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thkaufm
2003-Feb-07, 03:08 AM
"If you throw an object straight up, it will rise until the the negative acceleration of gravity stops it, then returns it to Earth"

If light is shot straight up away from a black hole what happens to it?

Tom

irony
2003-Feb-07, 03:29 AM
Well, since light can't slow down I believe what happens is that the wavelength stretches out, and just gets longer and longer, and weaker and weaker, until there's no possible way to detect it. But I'm not an expert. There's probably somebody here who is, though.

JS Princeton
2003-Feb-07, 04:54 AM
As with many other things in relativity, it depends on where you look. Locally, it will just appear that light goes off as normally. This is because all areas have to be locally flat (in the special relativity limit). Globally, the lightbeam will follow a geodesic. In the case of emitting the lightbeam exactly at the event horizon it will continue to climb out of the potential well forever. It will never leave, but just sit at the horizon assymptotically. Of course, it's impossible to get the angle exactly right, so you're either going to have an eventual escape or a geodesic that heads into the black hole

irony
2003-Feb-07, 05:01 AM
What he said. /phpBB/images/smiles/icon_biggrin.gif

DStahl
2003-Feb-08, 08:41 AM
I'm trying to visualize what would happen inside the event horizon. Let's assume it's a really big black hole, with an event horizon bigger than the mean circumference of Neptune's orbit, and that it is quiescent and no matter is falling into it except our doomed experimenter.

So, our experimenter shines her laser pointer directly away from the center of the black hole. Now, what I think happens is this: she is falling toward the center of the black hole, as she must inevitably do once she is inside the event horizon. From her frame of reference she sees the light shine upward (away from the black hole) normally. But if she could be viewed from some mythical "pseudo-time" reference frame one would see that she is falling so fast, and her timeflow has become so extreme, that the laser light actually does not shine away from the black hole's center--in fact, it is also being drawn down toward the center. The lovely, lightspeed photons in the laser beam aren't falling as fast as the doomed experimenter, but they are still falling toward the black hole's center--or, perhaps, their spacetime geodesics are still being bent into the spacetime gradient curving down toward the black hole singularity.

---

OK, JS Princeton, did I get it more or less right? Or did I bungle it badly? /phpBB/images/smiles/icon_wink.gif

JS Princeton
2003-Feb-09, 12:55 AM
Good run, Dstahl.

Inside the event horizon is a tricky subject. We haven't ever observed it, so it's going to be hard to answer the question. We can, however, look at what the equations tell us.

Here's the scoop: we're going to look at the most basic type of black hole (no charge, no spinning) because you can actually have some interesting time machine effects with angular momentum and charge in a black hole. In the normal Schwartzchild metric, if you are on the inside of the event horizon you are being pulled into the black hole's singularity (we're not sure exactly how singularities work, but we'll suppose that there is some singularity and empty space in between the singularity and the event horizon). Okay, so you shine your flashlight. The lightbeam has to take a geodesic that will intersect the singularity. Your path will also intersect the singularity. This means you're both heading for the singularity, but it so happens the geodesic for the lightbeam is longer than yours so it looks locally like you shot a lightbeam straight out.

Having the other reference frame described by Dstahl is actually an impossible view. This is because from that reference frame the person falling into the black hole is travelling faster than the speed of light. There's nothing wrong with such a thing in general relativity, surprisingly. The impossible thing is the observation itself since the blackhole environment means that an outside observer cannot "see" you from a given made-up reference frame Dstahl described in the last part of his post.

DStahl
2003-Feb-09, 10:22 PM
Thank, JS Princeton. Yes, my pseudo-reference frame is impossible--and like most conceptual shortcuts which abrogate elements of physics it end up with distinctly unphysical aspects.

The geodesic spacetime trajectories visualization is a nice way to go about it. "...the geodesic for the lightbeam is longer than yours"-- like that.

<font size=-1>[ This Message was edited by: DStahl on 2003-02-09 17:23 ]</font>

Zathras
2003-Feb-09, 10:36 PM
One way to look at it is that, once you are inside the event horizon, the radial coordinate and the time coordinate switch places, so that to the light, going towards the center to the black hole is the same as going forward in time--it is unavoidable. Since the "time" coordinate is now spacelike, the light would be able to move backwards with respect to the external time coordinate, if it experiences a force in that direction.

Kizarvexis
2003-Feb-09, 11:27 PM
On 2003-02-09 17:36, Zathras wrote:
One way to look at it is that, once you are inside the event horizon, the radial coordinate and the time coordinate switch places, so that to the light, going towards the center to the black hole is the same as going forward in time--it is unavoidable. Since the "time" coordinate is now spacelike, the light would be able to move backwards with respect to the external time coordinate, if it experiences a force in that direction.


You were much clearer on Epsilon 3. /phpBB/images/smiles/icon_smile.gif

Could you run that past me again, but with real words? I'm lost.

Kizarvexis
Which isn't unusual when you get into the wierd quantum and space/time stuff. /phpBB/images/smiles/icon_smile.gif

Zathras
2003-Feb-10, 04:04 PM
On 2003-02-09 18:27, Kizarvexis wrote:


On 2003-02-09 17:36, Zathras wrote:
One way to look at it is that, once you are inside the event horizon, the radial coordinate and the time coordinate switch places, so that to the light, going towards the center to the black hole is the same as going forward in time--it is unavoidable. Since the "time" coordinate is now spacelike, the light would be able to move backwards with respect to the external time coordinate, if it experiences a force in that direction.


You were much clearer on Epsilon 3. /phpBB/images/smiles/icon_smile.gif

Could you run that past me again, but with real words? I'm lost.

Kizarvexis
Which isn't unusual when you get into the wierd quantum and space/time stuff. /phpBB/images/smiles/icon_smile.gif



Oh, Zathras is sorry for that. The Great Machine has informed Zathras of this website:

http://bustard.phys.nd.edu/Phys171/lectures/blackhole.html

This outline gives a good description of this in terms of the light cones, which give the locus of paths on which light can travel.