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worzel
2004-Jul-20, 08:04 AM
Light from outside a black hole's event horizon leaving perpendicularly to the surface reaches us at speed c but increasingly redshifted as the light source approaches the event horizon. When the light source gets to the event horizon what happens?

Does it still reach us at c but is redshifted out of existence

Does is just suddently stop dead in tracks at the eh

I know it has to do with the warping of spacetime but I can't get my head around the idea of a photon frozen in position but still moving at c unless time itself actually stops at the eh and so presumably goes backwards inside it.

CaptainToonces
2004-Jul-20, 08:30 AM
I'll take a stab but I am still learning this myself. I think, first of all it would be blue shifted as it approached the eh. If it actually came within the eh it would never escape as gravity would be so great it would continually force the photon's trajectory to the center of the black hole (that's the definition of the eh). If it managed a "near fly-by" of the eh, its trajectory would be bent according to proximity to the eh, and the shifting of its frequency would equal out to a far away observer. Please correct me if I'm wrong experts!

worzel
2004-Jul-20, 08:42 AM
I'm talking about light moving in exactly the opposite direction to the centre of mass of the black hole so it can't get bent and it definitely is redshifted (I think).

My problem is that there seems to be a discontinuity: 1mm outside the eh (or even 1nm) light heading out moves at c and yet at the eh it doesn't move :?

JohnD
2004-Jul-20, 08:58 AM
You are forgetting that this is in space AND time. As the photon approaches the EH, time slows for it. It is still moving at c, but will take longer than the Universe will exist to reach the EH.

BUT!
Stephen Hawking Alert!
Hawking has changed his mind about BHs and Hawking radiation! He is due to speak in Dublin tomorrow, 21st July, to say that BHs do NOT have a well defined EH, avoiding the 'information paradox'.

There is an abstract of the abstract on the Conference website at http://www.dcu.ie/~nolanb/gr17.htm
(click on the last item in the left column - "addition to plenary programme - Stephen Hawking")

But it needs translation, apart from the last sentence, "The way the information gets out seems to be that a true event horizon never forms, just an apparent horizon."
So the EH is not really there, just appears to be there!

John

Ut
2004-Jul-20, 01:34 PM
*blink*

Note to self: Do not study black holes.

worzel
2004-Jul-20, 01:42 PM
You are forgetting that this is in space AND time. As the photon approaches the EH, time slows for it. It is still moving at c, but will take longer than the Universe will exist to reach the EH.

Yeah I understand the idea of time slowing down nearer the BH, but for it to stop the photon escaping it must freeze time which to me seems to suggest that time must go backwards insdie the EH.

Also, we measure a photon that was fired just outside the EH and measure it's speed as c. But that photon being just outside the EH would presumably see much of the universe unravel before it reaches us due to the immense time dialation of the BH. How are these two points reconciled?


Stephen Hawking Alert!

I'm having enough difficutly getting my head around black hole basics as per relativity, I'll leave off the Hawkins for now thanks.

Wally
2004-Jul-20, 02:34 PM
Light within the EH doesn't stop. It's just travelling in a region of space that is completely warped by the massive gravity of the BH. Hense, it's "straight line" is actually what we'd perceive as a circle/spiral curve. It never "leaves" that region of space.

How's that?

Normandy6644
2004-Jul-20, 03:19 PM
Light within the EH doesn't stop. It's just travelling in a region of space that is completely warped by the massive gravity of the BH. Hense, it's "straight line" is actually what we'd perceive as a circle/spiral curve. It never "leaves" that region of space.

How's that?

Think of it this way. If you are at all familiar with the concept of "light cones," the direction - past, present, and future - determines all spacetimes events for any particular object. Gravity forces light cones to "tilt" towards the source of the gravitational field. When the gravitational field becomes too great (i.e., a black hole), the light cones are tilted completely inside the field, not allowing any events to take place outside. Thus as a photon crosses the event horizon, it's light cone becomes "trapped" inside, and never again visible to anything outside the black hole.

This may all turn out to be wrong soon, but it's the current picture as I understand it. :D

worzel
2004-Jul-20, 03:43 PM
Light within the EH doesn't stop. It's just travelling in a region of space that is completely warped by the massive gravity of the BH. Hense, it's "straight line" is actually what we'd perceive as a circle/spiral curve. It never "leaves" that region of space.

How's that?

Think of it this way. If you are at all familiar with the concept of "light cones," the direction - past, present, and future - determines all spacetimes events for any particular object. Gravity forces light cones to "tilt" towards the source of the gravitational field. When the gravitational field becomes too great (i.e., a black hole), the light cones are tilted completely inside the field, not allowing any events to take place outside. Thus as a photon crosses the event horizon, it's light cone becomes "trapped" inside, and never again visible to anything outside the black hole.

This may all turn out to be wrong soon, but it's the current picture as I understand it. :D

Thanks for the attempts. I have read many explanations similar to these and while think I can understand them they don't apply to a photon going in exactly the opposite direction to the BHs pull - that trajectory wouldn't be tilted leading me to this weird notion of a photon forever trapped on the EH, going nowhere at the speed of light.

Tensor
2004-Jul-20, 04:11 PM
Thanks for the attempts. I have read many explanations similar to these and while think I can understand them they don't apply to a photon going in exactly the opposite direction to the BHs pull - that trajectory wouldn't be tilted leading me to this weird notion of a photon forever trapped on the EH, going nowhere at the speed of light.

