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Author Topic: Would the photon lose all its energy at infinity?  (Read 69599 times)

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.
 

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?
 

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 »
 

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.

Quote
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 »
 

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?
 

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. 

Quote
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.
 

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?
 

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 »
 

Offline 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.
 

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.
 

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 »
 

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 »
 

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:

 

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.
 

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.
 

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. :)
 

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.
 

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
 

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 »
 

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.
 

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.
 

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 »
 

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?
 

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.
 

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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Re: Would the photon lose all its energy at infinity?
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