Black holes can exert negative pressure on the environment

 Since the discovery of Hawking radiation, physicists have known that black holes are not a big “nothing”, or the end of all the matter that made up a star.  Rather, they are dynamic objects, with motion, temperature and thermodynamics similar to the rest of the universe.  Now a pair of scientists have discovered that they too must exert pressure on the environment.

 The breakthrough came when Xavier Calmet and Folkert Kuipers were studying a small Schwarzschild black hole — that is, a mathematical model proposed for solving Einstein's equations, but one that ignores things like rotation and electrical charge.  The researchers' goal was to understand the fluctuations in a black hole's event horizon that correct for its entropy (a measure of the progression from order to disorder in the universe).

 In general relativity, any matter falling into a black hole will lose its quantum information (such as the spin of its subatomic particles) forever, but in quantum mechanics this is impossible.  This problem has been known as the information paradox and has kept theoretical physicists awake since it was proposed by Stephen Hawking decades ago.

 While trying to understand the relationship between the event horizon — the boundary of the black hole from which nothing, not even light, can escape — the pair found something unexpected in their equations.  It took some time for them to realize that this was pressure exerted by the black hole in their environment.  The strangest thing is that the value of this pressure was expressed in a negative number.

 This means that, in addition to exerting pressure—however very small—the black hole was shrinking, not growing.  This appears to support Hawking radiation, a theory that shows black holes as objects that emit radiation and lose mass.  In other words, if a black hole doesn't feed on matter, it will eventually evaporate and disappear.

 Whether these results are directly related to Hawking radiation is still uncertain, but it is at least curious how both works point to a black hole that can shrink.  In addition, the new article, published in Physical Review D, may be useful in trying to reconcile Einstein's General Relativity with quantum mechanics, two hitherto incompatible theories.

 Understanding black holes may be the key to finding something that unifies the two theories that explain our universe so well, but they don't work together.  “Our work is a step in that direction,” said Calmet, “and although the pressure exerted by the black hole we were studying is minuscule, the fact that it is there opens up multiple new possibilities.”

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