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#55823 - 05/04/16 08:44 PM Energy conservation in curved spacetime.
Bill S. Offline
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A photon’s energy depends on its frequency (f). Higher frequency = greater energy.

If a photon travels through space having E = f1, and enters a gravity well, it is blue-shifted, its energy becomes E = f2.

As f 2 > f1; if measured at any point in the gravity well, the photon’s energy is greater than it was when in space.
Where does the extra energy come from?

Two possible answers suggest themselves.
1. It comes from the gravitational energy of curved spacetime.
2. Energy is not globally conserved in curved spacetime.
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#55830 - 05/08/16 12:56 PM Re: Energy conservation in curved spacetime. [Re: Bill S.]
Orac Offline
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The third choice is one needs to take care when one chooses a frame of reference in a problem.

A person places a bullet in a gun and fires it. You may calculate the energy that was imparted from the bullet into the gun as the gun speeds away from the bullet which is stationary and contains no energy because it isn't moving in its reference frame. That person speeding toward the bullet passed right around the bullet which was sort of surprising to the bullet because the person was much bigger than it and it was shocked how fast people can move.

Everything in that statement is correct from the bullets frame of reference, but that is probably not how you would see the events and to have it described in that way you would call very deceptive and possibly wrong regardless of it's technical accuracy.

It does not violate conservation laws for two observers to measure totally different energies, it is the normal as the energy of the bullet in the above answer demonstrates.

Interpretation of events is subjective to view point, and physics can not tell you a position that is more correct than any other. If you get differences you need to consider do you have the same reference point.

Would you care to think carefully about your reference frame, for example do you have only one in the above example smile


Edited by Orac (05/08/16 01:14 PM)
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#55831 - 05/08/16 08:37 PM Re: Energy conservation in curved spacetime. [Re: Orac]
Bill S. Offline
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Quote:
You may calculate the energy that was imparted from the bullet into the gun as the gun speeds away from the bullet which is stationary.....


That works for your bullet and gun, but don't you run into problems if the propagant is a photon? smile
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#55833 - 05/09/16 01:12 AM Re: Energy conservation in curved spacetime. [Re: Bill S.]
Orac Offline
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Let me be blunt, give me the reference frame you can see the photon BLUE and NORMAL. Where exactly in space are you going to stand to see that ... think hard.

Lets take you back to the discussion with Paul and the 3D wave that collapses at one point. You get to observe a photon in exactly ONE location in space and time and once observed it collapses everywhere else. So your discussion first involves at least two different photons (they might be the same at source .. hint that could be a good reference frame) and you aren't even observing them from the same point. Viewed from the source it becomes obvious the background is what is changing !

Do you get it you may see a photon either BLUE or NORMAL but NEVER BOTH. You are constructing some GOD reference frame in your head that simply doesn't exist for multiple reasons. This is the the same rubbish people do with time stopping on the event horizon of a black hole and think its real.

What I should have done in the GUN/BULLET case is mixed statements from the bullet reference frame and statements from the gun reference because that is most certainly what you are doing.

So pick any reference frame you like, but stop using two reference frames and trying to claim them as one. You aren't GOD and you can't be in both reference frames at the same time and nor can the photon. The photon isn't NORMAL and then GOES BLUE as it goes along, it never changes what is changing is the background AKA YOU. A bird flying and landing on a power line change does not change it's voltage either, even though from you view point you may try and claim that.

Different OBSERVERS see things differently but that has nothing to do with the photon which remains oblivious and cares nothing for you observers who only have limited relevance to it until they become casually linked. The bird when landing on a power line may touch another power line and it will suddenly find the placid line it was landing on, suddenly kills it. What the bird did was make contact with two different backgrounds, which independently were safe.

So the BLUE and NORMAL is only meaningful to YOU, the photon is always NORMAL and unchanging .. where YOU ARE STANDING TO OBSERVE IS WHAT IS CHANGING.

