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Author Topic: Would the photon lose all its energy at infinity?  (Read 68857 times)

Offline yor_on

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Re: Would the photon lose all its energy at infinity?
« Reply #75 on: 23/02/2015 22:14:44 »
I would call it different definitions Bill. SR describes a 'flat universe', and assuming a perfect vacuum you have 'c'. GR describes gravity. A photon 'interact' with gravity. f you let go of a propagation, then it can be defined to the way a field interact, which makes it simpler. Not when it comes to how this 'field' is constructed though, and how you want to consider its 'dimensions' 'infinite stretch' etc. But it let us free from the discussion whether a photon path is 'bent' or 'slowed down' from the eye of the beholder. We're so used to motion, we see it everywhere, and when we measure a 'speed of light' we automatically start to discuss paths and 'weak experiments' proving that concept. But you don't really need it, you only need a logic giving those excitations an expression that also can be translated into a 'speed'.
==

what I think one need to see there is that the description no longer is solely about 'speeds' and 'paths', but more about 'timings', as expressed through that possible 'field'. It's somewhat of a bother how to think of this field, as we have real observer dependencies existing, making my observation differ from yours. Also it's somewhat of a huddle trying to define it as 'deterministic', or not. Myself I would expect it to be microscopically probabilistic and macroscopically defined from observer dependencies though, just as the universe we think us see normally. So 'non linear' to me, and if you use time symmetries, also non linear both ways. If there is a symmetry that is.

The last one is weird, but it has its own logic. As long as you have a way to record every outcome 'everywhere' (and at all 'times') you can disregard that statement. but if you don't have that ability, then you will find the past bifurcate into possibilities, just as the future can be seen to do, until it becomes a 'now', as in you observing that specific outcome. If the universe is non linear though, then I don't see how we ever will be able to know all outcomes. And if that is true for a future, then that 'future' also is my 'now', and 'past'.

Ever heard of 'the fog of war'? How about 'the fog of history' :)
« Last Edit: 23/02/2015 22:49:27 by yor_on »
 

Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #76 on: 23/02/2015 22:21:27 »
What slows light in a gravitational field?
The causal structure of spacetime?

I think I need time to come up with a better answer.
The answer is that gravitational time dilation causes light to slow down. Think about it and you'll soon realize why.
 

Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #77 on: 23/02/2015 22:24:37 »
Quote from: JohnDuffield
The altered properties of space. In mechanics a shear wave travels at a speed v = √(G/ρ) where G is the shear modulus of elasticity and ρ is density. In electrodynamics the an electromagnetic wave travels at a speed c = √(1/ε0μ0) where ε0 is the permittivity of space and μ0 is the permeability.
All wrong.
 

Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #78 on: 23/02/2015 22:31:34 »
Quote from: JohnDuffield
You don't calculate it, you measure it.
I can't believe this!!! The mass is calculated using the theory of relativity. One measures the mass and compares it with what the theory predicts. But in all cases one can calculate what the mass should be. In fact sometimes you actually cannot measure it, you have to calculate it.
« Last Edit: 24/02/2015 03:12:01 by evan_au »
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #79 on: 25/02/2015 01:24:20 »
Being the OP I would like to pose a further question. Would the decrease in energy of the photon moving away from a gravitational field source be linear over distance? That is would the decrease in the gradient be a straight line?
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #80 on: 25/02/2015 01:36:10 »
Light slows in a gravitational field. Unless gravitons are physically real, there is nothing in a gravitational field to take the place of atoms in other media. What slows light in a gravitational field?
The altered properties of space. In mechanics a shear wave travels at a speed v = √(G/ρ) where G is the shear modulus of elasticity and ρ is density. In electrodynamics the an electromagnetic wave travels at a speed c = √(1/ε0μ0) where ε0 is the permittivity of space and μ0 is the permeability.

John! Really? Get your square roots right. You have just redefined the speed of light. By posting incorrect equations you are doing such a disservice to those struggling to learn physics and I just can't let this one go. It's just wrong. If you insist on posting equations then at least sanity check them.
 

Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #81 on: 25/02/2015 03:46:14 »
Being the OP I would like to pose a further question. Would the decrease in energy of the photon moving away from a gravitational field source be linear over distance?
No. Take a look at the formula for it. First you have to keep in mind that you're comparing the results of two observers. The energy and thus frequency as measured by one observer remains constant as it moves through the field.

See Eq. 7 at http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm
 

Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #82 on: 25/02/2015 11:26:38 »
John! Really? Get your square roots right. You have just redefined the speed of light. By posting incorrect equations you are doing such a disservice to those struggling to learn physics and I just can't let this one go. It's just wrong. If you insist on posting equations then at least sanity check them.
I haven't posted anything that's incorrect. See for example permeability on Wikipedia where you can see the expression written as 6f5ca076bd3e4dbd42871e18d724f425.gif.

Quote from: jeffreyh
Would the decrease in energy of the photon moving away from a gravitational field source be linear over distance?
It doesn't lose any energy. And again: when you send a 511keV photon down into a black hole the black hole mass increases by 511keV/c. Conservation of energy applies. This is also true for an ascending photon. It appears to have lost energy because we added energy to you to lift you up. However it hasn't actually lose any energy.

Quote from: PmbPhy
I can't believe this!!! The mass is calculated using the theory of relativity. One measures the mass and compares it with what the theory predicts. But in all cases one can calculate what the mass should be. In fact sometimes you actually cannot measure it, you have to calculate it.
You said the mass of a particle. There is no theory that predicts the mass of an electron, or any other particle. You know this. Do not be dishonest Pete. It does you no credit. And by the way, this is the wrong answer: The answer is that gravitational time dilation causes light to slow down. You measure time using say optical clocks. When your clock reading at one elevation doesn't match your clock reading at another elevation, it isn't because "time is going slower", it's because light is going slower. If you had read Einstein's original material you would know this. Or are you saying Einstein was wrong?

 

Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #83 on: 25/02/2015 14:03:36 »
Yes, Einstein writes, specifically, "the curvature of light rays occurs only in spaces where the speed of light is spatially variable." He says this because if one can introduce an accelerated reference frame relative to an inertial one, then the constant motion over time from the inertial frame will be accelerated (a spatially variable speed).

This quotation comes from a section titled, "Some consequences of the equivalence hypothesis". It is a consequence of using systems of coordinates and their properties in Riemann geometry (that is, spacetime curvature as we tend to refer to it today) to represent gravity that we find that in some systems of coordinates, the speed of light is not constant over finite distances. Later in that same section, indeed, on the page that JohnDuffield has carefully cut from his screenshot, Einstein also writes, "Nevertheless, this limiting case <also> is of fundamental significance for the theory of general relativity; because the fact from which we started out, namely that no gravitational field exists in the vicinity of a free-falling observer, this very fact shows that in the vicinity of every world point the results of the special theory of relativity are valid (in the infinitesimal) for a suitably chosen local coordinate system."

[Please do not forget or ignore this very important point when someone tries to tell you that general relativity demands that time stops in some scenario. Especially if they tell you that Einstein said this.]
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #84 on: 25/02/2015 17:51:39 »
Yes, Einstein writes, specifically, "the curvature of light rays occurs only in spaces where the speed of light is spatially variable." He says this because if one can introduce an accelerated reference frame relative to an inertial one, then the constant motion over time from the inertial frame will be accelerated (a spatially variable speed).

This quotation comes from a section titled, "Some consequences of the equivalence hypothesis". It is a consequence of using systems of coordinates and their properties in Riemann geometry (that is, spacetime curvature as we tend to refer to it today) to represent gravity that we find that in some systems of coordinates, the speed of light is not constant over finite distances. Later in that same section, indeed, on the page that JohnDuffield has carefully cut from his screenshot, Einstein also writes, "Nevertheless, this limiting case <also> is of fundamental significance for the theory of general relativity; because the fact from which we started out, namely that no gravitational field exists in the vicinity of a free-falling observer, this very fact shows that in the vicinity of every world point the results of the special theory of relativity are valid (in the infinitesimal) for a suitably chosen local coordinate system."

