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A Random Walk through Oddly Named Physics Things


A few months ago I helped a friend of mine, Rose Eveleth, think of weirdly named things in physics. She could only include five, but there were a few more I thought were worth highlighting. So here they are.

Naming things is difficult. This fact can be attested to by anyone who's ever had a baby, a hamster, or a punk band. It's no easier for scientists, who must constantly find names for the things they discover and the theories they come up with to explain those discoveries.

In spite (or perhaps because) of the overwhelming boringness of much technical jargon, scientists are drawn to whimsical or poetic names more than you might suspect. Here are some of my favorites.

Misner's Mixmaster Universe

The "cosmic microwave background," or CMB for short, is a kind of radiation: electromagnetic static that fills the entire universe. When you switched on your TV in the pre-digital era and no channel was tuned in, a few percent of that fuzz was the CMB.

Also known as "relic radiation," the CMB is energy left over from a time shortly after the big bang, when the entire universe was extremely hot and dense.

The thing that puzzled cosmologists was just how uniform the CMB was. They had expected that, because of quantum fluctuations present just after the big bang, the radiation would be patchy: stronger in some places and fainter in others. But this was not at all what was observed: the CMB looks almost exactly the same everywhere in the sky.

In a 1969 paper entitled "Mixmaster Universe," physicist Charles Misner set out his idea for a solution to the paradox. Although it sounds like a 1980's proto-hip-hop group, the theory actually gets its name from a kitchen appliance, the Sunbeam Mixmaster.

The idea was that the early universe went through a phase of so-called chaotic evolution, which did for the cosmos what the Mixmaster does for cake batter, mixing its contents until they were smooth and even.

The Mixmaster Universe is admired for being an ingenious solution to the equations of general relativity. However, it has been superseded by another theory, called "inflation," which does a better job of explaining the data than Misner's.

Unlike the physics theory it inspired, the Sunbeam Mixmaster is still going strong, and is probably available at a store near you.

The Luminiferous Aether

Schematic from Michelson and Morely’s report of their experiment, published in the American Journal of Science in 1887.

Anyone who's ever been forced to take freshman physics knows that light is a wave. (Well, it's also a particle, but that's another story.) This fact has been known for a very long time: it was first suggested as far back as the 17th century by the Danish scientist Christian Huygens. For two hundred years afterwards, scientists and natural philosophers struggled with the question: "if light is a wave, then what is it waving?"

Just like sound waves need air to carry them, and ocean waves need water, the idea that light needs to travel through some kind of medium led scientists to propose the rather magical-sounding "luminiferous aether." The word "luminiferous" refers to light, while the mysterious substance was called "aether" because it was thought to be so rarefied as to be almost impossible to detect. And since we can see light from far-away stars, this aether must fill all of space.

Numerous aether-detection experiments culminated in one of the most famous observations ever made in science: the Michelson-Morely experiment, which attempted to measure the motion of the Earth through the aether. (It's also probably the only major contribution to physics that took place in Ohio.)

It is a physics truism that not finding what you're looking for can be more interesting than finding it, and so it was with Michelson and Morely, who failed to turn up any sign whatsoever that the aether was real. The collapse of the aether theory provided strong evidence in favor of Albert Einstein's Special Theory of Relativity, which was developed in the following decades. Relativity doesn't need - and in fact can't have - a light-carrying medium like the aether.

Random Walks

Imagine a very drunk person - a friend of yours - who has just woken up, groggily, in the middle of a football field. He doesn't remember what made him go to the football field in the first place, and he wants out. Your friend tries walking a little bit, worried that he might need a bit more of a nap first. But no - success! His legs seem to work okay, and he commences his pondersome exit.

Instead of helping out your buddy, you decide to think about science for a while. From your vantage point in the stands, you notice something interesting. Your friend seems to be so drunk that he can't take two steps in a row in the same direction. (Hey, it's happened to the best of us.) In fact, the direction of each step seems to be totally random: he has an equal probability of going north, south, east or west, or any direction in between.

A number of questions come to mind. (At least, they do if you happen to be a physicist.) How long will it take your friend to get out? What is the probability that he will find himself back at his starting point? If the field is enclosed by a tall fence, with only one exit, how much longer will it take him to escape?

If you were to trace out the path he takes, the shape wouldn't look like anything you've seen in math class. It looks more like the graph of a stock price, or the coastline of Norway, than the smooth parabolas of classical geometry. It is a jagged fractal that meanders around and crosses over itself before hitting the edge of the field.

The drunken voyage that created this shape is an example of a "random walk." Random walks describe all kinds of phenomena in nature, economics and mathematics. The list of things which can be described or modeled using random walks is too long to include here, but notable entries include the distribution of genes in a population of animals, the path of drug molecules in cancer cells, the process that misaligns the little magnets in MRI machines, and the amount of money you're likely to lose in a game of roulette.

The Ultraviolet Catastrophe

At the beginning of the 20th century, British scientists Lord Raleigh and James Jeans tried to calculate what you should see when you look at a "black body." A black body is an idealization: an imaginary object that absorbs and emits light of all wavelengths perfectly. ("Wavelength" is, at least in this context, just a technical word for "color.") What black bodies look like isn't just a theoretical question: some astronomical objects, like the sun, are quite close to perfect black bodies.

In particular, Raleigh and Jeans were interested in the "blackbody spectrum": how much of the emitted energy is stored as red light, how much as blue light, and so on. To solve the problem, they used two of the most venerable theories of 19th-century physics: James Clerk Maxwell's theory of electromagnetism and Stefan Boltzmann's theory of statistical mechanics.

The blackbody spectra of objects of different temperatures. The shape of the curves can be calculated using quantum mechanics.

But when they did their calculation, there was a serious problem: the amount of energy stored in the short-wavelength part of the spectrum was.... infinite! This nonsensical result was such a big problem for physics that it was dubbed "the ultraviolet catastrophe" (ultraviolet light being of the of short-wavelength variety.)

The solution to the paradox germinated from the work of Max Planck, a German physicist. Planck introduced the idea that light could not be emitted and absorbed in arbitrarily small amounts; rather it comes in discrete packets that are now called “photons.” It works just like money: you can't buy less than a penny's worth of chocolate or dish soap, and an atom can't absorb less than a photon of light. Armed with this assumption, Planck was able to calculate the blackbody spectrum without any infinities appearing.

At the time, Planck's idea caused as many problems as it solved, because the assumption that light comes in such discrete packets completely contradicts Maxwell's theory of electromagnetism. The ultraviolet catastrophe wasn't resolved until the 1920's, when a comprehensive theory of quantum mechanics was introduced. Quantum mechanics showed that the older, 19th century theories were incomplete. Amongst many other achievements, quantum mechanics allows for correct calculation of the blackbody spectrum.

Images from Wikimedia Commons.

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

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