Is the Conservation of Energy ever violated?

[GoldPheonix]

I was talking with a person I know about physics; he claimed that at a Plank distance, the conservation of energy is, in fact, violated. I'm unfamiliar with quantum mechanics (I'm about to attend my first introduction to modern physics this fall, though I already know special relativity and some basic nuclear physics).


I have found nothing that states this; perhaps some one on here could set my understanding straight? Or am I correct that the conservation of energy is never violated?

 

[madmike159]

I thought that to. In quantum tunneling a particle can have extra energy to get through a potential barrier which it would not normaly have the energy to get through, as long as the energy was returned in a certain time.

 

[ZapperZ]

I thought that to. In quantum tunneling a particle can have extra energy to get through a potential barrier which it would not normaly have the energy to get through, as long as the energy was returned in a certain time.

Er.. no. In ballistic (elastic) tunneling, no such thing occurs. It comes out with the same energy that it started with. In inelastic tunneling, the particle interacts with something in the barrier to either gain or lose its initial energy. No energy conservation violation here.

Zz.

 

[GoldPheonix]

Cool. Does anyone else have anything to say on the matter?

 

[cesiumfrog]

Er.. no. In ballistic (elastic) tunneling, no such thing occurs. It comes out with the same energy that it started with.

Yes, but the point is that classically the particle should not of had enough energy to "tunnel" over the barrier (it would have been reflected instead). Isn't it commonplace for evanescent waves to be presented as lending energy for a small time (related to the time-energy uncertainty of the wave packet)?

 

[ZapperZ]

Yes, but the point is that classically the particle should not of had enough energy to "tunnel" over the barrier (it would have been reflected instead). Isn't it commonplace for evanescent waves to be presented as lending energy for a small time (related to the time-energy uncertainty of the wave packet)?

But in elastic tunneling, the ability to tunnel isn't due to it borrowing any energy, but rather how the particle is described. All you have is what went in, and what came out, with the tunneling matrix element describing all you need to know (and can know) during the process. I don't see any borrowing of energy here.

Zz.

 

[nughret]

This property highlights one of the differences between quantum and classical particles; if the two were the same then there would be no need for QM would there.
The only place where i have heard that energy conservation is an approximate is in GR due to the fact that the gravitational field does work on itself but this isn't contained in the energy-momentum tensor, i am not sure if this is correct though.

 

[granpa]

http://en.wikipedia.org/wiki/Gravitational_interaction_of_antimatter

The gravitational interaction of antimatter with matter or antimatter has not been conclusively observed by physicists. While the overwhelming consensus among physicists is that antimatter will attract both matter and antimatter at the same rate matter attracts matter (and antimatter), there is a strong desire to confirm this experimentally. For example, if gravitational interactions between antimatter and matter were found to be repulsive then there would potentially be a violation of conservation of energy, one of the most fundamental laws of physics (and allow for the possibility of antigravity generators).

 

[cesiumfrog]

What about vacuum fluctuations?

 

[Demystifier]

I was talking with a person I know about physics; he claimed that at a Plank distance, the conservation of energy is, in fact, violated. I'm unfamiliar with quantum mechanics (I'm about to attend my first introduction to modern physics this fall, though I already know special relativity and some basic nuclear physics).


I have found nothing that states this; perhaps some one on here could set my understanding straight? Or am I correct that the conservation of energy is never violated?

Planck distance is related to quantum gravity, which we do not well understand yet. Nevertheless, we have several different theories of quantum gravity, and most of them conserve energy at any distance.

 

[Llewlyn]

For the Noether's theorem the conservation of energy follows from the time-traslation symmetry. However you may find into books something about virtual processes that is, processes that "violate" conservation of energy behind Heisenberg indetermination. A typical virtual process is the exchange of a massive particle, as in the old theory of strong interaction (pi meson). I think it's incorrect to refer to those situations as violation of energy, since such processes are not observable, they "just only" figure out in perturbative approach in QFT.

Ll.

 

[ZapperZ]

We even have a FAQ entry on this:

https://www.physicsforums.com/showpost.php?p=986037&postcount=5

Zz.

 

[nickthrop101]

any energy that is put in, has to be given back, meaning there is no energy left ovver. There is NO case where the ;aw is violated :D

 

[tom.stoer]

However you may find into books something about virtual processes that is, processes that "violate" conservation of energy behind Heisenberg indetermination. A typical virtual process is the exchange of a massive particle, as in the old theory of strong interaction (pi meson). I think it's incorrect to refer to those situations as violation of energy, since such processes are not observable, they "just only" figure out in perturbative approach in QFT.

No, even in these processes with "virtual" particles energy is always conserved. If one looks at the corresponding Feynman diagram there is a delta-function which conserves energy and 3-momentum at each vertex. The difference to classical physics is that such a "virtual" particle can violate the mass-shell condition. Classically for a particle with rest-mass m one always has E²-p²=m². For a "virtual" particle this equation can be violated which means that "virtual" photons can have non-zero and even imaginary rest mass.

 

[tom.stoer]

In addition I would say that in quantum gravity the defintion of energy is not clear.

Energy in general relativity can be defined only in certain limiting cases like asymptotic flat spacetime. The reason is that the conserved Noether current is not a vector but a (2,0) tensor and that the local conservation law (continuity equation) for this energy-momentum tensor does not allow for a 3-integral to define energy as "energy content of a certain 3-volume" which transformes as the 0th component of a four-vector.

Therefore energy can only be defined in certain limiting cases of special spacetime geometries.I don't think things get simpler once spacetime is quantized :-)