5 days ago
Thursday, October 16, 2008
When smashing black holes looks like theoretical nuclear physics
A new article out in PRL describes the authors' numerical solutions to the problem of colliding black holes. Not just any black holes, either. Fast ones.
In general relativity (Einstein's famous spacetime; see a tutorial here), black holes are the objects so dense, so massive, that they warp spacetime to the point that no light can escape. All manner of strange physics takes place inside of black holes, including the smashing together of particles moving nearly the speed of light (the authors refer to this regime as "ultrarelativistic," which seems odd to me, like saying that something is "ultralogarithmic," but oh well). This is what connects black holes to the Large Hadron Collider - the conditions in the LHC, as well as in collisions between ultra-high-energy cosmic rays and the upper atmosphere, are ripe for producing events where large kinetic energies (kinetic energy is much greater than the rest mass energy) and small distances (such that the C.M. energy is beyond the Planck scale and gravity will dominate) will create outcomes theoretically similar to those within black holes. In particular, two colliding black holes would provide evidence for gravity waves (ripples in spacetime due to moving mass), the very thing LIGO hopes to detect.
(image credits: Denver Museum of Nature and Science)
The paper itself is short (as PRL articles are), but it describes the methods and solutions for several sets of initial conditions. The simulated collision starts with two non-rotating, "boosted" (in other words, moving quickly) black holes of equal mass, varying in speed from 36% to 94% of the speed of light. The amount of energy radiated - in essence, lost to gravity waves - is given as a percentage of the total mass of the two colliding black holes. Earlier estimates placed an upper limit on this fraction at 29%, but the authors find that 14% is more likely.
This discrepancy, according to the authors, is important to searches for gravity waves, both at LIGO and the LHC. If you're expecting to lose 29% of your total energy but really only lose half of that, you'll be looking for your data in the wrong place.
The thing I find most amusing is how similar two colliding black holes look, mathematically, to two colliding particles, like nuclei or nucleons. That, as you get into the strange regimes on either side of the energy and dimension scales (either extremely high or extremely low), particles, even black holes, behave more like waves. From my point of view, anyway, it feels elegant (although I know I don't have a full understanding of general relativity). It's a pity that Einstein never believed in quantum mechanics.
Ulrich Sperhake, Vitor Cardoso, Frans Pretorius, Emanuele Berti, José A. González (2008). High-Energy Collision of Two Black Holes Physical Review Letters, 101 (16) DOI: 10.1103/PhysRevLett.101.161101