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Quote from: Bill S on 22/02/2015 14:17:49...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.

...My understanding is that the expansion of space is the major factor, if not the sole cause of the redshift...

...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.

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.

Quote from: PhysBang on 22/02/2015 15:16:51...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.

Quote from: PhysBang on 22/02/2015 15:16:51This 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/c², 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 ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN. 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?

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?

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.

Naïve 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?

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?

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

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.

John, you can measure a expansionary redshift, and their photons too, and also find them having lost 'energy' due to it.

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.

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.

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...

Quote from: PhysBangYou 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 holeNo it isn't. This is clearer if you use a falling brick. Momentum p=mv is shared equally, but kinetic energy KE=½mv² isn't.

The brick's energy doesn't change. Gravity merely converts potential energy into kinetic energy. Read ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN: "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..."

Quote from: yor_on on 22/02/2015 16:39:15The 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.

Again, cherry-picking quotations from Einstein does not help us understand physics.

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.

But the fact remains that when you send a 511keV photon into a black hole, its mass increases by 511keV/c², not a zillion tonnes.

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, ...

...the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy

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.

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.

Do you know how to calculate the mass of a particle when it's in a gravitational field?

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.

They most certainly are not!

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.

You claim that there's no such thing as negative energy in physics. I think that you don't know what energy is.

What slows light in a gravitational field?

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?

Quote from: Bill S on 23/02/2015 19:39:50What slows light in a gravitational field?The causal structure of spacetime? I think I need time to come up with a better answer.

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.

You don't calculate it, you measure it.

Quote from: Bill S on 23/02/2015 19:39:50Light 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.

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?

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.

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.

I won't be posting many more questions on this forum because it just isn't worth it.

Quote from: jeffreyH on 25/02/2015 17:51:39Well 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. Quote from: jeffreyH on 25/02/2015 17:51:39I want to learnI don't think you do. I think you want confirmation of some idea you've come up with. Quote from: jeffreyH on 25/02/2015 17:51:39I 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/c². No energy is acquired by the descending photon. In similar vein no energy is lost by the ascending photon. Quote from: jeffreyH on 25/02/2015 17:51:39I 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.

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.

But never mind John is here to fix physics....sorry, you cannot view external links. To see them, please REGISTER or LOGINSo 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.

c = √(1/ε0μ0) That is where you were wrong until you corrected it in a later post.

But never mind John is here to fix physics.

Quote from: jeffreyH on 25/02/2015 23:16:38c = √(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. Quote from: jeffreyH on 25/02/2015 23:16:38But never mind John is here to fix physics.No, I'm here to talk physics.

It isn't wrong.

See for example ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN, 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..."