Shufflebrain

The entire idea hinged on phase. And Gabor solved the phase problem with the conceptual approach Albert Einstein had taken in theorizing mass-energy and, eventually, the universe itself. No wonder we lose the phase, Gabor thought, if there is nothing to compare it with! He would need a reference . He would have to deal with phase information in relative, not absolute, terms.

The big technical hurdle was coherency. How could he produce a coherent source? Gabor's answer was very similar in principle to Young's and Fresnel's: Let light shine through a pinhole. If he set a tiny transparent object in the path of the beam, some waves--object waves--would pass through it, while others would miss; the waves that missed the object would still collide with the object waves downstream, and that ought to create interference patterns. Those waves that missed the object would become his reference. And the interference patterns would become a record of the phase and amplitude differences between object and reference waves. He'd use a photographic plate to capture that pattern, he decided.

Recall the discussion about objects warping the amplitude and phase of waves? If the interference pattern is completely determined by the amplitude and phase spectra of interacting sets of waves, then what? The hologram should retain not only amplitude changes but also the relative phase variations imposed on the object waves.

It is hard to believe that such records had already been produced by other physicists. But as a matter of fact, x-ray crystallographers' diffraction patterns, generically speaking, are holograms. Crystallographers take the information from the x-ray diffraction patterns and use the equations Kraut was talking about to deduce the images of the atoms in crystals. Gabor realized that he could do the same thing with a beam of light. He could physically decode the image. He realized that if he passed the original light through the hologram plate, instead of through the object, the shadows in the hologram would put the warp into those waves and the complete image would appear where the object had been. For this would reconstruct the image-bearing wave front. When he tried it, it worked.

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The object Gabor used for his very first hologram was a tiny transparent disc with a microscopic but monumental message etched onto it. It read, "Huygens, Young, Fresnel."

Gabor did not succeed with the electron microscope. In fact, his first hologram just barely reconstructed the message. "It was far from perfect," he quipped. But it was not his reconstruction that had made history. It was the idea.

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Gabor's principle is very simple, in retrospect--so simple that only a genius could have seen through the taboos of the time to find the hologram amid the arcane and abstract properties of waves.

The hologram is an interference pattern, and interference is a subject taught in high school physics courses. To engineers and physicists, the hologram is a straightforward extension of elementary optics. Even the mathematics of the hologram are fairly simple. Crystallographers for some time had been doing the construction step of holography without calling it that. And a color technique developed in 1894 (the Lippman process) suggested even the reconstruction step. How then was it possible for the hologram to escape modern science until 1947?

Science is people. Scientists seldom try out in their laboratories what they do not believe in their guts. Recording the phase of light waves would violate the uncertainty principle. Nothing yet known has withstood the incredible power of the uncertainty principle, including the hologram. There's a subtle distinction, though, between phase and a code resulting from phase. But only an extraordinary mind could see this; and only an extraordinary person had the courage to proceed from there.

Gabor was no ordinary person. And in the early 1960s. in Michigan, two other extraordinary persons entered the scene--Emmett Leith and Juris Upatnieks. A small amount of work had been done in this field after Gabor's research; but Leith and Upatnieks turned Gabor's rudimentary discovery into holography as it is practiced today. And among their long string of remarkable inventions and discoveries was one that precipitated nothing less than hologramic memory theory itself.

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The germ of hologramic memory is unmistakable in Gabor's original discoveries--in retrospect. A physicist named van Heerden actually proposed an optical theory of memory in 1963; but his work went as unnoticed as Gabor's had. As in the case of the acorn and the oak, it is difficult to see the connection, a priori. Leith and Upatnieks did for the hologram what Gabor had done for interference in general: they extended it to its fullest dimensions.