Would the photon lose all its energy at infinity?

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Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #50 on: 22/02/2015 15:18:40 »
...My understanding is that the expansion of space is the major factor, if not the sole cause of the redshift...
I think there's a big issue here that you're missing, wherein the universe expanding over time can be likened to pulling away from a black hole through space. Note what Pete said: the total energy of a photon moving through a gravitational field is constant. And remember that if you accelerate away from the photon source, you measure the photons as redshifted, but they haven't lost any energy. If you climb away from the photon source, you measure the photons as redshifted, but they haven't lost any energy. So when the universe expands, the inference is this: you measure the CMB photons as redshifted, but they haven't lost any energy.
This is all great stuff, but it contradicts every cosmology text that discusses the expansion of the universe and explicitly includes the loss of energy from redshift in calculating the energy density of photons.

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Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #51 on: 22/02/2015 15:58:33 »
...Yes. It's quite simple, you merely choose a system of coordinates with an overall motion of 13m/s, to the systems of coordinates you are naively using, in the opposite direction to the falling of the brick.
You're talking out of your hat. The brick and the ground still have a closing speed of 14m/s.

This is all great stuff, but it contradicts every cosmology text that discusses the expansion of the universe and explicitly includes the loss of energy from redshift in calculating the energy density of photons.
But the fact remains that when you send a 511keV photon into a black hole, its mass increases by 511keV/c, not a zillion tonnes. Energy is conserved. We know of no situation where it isn't. If some of those cosmology texts say gravitational field energy is negative, they're at odds with Einstein, who said the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy. And the issue is this: how can a photon in space lose energy when it doesn't interact with anything, and where did that energy go?

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Offline yor_on

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Re: Would the photon lose all its energy at infinity?
« Reply #52 on: 22/02/2015 16:39:15 »
John, you can measure a expansionary redshift, and their photons too, and also find them having lost 'energy' due to it. And PhysBang haven't spoken out of his hat yet as I think :) In fact any redshift of a photon means a loss of energy, and it's just as strange due to me moving away from its source than with a expansion. Both are in a way about 'motion'. It just that we are so used to thinking of it form of ordinary objects, as a ball thrown, that one seems a lot simpler than the other.

=
You can argue that a photon due to you moving away loses energy as a result of its momentum becoming smaller in relation to your direction of motion, just as with the ball thrown to you. I don't see how I can argue the same with a expansion though? That supports what you see looking at a light source redshifting, moving away from you, relativistically speaking, and also supports your definition of a photon staying intrinsically the same. But a expansion is another matter to me, and not what I call 'observer dependent'.
=

The point is that no matter ones direction, from, or towards the photon source, those photons will have a exact same speed. The only difference is one of energy lost, or gained. and that is one he* of a mystical thing to me. So we can use classical physics to describe it through the idea of momentum, but we can't use light slowing down or speeding up, relative ourselves. When it comes to a expansion I still don't know how a 'photon' is thought to lose energy in itself though. The best description is a wave there, and then just presuming the duality of light to be adhered too. Somewhat to how we define conservation laws and a 'photon recoil'. Those ideas make a great deal of sense to me.
« Last Edit: 22/02/2015 18:11:38 by yor_on »
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Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #53 on: 22/02/2015 17:25:15 »
...Yes. It's quite simple, you merely choose a system of coordinates with an overall motion of 13m/s, to the systems of coordinates you are naively using, in the opposite direction to the falling of the brick.
You're talking out of your hat. The brick and the ground still have a closing speed of 14m/s.
OK, now you are talking about something different. It is important to be precise, especially when discussing quantities like work that can depend on choice of system of coordinates.