This naive idea that layman seem to get that standing here in some location that you can claim time really stops at an event horizon or that a photon "really" changes to blue does my head in. The universe doesn't revolve around humans and they aren't god and given a special reference frame, your reference frame background matters ALWAYS.


Edited by Orac (05/09/16 02:06 AM)
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#55834 - 05/09/16 02:31 AM Re: Energy conservation in curved spacetime. [Re: Orac]
Orac Offline
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So lets put a simple physical example to your photon above.

A) I am driving a car and I have an accident at 15mph running into a brick wall. I survive and after a brief stint in hospital get back to driving.

B) Unfortunately on the way home from hospital at 15mph I run up the tail of a slow moving bus doing 13mph. My injuries from this accident are so minor I am discharged within hours.

C) Resuming my trip home at 15mph I pull out to pass another slow bus. Unfortunately I pull out and run head on into a car doing 20mph coming the other way. I am pronounced dead at the scene.

I was moving at the same speed in the same car (and hence same energy I thought) in all three crashes, so why was the result of my crash so different ... what caused the energy of the crash to change?

In B my car appeared to have less energy in the crash hence my car was redshifted. In C my car appeared to have extra energy in the crash and hence my car was blueshifted. Those descriptions are exactly consistent to what you did to the photon above and I am using the same reference frames. So I ask your question now, what changes the car energy change as it drives along and where does the energy go and come from in the red and blue shift?

I am hoping you can clearly see my car energy never changed, what changed was the background AKA everything else.

Wait I am dead ... you caught a lucky break smile


Edited by Orac (05/09/16 07:54 AM)
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#55836 - 05/09/16 06:16 PM Re: Energy conservation in curved spacetime. [Re: Bill S.]
Bill S. Offline
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Orac, the only problem I have with anything you say in your last two posts is that I think you missed the (simple) point I was making.

SR says that we can consider the gun to be stationary, and the bullet moving, or the bullet stationary and the gun moving, but it does not permit a RF in which the photon is stationary, and the emitter moving.

That’s all.

Sorry you didn’t survive your third crash. Had you survived, I would seriously suggest you consider giving up driving. smile
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#55837 - 05/09/16 06:28 PM Re: Energy conservation in curved spacetime. [Re: Bill S.]
Bill S. Offline
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Quote:
As f 2 > f1; if measured at any point in the gravity well, the photon’s energy is greater than it was when in space.


Considering this in the light of your traffic analogy, there seems to be a fundamental difference.

If I measure the energy of your car at any point, it remains unchanged; only the energy of the interactions alters.

If I measure the energy of the photon before and after it enters the gravity well (f1 & f2) would I not find it had changed?
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#55840 - 05/10/16 03:30 AM Re: Energy conservation in curved spacetime. [Re: Bill S.]
Orac Offline
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Originally Posted By: Bill S.
If I measure the energy of your car at any point, it remains unchanged; only the energy of the interactions alters.

The DAMAGE TO ME IS A RAW MEASUREMENT and the energy most definitely changed, so I just falsified that statement. You are adjusting the measurement in your head using logic and understanding to say that.

This is what I am trying to make you realize measurement involves some understanding of background. What you are trying to say is when you measure things in the crash you understand what is moving and what isn't and you adjust your calculation and deduce my car and I still had the same energy. The raw measurement which was the damage to me most certainly changed but you realized why.

Originally Posted By: Bill S.
If I measure the energy of the photon before and after it enters the gravity well (f1 & f2) would I not find it had changed?

Now we are back to the second problem you don't get to measure a photon twice ... NOT EVER ... they are absorbed by any measurement. You may measure one photon prior to the gravity well, and you may measure a second different photon inside the gravity well. This immediately identifies you better go back to the source and make sure the two photons were the same at source and in doing so you have realized you need to consider the backgrounds at the three point 1.) the source, 2.) measurement point 1, 3.) measurement point 2. Now once you do that equate all the energies which is the same as the car crash and my damage as a raw measurement.

The same situation as the car crash happens with your raw measurement which will show an increase in energy but you will realize it's just the background that makes it look like that. The photon hasn't gained or lost any energy it's just being measured in a different point in space.


