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Mass/Energy
Name: Sarah
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 1999
Question:
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?
Replies:
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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Update: June 2012
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