# Fermilab

## Inquiring Minds

Photons and Exchange of Forces

(1) Is the Electromagnetic field surrounding any charged particle as termed as a "Electromagnetic field" as the expression as representing the continuous time and area surrounding the area near a charged particle of its emmission and absorption of photons?

(2)Would it really take just only take one photon and why this particular photon is absorbed to cause the effect when there would be many others that might cause a canceling out effect and why not "absorb" any other forces like weak or strong?

(3)Okay i got the part about the conservation of energy to cause repulsion but the most important and long sought after question i have been searching for two years now is what the heck (pardon my french) is the interaction as described like above for two particles of unlike charges to create attraction???????? Thank you,

Brian

Dear Brian,

Thank you for your question regarding the forces between two charged particles: "How does the repulsion and attraction between two charged particles actually work by the photons to produce the forces"

Forces of the microcosm are described by the transfer of energy and momentum from one particle to another. To understand how a photon can transfer a force between two charged objects, I suggest to think of two people, each one standing on a boat in the middle of a lake. Then one person throws a basketball to the person in the other boat. What will happen?

The boats will move away from each other. The person throwing the ball will move against the direction the ball is moving, the person catching the ball will move in the same direction as the ball is thrown. The basketball carries energy and momentum, and the two boats seem to "repel" each other. Two objects with same sign charges will repel each other in the same way, with the photon transferring energy and momentum.

How can an exchange of a photon describe the attractive force between two objects with opposite charge?

Well, think again about two people standing in two different boats. Now imagine them standing with their backs turned toward each other, and one person is throwing a boomerang. Throwing the boomerang this person will move towards the other boat as he gets a push against the direction he threw the boomerang. The boomerang, being in the air, flies a curve, and it will eventually arrive at the person in the second boat. Catching the boomerang this person will now be pushed towards the person that threw the boomerang, and they both move closer to each other, they "attract" each other.

Summary: A photon can act either as a basketball or as a boomerang, depending on the charges of the two objects that exchange that photon.

Sincerely,

Kurt Riesselmann
Physicist, Fermilab Public Affairs

Back to Questions About Physics Main Page