[Please do not forget or ignore this very important point when someone tries to tell you that general relativity demands that time stops in some scenario. Especially if they tell you that Einstein said this.]

Well the points I have been trying to discuss are lost in the mire so I am abandoning this thread and I'll just carry on without any reasonable answers. I want to learn. I don't want to be lectured and told why everything I read both in textbooks and online is wrong and then not be given any proof that it is wrong that I can reliably test. I won't be posting many more questions on this forum because it just isn't worth it.
 

Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #85 on: 25/02/2015 18:16:49 »
Well the points I have been trying to discuss are lost in the mire so I am abandoning this thread and I'll just carry on without any reasonable answers.
You've had some good answers.

I want to learn
I don't think you do. I think you want confirmation of some idea you've come up with.

I don't want to be lectured and told why everything I read both in textbooks and online is wrong and then not be given any proof that it is wrong that I can reliably test. I won't be posting many more questions on this forum because it just isn't worth it.
Both PmbPhy and I have told you that the ascending photon doesn't lose any energy. The proof is conservation of energy: you send a 511keV photon down into a black hole, and the black hole mass increases by 511keV/c. No energy is acquired by the descending photon. In similar vein no energy is lost by the ascending photon. 

I won't be posting many more questions on this forum because it just isn't worth it.
You ask a question, and you get an answer. Don't reject that answer just because it doesn't square with some popscience nonsense you've picked up. Pursue it, and/or ask the question elsewhere and compare answers.
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #86 on: 25/02/2015 18:58:38 »
Well the points I have been trying to discuss are lost in the mire so I am abandoning this thread and I'll just carry on without any reasonable answers.
You've had some good answers.

I want to learn
I don't think you do. I think you want confirmation of some idea you've come up with.

I don't want to be lectured and told why everything I read both in textbooks and online is wrong and then not be given any proof that it is wrong that I can reliably test. I won't be posting many more questions on this forum because it just isn't worth it.
Both PmbPhy and I have told you that the ascending photon doesn't lose any energy. The proof is conservation of energy: you send a 511keV photon down into a black hole, and the black hole mass increases by 511keV/c. No energy is acquired by the descending photon. In similar vein no energy is lost by the ascending photon. 

I won't be posting many more questions on this forum because it just isn't worth it.
You ask a question, and you get an answer. Don't reject that answer just because it doesn't square with some popscience nonsense you've picked up. Pursue it, and/or ask the question elsewhere and compare answers.

Basically what you keep telling me is that all the things I am reading (lots of mathematics and physics textbooks) are basically pop science and that I have a pet theory to peddle. You quote Einstein willy nilly without even providing mathematical equations to demonstrate that what you say is correct. Usually you just copy and paste an easily recognizable equations derived by someone much cleverer than you. That is why I explicitly asked you how you derived a particular equation to which you replied it was a well known Schwarzschild metric equation. Anyone can do that. Mostly you post pretty pictures which I assume you do not compose yourself. What really irritates me is that I AM putting in the effort. Lots of it. So that I can actually get to a point where I have the tools I need to progress. I could be lazing on a beach somewhere drink in hand. However this is something I wish I had pursued when I was a lot younger. So don't start preaching to me please until you get your own house in order.
 

Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #87 on: 25/02/2015 20:13:25 »
Basically what you keep telling me is that all the things I am reading (lots of mathematics and physics textbooks) are basically pop science and that I have a pet theory to peddle.
I'm afraid I do think some of the things you say are popscience. I don't recall you referring to some textbook when asking a question. 

You quote Einstein willy nilly without even providing mathematical equations to demonstrate that what you say is correct.
A mathematical equation will not demonstrate that what I say is correct. Hard scientific evidence demonstrates that. And all the hard scientific evidence says there are no perpetual motion machines, and that energy is conserved.

