Does quantum mechanics violate conservation of energy?

Wait... so we have a bunch of energy just sitting there conserving energy until one day it exp-lodes...

The low frequency waves coming off of it contain the equal amount of energy which was "destroyed" as opposed to conserved during the material explosion... Which vibrates the surrounding "space".

Is this "vibration" equivalent to the amount of matter which was transfered into energy during the explosion or the push off from our photons that allows them to reach an unstoppable speed?

Like say a photons equivalent to breaking the sound barrier?

 

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You seem to be under the impression that if something about a system changes, energy or momentum have to be added/removed to make the change. Quite frankly, I don't know where you got this idea, and QM has no problem with energy conservation if you don't introduce it.

This is pure nonsense. I recognize the words and the syntax as English, but I can't find any semantics.

 

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To break the sound barrier you need air and in the beginning there was no air Ha Ha

 

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You're arguing about nonsense in a public science forum. If there was a scientific consensus for your analysis then that wouldn't be the case.

 

OK, I'm trying to follow. Where I struggle is to see the need to equate this new term of yours, of "driving force", with energy emission or absorption, which is presumably what you must think occurs somewhere if you think if energy is NOT conserved. Can you elaborate on this?

(By the way, I suppose in QM we should speak of energy being conserved on average rather than at any instant in time, since the Uncertainty Principle will preclude an analytically exact energy balance to be drawn up at any given instant.)

 

Why? I don't see that classical physics in itself necessarily requires a significant energy input to cause anything that you might feel like calling a "big change". That might be true in certain or even many circumstances, but I think you'll find that wherever it is true there's a good understandable reason for it that's more substantial than "there was a change".

Incidentally, we can do interference experiments with classical light just as well as with individual photons. Do you think classical electromagnetism violates energy conservation or causality?

Why not? Why can't that in itself be regarded as a physical change, caused in a way that is already well described by quantum physics?

I don't think you can argue that. We all learn classical physics and we know what can happen in interference experiments. Yet no-one seems to be moved by your argument, and despite the fact quantum physics has been around for 80-90 years what you're describing has never been regarded as a paradox.

 

I think it has been in some radar transceiver tech manuals from the early 50's and 60's, and was stated that this type of phenomena hasn't been explained. I don't think any other experiments have really tested this principle, for instance I don't think they have even attempted to use it in laser's that consist of mixed wavelengths. There have not been many other technologies based on this principle that I can think of besides the microwave. That was what I referred to earlier as particle precognition at an action at a distance.

 

So you're referencing a half remembered tech manual from 60 years ago as the source of your position? Is this the same source which told you that microwaves are electrons?

 

Fednis48

Yes, I am under the impression that energy must be added for a system to undergo a displacement of position, that's one of the most basic concepts of physics. I can't believe you don't agree.

Exchemist, you say

I agree that the average energy is conserved, in that the particles/photons have an energy and the average number of particles is always constant, but the change in the position distribution requires energy, even though the average position is the same, even though the number of particles/photons is the same.

Forget about "energy emission or absorption, which is presumably what you must think occurs somewhere". What I'm saying is that the change in position distribution accurrs in the absence of an input of energy. This violates conservation of energy. No change of position distribution comes for free. Or does it. Clearly the change in position distribution that I have given as an example (the 2-slit quantum eraser and the Mach-Zehnder interference modulation) has a violation. There is a change in position distribution which is for free.

przyk, you comment

Classical or quantum mechanical, light is light. I'm trying to get to the bottom of the question "what causes the displacement of the position distribution" and arguing that there needn't be an explanation is insufficient.

And your other question,

Actually, the change to the description is not a direct change to the position/momentum component of the description, it is (in the case of the insertion of a half wave plate in front of one slit) a change to the polarization component on one path (which entangles path and pol.) so that the subsequent bra-ket calculation (correlation fn) shows no interference. I'm not saying this is not well understood by quantum mechanics, I'm saying that it is understood in a way that cannot be reconciled with energy conservation. Remember, the end statistics are not wholely determined by the state description, they are partly determined by chance, the measurements are statistical. Somehow the state description is always fulfilled by a statistically relevant set of measurements. But what accounts for the change in the statistics? Is it the half wave plate interacting with the system? Absolutely insufficient explanation. The statistics just work out at the end of the experiment in a way that does not conserve energy.

And your final comment

Maybe the time has finally come.

 

The final analysis; an experiment

The best way to state this conundrum is to state it in the form of a thought experiment. Let's use the same example, the half wave plate (HWP) inserted in front of one slit of a double slit experiment. The HWP wipes out the interference.

Set up our interference so that the constructive/destructive bands of interference fall on a material that can be ionized to create a potential. If there is interference then there will be a potential difference between the constructive bands and the destructive bands. If there is no interference then there will be no potential difference.

Now install the HWP on a wheel that can modulate the insertion of the HWP very quickly, so that it modulates the presence of interference. This will modulate the potential difference (between a potential difference and no potential difference).

Also setup the potential difference modulation so that it drives a motor which stores energy.

Drive the wheel so that it modulates the HWP insertion, which modulates the interference, which modulates the potential difference, which drives a motor that stores the energy.

Now wise guys, explain to me where the energy is comming from. It's not comming from the interaction between the HWP and the light. Where is the energy comming from?

 

Well from the light, surely? In this setup, when the interference pattern is present, the screen converts a portion of the incident light to electricity instead of just heat, but when the pattern is absent all the incident light is simply absorbed by the screen and turns to heat. I don't see any mystery there. After all, you must have a light source, powered from something.

 

This is contrived. I could do the same thing in classical physics by exchanging the slits and HWP with a simple slotted screen in front of the light source. You're arguing about energy conservation but you're really just discussing information loss at a given scope.

 

Exchemist your comment in response to "where's the energy comming from"

The energy results from the difference in the two possible potentials. Something has to drive the change in potential. You can't just say the light is powered by something. We all know that. If the light's energy was causing the energy input then we would not need to modulate the interference pattern. The change in interference statistics causes a change in the potential which drives the motor. So what is causing the change in statistics? Surely not the interaction with the HWP.

 

Do the experiment. Then you tell us the results. Assertions and convoluted thought experiments are not going to get it.

 

I'm not even convinced there would be a potential; it's alternating ionized areas. Even if it could, how does this differ from the slotted-screen classical optical physics scenario I mentioned?

 

You said there is a potential difference when the interference pattern is present. That means you can extract electrical energy when the pattern is present. I cannot conceive of a setup in which energy can only be extracted when the system is modulated. Modulation is not a source of energy. If you modulate a radio wave you do not thereby increase its energy, you merely alter the pattern of lumpiness in the transmission of that energy.

 

Photons don't "get up and move" at all. They aren't sitting on the screen, then re-arrange themselves when you close the slit, they are continuously refreshed by the light source.

Nonsense. The pattern on the screen is constantly being re-drawn by the light source; a continuous input of energy.

By the way, you can repeat this experiment with a garden hose and see the same process at work. It won't be an interference pattern, but it will be a change in the distribution of wetness on the wall. And you could even aim the spray at a pair of alternating buckets to collect the water and power a hydroelectric generator by the alternation of which slits were open.

 

I am not aware of any such principle in classical physics. Like I said above there may be some or even many situations where a change would require an energy input, but wherever that is true there will always be a good identifiable reason for it beyond just that there was a change.

Example: you see a 10 kg block of lead lying at rest somewhere. A second later, you notice that it has somehow been displaced 10 metres from its original position. Clearly, that would require an energy input, and there's a very clear reason for that: simply, the block of lead must have had an average velocity of at least 10 m/s during that second, so at some point it would have had a kinetic energy of at least 500 joules and it would have had to get that energy from somewhere.

But there's no particular reason something like this should be true for photons in an interference experiment. They already have all the kinetic energy they need to land anywhere you might see them land from the moment they leave the source, and it is simply a matter of directing them. Classical physics doesn't put any lower bound on the amount of energy that would be needed for this; it can be done with an arbitrarily low energy cost.

If anything I'd say it's simply reality that disagrees with you since it is not merely a prediction of quantum physics or classical electrodynamics that we can cause or destroy interference without a "driving force". It has long been an experimental fact: we can cause or destroy interference and apart from the light source itself we don't need to be constantly powering the setup to do it.

But the title of this thread doesn't say "light". It says "quantum mechanics". Yet your problem does not seem to be limited to quantum physics. So why are you suggesting quantum physics violates energy conservation based on a phenomenon that a classical theory is also quite happy predicting? Why are you saying your conclusion follows from "classical reasoning" when an entirely classical theory -- electromagnetism -- disagrees with you?

Well is your problem about energy conservation or is it about causality? Make up your mind; they are not the same thing. If it's about energy conservation, then you have not successfully argued that there is a problem because you have not supported this principle of yours that a change -- any change -- always requires an energy input. If it's about causality then quantum physics already provides a causal explanation, otherwise it would be unable to predict when we should and when we shouldn't expect to see interference patterns.

How so? It's always the same photons with the same energies that get absorbed by the detector plate. A statistical distribution in itself does not have any energy associated with it.

 

The source that produced the light of course. Assuming something like what you describe could actually work, you will never get more energy out of it than the energy the photons have just before they get absorbed and cause the ionisation.

 

Ignoring all the sytrofoam here, the operative term appears to be "coherent".

Hence the reason for a coherent source. So far so good.

Here you hit the wall. Are you simply asking why it's difficult to measure amplitude near a null? In that case the topic here is sensitivity, which is a measurement issue, not a statement about QM or conservation of energy.

Is this a school lab question? It reads like you're assuming that some measurement issues have lead to you to believe that noise has quantum sources, which is fine, except that it's not clear you understand the ramifications of noise at a null. Have you taken a statistics course? It might be better for you explain what statistics you are talking about. Otherwise I think you may have just jumped to the wrong conclusion. Noise happens, and it may seem rather elusive for the person who hopes to get below the noise floor. Feel free to elaborate, since I may have misunderstood you.

Question: would you expect σ² to go up, down, or stay the same as you dial from "constructive" to "destructive" on your interferometer?