# Does particle interaction take time?

1. Aug 14, 2009

### wilco97531

Is there a minimum time interval for two fundamental particles to interact, or is the interaction instantaneous? For example, if a photon excites an electron, is there a time period required for this interaction to take place? When a billiard ball hits another one there is obviously a time interval where momentum is being transferred from one ball to the other. Since fundamental particles seem to be uncompressible, is there are time interval for interaction or is it instantaneous?

2. Aug 14, 2009

### kote

Ignoring nonlocal quantum effects, interactions are limited by the speed of light. If you press on one end of an incompressible rod, the other end doesn't move instantaneously. If it did, it would violate special relativity, as the "signal" would travel from one end to the other faster than the speed of light. It's less about compressibility and more about relativity.

For the sake of illustration, say you've got an elementary particle one light-year in diameter. When another particle smacks into it, the part of the particle to get smacked will move instantly, but the other side of the particle, one light-year away, won't move for another year.

3. Aug 14, 2009

### wilco97531

That may be true for a rod, but 'size' for a fundamental particle is somewhat fuzzy, isn't it?

4. Aug 14, 2009

### kote

Yes. But you asked about incompressible particles, which necessarily have definite boundaries and a definite size :).

Unfortunately there's no good answer I can give about the nature of fundamental particles that won't start a neverending argument.

Sorry!

5. Aug 14, 2009

### Sleuth

I would only add that the example made up by kote shows how in special relativity a totally rigid body, as tought in classical mechanics, cannot exist, so there not exists any truly incompressible body.
But if you are dealing with elementary particles, quantum field theory supposes them to be point-like, and actually there is no sperimental evidence of the opposite, so the problem of propagating the signal through out the body doesn't make sense here. The only time that you have to wait is the time it takes the photon to reach your electron, or other point-like particle. The problem could be: how two elementary particle interact as, for example, two electrons? in this case they can interact through photons, W, Z in an enormous amount of ways, always exchangin these kind of particles.
Now these particles are virtual particles, such that the relation p^2 = M^2 is not necessarly true, for example a virtual photon can have a mass different from 0.
The time these virtual particles are allowed to exist is given but Heisenberg principle. DE*Dt = h, or something like this.

6. Aug 14, 2009

### wilco97531

If we consider for example a quantum computer, which uses quantum interactions for calculations, would there be an upper limit to its speed (besides the obvious limitations of the setup)?

7. Aug 14, 2009

### Kryptech

Question?... First I agree the principles of physics are clear for every action there is reaction(put simpley) as for the time it takes for the reaction... to be technical if you take two pool balls... roll one into another... the first transfers energy into the other... there are two aspects that need to be addressed... the initial contact and the fulfillment of transfer. contact is not visual contact but the moment the effects of the first begin to impose on the second ball. as with any object when moved there is movement in the atmosphere around them as well. the atoms being "moved" by the first ball would essentialy "touch" the second before the actual objects collided. in that instant the energy begins transfer when the second is effected it takes time for the energy to cross from the point of impact to the other side of the object being effected (hense my quotation of Kote).

My question is what is your precise question... does it take time to begin the interaction or does it take time to complete it?

in Kote's post he mentioned the rod. It would take the other end of the rod a year to move yes... but that interaction took place immediately... the effects takes time.
according to quantum physics the rod already moved before it was moved.... but as Kote stated in relative terms there is always time involved even if we can't measure it with our own means everything has time. how much time is relative to perspective.

8. Aug 14, 2009

### Sleuth

I think the original question was about elementary particles. When talking about finite bodies like a rod, a ball or whatever, it's obvious that any consideration can be only qualitative: you will never be able to predict how long it takes an interaction between two rods or balls, because you should take into account how single atoms composing the two objects interact with each other etc, without considering the complications brought by taking into account quantum effects. Maybe we should only stress on one point: the answer depends upon which theory you are dealing with, or which theory you trust... if you trust QFT the interaction can be drawned as a feynman graph, and as such in the same istant when a photon scatters an electron, it changes its energy and momentum etc etc...
Then another problem can rise: if we are considering for example and atom, so we have an electron in some state, and it is hit by a photon and excited, we can ask how long does it take the electron to change its state? but this is a different problem, I think.