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This is all great stuff, but it contradicts every cosmology text that discusses the expansion of the universe and explicitly includes the loss of energy from redshift in calculating the energy density of photons.
But the fact remains that when you send a 511keV photon into a black hole, its mass increases by 511keV/c, not a zillion tonnes. Energy is conserved. We know of no situation where it isn't. If some of those cosmology texts say gravitational field energy is negative, they're at odds with Einstein, who said the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy. And the issue is this: how can a photon in space lose energy when it doesn't interact with anything, and where did that energy go?
You seem to think that you can stick to one aspect of one physical scenario and use it to defeat many unrelated aspects of physics. If you accept Newton's Third Law, then you accept that a photon falling into a black hole also attracts the black hole. So the energy increase in the photon is exactly matched by a loss in the black hole, just like for any gravitational interaction. One cannot simply measure the total energy of the system, that does not change, and then say that none of the energy for any part of the system never changes; that's simply the fallacy of division.
« Last Edit: 24/02/2015 03:22:31 by evan_au »

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Offline Bill S

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Re: Would the photon lose all its energy at infinity?
« Reply #54 on: 22/02/2015 18:42:22 »
Nave questions.

I am stationary, relative to the Earth.  A photon is approaching me at c, and at the start of the scenario is one light minute away. 
I accelerate away from the photon at an appreciable % of c.   
The photon is still closing the distance between us at c, but it is redshifted.  That means that when it catches up with me I will measure a redshift.
Does the photon have less energy as a result of being redshifted?
How could my action take energy from a photon that was one light minute away?

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Offline evan_au

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Re: Would the photon lose all its energy at infinity?
« Reply #55 on: 22/02/2015 19:02:01 »
Quote from: Bill S
Does the photon have less energy as a result of being redshifted?
Yes, the red-shifted photon will impart less energy to your detector.
  • Viewed as a wave, the photon is emitted with a certain frequency at the source. But the wave crests will "catch up" with a moving spaceship at a slower rate. (Just like a ship will measure one frequency of ocean waves when it is stationary, but a different frequency when moving towards or away from the source...)
  • Viewed as a stream of photons, the photon are emitted at a certain rate at the source. But they will "catch up" with a moving spaceship at a slower rate. This slower rate can be described via classical physics at low speeds, and by time dilation at relativistic speeds. 

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How could my action take energy from a photon that was one light minute away?
You only measure the energy of the photon when it strikes your detector. So the motion of the detector only affects the photon energy when it is 0 light-minutes away.

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Offline Bill S

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Re: Would the photon lose all its energy at infinity?
« Reply #56 on: 22/02/2015 19:16:08 »
Quote
You only measure the energy of the photon when it strikes your detector. So the motion of the detector only affects the photon energy when it is 0 light-minutes away.

So, if the only factor causing the redshift is my velocity, and the photon misses my detector, it will go on its way unchanged from when it was emitted?  There is no loss of energy, the photon was never redshifted, the only effect is in the measurement?

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Offline Toffo

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Re: Would the photon lose all its energy at infinity?
« Reply #57 on: 22/02/2015 19:31:31 »
Before accelaration: An almost stationary star emitted the photon hundreds of years ago, almost no recoil energy went into the star.

After accelaration: A moving star emitted the photon thousands of years ago, some recoil energy went into the star.

Part of the "missing" energy can be found in the star.

Rest of the "missing" energy can be found ... somewhere else.


« Last Edit: 22/02/2015 19:39:32 by Toffo »

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Online jeffreyH

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Re: Would the photon lose all its energy at infinity?
« Reply #58 on: 22/02/2015 19:50:35 »
Nave questions.

I am stationary, relative to the Earth.  A photon is approaching me at c, and at the start of the scenario is one light minute away. 
I accelerate away from the photon at an appreciable % of c.   
The photon is still closing the distance between us at c, but it is redshifted.  That means that when it catches up with me I will measure a redshift.
Does the photon have less energy as a result of being redshifted?
How could my action take energy from a photon that was one light minute away?

At an appreciable % of c YOUR energy has increased relative to the photon. Plus your time has slowed down. Viewed this way then the photon has to appear to have less energy as your detection equipment is also in a more energetic state.

