June 20th, 2004

So Cathy asked...

> I'm still thinking about kinetons. Now my question
> is, why doesn't gravity "accelerate something all
> the way to the speed of light more often"?

The farther away you are, the less of an effect it has. The closer you are, the less room you have to accelerate towards something before colliding with whatever it is. That's pretty standard physics.

Plus most real systems are more complicated. You start heading towards the Earth, you might hit the moon instead, and by the time you get close enough that gravity's really pulling on you, you've hit this big gaseous ocean we live at the bottom of called "the atmosphere" and get slowed down by wind resistance. (And yes, the atmosphere's a lot like the water ocean. It's got currents and tides; "wind" to us. And it protects us from all sorts of falling rocks that splash into it and get ground to dust before they ever make it down to the ocean floor we live on. And we'd have a much harder time floating up off the ground without the air to push against: balloons, helicopters, wings. All ways of swimming in air...)

Large chunks of solid matter do get accelerated to light speed by black holes, though. The escape velocity being higher than the speed of light probably means stuff can be accelerated to the speed of light rapidly enough to reach it before impacting with the thing. And this of course brings up the "event horizon", which is the distance at which you have light in orbit around the black hole. (Closer, it falls in, farther away it escapes. But at the event horizon, if it's moving in just the right direction, it goes in a complete circle around the thing.)

Now most light approaching a black hole does not nick the edge just right and go into orbit, it either falls in or misses and continues on its way (sometimes doing a pronounced U-bend in the process, but oh well). But there's always a bit of background light from all the stars, and a little of it is constantly feeding into the event horizon, and this goes on for millions of years. You know how you can take normal sunlight and set things on fire with a magnifying glass just by concentrating it? (Focus the light enough you get a point where heat is coming it faster than the heat can dissipate, so the temperature rises past the flash point. If you take a square meter of light and focus it on a square centimeter, you get fire. Now imagine the same 1000x concentration of light, but instead of doing it in space you do it in time. Take all the incoming sunlight for sixty seconds, catch and hold it, and then release it in one second. Just like a 60x concentration with a magnifying glass, it would set stuff on fire pretty easily...)

Now, imagine that the black hole is acting like a strange magnifying glass, not just focusing background starlight but capturing and holding it, more and more every second. Imagine it does this for millions of years, until you've got the most powerful laser beam in nature, probably several feet thick and brighter than you can possibly imagine. If every laser on earth were arranged in a big circle and pointing at exactly the same spot, until you had a tiny little flat area in the center just saturated with laser light that'd incinerate anything it touches instantly... Imagine this naturally occuring, with every point all along the surface of a gigantic sphere like that, only WAY THE HECK more powerful...

This is, incidentally, how we can detect black holes. When asteroids or planets or stars or whatever head towards a black hole, they eventually hit the event horizon, and this uberlaser incinerates them _instantly_. Just like the shoemaker-levy comet debris that whacked into jupiter, this involves a mushroom cloud, only the plume of debris it sends up isn't dust; large portions of it have been completely disrupted and converted into energy, so you get a cloud of fairly heavy photons and charged particles and such blasting out in all directions from the point of impact. (Some of the event horion light itself richochets off the incoming debris, and gets to escape out into space due to this as the debris heads in towards the black hole. What little we know about the event horizon comes from watching these collisions, which produce huge bursts of X-rays. Does the incineration of the debris produce X-rays? Is the light in the event horion predominantly X-rays? This kind of thing keeps astronomers awake at night.)

Now imagine the disney movie "the black hole", where they didn't have to deal with the event horion at all. Okay, it was a disney movie, we don't expect them to get the physics right. How about the Star Trek "Voyager" episode where the ship accidentally wanders inside the event horizon of black hole (without being instantly incinierated by a laser that contains the masses of entire stars converted entirely into energy in a way that makes the less than 2% mass to energy conversion efficiency of the best nuclear bombs we've got look really puny. If the mass of the thing was converted into energy, we wouldn't have to worry about all this radioactive fallout, which is unconverted nuclear fuel that just got vaporized and condensed out on the surrounding countryside as a fine dust... That's why all the residual radiation's left over...)

I hate that Voyager episode. I consider the phrase "warp particles" to be the low point of the star trek universe, even more so than the stupid "Farce of Nature" episode in TNG that introduced warp speed limitations because it's hard to hug a tree in space. No, on Voyager they wound up within the event horizon of a black hole, didn't notice for a while (and didn't get instantly shredded by the gravitational forces or sucked in to the core), and then found the hole they'd made on the way in and left through that!

Okay, guys. Conditions _inside_ the black hole aside. You've got a big spherical laser, with light orbiting in all directions (all directions within a plane, anyway). If you somehow did manage to put a hole in it, that hole would close up again at the speed of light! Trying to chop a hole in the surface of liquid water is WAY more effective; you could manage to do it for whole tenths of a second rather than the nanoseconds you could manage with constantly moving photons that are all in circular orbits!

Not just that, but they used their phasors to widen the hole, which had started to close up slightly in the hours they'd been inside the black hole. Okay, repeat after me: "the event horizon of a black hole can zap a star into a cloud of X-rays in one go". They're fighting it with a weapon that takes several bursts merely to _disable_ a ship the same size they are.

The scientific advisor on that series needed to be hurt. Badly...

I'll stop now.
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