Henry Baker <hbaker1@pipeline.com> wrote:

Keith F. Lynch wrote:

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Henry Baker <hbaker1@pipeline.com> wrote:

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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).

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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.

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.

Gravitational radiation is proportional to mass. So, of course, is the object's inertia which resists spiraling in. The two effects cancel out. So objects will spiral in at a rate independent of their masses. It would contradict the uniqueness of free fall and the equivalence principle if this wasn't the case.

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.

The tidal effect at the event horizon depends on the size of the black hole. A sufficiently large one would have no more tidal effect on a planet passing through its event horizon than the moon has on our planet. Whether a black hole that large exists, I don't know. There's the claimed "firewall effect," which would incinerate anything passing through the event horizon. Whether it exists, I don't know. I suspect not, since there are event horizons that have nothing to do with black holes, and those definitely don't have a firewall effect. Why should black hole event horizons behave differently? One example of such an event horizon is the edge of the observable universe, beyond which objects are receding from us faster than light. (The speed limit only applies relative to a local observer.) And we're on that cosmological event horizon to distant (hypothetical) observers. Another example is that there's an event horizon behind anyone who is accelerating. It's about 10/A light years behind you, where A is your acceleration in meters per second per second. As long as you maintain that acceleration, light, matter, signals, etc., from behind that horizon will *never* catch up to you. Once again, there are (hypothetical) observers to whom we are right on the event horizon. And yet it's still cold outdoors, at least where I live.