Does uncertainty principle imply non-conservation of energy?

[loom91]

Hi,

I was wondering about the following question. Does Heisenberg's Energy-Time uncertainty inequality (ΔE.Δt=>h/2) imply non-conservation of energy? I mean, if the total energy of the system fluctuates, then how can the energy be conserved? Does the COnservation of Energy so fundamental to classcial and relativistic physics break down in quantum mechanics? Thanks.

Molu

 

[Kurdt]

It is possible to break the energy conservation law but if you look yourself at the equation you will see that this can only be achieved for very small periods of time. For example try working out how long a stationary electron can exist by plugging its rest mass in the equation and finding delta t.

 

[vanesch]

I was wondering about the following question. Does Heisenberg's Energy-Time uncertainty inequality (ΔE.Δt=>h/2) imply non-conservation of energy?

The short answer is: no, there's no violation.

The longer answer can be this:
Given the probabilistic aspect of quantum theory, what do we mean now, by "conservation of energy" ?
In quantum theory, it can be expressed in two different ways. The first way is this: A state with a precisely known energy will always keep this energy. The reason is that a state with a precisely known energy is an eigenstate of the Hamiltonian, and that's a stationary state under unitary evolution: so it remains (up to a phase factor) itself.
The second way goes as follows: for a given state, look at its EXPECTATION VALUE of the energy <psi | H | psi>.
This is the statistical average over many trials of measuring the energy.
Well, this expectation value is to remain the same during time evolution.
It simply means that the energy value was not a well-defined quantity (only its expectation value was), and hence we cannot talk about violation of its conservation, given that it wasn't fixed initially.

And where does the time-energy uncertainty relationship come in ?
It tells you esentially that *in order to perform an energy measurement with precision dE*, you will need to measure (to have your measurement apparatus interact with) the system for a time of at least dt.

So this means that when you are discussing about a system for a time less than dt, that there is no visible difference between a stationary state with precisely energy E, or with a superposition of stationary states of which the energy eigenvalues lie within dE of E. Indeed, below a time dt, the unitary evolution equation (Schroedinger equation integrated) will not have altered significantly the phases between these contributions as can easily be verified (each term taking a factor exp(-i E t / hbar) ).
So you're not able to find out the difference between the two situations, and hence you cannot know whether the system is in such a superposition, or in a precise energy eigenstate.
As such, the uncertainty is a matter of uncertainty on the INITIAL condition (was the system in a pure energy state or not ?) or of the energy transfer during the measurement interaction between apparatus and system. It is not a question of "stealing energy from nowhere" or something of the kind.

There are two typical cases: 1) the system was "created" in a time dt. This means that during its "creation interaction" one cannot be sure that it was in a pure energy eigenstate: it could be created in a superposition of eigenstates with eigenvalues spread over dE. So you measuring (precisely) the energy value just means you selected out one of the possible eigenstates of which the system was in a superposition: no violation of conservation of energy. This is often the case with "particle resonances" or other short-lived phenomena.
2) The system is *prepared* in a precise energy eigenstate, and you quickly measure, during time dt. In this case, it can be shown that the interaction between the system and the measurement apparatus can give rise to a transfer of energy of order of dE. So reading again another value (within dE) of the energy is then just part of the "perturbation" introduced by the energy measurement apparatus. Again no violation of energy conservation.

Finally, in the long run, the *expectation value* will be recovered, as the average of a great many number of measurements. So there will never be a net gain or loss of energy.

 

[marlon]

Nice explanation Vanesch. I am going to steal this text and put my name under it when a future question like this pops up :)

marlon

 

[pmb_phy]

Hi,

I was wondering about the following question. Does Heisenberg's Energy-Time uncertainty inequality (ΔE.Δt=>h/2) imply non-conservation of energy?

Note: ΔE.Δt=>h/2 isn't really a Heisenberg Uncertainty Relation because there t is not an observable, i.e. there is no hermetian operator corresponding to time. One must proceed with caution when interpreting the meaning of this expresssion.

Pete

 

[loom91]

Hmmm, the second half of that message went over my head, but I think I got my answer in the first part. As one can not assign a definite value to energy, there is not a definite value to be conserved. In a given time interval Δt, Von-Neuman measured values of E will not stay the same but vary within ΔE.

 

[koantum]

A state with a precisely known energy will always keep this energy.

Huh? You can't know a given system will always keep a particular energy, can you? So how can you know its precise energy without waiting till the end of time?

And where does the time-energy uncertainty relationship come in? It tells you essentially that *in order to perform an energy measurement with precision dE*, you will need to measure (to have your measurement apparatus interact with) the system for a time of at least dt.

The following is a quote from an excellent article by Jan Hilgevoord, The uncertainty principle for energy and time (American Journal of Physics 64 (12), pp 1451-6):

There exist many other formulations of the uncertainty principle for energy and time on which we shall only comment briefly. Some formulations are simply wrong, such as the statement that for a measurement of the energy with accuracy dE a time dt>hbar/dE is needed. This statement is wrong because it is an assumption of quantum mechanics that all observables can be measured with arbitrary accuracy in an arbitrarily short time and the energy is no exception to this. Indeed, consider a free particle; its energy is a simple function of its momentum and a measurement of the latter is, at the same time, a measurement of the former. Hence, if we assume that momentum can be accurately measured in an arbitrarily short time, so can energy...​

And right you are, Pete.