Edited by Orac (05/10/16 03:43 AM)
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#55843 - 05/10/16 01:10 PM Re: Energy conservation in curved spacetime. [Re: Orac]
Bill Offline
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Originally Posted By: Orac

Now we are back to the second problem you don't get to measure a photon twice ... NOT EVER ...

Keep in mind that here Bill S. is talking about a thought experiment. In a thought experiment you can do just about any impossible thing, such as measuring the energy of a photon twice.

Bill Gill
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#55844 - 05/10/16 01:24 PM Re: Energy conservation in curved spacetime. [Re: Bill]
Orac Offline
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Originally Posted By: Bill
Keep in mind that here Bill S. is talking about a thought experiment. In a thought experiment you can do just about any impossible thing, such as measuring the energy of a photon twice

Whenever I do thought experiments I stick to the laws of physics it's just discussions about experiments that are hard/dangerous or unethical to setup. Sometimes I might choose to vary one thing and show consequences but that is about it.

I don't get your answer here Bill G, so if I am allowed anything what is to stop me saying something ridiculous like little invisible purple aliens add and subtract energy as required. What is the point of a thought experiment that allows breaking all the laws of physics and anything to happen?

To me your suggestion sounds more like discussion had in philosophy and religion not science and not something I am comfortable with.

I guess you can have such thought experiments just leave me out .. besides I am still dead ATM smile


Edited by Orac (05/10/16 01:36 PM)
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#55845 - 05/10/16 07:47 PM Re: Energy conservation in curved spacetime. [Re: Bill]
Bill S. Offline
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Quote:
If I measure the energy of the photon before and after it enters the gravity well.....


Even as I posted, I thought I should have used a conditional phrase: "If I were able to measure.....". Then I thought I would leave it, to see how predictable Orac was. smile

It seems that the safe "scientific answer" has to be that we don't know if a photon would gain energy on entering a gravity well because we cannot make the necessary measurements. Of course, we can measure the blue shift of a large number of photons, but how can we be sure that an individual photon would behave in the same way as a group?
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#55846 - 05/10/16 10:36 PM Re: Energy conservation in curved spacetime. [Re: Bill S.]
Bill Offline
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Originally Posted By: Bill S.


It seems that the safe "scientific answer" has to be that we don't know if a photon would gain energy on entering a gravity well because we cannot make the necessary measurements. Of course, we can measure the blue shift of a large number of photons, but how can we be sure that an individual photon would behave in the same way as a group?


Now that is one of the core questions about all of our scientific 'facts'. We can determine how things work to a high degree of certainty, but there is always some limit to how closely we can measure, and I'm not talking about the uncertainty principle. I'm just talking about basic measurement uncertainties. So, in fact, the best we can say is that 'it is true as far as we have been able to measure it.'

Bill Gill
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#55847 - 05/11/16 01:29 AM Re: Energy conservation in curved spacetime. [Re: Bill]
Orac Offline
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It's actually worse than that Bill G, if we are going to go down this sort of layman "facts" path then you can't say the photon does enter the gravity well at all. You have no ability to track a photon at all at any point in it's travel.

So actually you can't say anything at all about the photons movements to the level of certainty Bill S is demanding as being scientific. The only certainty Bill S can know to the level he wants is that it is present where he measures it and it has no energy until that time.

To show that you can do a thought experiment that from the source the photon is spreading out in it's 3D sphere and if the gravity well is any distance from the source there will be more of the wave out of the gravity well than in it. So if the gravity well is changing the energy of the photon it can only be in that portion of the photon wave that is in the well. So now move the source further or closer .. what should happen? Big hint here Bill S think about this and your wanting to add and subtract energy to the photon as it enters the gravity well. Do you see the photon seems to "gain" the same consistent amount so maybe it's the collapse to a point in the gravity well that changes the energy not the entering it?

So Bill S I have to ask do photons spread out in a 3D sphere and collapse point or are they particles flying in straight lines as per above it matters to entering the gravity well problem?

