# Interaction - Fundamental question

1. May 9, 2015

### Omega0

Hi,

What is the basic idea about particle interaction in QM?

Say you have first one single charged particle in an "empty universe". Here I think the first issue is given: That's impossible, you'll need masses (or better: energy) from a GRT view. Okay, say we watch this single charged particle - but we don't know that it is charged.
In the classical picture I have a field, something strange, you can draw radiating lines or whatever. Unfortunetely this makes no sense for a single particle because we need a measurement. Now let us stll be the "god observer" without interaction to the scene. I place now a particle of the oppenent charge in my universe. I have now attraction between both particles (retarded, for sure). Now it makes more sense to think in "field lines" between both particle but nevertheless: This world outside is hidden until I measure the change, so I wiould measure the acceleration of the particles.
I would need to disturb the process via measurement.
Between both particles exists something called "electromagnetic attracion". There is potential energy between both. They will accelerate. They will "radiate". It is impossible to find a reference frame where they don't.

So let's get to quantum picture of the scene. Both particles are accelerating, so I expect them to emit particles. They emit them in opposite directions to accelerate. Are those photons? What is about the "field" around those charged particles - do you think it exists because we have two particles or do we only measure it as a "third particle"? How would you describe this picture?

Thanks,
Jens

2. May 9, 2015

### bhobba

The interaction terms of the Hamiltonian.

QM cant be viewed that way.

Thanks
Bill

3. May 9, 2015

### Omega0

Why shouldn't it? There is interaction. So i need an interacting particle?
PS: This is related to QFT which I would say is a part of QM

Last edited: May 9, 2015
4. May 9, 2015

### bhobba

In QM what an object is doing when not observed is anyone's guess so cant be described in terms of acceleration (and in fact isn't), attraction in QFT is not like that (its described as a perturbation between the field of the particle and the field of what's emitted), and emissions described by QFT is not formulated in such terms eg you need a Fock Space:
http://en.wikipedia.org/wiki/Fock_space

Thanks
Bill

Last edited: May 9, 2015
5. May 9, 2015

### Omega0

Okay, thanks.
But how does the "Fock space" help me in finding out what happens to those both pariticles?
Mathematics is nice, but what about the physics, what would you say is happening?
I have creator and destructor operators which mean I will have particles, right?
Everything else would be pure unlogic.
Interchange between those particles needs particles - that's QFT.
Do you agree, Bill?

6. May 9, 2015

### bhobba

As our own Nugatory said - anything you read about such things outside a QFT textbook is likely wrong. Unfortunately the math is the physics in this instance.

That's the thing about QFT - particles and fields are the same thing - particles are like excitations of the field.

This interchange stuff is so called virtual particles. They don't really exist - as has been discussed in many threads on this forum.

Thanks
Bill

7. May 9, 2015

### Omega0

I would say these are photons. Thanks.

8. May 9, 2015

### bhobba

That's an entire textbook: