I suspect that anything solid will be torn apart by a combination of tidal gravitational forces, collisions and gamma/xray radiation well before the event horizon.
Oh, and I also suspect that anyone close enough to see is already too close…
The radiation levels will probably still do it, though.
From here, the Milky Way supermassive black hole (maybe 1 or possibly 4 million solar masses) is ingesting 2.3 x 10[sup]-9[/sup] solar masses per year for a power output of 2500 times the sun (outside the event horizon). Evidence indicates that power output could have been up to a million times current level 350 years ago.
An object falling into a black hole takes forever to cross the event horizon, but only as seen by an observer outside the horizon. For an observer on the infalling object (an astronaut in a spaceship, for example) the time taken to cross the horizon would be finite, and, I suspect, all too brief.
You couldn’t observe the infalling object from outside forever, because any light reflected from, or emitted by, the infalling object will be increasingly redshifted until it is completely undetectable.
Poul Anderson used this time dilation to chilling effect in his short story Kyrie, where an alien being in telepathic communication with a human galls into a black hole. Telepsathy in this story was some no -EM phenomenon, and (sorta magically) not subject to relativistic effects, so the woman could hear the alien’s death scream for the rest of her life (and it would continue on effectively forever afterwards).
So how can we see fully formed black holes form in the first place, if time is warped so much by that level of gravity? Shouldn’t we never see a black hole, but only stars starting to collapse?
(Of course I mean “see” in the general detection sense)
It’s not coincidence but necessity. Beyond the EH particles can escape, normal physics rules. Inside the EH time itself shifts. To escape a particle would essentially have to go into the past, which can’t happen. Get one of the physics mavens to talk about this or search on the million older threads. It’s pretty amazing.
I don’t think we’ve ever seen a black hole form. It’s not clear that we’ve ever actually seen a black hole - only the radiation that pours off of them. But we can see stuff around the black hole because the time effect is so extremely asymptotic. As soon as particles are away from the EH they are less subject to the dilation. As an analogy, it requires enormous amounts of energy to lift a rocket off the earth’s surface. Start from ISS and much less fuel is needed.
Sorry, I worded my question quite poorly, I now see. I didn’t mean to ask about seeing the formation process of the black hole, but why there can be black holes at all, because of time dilation all but stopping (to an outside observer) the formation process.
Right, there is actually a last photon emitted by an infalling object, and it’s actually not very long after when it “should” have crossed the horizon.
Also note that this is all based on a classical (i.e., non-quantum) model of black holes. We don’t have a quantum model of gravity yet, but by the time that you’re trying to argue that the black hole “hasn’t really formed yet”, millennia after the supernova, you’re pretty clearly pushing the classical model beyond its limits.
So time dilation reaches a maximum when the escape velocity of the gravitational potential reaches the speed of light?
Hey, if time dilation is tied to strength of gravitational field, does it have to be the SAME field? I mean, time is the same over the entire surface of Earth, but is it the same as another Earth 1 light-year away?
Bear in mind that black holes don’t have to have accretion disks; those form if a large amount of matter is falling into a black hole, from a companion star for instance. it’s an observer effect that by definition the only black holes we currently know about we spotted because they have accretion disks. And the energy from accretion disks should not be confused with Hawking radiation, which is unobservable for stellar mass or greater black holes.
First of all, it’s incorrect to define the event horizon as being where the escape speed is equal to c. Doing so naively will give you the correct value for the Schwarzschild radius, but that’s only a coincidence: There are two different errors that happen to cancel out.
Second, time dilation depends not on the gravitational field, but on the gravitational potential. Potential is analogous to height, while field is analogous to slope. Just as it’s possible for a very high mountain to not be very steep, or vice versa, it’s possible to have a large gravitational potential without a large field, or vice versa.
Actually, for finite mass distributions, as long as you’re outside the mass distribution (i.e. above the surface of the object), doesn’t same field imply same potential? Field varies as square of distance from centre of mass, so all that matters is the distance.
But potential varies as one over the distance, not distance squared. Double your distance from a given mass, and you’re at half the potential, but only a quarter of the field.
I mean 2 situations with the same field caused by a finite mass distribution have the same potential, not that the potential and field are the same for a situation (the units are different, they can’t be the same).