Chapter five
The conservation of spatial phase

Museum of Holography in SOHO. Photo by Paul D. Barefoot.
Museum of Holography in New York. Photo courtesy of Paul D. Barefoot

Holograms are 3-dimensional photographs of uncanny realism. They are unlike a slide projected onto a screen. Objects reproduced holographically appear in mid air, and they appear exactly as they do in real life. If you shift your point of view, by taking a sidestep to the left or right for example, you can open up to scrutiny parts of the object that were previously masked. The only way to distinguish a well-made hologram from a real object is to put your hand through it.

In other words, you don’t need to actually see a hologram to know what one looks like. If you have moved through the world, you already know what a hologram looks like. It looks like reality.

When the technology of holography first came to be widely understood and appreciated in the 1960s and 1970s it was quickly seized as a metaphor for both human vision and human memory. What we see and what a holographic apparatus remembers and projects are virtually identical.

The holographic vision theories were quickly extinguished. They were clobbered in the 1970s with “insuperable objections” based on the eye’s supposed inability to capture or conserve spatial phase. We will revisit this problem after we take a look at what spatial phase means, but I should point out here that I think the “insuperable” phase argument was completely specious. It is unfortunate the early holographic vision theorists were cut short. In essence, they were probably right.

Holographic memory theories have had a much longer run. One version, based on the curious and oddly popular notion that quantum effects must account for the workings of the brain, was launched around 1987, and still has adherents today. But the golden age of the original — let’s say Classical — holographic memory theory was probably 1963 to 1973.

The idea failed to stick for several reasons. It eluded most people — the holographic memory theory was difficult to understand without studying Fourier optics. There were strong technical objections, arising from problems of coherence and phase reference, which made it seem biologically unrealistic.

And at some point the decline became sociological: The holographic memory idea, or analogy, lost credibility in proportion as neuroscientific enthusiasm tilted toward and then finally avalanched into neural networks.

At the end of the day, the holographic memory theory sort of flipped over and slid sideways into mysticism and quackery.

For example, it would be possible for you to attend a seminar in the Pacific Northwest on holographic memory. In this seminar you would learn that the body and the mind are the same thing, and so the body is included with the brain in the holographic memory. Each part of a hologram contains the whole, and the whole contains the parts. You are in the universe but the universe is also in you — and so on in this vein.

Now, it seems that by squeezing the body in a certain way, pretty hard, one can elicit and purge various bad holographic memories. For several hundred dollars you can be squeezed to get rid of your own damaging memories and you can also, later, learn to do the squeezing, as a sort of profession. This knowledge will enable you to establish a profitable therapeutic practice in the business of purging holographic memories. Perhaps attaining oneness with the universe at the same time.

So there you have the wreck of an idea.

If you leaf through the early literature, you will find that holographic memory was once regarded as one more arguable but interesting and well reasoned scientific hypothesis about how the memory works. Today it is an eye-roller. I know I must be missing a chapter or two of this history. I don’t understand how a simple scientific analogy turned into some sort of strange, quasi-religious ideology. But it did.

To see what might be salvaged, and to get a clearer look at the solid, unmystical and underlying technology, which is holography, let’s consider here the Museum of Holography.

The Museum of Holography
In lower Manhattan, in Soho, there existed for 15 years behind a storefront at 11 Mercer Street a museum of holography. A fine account of the museum, along with an excellent general history of the science and art of holography, is at the site of Holophile, Inc.

On the upper level of the Museum of Holography was an active art gallery displaying the artistry and novelty of holographic images, including some fine art. But the main attraction was a gallery of ominously realistic heads – holographic portraits — of celebrities and atheletes including, for example, that of Mike Eruzione, captain of the 1980 U.S. Olympic Hockey Team. In fact, the heads of the whole team were on display.

The museum once had a show of holographic portraits of Famous New Yorkers. William Buckley and the young Tom Brokaw were included among the heads of the famous. No question, the gallery of celebrity heads was what most impressed visitors. You could not view the celebrities’ heads without thinking of a wax museum and then, pretty quickly, of the French revolution, of Robespierre.

There was also some technological art, that is, pictures conceived as demonstrations of the strange power of holography to project reality. Or anyway, the reality perceived by human beings, the reality communicated to us by visible light.

