Black Holes and Time

a reply to: kykweer

YES

NO.

Wildespace (who usually does know what he's talking about) has forgotten in this case that time dilation is always reciprocal. Saint Exupery is correct.

originally posted by: Astyanax
a reply to: kykweer

YES

NO.

Wildespace (who usually does know what he's talking about) has forgotten in this case that time dilation is always reciprocal. Saint Exupery is correct.

Relativistic time dilation is reciprocal. Gravitational time dilation isn't. Hypothetical people living on a hypothetical planet that's orbiting very near a black hole will see the rest of the universe sped-up and blueshifted.

I'm pretty sure the time dilation we've been talking about here is due to the black hole's gravity. Of course, this is more complicated by the fact that the traveller will also be speeding very very fast towards the event horizon, actually approaching the speed of light. That brings relativistic time dilation into play, which will add some slowing down to the observed universe, but I still think that gravitational time dilation will be the prevailing factor.

Some physics buffs might correct me on that.

originally posted by: Astyanax
a reply to: kykweer

YES

NO.

Wildespace (who usually does know what he's talking about) has forgotten in this case that time dilation is always reciprocal. Saint Exupery is correct.

Lol yeah, you have an odd way of bringing your point across. This post is more of a thought experiment and maybe just fantasy of what goes on inside a black hole and how time is experienced inside the event horizon.

originally posted by: Saint Exupery

As the star collapses, at the moment the density increases to the point where the event horizon forms, the material will be divided into that which is within the event horizon and that which is not. That which is within the event horizon falls into the singularity. For us intrepid observers observing from a safe distance, the collapsing material outside the event horizon cannot (under General Relativity) ever fall through the horizon within the lifetime of the universe. Thus all of the mass of material falling into a black hole would appear to plate itself around the outside of the event horizon. However, observing this monatomic shell would be very difficult, since the light from it would be drastically red-shifted into the far radio spectrum.

I think most of us will agree to that and can understand, but it is not enough. What happens INSIDE the event horizon.

originally posted by: Saint Exupery

originally posted by: kykweer;

As the star collapses, at the moment the density increases to the point where the event horizon forms, the material will be divided into that which is within the event horizon and that which is not. That which is within the event horizon falls into the singularity. For us intrepid observers observing from a safe distance, the collapsing material outside the event horizon cannot (under General Relativity) ever fall through the horizon within the lifetime of the universe.

How does (from the outside observer's point of view) the black hole ever grow in mass if (again, from the outside observer's point of view) nothing ever falls through the event horizon?

Well, I seriously doubt black holes, as described in current Physics, exist at all.

First of all, frame dragging has not been proved experimentally.

Secondly, Quasars cannot be explained by General Relativity.

Thirdly, there are some controversies concerning black holes: if a black hole cannot let anything escape, not even light, then how come it emits radiation? radiation is particles, and their escape velocity is the speed of light. Since light cannot escape, so should radiation.

Fourthly, time dilation is contradictory by itself and actually invalidates General Relativity. According to time dilation, an observer A moving away from from another observer B will experience slower time than B. However, due to frame equivalence, the observer B is also moving away from observer A, and hence the observer B must experience slower time than A.

So General Relativity is an invalid theory and so are black holes.

originally posted by: masterp
Thirdly, there are some controversies concerning black holes: if a black hole cannot let anything escape, not even light, then how come it emits radiation? radiation is particles, and their escape velocity is the speed of light. Since light cannot escape, so should radiation.

The radiation or particles escape from just outside the event horizon, not from inside it.

Fourthly, time dilation is contradictory by itself and actually invalidates General Relativity. According to time dilation, an observer A moving away from from another observer B will experience slower time than B. However, due to frame equivalence, the observer B is also moving away from observer A, and hence the observer B must experience slower time than A.

So General Relativity is an invalid theory and so are black holes.

I don't see how you arrive at that conclusion. Relativistic time dilation is different from gravitational time dilation, so one doesn't invalidate the other.

The experiments using precice clocks flown around the Earth at high speed and stationary clocks on the ground accounted for both relativistic and gravitational time dilation.

originally posted by: masterp
Thirdly, there are some controversies concerning black holes: if a black hole cannot let anything escape, not even light, then how come it emits radiation? radiation is particles, and their escape velocity is the speed of light. Since light cannot escape, so should radiation.

Two things:

Thing 1:
If you mean the x-ray jets that are emitted from a black hole, those jets are not emitted from the black hole itself (not from inside the even horizon), but from the accretion disk surrounding a black hole. The accretion disk is the disk of material that is being drawn into the black hole. However, that material has not yet past the event horizon, so that material is not hopelessly lost to the black hole's gravity; theoretically you can go to the accretion disc, grab some material, then leave.

The accretion disk swirls around the black hole so fast that massive amounts of frictional energy builds up, which is released from the black hole's accretion disk as x-rays.


Thing 2:
If by radiation, you mean Hawking Radiation, then that really isn't coming from the actual black hole (inside the event horizon) either. At least not exactly.

