this week’s Science podcast.
In quantum mechanics, if a reaction is possible, the opposite reaction is also possible, Witten explained. Processes should be reversible. Thus, if a person can be swallowed by a black hole to create a slightly heavier black hole, a heavy black hole should be able to spit out a person and become a slightly lighter black hole. Yet nothing is supposed to escape from black holes.
To solve the dilemma, physicists looked to the idea of entropy, a measurement of disorder or randomness. The laws of thermodynamics state that in the macroscopic world, it’s impossible to reduce the entropy of the universe — it can only increase. If a person were to fall into a black hole, entropy would increase. If the person were to pop back out of it, the universal entropy tally would go down. For the same reason, water can spill out of a cup onto the floor, but it won’t flow from the floor into a cup.
This principle seems to explain why the process of matter falling into a black hole cannot be reversed, yet it only applies on a macroscopic level.