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

1. No-where-man

184
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?

2. Kruger

219
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

3. marlon

Actually, virtual particles are inherent to QFT not QM. When going from an initial state to a final state we describe the actual transition in terms of a sequence of virtual states. These are like a bridge between the initial and final state. These states are a consequence of perturbationtheory and are virtual because indeed they do not obey energy conservation during a short amount of time. That is why these states don't have long lifetimes. Energy conservation is respected between initial and final states but not inbetween. In field theory, the analogon of these virtual transition states are virtual particles because vibration of fields corresponds to particles. But again, that last interpretation is a QFT thing NOT a QM thing

marlon

4. Kruger

219
We have to differ from two things now.
marlon you describe the way particle exchange forces (spoken in simple language)
and I think of the particles that take there energy from "nowhere". Consider for example the "hawking radiation". You see what I mean?

5. marlon

you are making two mistakes here. First of all, virtual particles only arise in QFT because of second quantization. The mere fact that a change in vibrational modes of a field (you know going from energy level 1 to level 2) corresponds to a particle of certain momentum and energy is a QFT thing.

Secondly, in QFT, virtual particles can arise in two distinct and completely different ways. You are mixing these two ways. Virtual particles can arise as the force mediators between matter particles. In this case they carry a distinct momentum value and due to HUP, they are everywhere in space (not in spacetime!!!). These are the analogon of what we call virtual transition states in QM, because these virtual particles also directly arise from perturbation theory in QFT. Then, you have the vaccuum fluctuations or the zero point energy (ZPE). These two ways of 'generating' virtual particles are totally different in nature...That is my point

marlon

12,314

Daniel.

7. vanesch

6,236
Staff Emeritus
I think it is correct to say that in quantum theory, there is strict energy conservation between the initial prepared state and the final observed state. However, in the *calculation* you can encounter intermediate ("virtual") states which do not respect energy conservation. But that's just a way to solve the time evolution problem.

cheers,
Patrick.

8. Kruger

219
I see marlon. But the vacuum fluctuation is a result of energy time uncertainty and the zero point energy is a result of momentum position uncertainty. And vacuum fluctuation is the case where virtual pair are created and these pair annihilate after a short time. The ground state field is always there.

9. El Hombre Invisible

Energy conservation following Hawking radiation... I think I read years ago in that bastion of semi-interested science readerships A Brief History Of Time that one of the pair is negative energy and the other is positive energy. Is this true? What the heckfire is negative energy anyhoo?

8,147
Staff Emeritus
No. One of them is a regular particle and the other is its antiparticle. They both have positive energy but their charges, if any, and spins, if any, are opposite.

11. Kruger

219
@Self Adjoint: For this negative energy question we have to look with which theory we word. According to Dirac's hole theory positrons are holes in the negative energy sea. And according to QFT or QM (I'm not sure which theory) the positron has negative energy and a negative velocity and thus a positive energy. You see what I mean?

12. Kruger

219
Sorry, I have a little problems with a something:

Is the vacuum state of the em-field (energy: E=h(bar)w/2) a product of virutal particles or is it a product of momentum-position uncertainty?

13. El Hombre Invisible

How can energy be conserved if Hawking radiation is possible? (The answers probably that my understand is so far off it ain't even funny.) If a virtual particle can break the laws of energy conservation for a short period of time, but in that time the black hole attracts one of the pair and the other is flung off to maintain momentum, and both have positive amounts of energy, then the amount of energy both in the black hole and the space outside has increased. Also, when I read it he did not seem to be talking about matter/antimatter. In fact, I thought Hawking radiation was somewhere in the x-ray dept. But then maybe my memory is just screwed.

14. Chi Meson

1,772
I think what Hombre is referring to is the "evaporation" of a black hole. I have also read that the drawn-in particle consists of "negative energy" which annihilates with the energy of the singularity. This way the emmitted Hawking radiation is balanced by the loss of mass/energy in the interior. (I *know* something is wrong here, I'm seeking clarification).

Is the "evaporation" of a black hole still a valid theory? (My mantra is "I graduated such a long time ago.")

15. vanesch

6,236
Staff Emeritus
Dunno what's in vogue, I'd think it is still the case.
But I find it rather peculiar that people jump at such hypothetic and badly understood phenomena to illustrate *quantum theory*. Hawkin radiation and black hole evaporation are educated guesses of results a future quantum gravity theory should be able to answer clearly, by mixing stuff from general relativity and quantum field theory.
Why take such examples ?

cheers,
Patrick.

16. wangyi

56
in QFT there is a $$\delta^4(\sum p)$$ on each vertex. i think this means conservation of energy is respected everywhere, not only the initial and final state, but also intermediate states. (on condition that you still take p^0 as energy). I think the "virtual" only indicates that it does not satisfy the E^2=p^2+m^2 relation, so the energy can be conservationally passed to other particles freely on condition that the time period is short.

But i do not have full confidence on by opinion, and waiting for your idea:)

regards
wangyi

17. wangyi

56
Say something on my view of Hawking radiation,
first, blackholes do not only radiative photon, but all kinds of particles(see,eg,gr-qc/0406017).
second, the particle pair in Hawking radiation is particle and antiparticle, but also,
they are positive and negative energy ones. because their four-momentum sum is zero.
third, we can not say it do not respect energy conservation. because in our point of view, we see the blackhole give out positive-energy particles, and then it gets smaller, i.e. less energy. in the point of view of person inside the blackhole, they think the black hole has eaten a particle with positive energy(because the x and t changes in blackhole), and the blackhole gets larger. As the two people can never tells each other, they think energy is conserved(here i have a question: what if the observer outside run fast into the blackhole and tells the one inside the blackhole)
fourth, in GR, energy is questionable.

cheers
wangyi

18. Rebel

53
Sorry, but I dont know how can you talk about a person " thinking " inside a black hole and the observer to " run fast into the black hole " - to tell the other one- , nor the x and t changes in the black hole, can anyone tell me if some (and wath) of this has any sense? Maybe the only intriguing question to me is really about that of the x and t changes and giving an positive amount of energy inside.

Last edited: May 12, 2005
19. jackle

264

I also thought that energy is uncertain in quantum physics for real particles as a consequence of entanglement with time. As I remember, energy is always conserved as long as you discard unthinkably unlikely events and don't introduce unthinkably short fractions of time.

20. vanesch

6,236
Staff Emeritus
I'd be surprised :-)

cheers,
Patrick.