Degrees of Freedom

Degrees of Freedom

The boundless dimensions of math and physics

Book Review: Our Magnetic Earth, by Ronald Merrill


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A magnetic sense is now well documented in dozens of animal species. It turns out that tracking the geomagnetic field—that same invisible thing that points compasses—is handy for life, in lots of situations. Using their internal compasses, naked mole rats in Africa navigate their pitch-black underground mazes. Lobsters off Bermuda find their way to regions of the seafloor where they congregate to spawn. Thrushes migrate south in the autumn and north in the spring. Honeybees know which way is home to their hive. And humpback whales swim for hundreds of kilometers at a time in the open ocean without deviating by more than one degree from the course they initially set.

Biological tissues however tend not to respond to, or be affected by, magnetic fields. Thus, for a long time explaining how animals sense these fields has been a holy grail of sensory biology. There now appear to be at least two plausible explanations. One proposed mechanism is based on microscopic particles of iron oxide located inside specialized cells; the other on a quantum effect in which certain chemical reactions--specifically some that may involve a protein in the retina called cryptochrome--slow down or speed up depending on which way points north with respect to the animal's head.

Each of the two mechanisms has mesmerizing evidence to back it up, as well as detractors. To learn more, you’ll have to read my new feature article The Compass Within,” in the January 2012 issue of Scientific American.

Our Magnetic EarthBut how does the planet generate a magnetic field in the first place, and why does that field point, more or less consistently, to a magnetic north? As Ronald Merrill’s fascinating recent book Our Magnetic Earth: The Science of Geomagnetism explains, there are essentially two ways that a relatively permanent magnetic field can arise in nature. One is the magnetization of a solid object, as in the case of a bar magnet or of the iron oxide found in certain animal cells; the other is the so-called dynamo effect, in which electric currents generate the field.

Early on, researchers realized it had to be currents. No known mineral or material is able to maintain a permanent magnetization at temperatures above 1,000 degrees Celsius. But Earth’s metallic core—where its geomagnetic field originates—is way hotter than that: at an estimated 5,000 degrees, it is as hot as the surface of the sun.

So, dynamo it is. And ours is not the only planet in the solar system thought to harbor a dynamo in its core. So do Jupiter, Saturn, Uranus, Neptune and possibly Mercury and even one of Jupiter’s moons, Ganymede.

This realization however was only the beginning of a long study that is still in progress. One difficulty is that we can only measure the magnetic field on Earth's surface or in space. From those data alone, it is not possible even in principle to reconstruct the shape of the magnetic field lines deep inside. This, Merrill points out, is known to mathematicians as a “non-uniqueness” problem—also known as the difficulty of guessing what’s inside a Christmas gift by lifting it and shaking it (which, Merrill informs us, is what his wife used to do) rather than opening the box.

As a matter of fact, not much is even known about the composition of Earth beyond the fact that its most abundant element is iron. According to Merrill, in 1952 the late Harvard University geophysicist Francis Birch wrote, in a classic Journal of Geophysical Research paper on the composition of Earth’s core,

Unwary readers should take warning that ordinary language undergoes modification to a high-pressure form when applied to the interior of the earth. A few examples of equivalents follow:

Certain -> Dubious

Undoubtedly ->Perhaps

Positive proof -> Vague suggestion

Unanswerable argument -> Trivial objection

Pure iron -> Uncertain mixture of all the elements

“In spite of a considerable amount of excellent work,” Merrill writes, “our understanding of Earth’s core’s composition is remarkably similar to that given by Birch more than a half century ago.”

But while lots of details still need to be ironed out, Merrill says, scientists now believe they have a rough idea of the physics behind (or underneath) the geomagnetic field. When an electrical conductor moves, it drags the magnetic field around with it. But what happens when the conductor is not rigid, and in particular, when it’s liquid, as in the case of Earth's outer core? As layers of liquid slide over each other, magnetic field lines get stretched, and the result is an amplification of the magnetic field itself, at the expense of the kinetic energy of the fluid. But as long as the motion continues, this phenomenon can sustain a magnetic field that would otherwise slowly dissipate.

In recent years, researchers have produced computer simulations of the geomagnetic dynamo and, crucially, they have shown that such a dynamo would have periodic reversals, which would explain why the north and south poles have switched at seemingly random intervals of time over the eons.

The last such reversal appears to have happened 780,000 years ago. When the next one will be is anybody's guess. During reversals, the field does not disappear, but rather it becomes weaker, potentially disrupting some animals' migratory patterns as well as letting solar wind destroy part of the ozone layer of the upper atmosphere. This is a favorite disaster scenario for some 2012 doomsayers, but Merrill reassures us that reversals take place very slowly, over centuries if not millennia, and that their effects are probably not that disastrous after all.

