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View Full Version : What is the speed of light at the centre of a black hole??

Jukeball
04-22-2001, 11:38 AM
From what I have heard that black holes do not allow light to leave due to the intense gravity existing , so does that mean the speed of light is zero , implying that there is no time on a balck hole , and since all the light is concentrated at the center where has all the light gone , or has the light been compressed too!!! to the size of a baseball . weird

lno
04-22-2001, 11:49 AM
At a singularity, pretty much all of our understanding of laws of physics break down. Which, of course, is why SF writers love to use black holes as a way to travel through time, travel faster than light, travel to other universes, and get a taco.

Mm, taco.

I refer interested readers to chapters six and seven of Stephen Hawking's A Brief History Of Time, ISBN # 0553380168.

VarlosZ
04-22-2001, 11:54 AM
The speed of light at the center of a black hole is equal to the speed of light at the center of a black hole. I think this is the best answer you can hope to get (though I may be wrong).

Peter
04-22-2001, 04:59 PM
Originally posted by LNO
At a singularity, pretty much all of our understanding of laws of physics break down. Which, of course, is why SF writers love to use black holes as a way to travel through time, travel faster than light, travel to other universes, and get a taco.

Mm, taco.

I refer interested readers to chapters six and seven of Stephen Hawking's A Brief History Of Time, ISBN # 0553380168.

Pretty much nailed it. The speed of light could still be the speed of light for all we know, but since our knowledge of the physics and the Universe stop at a singularity, it almost doesn't matter.

Autumn Wind Chick
04-22-2001, 07:11 PM
I'll take a guess at this one (with a little help from my hubby)

Logically, the speed of light at the center of a black hole should be zero, since it is impossible for anything to move AWAY from the singularity.

First, the speed of light depends upon the properties of the medium -- i.e. the permitivity and conductivity. Thus, light travels more slowly in air than in a vacuum. It would hardly be surprising if the permitivity and conductivity at the center of black hole are such that the speed of light is zero.

More fundamentally, light requires a moving electrical field to induce a magnetic field, and vice-versa. It seems unlikely that a static electric field could produce a magnetic field. Thus, it would seem that light can't exist at the center of a black hole.

I would imagine that any light hitting the center (or even getting close) would get absorbed by all the stuff that's already there.

P.S. This is all guess-work based on high school physics and a couple electrical engineering classes. My husband never took quantum mechanics or QED and wonders if these things might affect the analysis.

Blowie
04-22-2001, 07:35 PM
It's light Jim, but not as we know it.

Drastic
04-22-2001, 07:38 PM
Disclaimer: I am not a physicist, just an interested layman, so do not know the math involved, and if you put the math in front of me I would stare blankly at it. :)

General relativity describes gravity very precisely (as opposed to my mangled attempt here) as the curvature of spacetime in a four-dimensional manifold. Light travels at an invariant velocity--it's always c, regardless of the observer's reference frame--along the geodesic (most efficient, shortest path) curvature of the shape of spacetime in its path.

In a black hole, the curvature of spacetime becomes infinite--now, that's not very meaningful--what it's really saying is that the equations and math of our current physics of general and special relativity, which work very very well at sizes larger than the quantum and other than singularities, start producing values of infinity. An equation whose answer is infinite is a non-meaningful equation. It means our math isn't up to the task, not necessarily that there really is an "infinite" curvature.

So my guess, after all that unclear babble, is that the speed of light at a singularity remains c--it's simply c travelling along a shape of spacetime that we do not have the physics to describe in any meaningful way.

barbitu8
04-22-2001, 08:17 PM
Originally posted by Autumn Wind Chick

First, the speed of light depends upon the properties of the medium -- i.e. the permitivity and conductivity. Thus, light travels more slowly in air than in a vacuum. It would hardly be surprising if the permitivity and conductivity at the center of black hole are such that the speed of light is zero.

More fundamentally, light requires a moving electrical field to induce a magnetic field, and vice-versa. It seems unlikely that a static electric field could produce a magnetic field. Thus, it would seem that light can't exist at the center of a black hole.

Perhaps I'm not getting this. Light does not require any medium and can move through a vacuum. In fact it does. So how does it require a moving electrical field. It would require an electrical field to produce a magnetic field, but that is not at all on point.

