Warped Passages (25 page)
The Standard Model of Particle Physics: Matter’s Most Basic Known Structure
You’re never alone,
You’re never disconnected!
You’re home with your own;
When company’s expected, you’re well protected!
…When you’re a Jet, you stay a Jet!
Riff (
West Side Story
)
Of all the stories she had read, Athena was most thoroughly perplexed by Hans Christian Andersen’s “The Princess and the Pea.” The story tells of a Prince who searched unsuccessfully for a suitable princess to wed. After he had searched in vain for weeks, a potential princess arrived by chance at his palace, seeking shelter from a storm. This soggy visitor thereby became the unwitting subject of the Queen’s litmus test for princesses.
The Queen prepared a bed, which she piled high with mattresses and eiderdown quilts. At the very bottom of the pile she placed a solitary pea. That night, she showed her visitor to the carefully prepared guest room. The next morning, the princess (as indeed she proved herself to be) complained that she had not been able to sleep at all. She had tossed and turned the whole night, and found she had actually turned black and blue—all because of the uncomfortable pea. The Queen and Prince were convinced that their visitor was truly of royal blood, for who else could be so delicate?
Athena turned the story round and round in her head. She thought it fairly ridiculous that anyone, even the most sensitive of princesses, would ever have discovered the pea by lying passively on top of the pile of mattresses. After many days’ deliberation, Athena found a plausible interpretation, which she rushed to tell her brother.
She rejected the common interpretation that the princess proved her royal nature by demonstrating delicacy and refinement with her sensitivity to even something as minor as a pea under a pile of mattresses. She offered an alternative explanation.
Athena suggested that when the Queen went away and left the princess alone in the room, the princess threw decorum to the wind and gave vent to her boisterous youthful nature. The princess ran around and jumped up and down on her bed until she was exhausted, and only then lay down to try to sleep. Through her rambunctiousness, the princess compressed the mattresses so much that for a brief moment the pea stuck out like a sore thumb and gave her a small bruise. Athena thought this princess was still rather impressive, but found her revisionist interpretation much more satisfactory.
Finding substructure within the atom was as remarkable an accomplishment as the princess finding her pea. Particles called
quarks
, the building blocks of the proton, occupy about the same fractional volume of the proton as a pea does in a mattress. A 1 cubic centimeter pea in a 2 meters × 1 meter × ½ meter mattress takes up one-millionth of the mattress’s volume, which is not too different from the fraction of volume a quark occupies in a proton. And the way in which physicists discovered quarks bears some resemblance to the rambunctious princess’s discovery. A passive princess would never discover a pea buried layers and layers down. Similarly, physicists didn’t discover quarks until they slammed into the proton with energetic particles that could explore its innards.
In this chapter you will make a jump of your own, into the Standard Model of particle physics, the theory that describes the known elementary constituents of matter and the forces that act upon them.
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The Standard Model, which represents the culmination of many surprising
and exciting developments, is a stupendous achievement. You don’t need to remember all the details—I’ll repeat the names of all the particles or the nature of their interactions when I refer to them later on. But the Standard Model underlies many of the exotic, extra-dimensional theories that I will describe shortly, and as you learn about the recent exciting developments, a feeling for the Standard Model and its key ideas will contribute to a deeper understanding of matter’s fundamental structure and the way physicists think about the world today.
The Electron and Electromagnetism
When Vladimir I. Lenin used the electron as a metaphor in his philosophical book
Materialism and Empirio-Criticism
, he wrote that “the electron is inexhaustible,” referring to the layers of theoretical ideas and interpretation through which we interpret it. Indeed, today we understand the electron very differently than we did in the early twentieth century, before quantum mechanics revised our ideas.
But in a physical sense the opposite of Lenin’s quote is true: the electron
is
exhaustible. So far as has been determined, the electron is fundamental and indivisible. To a particle physicist, the electron, rather than having “inexhaustible” structure, is the simplest Standard Model particle to describe. The electron is stable and has no constituent parts, so we can characterize it completely by listing only a few properties, including mass and charge. (The Czech anti-Communist string theorist Luboš Motl quipped that this is not the only difference between his and Lenin’s perspectives.)
An electron will move towards the positively charged anode of a battery. A moving electron also responds to a magnetic force: as an electron moves through a magnetic field, its path will bend. Both these phenomena are the result of the electron’s negative charge, which makes the electron respond to electricity and magnetism.
Before the 1800s, everyone thought that electricity and magnetism were separate forces. But in 1819 the Danish physicist and philosopher Hans Oersted found that a current of moving charges generates a magnetic field. From this observation he deduced that there should be
a single theory describing both electricity and magnetism: they must be two sides of the same coin. When a compass needle responds to a bolt of lightning, it confirms Oersted’s conclusion.
The classical theory of
electromagnetism
, still in use today, was developed in the nineteenth century and used the observation that electricity and magnetism are related. The notion of a
field
was also critical to this theory. “Field” is the name physicists give to any quantity that permeates space. For example, the value of the gravitational field at any point tells how strong the effect of gravity is there. The same goes for any type of field: the value of the field at any location tells us how intense the field is there.
In the latter half of the nineteenth century, the English chemist and physicist Michael Faraday introduced the concepts of electric and magnetic fields, and these concepts persist in physics today. Given that he had to temporarily abandon his formal education at the age of fourteen to help support his family, it is quite remarkable that he managed to do physics research that had such a revolutionary impact. Fortunately for him (and for the history of physics), he was apprenticed to a bookbinder who encouraged him to read the books on which he was working, and educate himself.