 

[loom91]

So what DOES the ET UP mean? It seems to be a much more complicated concept than the xp one. We can measure the energy of a system in a particular instant of time, no time interval is needed for this. Then where does the inequality come in?

 

[koantum]

So what DOES the ET UP mean?... where does the inequality come in?

As there are several wrong uses, so there are several correct uses, as Hilgevoord points out in the aforementioned article. Foremost among these is the relation between the energy width and the lifetime of a quantum state. Note that there is a completely analogous relation between the momentum width of a state and its spatial translation width, and that this is conceptually different from the limitation imposed on the simultaneous measurability of position and momentum by the commutation relation of the corresponding operators.

 

[loom91]

As there are several wrong uses, so there are several correct uses, as Hilgevoord points out in the aforementioned article. Foremost among these is the relation between the energy width and the lifetime of a quantum state. Note that there is a completely analogous relation between the momentum width of a state and its spatial translation width, and that this is conceptually different from the limitation imposed on the simultaneous measurability of position and momentum by the commutation relation of the corresponding operators.

You mean that a higher-energy quantum state will be short-lived? How can that be possible? A quantum state does not collapse until you measure it, so why would it be short-lived if it had higher-energy?

 

[marlon]

You mean that a higher-energy quantum state will be short-lived? How can that be possible? A quantum state does not collapse until you measure it, so why would it be short-lived if it had higher-energy?

Again, you are not interpreting the HUP correctly. If the delta t is small, the delta E gets big. This does NOT imply that the energy of the state is bigger, it implies that the spread of energyvalues that you get after consecutive measurements is bigger. So we are talking about spread of energyvalues and not absolute energyvalues.

Also, the HUP does NOT imply a violation of total energyconservation. Suppose you have a process with initial and final state. When you compare the initial and final energies, there will not be a violation. The energyconservation law, by definition, only applies to initial and final states (ie the external Feynman lines).

It is in the intermediate states that energyconservation can be violated because of the energy-uncertainty. However, the energyconservation law does not apply to such states (ie internal feynmann lines). Momentumconservation is respected always because it applies to both internal and external Feynman lines.

Virtual particles only arise in that specific period of time where the energy is uncertain (so in the internal Feynman lines) and they are the QFT variant of the successive intermediate quantumstates that a system goes through when evolving from the initial to the final state in QM. Given this analogy, it always sounded strange to me that people bring up this apparent energy non-conservation within the context of virtual particles but not within QM context.

regards
marlon

 

[nrqed]

Again, you are not interpreting the HUP correctly. If the delta t is small, the delta E gets big. This does NOT imply that the energy of the state is bigger, it implies that the spread of energyvalues that you get after consecutive measurements is bigger. So we are talking about spread of energyvalues and not absolute energyvalues.

Also, the HUP does NOT imply a violation of total energyconservation. Suppose you have a process with initial and final state. When you compare the initial and final energies, there will not be a violation. The energyconservation law, by definition, only applies to initial and final states (ie the external Feynman lines).

It is in the intermediate states that energyconservation can be violated because of the energy-uncertainty. However, the energyconservation law does not apply to such states (ie internal feynmann lines). Momentumconservation is respected always because it applies to both internal and external Feynman lines.

Virtual particles only arise in that specific period of time where the energy is uncertain (so in the internal Feynman lines) and they are the QFT variant of the successive intermediate quantumstates that a system goes through when evolving from the initial to the final state in QM. Given this analogy, it always sounded strange to me that people bring up this apparent energy non-conservation within the context of virtual particles but not within QM context.

regards
marlon

Actually, energy is conserved even by intermediate states (internal lines). The best way to see this is to use "old-fashioned perturbation theory" (where one uses those nifty time-ordered diagrams or "Z" diagrams) where one imposes explicitly conservation of both energy and momentum at all vertices. People don't use them much because for any loop process there are several OFPT diagrams and their individual expression is not covariant (for one thing, the integration is only over three-momenta). But when they are all summed up, the result *is* covariant (although it is not obvious). When they are summed up, one can actually introduce a 4th integration (over the energy), introduce an i epsilon prescription for handling of the poles and lo and behold, the total combination of all the OFPT diagrams sums up to a Feynman diagram with the usual Feynman rules (and it is obviously covariant)! So it is much faster and simpler to work with the Feynman rules. But they hide the fact that energy and momentum is conserved.

what *is* spe ial about internal lines is that they are off-shell, that is their momenta and energies do not obet the dispersion relation E^2=c^2p^2 +m^2 c^4. But energy and momentum *is* conserved at the vertices.


Patrick

 

[marlon]

Actually, energy is conserved even by intermediate states (internal lines). The best way to see this is to use "old-fashioned perturbation theory" (where one uses those nifty time-ordered diagrams or "Z" diagrams) where one imposes explicitly conservation of both energy and momentum at all vertices.

I am sorry but i do not follow. I agree on the momentum conservation but the energy conservation is a totally different story. If total energy were conserved at all times (which is what you are saying because you also account for the intermediate states) , "virtuality" could not exist. What you are saying is therefore impossible.