This is a supercomputer-based simulation of the geodynamo by Gary Glatzmeier of the University of California, Santa Cruz, and his colleagues:

[For more on this, check out the Scientific American article Probing the Geodynamo,” by Gary A. Glatzmaier and Peter Olson, April 2005 (requires subscription), as well as Glatzmaier’s website.]

Scientists are also trying to build small-scale versions of Earth’s core in the lab. In one such experiment, at the University of Maryland, Daniel Lathrop and his collaborators built a rotating sphere three meters (ten feet) in diameter and filled it with liquid sodium. They hope the sphere will help them understand how the chaotic motions in the core lead to a geomagnetic field.

Seen in action, as it spins at four rotations per second, Lathrop's sphere looks worthy of a Marvel Comics supervillain:

(More on these efforts on my friend Charles Choi's blog.)

In his book, Merrill gives an honest and captivating account of the scientific process, its uncertainties, and its cultural dynamics. Science is often portrayed as a fight between smart innovators and conservatives who are on the wrong part of history, but in reality, before an open question is settled there are often solid scientific arguments made on both sides of a debate. One good example is plate tectonics. It was an extraordinary claim, and as such it really required extraordinary evidence before the "drifters," as Merrill calls them, were able to convince the skeptics--or most of them anyway--in the early 1960s.

Merrill intersperses the narration with juicy anecdotes and personal detail, which often leave us wanting to know more. (At different times, we find our hero-scientist dangling from a rope on one of Yosemite’s climbing walls, or SCUBA diving by a shipwreck, or on a boat surrounded by white sharks who had been tagged for tracking their migrations.)

Often, however, he falls back into professor mode. One aspect of the book that, unfortunately, may turn away some readers, is an eat-your-vegetables-first prescription coming right in the first chapter: the reader has to slog through technical details on the physics of magnetization before he gets to the fun part. I suspect that some readers never did.

I found that the book was at its best when it delved into the friction among scientists in these different disciplines—and the lessons in modesty that researchers often learn (or should) from collaborating with people from other buildings across campus. Geomagnetism and the magnetic sense, to which Merrill dedicates a chapter, are problems that require expertise from a broad range of researchers, incuding chemists, physicists, geophysicists, mathematicians and biologists.

Such friction was prominently on display in the case of Lord Kelvin, who in 1862 calculated that Earth could not be older than 400 million years, and probably was only 100 million years old. Kelvin scoffed at evidence to the contrary that had been discovered by geologists, who he regarded as incapable of doing math, Merrill writes. It is an example of the arrogance some physicists exhibit toward sciences they deem less “fundamental." (Ernest Rutherford, the discoverer of atomic nuclei, notoriously said that all science is physics—the rest is just stamp collecting.)

In turn, geophysicists may sometimes scoff at biology as a “soft” science, Merrill writes, but those who have tried to actually learn some—let alone do research in it—know better. In particular, he says, geophysicists used to underestimate the problem of determining the physical mechanism behind animals’ magnetic sense.

(Still on the subject of cultural differences among academic communities, Merrill also makes a very poignant remark about mathematicians. Although the increasingly extreme specialization of science that has occurred over the last century or so is common to most branches of knowledge, so that, say, a nuclear physicist and a solid-state physicist can only talk to each other with some difficulty, the situation is far worse in math, Merrill says: when someone is up for tenure at a a math department, he says, most of the faculty in the department have little understanding of the candidate's work, and so they often rely on the advice of authorities from other universities.)

I shall conclude by quoting one of my favorite anecdotes from the book, regarding Ted Ringwood, an eminent geochemist at Australian National University and Ray Crawford, a “far less famous” scientist. Crawford had a penchant for collecting stationary from places he visited, and a skill for practical jokes.

The austere Ringwood had gotten on loan from NASA a few samples of lunar rock to study. NASA did not trust just anyone to guard its precious trophies, and required extraordinary caution in handling them and storing them. One day, Ringwood received a letter, printed on NASA stationery, Merrill writes. “The letter informed Ringwood that NASA had funded psychologists to study the effects that stress had on scientists studying lunar samples. Would Ringwood help in this study by sending a vial of his urine to the American embassy in Canberra on a weekly basis? Ringwood complied with this request for several weeks before someone in the embassy had the courage to phone him to inquire what the professor wanted done with he urine samples.”

Our Magnetic Earth: The Science of Geomagnetism, by Ronald T. Merrill. University of Chicago Press, 2010.

Further readings:

The views expressed are those of the author and are not necessarily those of Scientific American.

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