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Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #59 on: 23/02/2015 12:02:44 »
So, if the only factor causing the redshift is my velocity, and the photon misses my detector, it will go on its way unchanged from when it was emitted? There is no loss of energy, the photon was never redshifted, the only effect is in the measurement?
Correct. And it's the same if you ascend. You measure the photo to be redshifted, but it didn't change, you and your measurement equipment changed.

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Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #60 on: 23/02/2015 12:28:57 »
Quote from: PhysBang
You seem to think that you can stick to one aspect of one physical scenario and use it to defeat many unrelated aspects of physics. If you accept Newton's Third Law, then you accept that a photon falling into a black hole also attracts the black hole. So the energy increase in the photon is exactly matched by a loss in the black hole
No it isn't. This is clearer if you use a falling brick. Momentum p=mv is shared equally, but kinetic energy KE=mv isn't.

Quote from: PhysBang
One cannot simply measure the total energy of the system, that does not change, and then say that none of the energy for any part of the system never changes; that's simply the fallacy of division.
The brick's energy doesn't change. Gravity merely converts potential energy into kinetic energy. Read this:

"As an illustration, consider two objects attracting each other in space through their gravitational field. The attraction force accelerates the objects and they gain some speed toward each other converting the potential (gravity) energy into kinetic (movement) energy..."   



John, you can measure a expansionary redshift, and their photons too, and also find them having lost 'energy' due to it.
True enough. But where did this energy go?

In fact any redshift of a photon means a loss of energy, and it's just as strange due to me moving away from its source than with a expansion.
Like Bill was saying, your motion doesn't actually change the photon. It hasn't actually lost any energy.

I don't see how I can argue the same with a expansion though? That supports what you see looking at a light source redshifting, moving away from you, relativistically speaking, and also supports your definition of a photon staying intrinsically the same.
Interesting, isn't it?

The point is that no matter ones direction, from, or towards the photon source, those photons will have a exact same speed.
That's another can of worms. If you head towards a star that's two light years away at 0.99999c, and if that star goes nova just as you set off, you will see the flash when you're halfway there. Your local measurement of that light coming towards you is c, but you and the light covered the two light years in one year of my time. Your closing speed was 1.99999c.

The only difference is one of energy lost, or gained. and that is one he* of a mystical thing to me. So we can use classical physics to describe it through the idea of momentum, but we can't use light slowing down or speeding up, relative ourselves...
What you measure is not always the way it is.
« Last Edit: 24/02/2015 03:23:28 by evan_au »

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Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #61 on: 23/02/2015 14:02:06 »
Quote from: PhysBang
You seem to think that you can stick to one aspect of one physical scenario and use it to defeat many unrelated aspects of physics. If you accept Newton's Third Law, then you accept that a photon falling into a black hole also attracts the black hole. So the energy increase in the photon is exactly matched by a loss in the black hole
No it isn't. This is clearer if you use a falling brick. Momentum p=mv is shared equally, but kinetic energy KE=mv isn't.
Sure, but the momentum of the brick changes if we examinie it alone. The momentum of the photon changes as well. And where is the momentum of a photon?
Quote
The brick's energy doesn't change. Gravity merely converts potential energy into kinetic energy. Read this:

"As an illustration, consider two objects attracting each other in space through their gravitational field. The attraction force accelerates the objects and they gain some speed toward each other converting the potential (gravity) energy into kinetic (movement) energy..."   
OK, so you found a single sentence that supports what you are saying. But that sentence does not say that the momentum doesn't change. Nor is attempting textual analysis of individual quoatations a replacement for physics.