Total energy conservation only applies to final and initital states, NOT intermediate states. What i mean with this is the fact that during the intermediate states, the energy is uncertain. Now, indeed one can "pick (ie measure)" the "correct" energyvalue during these states, so that energyconservation is respected. But, other energyvalues are possible as well so in other cases energy-conservation is NOT respected.

To summarize, one can say the energy conservation CAN be respected in intermediate states but it does not have to be like that. This is also expressed by the fact that not all gauge bosons/ force carriers are virtual.

regards
marlon

 

[nrqed]

I am sorry but i do not follow. I agree on the momentum conservation but the energy conservation is a totally different story. If total energy were conserved at all times (which is what you are saying because you also account for the intermediate states) , "virtuality" could not exist. What you are saying is therefore impossible.

Total energy conservation only applies to final and initital states, NOT intermediate states. What i mean with this is the fact that during the intermediate states, the energy is uncertain. Now, indeed one can "pick (ie measure)" the "correct" energyvalue during these states, so that energyconservation is respected. But, other energyvalues are possible as well so in other cases energy-conservation is NOT respected.

What I am saying is that it is entirely possible to do the entire calculation is a consistent way by having energy conserved at all steps (using old fashioned perturbation theory which is, granted, almost completely unknown to most people). In that approach, one only integrates over three-momenta. Then the question is: what is the energy of an intermediate state? *If you use the mass-shell condition* for the intermediate states, then yes, energy will be violated. If they are off-shell, then you can always say that energy is conserved.


Since you may not be familiar with OFPT, let me put it this way. Take a one-loop Feynamn diagram, say. The "energy" integration can be carried trivially in the complex plane, the i epsilon prescription telling us how to do it. It picks discrete values for q_0, it does not integrate over a *continuum* of q_0. It really is different than the three-momentum integration. So one cannot say that *all* values of q_0 are included!
What you are left with are two diagrams (corresponding to two OFPT diagrams). How would you define the energy of an intermediate state then? I am saying thatif you draw them as the two OFPT and use the values of q_0 that were picked by the poles, those values will correspond to the initial energy of the system!

But I won't argue if this makes no sense to you.

Regards

Patrick

 

[marlon]

What I am saying is that it is entirely possible to do the entire calculation is a consistent way by having energy conserved at all steps (using old fashioned perturbation theory which is, granted, almost completely unknown to most people).

What exactly do you mean by "old fashioned perturbation theory" ?
The QM perturbationtheory that you learn in college (and i am pretty sure we are both talking about this one) the concept of virtual transistion states and the connection to energy conservation is very clear and is exactly as i have described it. No point in arguing that.

*If you use the mass-shell condition* for the intermediate states, then yes, energy will be violated.

Indeed

If they are off-shell, then you can always say that energy is conserved.

I don't get this. The energy violation comes from the HUP (uncertainty in energy in the intermediate states) and as a consequence of that, particles are off mass shell. That is the story.

I am saying thatif you draw them as the two OFPT and use the values of q_0 that were picked by the poles, those values will correspond to the initial energy of the system!

But I won't argue if this makes no sense to you.

Regards

Patrick

So but where is the spread in energy during the intermediate states. Indeed, if you pick the values by the poles, particles are on mass shell and all is ok. I agree, but what about the spread in energy?


marlon

 

[marlon]

But I won't argue if this makes no sense to you.

Well, this is always the easy way out, so let me ask you this :

When talking about intermediate states and virtual particles, the fluctuation of energy during such states is a key concept. The influence of the HUP and the direct connection to virtual particles that, by definition, do NOT respect total energyconservation is very clear and straightforeward.

Now, one can always start reciting old models "that nowbody knows about" and that somehow seem to be violating mainstream physics. While doing so, i ask you this : how do you bring in the above mentioned key concepts into your old model ? What is the connection between the two ?


marlon

 

[nrqed]

Well, this is always the easy way out, so let me ask you this :

When talking about intermediate states and virtual particles, the fluctuation of energy during such states is a key concept. The influence of the HUP and the direct connection to virtual particles that, by definition, do NOT respect total energyconservation is very clear and straightforeward.

Now, one can always start reciting old models "that nowbody knows about" and that somehow seem to be violating mainstream physics.
marlon

you are right, I am just a crackpot.

 

[marlon]

you are right, I am just a crackpot.

Ok, what a mature answer. I asked you two specific questions that you do not seem to be able to answer. Good work my man...

regards

marlon

 

[George Jones]

When talking about intermediate states and virtual particles, the fluctuation of energy during such states is a key concept. The influence of the HUP and the direct connection to virtual particles that, by definition, do NOT respect total energyconservation is very clear and straightforeward.

So, you're saying that 4-momentum, which has energy as one component, is not conserved at every vertex.

Regards,
George

 

[nrqed]

Well, this is always the easy way out, so let me ask you this :

If you explain something and someone's first reaction is to say :"No, what you are saying is impossible. Case closed" then what is the point of trying to keep explaining? Your mind is already set. But I will give another try (even though it's probably a waste ot time since you already know that I am wrong)

Now, one can always start reciting old models "that nowbody knows about" and that somehow seem to be violating mainstream physics. While doing so, i ask you this : how do you bring in the above mentioned key concepts into your old model ? What is the connection between the two ?

