Advancing science – Illumination at the limits of knowledge | Technology Quarterly

ALL THE technologies discussed in this report are moving forward apace. The companies which provide machinery to solar-cell manufacturers are ceaselessly trying to make more efficient use of silicon and less costly modules. In universities and elsewhere researchers are looking at ways to add a second layer to such cells so as to capture energy at wavelengths silicon ignores—though their best attempts so far do not last very long outdoors.

Advances in manufacturing and design are making LEDs ever better sources of illumination. In more and more screens they backlight the liquid-crystal shutters which brighten pixels by detenebration. Some screens already do without shuttering, using liquid-crystal-free arrays of micro-LEDs to produce images that offer better contrast and use less energy. In information technology the division of labour that sees data processing done by electrons and data transmission by photons is under attack; switches that could be programmed to do some information processing while keeping that information in the form of photons would allow data to flow around data centres more quickly and efficiently. Laser beams of slightly different wavelengths are being packed ever more densely into optical fibres, with more bits encoded into every symbol stamped on to their light. The current record for data transfer down a single fibre, held by researchers at UCL, a British university, is 178 terabits a second.

But if you want to see lasers which push the boundaries of the possible in the most dramatic of ways, you have to turn to those made, not for practical applications, but to further science. Wherever researchers require ludicrous amounts of power or precision, there’s every chance that they are using a laser, some sort of digital photon detector, or both. To see the cutting edge of what light can do, head for a lab.

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California is a case in point: walking its halls evokes a sense of the technological sublime which is all but visceral. The 192 laser beams from the 100-metre-long, xenon-pumped beamlines that fill its two warehouse-sized clean rooms converge on a peculiarly perforated spherical chamber. When NIF is operational a tiny bubble at the centre of that chamber is illuminated with 500 terawatts, which is to say 500,000GW.

Given that the world’s total electricity generating capacity is less than 5TW, how is a 500TW system possible? The answer is brevity. Because power is energy divided by time, a relatively small amount of energy can provide a huge amount of power if it is delivered quickly enough. The NIF fires for only a few tens of nanoseconds (billionths of a second) at a time. Each blink-and-you-miss-it 500TW blast thus delivers only a kilowatt-hour or so of energy.

Using such a gargantuan device to provide such a modest amount of energy seems bizarre. But NIF’s job requires the energy to be delivered with great spatial precision and almost instantaneously. Only then can it heat the laser’s tiny targets to temperatures and pressures otherwise reached only in the centres of stars and the blasts of nuclear weapons—conditions which can fuse atomic nuclei. Congress paid billions for the NIF on the basis that it might open the way to making nuclear fusion of this sort a practical energy source. It has not delivered on those dreams. But it has provided new insights into astrophysics as well as experimental data relevant to the design and maintenance of hydrogen bombs, which is Lawrence Livermore’s main concern.

Physicists are not the only scientists entranced by lasers. One of the workhorses of genetic engineering is green fluorescent protein (GFP). The instructions for making GFP are easily added to genes for other proteins. When poked with finely focused lasers these modified proteins fluoresce, thus revealing their whereabouts—a handy way of learning which proteins cells put where.

A remarkable refinement of this technique, first demonstrated in 2011, is to turn the cell itself into a laser. Engineer a cell to produce GFP, put it between two mirrors and pump energy in and the protein’s light will be amplified in just the same way as it would in a piece of ruby or neodymium-doped glass. “Light-emission microscopy” based on this possibility amplifies the light given off by fluorescent proteins and other light-emitting markers.

Photons can also be used to change how cells behave. By engineering proteins to be sensitive to light and then turning that light on and off, researchers can change what cells do—including the ways they do, or don’t, transmit nerve impulses. Laser light flashed on to the nerves of a suitably engineered flatworm, or shone down optical fibres into the brain of a mouse, allows researchers to turn different parts of the nervous system on and off and observe the changes in behaviour that follow. This “optogenetic” puppeteering provides all sorts of new insights into the machinery of the brain. With all due respect to those using photons to explore the strange interconnectedness of things in quantum mechanics—which Einstein famously described as “spooky”—photons that can literally change a mind in mid-thought may be the spookiest of all.

The degree to which light-based techniques are changing sciences across the board can be seen in the past decade’s decisions by the Nobel Physics Committee. In 2014 the committee recognised a physical breakthrough in the production of light—the development of blue LEDs, a technical tour de force which made the production of white light cheaper and easier than ever before. Since then the physics prize has been awarded to three different ways of using lasers either for experiments in the lab or observations of the world. A tour of these prize-winning accomplishments allows a last celebration of this golden age of light.

