force carrier photons and magnetic attraction

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Force carrier particles mediate the fundamental forces, and for electromagnetism the force carrier is the photon. That’s what I was taught in high school. I (sort of) understand the idea that these are “virtual” particles though, not real. They mathematically model how the force acts but they aren’t actually there.

Pretending I’m an electron, I can throw a ball (photon) at a bottle (another electron) to knock away (magnetically repel) the bottle. Makes sense. But if I were a proton, then the electron is attracted. How can bouncing a particle (much less a mass less, virtual one) off the electron ever “knock it” towards a positive charge?

In: Physics

3 Answers

Anonymous 0 Comments

Since it’s a virtual particle, it can do things that real particles can’t, for example have negative energy and negative momentum. A repulsive force is carried by a virtual particle with positive momentum, while an attractive force is carried by one with negative momentum.

Normally if a ball (with positive energy and momentum) hits you, it would push you in the direction of its travel because it imparts a portion of its kinetic energy and momentum to you. A virtual ball with negative energy and momentum will do the same, except the energy and momentum you receive from the ball would be pointing in the opposite direction.

Anonymous 0 Comments

Virtual particles are weird, in a lot of ways.

One of those ways is that virtual particles can have properties that no known real particles have.

The specific property necessary in this case is that the virtual particle being exchanged has momentum in the *opposite* direction to the one it’s travelling in – effectively it has negative mass. So when it’s “thrown” the particle throwing it moves in the same direction it does, and when it’s caught it pulls rather than pushing.

Negative mass doesn’t exist, as far as we know, in the world of real particles. But virtual particles aren’t real particles, they’re a particular way of looking at a set of mathematical equations, and thus they’re not bound by the same rules.

Anonymous 0 Comments

Your idea of particles hitting eachother isn’t very accurate. These virtual photons are waves interacting with the other electron. They are interacting, not hitting.

Photons are always absorbed, they cannot “bounce” off the electron. The electromagnetic wave of the photon shakes the electron like a boat on a stormy sea. This electron now produces its own wave because it is a moving charge at a 90° angle to both the direction of the wave and it’s polarisation.

If the photon is energetic enough, the electron will move to a different energy level within the atom, and when it then de-excites, may fall go between many different energy levels, releasing a photon with different energy each time depending on the size of the jump down. This is how fluorescence works.

So confusingly, when describing forces with virtual particles, you end up with negative energy virtual photons. They don’t have to follow the usual rules of E^(2) = (mc^(2))^(2) + (pc)^(2) so are able to have any energy they want. This negative energy is what causes attraction.