The fact that you haven't heard about it does not imply that nobody has.
If you had read the original work of Feynman, Bethe, Stueckelberg, etc, you *would* be familiar with this (you obviously are not familiar with their original work). It is sometimes referred to as "Old fashioned Perturbation Theory" or "Non-covariant Perturbation theory" (which is a misnomer..it *is* covariant but not *explicityly* covariant) or "time ordered perturbation theory". ANY DIAGRAM in the "modern approach" can be written as a sum of time ordered diagrams, the two approaches are EQUIVALENT. (you just to do the contour integrals over the zeroth components and you end up with a sum of integrals over three-momenta). It is not a "model" or a discarded theory. It is of course more convenient to work directly with covariant diagrams because they contain all the time ordered ones and the expressions are manifestly covariant.

See for example (I did not have access to my books earlier today):
"Space-Time approach to Quantum Electrodynamics" by Feynman, Phys Rev vol 75, p.486. 1949. Look in particular at the two diagrams of figure 5 for Compton scattering. In the "modern" approach one would only draw on diagram.
(the article is reprinted in "Selected Papers on Quantum Electrodynamics", Edited by Julian Schwinger, Dover.)

Time ordered perturbation theory is covered briefly in "gauge Theories in Particle Physics" by Aitchison and Hey (sections 5.6 to 5.9..look in particular at Fig 5.15).

A quote from this book (p 155, second edition)"

"With this step we have made a fundamental reinterpretation,. In figures 5.7(a) and 5.7(b) the photon has zero mass but energy is not conserved at the vertices. This is, as repeatedly emphasized, perfectly normal in second-order quantum mechanical perturbation theory. One calls the states |n> "virtual" as well as "intermediate" for this reason. However, this is an intrinsically non-covariant statement, since energy is singled out. By contrast, we can interpret fig 5.8 quite differently. We can interpret q=p_a-p_a' as the *4-momentum* of the photon, assuming covariant 4-momentum conservation at each vertex. But then the mass of the photon is q^2 which is not zero! Here also we call the photon "virtual""

where figure 5.8 is simply a usual covariant Feynman diagram (figs 5.6 are time ordered diagrams). The term between ** is their emphasis. The bolded part is my emphasis.

The point is that four-momentum is conserved so energy as well as momentum is conserved at the vertices. It does not make sense to say that one quantity is conserved but not the other in a relativistically covariant theory!

When talking about intermediate states and virtual particles, the fluctuation of energy during such states is a key concept. The influence of the HUP and the direct connection to virtual particles that, by definition, do NOT respect total energyconservation is very clear and straightforeward.

I wil try to make it as simple as possible: consider any Feynman diagram vertex. The Feynman rule contains a four-dimensional delta function, right? This ensures that FOUR-MOMENTUM IS CONSERVED AT THE VERTICES! How can energy conservation be violated if four-momentum is conserved? It does not matter if we are dealing with tree diagrams or loop diagrams...for *any* loop four-momentum the delta function insures that 4-momentum is conserved in all the intermediate states! If this line has more 4-momentum, this other line will have less 4-momentum, in such a way that the total four-momentum of the intermediate state is equal to the total four momentum of the initial state. How can you have violation of energy in that case??

You seemed to think that I was naively going back to QM perturbation theory when it is actually you who was extrapolating a conclusion from QM to quantum field theory.

 

[marlon]

So, you're saying that 4-momentum, which has energy as one component, is not conserved at every vertex.

Regards,
George

No this not at all what i have said. What i am saying is that the violation of energy conservation happens in between two vertices. When calculating the propagator of some interaction between the initial and final state we integrate over all momenta k, so also the momenta for which $$k^2=m^2$$ does NOT count , are in fact incorporated in the QFT.


marlon

 

[nrqed]

No this not at all what i have said. What i am saying is that the violation of energy conservation happens in between two vertices. When calculating the propagator of some interaction between the initial and final state we integrate over all momenta k, so also the momenta for which $$k^2=m^2$$ does NOT count , are in fact incorporated in the QFT.


marlon

energy violation cannot occur "in between vertices" since the four-momentum in a Feynman diagram line is the same everyhwere along the line. If four-momentum is conserved at the vertices, it is conserved everywhere. (it cannot be conserved next to the vertices and then violated a little bit further down the line)

The sum of the four-momenta of all the intermediate particles is always equal to the total initial four-momentum (no choice, it is enforced by the direa delta functions). In that sense energy is conserved.

The virtual particles are not on their mass shell. But that has nothing to do with violation of energy of the interaction.

 

[marlon]

The virtual particles are not on their mass shell. But that has nothing to do with violation of energy of the interaction.

Yes it has. Particles that are off mass shell do not respect total energyconservation. In the same way vacuum fluctuations (virtual particle anti-particle pairs) do not respect energyconservation. In every interaction where the force carrier is virtual, energyconservation is NOT respected in between final and initial state. During this period of time (or in between vertices), one integrates over all momenta to acquire the propagator. It is this integration that is directly responsible for virtual particles and the non conservation of total energy. Only the momentum conservation law is respected "everywhere and anywhere".

marlon

 

[nrqed]

Yes it has. Particles that are off mass shell do not respect total energyconservation. In the same way vacuum fluctuations (virtual particle anti-particle pairs) do not respect energyconservation.

