According to the Standard Model, there are several other types of elementary particles besides the elementary particles of matter and their corresponding antiparticles. One example is a photon—a “particle” of light or electromagnetic radiation. Recall that visible light and other forms of electromagnetic radiation are electromagnetic waves: they consist of oscillating electric and magnetic fields. But the distinction between waves and particles gets a little blurry at the microscopic scale. We’ve already seen that electrons behave as both waves and particles, as Bohr discovered. Similarly, electromagnetic waves act a bit like particles in some respects. For instance, they come in particle-like “packets,” they can exert forces on things, and they even behave as though they have momentum—despite the fact that they have no mass! The mysterious nature of elementary particles is just one of many enigmatic aspects of the Standard Model. We’ll examine some of these mysteries in chapter 7. For now, just take my word for it that light consists of things called “photons,” which—despite their wave-like nature—are classified as particles in the Standard Model. As we’ll see in chapter 7, the things we’ve been calling “elementary particles” are neither waves nor particles in any ordinary sense. At the most basic level, particles are something like ripples, disturbances, in fluid-like entities called “quantum fields” that pervade space and time. At least, that’s the current theory, and it has yielded highly successful predictions. But physicists recognize that this theory can’t be entirely correct, for reasons that we’ll discuss.
Photons belong to a class of particles called bosons, named for Indian physicist Satyendra Nath Bose. The elementary bosons are sometimes referred to as “force-carrier particles,” because they transmit or mediate the forces that are exerted between other particles. Some composite particles are classified as bosons too, but composite bosons are not force-carriers. A taxonomy of composite particles will be given later in this chapter. To understand what is meant by this, consider the following analogy. Imagine that you and a friend are both standing on roller skates, facing each other. You throw a heavy object—a bowling ball, say—toward your friend, who (lucky for him) manages to catch it. (But now he’s not your friend anymore, because you just threw a bowling ball at him.) If you remember Newton’s laws of motion, you should be able to predict what will happen in this scenario. After throwing the ball, you will be rolling backward on your skates, away from your (erstwhile) friend; and he’ll be rolling the other direction, away from you. In effect, you’ve pushed your friend away, even though you never touched him. The force exerted between you and your friend was mediated, or transmitted, by the bowling ball. Similarly, according to contemporary physics, any force exerted between two material objects is mediated—at the most basic level—by an exchange of bosons.
Of course, this “bowling ball” analogy only works for repulsive forces like the electromagnetic force that pushes two protons away from each other; the analogy doesn’t apply to attractive forces like gravity. The analogy is merely intended to make it sound as though the idea of a “force-carrier particle” actually makes sense. But nothing in quantum mechanics really makes sense. It is profoundly mysterious, and not even the greatest physicists claim to truly understand why it works the way it does. In chapter 7, I’ll do my best to help you make sense of it. But if I succeed, if you feel you really understand quantum mechanics, that probably means you’ve misunderstood it entirely.
As we’ll see in chapter 6, Einstein’s theories of relativity imply that the speed of light is the absolute maximum speed at which any physical object or process can travel from one place to another. Nothing—not even forces—can go faster than light. Forces (and the particles that mediate them) have to obey the speed limit just like everybody else.
In other words, there’s no such thing as force-at-a-distance. Forces do not operate instantaneously over a distance, contrary to what the theories of classical physics (i.e., Newton’s laws and Maxwell’s equations) assumed. If you flip a switch to turn on an electromagnet, for instance, that electromagnet will not immediately begin to exert forces on metal objects nearby. It will take a split second for the bosons that mediate that force to travel between the magnet and the other objects. But it won’t take long, because the bosons that mediate the electromagnetic force are photons, which travel at the speed of light!
There are other types of elementary bosons, corresponding to each of the fundamental forces. A fundamental force, as you may recall from chapter 2, is a force that is not explained in terms of other forces. In classical physics, only two fundamental forces were identified: gravity and electromagnetism. In this chapter, we’ve encountered two more: the strong force and the weak force. The strong force is mediated by massless particles called gluons. The weak force is mediated by massive particles called W and Z bosons, which are about eighty times heavier than protons. The particles that mediate the force of gravity have not yet been discovered, and they are not included in the Standard Model. But physicists have given them a name anyway: gravitons.
And there is yet another type of elementary boson: the Higgs boson, which will be discussed on the next page.