Thu Nov 17 19:02:58 EST 2011

table of the day

I've seen a lot of neat tables in my day, but this one is really something else.

Black holes

(From "Are Black Hole Starships Possible?", 2009)

It's not every day you learn that a one-attometre black hole would mass 673,000 tonnes, and radiate 129 petawatts of Hawking radiation. Some of that's in fairly harmless neutrinos, but the 15.7 gigaelectronvolt (GeV) gamma rays most decidedly ain't. (The gamma radiation coming off of a mass of Cobalt-60, (which is excitingly radioactive) by comparison, is a mere 1.33 MeV, 11,804 times less energetic. Attometre-gauge black holes pack a punch.)

The paper makes a pretty good case that using a microscopic black hole as a starship drive is at least physically possible, though there's a whole host of amusing practical problems that should be of any interest to the aspiring megaproject engineer with a couple state vector backups safely stored behind a kilometre of lead shielding.

Firstly is the problem of making them. Apparently Messrs. Crane and Westmoreland are the first people to seriously consider how to generate an artificial black hole, (!) and they conclude that the most practical method (!!) is "by firing a huge number of gamma rays from a spherically converging laser." (!!!)

One can easily imagine just how huge this would have to be, of course, since you're aiming to get the energy density of a couple cubic attometres high enough to spontaneously generate an object that masses 673,000 tonnes.

Once you've built your absolutely gigantic gamma ray laser array, and accompanying solar panel satellite well within the orbit of Mercury, you get to the fun part of calibrating the thing.

The best case scenario is that you generate a fairly large black hole, radiating at a sedate 130 petawatts or so. But if you don't get the power density high enough, then you might end up with a smaller black hole. The smaller the black hole, the more energy it radiates, and the faster it evaporates. At .9 attometres, it radiates 160 petawatts. At .6, 367 petawatts. At .3, 1527. At .16, 5519 petawatts and a lifetime measured in days! It's a short and steep slope to kaboomville. You can see here that a misaligned laser array is mostly a big machine for producing gigantic explosions.

The authors helpfully point out that if you're worried about the radiation flux affecting the Earth, you can just move evaporating black holes to the other side of the Sun. Which reminds me of a famous maxim: if you're using a star as radiation shielding, then you're having a fun time.

Then there's the problems of how to use something that mostly shines in the gamma ray spectrum as a propulsion device, and how to postpone the inevitable kaboom-date. You wouldn't think this would be a problem, since the popular conception of a black hole starts and ends with "it eats things", but take another look at that table. The attometre hole loses 1.43 kilograms a second to Hawking radiation. How are you going to cram 1.43 kilograms of mass into a point 2 attometres across?

The authors, who I absolutely cannot fault in the "imagination" or "audacity" departments, conclude that "this point must remain as a challenge for the future."

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