The problem worzel is you are trying to fit a concept where it doesn't apply. Even if the photon starts out moving directly opposite the BH's "pull", spacetime is so warped near the EH that the photon's motion is rapidly "tilted", so the photon's path is curved back into the BH. There is the additional warping effect of frame dragging that also affects the photon's path.

Ricimer
2004-Jul-20, 05:47 PM
yeah, I'll echo what a few others have said.

It turns around and heads back.

the path through space, and through time, all lead to the singularity. So even if you manage to be "stationary" you'll end up there due to time pointing that way. Its basically fate.

Disinfo Agent
2004-Jul-20, 07:04 PM
Light from outside a black hole's event horizon leaving perpendicularly to the surface reaches us at speed c but increasingly redshifted as the light source approaches the event horizon. When the light source gets to the event horizon what happens?

Does it still reach us at c but is redshifted out of existence

Does is just suddently stop dead in tracks at the eh

I know it has to do with the warping of spacetime but I can't get my head around the idea of a photon frozen in position but still moving at c unless time itself actually stops at the eh and so presumably goes backwards inside it.
Time is relative. The time at which the light source crosses the event horizon is not the same for you -- an outside observer -- and for the light source itself.
From its point of view, the source crosses the horizon and falls into to black hole after a finite time t. From your point of view, it never actually crosses the horizon.

black hole paradox (http://www.badastronomy.com/phpBB/viewtopic.php?p=245531&highlight=collapse+star#245 531)

This is a good description:

Does it still reach us at c but is redshifted out of existence

Disinfo Agent
2004-Jul-20, 07:11 PM
Also, we measure a photon that was fired just outside the EH and measure it's speed as c. But that photon being just outside the EH would presumably see much of the universe unravel before it reaches us due to the immense time dialation of the BH. How are these two points reconciled?

Will you see the universe end [as you fall into a black hole]?

If an external observer sees me slow down asymptotically as I fall, it might seem reasonable that I'd see the universe speed up asymptotically-- that I'd see the universe end in a spectacular flash as I went through the horizon.* This isn't the case, though.* What an external observer sees depends on what light does after I emit it.* What I see, however, depends on what light does before it gets to me.* And there's no way that light from future events far away can get to me.* Faraway events in the arbitrarily distant future never end up on my "past light-cone," the surface made of light rays that get to me at a given time.

That, at least, is the story for an uncharged, nonrotating black hole.* For charged or rotating holes, the story is different.* Such holes can contain, in the idealized solutions, "timelike wormholes" which serve as gateways to otherwise disconnected regions-- effectively, different universes.* Instead of hitting the singularity, I can go through the wormhole.* But at the entrance to the wormhole, which acts as a kind of inner event horizon, an infinite speed-up effect actually does occur.* If I fall into the wormhole I see the entire history of the universe outside play itself out to the end.* Even worse, as the picture speeds up the light gets blueshifted and more energetic, so that as I pass into the wormhole an "infinite blueshift" happens which fries me with hard radiation.* There is apparently good reason to believe that the infinite blueshift would imperil the wormhole itself, replacing it with a singularity no less pernicious than the one I've managed to miss.* In any case it would render wormhole travel an undertaking of questionable practicality.

John Dlugosz
2004-Jul-20, 10:32 PM
Light from outside a black hole's event horizon leaving perpendicularly to the surface reaches us at speed c but increasingly redshifted as the light source approaches the event horizon. When the light source gets to the event horizon what happens?

Does it still reach us at c but is redshifted out of existence

Does is just suddently stop dead in tracks at the eh

I know it has to do with the warping of spacetime but I can't get my head around the idea of a photon frozen in position but still moving at c unless time itself actually stops at the eh and so presumably goes backwards inside it.

As the light source gets closer to the Event Horizon, it appears to be redder and redder. It also gets dimmer and dimmer as photos are spaced out in time more. At some finite distance the last photon reaches you that ever will. This happens before the red shift becomes infinite.

worzel
2004-Jul-21, 01:31 AM
Thanks for all the great responses and links. Sorry if I seem a bit obtuse, this stuff does my head in.

So, now I have an even weirder (though more consitent) picture of whats going on. Everything that has ever fallen in will never appear to actually cross the eh from an outside point of view but just fade away due to redshifting (but never actually cross could I detect its redshifted light). But from the unfortunate objects' point of view they reach the singularity in a finite amount of time and see the universe come to an end as they do. All because of the slowing down of time to 0 at the EH from an outside frame of reference.

I don't understand disinfo's quote from Physics FAQ though - if I never see objects reach the EH then they must see me (or my descendants) pass through eternity :-?

Disinfo Agent
2004-Jul-21, 01:04 PM
But from the unfortunate objects' point of view they reach the singularity in a finite amount of time and see the universe come to an end as they do.
Not necessarily. In a nonrotating, uncharged black hole, the light source does not see the universe come to an end as it crosses the EH.
Furthermore, even in other kinds of black holes, you do not see the universe coming to an end while crossing the external EH. You see that as you cross a wormhole inside the black hole -- after you've already crossed the EH.

At least, this is how I read the Physics FAQ.

Disinfo Agent
2004-Jul-22, 02:37 PM
Here's another link about this:

What would happen to me if I fell into a black hole? (http://cosmology.berkeley.edu/Education/BHfaq.html#q3)