1. Forces and Exchange particles
Yukawa became interested in how forces work. His particular interest was in how the nucleus of an atom could be held together despite the electromagnetic repulsion of the protons within it. In 1935 he developed an idea based on a very simple model

2. A hydrogen atom with a single proton and electron

3. A molecule of hydrogen gas is bound together by the influence of 2 electrons which are passed back and forth between them

4. Even simpler is an H2+ ion which has only one electron being passed around
The “exchange” particle holding the protons together could not be the electron. The electron does not even take part in the strong force.
Yukawa suggested that there should be a totally new particle of exchange. He predicted the properties it should have.

5. Yukawa suggested that there should be a totally new particle of exchange.
This particle would be holding not only protons to protons
but protons to neutrons and neutrons to neutrons
He predicted the properties the new particle should have. The neutral pion (π0)
was discovered in 1947 and it was thought to be totally responsible for the strong force.
We now know this is not the case, because protons are actually built of simpler particles.
BUT the idea of “exchange particles” being
responsible for forces has been very successful.

6. The exchange particle of the electromagnetic force
The Virtual Photon
The exchange particle of the electromagnetic force

7. The electromagnetic force
The electromagnetic force only applies to particles with charge.
When charged particles interact electromagnetically, they exchange photons

8. time
This is the Feynman diagram for the interaction of two electrons.
The electrons approach each other.
They exchange a virtual photon.
The electron which emits the photon recoils.
The electron which absorbs the photon carries off an equal amount of momentum in the opposite direction. e- e- γ e- e-
space

9. The Weak Force And Particle Exchange

10. In β- decay a neutron changes into a proton with the emission of an electron and an antineutrino.
This is an example of the WEAK INERACTION n p
This decay is “mediated” by an exchange particle called a W-.

11. The Feynman representation on the right is the easiest way to show thisν p W- W- ν n

12. In β+decay a proton changes into a neutron with the emission of an positron and an neutrino.
In this example of the weak interaction the decay is “mediated” by an exchange particle called a W+. p n n ν e+ W- p

13. Electron Capture
In electron capture ( the third type of beta decay) a proton becomes a neutron by capturing an orbital electron.
This can be represented on a Feynman diagram like this: n ν W+ p+ e-