I put it to you that the invisible purple aliens watching you know when you are about to measure and put the photon where it needs to be, as per my original thought experiment smile

Now back on the other tangent, Bill G's answer is totally correct and the word he didn't use is measurement background, and every and any measurement requires a background or reference to which the measurement refers, and which the measured magnitude is derived from.

Yes, Bill S I am being predictable again, I am not insisting on a correct answer but making you work thru the answer you give. The correct answer(s) are the only ones that will stand up to scrutiny and I need not insist on them being right, you will work it out for yourself.

Bill S you are at the point you need to work out how you will decide what an acceptable measurement background is. To get you started that lets give a start point and get you to realize you are using "conjoint measurement theory", there are other types. Within your concept there is a problem

https://en.wikipedia.org/wiki/Measurement
Quote:
The concept of measurement is often misunderstood as merely the assignment of a value, but it is possible to assign a value in a way that is not a measurement in terms of the requirements of additive conjoint measurement. One may assign a value to a person's height, but unless it can be established that there is a correlation between measurements of height and empirical relations, it is not a measurement according to additive conjoint measurement theory. Likewise, computing and assigning arbitrary values, like the "book value" of an asset in accounting, is not a measurement because it does not satisfy the necessary criteria.

That is the basis you rejected the measurements as being quote "unscientific", it didn't meet your criteria so name the criteria and lets see what I can do with it.

Hmm yep think I tidied up all the loose ends and covered everything.


Edited by Orac (05/11/16 02:27 AM)
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#55848 - 05/11/16 10:30 AM Re: Energy conservation in curved spacetime. [Re: Bill S.]
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Thanks for the brainstorm. Your discussion is a good example for the connection between the relativity theory and the gravity wells in the universe. Energy does change depending on the environment but without motion this is impossible. The two possibilities are either the curved space time causes energy release or energy increase. Yet, this can be relatively accurate when examined from the certain gravity well.

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#55854 - 05/12/16 01:32 PM Re: Energy conservation in curved spacetime. [Re: ongoingscience]
Orac Offline
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Originally Posted By: ongoingscience
Energy does change depending on the environment but without motion this is impossible.

If energy can only exist with motion then what is potential energy, chemical energy, nuclear energy and possibly dark energy none of which require motion at all. The idea is naive and has been falsified for probably well over 200 years.

Many physicists would argue photons should not be called energy either even though you can write a formula for energy for them and this is the problem Bill S was grappling with. Technically photons are simply the potential to become energy and they must collapse to a point in spacetime to do so, but it leaves you with the problem what was the photon before that?

If you want to push a physicist on what a photon is well then it is a ripple in quantum fields that may or may not become "stuff" which you would call energy, that is all we can say.

If you push us on what energy is well it certainly isn't matter and it isn't motion but it exists in those things. What it is as best we can tell is the exchange of information in the fields of the universe.

I was playing a game with Bill S who has discussed photons and QFT enough to work out the problem with his idea if he thinks about it. The problem is pretty basic most of the "photon wave" doesn't even go into the gravity well, especially if the source is outside the well. In that situation at least half of the "photon wave" 3D sphere is actually going away from it. It's only when it collapses to a point because of measurement within the gravity well that it actually "enters" the gravity well in a formal sense. Think about a wave reaching the centre of the well but being observed somewhere outside the well and suddenly the energy it supposedly gained vanishes.

Trying to construct any concept that the photon gains and loses energy in the proper 3D spherical photon emission scheme is ridiculous. It was only working in Bill S's head because he had turned the photon to a tennis ball like particle which flew along in a straight line to where he measured it, something we teach the kiddies at school and probably shouldn't as he just illustrated why.

I was surprised it was taking him so long to see the problem and I was having to give so many hints.


Edited by Orac (05/12/16 02:03 PM)
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#55856 - 05/12/16 09:41 PM Re: Energy conservation in curved spacetime. [Re: Orac]
Bill S. Offline
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Quote:
In that situation at least half of the "photon wave" 3D sphere is actually going away from it.


