Can energy be created/destroyed in quantum physics(I need experts)?

[Nereid]

Hawking radiation has not yet been observed; not for want of looking though

 

[Kruger]

no, energy is not conserved in QM as well due to the HUP ; it is this QM-idea that is adopted by QFT. Momentum-conservation is always respected in both QM and QFT. For example in QM the transition from one state to another is described in terms of virtual transition states.

mhh. I have a physics book here and there it says that the number of particles in QM is always conserved.

 

[marlon]

mhh. I have a physics book here and there it says that the number of particles in QM is always conserved.

that is correct, the same goes for free field theories but not for interacting quantum field theories. Besides keep in mind that you are talking about real particles. The energy conservation is valid between real particles. The violation part occurs in some period of time during the actual interaction between real matter particles;

marlon

 

[jackle]

I think energy uncertainty is a problem but it could depend how you interpret this.

what exactly do you mean by these words ?
marlon

My weak understanding is like this: to know the energy of an electron you must measure it for a suitable duration. This suggests to me that conservation of energy is unprovable for very short durations. These wouldn't be long enough in principle to measure the energy. How do we interpret this?

I have already decided that my understanding is very shakey because I can't see why the same isn't true of momentum/position. ie. Can we prove conservation of momentum in a system that we know is somewhere within an unimaginabley small space? Wouldn't that make the momenta uncertain?

You might be able to help me with this

 

[Kruger]

If I may ask: Why does every integral in the attached formula needs a factor 1/pi?

Of course it is the forumula to calculate (with euler mclaurian formula) the casimir force?

 

[James Jackson]

I don't know anything about that specific integral, but I'd imagine it's some kind of normalisation factor, or a by-product of the space you're integrating over.

 

[danAlwyn]

I was told (possibly mistakenly) that the layman's particle explanation of Hawking radiation went something like this:

"Virtual" particles are created as intermediaries to an exchange in QFT somewhere near the event horizon of a black hole. These particles would normally disappear like they do in most QFT problems, except:

The proximity to the black hole leads to a massive gravitational field. Falling into the black hole, even for that brief period in which the virtual particles exist, results in the virtual particles "absorbing" enough gravitational potential energy that they can actually be turned into real particles.

One of the new real particles falls into the Black Hole. The other one may actually have enough energy to escape.

The total consequence then, from the perspecitve of a long range observer, is that somehow the gravitational energy of a black hole has been tapped in order to create an escaping particle. Thus, the black hole has lost energy equivalent to the energy of the particle that got away, and we interpret that escaping particle as Hawking radiation. In this case we end up with global conservation of energy.

Is this view incorrect? Or is it possibly an oversimplification?

 

[James Jackson]

Sounds about right, although the particle - antiparticle pairs aren't due to an 'exchange somewhere', they're particles created out of the vacuum.

 

[danAlwyn]

Sounds about right, although the particle - antiparticle pairs aren't due to an 'exchange somewhere', they're particles created out of the vacuum.

I should have clarified, I just meant that the particle-antiparticle pairs had to be created within the region sufficiently near, but not inside, the event horizon. If they're created too far away, then the gravitational field isn't strong enough, and nothing interesting happens. I'm just being unclear today, but I agree with you.

 

[James Jackson]

I think vacuum production and annihilation is pretty interesting by itself :-)

 

[Kruger]

virtual photons can also appear from nothing. they have not to be a product of field interaction (like electrostatic force, ...).

 

[Kruger]

Does somebody know my question about the 1/pi in the integral (post with attached picture)?

 

[Kruger]

? Is there anyone?

 

[marlon]

? Is there anyone?

i cannot read this forumula clearly. Where does this integral arise ? Please give some more info.

marlon

 

[Kruger]

Sorry. It is the derivation of the Casimir force. One has to build the difference between the energy inside the walls and outside the walls. And this difference is presented in the formula above. Ground state energy is hw/4pi and with w=c|k| you can integrate over all k outside the walls and inside the walls ... I think you understand.

 

[James Jackson]

Is k a reciprocal space? If so, I would imagine the relation between k space and real space would cause the Pi.

 

[Kruger]

k is the wavevector

 

[heusdens]

According to quantum physics energy only exists in small pockets of energy called quantum-or something like that,I personally don't know qunatum physics,that's why I need an expert.
The law of energy conservation is wrong according to quantum theory of physics?

I think the short answer must be that also under consideration of quantum physics, energy is still a conserved quantity, and the violations of that law under very tiny time conditions are the exclusions which admit the energy conservation law.

 

[tom.stoer]

I don't know whether it's a good idea to step into this discussion at such a late stage.

Comments from my side:
- virtual particles are mathematical entities only (we had this discussion in numerous threads)
- looking at Feynman diagrams involving virtual particles energy-momentum is conserved at the vertices - always!
- energy in quantum field theory is defined via the Hamilton operator H defined as integral over three-space
- of course energy can only be conserved if one does not restrict the region of integration to some subset of three-space
- H is conserved in any QFT I've ever seen

In curved spacetime (GR, QFT on curved manifolds) energy cannot be defined unambiguously. The problem is that local conservation of energy-momentum density (which always holds) does no longer allow one to define a globally conserved energy; this is due to the curvature which allows this integration only in certain special cases (there must be a timelike symmetry of the spacetime manifold).

So if curvature is present one must use different "non-local" definitions of energy which is no longer possible unambiguously (several different definitions are known). The problem is that these definitions do no longer rely on the Noether theorem or any other local symmetry argument.

 

[Andrey]

Yes, energy can be created from nowhere for a short amount of time cause of Heisenberg's uncertainty principle. The particle that are created are called "virtual particles" and are always created in pair because of conservation laws (charge, ...). Energy conservation can be violated for short times and this happens all the time. In the same way we can describe interactions in quantum electro dynamics. The hawking effect is based on this energy creation and destruction.

Watch in google, or wikipedia for:
Heisenberg's uncertainty prinziple
Energy-time uncertainty
virtual particles

No that's not true. You cannot create something from nothing. Something has to exist prior to creation of something else otherwise where would the causes for new phenomena come from?

 

[ZapperZ]

Not sure why this thread from 2005 was resurrected, but please read the FAQ thread in the General Physics forum.

Zz.

 

[tom.stoer]

No that's not true. You cannot create something from nothing. Something has to exist prior to creation of something else otherwise where would the causes for new phenomena come from?

I fully agree!

We had this discussion a couple of times and I can only repeat what I stated before: Even for virtual particles energy conservation holds at every vertex in a Feynman diagram!