Shufflebrain

Interference occurs whenever waves collide. You've probably seen waves of water cancel each other upon impact. This is destructive interference, which occurs when the rising part of one wave meets the falling part of another. Conceptually, destructive interference is like adding a positive number to a negative number. On the other hand, when waves meet as both are moving up together, interference still occurs but it is constructive , or additive, and the resulting wave crests higher than its parents. In order to have an interference pattern, some definite phase relationship must exist between two sets of colliding waves. A well-defined phase relationship is coherent and is often referred to as "in step". When colliding waves are incoherent, their interaction produces random effects. An interference pattern is not random; and a basic requirement in an interference pattern is coherency.

Ordinary light is decidedly incoherent, which is why optical interference patterns aren't an everyday observation. Today, Heisenberg's uncertainty principle[6] accounts for this: wicks and filaments emit light in random bursts. Even if we filter light waves -- screen out all but those of the same amplitude, wavelength, and frequency -- we can't put them in step. In other words, we can't generate a definite phase relationship in light waves from two or more sources.

Young's experiment circumvented the uncertainty principle in a remarkably simple way. Recall that his sunbeam first passed through a single pinhole. Therefore, the light that went through both pinholes in the far baffle, having come from the same point source, and being the same light, had the same phase spectrum. And, coming out of the other side of the far baffle, the two new sets of waves had a well-defined phase relationship, and therefore the coherency to make interference fringes.

Here's what Fresnel did. He let light shine through a slit. Then he lined up two mirrors with the beam, aiming them so as to reflect light toward a screen. But he set the two mirrors at unequal distances from the screen. In so doing, he introduced a phase difference between the waves reflected by each mirror. But because the waves came from the same source (the slit), their phase differences were orderly; they were coherent. And when they interfered, they produced fringes in the form of Fresnel rings.

Interference patterns not only depend on an orderly phase difference, they are precisely determined by that difference. If you are ever in a mood to carry out Young's experiment, see what happens when you change the distance between the two holes (create a phase variation, in other words). You'll find that the closer the openings, the narrower the fringes (or beats) will be and the greater the number of fringes on the screen.

The hologram is an interference pattern. The distinction between what we call hologram and what Young and Fresnel produced is quantitative, not qualitative. Now, in no way am I being simplistic or minimizing the distinction (no more so than between a penny and a dollar). Ordinary interference patterns do not contain the code for a scene, because no scene lies in the waves' paths. Such patterns do record phase variations between waves, though, which is the final test of all things hologramic. Just to keep matters straight, however, unless I explicitly say otherwise, I will reserve the term

hologram

for interference patterns with actual messages.

***

The hologram was born in London on Easter Sunday morning, 1947. It was just a thought that day, an abstract idea that suddenly came alive in the imagination of a Hungarian refugee, the late Dennis Gabor. The invention itself, the first deliberately constructed hologram, came a little later. But even after Gabor published his experiments in the British journal Nature the following year, the hologram remained virtually unknown for a decade and a half. Gabor's rudimentary holograms had none of the dramatic qualities of holograms today; perhaps as few as two dozen people, worldwide, appreciated their profound implications. Not until 1971, after holography had blossomed into a whole new branch of optics, did Gabor finally receive the Nobel Prize.

Gabor often related his thinking on that fateful Easter morning. He hadn't set out to invent the hologram. His thoughts were on the electron microscope, then a crude and imperfect device. In theory, the electron microscope should have been able to resolve atoms.[7] (Indeed, some instruments do today.) But in 1947, theory was a long way from practice. "Why not take a bad electron picture." Gabor recounted in his Nobel lecture, "but one which contains the whole of the information, and correct it by optical means?"[8]