Feynman's Thesis, 3 Principles

[jamie.j1989]

Hi, I've just started studying Feynman's thesis and am in need of some discussion regarding the three principles he put forward on the development of his 'Principle of least action in quantum mechanics'. The three principles are

1) The acceleration of a point charge is due to the sum of its interactions with other charged particles. A charge doesn't act on itself.
2) The force of interaction which one charge exerts on another is calculated by means of the Lorentz force formula $F=e(E+\frac{v}{c}\times H)$, where $E$ and $H$ the electric and magnetic fields respectively and $v$ and $c$ the velocity of the particle and the speed of light respectively. The fields are the fields generated by the first charge in accordance with Maxwell's equations.
3) The fundamental (microscopic) phenomena in nature are symmetrical with respect to interchange of past and future. This requires the solution of Maxwell's equations to be used in computing the interactions is to be half the retarded plus half the advanced solution of Lienard and Wiechert.

What I would like to discuss is why he has decided on these and what exactly There individual consequences are. Most of all I'm at a complete loss as to what the third one even means.

My understanding of 1)
As far as I am aware this is currently no longer correct. Electrons do interact with themselves. However Feynman was attempting to find a way to get rid of the infinite mass that arrises classically with an electron interacting with itself; of which, I also don't quite understand.

My understanding of 2)
I'm happy with the text. I'm not sure about the motivation though, he needs a way of calculating 1); there is no reason why the physics in 2) shouldn't be used in his new formulation?

My understanding of 3)
Lost.

Thanks

 

[phyzguy]

Feynman discusses this in some detail in the Feynman Lectures, Vol 2 Chapter 28 at this link. Perhaps this will help. I think the simple answer to the question of why he made these assumptions is that with those assumptions things work out.

 

[cosmik debris]

Number three is just a statement about time symmetry, as in classical mechanics.

Cheers