What do you define as energy conservation? What I mean is: if you loojk at the total energy of the incoming particles and the total energy in the intermediate state (for any loop momentum), then the two are equal. The total four-momentum is conserved at the vertices! Consider the emission and reabsorption of a virtual photon by an electron. The delta function enforces that the four momentum of the virtual photon plus the four-momentum of the virtual electron is equal to the four-momentum of the initial electron. Therefore, both the energy and the momentum is conserved.

My definition of energy conservation is: energy in the intermediate state is equal to the energy in the final state. That follows trivially from the delta function at the vertices.

In every interaction where the force carrier is virtual, energyconservation is NOT respected in between final and initial state. During this period of time (or in between vertices), one integrates over all momenta to acquire the propagator.

one integrates over all four-momentum *within the restrictions imposed by the delta functions at the vertices*! One does NOT integrate over all energies and momenta of all the intermediate states.

It is this integration that is directly responsible for virtual particles and the non conservation of total energy. Only the momentum conservation law is respected "everywhere and anywhere".

marlon

this is a noncovariant statement. If it would be true in one frame it would not hold in all frames. Either four-momentum is conserved or not, but one cannot conserved some parts only in a covariant way.

The integration is responsible for virtual particles to be off-shell. If particles could not be off shell, there would be only one way to conserve four momentum. Because they can go off-shell, there is an infite number of ways to share the total four-momentum between the particles in the intermediate states, hence the integration.

 

[marlon]

What do you define as energy conservation? What I mean is: if you loojk at the total energy of the incoming particles and the total energy in the intermediate state (for any loop momentum), then the two are equal.

Well, here's our fundamental difference. In my second post here, i clearly stated that total energy conservation applies to the initial and final state and NOT the intermediate state. In QFT, thanks to this violation of total energy conservation many interactions exhibit the following property : Due to the existence of virtual particles (which DO NOT exist in the initial state for the obvious reason) there is more "total" energy than in the initial state. Many such interactions are known in QFT and this can only exist if energy conservation is violated during the period between initial and final state. It is in this period that interactions with the vacuum can occur as well. If this were not possible, concepts like the vacuum polarization tensor would not exist or be usefull.

The total four-momentum is conserved at the vertices!

This is not the point. We are talking about the spread of energy that exists during vertex points !

Consider the emission and reabsorption of a virtual photon by an electron. The delta function enforces that the four momentum of the virtual photon plus the four-momentum of the virtual electron is equal to the four-momentum of the initial electron. Therefore, both the energy and the momentum is conserved.

Hold on, are we talking about both a virtual electron and photon or just a virtual photon and a real electron. In the latter case your example is invalid because an interaction like that between a real electron and a virtual photon would not occur "just like that". Other processes have to occur as well. Besides, the virtual photon will not be a "member" of the final state. It does not live forever so it cannot be incorporated into the total energy conservation (given the fact that this conservation is defined as i outlined in the beginning)

My definition of energy conservation is: energy in the intermediate state is equal to the energy in the final state.

C'mon, there are a lot of examples where this is not the case. You are also forgetting about the interaction with the vacuum here. really, what you state here is not true.

For example, in beta decay one can easily see that the energy available for the intermediate W boson cannot exceed the mass-energy difference between a neutron and a proton (initial and final state), which is very much less than the mass-energy of a W boson. Thus, the W boson here cannot be observed, yet this is clearly NOT what happens in reality and what we have seen in experiments.

One does NOT integrate over all energies and momenta of all the intermediate states.

Look at the most easy QFT, ie the free field theory. Look at how one describes the interaction between to particles. Look at how the concept of force is introduced in QFT. Look at the first 20 pages of Zee's book.

this is a noncovariant statement. If it would be true in one frame it would not hold in all frames. Either four-momentum is conserved or not, but one cannot conserved some parts only in a covariant way.

I never stated that four momentum is partially conserved. Again, my definition of energy conservation is very clear and i have stated it several times.

The integration is responsible for virtual particles to be off-shell. If particles could not be off shell, there would be only one way to conserve four momentum.

Correct

Because they can go off-shell, there is an infite number of ways to share the total four-momentum between the particles in the intermediate states, hence the integration.

No, one cannot speak about the four momentum of particles in intermediate states since energy is uncertain. Also, many interactions are known (i gave an example) where this is not true. What you state here is totally contradictory.

regards
marlon

 

[nrqed]

Well, here's our fundamental difference. In my second post here, i clearly stated that total energy conservation applies to the initial and final state and NOT the intermediate state. In QFT, thanks to this violation of total energy conservation many interactions exhibit the following property : Due to the existence of virtual particles (which DO NOT exist in the initial state for the obvious reason) there is more "total" energy than in the initial state. Many such interactions are known in QFT and this can only exist if energy conservation is violated during the period between initial and final state. It is in this period that interactions with the vacuum can occur as well. If this were not possible, concepts like the vacuum polarization tensor would not exist or be usefull.