Blowie
04-22-2001, 08:26 PM
Maybe we need to define “light”. Are we after the wave theory or the particle?

A wave of electric and magnetic fields does seem unlikely. But a photon here or there seems quite likely.

Chronos
04-22-2001, 10:34 PM
Light doesn't just need fields, light is electric and magnetic fields, both constantly changing. Electric and magnetic fields do not require particles. As for what happens inside a black hole, if you want to find out, you'll pretty much have to stick your head in and take a look. I would ask you to report back after you do, but you wouldn't be able to. Basically, the laws of physics can do anything they want inside the event horizon, so long as they do so in such a way that you can't tell from the outside.

As for light being slowed down inside a black hole, it's not that the speed is decreased, it's just that there's no path it can possibly follow that will lead outside the horizon. All roads lead back to where you came from.

Autumn Wind Chick
04-23-2001, 08:19 PM
Chronos, I respectfully disagree with you.

First, I believe that the speed of light and the "curvature" of its paths are two separate issues.

A hypothetical:

Let's suppose that the universe is "closed." (I realize that current evidence supports an open or semi-open universe, but a closed universe is certainly not out of the question.) In such a universe, "all roads lead back to where you come from," much like inside the event horizon of black hole. (In fact, a closed universe arguably IS a black hole of sorts.)

Notwithstanding this property, it would still be (generally) possible to measure the speed of light, and note that the speed varies depending on the properties (i.e. permitivity and conductivity) of the particular region of the universe where the measurements are made.

Certainly, nobody would argue that the fact that the speed of light can be measured rules out the possibility of a closed universe.

My second disagreement is your apparent reluctance to theorize at all about what goes on inside a black hole. Although you are correct that information cannot leave a black hole, that is more of a philisophical objection than a scientific one.

William Poundstone has a good discussion of this issue in his book Labrynths of Reason. He points out that if a human were to enter a large enough black hole in a spaceship, he would have 20 minutes or so to make observations, do experiments, write about them etc.

Is it fair to say that you won't find a (hollow) chocolate easter bunny at the singularity of a black hole?

bryanmcc
04-23-2001, 10:36 PM
But what the hell would be the point of making such a supposition? Since no information can be received from inside an event horizon, any speculation is as good as any other; and all of them are therefore pointless.

-Maybe it's a chocolate easter egg.
-Maybe it's solid gold.
-Maybe it's not much at all.
-Maybe a million other things.

There's no way to know, so arguing about it is pointless. Unless you want to open a GD thread about it. (Although perhaps it's more suited to IMHO.)

-b

DrMatrix
04-24-2001, 12:28 AM
Just because we can't receive information from the other side of the event horizon is no reason to suppose that the laws of physics are radically different inside. It is silly to suggest that a black hole is chocolate filled. The theory that predicts them says that everything collapses into a singularity at the center.

To answer the OP. Light falls into the singularity. At the singularity, there is no light or matter. It's a point mass.

bryanmcc
04-24-2001, 08:53 AM
Of course there's no reason to believe that the inside of an event horizon would be chocolate or gold. I was just attempting (poorly) to point out that, as we can never observe the inside of an event horizon -- or, if we do, that information (and the observer) remains trapped inside -- making any speculations about conditions there would be rather pointless. Of course, the simplest (and most logical) assumption would be that the laws of physics would remain the same, except for at the singularity, where our mathematics breaks down. Occam's Razor -- and common sense -- would suggest that we make no special assumptions that this region would be any different than our regular laws of physics would predict. Sorry for any confusion my previous post may have caused.

-b

Chronos
04-24-2001, 02:16 PM
Yes, the macroscopic speed of light depends on properties of the medium through which it's passing. I was reading the term "speed of light" in the OP to refer to c, the speed of light in a vacuum, which is unchanged anywhere outside a black hole, and presumably also unchanged inside.

The absolute center of a black hole is a singularity, and all bets really are off there. There is no theory currently, nor any theory in development, which can say much of anything about a singularity. If you're inside the horizon but not at the center, then maybe there's some hope: We still don't have any hope of experimental verification, but we might (or might not) be able to extrapolate from theories that work outside. This largely depends on whether you can see anything inside of your current position: If not, then you can probably continue to use normal GR to describe your surroundings, but if you can see the singularity, then again, all bets are off.

bughunter
04-24-2001, 04:16 PM
My information may be out of date, but...

