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Offline jeffreyH

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what would happen to a photon in this situation?
« on: 27/01/2015 02:01:12 »
If we have a photon that is traveling on a path that takes it exactly towards the centre of gravity of a black hole how would the gravity affect its speed? When a photon is moving away from a source its wavelength is red shifted. Would we get a blue shift and an increase in energy? How much of this would be kinetic?


 

Offline Atomic-S

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Re: what would happen to a photon in this situation?
« Reply #1 on: 27/01/2015 04:42:52 »
Quote
When a photon is moving away from a source its wavelength is red shifted.
It depends. What is the reference frame of the photon's observer?

The speed of the photon as viewed by any observer near it will be simply c.  The speed of the photon as seen by a different observer could be calculated by noting the relationship between space and time measured by that observer vs. space and time as it applies in the vicinity of the photon.  I believe the answer to that, in the situation you propose, is that a clock near the black hole will run slower than one far away, so that in a given interval of far-away-observer time, the photon will have undergone, as seen by that observer, less time, meaning that because it is traveling at c in its own reference frame, it will have traveled a shorter distance in its frame than the clock of the observer says it should. However, we also have to take into account the difference in spatial measures. I understand that a given length near the black hole, measured radially by a local observer, will measure longer than it will appear to be to a distant observer.  That, if correct (and I am not absolutely certain), would indicate that not only does the photon travel a lesser distance in the lesser time, but the distance that it does travel will appear less to the remote observer than it will locally. both of these phenomena would result in the photon appearing, to the remote observer, to slow down as it approached the black hole, eventually approaching an apparent speed of zero.

 

Offline Atomic-S

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Re: what would happen to a photon in this situation?
« Reply #2 on: 27/01/2015 04:52:00 »
As pertains to the energy relationships:  An observer near the hole observes the clock far from the hole running faster than his, so that a wave emitted from far will appear to be in sync with that faster clock, and will therefore have a higher frequency than a wave emitted by a like apparatus located  near the hole. this is a blue-shift. Because the number of photons will remain unchanged by traveling, and because the quantum energy increases with frequency, the light upon arrival near the hole will have more energy as seen by the near observer than it did when it began its journey from afar as seen by the far observer.
 

Offline yor_on

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Re: what would happen to a photon in this situation?
« Reply #3 on: 27/01/2015 11:05:46 »
yep, it's relationships, set in an idea of conservation laws keeping count. So if we assumed that we actually could observe a photon from several positions in a SpaceTime, we would get different definitions. It's a good example of what I call using a 'container model' to describe it. In reality that 'photon' will interact just once, in its annihilation. To that you can add a 'recoil' observed as whatever it leaves react to it.
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #4 on: 28/01/2015 01:08:50 »
Another thing that came to mind with regard to photons and black hole is the mass in the accretion disk and perhaps the ergosphere. Gravitational fields that are in opposition will partially cancel. This is most evident in a cavity at the centre of gravity of a massive body. Depending upon the amount of mass in the ergosphere or accretion disk there will be a gravitational field that can in effect cancel some of the gravitation of the black hole at the event horizon. This could mean that the effective horizon may shrink and that light could in effect exit the horizon due to this cancelling effect. It may be a miniscule effect but any effect should show some decrease in the radius of the horizon.

This does not mean that the black hole will necessarily lose mass but that photons generated just inside the horizon may be able to escape. I am really not sure if this is right but I can't off the top of my head think of why it shouldn't be that way. If this were true then that puts a dent in the current view of information loss. If we had two black holes orbiting each other at fairly close range then this cancellation could reduce the horizon of each by a significant amount.
« Last Edit: 28/01/2015 01:11:01 by jeffreyH »
 

Offline JohnDuffield

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Re: what would happen to a photon in this situation?
« Reply #5 on: 28/01/2015 12:29:06 »
If we have a photon that is traveling on a path that takes it exactly towards the centre of gravity of a black hole how would the gravity affect its speed?
It slows down. Yes. Check out the coordinate speed of light.