Can you relate this to the double slit experiment where photons are sent through singly?
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#55857 - 05/13/16 03:37 AM Re: Energy conservation in curved spacetime. [Re: Bill S.]
Orac Offline
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Yes it always has been the explaination of why it interference with itself in that the waves pass thru both slits. We don't talk about the part of the wave that are also going away from the slits but however lets do that now.

So our photon wave passes thru slits and is on is half way on it's way to a view point we have setup to see the normal interference. At that exact instant we actually observe the photon at the point the same distance behind the source point and away from the slits so our "part of the wave" we are observing hasn't gone thru any slits. The observation collapses the wave to that single point and it shows no interference pattern at all, it reflects the conditions of the part wave that we encountered only.

The key point to this is each part of the wave seems unconnected to other parts of the wave and they know nothing of each others histories. The collapse to a point only reflects the history of the wave reaching the observation point.

This is what make photons tricky because you can't make them real or energy or having interactions until they collapse to a point. Good old Prof Matt Strassler in his articles does a good job on it

https://profmattstrassler.com/articles-a...alse-dichotomy/

He basically ends up saying the same thing we all do, quote "Photons should not be called `energy’, or `pure energy’, or anything similar."

The key point that determines what happens with a photon is at collapse point and you really can't construct histories or interactions with anything until you have a collapse point. It is what makes your OP statement misguided and what makes the double slit experiment do peoples head in especially in things like delayed choice and quantum eraser experiments when they construct histories and say things like well the photon has passed thru the slit ... well no it didn't until it collapses to a point.

So the answer to your OP question is the photon doesn't change energy and can't. The energy difference is simply a reflection of the energy differences between the background of the source point and the point of measurement.

The thing to ponder is does that mean even a point in empty space has energy or is the energy background something to do with being able to measure?


Edited by Orac (05/13/16 04:01 AM)
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#55865 - 05/14/16 04:14 PM Re: Energy conservation in curved spacetime. [Re: Orac]
Orac Offline
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After all that discussion you should be able to follow this now which is basically the same thing with entangled atoms. The waveform view and the particle view differ markedly as in your original post of an photon entering a gravity well.

There detector can detect the wave without collapsing it by using some QM tricks of entanglement. I thought that bit might interest you.

http://phys.org/news/2016-05-atoms-entangled-emit.html


Edited by Orac (05/14/16 04:25 PM)
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#55866 - 05/14/16 09:31 PM Re: Energy conservation in curved spacetime. [Re: Orac]
Bill S. Offline
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An interesting article, Orac.

Quote:
This might sound like a reversal of cause and effect, with the effect pushing on the cause. It is possible only because of one of the weirdest of all the quantum effects: When an atom emits light, quantum physics requires the light and the atom to become connected, or entangled, so that measuring a property of one instantly reveals the value of that property for the other, no matter how far away it is.

Or put another way, every measurement of an entangled object perturbs its entangled partner. It is this quantum back-action, Murch said, that could potentially allow a light detector to control the light emitter.


This can't involve using entanglement for FTL information transfer, but it begins to sound that way.
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#55867 - 05/14/16 10:19 PM Re: Energy conservation in curved spacetime. [Re: Bill S.]
Bill S. Offline
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I like to try to answer questions, but I seem to have missed this one.

Originally Posted By: Orac
So Bill S I have to ask do photons spread out in a 3D sphere and collapse point or are they particles flying in straight lines as per above it matters to entering the gravity well problem?

No; photons do not spread out in a 3D sphere, light does that. Light is not waves or particles. The nearest we can come to modelling light is to say that it travels as something we identify as waves and is – or can be – detected as discrete particles. While travelling as a wave, it spreads out, but this does not introduce gaps, as it might if sufficient spreading revealed a particulate nature.

Only when light is observed can it be "identified" as displaying a particulate nature; beyond which point it can neither spread, nor do anything else.

Perhaps we should consider light as a probability distribution, some properties of which appear to us to be quantized. Could we derive a more appropriate model from that starting point?
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