Faraday’s idea was that charges produce electric or magnetic fields everywhere in space, and these fields in turn act on other charged objects, no matter where those objects are. The magnitude of the effect of electric and magnetic fields on charged objects does depend on their location, however. The field exerts the most influence where its value is largest, and has a smaller effect where its value is less.
You can see evidence of a magnetic field by sprinkling iron filings in the vicinity of a magnet. The particles organize themselves in patterns according to the strength and direction of the field. You can also experience a field by holding two magnets close together. You’ll feel the magnets’ mutual attraction or repulsion well before they touch each other. Each is responding to the field that permeates the region between them.
The ubiquity of electric fields was brought home to me one day when I was finishing a climb on a ridge near Boulder, Colorado, with a partner who was new to climbing but had a lot of hiking experience.
An electrical storm was approaching rapidly, and I didn’t want to make him nervous, so I encouraged him to move quickly without pointing out that the rope was crackling and his hair was standing on end. When we were safely down at the bottom happily reviewing our adventure, much of which had been a delightful climb, my partner told me that of course he had known we were in danger: my hair had been visibly standing on end too! The electric field wasn’t only in one place—it was everywhere around us.
Before the nineteenth century, no one described electricity and magnetism in terms of fields. People conventionally used the term
action at a distance
to describe these forces. Action at a distance is the expression you might have learned in elementary school which describes how an electrically charged object instantly attracts or repels any other charge, no matter where it is. This might not seem mysterious, since it’s what we’re accustomed to. However, it would be extraordinary if something in one place could instantly affect another object some distance away. How would the effect be communicated?
Although it might sound like just a matter of semantics, there really is an enormous conceptual difference between a field and action at a distance. According to the field interpretation of electromagnetism, a charge doesn’t affect other regions of space immediately. The field needs time to adjust. A moving charge creates a field in its immediate vicinity, which seeps (albeit very rapidly) throughout space. Objects learn of the motion of the distant charge only after light (which is composed of electromagnetic fields) has had time to reach them. The electric and magnetic fields therefore change no faster than the finite speed of light allows. At any given point in space, the field adjusts only after sufficient time has elapsed for the effect of the distant charge to reach that point.
However, despite the critical importance of Faraday’s electromagnetic fields, they were more heuristic than mathematical. Perhaps because of his spotty education, math was not Faraday’s strength. But another British physicist, James Clerk Maxwell, incorporated Faraday’s field idea into classical electromagnetic theory. Maxwell was a brilliant scientist who counted among his many interests optics and color, the mathematics of ovals, thermodynamics, the rings of
Saturn, measuring latitude with a bowl of treacle, and the question of how cats land upright while conserving angular momentum when dropped upside down.
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Maxwell’s most important contribution to physics was the set of equations that describe how to derive the values of electric and magnetic fields from a distribution of charges and currents.
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†
From these equations, he deduced the existence of electromagnetic waves—the waves in all forms of electromagnetic radiation, as in your computer, television, microwave oven, and the many other conveniences of the modern era.
However, Maxwell made one mistake. Like all other physicists of his day, he took the field idea too materially. He assumed that the field arose from the vibrations of an aether—an idea that Einstein, as we have seen, ultimately debunked. Nonetheless, Einstein credited Maxwell with the origin of the special theory of relativity: Maxwell’s electromagnetic theory gave Einstein the insight about the constant speed of light that instigated his monumental work.
The Photon
Maxwell’s classical electromagnetic theory made many successful predictions, but it predated quantum mechanics so it obviously didn’t include quantum effects. Today, physicists study the electromagnetic force with particle physics. The particle physics theory of electromagnetism includes the predictions of Maxwell’s well-studied and well-verified classical theory, but incorporates the predictions of quantum mechanics as well. It is therefore a more comprehensive and more accurate theory of electromagnetism than its classical predecessor. In fact, the quantum theory of electromagnetism has yielded incredibly
precise predictions that have been tested with the unbelievable precision of one part in a billion.
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The quantum electromagnetic theory attributes the electromagnetic force to the exchange of the particle called the
photon
, the quantum of light that we considered in the previous chapter. The way it works is that an incoming electron emits a photon, which travels to another electron, communicates the electromagnetic force, and then disappears. Through their exchange, photons transmit, or
mediate
, a force. They act as confidential letters that convey information from one place to another, but are afterwards immediately destroyed.
We know that the electric force is sometimes attractive and sometimes repulsive: it’s attractive when oppositely charged objects interact, and repulsive when the charges have the same sign, either both positive or both negative. You might think of the repulsive force communicated by the photon as an interaction between two ice skaters throwing a bowling ball back and forth; each time one of them catches the ball, he slides away from the other across the ice. Attractive forces, on the other hand, are more like two novices tossing a frisbee to each other; unlike the ice skaters, who slide further apart, these beginning frisbee players would approach each other with each successive throw.
The photon is the first example we will encounter of a
gauge boson
, a fundamental, elementary particle that is responsible for communicating a particular force. (The word “gauge” sounds more daunting than it really is; physicists first used it in the late 1800s because of a tangential analogy to railroad gauges that tell you the distance between the rails—a term that was far more familiar a hundred years ago.) Weak bosons and gluons are other examples of gauge bosons. These particles communicate the weak and strong forces respectively.