Usually you just copy and paste an easily recognizable equations derived by someone much cleverer than you. That is why I explicitly asked you how you derived a particular equation to which you replied it was a well known Schwarzschild metric equation. Anyone can do that. Mostly you post pretty pictures which I assume you do not compose yourself. What really irritates me is that I AM putting in the effort. Lots of it. So that I can actually get to a point where I have the tools I need to progress. I could be lazing on a beach somewhere drink in hand. However this is something I wish I had pursued when I was a lot younger. So don't start preaching to me please until you get your own house in order.
I will reiterate: when you ask a question, you get an answer. If you don't like that answer, ask your question elsewhere, and/or challenge the answer using your own references to Einstein and the evidence and the maths. Now, I apologise for causing offence, please can we get back to the physics.   
 

Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #88 on: 25/02/2015 20:51:27 »
I will reiterate: when you ask a question, you get an answer. If you don't like that answer, ask your question elsewhere, and/or challenge the answer using your own references to Einstein and the evidence and the maths. Now, I apologise for causing offence, please can we get back to the physics.
References to Einstein are not how questions in physics are supposed to be answered.
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #89 on: 25/02/2015 23:16:38 »
Light slows in a gravitational field. Unless gravitons are physically real, there is nothing in a gravitational field to take the place of atoms in other media. What slows light in a gravitational field?
The altered properties of space. In mechanics a shear wave travels at a speed v = √(G/ρ) where G is the shear modulus of elasticity and ρ is density. In electrodynamics the an electromagnetic wave travels at a speed c = √(1/ε0μ0) where ε0 is the permittivity of space and μ0 is the permeability.

c = √(1/ε0μ0)

That is where you were wrong until you corrected it in a later post.
« Last Edit: 26/02/2015 08:08:21 by evan_au »
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #90 on: 25/02/2015 23:47:12 »
But never mind John is here to fix physics.

http://bogpaper.com/science-sundays-with-john-duffield-bankrupting-physics/

So all you protectionist Phd wielding protectionists out there with your pop science theories had better watch out. He's coming for you. I think yor_on has the right idea. Quick, find a table to hide under.
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #91 on: 26/02/2015 00:02:10 »
I have olny read one of Lee Smolin's books "Three Roads To Quantum Gravity". I have no idea what his views are now but they likely don't quite fit with John's ideas.

Smolin's "The Trouble With Physics" is on Amazon. I will not post a link. The outline and reviews should be read at least to see what Smolin's attitude really is. John holds him up as an example of someone "in his camp" so to speak. That is a disservice to Smolin.
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #92 on: 26/02/2015 00:40:02 »
And finally, as a dedicated amateur you should re-read this post from another blog because it sums up the position you put every other amateur in with your behavior and attitudes. How is anyone in the science professions ever to take anyone unqualified seriously when this is the result.

http://www.preposterousuniverse.com/blog/2013/07/29/talking-back-to-your-elders/

JohnDuffield, I think you are making misjudgments in your arguments.

And by the way. I too do not have the official credentials that so many people want to require of me. You need to put in the time to understand the issues in physics. Its human nature to take a specific example (someone without the credentials) and then erroneously apply it to all people that do not have the credentials.
« Last Edit: 26/02/2015 08:04:48 by evan_au »
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #93 on: 26/02/2015 01:18:36 »
Just to help you John here is a Lagrangian. Ask the nice physicists what it means.

bd5aabbac9bc3e5a0bc6cd4eae2fe473.gif
 

Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #94 on: 26/02/2015 12:48:06 »
But never mind John is here to fix physics.

http://bogpaper.com/science-sundays-with-john-duffield-bankrupting-physics/

So all you protectionist Phd wielding protectionists out there with your pop science theories had better watch out. He's coming for you. I think yor_on has the right idea. Quick, find a table to hide under.
Seeing that there is too funny! An entire blog devoted to a school of economics that explicitly denies empirical research. A perfect place for that physics content.
 

Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #95 on: 26/02/2015 14:42:13 »
c = √(1/ε0μ0)

That is where you were wrong until you corrected it in a later post.
It isn't wrong. What's the square root of a sixteenth? A quarter. And what's one divided by the square root of sixteen? A quarter. 

But never mind John is here to fix physics.
No, I'm here to talk physics.
 

