Name: Alan M. V.
This is both a rather complicated and simple question but
one that has always bothered and confused me.
It is always taught that the four forces - gravity, electro-magnetic,weak
and strong nuclear forces are the result of the exchange of particles.
Indeed, on one TV special concerning String Theory, I witnessed a
so-called demonstration of this which showed two men in rowboats facing
each other throwing a ball back and forth, the ball, representing the
quantum particle that manifests the force. The subjects in their rowboats
were shown being drawn to each other on the water as they continued to
This brings about the question of the entire concept of attractive forces.
If two protons in the nucleus are "stuck" together by the strong nuclear
force by the exchange of a particle how does that in fact work?
The first proton would feel a repulsive "kick" as the gluon jumped to the
other proton which should also feel a repulsive kick from the momentum of
the gluon it absorbs and vice versa. It seems to me that ANY exchange of
particles, waves or energy would ultimately turn out to be a REPULSIVE
force and NOT an ATTRACTIVE force.
Please help me to understand this seemingly contradictory concept.
You are indeed correct! Gravity is mediated by the exchange of gravitons,
EM forces by the exchange of photons, weak forces by intermediate bosons
(W+, W-, and Z0) and strong forces by gluons, of which there are 8 kinds.
Of course, these are just names, but I hope you enjoy knowing the vocabulary
As you correctly point out, one can simulate the force transmitted by force
carriers being exchanged by tossing bowling balls back and forth. One
weakness of this analogy, as you correctly point out, is that it always
leads to a repulsive force. I presume you meant to say that "The subjects
in their rowboats were shown being REPELLED by each other on the water as
they continued to 'play catch'". However, exchange of particles can also
lead to attractive forces. You cannot simulate that by throwing things to a
friend because it is a basically quantum mechanical effect which cannot be
The quantum mechanics cannot easily be explained in an elementary way, but I
will try to give you the flavor. Consider an electron which emits a photon
which will interact with another electron. Afterwards, in the coordinate
system in which the electron was at rest, you will see the photon speeding
off with some kinetic energy and by conservation of momentum the electron is
recoiling and so has some kinetic energy also. Notice that energy is not
conserved! Initially the electron at rest has just its rest mass energy.
Afterwards the energy is increased by the kinetic energies of both the
electron and photon!
But energy must be conserved!!! The answer to this conundrum is provided by
the Heisenberg Uncertainty Principle, one form of which can be written as
dE * dt >= hbar, where dE is the uncertainty in the energy of a system, dt is
the time available to measure that energy, and hbar is Planck's constant
divided by two pi. Notice that if dt is VERY small, dE can be very large
and so energy need not conserved by a large amount. So the mediating
particle is virtual in that it cannot exist for long and in some sense its
energy can be negative. So if you could exchange virtual bowling balls with
your friend, you might produce an attractive force.
Notice that the mass of the mediating particle is related to the range of
the force (how far apart the particles can be and still feel the force).
For EM forces for which the mediating particle (the photon) has zero mass,
it can have very little energy and so can exist a relatively long time.
Since it has zero mass, it is always travelling at the speed of light. This
explains why the electromagnetic force has an infinite range (though it
falls off like the inverse square of the separation). Conversely the
intermediate bosons have a large mass (close to 100 times the mass of the
proton), so they must be given a relatively large amount of energy to exist
at all, even in a virtual state. They there exist only for a very short
time while mediating a force and so the force is very short range.
Similarly the graviton mas zero mass so gravity has an infinite range and
gluons have an intermediate mass and so the strong force has a short range,
but much larger than the weak force.
I hope you find this helpful. Quantum mechanics is a mysterious business!
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012