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Name: Stuart
Status: educator
Grade: 12+
Country: United Kingdom
Date: Summer 2012

Forces between elementary particles are mediated via particles. For example, the Electromagnetic force is mediated via photons. What I want to know is - how do the photons know where to go? The cannot see/feel the other particle. The cannot feel its effect because the cause the effect.


Photons are incapable of knowing anything. They are like balls on a pool table that just get knocked around by their environment. Sometimes they go to beneficial places but most other times they just pass on out of the space not interacting with other things. Like when I strike a match, light goes out all three dimensional directions. Some light rays hit a human retina, but most do not.

Sincere regards, Mike Stewart

Hi Stuart,

The photons do not know where to go, they just go. They are in motion and as they propagate, the E and B perpendicular wave functions move with them. Unless directed, they run into each other, reflect and scatter... go all over the place.

We have devices to focus their direction, but there is always scatter as they bump into atoms and particles. Focus a flashlight as best you may and there will always be loss. Even lasers in a vacuum deviate. These deviations result in spectra that are quite useful in measuring many factors, such as galaxy distance and the age of the Universe.

In photon direction experiments, we try to keep distances short, use lasers, vacuums and collimators... but it is still a bit like herding cats, the photons mostly will go in a general direction, but there are always escapees.

Hoping this helps, PEHughes, Ph.D. Milford, NH

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