# I am also confused in energy in QT

• wangyi
In summary, energy is conserved in the transition between the initial and final states, but not during the intermediate state.
wangyi
Is it really conserved or not?
In one hand, from the Heisenberg principle, \delta E \delta t ~h, then energy is not strictly conserved.

While in the other hand, from the Feynman diagrams we are drawing, the four-vector of momentum is conserved in every vertex, so energy is conserved everywhere.

I think some of the examples raised in the post before can not well illustrate the broken of energy conservation rule in quantum level, because they can be explained as the energy-0(or nearly 0) state creats a pair of particles, one carries positive energy and the other carries negative energy.

regards
wangyi

https://www.physicsforums.com/journal.php?s=&action=view&journalid=13790&perpage=10&page=3

Scroll down to the what are virtual particles entry.

Energy is conserved between the initial and final state but not during the transition between these states

marlon

Last edited by a moderator:
Incidentally,when computing Green functions (closely linked to S-matrix elements) u'll end up with a $\delta^{4}\left(\mbox{incoming momenta-outgoing momenta}\right)$ which should say it all.

Daniel.

dextercioby said:
Incidentally,when computing Green functions (closely linked to S-matrix elements) u'll end up with a $\delta^{4}\left(\mbox{incoming momenta-outgoing momenta}\right)$ which should say it all.

Daniel.

Hence, energy conservation is respected between final and initial state. When it comes to virtual particles, momentum-conservation is respected AT ALL TIMES because of the above formula. Each vertex has such relations and besides they determin which exact momentum the virtual particles will have to 'carry over'

marlon

Do you mean this:
Energy conservation is always respected.

But how is the superposition of state having different energy?
When we measure it, it fall into each energy with posibility.
Does it mean when we measure it, we must pass energy to it?

In the same way, if a state is a superposition of positive charge and negative charge(can this be made? I think it can, but not sure), when we measure the charge, it falls into either positive or negative. But it is not likely true that we pass any charge to it.

I now think it is like this, do you agree with me?
Energy conservation is always respected.
the deltaE deltaT relation only tells us that the particle can go off the mass-shell by amount deltaE during time order deltaT?

wangyi said:
I now think it is like this, do you agree with me?
Energy conservation is always respected.
the deltaE deltaT relation only tells us that the particle can go off the mass-shell by amount deltaE during time order deltaT?
correct

marlon

## 1. What is QT energy?

QT energy refers to the energy associated with quantum mechanical phenomena. It is a measure of the energy of a system at the quantum level, which is the level of particles and atoms.

## 2. How is QT energy different from classical energy?

QT energy differs from classical energy in that it is quantized, meaning it can only take on discrete values rather than being continuous. This is a fundamental aspect of quantum mechanics that distinguishes it from classical physics.

## 3. What is the relationship between QT energy and QT mechanics?

QT energy and QT mechanics are closely related, as QT mechanics is the branch of physics that studies the behavior of particles at the quantum level, including their energy. QT energy is a fundamental concept in QT mechanics and is used to describe and predict the behavior of quantum systems.

## 4. How is QT energy measured?

QT energy is typically measured using a variety of experimental techniques, such as spectroscopy, where the energy levels of a system can be observed and measured. These measurements can then be used to calculate the energy of a quantum system.

## 5. What are the applications of QT energy?

QT energy has many important applications in various fields such as chemistry, materials science, and electronics. It is used to understand and predict the behavior of particles and atoms, which is crucial in developing new technologies and solving complex problems in these fields.

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