Quote
The point is that no matter ones direction, from, or towards the photon source, those photons will have a exact same speed.
That's another can of worms. If you head towards a star that's two light years away at 0.99999c, and if that star goes nova just as you set off, you will see the flash when you're halfway there. Your local measurement of that light coming towards you is c, but you and the light covered the two light years in one year of my time. Your closing speed was 1.99999c.
Some people make a very basic mistake in relativity theory, thinking that the speed of light is always relative to some object. The speed of light is, in special relativity, constant relative to nice systems of coordinates where Newtonian mechanics works. In general relativity, the speed of light is constant in every infinitesimal region of every system of coordinates.
« Last Edit: 24/02/2015 03:25:32 by evan_au »

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Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #62 on: 23/02/2015 14:26:25 »
Only it isn't constant in the room you're in:


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Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #63 on: 23/02/2015 14:59:45 »
Again, cherry-picking quotations from Einstein does not help us understand physics.

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Offline Bill S

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Re: Would the photon lose all its energy at infinity?
« Reply #64 on: 23/02/2015 15:09:02 »
Quote
Again, cherry-picking quotations from Einstein does not help us understand physics.

That's true, but for non-experts like me some explanation of what Einstein meant by (for example) the last sentence of John's quote would be of great value.

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Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #65 on: 23/02/2015 15:17:55 »
Again, cherry-picking quotations from Einstein does not help us understand physics.
He was explaining that the coordinate speed of light only has the value c in an inertial frame, in a vacuum in the absence of a gravitational field. If there is a gravitational field present then the coordinate speed of light varies with the gravitational potential. This is a fact of general relativity and well tested.

I derived this here - http://home.comcast.net/~peter.m.brown/gr/c_in_gfield.htm

The derivation for a uniform gravitational field/uniformly accelerating frame of reference is the same as the one Einstein did in his 1911 paper on the subject.

And there's absolutely nothing wrong with quoting a physics source. It is doing physics in the sense that you're referring to the calculations of results obtained by others. Doing it correctly is the challenge. :)

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Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #66 on: 23/02/2015 15:47:25 »
It is one thing to recognize that the coordinate speed of light changes over finite distances. It is another thing to deny that the speed of light is constant over infinitesimal regions.

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Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #67 on: 23/02/2015 16:07:33 »
It is one thing to recognize that the coordinate speed of light changes over finite distances. It is another thing to deny that the speed of light is constant over infinitesimal regions.
I see. Is that what he was claiming? In which post did he make such a remark?

By the way. If you're interested a derivation from a text there here's one from Gravitation and Spacetime - 3rd Ed. by Ohanian and Ruffini

http://home.comcast.net/~peter.m.brown/gr/Ohanian_sol.pdf

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Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #68 on: 23/02/2015 16:31:15 »
Quote from: JohnDuffield
But the fact remains that when you send a 511keV photon into a black hole, its mass increases by 511keV/c, not a zillion tonnes.
Do you know how to calculate the mass of a particle when it's in a gravitational field?

Quote from: JohnDuffield
We know of no situation where it isn't.
There's no reason to assume that the energy of any particle, including photons, is conserved when its moving through a gravitational field. The law of conservation of energy applies to the total energy of a system, not just to single particles moving through a gravitational field. An example of when the energy of a particle changes when its moving through a gravitational field is when the gravitational potentials, i.e. the guv, are time dependent.

The derivation of this fact can be found in A First Course in General Relativity - 2nd Ed. by Bernard Schutz, page 176

I put it on my website at http://home.comcast.net/~peter.m.brown/gr/conserved_quantities.htm

Quote from: JohnDuffield
If some of those cosmology texts say gravitational field energy is negative, they're at odds with Einstein, ...
They most certainly are not!

Quote from: JohnDuffield
...the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy
Just because the energy of a gravitational field acts the same way as other energy, it doesn't mean it has the have the same value of energy.

You claim that there's no such thing as negative energy in physics. I think that you don't know what energy is.
« Last Edit: 24/02/2015 03:29:03 by evan_au »

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Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #69 on: 23/02/2015 17:19:54 »
That's true, but for non-experts like me some explanation of what Einstein meant by (for example) the last sentence of John's quote would be of great value.
He meant what he said. Light curves in the room you're in because the speed of light is spatially variable. The speed of light near the floor is less than the speed of light near the ceiling. If it wasn't, your pencil wouldn't fall down.

