Be afraid of the dark. Black holes, the insatiable monsters of the universe, are impossible to kill with any of the weapons in our grasp. The only thing that can hasten a black hole’s demise is a cable made of cosmic strings, a hypothetical material predicted by string theory. But there is reason to take heart. Finding out black holes are nearly invincible could lead to better understanding of how physical laws work in the universe’s most extreme environments.
The simplest way to witness a black hole’s death is to wait. We used to think black hole gravity was so strong that nothing could escape past the point known as the event horizon, and a black hole could never shrink in size. But Stephen Hawking showed in 1976 that black holes should in fact emit radiation due to the effects of quantum mechanics, and will eventually evaporate away.
The problem is that death by evaporation is unbelievably slow. For a black hole with the mass of the sun, the process would take 1057 times the age of the universe.
Bucket of death
In 1983, William Unruh at the University of British Columbia in Vancouver came up with a way to speed things up. He imagined lowering a bucket into the black hole’s radiation zone and siphoning off some of the particles. Unruh assumed that the bucket was unbreakable and that the cable carrying the bucket had no mass – properties that obviously would not hold true in the real universe.
Now Adam Brown of Stanford University has investigated whether the idea would work using the toughest known materials on Earth. Perhaps unsurprisingly, Brown found that the answer is no. Even a cable made of carbon nanotubes, which have the highest known ratio of tensile strength to weight, would not be able to withstand the powerful forces around a black hole.
“Either it’s so thin at the bottom that a single photon will melt it, or it’s so thick at the top that the whole rope would undergo gravitational collapse and make its own black hole,” says Brown. The only things that would do the job are the fundamental strings of string theory, which hypothetically can support their own weight in the vicinity of a black hole. But even they are not strong enough to lower a bucket into the fog of Hawking radiation. The best they can do is guide one extra photon out of the black hole at a time, like water drops travelling along a fishing wire.
Adding more strings increases the evaporation rate – but only up to a point. A grouping of too many strings would also collapse into a black hole. In the end, the maximum possible number of photon-sucking strings wouldn’t make much difference to the black hole’s lifespan over the age of the universe. “You can’t destroy it faster than it would self-destruct,” says Brown. That may be a relief to physicists. Discovering that it was possible to take energy quickly out of a black hole would be problematic for our understanding of how matter behaves around the event horizon.
“If it were the case that you could destroy the black hole very quickly and get information out, that would certainly be slightly alarming from the point of view of what we think we know about black hole thermodynamics,” says Brown. Instead, his result suggests that current physics for describing the event horizon are on the right track.
“It certainly is an interesting contribution,” says Unruh. “Thinking about this stuff can have implications for how we think about black holes.”
Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.111.211301
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.