Whitfield Diffie <whitfield.diffie@gmail.com> wrote:
"Keith F. Lynch" <kfl@KeithLynch.net> wrote:
You could survive a fall through the event horizon of a quiescent galactic-mass black hole. (Of course you wouldn't survive for long after that, as you approached the singularity.
I don't see why. If a black hole is massive enough, (my crude calculation, which those with better ALUs are welcome to correct, is 3.65x10^18 solar masses or about four million times the mass of the Miky Way galaxy) it has about the density of our galaxy as a whole and might be habitable.
Time and space are swapped inside the event horizon. Once you're inside, the singularity is not some distance ahead of you, it's some time in your future. (As such, the more powerful your engine, the *sooner* you'd hit the singularity. To live as long as possible, remain in free fall.) Of course if the black hole is big enough, you would die of old age before you reached the singularity. But it would have to be very large indeed. Even a galactic mass black hole only gives you a few minutes.
What this leads to is the question can the universe be distinguished from a black hole?
The fact that it's expanding rather than contracting. Charles Greathouse's post reminded me of the firewall conjecture. If that's true, you can't fall through any event horizon without being incinerated. Henry Baker <hbaker1@pipeline.com> wrote:
According to a recent talk given at UCSB, the largest known black holes have a Schwarzschild radius of the same order of magnitude as the radius of the Earth's orbit -- i.e., 1 AU.
Light would orbit this black hole in a few minutes; I would presume that an Earth-sized object would have to orbit at nearly the speed of light even at 5 AU (Jupiter's distance from the Sun).
That orbit wouldn't be stable. The planet would lose energy by emitting gravitational radiation, and within a few orbits would spiral into the black hole. Of course everything about black holes is conjectural. Some of it -- the division by zero part -- pretty much has to be wrong. GR has never been tested except in the weak field limit. What is known for certain is that there are non-luminous objects so dense that no known force could prevent their total collapse, and that one of them is in the center of our galaxy. There are several non-GR theories about these objects, but all such theories are, in my opinion, uglier than GR, and there's no evidence for them.
What this leads to is the question can the universe be distinguished from a black hole?
The fact that it's expanding rather than contracting.
I thought of that between posts but it fills me with a charming amateur idea. Black holes expand as they pick up mass. Perhaps the dark energy is the effect of something falling into the universe. Is it known that the universe is declining in density? Whit
Re Earth-sized object orbiting black hole at 5 x Schwarzschild radius: Are you sure that such a small object like the Earth would lose that much energy in a few orbits? All of the simulations that I've seen show _similar-sized_ objects -- e.g., binary black holes -- converging in a few orbits. But the Earth is such a small mass relative an Earth-orbit-sized black hole that I wouldn't think that gravitational radiation would cause the orbit to collapse so quickly. Tidal forces would likely rip the Earth into pieces & possibly schmear it out like Saturn's rings, but I'm skeptical about it collapsing so quickly from gravitational radiation. At 03:37 PM 10/20/2013, Keith F. Lynch wrote:
Henry Baker <hbaker1@pipeline.com> wrote:
According to a recent talk given at UCSB, the largest known black holes have a Schwarzschild radius of the same order of magnitude as the radius of the Earth's orbit -- i.e., 1 AU.
Light would orbit this black hole in a few minutes; I would presume that an Earth-sized object would have to orbit at nearly the speed of light even at 5 AU (Jupiter's distance from the Sun).
That orbit wouldn't be stable. The planet would lose energy by emitting gravitational radiation, and within a few orbits would spiral into the black hole.
participants (3)
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Henry Baker -
Keith F. Lynch -
Whitfield Diffie