From Newsgroup: sci.physics.relativity
Stefan Ram wrote:
You can dive into a black hole and another person can dive
into another black hole way off, and you wind up meeting inside
[_Fall_, not "dive". "Dive" implies that you go below some surface, and
that you can come back to that surface ;-)]
Yes.
(does not work with Schwarzschild black holes),
False. The maximally extended Schwarzschild solution includes the
possibility of another (perhaps mirror or parallel) universe. The
respective Penrose diagram looks like this:
time (e.g. the T-coordinate of Kruskal--Szekeres coordinates)
^
: .-------------------. <-- singularity (r = 0)
: `. .'
: `. black .'<-- outer event horizon (r = R_S)
: `. hole .'
: `. .'
: other `.' our universe
: universe .' `.
: .` `.
: .' white `.
: .' hole `.
: ' `
'---------------------------> space (e.g. the X-coordinate of K.--Sz.)
If a person in our universe falls into a black hole, and a person in the
other universe falls into the same black hole, then, in theory, both can
meet inside, and can tell each other about their respective universes:
time
^
: .-------------------. <-- singularity (r = 0)
: `. black hole .'
: `. -. .- .'<-- outer event horizon (r = R_S)
: `.': :`.'
: .' `. .' `.
: other `.' our universe
: universe .' `.
: .` `.
: .' `.
: .' `.
: ' `
'---------------------------> space
Unfortunately, though, they cannot tell anyone else because (classically)
still no information gets out the black hole (in a Penrose diagram, lines at
an angle of +-45-# to the horizontal indicate lightlike geodesics, so the information would have to be faster than c to get out). And unless it is
not actually a Schwarzschild black hole, both will die together at some
point on their way falling together towards the singularity; at the latest
at the singularity.
[In fact, to learn of other universes inside a black hole is even
possible without a second person. But you would still not be able
to tell anyone about it if it is a Schwarzschild black hole.]
See also:
Thomas Lahn: General Relativity (public playlist)
Veritasium: Something Strange Happens When You Follow Einstein's Math <
https://www.youtube.com/watch?v=6akmv1bsz1M&list=PL41EYJuJ5YuDn3d13ryZwpzGBXewXa9AH&index=11>
[The way the coordinate transformations from Schwarzschild to
Kruskal-Szekeres coordinates etc. are demonstrated there by morphing the coordinate grid is just beautiful :)
The video is quite good, but unfortunately it repeats the common
misconception of "not even light can escape". While that is true in a
sense, it does not have anything to do with that the "escape velocity" (actually: escape speed) would be c at the outer event horizon. See also Andrew J. S. Hamilton's "Inside Black Holes", <
https://jila.colorado.edu/~ajsh/insidebh/index.html>, from which some of
the Penrose diagrams in the video are taken, for details. See also the many references in the video description.]
However, there is an interesting fact: When the person in our universe falls into the black hole, that increases its mass, which increases the radius of
its event horizon, i.e. causes the black hole to grow; and that could
*cause* the person in the other universe to fall into it as well (if their speed is not fast enough), or simply *be* in it, even *retroactively*:
time
^
: .-------------------. <-- singularity (r = 0)
: `. black .'
: `. hole .'<-- outer event horizon (r = R_S)
: '-. .^'
: person -->*. .' :
: in other .'. *<-- person in
: universe .' `. our universe
: .` `.
: .' `.
:
'---------------------------> space
provided both black holes came out of particles that are each
entangled with a particle the other black hole came out of.
Nonsense, or at least highly speculative.
This was what I remember after having watched "The Quantum Origins
of Gravity" (Oscar Klein Memorial Lecture 2018) by Leonard Susskind.
I suggest that you watch it again because you might misremember.
--
PointedEars
Twitter: @PointedEars2
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