Wednesday, June 14, 2006

Branes, Bursters, and Black Holes

In physics, you don't have to go around making trouble for yourself - nature does it for you. ~Frank Wilczek

Around about August of next year, assuming NASA's science budget hasn't been completely gutted, the Gamma Ray Large Area Space Telescope (GLAST) will be launched, much to the delight of a couple of physicists, Charles Keeton of Rutgers and Arlie Peters of Duke . GLAST is going out to collect information in the gamma ray spectrum, but thanks to its precision, Keeton and Peters think it will be able to detect “wiggles” in the spectrum of Gamma Ray Bursters (GRB's). This, they say, will be an indication that small primordial black holes exist, perhaps in our own solar system, which will be proof of a fourth spatial dimension, which will support the Randall-Sundrum “braneworld” model of the universe, which says our universe is floating in a large universe in a fourth dimension.

Got all that? Perhaps, we should take this a bit more slowly.

Let's take the “easy” one first: GRB's. Gamma Ray Bursters are something that sends out unimaginable amounts of energy for very short periods of time and are scattered around the universe more or less randomly. At this point, no one knows what they are. The thinking is that they may be colliding neutron stars, merging black holes, or, well, something really big blowing up. The energy is released directionally and when it's pointed at Earth, we see an intense flash of gamma radiation coming from a point source in space. Occasionally, optical telescopes get the word quickly enough and may find some sort of dimly glowing object in the area that reveals little about the cause of the outburst.

(The reason directionality is important is that if the energy were radiated in all directions, the masses involved would be so large as to be unrealistic. But, if you can't get that much mass together, E=mc2 bites the dust. But, beams and jets are commonly observed coming from active galaxies, pulsars, and potential black holes, so the directional energy hypothesis seems a good one.

So, one thing GLAST will no doubt study is GRB's, but it will be looking for other high-energy sources of gamma rays, too. Now, if there happens to be something massive between us and the gamma rays, it's lensing effect could be detected as a “wiggle” in the spectrum. No, I don't know what they mean by a wiggle, either, but presumably it's some detectable anomalous distortion of the spectral data. If there are a lot of little black holes in the galaxy, it's possible one of them might cause such distortion.

All righty, you say, where did the little black holes come from?

Well, let's go back, waaaaaaay back to the Big Bang. The Big Bang has proved to be a good model for describing the current state of the universe, although it has its problems. Alan Guth's inflationary universe solution solved a number of these, but there was still nagging issues, like the singularity (if nature abhors a vacuum, physicists loathe a singularity), what triggered the bang, what happened in the very initial instance of the bang (where the laws of physics break down), that sort of thing. Enter the string theorists.

Even though I dislike string theory, I admire the kinds of minds it has taken to come up with it. It is the most complex, intricate, and downright convoluted mathematics that has ever been invented. So far, it's been so complex that it consists mostly of approximations. As such it's not a good theory for predicting things, yet. But, it does purport to be able to deal with that initial instant of time. And, it has spun a new idea, that of branes.

“Brane” is probably short for “membrane”, although there seems to be some debate about that. There's M-brane theory, p-brane theory, and others. One of the consequences of brane theory is that the universe is one big happy (mem)brane (an easy way to think of it) floating with other brane universes. In one version of the theory, the brane universes float about and undulate (due to gravity waves) and occasionally touch, The point of contact is a creation event for a new universe.

(This is wildly simplified, but I don't think I'm doing any violence to the theories here.)

Now, the creation of the universe (ours) results in a lot of small black holes being formed. Stephen Hawking found that all black holes “evaporate”, emitting what has come to be called Hawking radiation, gradually dissipating into space until they cease to exist. This served to explain why there didn't seem to be a surplus of black holes floating around. But, a consequence of braneworld theory is the presence of a fourth spatial dimension. According to Keeton and Peters (remember them?), this extra dimension would somehow slow the Hawking radiation, with the result that there ought to be lots of these little black suckers floating around.

Of course, “lots” is a relative term, but apparently we're talking about enough that there should be some in our own solar system, which, by their nature, would produce the “wiggle” in the gamma ray spectrum. And that would be an indication that the braneworld theory might have substance to it. There's a lot of work to do (GLAST has to get off the ground for one thing), and it's almost a sure thing that any data will be subject to interpretation, but the possibilities of lending some credence to string theory is exciting because of what the theory might be able to tell us about how we got here. Which would be pretty amazing for an ugly theory. But, who knows, if they got some hard data, they might be able to pretty it up some.

The idea of so-called parallel universes has always had appeal to the sci-fi writers. Actually, though, it's not proper to think of these as parallel in the three dimensional sense, since they are actually suspended in a fourth dimension. They could actually be within each other, as far as that goes, without the inhabitants of one universe knowing anything about the inhabitants of another.

Which might be a good thing for them and us.

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