Arthur Ashkin

Arthur Ashkin: Holding Life Still With Light

At ten years old, in Depression-era Brooklyn, Arthur Ashkin was transfixed by a Crookes radiometer — the little glass bulb with black-and-white vanes that spins when you shine a light on it. He worked out that heat, not light itself, was doing the pushing, and noticed the vanes reversed direction if you warmed the bulb from outside. Sixty years later the Nobel Committee would award him half the Physics Prize for the mature version of that childhood question: can a beam of light actually grab hold of matter? It could. He was ninety-six when they told him, and he said he could not get all that excited about it.

Brooklyn, Radar, and a Brother Who Got There First

Ashkin was born on 2 September 1922 in Brooklyn, New York, the son of Isadore Ashkin, who had emigrated from Odesa at eighteen and built a dental laboratory in Manhattan, and Anna Fishman. There were three surviving children: an older brother Julius, Arthur, and a younger sister Ruth. Julius set an intimidating pace, taking a Columbia physics PhD at twenty-two and eventually heading the physics department at Carnegie Tech.

Arthur followed him to Columbia in 1940, out of James Madison High School. The war interrupted him; he spent it as a technician at the Radiation Laboratory building magnetrons for military radar, under a supervisor named Sid Millman who would prove decisive twice. Ashkin finished his bachelor's in physics in 1947 with a Phi Beta Kappa key, then went to Cornell on the G.I. Bill, taking his PhD in nuclear physics in 1952 with a thesis on electron-positron scattering. At Cornell he met Aline, whom he married in 1954 and stayed married to for more than sixty years.

Forty Years at Bell Labs

Millman, by then at Bell Laboratories, recommended his old technician. Ashkin arrived in 1952 and stayed forty years. The first decade went into microwaves and electron tubes — respectable, unremarkable work. Then the laser arrived, and around 1960 Ashkin switched fields entirely, into nonlinear optics, optical fibers, parametric oscillators and amplifiers. During the 1960s he was among the co-discoverers of the photorefractive effect in piezoelectric crystals. He would eventually hold forty-seven patents.

But he had never let go of the radiometer question. Light carries momentum. That is not controversial — Maxwell had said so, and radiation pressure is why comet tails point away from the sun. The problem is that the force is absurdly small. Nobody expected it to be useful for pushing anything you could see.

What Optical Tweezers Actually Do

Ashkin's insight was that a laser does not just push. Focus a beam tightly and it creates a steep gradient in intensity — very bright at the focal point, dimmer a fraction of a millimetre away. A transparent particle sitting in that gradient acts like a tiny lens, bending the light passing through it. By Newton's third law, bending the light's path pushes back on the particle, and the net result is a force that drags the particle toward the brightest spot and holds it there.

So there are two forces at work: the scattering force, which shoves the particle along the beam, and the gradient force, which pulls it sideways into the beam's core. Balance them and you have a trap. Get the focusing steep enough and the gradient force alone can hold a particle against the push, in all three dimensions, with a single beam. That is an optical tweezer: a knot of light with something caught in it.

Ashkin published the first optical trapping paper in 1970. Bell Labs' Theoretical Physics Department initially rejected it; his boss Rudi Kompfner overruled them and told him to just send it in. It became the most-cited paper of its year and has since passed five thousand citations. In 1971 he levitated small glass spheres against gravity. The single-beam gradient trap — the true tweezer — followed in the 1980s, with the technique conventionally dated to 1986.

From Glass Beads to Living Cells

The step that made Ashkin famous outside physics was biological. He found he could trap not just latex spheres but bacteria, viruses, and eventually whole living cells, and — critically — that by switching to infrared light he could do it without cooking them. Suddenly biologists had a pair of hands small enough to hold a single organelle. Optical tweezers went on to be used to measure the forces generated by molecular motors like kinesin walking along a microtubule, to stretch single DNA molecules, and to manipulate individual cells without touching them.

There was a parallel legacy in physics. Ashkin's trapping work formed the foundation for Steven Chu's laser cooling and trapping of atoms, which won Chu a share of the 1997 Nobel Prize, and from there it fed directly into Bose–Einstein condensation and the atom laser.

The Prize, Twenty Years Late

On 2 October 2018, aged ninety-six, Ashkin was awarded half the Nobel Prize in Physics for his invention of optical tweezers that grab particles, atoms, viruses and other living cells with their laser beam fingers. Gérard Mourou and Donna Strickland shared the other half for chirped-pulse amplification. He was the oldest person ever to receive a Nobel at the time of award, a record that lasted just one year until John Goodenough took the Chemistry Prize at ninety-seven.

The other honours had accumulated steadily: election to the National Academy of Engineering in 1984 and the National Academy of Sciences in 1996, the Charles Hard Townes Medal in 1988, the Frederic Ives Medal in 1998, the Joseph F. Keithley Award in 2003, the Harvey Prize in 2004, and induction into the National Inventors Hall of Fame in 2013. He retired from Bell Labs in 1992 and kept working, in a laboratory in the basement of his house in Rumson, New Jersey.

Why Arthur Is Called a Genius

The quality on display in Ashkin's career is not raw analytical horsepower — he was not a theorist, and the physics of radiation pressure was well established before he touched it. It is something rarer and harder to teach: an unusually literal imagination, coupled with the patience to chase one question for fifty years without institutional encouragement.

Everyone knew light carried momentum. Ashkin was the one who asked what would happen if you actually tried to pick something up with it, and then kept building apparatus until it worked. The 1970 paper was rejected by his own institution's theory group. He was told, in effect, that the effect was too weak to matter. He was right and they were wrong, and the thing that made him right was a physical intuition about how a transparent bead refracts a beam — a mental picture, not an equation.

The honest counter-case: he did not discover new physics. Optical tweezers are a brilliant application of understood principles, and the Nobel came, in the end, for an instrument. Bell Labs in that era was an extraordinary machine for turning good people into productive ones, and Ashkin had forty years of it, with a boss willing to override the sceptics. Much of the biology that made the technique indispensable was done by others. What is genuinely his is the stubbornness of the original idea, and the fact that a technique now used in laboratories worldwide came out of one man refusing to drop a question he first asked as a boy, staring at a spinning toy.

Legacy

Ashkin died on 21 September 2020, aged ninety-eight. His three children — Michael, an art professor at Cornell, Judith, a Tai Chi instructor, and Daniel, a ceramic scientist — none followed him into physics. His instrument did better. Optical tweezers are now standard equipment in cell biology, biophysics and atomic physics, and the ability to hold a single living bacterium motionless in a beam of light, which sounded like a conjuring trick in 1970, is now something a graduate student sets up before lunch.

Achievements

• National Inventors Hall of Fame — 2013
• Affiliated with Bell Labs and Lucent
• Educated at Cornell University, Columbia University and James Madison High School
• Worked as physicist and researcher