On weekdays the museum filled with grade school children, who arrived in groups and toured the gallery in delighted, squealing clusters of threes and fives. They left with souvenirs: goofy looking diffracting sunglasses, tiny holographic charms and trinkets.

Down in the basement, precisely at the foot of the stairs, was positioned the full sized holographic image of a man, the Hungarian born physicist Dennis Gabor, seated as though he might have just then looked up from his desk in order to watch you come downstairs to visit him. Gabor invented the hologram. He was a small inquisitive looking man with a hooked nose and a walrus mustache. He wore glasses of distinctive design, with special thick arms that would block peripheral vision but shield the eye, perhaps from chance exposure to the blast of the laser beam used to create his image.

Dennis Gabor was a Nobel laureate and a stunningly gifted inventor and scientist. He was one of that famous group of expatriate Hungarian Jews who studied in Germany in the 1920s and then proceeded to astonish the world in the 1940s, mostly by inventing nuclear weapons. By coincidence, several of these geniuses were enrolled together in a course in statistical physics taught by Albert Einstein. The role call for this small class would sound, if we could hear it now, like a Who’s Who of twentieth century physics: Leo Szilard, Eugen Wigner, John von Neumann, Edward Teller and Dennis Gabor.

These expatriate Hungarians were primarily theoreticians. Their lifeworks are inaccessible to most people. Perhaps for this reason they tend to be remembered for their achievements in applied science. Szilard conceived the nuclear chain reaction, following the model of chain reactions published in the literature of organic chemistry. Wigner designed the first full scale nuclear reactors. Von Neumann – the air burst explosive, the implosion lens for the atom bomb dropped on Nagasaki, the hydrogen bomb, the digital computer, and the Game Theory. Teller, the hydrogen bomb. And Dennis Gabor, the hologram.

It is pretty unlikely, however, that Albert Einstein, who loathed schools and pedagogy, ever called the role.

In 1947, at the age of 47, Dennis Gabor created the first hologram. Notice that he did this work before the invention of the laser. But the techique begged for a laser, and when it finally appeared, in the 1960s, holography was enthusiastically re-discovered. Dennis Gabor died in 1979. In the basement of the holography museum in Soho, near beside the hologram of Dennis Gabor, was a hologram of the bound and embellished certificate proclaiming his Nobel Prize. When they switched off the power at night, both the man and his Nobel prize would of course vanish. Every morning they reappeared when the power was switched on again. Apparently this went on for years. Flickering immortality.

Under the hammer
Not long after I discovered the museum in the early 90s, it lost its benefactors and went bankrupt. Its entire collection, including Dennis Gabor’s surviving image, and that of his Nobel prize, were sold at an auction. The auction was held on a snowy Saturday in January, 1993, at the order of the United States Bankruptcy Court of the Southern District of New York, in a tin building on the Saw Mill River Road in Hawthorne, New York. A tiny ad in the Times classifieds offered the following 19th century P.T Barnum style advertising copy:

“Very Extensive Collection of Some of The Finest And Most Important Holograms in The Country, Featuring The Nobel Prize of Dr. Dennis Gabor, Plus His Holographic Portrait And Other Supporting Papers. Also Works By: Yaacon Agam, S. Benton, J. Burns, Dr. Jeong, Claudius. And Many More Early and Prominent Creators. The Museum Also Includes Holographic Production Equipment, Lasers, Lighting, Etc.”

I attended the auction, curious to know what might become of Dennis Gabor’s immortal image. During the morning inspection period before the auction began I actually found it — the glass holographic plate, face up on a cafeteria table.

The glass measured 18″ by 24″. A label explained that Gabor sat for his three dimensional portrait at the McDonnell Douglas Labs in St. Charles, Illinois, in October, 1971. That was year he won the Nobel Prize. The plate had been set out next to three liquor store boxes filled with faded newspaper clippings, correspondence, magazine articles and scientific papers. Here was a photo of Gabor in tails with the queen of Sweden upon his arm. The queen of Sweden was much bigger and taller than Dennis Gabor.

The whole collection, including the plate, the papers, and a paper copy of the Nobel Prize, was marked as lot #210. I picked up the holographic plate and held it at various angles against the fluorescent lights of the warehouse. There was no visual hint that this flat panel of glass contained a three dimensional image of a man. As with any hologram, it was impossible to discern any sort of literal image.