First, I recall Kaufmann describing the singularity of a charged or rotating black hole as a loop, not a point. Thus, the exact center of the rotating black hole is not part of the singularity, and it is possible to describe a line through this point that leads "somewhere else."

Second, I also recall learning that inside the event horizon, the dimensions of time and space are "swapped." I'm an engineer, not a theoretician, so I never bothered to find an exact mathematical description of this effect, but as I understood it, if an observer was travelling through space, and crossed an event horizon, he or she would then be travelling in time.

Anyway, if either of these points are still valid in the context of current theory, do they have an effect on the answer to the OP? Or on the question, even?

Silo
04-24-2001, 09:18 PM
Originally posted by jayshah
From what I have heard that black holes do not allow light to leave due to the intense gravity existing , so does that mean the speed of light is zero , implying that there is no time on a balck hole , and since all the light is concentrated at the center where has all the light gone , or has the light been compressed too!!! to the size of a baseball . weird
To an observer inside the black hole, the speed of light would be the same.

Originally posted by DrMatrix
Just because we can't receive information from the other side of the event horizon is no reason to suppose that the laws of physics are radically different inside.

Exactly.

To answer the OP. Light falls into the singularity. At the singularity, there is no light or matter. It's a point mass.

Sure it's a point mathematically speaking. But is that what's really there? (i.e. is that what it would look like if we could see it?

*
The way I see it, within the event horizon you have a big ball of super condensed (crystalized) matter. We have no way of understanding the physics dealing with matter existing in this state. And as a result, we cannot say what happens to the photons entering the event horizon (maybe they dance around below the event horizon on the outside of the matter ball... like butterflies on lillypads :)). But rather, considering that the physics aren't radically different within the black hole, it's logical that the speed of light would remain the same to an observer within it.

Enola Straight
04-24-2001, 11:31 PM
Bughunter is right about the swapping of space and time within the event horizon.

Normally, one can move in any direction he chooses in space, but only foreward in time(to the future).

Inside an event horizon, one could move in any direction in time, but only foreward in space(to the singularity)

echonymph
04-24-2001, 11:35 PM
1984320000-km/s

TwistofFate
04-25-2001, 05:32 AM
AWC, look at it like this.

Just because light is unable to escape a black hole, dosent mean it is moving anywhere.

Basically, throw your physics book out the window and write yourr own explination, because it has equal chance of being correct within a singularity, and an equal chance of being incorrect.

oh, and William Poundstone was full of shit.

SPOOFE
04-25-2001, 05:45 AM
ust because light is unable to escape a black hole, dosent mean it is moving anywhere.

In other words, given the time/space "swapping" that occurs when crossing an event horizon, the light would be physically trapped in the singularity, but temporally moving at the time-equivalent of c?

Sounds a lot like something you'd hear in Star Trek...

bughunter
04-25-2001, 06:15 AM
And exactly the kind of thing they'd use to defeat the Borg.

DrMatrix
04-25-2001, 11:02 AM
Originally posted by Silo
To an observer inside the black hole, the speed of light would be the same.Well, kinda. The speed of light is not c for all observers, light travels at the speed of c for all inertial observers. For a reference frame in a gravitational field to be inertial, it must be free falling. If you were freely falling into a black hole, you would observe that light travels at c. However, an inertial reference frame cannot include the singularity. To see that light does not have to travel at c, consider a photon trapped at the event horizon. It is suspended so we have light not moving at all. Sure it's a point mathematically speaking. But is that what's really there? (i.e. is that what it would look like if we could see it?We can never see the singularity. Light cannot leave the singularity so we cannot see it. Also, anything we see must be in our past light cone. The singularity of a black hole is never in our past light cone; it is always in our future light cone.

As to time and space being swapped inside the event horizon. I understand that the doomed observer's distance from the singularity cannot increase -- the distance must increase. But how does this allow the ill-fated observer to travel through time. It seems to me that his space and time axes are both time-like.