Offline jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #96 on: 26/02/2015 15:44:58 »
c = √(1/ε0μ0)

That is where you were wrong until you corrected it in a later post.
It isn't wrong. What's the square root of a sixteenth? A quarter. And what's one divided by the square root of sixteen? A quarter. 

But never mind John is here to fix physics.
No, I'm here to talk physics.

Well talk about physics then. I posted an equation to help.
 

Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #97 on: 26/02/2015 16:44:44 »
See for example hyperphysics where you can read this: "The conservation of energy principle is one of the foundation principles of all science disciplines. In varied areas of science there will be primary equations which can be seen to be just an appropriate reformulation of the principle of conservation of energy". You gave a Lagrangian for a massive particle in a gravitational field, wherein the first portion is the kinetic energy, and the mgz is the potential energy. This is not appropriate for a photon, because the photon is massless, and it's all kinetic energy. If you throw a massive particle upwards, kinetic energy is converted into potential energy. When all the kinetic energy is converted into potential energy, the particle has reached its highest point, and then it start falling back down, converting potential energy into kinetic energy. When you send a photon upwards, it doesn't slow down and stop. Instead, it speeds up. If you don't believe me, contact Don Koks, the editor of the Baez/PhysFAQ website, who said this:

"Now use the Equivalence Principle to infer that in the room you are sitting in right now on Earth, where real gravity is present and you aren't really accelerating (we'll neglect Earth's rotation!), light and time must behave in the same way to a high approximation: light speeds up as it ascends from floor to ceiling (it doesn't slow down, as apparently quoted on your discussion site), and it slows down as it descends from ceiling to floor; it's not like a ball that slows on the way up and goes faster on the way down..."     
 

Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #98 on: 26/02/2015 16:45:35 »
Quote from: JohnDuffield
It isn't wrong.
Of course it's wrong.

In reply #74 you wrote claimed that

0f27e69a0da5439e4647554eba494831.gif

which is incorrect. The correct expression is

b23b1a959c39482715e60dffde20c90c.gif
« Last Edit: 27/02/2015 09:52:25 by evan_au »
 

Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #99 on: 26/02/2015 17:10:25 »
See for example hyperphysics where you can read this: "The conservation of energy principle is one of the foundation principles of all science disciplines. In varied areas of science there will be primary equations which can be seen to be just an appropriate reformulation of the principle of conservation of energy". You gave a Lagrangian for a massive particle in a gravitational field, wherein the first portion is the kinetic energy, and the mgz is the potential energy. This is not appropriate for a photon, because the photon is massless, and it's all kinetic energy. If you throw a massive particle upwards, kinetic energy is converted into potential energy. When all the kinetic energy is converted into potential energy, the particle has reached its highest point, and then it start falling back down, converting potential energy into kinetic energy. When you send a photon upwards, it doesn't slow down and stop. Instead, it speeds up. If you don't believe me, contact Don Koks, the editor of the Baez/PhysFAQ website, who said this:

"Now use the Equivalence Principle to infer that in the room you are sitting in right now on Earth, where real gravity is present and you aren't really accelerating (we'll neglect Earth's rotation!), light and time must behave in the same way to a high approximation: light speeds up as it ascends from floor to ceiling (it doesn't slow down, as apparently quoted on your discussion site), and it slows down as it descends from ceiling to floor; it's not like a ball that slows on the way up and goes faster on the way down..."   
In General Relativity, we are free to use systems of coordinates in which the coordinate speed of light over finite distances can change. This is one way to represent the change in the energy of light from the effect of gravity on that light. In other systems of coordinates, we use the change in frequency of the light to represent the change in energy of the light due to gravity.

Because the light can only be represented as kinetic energy, the only way to represent the change in energy is in the kinetic energy, either through speed or frequency. When it comes to an absorption event, in the system of coordinates in which the absorber is at rest, the light is absorbed at a higher or lower frequency depending on the way that gravity has changed the photon.

In a standard application of this, in a photon traveling away from or towards the Earth, for example, there is no concern about the conservation of energy as the energy of the entire system is conserved.
 

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Re: Would the photon lose all its energy at infinity?
« Reply #99 on: 26/02/2015 17:10:25 »

 

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