Black hole dynamics

Sir Cumference

Aug 10, 2016 22:11

Well, does time dilation increase to infinity as I approach a black hole's absolute horizon, xor apparent horizon?

user54412

(English really could use the word "xor" some days.)

2

Sir Cumference

Fine

Actually, that doesn't work...

Screw it

0celo7

what is xor

Sir Cumference

Ya ever heard of a logic gate?

user54412

An apparent horizon should be good enough to see whatever redshift you want.

Sir Cumference

Aug 10, 2016 22:14

If it's an XOR gate, then the output will be true if only one of the inputs is true

False if both or neither are true

@ChrisWhite So as I approach the apparent horizon, time dilation increases to infinity?

user54412

sure

Sir Cumference

Cool

Next question

If I approach the event horizon of a black hole, and time dilation increases, could the black hole evaporate through hawking radiation before I reach the event horizon?

user54412

not exactly sure where the time dilation comes into this

Sir Cumference

Well, if I approach the black hole and time dilation increases, I would see the rest of the Universe speed up quickly, wouldn't I?

Before I reach the event horizon, could the black hole have already evaporated?

user54412

first, you should be very careful about applying that logic, since in SR the same thing would tell you the moving spaceship sees Earth age faster than normal

user54412

Aug 10, 2016 22:20

second, if you want to think about redshift factors, there's a fundamental asymmetry between what's inside your radius and what's outside

user54412

the speed at which you see the rest of the universe going by is not a priori the speed at which you'd see the black hole below you evolving

Sir Cumference

So I would inevitably reach the event horizon, right?

user54412

more fundamentally, the issue is that if the black hole evaporates, then there is no event horizon

Sir Cumference

You just said the speed at which the black hole evolves is not necessarily the speed at which the rest of the Universe evolves

user54412

well, I guess there's some speculation about point objects (string-sized objects?) being left over to solve information issues, but let's put that aside

Sir Cumference

Aug 10, 2016 22:25

So if a black hole were pulling me in, would I eventually reach the event horizon?

user54412

@SirCumference but an event horizon is defined by the global structure of spacetime -- it is nothing more or less than the surface delineating what things can send null signals to the point at infinity

user54412

if it turns out that even stuff near/entering/inside a black hole can eventually make it to infinity just by waiting the finite time until the black hole doesn't even exist, then there is no event horizon

user54412

nowhere/nowhen in spacetime

Sir Cumference

You literally just said that the black hole may evolve at a different speed from the rest of the universe

Doesn't that imply that I would eventually reach the event horizon, before the black hole evaporates?

user54412

@SirCumference sure, that statement is true within the bounds of its own ambiguity

user54412

Aug 10, 2016 22:28

@SirCumference the event horizon is not a thing that can exist for some times and not others

Sir Cumference

@ChrisWhite Then how can a black hole possibly evaporate?

Bernard Meurer

@ChrisWhite Is it wrong to make bets about someone's life?

Sir Cumference

Doesn't the event horizon form when the black hole forms, and disappear when the black hole evaporates?

user54412

@SirCumference that's too much of a simplification, and it's one of the reasons the notion of apparent horizon was invented in the first place

Sir Cumference

@ChrisWhite All right, so how exactly is it incorrect?

user54412

Aug 10, 2016 22:30

@BernardMeurer That's an oddly out of context moral question.

Bernard Meurer

@ChrisWhite Is that a no?

And thanks, they're my specialty :)

user54412

Pragmatically, it might be illegal in some jurisdictions. Deontologically a la Kant I can't see what's contradictory about a world in which such bets were made. Anarcho-libertarian capitalism would allow such things, and possibly even encourage them as raising GDP. A true consequentialist would probably be bound to say it's wrong iff it makes it more likely that someone I like dies or someone I don't like lives.

user54412

@SirCumference This really depends on what you think happens when the black hole evaporates. Is there something unevaporable left?

Sir Cumference

@ChrisWhite Er, I wouldn't know. I'm the one asking about evaporation.

user54412

well, if there really is nothing left, then there never was an event horizon

Bernard Meurer

Aug 10, 2016 22:37

@ChrisWhite I like Kant, so I'll go with that; thanks!

Sir Cumference

So, once again, how can a black hole possibly evaporate?

Or how can one form at a specific point in time, if the event horizon cannot exist for some times and not others

user54412

@BernardMeurer Really the question is whether this concerns certain people in the US or certain people in Brazil

user54412

though perhaps it's best if you don't answer that in public :p

Bernard Meurer

I was betting that the guy climbing trump tower would survive

user54412

wait wat?

Sir Cumference

Aug 10, 2016 22:40

@BernardMeurer I just heard about that a few seconds ago

Bernard Meurer

What what?

A guy is/was climbing trump tower with suction cups

but the police just grabbed him inside

user54412

user54412

33 seconds -- nice timing

Bernard Meurer

I've been watching it for a while now

Sir Cumference

@ChrisWhite So if a black hole has an event horizon, and event horizons aren't something that exist some times and not others, then how can a black hole form or evaporate?

This seems like a contradiction

user54412

Aug 10, 2016 22:43

there are tacit approximations made when speaking about these things

Sir Cumference

What kind of approximations?

user54412

there's "black hole" -- the eternal, static Schwarzschild GR solution

user54412

then there's "black hole" -- the finitely old astrophysical object that theory says will evaporate in $10^{10^{10}}$ years or so

Sir Cumference

You're saying the latter is an approximation of the former?

They seem completely contradictory

user54412

the Schwarzschild solution is very close to reality, and much simpler to work with

Sir Cumference

Aug 10, 2016 22:45

But the latter is reality as we understand it, right?

user54412

yes

Sir Cumference

Or as close an approximation as we can get

So back to my original question

Emilio Pisanty

@ChrisWhite The Trump tower climber? Indeed, except that sometimes the tacit approximations used by different people differ rather wildly.

user54412

especially when taking limits as time goes to infinity, there's a real risk of interchanging limits that ought not to be swapped

Sir Cumference

Ignoring the approximated Schwarzschild solution, will a black hole always evaporate before I reach it?

user54412

Aug 10, 2016 22:47

the tower climber does look pretty happy with life

user54412

definitely not a grad student then

user54412

@SirCumference yes, but I reserve the right (and chat moderation power) to edit this message in the future if it proves convenient

Bernard Meurer

@ChrisWhite He is clearly a poor chess player

Sir Cumference

@ChrisWhite Hm, so is there a black hole so large, it can't possibly kill me through tidal forces?

user54412

@SirCumference yes, the one at the center of the galaxy

Sir Cumference

Aug 10, 2016 22:52

@ChrisWhite Interesting. If I were to approach it, soon enough I would find myself at the point in time when the black hole has evaporated, right?

user54412

I suppose

user54412

though

user54412

as it evaporates it gets smaller, so the tidal forces increase

Sir Cumference

Crap

So any black hole could theoretically tear me apart...

That sucks.

0celo7

Just like any sufficiently motivated woman

never forget that

The h Bar

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