by David Kordahl
In popular media, physics often comes up for one of two competing reasons. The first is to introduce a touch of mysticism without labeling it as such. Whether it’s Carl Sagan talking about our bones as stardust, or Lisa Randall suggesting some extra dimensions of space, these pronouncements are often presented to evoke the listener’s primal awe—an ancient and venerable form of entertainment. The second reason is just as venerable, and often as entertaining. Sometimes, physics just gets results. Think of MacGuyver in MacGuyver, Mark Watney in The Martian, or those stunt coordinators in Mythbusters—characters whose essential pragmatism couldn’t be further from the tremulous epiphanies of the theorists.
Dramatically, the esoteric and the everyday can seem like opposites, and many fictional plots seem to advise against bringing them together. Mad scientists, those cautionary anti-heroes like Drs. Frankenstein and Manhattan, are often characters who both stumble upon hidden truths and put them to terrible use. But in the real world of physics, it’s common to forge connections between the realms.
Physical analogies, examples that link unfamiliar physics to everyday experience, are important in forging such connections. Waves in an Impossible Sea: How Everyday Life Emerges from the Cosmic Ocean, a new book by the physicist Matt Strassler, is an impressive attempt to explain contemporary physics using little math but many analogies. Strassler mainly goes against the archetype of the theoretical physicist as the purveyor of primal awe. Instead, he’s a practiced teacher, more interested in accuracy than amazement. In seven concise sections—Motion, Mass, Waves, Fields, Quantum, Higgs, and Cosmos—he covers the basics of physics with minimal fuss, but with a charmingly dorky earnestness.
Strassler’s main method is to give us a familiar example, then to describe how an unfamiliar example is like or unlike it. Take, for instance, the relativity of motion. The first section of Waves in an Impossible Sea describes Galileo’s thought experiment of passengers below the deck of ship who cannot detect whether or not it is in motion. What explains why motion is sometimes hard to detect? Why, the principle of relativity. “Inside a steadily moving isolated bubble, where you have no view of, contact with, or perspective on the outside world, your motion is undetectable because it has no perspective-independent meaning in our universe.”
But if all motion is merely relative motion, why doesn’t our experience seem to jibe with this? Well, Strassler explains, this is because we we are used to moving through air, where friction causes drag, not to the airless environment of empty space. This is why an “airplane, flying through the atmosphere, must run its engines continuously to fight air resistance. But a satellite above the atmosphere can coast at much higher speeds than a plane without using an engine.” Like us, the airplane experiences significant drag. Unlike us, the satellite does not.
This example nicely illustrates how the book works. Strassler is not instinctively a storyteller (don’t expect the potted biographies of scientists), and when he does tell stories, they’re often just recollecting a time he told someone else about the same thing he’s telling us now. In place of stories, Strassler focuses on analogies, examples he hopes are more accurate than those the reader has heard before, and aims to knock down those analogies that fail to represent real physics—a class of (non) explanations he labels as “physics fib[s] or, more simply, [as] phib[s].”
One phib that especially bugs Strassler was repeated widely a little over a decade ago, pre-2012, before researchers at the Large Hadron Collider had announced their eventual discovery of the Higgs boson. But this requires a little setup.
The discovery of the Higgs boson was not that significant in and of itself. This discovery was important, for scientists like Strassler, because the predicted particle, the Higgs boson, vindicated the Higgs mechanism, which forms the standard explanation for how particles get their mass. Particles are said to gain mass via their interaction with the Higgs field. But how does this work?
In Strassler’s least-favorite phib, massive particles are said to act like a famous scientist at a conference. The Higgs field, in this analogy, is like the crowd of lesser scientists surrounding the famous scientist, wanting to bend his ear. In the crowd, the scientist has more inertia—or, put another way, he has more mass.
Okay, so why is this analogy such a bad one? Interested readers can find a longer version of Strassler’s complaint on his blog—but one obvious problem with this analogy is that it seems to contradict the principle that all motion is just relative. In this story, the scientist either is or is not moving relative to his peers, which obscures the issue. The Higgs field should look the same to all objects moving through it, changing the behavior of massive objects without violating relativity.
It’s not an overstatement to say that Waves in an Impossible Sea is an extended delivery system for Strassler’s alternative analogies for the Higgs mechanism. Unfortunately, I don’t think these analogies would make much sense to readers who haven’t traveled through the requisite 200 pages of setup, so I’ll just mention that these analogies use a mass on a string, a string with many masses, and magnetic dipoles. In other words, the analogies Strassler employs are physical, not psychological. If that sounds good to you, you’ll probably enjoy the book.
There is surely more that can be said about the subject of physical analogies. Strassler employs them, but he does not spend much time philosophizing about their features. Physical analogies are tricky because they require the reader to appreciate examples for which every part of one system is swapped out for every part of another, but somehow the dynamics of the two systems still match.
In the first semester of physics, one learns about two versions every physicist’s favorite model: the harmonic oscillator, which bounces back and forth, and back and forth, forever without end. The first oscillator one encounters involves a mass pulled by a spring. This oscillator’s period, the amount of time it takes to complete one full oscillation, depends both on the mass being pulled, and on the spring doing the pulling. The bigger the mass, the longer the period; the stiffer the spring, the shorter the period. A small mass on a stiff spring has a short period.
The next version of the harmonic oscillator initially seems similar.
The second oscillator is a mass pulled downward by gravity, and upward by a lightweight string—a simple pendulum. For the simple pendulum, the period involves the length of the string and the gravitational field strength pulling on the mass. (Near the Earth, gravity pulls downward with 9.8 N/kg; on the Moon, it is around 1/6 as strong.) The longer the string, the longer the period. The stronger the gravitational field, the shorter the period. A pendulum with a short string on Earth has a much shorter period than a pendulum with a long string on the Moon.
And so what? The place where physical analogies become challenging is when we have to confront how cosmetically similar systems differ in their details. Both our oscillators bounce back and forth, and both trade kinetic for potential energy twice each cycle. For both oscillators, the total energy depends on how far the mass swings. But in other ways, they turn out to be quite different. Dynamically, the length of the pendulum string takes the place of the mass on the spring, and the gravitational field strength takes the place of the spring’s stiffness. Are these two systems really all that alike, or are we just convinced of their similarity because, for both, the mass bobs back and forth, then back and forth, over and over again?
Last year I wrote about Anthony Zee’s Quantum Field Theory, As Simply as Possible, which covers some of the same territory a Waves in an Impossible Sea, though with much more mathematical detail. But where Zee chatted charmingly about the tools of field theory, Strassler ends on a more ponderous note:
It’s only in recent years that we’ve fully appreciated the most important lesson of modern physics: there’s absolutely nothing mundane about ordinary life. The cosmos, stunningly strange and unrelentingly contrary to common sense, infiltrates our every moment. We ourselves, and everything we experience from birth till death, are vibrating manifestations of a nightmarish, uncertain, amotional universe.
This is a strange moral for book that seems refreshingly full of common sense—but maybe every theorist should be granted some unlabeled mysticism. Part of the charm of physics is that while physicists work on technical problems, they are still allowed some of the dignity of poets, allowed to imagine that beneath their simple analogies and mathematical constructions yet beats the hidden pulse of world.