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Name: Sarah
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 1999 

These questions were generated by my physics class during our recent discussion of the four fundamental forces (they visited the Particle Adventure website):

1) If photons are massless, and E=mc2, how can they have energy?

When people talk about the mass of a particle, they normally mean the rest mass--the energy required to form the particle without giving it any kinetic energy. But photons don't exist without kinetic energy. Photons must have zero rest mass because the energy required to make one goes to zero as the photon's frequency goes to zero.

The relativistic mass of a photon is not zero, and you can see this also by noting that a photon carries momentum.

2) How do force carrier particles exert attractive forces (i.e. electromagnetic attraction)? (The analogy for force carrier particles we used was of two people tossing a basketball back and forth together, and such an analogy only explains repulsive forces)?

I've never heard a satisfying explanation of this. One way to think about it is to look more closely at fields. We accept the notion that an electromagnetic field can attract or repel charged objects without even having any gut-level explanation of how it does this--we accept it because "that's just what fields do" and we have no competing notion of what fields should do. When a field is quantized, that is, when it is regarded as a collection of particles, then we do have a conceptual problem because we think we know what particles are and what they do from our experience with small objects. But this picture of particles comes from our experience with (collections of) fermions--the kind of particles that don't carry forces. Force carrying particles are different. They can overlap, for example, whereas two fermions can never occupy the same state at the same time.

Eventually in my mind it comes down to the original wave-particle duality. You look at these things as particles when it's mathematically or conceptually convenient to do so, and you regard the duality itself either as evidence that the world contains fundamentally ambiguous objects, or that the approximate theories under which we currently are laboring just aren't using the right sorts of objects to describe the world.

> >3) Are there antiforce particles like there are antimatter particles?

Yes. Massless particles are their own antiparticles (you could equally well say they do not have antiparticles), but massive force-carrying particles exist, and they have antiparticles. Long-range forces like the electromagnetic and gravitational forces are associated with massless particles; the short-range nuclear force is associated with massive particles called mesons. Only short-range forces are conveyed by particles having discernible antiparticles.

Tim Mooney

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