When a photon is moving away from a source its wavelength is red shifted. Would we get a blue shift and an increase in energy? How much of this would be kinetic?
People say the ascending photon is redshifted, but actually, it doesn't change. Conservation of energy applies. In similar vein people say the descending photon is blueshifted, but again, it doesn't change, and conservation of energy applies. You can work this out by sending a 511keV photon into that black hole. Ask yourself this: by how much does the black hole mass increase? The answer is 511keV/c≤. Not a gazillion tons. Also ask yourself this: if you accelerate towards a 511keV photon, does it get blueshifted? The answer is that actually, the photon doesn't change a jot. Your measurement of its frequency and energy changes because you moved. But again, conservation of energy applies.

Quote from: jeffreyh
If this were true then that puts a dent in the current view of information loss.
You should read about Friedwardt Winterberg's firewall.  I think it destroys the current view of information loss. Meh, I'm an IT guy, and something of an amateur relativist. I am not impressed with cosmologists talking about information as if it was some physical thing.   

what is the ctually, it doesn't change. uis
 

Offline PmbPhy

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Re: what would happen to a photon in this situation?
« Reply #6 on: 28/01/2015 16:04:50 »
Quote from: jeffreyH
If we have a photon that is traveling on a path that takes it exactly towards the centre of gravity of a black hole how would the gravity affect its speed?
The coordinate speed of the photon will change as it moves towards the event horozon.

Quote from: jeffreyH
When a photon is moving away from a source its wavelength is red shifted.
Gravitational redshift is only observed when local observers at different positions compare their measurements. The wavelength as measured by  Schwarzschild observers remains unchanged.

For details on potential and kinetic energy of a photon see
http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #7 on: 29/01/2015 02:20:18 »
Quote from: jeffreyH
If we have a photon that is traveling on a path that takes it exactly towards the centre of gravity of a black hole how would the gravity affect its speed?
The coordinate speed of the photon will change as it moves towards the event horozon.

Quote from: jeffreyH
When a photon is moving away from a source its wavelength is red shifted.
Gravitational redshift is only observed when local observers at different positions compare their measurements. The wavelength as measured by  Schwarzschild observers remains unchanged.

For details on potential and kinetic energy of a photon see
http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm

Thanks Pete. I asked this because I really don't know. I haven't had time to look at your page but I will later and respond.
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #8 on: 29/01/2015 03:33:17 »
I have just read the first part of your page and I noted the two time axes. This is actually a method I hadn't thought of. Do you believe that? I will read more later. The hour is late.
 

Offline JohnDuffield

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Re: what would happen to a photon in this situation?
« Reply #9 on: 29/01/2015 12:08:46 »
The coordinate speed of the photon will change as it moves towards the event horozon.
See what Einstein said:

"Second, this consequence shows that the law of the constancy of the speed of light no longer holds, according to the general theory of relativity, in spaces that have gravitational fields. As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable".

Rather counter-intuitively, the descending photon slows down, and the ascending photon speed up. The "modern" interpretation of general relativity says the speed doesn't change, but this is not in line with what you might call the Einstein interpretation of general relativity.

Gravitational redshift is only observed when local observers at different positions compare their measurements. The wavelength as measured by  Schwarzschild observers remains unchanged.
Good man.

For details on potential and kinetic energy of a photon see http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm
I noticed the total energy of a photon moving through a gravitational field is constant. That's nice to see. I wish more people knew it. And that it's the same for a falling brick. 
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #10 on: 29/01/2015 23:33:16 »
Quote from: jeffreyH
If we have a photon that is traveling on a path that takes it exactly towards the centre of gravity of a black hole how would the gravity affect its speed?
The coordinate speed of the photon will change as it moves towards the event horozon.

Quote from: jeffreyH
When a photon is moving away from a source its wavelength is red shifted.
Gravitational redshift is only observed when local observers at different positions compare their measurements. The wavelength as measured by  Schwarzschild observers remains unchanged.

For details on potential and kinetic energy of a photon see
http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm

Well your page has cleared up a lot of questions for me. Especially on the conservation of energy in a gravitational field. Would you mind if I used the equations?
 