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Offline PmbPhy

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Re: Would the photon lose all its energy at infinity?
« Reply #70 on: 23/02/2015 17:30:39 »
Quote from: JohnDuffield
Light curves in the room you're in because the speed of light is spatially variable. The speed of light near the floor is less than the speed of light near the ceiling. If it wasn't, your pencil wouldn't fall down.
So what? Everyone who knows GR knows this. So long as you know that this is not about infinitesimals. PhysBang was correct when he said
Quote
It is one thing to recognize that the coordinate speed of light changes over finite distances. It is another thing to deny that the speed of light is constant over infinitesimal regions.
He's much better than you at physics. You'd better listen to him.

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Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #71 on: 23/02/2015 18:06:42 »
Do you know how to calculate the mass of a particle when it's in a gravitational field?
You don't calculate it, you measure it. And I will reiterate: when you send a 511keV photon into a black hole, the black hole mass increases by 511keV/c. You know this.

See? This is what I mean by the errors you keep making. There's no reason to assume that the energy of any particle, including photons, is conserved when its moving through a gravitational field.
I'm not making any errors, and you know it. Because in your article you said this:

"The total energy of a photon moving through a gravitational field is constant."

They most certainly are not!
Yes they are. Einstein made it crystal clear that gravitational field energy is positive. It causes more gravity.

This is a good example of your poor understanding of what you read. Just because the energy of a gravitational field acts the same way. Just because it acts the same way as other energy it doesn't mean it has the have the same value of energy. That's just plain dumb.
No, it's a good example of my understanding. When two bodies fall together some of their mass-energy is converted into kinetic energy which ends up being radiated away into space. We're then left with a mass deficit. People then refer to binding energy as negative energy, but there's no actual negative energy present, just less positive energy. The conservation of energy books balance.     

You claim that there's no such thing as negative energy in physics. I think that you don't know what energy is.
I know what energy is. See post #7 here where on 19/09/2013 you said this:

"My objection with French here is that he gives the impression that energy "exists" whereas physics uses it merely as a bookkeeping scheme which requires no physical system."

It does exist. Matter is made from it. Ah, I see you've changed your tune, because in your article you say this:

"This means that there is a flow of energy going around the electron. What is it that's actually flowing? All we can do at this point is give it a name. And the name we give it is energy." Have you been reading my Energy Explained?
« Last Edit: 24/02/2015 03:30:59 by evan_au »

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Offline Bill S

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Re: Would the photon lose all its energy at infinity?
« Reply #72 on: 23/02/2015 19:39:50 »
Another nave question.

Light in a vacuum travels at c.

Light in a medium appears to travel more slowly, but this is because the photons interact with atoms in the medium (I know thats an oversimplification).  Between atoms, light still travels at c.

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?

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Offline PhysBang

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Re: Would the photon lose all its energy at infinity?
« Reply #73 on: 23/02/2015 19:50:13 »
What slows light in a gravitational field?
The causal structure of spacetime?

I think I need time to come up with a better answer.

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Offline JohnDuffield

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Re: Would the photon lose all its energy at infinity?
« Reply #74 on: 23/02/2015 19:55:50 »
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.

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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 »
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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.

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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.

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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 »

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Online 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?

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Online 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.

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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

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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 [tex]c_0={1\over\sqrt{\mu_0\varepsilon_0}}[/tex].

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?


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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.]

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Online 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.

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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.

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Online 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.

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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.   

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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.

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Online 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 »

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Online 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.

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Online 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.

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Online 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 »

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Online 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.

[tex]L = \frac {1}{2}m(\dot{x}^2 + \dot{y}^2 + \dot{z}^2) - mgz[/tex]

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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.

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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.

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Online 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.

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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..."     

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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

[tex]c = \sqrt{\epsilon_0 \mu_0}[/tex]

which is incorrect. The correct expression is

[tex]c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}[/tex]
« Last Edit: 27/02/2015 09:52:25 by evan_au »

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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.