Hawking radiation is not exactly the radiating away of the actual material that fell into a black hole; it instead works in a more indirect way. In Hawking radiation, an entangled pair of virtual particles pop into existence near a black hole's event horizon (a particle plus its anti-particle). One of those particles crosses over the event horizon and gets inescapably caught in the gravity of the Black hole, while the other does not cross the event horizon, and it escapes.

Due to the conservation of total energy, the one that fell in would be negative energy, causing the black hole to lose one particles worth of energy, while the other entangled positive particle is left over, appearing (but only appearing) to have been radiated from the black hole.

So even Hawking radiation is not exactly something finding its way out of the grasp of a black hole.

a reply to: Soylent Green Is People
I find it interesting that you're able to comprehend (and explain) the above, but cannot comprehend that, for an external observer, a black hole grinds the time at the event horizon to a halt.

originally posted by: Soylent Green Is People

originally posted by: Saint Exupery

originally posted by: kykweer;

As the star collapses, at the moment the density increases to the point where the event horizon forms, the material will be divided into that which is within the event horizon and that which is not. That which is within the event horizon falls into the singularity. For us intrepid observers observing from a safe distance, the collapsing material outside the event horizon cannot (under General Relativity) ever fall through the horizon within the lifetime of the universe.

How does (from the outside observer's point of view) the black hole ever grow in mass if (again, from the outside observer's point of view) nothing ever falls through the event horizon?

I think the mass was created from the collapsing star till a point where the event horizon is created? So you would technically see the star hypernova into a black hole.

originally posted by: wildespace
a reply to: Soylent Green Is People
I find it interesting that you're able to comprehend (and explain) the above, but cannot comprehend that, for an external observer, a black hole grinds the time at the event horizon to a halt.

I comprehend that time grinds to a halt. But that's the paradox that I can't grasp.

The paradox being this: If time grinds to a halt to the outside observer, then WHEN does anything ever fall into the black hole for the outside observer?

The answer to that question seems to be "never". However, if the outside observer can never know of anything falling into a black hole, then how does a black hole ever gain mass from the viewpoint of the outside observer.

I suppose my question that needs to be answered is this:
According to time dilation, nothing can ever fall in from our point of view -- so then, when and from where did the additional mass of a growing black hole (which is known to have gained mass in a finite amount of time from our point of view) get there? If an outside observer can observe a black hole gaining mass, then where did that mass come from?

It seems to me that sometime in a finite time span during the history of the universe from our point of view, black holes do in fact gain additional mass by having things fall through their event horizons -- it's not something that takes a infinite amount of time from our point of view to happen.

That's the paradoxical issue I am having.



EDIT TO ADD:

Actually, answer me this:

During the finite history of our universe, have black holes gained mass?

If the answer is "yes", then it seems to me that if we had the ability to monitor the mass of a black during that finite time, then we would be able to monitor the a difference in its mass from when it was less massive to the time it was more massive -- i.e., there would be a point in time that we would be able to monitor it growing in mass.

So obviously during that finite period of time that we were monitoring it, we would know that something fell through the event horizon that caused the black hole to grow in mass. That seems counter to the notion that, from our point of view, it takes an infinite amount of time for something to fall through.

a reply to: Soylent Green Is People

Very interesting questions.

Firstly, (and theoretically, of course) stellar and larger black holes are indeed gaining mass, due to any infalling material and also due to the temperature of the Microwave Background Radiation being higher that the overall temperature of Hawking Radiation from such large BHs (so they are basically absorbing that energy faster than they're losing their own).

As for observing this increase in mass: we can't see anything beyond the event horizon by definition, so while we'd probably be able to observe the increase in mass of a BH (and we observe that by the gravitational effects on other bodies), we'd have no information about how that increase happened.

That's just my 2 cents here.

This thread has got my head spinning and smoke coming from my ears. Lots of interesting things and thoughts in here... I'm surprised nobody has mentioned (theoretical) white holes. Imagine instead of not being able to escape the force of gravity, no amount of force could ever get you TO the white hole. Can you enter a black hole and come out of a white hole? Does time go faster and faster as you get near a white hole? I don't mean to derail...maybe white holes need their own thread.

a reply to: wildespace

Relativistic time dilation is reciprocal. Gravitational time dilation isn't.

The principle of equivalence says you are wrong. In addition, there is very high relative velocity between external objects and the infalling astronaut.

originally posted by: Astyanax
a reply to: wildespace

Relativistic time dilation is reciprocal. Gravitational time dilation isn't.

The principle of equivalence says you are wrong. In addition, there is very high relative velocity between external objects and the infalling astronaut.

The general theory of relativity describes how, for both observers, the clock that is closer to the gravitational mass, i.e. deeper in its "gravity well", appears to go more slowly than the clock that is more distant from the mass. In the situations of relativistic time dilation, each of the observers see the other's clock as moving slower (a reciprocal effect). With gravitational time dilation, both observers agree that the clock nearer the mass is slower in rate, and they agree on the ratio of the difference (time dilation from gravity is therefore not reciprocal).

en.wikipedia.org...