It is clear that we won't make progress unless we look at a specfic Feynman diagram because I am saying that the total energy in the intermediate state is equal to the initial energy and you are saying that it is not. So we just have to write down the expression for a Feynman diagram and see! Let's apply the Feynman rules, look at the intermediate state (in which there are virtual particles), add up their energies and see what happens! Otherwise, it's no different than being indoors and arguing "the sky is blue" "no, the sky is orange" "no the sky is blue" "no, the sky is orange!" and so on, without going out and looking at the darn sky

This is not the point. We are talking about the spread of energy that exists during vertex points !

I am really not sure what "during vertex points" means. I still argue that if four-momentum conservation is imposed at all vertices, then four-momentum will be conserved in all intermediate states.

The simplest example I can think of is the one-loop vacuum polarization diagram. Photon goes to an electron-positron virtual pair and this pair turns back into a photon. If we call q_0 the energy of the initial photon, the sum of the energy of the electron-positron will be q_0, for any loop momentum. Do you disagree with that?? (notice that the energy of a virtual particle is the zeroth component of its four-vector. One cannot use ${\sqrt{m^2- {\vec p}^2}}$ since it is not on-shell)

Or if you prefer, take the scattering of an electron off an external em field. There are 4 diagrams to one loop (vertex correction, mass renormalization on the two external electron lines, vacuum polarization on the photon line). Pick any intermediate state and we'll calculate the total energy to see.

No, one cannot speak about the four momentum of particles in intermediate states since energy is uncertain. Also, many interactions are known (i gave an example) where this is not true. What you state here is totally contradictory.

regards
marlon

tehn how can one ever write down an expression for a Feynman integral? The propagator for a photon, say, is 1/q^2. So if the momentum of a virtual photon is not defined, how can we get anything calculated?

Regards,

Patrick

 

[nrqed]

I never stated that four momentum is partially conserved. Again, my definition of energy conservation is very clear and i have stated it several times.


marlon

Well, you said that three-momentum is conserved everywhere but that energy is not. So I see as partial conservation of four-momentum, which cannot hold as a covariant principle.

 

[vanesch]

I have to agree with nrqed here, Marlon. Normally you have a deltafunction $$\delta^4(\Sigma p_i)$$ at each vertex. What simply changes between the external lines and the internal lines (= virtual particles) is that for the external lines, we have $$p^2 = m^2$$ while for the internal lines this is not true ; it is what's usually called the "on shell" condition for external particles, which is not held for the internal lines.
But, as Pat pointed out, the on-shell condition $$p^2 = m^2$$ has nothing to do with the conservation of the 4-momentum in a vertex. It is not because a particle is off-shell that the 4 components of p are not conserved at the vertex ; simply that the "energy" and "3-momentum" parts are re-distributed in such a way over the different lines meeting in the vertex, that they do not match the on-shell condition anymore.\

At least, that's how I understood this.

Now, I can conceive that there are other ways, in QFT, to calculate the map from initial states to final states - after all, this Feynman diagram stuff is only a calculational procedure. There are maybe ways to do this without concerving energy in intermediate stages, I don't know. If the procedure is not explicitly lorentz covariant, this is thinkable.

 

[vanesch]

Huh? You can't know a given system will always keep a particular energy, can you? So how can you know its precise energy without waiting till the end of time?

Well, simply because a stationary state (eigenstate of Hamiltonian) takes as time evolution a phase factor.

The following is a quote from an excellent article by Jan Hilgevoord, The uncertainty principle for energy and time (American Journal of Physics 64 (12), pp 1451-6):

There exist many other formulations of the uncertainty principle for energy and time on which we shall only comment briefly. Some formulations are simply wrong, such as the statement that for a measurement of the energy with accuracy dE a time dt>hbar/dE is needed. This statement is wrong because it is an assumption of quantum mechanics that all observables can be measured with arbitrary accuracy in an arbitrarily short time and the energy is no exception to this. Indeed, consider a free particle; its energy is a simple function of its momentum and a measurement of the latter is, at the same time, a measurement of the former. Hence, if we assume that momentum can be accurately measured in an arbitrarily short time, so can energy...​

And right you are, Pete.

Well, I'm affraid that this is a wrong statement, but it touches upon interpretational issues again. If you introduce again a "magical instantaneous collapse", then yes, of course you can measure energy instantaneously... and now you'll have to explain to me how you build such an apparatus (always the same problem bites you when postulating this immediate collapse!).

If you consider that you can treat the quantum system and the measurement apparatus as quantum objects, then of course the "instantaneous collapse" doesn't happen, and one needs to introduce a unitary evolution of the interaction between system and measurement apparatus. And here the devil comes in:
this interaction is described through A TERM IN THE HAMILTONIAN OF THE OVERALL SYSTEM. Now if you want this measurement to complete quickly (in this view: to have full entanglement between the system states and the pointer states of the apparatus), then this interaction has to be rather strong, but that means that it INTRODUCES AN ERROR IN THE ENERGY OF THE SYSTEM ALONE. The measurement has *ALTERED* the hamiltonian significantly. So the only way to do this is by introducing a small interaction, which will then give rise to a slow time evolution from product state into fully entangled state. The more accurate you want your energy measurement to be, the smaller the interaction term between the system and apparatus has to be, and hence the slower the time evolution which will entangle the system fully with the pointer states.

