December 13th, 2008


This stuff is confusing

Let's say you've got two identical rods. I dunno what they're made of or how big they are. But they're out there somewhere in space, and one of them is moving. Very quickly. And the other one is stationary. (Or maybe the one is stationary and the other is moving. Whichever you prefer.)

Now, since it's moving that quickly, its length is going to contract. So its apparent volume will be relatively small. (Relatively! Get it? Ha ha! Oh, c'mon. You're no fun.) And by the same token, its mass will be quite high. So its density will be substantially higher than that of its less mobile cousin.

Now, as I understand things, anything that has too much mass within a given radius is effectively a black hole. Right? And something moving sufficiently close to the speed of light can have arbitrarily high mass.

And so... from the perspective of the un-moving rod, will this rod appear to be a black hole? And if the moving rod collides with the stationary one... well, what happens? Will some of its matter fall into the event horizon? All of it? (Don't forget that, because of its rest shape and the direction of its motion, this apparent black hole should now be disc-shaped.) What if the collision slows it down enough that it's no longer moving fast enough to have enough effective mass to continue to have an escape velocity greater than c? What if it doesn't? Will the matter that's "sucked in", so to speak, cease to perceive the thing it was just sucked into as a black hole, since it would now be traveling at the same velocity, making it all effectively stationary from that perspective?

(This is why I keep a small notepad next to my bed.)