Colliding Particles Can Make Black Holes After All

cremeglace writes with this excerpt from ScienceNOW: "You've heard the controversy. Particle physicists predict the world's new highest-energy atom smasher, the Large Hadron Collider (LHC) near Geneva, Switzerland, might create tiny black holes, which they say would be a fantastic discovery. Some doomsayers fear those black holes might gobble up the Earth — physicists say that's impossible — and have petitioned the United Nations to stop the $5.5 billion LHC. Curiously, though, nobody had ever shown that the prevailing theory of gravity, Einstein's theory of general relativity, actually predicts that a black hole can be made this way. Now a computer model shows conclusively for the first time that a particle collision really can make a black hole." That said, they estimate the required energy for creating a black hole this way to be roughly "a quintillion times higher than the LHC's maximum"; though if one of the theories requiring compact extra dimensions is true, the energy could be lower.

Comments

misleading title

[bcrowell]

The title of the slashdot article, "Colliding Particles Can Make Black Holes After All," is misleading, although the summary is less misleading. There's no "after all." Here is the earlier paper, by Giddings and Mangano, which concluded that the LHC would not cause the end of the world. Here is the more recent paper, by Choptuik and Pretorius, referred to in the present slashdot summary.

The "after all" makes it sound as though the Choptuik paper contradicts the Giddings paper. It doesn't. Giddings and Choptuik agree that if the number of spacetime dimensions, D, equals 4, then black holes will not be formed at LHC energies. They agree that at much higher energies, with D=4, black holes will be formed. Choptuik checked the latter statement more carefully than had previously been done, and confirmed what everyone expected.

The LHC black hole doomsday scenarios all require D>4, and in addition they require a number of other implausible things to occur. The Choptuik calculation has little relevance to this discussion, because it just confirms something everyone was pretty sure was true anyway, without affecting the extreme unlikeliness of the long list of *other* things that would have to be true if you were to get an LHC black hole doomsday scenario.

I don't see anywhere in the Choptuik paper where they explicitly state that they're assuming D=4. But I think they must be, since, e.g., they refer to things like Petrov classification of spacetimes, which I think are specific to D=4.

By the way, a commonly quoted argument against the LHC black hole doomsday scenario is actually an oversimplification meant for consumption by nonscientists. The argument is that if such a thing was possible, it would actually already have happened to the earth because of cosmic-ray events. If you read the Giddings paper, there are some loopholes in this argument that they specifically identify. If the long list of implausible things actually all turn out to be true, then it is possible, in a certain specific example involving D=6 (see p. 28) that LHC collisions *would* destroy the earth after a lag of millions of years, while cosmic ray interactions would not. For that reason, they turn to arguments involving neutron stars and white dwarfs rather than planets. It turns out that this argument has no such loophole: even if the long list of implausible statements were all true, neutron stars and white dwarfs would already have been destroyed by cosmic rays. Since we observe that neutron stars and white dwarfs do exist, we conclude that the long list of implausible statements cannot be true. So I know it isn't as comforting to non-physicists as the argument based on the earth's present existence, but the argument based on neutron stars' and white dwarfs' existence is actually secure.