A word about nomenclature. Sometimes physicists can be quite precise in their use of language, but when it comes to “forces,” they tend toward the colloquial, not the precise. In Newtonian physics (classical mechanics) force is well defined: it is the time rate of change of momentum, or the product of mass and acceleration. To the modern physicist, force is an “influence,” or an “interaction,” not a precisely defined quantity. There is actually a sound reason for the vagueness. In classical physics, force is a measurable concept that enters into equations. In quantum physics, it is not. It has vanished from the equations. So the quantum physicist is free to use the word “force” as he or she might use “crimson” or “pleasing” or “dark.”
Regardless of vagueness in terminology, it is clear that there are (so far!) four very different ways in which particles and other objects influence one another, or interact with one another. The four “forces,” in decreasing order of strength, are:
1. Strong force (or nuclear force), acting among quarks and therefore among all entities made of quarks. It acts only over very short distances (~10–15 m).
2. Electromagnetic force, accounting for the interactions of charged particles with each other and, at a more basic level, for the emission and absorption of photons by charged particles. It acts over long distances.
3. Weak force, accounting for interactions involving neutrinos and other “leptons” (electrons, muons, and taus). Like the strong force, it acts only over very short distances.
4. Gravitational force, acting to attract all matter and energy to all other matter and energy. It acts over long distances.
According to present theory, all forces are “mediated” by so-called exchange particles. For instance, when one quark interacts with another quark, it is because one or more gluons have been exchanged between the two quarks. Gluons are the exchange particles of the strong force. It’s as if two baseball players could not interact by shaking hands or embracing, only by tossing a baseball back and forth. Gluon exchange sits at the heart of all strong interactions—between a proton and a neutron, for example, or between two neutrons, or between a neutron and an atomic nucleus. (If the strongly interacting particles are electrically charged, electromagnetic force comes into play as well.)
For the electromagnetic force, the photon is the exchange particle. Photons have in common with gluons that they have no mass, no electric charge, and one unit of spin. (Gluons, in addition, carry a “color charge.”) If you think back to the famous 1911 experiment of Ernest Rutherford, for instance, in which an alpha particle (the positively charge nucleus of a helium atom) is deflected by the nucleus of gold atom (without “touching” it), you must imagine a stream of photons being incessantly exchanged between the two objects and accounting for the fact that the alpha particle follows a curved path, emerging in a different direction than its direction of approach. The number of photons exchanged is not millions or billions. It is more. Or think of this simple experiment that anyone can do. Rub a plastic comb on a wool sleeve, then hold the comb close to (but not touching) a bit of paper on a desk. The paper leaps up. It is electrically attracted to the comb. Here, too, innumerable photons are exchanged and provide the ultimate explanation for the attraction.
Since the strong force and the electromagnetic force act in different ways (by exchanging very different kinds of particles), there is no way to state an exact ratio of their strength. Roughly speaking, the strong force is about 100 times stronger than the electromagnetic force.
For the weak force, there are two exchange particles, the so-called W and Z bosons. Actually, all exchange particles for all four interactions are bosons—particles of zero or whole-integer spin that do not obey the Pauli exclusion principle. The W particle is positively charged (its antiparticle is negative), has one unit of spin, and has a mass equal to about 86 proton masses (as particles go, it is very heavy). The Z particle is neutral (that is, uncharged), also has one unit of spin, and has a mass equal to about 97 proton masses (even heavier). These bosons mediate numerous weak-interaction processes, including the beta decay of a radioactive nucleus, in which an electron and an antineutrino are created as a neutron is transformed into a proton. (Words that fit the theory a little better are these: A neutron is destroyed while a proton, electron, and antineutrino are created.) How weak is the weak force? Again, in round numbers, about 100,000 times weaker than the strong force, or about 1,000 times weaker than the electromagnetic force.
Finally, the gravitational force. It is the force we are all most familiar with on Earth: the force that high jumpers must contend with, the force that propels skiers, the force that can break bones. It is also the force that guides the planets in their orbits, holds the Sun together in one hot ball, and controls the wheeling of galaxies and clusters of galaxies. Yet it is, at the same time, so extraordinarily weak that its effects have never been observed in the atomic or subatomic domain. Its still-hypothetical exchange particle is the graviton, a boson of zero mass, zero charge, and two units of spin. The gravitational force makes the weak force look more like Charles Atlas than like Casper Milquetoast.1 It is about 1038 times weaker than the strong force (making it some 1036 times weaker than the electromagnetic force and 1033 times weaker than the weak force). These are strength ratios that are beyond our ability to comprehend, and may hold a lesson—that there is a whole world of fascinating physics “below” the realms explored so far.
According to present theory, the four forces have in common that the ultimate locus of interaction is a spacetime point. For each force, the boson that mediates the force comes into existence at a point in space and at an instant of time—a literal spacetime point—and vanishes at another spacetime point. From this incessant dance of creation and annihilation flow the interactions observed in the laboratory—for example between two quarks or two electrons. But is the mathematical spacetime point used by the theorist the same as a real spacetime point in the physical world? We don’t know. If it turns out that what we now treat as a spacetime point is really an incredibly small but finite portion of space and time, physicists will be delighted and not surprised.
1 Those growing up, as I did, in the 1930s and 1940s, know that Charles Atlas stands for strength and Casper Milquetoast for weakness. Charles Atlas (1892-1972) was a real person, famous as a bodybuilder and an inspirational author. Casper Milquetoast was a comic-strip character notable for his timidity and indecisiveness.