. Gravity is thought to be mediated by an elusive particle named the graviton. The weak nuclear interaction associated with radioactivity is mediated by three elementary particles called W+, W-, and Z bosons. The strong nuclear interaction, which holds quarks together to form protons and neutrons, is mediated by particles called gluons (and appropriately so, because Superglue has nothing on them, because we can never pry two quarks apart, as we saw in ). There is one caveat, though, that I should mention: Gravity continues to be a bit of an exception, because we still do not have a quantum theory of gravity, and that is one of the big outstanding challenges in physics today, so the graviton picture is not a done deal, and gravity is still best explained by objects moving along their natural trajectories in curved space-time according to Einstein’s theory of general relativity, as discussed in . But for all the other fundamental forces of interaction for which a quantum understanding exists, interactions happen through exchange or some mediating messenger particles. Yes, light is the messenger that makes electricity and magnetism work—isn’t that something! After all, as we saw in , light in turn is a manifestation of electromagnetic interaction at work.
, for people to interact, there needs to be physical, verbal, visual, or auditory contact or exchange: your eyes meet, you exchange a look (involving a whole bunch of photons); you talk to each other, you exchange words and sound; your lips meet, you exchange a kiss; you have direct physical contact, perhaps you exchanged a hug or pats on the back; you make love, well, there you are exchanging a lot of things—make up your own list! Perhaps these days you might be spending more time talking on the phone, texting, or chatting on the net; well then, besides words, written and spoken, you are exchanging electromagnetic impulses, as well, via wires and satellites. So you see, with human interactions there is never any mystery about how it happens; there is always either direct contact or some sort of exchange. Newton might have guessed the right answer if only he had looked toward human behavior for his insights instead of staying focused on the heavens, but in his defense, on gravity the jury is still out.
Figure 12.2 Just like in quantum interactions, people interact by exchange, as well—exchanges of handshakes, words, kisses and hugs, and so forth.
(or weight) of the messenger particles. Some of those messenger particles could be quite heavy (on the scale of subatomic particles), and others have no mass at all. The photon, for instance, is massless, and therefore always travels at the maximum speed possible in the vacuum of space—the speed of light. Because they are massless, they can in principle travel forever all the way to the ends of the universe, as borne out by the fact that we can see quasars and galaxies billions of light-years away. But the messenger particles for the weak nuclear interaction, the W and the Z bosons, are quite heavy by elementary particle measures, so the distance they can travel is extremely limited—it is tough to get heavy things to move around. This essential difference has profound implications for the nature of the electromagnetic force and the weak nuclear force. Because the photon is massless and has no travel restrictions, so to say, the electromagnetic force can act over large distances, causing the “action at a distance” that puzzled scientists for so long. So, we see electromagnets in junkyards pulling up things from afar. The weak interaction, on the other hand, is significant only at very short range—about the span of atomic nuclei—and that is why it never manifests directly on the scale of everyday life. Following the same arguments, although the graviton has not been detected in experiments, it is expected to be massless.
, resonant interactions can amplify suppressed violent tendencies in a mob where often good-natured and friendly people suddenly become supremely vicious, capable of perpetuating unbelievable violence and mayhem. The behavior-altering impact of interactions is simply an extension of the amazing changes that interactions can induce in the material world. For example, in ordinary conductors of electricity, like in the wiring around the house, there is always significant loss of energy because of the resistance offered by the wire (just like water flowing on level ground slows down and eventually stops). But there are special materials called superconductors, used in a lot of high-tech equipment, in which electricity can flow with almost zero resistance. As a result of superconductors, electric currents can continue to flow for an extremely long time. But the reason superconductors are different is because of the onset of extraordinary long-range interaction among pairs of electrons via mediating particles called phonons forming what are called Cooper pairs. In the laconic words of the Nobel Prize–winning physicist Philip W. Anderson, “More is different,” and the primary reason is the presence of interactions.
See for a more detailed explanation of mass and weight, which is traditionally defined as the amount of matter in any object. A generalized meaning of mass as a measure of inertia is also discussed in .
Phonons are quanta of vibrations, including sound, so named in analogy with photons, the quanta of light.