Pervasive Patterns of Waves Make the Invisible Visible

Physics is simple. In fact, it never ceases to astonish me how, with a few simple ideas from physics in your suitcase, you can travel from your own backyard to the most exotic realms of the universe, feeling equally at home in both inner and outer space.

Consider, for example, the familiar way water evaporates from a puddle, or freezes when the temperature drops. The universe began, cosmologists say, when the vacuum of empty space went through a very similar transition, from a melted state into a frozen one. The glowing gases of auroras and stars are simply matter in what you might call an even more melted state, in which electrons are knocked loose from atoms. Superconductors carry electricity friction-free because they are “super-frozen” to the point where they behave like a single large atom.

This simple business of freezing and melting explains everything from the “quark soup” that made up the early universe to liquid helium’s ability to flow uphill.

Or consider the harmonics of a bottle of beer. Blow over the top, and you can make a series of different sounds depending on how hard you blow and how much beer is left in the bottle. And lo and behold, it is by analyzing a very similar set of harmonics set up by the sloshing of gas and light in the early universe that astronomers have been able to put their ears to the cosmos, listening in on its babblings from the first moments of time.

Among the most beautiful of pervasive, simple phenomena is interference: what happens when waves alternately march in and out of step, adding up and canceling out.

In the thin skins of soap bubbles, these alternating bands show up as serpentine ribbons of color, each pastel shade appearing in the place where the thickness of the skin causes those particular waves of light to march in step. In a holographic plate, the bands encode the information needed to make three-dimensional images hover in thin air.

The trick to interference is that it vastly magnifies patterns, making the invisible visible. For example, light waves are far too small to see individually with your eyes, but the interference patterns shine through clearly in the iridescent colors of opals, oil slicks and butterfly wings.

This magnification effect makes interference patterns uncannily useful. Your 767 en route to Maui relies on laser-produced interference patterns to inform the plane’s inertial guidance system of even the slightest changes in position. Rosalind Franklin used interference patterns produced by X-rays to first “see” the spiral structure of DNA, and today most studies of materials rely on interferometry in one way or another.

So does most astronomy. Radio astronomers have long used interference patterns of radio waves to see into the distant sky--sometimes using arrays of telescopes spread across the globe, combining waves from each to create a single vastly improved image.

Today, virtually all major optical telescopes are adopting interferometry as well, including Mt. Wilson, the premier Keck telescope on Mauna Kea in Hawaii, and the European Southern Observatory’s Very Large Telescope in Chile.

One of the most exciting ways astronomers are using interference is to try to catch gravity waves from cataclysmic events in space--the collision of a couple of black holes, for example. Such a horrendous clash would shake up the four-dimensional fabric of space-time itself, sending out ripples that should reach our shores.

Alas, space-time doesn’t get shaken up easily, and these ripples are almost inconceivably weak. Thus, the Laser Interferometer Gravity Wave Observatory consists of two enormous interferometers on opposite ends of the country, each strung with 4-kilometer-long laser beams. Plans are in the works to put even bigger interferometers in space--probably the only way to see Earth-like planets.

Any rhythmic pattern can produce interference patterns: ripples caused by stones dropped into a pool of water; the threads in moire silk; the overlapping struts of picket fences. Musicians listen for them (they sound like subtle throbs) to tune their instruments.

Indeed, because only wavelike phenomena can interfere, it was the discovery of interference patterns that proved that light was a wave--and later that elementary particles, too, have wavelike alter egos.

From the fringes at the edges of shadows to the “electron holograms” described in my latest issue of Physics Today, interference patterns are ubiquitous--synchrony and asynchrony playing off each other in a constantly unfolding fugue, poetry and utility in one simple, easy-to-carry package.

Put that in your suitcase, and I imagine it will take you on a trip somewhere interesting really soon.