Abruptly the auctioneer turned on a microphone and declared, with an amplified Queens accent, that there would be no further viewing. His assistant popped up – a woman about 30, with bobbed red hair. She wore a bright white blazer over a peach blouse. Her fingernails were Chinese red. She wore rings on each hand and a butterfly broche on her lapel that wildly scattered light. Evidently her job was to attract and hold the attention of the crowd, which she certainly did. She picked up a hologram (bleached, nondescript, unintelligible) and held it high over her head. The auctioneer started his chant and the bidding commenced.

When they got around to lot #210, the initial bid was just $500, which doesn’t seem like much for the memorabilia of a Nobel Prize winner, but this low bid was in the end brushed aside by much bigger money. The Media Lab of MIT was represented in Hawthorne by Professor Stephen A. Benton, the inventor of the white light holography technique. In the early afternoon it was announced that Dr. Benton had bought out the whole auction for MIT with a single bulk bid of $180,000.

In Cambridge, in 1994, they held an exhibition. Perhaps they set up the plate of Dennis Gabor and switched him on once again, good as new. Stephen Benton died in 2003. MIT has added substantially to the material bought in from the Museum of Holography, and has now the most extensive holography collection in the world. The Gabor plate is still in Cambridge, still safe, still sequestering the image of the man and still, somehow, ineffably sad.

What is spatial phase?
Light carries five kinds of information into the eye.

  • direction of propagation
  • polarization
  • frequency
  • amplitude
  • phase

Direction — the sense of where the light ray came from — is fundamental.

Scattered sunlight is polarized, and polarization is sensed by many insects, crustacea and cephalopods. Bees use this information to navigate. Amazingly, polarization effects can also be sensed by a very few humans (Haidinger’s Brush Effect).

Let’s concentrate here on three properties: Frequency, amplitude and spatial phase.

Frequency is perceived as color. Light amplitude is perceived as brightness. It is thought that spatial phase is not perceived at all. But what sort of information is carried by spatial phase?

Phase can be described only mathematically, but we can treat the subject in a casual narrative way in order to seed the basic idea, as follows:

Let’s say that light “takes an impression” of a three dimensional object as it bounces off of it. Light then carries that impression, as phase information, into a camera or into your eye.

Spatial phase. A plane wave of light takes an impression of a reflective three dimensional object as it bounces off of it. Light then carries that impression, as phase information, into a camera or into your eye. Illustration reproduced courtesy of Prof. Jay Newman

For example, the upper panel of this illustration shows, edgewise, a plane wave of light inbound toward a reflective object — a mirror with a channeled surface.

The lower panel shows the same wave outbound after reflection. The wave now bears the impression of the surface with a channel in it. It is as though the plane wave had “picked up” the impression of the channel (although it should be noted that the distortion introduced into the plane wave is double the depth of the channel.)

The “3D impression” is carried through space as a distortion of light’s plane wavefronts. Think of these wavefronts as a sheets of transparent plastic that have been instantaneously vacuformed around an object –maybe a toaster – and are now racing through space in rapid succession. Bubblepacks.

In a camera, this incoming 3D information about the shape of the object is lost — squandered — by photographic film. Phase information is lost when the precisely distorted light wavefronts plaster themselves, over the time span of an exposure, against the flat surface of the film. This is why a snapshot is just 2D. Flat.

We are taught that phase information is also squandered against the retina, exactly as it is against film, though I no longer accept this notion. (See for example the discussion of The Corduroy Neuron under the subhead, “Where can we go with this.” I think the eye probably conserves spatial phase, and that this an important aspect of our excellent depth perception.)

It must be said immediately that spatial phase is not adequately described as a 3D impression. Spatial phase is a formal concept that we will come back to again and again, particularly in the discussion of photoreceptors in the following chapters. For a start, however, “3D impression” will do. For an accessible discussion of holography, I would refer you to this “How stuff works” essay by Tracy Wilson.

Dennis Gabor’s quest to save phase
Dennis Gabor did not set out to invent the hologram. His purpose was to try to conserve spatial phase and to improve electron microscopy by eliminating lenses. Here is a quote from Gabor:

“But an ordinary photograph loses the phase completely, it records only the intensities. No wonder we lose the phase, if there is nothing to compare it with! Let us see what happens if we add a standard to it, a ‘coherent background”.

