Did you hear the one about the particle accelerator that created a micro-black hole? You know, the one where this black hole exponentially grows into an Earth-eating behemoth, destroying all life as we know it?
You probably did hear that little piece of comedy in the build-up to the grand start-up of the Large Hadron Collider (LHC) in September 2008, and at first, you might have thought there was some real physics behind this manmade doomsday theory.
Alas, the physics was flawed and the Hawaiian guy at the center of it all saw CONSPIRACY! hiding behind every super-cooled electromagnet.
The Earth (in fact, all celestial bodies) is bombarded with particles (cosmic rays) of far higher energies than the ones collided in the LHC. We're still here. What's more, I haven't seen any black holes float around my neighborhood recently.
The Black Hole Hunt
We know the Earth-munching, LHC-generated black hole theory has more flaws in it than Europa's crust, but scientists do think the next-generation particle accelerator could generate tiny black holes.
This is actually rather exciting. If micro-black holes are generated after the high-energy collisions inside the LHC, they could provide the first experimental evidence of Hawking Radiation, the only radiation predicted to be emitted from a black hole's event horizon. If the radiation predicted by Stephen Hawking is discovered (via the detection of evaporating black holes), a Nobel Prize for Physics wouldn't be far away.
Hold on, isn't there a mixed message here? On the one hand, we have conspiracy nuts scaring the world (yet thrilling the tabloid press), saying that "reckless" physicists could destroy the world with a black hole, and then we have physicists confirming that they would love to see black holes generated in the LHC. What's going on?
It's a little thing called mass, and the micro-black holes that are theorized to be produced by the LHC simply do not have enough of it to cause any damage.
More Mass = More Suck
Cosmic black holes are created after the collapse of a massive star. They are, by definition, massive. If something is massive, it has a strong gravitational field. Any planets, stars or space cows that stray too close will be sucked in, making the black hole more massive.
Micro-black holes are miniscule. They have next to no mass, exert a near-zero gravitational pull on matter, and therefore do not grow. In fact, they most likely do the opposite; they evaporate. Fast.
Even if they had the opportunity to grow, they would accrete matter so slowly that they still wouldn't attain any measurable growth for billions and billions of years.
In a recent publication, a group of physicists decided to crunch the numbers on the likelihood of the LHC generating these vanishingly small micro-black holes, and they pretty much drew the same conclusions as CERN physicists have been saying for the last year. Any black hole generated at the LHC would pose zero threat to Earth.
The leading theory about how micro-black holes might form in our Universe is made possible by the existence of extra-dimensions. The theory is that more dimensions exist than the four we experience (i.e. three spatial dimensions and one time dimension that constitute "space-time" as described by Einstein's theory of General Relativity).
The four dimensional Universe we live in can be considered to be a "brane," where other branes exist alongside ours, exerting a force. This description of out multidimensional Universe is very useful as it helps to explain why the force of gravity is many orders of magnitude weaker than the strong, weak and electromagnetic forces - gravity is 'leaking' into our 4D universe from the neighboring branes.
All this talk of branes and extra-dimensions may sound complex, but their existence allows the production of micro-black holes should the collisions inside the LHC be energetic enough. Therefore, if micro-black holes are detected in the LHC, we have experimental evidence for some of the most complex theories mankind has ever devised. In short, these are very exciting times.
The Fast Die Young, The Slow Don't Grow
So, what did the researchers from Italy, US and Germany find out?
"First, we found that tidal black holes would evaporate (almost) instantly," says Roberto Casadio from the University of Bologna, Italy, and his three colleagues in their publication titled Theoretical survey of tidal-charged black holes at the LHC.
This is all well and good, but what if a micro-black hole shoots through the Earth at high speed?
" show that the black holes with a large value of the initial momentum would cross the Earth in a matter of seconds and come out with velocities much larger than the Earth's escape velocity," say Casadio et al.
Once these speeding black holes pop out the other side of the Earth, they stop accreting mass (from the Earth's interior) and are flung into space and evaporate as they radiate Hawking Radiation. But don't worry about these welterweights punching a hole in the ground beneath you, on the entire trip through our planet, a single black hole will have swept up a meager 10-22 kg of rock.
10-22 kg is the mass of a hemoglobin molecule inside a red blood cell.
But say if the black hole isn't very speedy and it drops like a stone into the Earth... and stays there?
The researchers point out that the slower the black hole, the less mass it accretes; so although it might pop out of the LHC and sink into our planet, it will suck up very little mass.
If a slow-moving micro-black hole set up home inside Earth and sat there for 13.7 billion years (the age of the Universe), it would weigh in at a puny 10-18 kg (the mass of a virus).
"Our overall conclusion is therefore that the tidal charged black holes are a viable model of micro-black holes which might be produced at the LHC. The model predicts that such black holes cannot grow to catastrophic size, but might live long enough to escape the detectors and result in significant amounts of missing energy." –Casadio
When the LHC gets fired up in the coming weeks, let's see if any energy goes "missing" after a particle collision, it might be a sign of black hole birth (but not of the "Earth-munching" variety).
Source: Theoretical survey of tidal-charged black holes at the LHC, Casadio et al., 2009. arXiv:0911.1884v1 [hep-th]