In a way, it occurs to me now that the statement in the article gives us a means to know if collapse is "immediate": find a measurement apparatus of energy which violates the energy-time uncertainty (measurement time versus measured value precision). If this can be built, then this is a proof that it doesn't happen through unitary evolution, but is "immediate projection".

 

[koantum]

Well, simply because a stationary state (eigenstate of Hamiltonian) takes as time evolution a phase factor.

There you go again with your time evolution. A stationary state (like any ket or quantum state) is a tool for calculating the probabilities of possible measurement outcomes on the basis of actual outcomes (the "preparation"). The time dependence of this tool is a dependence on the time of the measurement to the possible outcomes of which the probabilities are assigned. There is no such thing as an evolving quantum state.

If you introduce again a "magical instantaneous collapse", then yes, of course you can measure energy instantaneously.

Again? And where does Hilgevoord introduce this "magical instantaneous collapse"? It is YOU who force those who buy your evolving quantum states to postulate reduction in order to avoid the absurdities that arise from YOUR chimera of an evolving quantum state.

and now you'll have to explain to me how you build such an apparatus (always the same problem bites you when postulating this immediate collapse!).

You are confusing separate issues. Ultimately it is only positions that can be measured. The values of other observables are inferred from the outcomes of position measurements. There are different ways of using position measurements to measure, for instance, momentum, which is why "momentum measurement" isn't well-defined unless a measurement procedure is specified. Whatever conceptual or practical problems may be associated with energy measurements, they have nothing to do with the projection postulate.

The measurement has *ALTERED* the hamiltonian significantly.

Since quantum states are represented by unit vectors, the dependence of the probabilities of measurement outcomes on the time of measurement can of course be taken care of by a unitary transformation U. The hamiltonian is the self-adjoint operator that appears in the exponent of U. It has something to do with the interval between measurements. It has nothing to do with the measurements themselves.

 

[vanesch]

There you go again with your time evolution. A stationary state (like any ket or quantum state) is a tool for calculating the probabilities of possible measurement outcomes on the basis of actual outcomes (the "preparation"). The time dependence of this tool is a dependence on the time of the measurement to the possible outcomes of which the probabilities are assigned. There is no such thing as an evolving quantum state.

As anybody adhering to this view will sooner or later discover, this view, by definition, makes it impossible to analyse the precise physics of a measurement apparatus, because it is "brought in by hand" externally to the formalism. So discussing the physics of the time dependence of a measurement will be difficult from this starting point (one of my main reasons not to adhere to this vision, btw), because you've taken away the only tool that might help you analysing what exactly goes on during a measurement process (when the apparatus interacts with the device).
Now, as long as we stay far away from any limits eventually imposed by quantum theory, this can be intuitively handled, but when we come to things such as the constraints of uncertainty principles, there's no tool left.

You are confusing separate issues. Ultimately it is only positions that can be measured. The values of other observables are inferred from the outcomes of position measurements. There are different ways of using position measurements to measure, for instance, momentum, which is why "momentum measurement" isn't well-defined unless a measurement procedure is specified.

Agreed, but in order to be able to link a position measurement to an energy value, you'll find out that you'll always need to have your state to be analysed evolve during a finite amount of time in an apparatus (for instance, a setup such as a mass spectrometer).

It has something to do with the interval between measurements. It has nothing to do with the measurements themselves.

That's interpretation-dependent. Even von Neumann considers this unitary evolution into pointer states as the pre-measurement evolution, using of course standard unitary evolution.

But no need to argue here: it is a statement that can be falsified. Show me an apparatus that can make a measurement of the energy E of a system when it has access to the system during time T, and whose accuracy dE is better than that given by the uncertainty relationship: meaning: the apparatus will be able to distinguish with high certainty two different incoming states which differ by less than dE.

 

[selfAdjoint]

because you've taken away the only tool that might help you analysing what exactly goes on during a measurement process (when the apparatus interacts with the device).

But that's not a reason to reject the viewpoint. In fact the viewpoint is the only one that really respects the model without bending or adding to it. When you attempt to introduce the measuring apparatus into the quantum world and analyze what goes on during the measurement process, what do you get? Multiple Worlds! Science Fiction! Putting the measurement apparatus in by hand obeys the Bohr criterion that our world - the world of the apparatus - is not a quantum world.

There is nothing presented in QM that justifies breaking Bohr. You would have to discover physics beyond QM to justify it, and I don't think anything I've seen in all the self-made puzzlements over the measurement problem that qualifies as that.

 

[reilly]

Don't forget that the true Compton Scattering amplitude is the sum of all possible Feynman diagrams. The usual 2nd order diagrams therefore give an approximation. Looked at from an old-fashioned viewpoint, this is saying out of the infinite number of QED states carrying an electron's charge, we are looking at two. But, virtually by definition, a two intermediate-state approximation is not an energy eigenstate, so there's no reason to suppose that in low order approximations energy is conserved -- all the terms in the perturbation series are required to preserve serious energy conservation. (This is thoroughly discussed in most any text that covers time-dependent perturbation theory, and or scattering theory -- Lippman Schwinger, dispersion theory.)

Virtual particles are a convenient fiction; as in the grandchildren of what used to be called virtual levels. When used with a good dose of common sense, they are a highly valuable concept. They make it easier to talk about equations. That's all there is to it.