Start with pure power. A technique called chirped-pulse amplification, developed by Donna Strickland and Gérard Mourou when they were both at the University of Rochester, allows lasers far more powerful than the NIF—lasers which work in the petawatt range. It provides a way around the unfortunate fact that, above a certain power level, even a very short pulse will melt any laser trying to amplify it further. Chirp amplification solves the problem by stretching pulses out in both space and time. An intense packet of photons that is, say, a millimetre long, and thus passes through any machine in just three trillionths of a second, can be chirped into one that is a metre long and lasts a full three billionths of a second. This stretched pulse is low-power enough to be amplified, after which it can be compressed back into its original form as a burst just as short as ever but now containing many more photons.

Labs around the world now use this technique to produce bursts of light both far shorter and far more powerful than those at NIF using much cheaper equipment. This allows them to study nuclear processes that are even more extreme than fusion. If the pulses can be made 1,000 times shorter still—which Dr Mourou, at least, thinks is possible, given a decade or so—they could achieve something no other technology has yet managed: the creation of matter (and antimatter) from scratch.

Einstein’s work dispensed with the need for an all-pervading luminiferous aether. But the fields evoked by quantum electrodynamics (QED), the mid-20th-century culmination of work on electromagnetism, quantum theory and relativity, populate empty space with something else instead: very faint possibilities. And QED says that, if light gets sufficiently intense, its photons will interact with these possibilities to bring forth brand new electrons from empty space. Einstein’s insight that mass can be converted into energy has been proven many times, most terribly in nuclear weapons. Creating material particles from massless light alone would be a remarkable turning of the tables, and one that ought to provide new insight into the quantum fields involved.

After power, pressure. The momentum of photons is tiny; but when applied to tiny things it can do useful work. In the 1960s Arthur Ashkin of Bell Labs realised that, if a small transparent object is placed on the edge of a laser beam it will move to the beam’s centre (provided that the beam is brighter at the centre than the edge). This is because the photons that pass through the object have their path bent outward, away from the beam: conservation of momentum requires the object thus diverting them to move in the opposite direction. If, once caught up in the beam, the object strays from its bright centre, the light pressure will bring it back.

In the 1970s Dr Ashkin put this idea into practice, using laser beams as “optical tweezers” with which to manipulate microscopic beads. In the 1980s he got the technique to work on individual bacteria and virus particles, while his student Steven Chu used a variant to trap individual atoms—work that won Dr Chu and colleagues a Nobel prize in 1997. The increasing use made of his tweezers in biology saw Dr Ashkin follow in his student’s footsteps in 2018, sharing the prize with Dr Strickland and Dr Mourou.

And then there is precision. Einstein’s general theory of relativity, promulgated in 1915, explains gravity in terms of the distortions masses impose on spacetime—spacetime being, to Einstein, simply the thing that clocks and rulers measure. His special theory of relativity had laid out the case for light being the ultimate ruler, a view that measurement professionals now share; the General Conference on Weights and Measures defines the metre not as the length of a specific rod in a vault in Paris, as it once did, but as the distance a photon in a vacuum travels in 1/299,792,458 of a second. Thus if you want to see ripples in spacetime—such as those which relativity says must be produced when two very large masses pirouette around each other—light is the best sort of ruler to use.

The Laser Interferometer Gravitational-wave Observatory (LIGO) consists of two such rulers. Its twin detectors, one in Louisiana and one in Washington state, both feature 4km-long perpendicular arms along which laser beams of truly phenomenal stability bounce back and forth (see chart). Instruments mounted at the point where the beams cross compare their phases in order to detect transitory differences in the arms’ lengths. Their precision is equivalent to that which would be needed to detect a hair’s-breadth change in the distance to a nearby star.

On September 14th 2015 LIGO picked up the shiver in spacetime produced by the merger of two black holes 1.3bn light-years away. In 2017 the Nobel Physics Committee, free of naysaying ophthalmologists, awarded the prize to Rainer Weiss, Kip Thorne and Barry C. Barish, the three scientists who had done most to make that observation happen.

Their extraordinary measurement was treated, quite rightly, as a slightly late 100th-birthday present for Einstein’s truly remarkable intellectual achievement. It was also an extraordinary demonstration of what can be done with photons. A century of work by scientists and engineers has taken the energy packets that Einstein first imagined in 1905 and produced a range of technologies with capabilities little short of the miraculous—a collective achievement far greater than any single act of genius. Relativity is remarkable. Putting photons to use has been revolutionary.

How understanding light has led to a hundred years of bright ideas the liberation of light

This article appeared in the Technology Quarterly section of the print edition under the headline “New enlightenments”

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