How can space know how to curve

[duordi]

How can space know how to curve from a black hole.
My understanding is that the no information can escape a black hole
so how can space know how much to curve?

Duordi

 

[PeterDonis]

We had a fairly recent thread on this:

https://www.physicsforums.com/showthread.php?t=658368

Also see the Usenet Physics FAQ article on "how does gravity get out of a black hole":

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html

 

[duordi]

Thanks for the reference.
There was some interesting discussion.

So technically nothing actually goes inside a black hole it is all frozen in time outside the black hole.

Duordi

 

[DiracPool]

"Nature cannot be fooled."

Margarine can fool mother nature.

 

[DiracPool]

So technically nothing actually goes inside a black hole it is all frozen in time outside the black hole

Technically, something does go into a black hole, and whatever that is does not come out. Sometimes a chair is just a chair.

 

[DiracPool]

so how can space know how much to curve?

Space doesn't "know" anything, it just does what it does, and what it does is curve or bend asccording to Einsteins GR equations. That is a first principal concept, not up for analysis unless you want to challenge the foundations.

 

[arindamsinha]

Space doesn't "know" anything ... what it does is curve or bend according to Einsteins GR equations...

Isn't that putting the cart before the horse?

 

[PeterDonis]

So technically nothing actually goes inside a black hole it is all frozen in time outside the black hole.

No. Things can fall into the black hole; it's just that nothing that happens inside the black hole's horizon can affect anything outside. So the gravity you feel outside the hole can't come from inside the hole; it comes from somewhere else. (As I noted in my posts in that thread, it actually comes from the past history of the object that collapsed to form the hole.)

 

[DiracPool]

Isn't that putting the cart before the horse?

What do you mean by that? First of all, Einsteins GR equations are non-linear, and therefore are only solvable via numerical integration, with perhaps the exception of the shperical Schwarchild thinga-ma-bobber.

 

[arindamsinha]

What do you mean by that? First of all, Einsteins GR equations are non-linear, and therefore are only solvable via numerical integration, with perhaps the exception of the shperical Schwarchild thinga-ma-bobber.

I meant Einstein's equations describe how space curves, rather than space curving to keep in line with Einstein's equations. Just joking!

BTW, question to anyone who may know the answer - is space 'bending' a scientific term (as opposed to 'curving')?

 

[DiracPool]

BTW, question to anyone who may know the answer - is space 'bending' a scientific term (as opposed to 'curving')?

What's the diff? Sounds like the same thing to me.

 

[PeterDonis]

is space 'bending' a scientific term (as opposed to 'curving')?

"Curving" is the term I usually see in papers (or popular books and articles) written by scientists; "bending" seems to be a more colloquial term used by laypeople.

 

[PeterDonis]

First of all, Einsteins GR equations are non-linear, and therefore are only solvable via numerical integration, with perhaps the exception of the shperical Schwarchild thinga-ma-bobber.

This is not correct. There are many exact, analytical solutions known to the Einstein Field Equation:

http://en.wikipedia.org/wiki/Exact_solutions_in_general_relativity#Types_of_exact_solution

Nonlinear differential equations are harder to solve than linear ones, but "harder" is certainly not the same as "impossible".

 

[DiracPool]

This is not correct. There are many exact, analytical solutions known to the Einstein Field Equation:

Ok, so maybe there are more exact solutions than the Schwartzchild one, but, in fact, Einstein's field equations are notorious for being intractable analytically, for exactly the reason I mentioned, the non-linearlity of the differential equations. In neuroscience, we have the same problem with Freeman's KV model which couples thousands of non-linear ODE's to model the solutions. Impossible to solve even the simplest scenarios analytically.

 

[PeterDonis]

Ok, so maybe there are more exact solutions than the Schwartzchild one, but, in fact, Einstein's field equations are notorious for being intractable analytically, for exactly the reason I mentioned, the non-linearlity of the differential equations.

This is true, but it's not as extreme as it appears to be in neuroscience by your report:

Impossible to solve even the simplest scenarios analytically.

The exact solutions listed on the page I linked to include "simple scenarios" that are pivotal in our understanding of the universe:

(1) The black hole solutions (generalized Kerr-Newman, which includes Schwarzschild as a special case but also includes rotating and charged holes);

(2) The FRW solutions, which are central in cosmology.

It is true that for more complicated cases, which lack the symmetry of the above classes of solutions, we use numerical simulations; but the exact solutions above have given a lot of insight into the key factors involved. So analytical solutions, idealized as they are, are extremely important in GR.

 

[DiracPool]

It is true that for more complicated cases, which lack the symmetry of the above classes of solutions, we use numerical simulations; but the exact solutions above have given a lot of insight into the key factors involved. So analytical solutions, idealized as they are, are extremely important in GR.

Point taken. You still haven't addressed my assertion that margarine can fool mother nature, though...

 

[PeterDonis]

You still haven't addressed my assertion that margarine can fool mother nature, though...

You're right, I haven't. I think I'm a counterexample; at least, I am if "margarine" is broadly interpreted to mean "butter substitute". Those haven't helped me lose any weight.

 

[DiracPool]

I think I'm a counterexample; at least, I am if "margarine" is broadly interpreted to mean "butter substitute". Those haven't helped me lose any weight.

I still can't believe it's not butter..