I don't expect much agreement. So let me provide a challenge: it's well known that the problem of finding the scattering states of an electron in a static Coloumb field can be solved exactly in parabolic coordinates. This means for this problem all the diagrams can be summed. So, not only are there "virtual photons" but "virtual electrons" as well buried in this exact amplitude. By looking at the Fourier transforms of the wavefunctions, and the scattering amplitude you'll be able to ferret out the role of virtual states in a rigorous way. See if you can prove me wrong

What you'll see is that a single pole term gets mediated by many other terms, the combination of which is needed to satisfy energy conservation in intermediate states -- all of them together in blissful superposition.

Or, here's the bones of an even simpler approach to the issue of "virtual
Think of a square well problem in 1-D. Say in the middle there's a potential V, say from 0-L. An exact solution in the potential sector will have the form of exp+W,
or exp-W, where W*W = -E - V. Whereas before and after the potential,

W*W = -E. So there's an energy jump. But, this energy jump will not show up in regular perturbation theory. It will take the full infinite expression from perturbation theory to get the correct "in-potential" wave function. (The diagrams show the particle as "free" as it traverses the potential; does not look good for energy conservation.)

So, beware of anything but a figurative approach to the idea of a virtual particle; otherwise you'll have to deal with some pretty serious math.

Regards,
Reilly

 

[vanesch]

But that's not a reason to reject the viewpoint. In fact the viewpoint is the only one that really respects the model without bending or adding to it. When you attempt to introduce the measuring apparatus into the quantum world and analyze what goes on during the measurement process, what do you get? Multiple Worlds! Science Fiction! Putting the measurement apparatus in by hand obeys the Bohr criterion that our world - the world of the apparatus - is not a quantum world.

There are two ways to answer this. The first one is the way von Neumann answered it: you can put the "cut" where you want. I think von Neumann would agree that if you were to inquire into the nature of the physics of a specific measurement apparatus, that you should treat it quantum-mechanically (and then he'd put the cut later, as in reading off the display or something). So you do not necessarily end up with MWI (of course, if you try to be logically consistent, you would).
The second one is that we cannot make statements about the physics happening in a measurement apparatus. This is an annoying feature (especially for an instrumentalist such as me :-).

So now let's go back to the statement over which there was a dispute:
my statement was that "a measurement yielding an energy measurement with accuracy dE needs to interact with the system for at least a time dt given by the E-t uncertainty relationship", which was said to be a false statement.
My answer is that if you consider quantum theory as ONLY a theory that is an "algorithm to calculate probabilities of outcomes" (which it certainly is too, of course), that you cannot make ANY STATEMENT about the correctness or not of the above statement, because YOU HAVEN'T GOTTEN ANY MEANS to go figure out how much time it would take for a measurement to have an energy measurement accuracy or anything, AS YOU DON'T HAVE A PHYSICAL DESCRIPTION of what goes on in the apparatus. You have to believe the salesman! You cannot analyse the apparatus.

So I'm not (this time :-) fighting over MWI, I'm just arguing that, if you want to make any meaningful statement over the potential limits on measurement time and energy accuracy during the interaction of a measurement apparatus and a system, that you will have to describe this interaction quantum mechanically. Otherwise you wouldn't even be able to say anything. Unless the salesman of the apparatus told you that it was an "instantaneous energy measurement apparatus" and hence that the corresponding hermitean operator is the hamiltonian. And no way to verify the salesman's statements.
Now, IF you allow for a quantum mechanical description of the apparatus-system interaction (or even only the "essential part" of it, not including the electronics or anything), then you will find out that there's a link between the measurement time (evolution time of the system) and the possible energy accuracy, which is limited by the Heisenberg E-t relationship, simply because of the disturbance NEEDED in the hamiltonian to make the apparatus+system evolve quickly enough = the interaction term between both.
You can consider this QM interaction in the Copenhagen picture, on the condition that you put the Heisenberg cut AFTER the essential part of the measurement process, in which case the interaction is described in a unitary way. And if you DON'T do that, you simply have nothing to say about this interaction.

 

[vanesch]

I don't expect much agreement. So let me provide a challenge: it's well known that the problem of finding the scattering states of an electron in a static Coloumb field can be solved exactly in parabolic coordinates. This means for this problem all the diagrams can be summed. So, not only are there "virtual photons" but "virtual electrons" as well buried in this exact amplitude. By looking at the Fourier transforms of the wavefunctions, and the scattering amplitude you'll be able to ferret out the role of virtual states in a rigorous way. See if you can prove me wrong

What you'll see is that a single pole term gets mediated by many other terms, the combination of which is needed to satisfy energy conservation in intermediate states -- all of them together in blissful superposition.

The claim is simply that during this process, if you started out with a state which was a quite well-defined energy state, that at ANY time, when considering the WHOLE state, you'd find the same energy expectation value, with small dispersion.

You're right of course that, to go from the initial to the final state, you can use whatever calculational procedure you like, and that there's a difference between "energy conservation during time evolution" and "energy conservation during calculation" , the latter not being any law of nature.
That said, in the USUAL way of using perturbative QFT, with Feynman graphs, there IS also "energy conservation during calculation", which is what we were discussing about, I thought. But nothing stops you from using another calculation where this might not be true.