To bench test his idea, Gabor used a high pressure mercury arc lamp, the most coherent source available to him at that time.

Gabor hoped to conserve the normally-lost spatial phase information by simultaneously recording, along with the distorted incoming wave fronts, a baseline reference – a yardstick wave. In practice he mixed two light waves at the face of the film: 1) the incoming, physically distorted wave bounced from the object to be filmed, plus 2) a pure, perfectly coherent, undistorted wave.

These two waves interfered, and the recording of their relationship consisted of a photograph of their interference pattern. This was the first hologram.

To project it, he developed the film and replaced it in front of the pure reference beam. A 3D image of the object of the hologram appeared, floating in space.

The principle of capturing spatial phase by remarking a wavefront’s position relative to a reference wave was clear in principle. However, it may be that Gabor did not have an understanding, in 1947, of what his hologram would turn out to mean physically. “Let us see what happens….” he said.

In 1950, a physicist at University College in Dundee, Scotland, Gordon L. Rogers, advanced the helpful idea that the finished hologram was actually focusing light by means of diffraction, rather than refraction. Using Rogers’ insight, the hologram could be understood as analogous to a lens created within the substance of the film. When coherent light was passed back through this “lens,” the original object reappeared in 3-space. In effect, the holographic lens created within the film was both a recording medium and a focusing device.

Notice that holography is all about spatial phase – its capture, conservation, and reconstruction. The technology depends on coherent light, and upon precision achieved at the level of a quarter wavelength of light. It is a perfectionists’ medium.

The Holographic Memory machine
A holographic memory is an electro-optical machine based on holography that has the power to recognize a face in a crowd. It was first conceived in 1963. This machine’s ability to recognize a very specific object can also be, in effect, de-tuned; so that instead of recognizing a particular racehorse, for example, it can recognize any racehorse. Further de-tuned, it can recognize any four-legged creature. In this sense the holographic memory machine has the rudiments of abstract thinking.

This is certainly tantalizing — but the idea that memory in the brain works like a holographic memory begins to falter when we try to extend the analogy to concrete parts and pieces. There is no laser or laserlike generator of coherent wave trains in our brain. Where is the essential reference beam? Where is the beam scattered from the object? Where and how might the two signals be mixed? Where is the extremely fine grained film?

It is possible to grapple with these many questions but ultimately, with whatever sort of analogous biological hardware one might propose — the question becomes this: How could spatial phase be reconstructed in the brain? Because at bottom, that’s what holography really does. It saves and then reconstructs spatial phase.

The non-holographic hologram.
But back up just one step, historically, from that first hologram of 1947. One of Dennis Gabor’s objectives in inventing holography – the conservation of spatial phase – could indeed turn out to be an important concept in understanding both vision and the brain. If you could truly conserve spatial phase — if you never lost it — you would not need to reconstruct it. You would not need a hologram, nor a reference beam, nor a beam splitter, nor a laser, nor any of the rest of the holographic apparatus.

Without any sort of contraption, freestyle, you could see holograph-like images in 3-space, which is to say – you could see images which are indistinguishable from the reality we see all around us every day.

The possibility that the eye conserves phase has never been seriously considered. This is because the retina isn’t supposed to be up to the task – and the nervous system as presently understood certainly isn’t. If the subject of spatial phase comes up at all in the context of the biology of vision it is quickly dismissed as impossible. Too quickly, let’s guess.

The textbook retina is a 2-dimensional sensor. It is optically flat, like photographic film. It necessarily squanders spatial phase information because it has no way to detect the 3-dimensional contour of an incoming plane wave.

The conservation of spatial phase should actually be pretty simple. It requires a “yardstick” photoreceptor, that is, a rod or cone cell that is able to distinguish and report out light intensity impressed at points along its length, at specific disks.

Receptors in depth would comprise a retina that can act as sensitive solid — able to detect the contours of an incoming plane wave as registered along the z-axes of each and every photoreceptor cell.

In the context of the multichannel neuron model, the notion of a photoreceptor that is sensitive along its z-axis is very important. It is the key to understanding what a visual memory is, and how it can be stored and remembered in the brain.

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