About things appearing to take forever to fall into black holes

[AaronX]

To an external observer, an object falling into a black hole would take forever and never appear to reach it, due to gravitational time dilation.

1. Where is the boundary that the object never crosses? Wikipedia says it’s the Event Horizon:

Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.

Is it just coincidence, or is there a reason why it’s the EH?

2. Is this an observable phenomenon, or would the object be outshined by the accretion disk?

I’m thinking this would be a great way to be buried, since the body would be there “forever”.

[si_blakely]

@AaronX

To an external observer, an object falling into a black hole would take forever and never appear to reach it, due to gravitational time dilation.

…

2. Is this an observable phenomenon, or would the object be outshined by the accretion disk?

I’m thinking this would be a great way to be buried, since the body would be there “forever”.

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…

Si

[AaronX]

Maybe not?

The point at which tidal forces destroy an object or kill a person depends on the black hole’s size. For a supermassive black hole, such as those found at a galaxy’s center, this point lies within the event horizon, so an astronaut may cross the event horizon without noticing any squashing and pulling, although it remains only a matter of time, as once inside an event horizon, falling towards the center is inevitable.

From the article on spaghettification.

[si_blakely]

I had forgotten about that.

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.

It is going to be warm when you get up close.

Si

[eburacum45]

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.

[CalMeacham]

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).

[Revtim]

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)

[Exapno_Mapcase]

@AaronX

Is it just coincidence, or is there a reason why it’s the EH?

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.

@Revtim

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?

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.

[Revtim]

@Exapno_Mapcase

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.

Instead of trying to explain the question myself, I Googled it and found a better explanation of the situation, coincidentally from SDMB alumni ‘Bad Astronomer’:
http://blogs.discovermagazine.com/badastronomy/2007/06/19/news-do-black-holes-really-exist/

Einstein showed that as gravity increases, your clock runs slower. Literally, if you have two people, one guy up high above a black hole, and another guy close in, the guy outside sees the close-in guy’s clock running slower. Literally, time flows more slowly near an object with gravity, and the stronger the gravity the slower time flows relative to an outside observer. For a black hole, time literally stretches to infinity at the event horizon. Clocks stop. Update: Well, I was being glib. Actually they continue to slow, ever approaching stopping but never actually reaching it. I was trying to simplify, but oversimplified — I make similar comments below in this entry, so where you read that things stop, think of it as “slowing almost to but never quite reaching zero”. Read the comments thread below for details.

This brings up a very interesting situation. If time takes forever to flow, then how does a black hole ever form? Imagine the core collapsing, and you’re looking at it from far away. You see it getting smaller, but the collapse also appears to be going more slowly because of the time dilation. Like Zeno’s paradox, you see the escape velocity approach the speed of light, but you’ll never see it actually get to the speed of light! Time would stretch out infinitely, and the collapse of the core would appear to you to stop.

[Revtim]

And to include his conclusion (sorry I didn’t before)

However, have a care here: there is still a massive, dense, highly gravitational object there! So we still have what are essentially black holes in the cores of galaxies and forming when stars explode and all that, it’s just that, technically, well, they aren’t actually black holes.

[Chronos]

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.

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.

[notsoheavyd3]

http://en.wikipedia.org/wiki/Black_hole#Event_horizon

From this link

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[46] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[47] At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[48] Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

IE it actually disappears before it his the event horizon. (It winks out.)

[AaronX]

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?

[Lumpy]

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.

[Chronos]

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.

[AaronX]

So does it have to be the same potential?

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.

[Chronos]

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.

[AaronX]

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).

[Chronos]

But the different scaling makes that impossible, too, unless the total mass is the same for both situations.

[AaronX]

Oh, that’s right. What about the time dilation thing?