Regarding black holes: they're mostly mathematical. The only physical parameter they have (that we can see) is mass. The event horizon is defined as the surface (sphere) inside of which all matter and energy are inexorably attracted to its center. But that's just the thing, there's no actual horizon. It's not like you hit a wall as you fall in*. It's only a mathematical definition, dependent completely on the mass.

*Our understanding is, of course, incomplete.

For any mass M, there's a radius R such that if you could stuff M inside of a sphere with radius R, you'd have a black hole. That may seem like a weird way to define it, but what I'm saying is that the configuration of matter INSIDE that radius is not important. If the mass M is distributed between three separate chunks of matter, and they're all close enough to be inside a sphere of radius R "drawn" around their center of mass, then they are a functional black hole. Such a configuration would be impossible to maintain very long, anyway; you know that after a certain amount of time, they'll fall into each other at the center.

Of course, you can't see it happen.

An interesting effect of this is that an event horizon has a tendency to grow in anticipation of incoming mass. Imagine a black hole of mass M and radius R. Now say we have a "shell" of mass M hovering somewhere outside of that radius. If we were to let it fall in, the new mass of the black hole would be 2M and the new radius would be R2. (I don't want to say 2R because I'm pretty sure it's not a one-to-one relationship. Let's just say it's a bigger radius.) But, if we slowly let the shell fall in, when do we say it's a part of the black hole?

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Well, obviously, when the shell comes inside the new radius we defined, it meets the criteria from before. Even though it's still pretty far away from the center, we have mass 2M inside the corresponding radius R2, and that's all we need. So, we've gone from a black hole of size R to R2. When did that happen? Did the size jump up suddenly when our shell crossed the R2 line? Surely not. This next part is harder to describe without some pictures, so hold on to your seats.

The event horizon is described as the surface behind which an outgoing photon could never escape, due to the intense warping of spacetime. So in weird cases like this, where the horizon seemingly grows as if it's sentient, we go back to that definition. Imagine a bunch of lamps floating near our black hole, in a line stretching between R and R2. Each one emits photons that can be seen far away. And let's say our shell crosses R2 at time 0, for simplicity. As our shell closes in, the photons from the lamp closest to R are unable to escape after a certain point, because even though they started at or after T1 (which is before 0), they couldn't go fast enough to get past R2 by the time R2 became the new horizon. If you have another lamp that's farther away from R, then those photons have a little more time to get out, but even they're trapped if they leave after a certain time.

Our observer (who is very far away) can see from the photons that the lamps are getting snuffed out, one by one, until all the lamps inside of R2 are dark (as we'd expect from the inside of a black hole.) Thus, we can see that the horizon doesn't just jump between sizes, but expands smoothly at a rate dependent on the speed of the falling shell.

Isn't that creepy? Even creepier is in the case where the shell never falls in, like if it just stops and floats above the horizon or if it turns around and speeds away. In that case, you might expect the horizon to expand and then shrink again, but no! It never moves at all.

It knows the future.



In other news, I'm 22 now, it's a new year, I'm a Bible talk leader and a senior, I'm still single, and I have awesome people in my life. See you around! I'll fix typos later, I've gotta go.

"still single" is a weird way to define your singleness.

re: maybe that's because you're not a crazy insecure little punk
although I still think you should be scared of me
» Unicornasaurus on 2014-01-20 03:14:48

re: I can see how all of that would change things...although I can see a relationship clocking you from behind when you least expect it more than I can see one coming when you're actually ready for one.
» Unicornasaurus on 2014-01-20 09:56:52

black hole. r u studying some math subject in uni?

hey!!! the maple leaf graphic on the right changes colour!!!

RYC: oh? u use futureme.org too?
» renaye on 2014-01-26 07:28:02

RYC: haha. yes. u may say i haven't scrolled ur site for some time. anways, it is cool! i think it's time for me to revamp my site. it looks a little untidy, but i'm so poor in css codes!!! where did u get ur template?
» renaye on 2014-01-27 12:33:28

Coooooool.

Have they ever seen matter approach a black hole and then turn around and leave or is that all theoretical?
» Zanzibar on 2014-01-28 09:57:43

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