Offline dlorde

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Re: what would happen to a photon in this situation?
« Reply #11 on: 30/01/2015 09:42:25 »
.. This could mean that the effective horizon may shrink and that light could in effect exit the horizon due to this cancelling effect. It may be a miniscule effect but any effect should show some decrease in the radius of the horizon.
...
This does not mean that the black hole will necessarily lose mass but that photons generated just inside the horizon may be able to escape.
I can see why the horizon might shrink, but light would still not escape from it - but it would be able to escape from where the horizon would have been be if it hadn't shrunk. As I understand it, the horizon is defined as the point where light can't escape.
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #12 on: 30/01/2015 13:27:13 »
.. This could mean that the effective horizon may shrink and that light could in effect exit the horizon due to this cancelling effect. It may be a miniscule effect but any effect should show some decrease in the radius of the horizon.
...
This does not mean that the black hole will necessarily lose mass but that photons generated just inside the horizon may be able to escape.
I can see why the horizon might shrink, but light would still not escape from it - but it would be able to escape from where the horizon would have been be if it hadn't shrunk. As I understand it, the horizon is defined as the point where light can't escape.

Yes that is the point. If we have two equivalent black holes that are in the process of merging the horizons of both may shrink radically, releasing tell tale gamma ray bursts. As galaxies merge it may be that gamma ray bursts are releases of trapped energy from temporarily cancelled fields inside the horizon. The cancellation allowing high energy photons to escape. How fast would this phenomena happen?
 

Offline PmbPhy

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Re: what would happen to a photon in this situation?
« Reply #13 on: 30/01/2015 16:52:06 »
Jeff - Take a look at the chapters of the new version of Exploring Black Holes. They're online at
http://www.eftaylor.com/exploringblackholes/

Especially the chapter GlobalLightBeams141029v2.pdf
 

Offline PmbPhy

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Re: what would happen to a photon in this situation?
« Reply #14 on: 30/01/2015 16:53:35 »
.. This could mean that the effective horizon may shrink and that light could in effect exit the horizon due to this cancelling effect. It may be a miniscule effect but any effect should show some decrease in the radius of the horizon.
...
This does not mean that the black hole will necessarily lose mass but that photons generated just inside the horizon may be able to escape.
I can see why the horizon might shrink, but light would still not escape from it - but it would be able to escape from where the horizon would have been be if it hadn't shrunk. As I understand it, the horizon is defined as the point where light can't escape.

Yes that is the point. If we have two equivalent black holes that are in the process of merging the horizons of both may shrink radically, releasing tell tale gamma ray bursts. As galaxies merge it may be that gamma ray bursts are releases of trapped energy from temporarily cancelled fields inside the horizon. The cancellation allowing high energy photons to escape. How fast would this phenomena happen?
There is a formula to determine what the value of the remaining black hole is. Obviously its a function of the two radii and is larger than either.
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #15 on: 30/01/2015 17:15:24 »
.. This could mean that the effective horizon may shrink and that light could in effect exit the horizon due to this cancelling effect. It may be a miniscule effect but any effect should show some decrease in the radius of the horizon.
...
This does not mean that the black hole will necessarily lose mass but that photons generated just inside the horizon may be able to escape.
I can see why the horizon might shrink, but light would still not escape from it - but it would be able to escape from where the horizon would have been be if it hadn't shrunk. As I understand it, the horizon is defined as the point where light can't escape.

Yes that is the point. If we have two equivalent black holes that are in the process of merging the horizons of both may shrink radically, releasing tell tale gamma ray bursts. As galaxies merge it may be that gamma ray bursts are releases of trapped energy from temporarily cancelled fields inside the horizon. The cancellation allowing high energy photons to escape. How fast would this phenomena happen?
There is a formula to determine what the value of the remaining black hole is. Obviously its a function of the two radii and is larger than either.

Thanks for the information Pete I will be reading it while I wait for a new book on Lagrangians and Hamiltonians. I may be back asking question!
 

Offline PmbPhy

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Re: what would happen to a photon in this situation?
« Reply #16 on: 30/01/2015 17:34:56 »
Quote from: jeffreyH
Thanks for the information Pete I will be reading it while I wait for a new book on Lagrangians and Hamiltonians.
What book is that?
 

Offline JohnDuffield

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Re: what would happen to a photon in this situation?
« Reply #17 on: 30/01/2015 18:26:50 »
Thanks for the information Pete I will be reading it while I wait for a new book on Lagrangians and Hamiltonians. I may be back asking question!
I recommend some caution. It's a renowned text, but there are... issues. I think the best way to show this is with why doesnít the light get out?
 
