How does force mediator transfer generate an attractive force?

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In summary, the potential for scattering is explained by the Fourier transform of the potential, and the attraction or repulsion only occurs in the full amplitude including no-photon-exchange.
  • #1
braides
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Hello!

Im hoping people can answer this for me, as I've not been able to find the answer anywhere else:

As far as I understand it (and please correct me if I am wrong), the strong force is caused by the transfer of a gluon from one quark to another, electromagnetism by the transfer of photons and so.

My problem is that I can't find anywhere why this transfer actually causes them to move closer together / attract. And with electromagnetism sometimes repel.

Help!
 
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  • #2
When a nail gets attracted to a bar magnet , I am pretty sure they don't actually exchange photons , its just a way to interpret it ,
 
  • #3
So if they don't exchange photons, then why do they move closer together? What is it that causes the attraction / repulsion?
 
  • #4
im not sure anybody knows , i hope someone can correct me ,
 
  • #5
cragar said:
im not sure anybody knows , i hope someone can correct me ,

i too want an answer.

i want to know about gravitons (if real) are they like hands that reach out across space from the sun & hold Earth in place?
 
  • #6
i thought a graviton was the excitation of the field .
 
  • #7
I was watching a video showing two guys throwing a ball between them. The ball was the photon for magnetism & they were getting closer with every throw to each other. Thats the best my understanding is. But really feel there is so much more to it.
 
  • #8
At some point you have to simply accept that an analogy that you read in a popularization is not perfect. To actually understand this requires a lot of work - the easy path of the analogy only takes you so far.
 
  • #9
Is there any basic links online that I can read to better understand?
 
  • #11
humanino said:

Humanino, I read the FAQ you posted- thankyou. It helped immensely, except for I don't understand the logic behind one "step".

"When I include all of these possibilities, it turns out that I can approximate the photon's momentum-space wave function usably well by the following: the wave function is a function proportional to the electric charge of the emitting particle (in a sense this defines what electric charge is), and it has a few big, narrow spikes in it. One spike is proportional to -i times the charge, and is to the left of the origin; the other spike is minus that and is to the right of the origin."

Why does the momentum-space wave function look like that? I am assuming the author is glossing over why for simplification- could you (or anyone) expand please?

Thanks
 
  • #12
What he calls the "wave-function" for the photon is just the propagator (as he mentions) which can be defined in non-relativistic scattering as the Fourier transform of the potential evaluated at the momentum transfer. There are not great depths in this sentence, it is just
[tex]\langle p_f | {\cal H}_\text{int} | p_i\rangle = \int \text{d}x\, e^{-ip_f x}V(x)e^{ip_i x} = \int \text{d}x\, e^{iqx}V(x) = \tilde{V}(q)[/tex]
In order to (over)simplify, he chooses the potential to be a sinusoid (whose Fourier transform is just the difference of two delta functions).

The essential point of the discussion is that attraction or repulsion occurs only in the full amplitude including no-photon-exchange.

I hope that helps. I may have some signs wrong w.r.t. his conventions, I certainly omitted (square root of) pi factors. If it remains obscure, please let us know.
 
  • #13
Sorry, still don't get it. What I don't understand is why the spike to the left of the origin is positive, and the spike to the right negative.

If a photon is emitted by something my thoughts would be that its momentum wave-function would be the same in all directions, not plus in one and minus in another.
 
  • #14
braides said:
Sorry, still don't get it. What I don't understand is why the spike to the left of the origin is positive, and the spike to the right negative.
It's an excellent question, right on in fact, and I wanted to discuss it more in depth, but the week end is gone and I still did not have time.

The easiest way to see that it must be that way, Baez mentions : the potential is real, so the imaginary part of its Fourier transform must be odd.

But it has wider implications related to causality. He briefly mentions the "trick" due to Feynman to formally write emission from left and absorption at right, while taking care of the reciprocal. This is in fact the sign difference between merely the Green function and the Feynman propagator. If you are familiar with it, it is also the [itex]-i\epsilon[/itex] prescription which produces automatically the right path around the poles.

I hope somebody else can contribute, otherwise I'll have to come back to it later.
 
  • #15
Thanks for all the help humanino, much appreciated.
 

1. How does force mediator transfer generate an attractive force?

The force mediator transfer is a process where particles exchange force-carrying particles, called force mediators. When two particles interact with each other, they exchange these force mediators, creating an attractive force between them.

2. What are force mediators?

Force mediators are particles that are responsible for carrying forces between particles. These include photons for electromagnetic forces, gluons for strong nuclear forces, and W and Z bosons for weak nuclear forces.

3. What is the mechanism behind force mediator transfer?

The mechanism behind force mediator transfer is based on the exchange of virtual particles between interacting particles. These virtual particles are constantly being created and destroyed, but their exchange creates a net force between the particles.

4. How does force mediator transfer differ from other types of force transmission?

Unlike other types of force transmission, such as contact forces, force mediator transfer does not require direct contact between particles. It can occur over large distances, as long as the particles are within the range of the force mediators.

5. Can force mediator transfer also create repulsive forces?

Yes, force mediator transfer can also generate repulsive forces between particles. This occurs when the exchange of force mediators results in a net force pushing the particles away from each other, rather than pulling them together.

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