The great photon disappearence act!

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The great photon disappearence act!

Postby Red Rule » Thu Jan 13, 2011 5:21 pm UTC

As I understand it a photon:
-always travels at c.
-has no rest-mass but it has some mass when it has energy. (in accordance to m=e/c^2)
-it's frequency is also related to it's energy.
but I'm far from an expert in this field.

Now if one were to take a light source and a gravitational source.
Let's take a lightbulb and a black hole.
(as a black hole is a point mass and a lightbulb emits light in all directions-we'd also need an extension cord for the lightbulb)
if said lightbulb were to fall towards the black hole, it's no stretch of the imagination to say that there would atleast 1 photon whose velocity vector is direcly opposite to the gravitational vector.
Would it then continue to move away from the black hole, losing energy as it did and then dissapear(as it would lose all it's properties)? or would it lose no energy at all and escape the black hole?

The first scenario seems to violate conservation of energy (unless the field absorbs it).
The second one, to me, seems plausible but as I said I'm certainly no expert in this field so I suspect I might be wrong as it leads to a possible violation of the definition of a black hole.

So, assuming I am wrong (as I probablly am), where did I go wrong? are my assumptions wrong or am I missing something important?
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Re: The great photon disappearence act!

Postby Robert'); DROP TABLE *; » Thu Jan 13, 2011 5:25 pm UTC

In the event that a photon is inside the event horizon of the black hole, something rather weird happens: the photon hits the black hole, and contributes its energy to it. If the lightbulb is not inside the event horizon, then the photon escapes with some non-zero energy. (The rest of which is converted into gravitational potential energy.)
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Re: The great photon disappearence act!

Postby Red Rule » Thu Jan 13, 2011 6:10 pm UTC

I am intriguied, but after a little reading I fear that further questioning will result in me having a terrible headache.
So many things to study, so little time.
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Re: The great photon disappearence act!

Postby quantropy » Thu Jan 13, 2011 6:21 pm UTC

Red Rule wrote:if said lightbulb were to fall towards the black hole, it's no stretch of the imagination to say that there would atleast 1 photon whose velocity vector is direcly opposite to the gravitational vector.
Once the lightbulb has crossed the event horizon this is no longer true, all paths lead to the singularity, just as all paths for us lead into the future.
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Re: The great photon disappearence act!

Postby Roĝer » Fri Jan 14, 2011 12:19 am UTC

Red Rule wrote: or would it lose no energy at all and escape the black hole?


A photon that is emitted outside the event horizon in the right direction will escape, but it will lose energy, in the form of being redshifted. So your nice blue lightbulb will appear to become red as it approaches the black hole.
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Re: The great photon disappearence act!

Postby Levi » Mon Jan 17, 2011 6:08 am UTC

So what happens to photons that are emitted away from the black hole inside the event horizon?
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Re: The great photon disappearence act!

Postby Xanthir » Mon Jan 17, 2011 7:36 am UTC

There is no "away from the black hole" inside the event horizon. All paths lead toward the singularity.
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Re: The great photon disappearence act!

Postby Antimony-120 » Wed Jan 19, 2011 5:13 am UTC

Or to put it another way, they might be going "away" at c, but space is moving inwards at greater than c, so ultimatly they move inwards anyways. They just move inwards as slowly as possible.
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Re: The great photon disappearence act!

Postby quantropy » Wed Jan 19, 2011 10:27 am UTC

It's rather difficult to visualize, but spacetime is still 4 dimensional within the event horizon, but the radial direction now becomes the time dimension. That means there must be another space dimension orthogonal to the two spherical angle dimensions. That's presumably the direction the photon travels in, but it's no longer a radial direction - I would think that it has the same status as the spherical angle dimensions.
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Re: The great photon disappearence act!

Postby MHD » Thu Sep 22, 2011 7:23 am UTC

Inside a black hole...

Well, what most people don't realise is this: Inside an event horizon there is no such thing as "outwards."

Once you have fallen inside, every direction is towards the singularity.

You have to travel backwards in time to get out.
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Re: The great photon disappearence act!

Postby Quizatzhaderac » Fri Mar 16, 2012 9:00 pm UTC

Roĝer wrote:
Red Rule wrote: or would it lose no energy at all and escape the black hole?


A photon that is emitted outside the event horizon in the right direction will escape, but it will lose energy, in the form of being redshifted. So your nice blue lightbulb will appear to become red as it approaches the black hole.

Also important to note is that from your perspective the light-bulb will take longer to reach the event horizon then it would under Newtonian mechanics. So while you see a lower energy of light you see it for longer.
Last edited by Quizatzhaderac on Tue Aug 19, 2014 7:18 pm UTC, edited 1 time in total.
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Re: The great photon disappearence act!

Postby WarDaft » Sat Mar 17, 2012 7:55 pm UTC

MHD wrote:Inside a black hole...

Well, what most people don't realise is this: Inside an event horizon there is no such thing as "outwards."

Once you have fallen inside, every direction is towards the singularity.

You have to travel backwards in time to get out.

But you can't observe yourself to have fallen inside the event horizon - it seems to shrink away beneath you, and there will always (well, until you die) in fact be a direction you can point to that to you appears to be 'out' even after you are inside the horizon according to a distant observer.


... random thought, what would happen if the Schwarzschild radius of the observable universe became larger than the observable universe?
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Re: The great photon disappearence act!

Postby Tass » Sun Mar 18, 2012 11:36 am UTC

WarDaft wrote:... random thought, what would happen if the Schwarzschild radius of the observable universe became larger than the observable universe?


You'd be in a closed universe, doomed to collapse to a singularity in finite time.
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Re: The great photon disappearence act!

Postby Roĝer » Tue Mar 20, 2012 2:34 pm UTC

Tass wrote:
WarDaft wrote:... random thought, what would happen if the Schwarzschild radius of the observable universe became larger than the observable universe?


You'd be in a closed universe, doomed to collapse to a singularity in finite time.


But the calculation for the Schwarzschild radius doesn't include dark energy/cosmological constant, which at such low densities becomes very relevant.
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Re: The great photon disappearence act!

Postby Tass » Tue Mar 20, 2012 8:00 pm UTC

Roĝer wrote:
Tass wrote:
WarDaft wrote:... random thought, what would happen if the Schwarzschild radius of the observable universe became larger than the observable universe?


You'd be in a closed universe, doomed to collapse to a singularity in finite time.


But the calculation for the Schwarzschild radius doesn't include dark energy/cosmological constant, which at such low densities becomes very relevant.


Sure.
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