You're standing on a gedanken planet holding a laser pointer straight up. The light doesn't curve round, or slow down as it ascends, or fall down. It goes straight up. Now I wave my magic wand and make the planet denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. I make the planet even denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. I make the planet even denser and more massive, and take it to the limit such that it's a black hole. At no point did the light ever curve round, or slow down as it ascends, or fall down. So why doesn't the light get out?
 
Note that some people might answer with the waterfall analogy. Itís wrong, because a gravitational field is a region of inhomogeneous space which we model as curved spacetime. See this Einstein quote and this paper. A gravitational field alters the motion of light and matter through space, but it doesnít suck space in. We do not live in a Chicken-Little world where the sky is falling in.
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #18 on: 30/01/2015 19:28:57 »
Thanks for the information Pete I will be reading it while I wait for a new book on Lagrangians and Hamiltonians. I may be back asking question!
I recommend some caution. It's a renowned text, but there are... issues. I think the best way to show this is with why doesnít the light get out?
 
You're standing on a gedanken planet holding a laser pointer straight up. The light doesn't curve round, or slow down as it ascends, or fall down. It goes straight up. Now I wave my magic wand and make the planet denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. I make the planet even denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. I make the planet even denser and more massive, and take it to the limit such that it's a black hole. At no point did the light ever curve round, or slow down as it ascends, or fall down. So why doesn't the light get out?
 
Note that some people might answer with the waterfall analogy. Itís wrong, because a gravitational field is a region of inhomogeneous space which we model as curved spacetime. See this Einstein quote and this paper. A gravitational field alters the motion of light and matter through space, but it doesnít suck space in. We do not live in a Chicken-Little world where the sky is falling in.

I never take anything at face value. I will read it without reference to anything else simply because I don't want to start with a slanted view. There are real difficulties with gravitation that no one has an answer to. The main difficulty is in experimental validation for some of the more extreme macroscopic or microscopic environments.
 

Offline JohnDuffield

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Re: what would happen to a photon in this situation?
« Reply #19 on: 31/01/2015 15:41:30 »
I don't think there's much of a problem with gravitation myself, or that nobody has answers. But I think there is a problem wherein what's taught just doesn't square with what Einstein said. Pete wrote about it in this paper. I share his sentiment about "Einstein's general relativity" and "modern general relativity", but I think the issues are greater than he appreciates. I recommend that you understand why doesn't the light get out? IMHO it really gets to the heart of things.
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #20 on: 31/01/2015 16:09:07 »
Quote from: jeffreyH
Thanks for the information Pete I will be reading it while I wait for a new book on Lagrangians and Hamiltonians.
What book is that?

The book is "A Student's Guide to Lagrangians and Hamiltonians" by Patrick Hamill.
 

Offline jeffreyH

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Re: what would happen to a photon in this situation?
« Reply #21 on: 31/01/2015 16:13:27 »
I don't think there's much of a problem with gravitation myself, or that nobody has answers. But I think there is a problem wherein what's taught just doesn't square with what Einstein said. Pete wrote about it in this paper. I share his sentiment about "Einstein's general relativity" and "modern general relativity", but I think the issues are greater than he appreciates. I recommend that you understand why doesn't the light get out? IMHO it really gets to the heart of things.

Why do you make such evasive statements? Go on then why doesn't the light get out. No need to be all mysterious. If you believe I am assuming something erroneous or I am not aware of all the facts the honest thing to do is be direct. What are you getting at John?
 

Offline JohnDuffield

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Re: what would happen to a photon in this situation?
« Reply #22 on: 31/01/2015 16:26:13 »
See this? Look at the second paragraph:



That's Einstein talking about the speed of light varying. He says that this is why light curves. He says it's the cause of gravity. Now take a look at Professor Tom Moore's answer to the question:

"As the planet's mass approaches the black hole limit, the signal emitted from the surface will seem to move more and more slowly away from the surface (and will also be seen to be increasingly red-shifted as observed from infinity). When the surface of the planet coincides with the black hole's event horizon, the signal will stop moving outward from the surface (and the redshift observed at infinity will go to infinity). So light no longer escapes." 

The light doesn't get out because it's stopped. That fits with what Einstein said. However you will not find anything like this in E F Taylor's book.
« Last Edit: 31/01/2015 16:28:38 by JohnDuffield »
 

Offline yor_on

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Re: what would happen to a photon in this situation?
« Reply #23 on: 31/01/2015 16:42:24 »
As I said before John, is doesn't matter for SR. SR is part of GR too, although you will see gravity and the equivalence principle joining. It also depends on whether you expect there to be one perfect description of a SpaceTime, which then should be GR, as far as I read you? My thoughts of it is that GR is where we are, but also that SR may be closer to some initial parameter(s). Take a look at 'the clock postulate' for some arguments around it. Using my way to define a clock, as equivalent to splitting 'c' in equal chunks, its a simple argument, and that is what I like. As simple arguments as possible :)
=

As for you describing the light being stopped as the way Einstein saw it. Nope, he saw it in terms of observer dependencies. In those terms the light being 'stopped' according to the 'far observer' is equivalent to the light 'propagating' for the local observer. He actually got questioned on that one once as I remember, don't have the reference here though, and then explained his thinking of it through just observer dependencies. You can exchange observer dependencies for different 'frames of reference' if you like.

The point is that the 'frame' we use for physics is local, always local. We don't use a clock at some event horizon, to define local time ticking. Neither do we use such for repeatable experiments. So defining it your way becomes a observer dependency, in where you, as you use your local clock and ruler to define your observation, exchange the one creating your definition (local clock and ruler) for a clock and ruler you defined elsewhere.

Locally light doesn't stop anywhere.

Hmm, that one can be questioned :) Can't it? Let's put it this way. Locally defined light is 'c'. And that has nothing to do with whether I conserve energy equivalent to some light quanta into a 'super cold' cloud of atom(s), to then be released as 'light' at some later moment. Light is at 'c' from the 'instant' it exist, until it annihilates at a detector, no acceleration involved.
=

What Einstein did, and a lot of other physicists and mathematicians still do, is treating 'time' as illusionary. And as your local clock here is what defines the clock at the event horizon, it doesn't make sense arguing that it 'stops', as the time you used to measure that clock in now would be described as 'illusionary' too. By setting your local clock to 'c' though, it becomes a reality. A local reality, equivalent to how we define physics, created through locally done 'repeatable experiments', tested to hold true over varying SpaceTime positions. 
« Last Edit: 31/01/2015 17:49:51 by yor_on »
 

Offline JohnDuffield

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Re: what would happen to a photon in this situation?
« Reply #24 on: 31/01/2015 18:09:05 »
As for you describing the light being stopped as the way Einstein saw it. Nope, he saw it in terms of observer dependencies. In those terms the light being 'stopped' according to the 'far observer' is equivalent to the light 'propagating' for the local observer..
That's what people say, but it just doesn't square with Einstein saying a gravitational field is a place where the speed of light varies. If the light is stopped it's stopped. The local observer won't see it propagating. The light is stopped. He can't see. He's stopped too. If you put a stopped observer in front of a stopped clock, he doesn't see it ticking normally "in his frame". He doesn't see anything. 

The point is that the 'frame' we use for physics is local, always local. We don't use a clock at some event horizon, to define local time ticking. Neither do we use such for repeatable experiments. So defining it your way becomes a observer dependency, in where you, as you use your local clock and ruler to define your observation, exchange the one creating your definition (local clock and ruler) for a clock and ruler you defined elsewhere. Locally light doesn't stop anywhere.
If that was true, the vertical light beam would get out of the black hole. Then it wouldn't be a black hole. This is why the question above cuts to the heart of the matter. 

Hmm, that one can be questioned :) Can't it? Let's put it this way. Locally defined light is 'c'. And that has nothing to do with whether I conserve energy equivalent to some light quanta into a 'super cold' cloud of atom(s), to then be released as 'light' at some later moment. Light is at 'c' from the 'instant' it exist, until it annihilates at a detector, no acceleration involved.
And at the event horizon, the coordinate speed of light is zero. So c is zero.   

What Einstein did, and a lot of other physicists and mathematicians still do, is treating 'time' as illusionary. And as your local clock here is what defines the clock at the event horizon, it doesn't make sense arguing that it 'stops', as the time you used to measure that clock in now would be described as 'illusionary' too. By setting your local clock to 'c' though, it becomes a reality. A local reality, equivalent to how we define physics, created through locally done 'repeatable experiments', tested to hold true over varying SpaceTime positions.
Gravitational time dilation at the event horizon is infinite. Has that light clock at the event horizon ticked yet? No. And it never ever will.
 

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Re: what would happen to a photon in this situation?
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