Light, Ice and Mirror Molecules

In the first week of October 2026 the Nobel committees in Stockholm rewarded three discoveries that began with curious, unfashionable questions. Karl Deisseroth, Peter Hegemann and Georg Nagel share the prize in physiology or medicine for light-sensitive proteins from a green alga that became optogenetics, a way of switching chosen brain cells on and off with light. Francis Halzen wins the physics prize alone for turning a cubic kilometre of Antarctic ice into the IceCube Neutrino Observatory and finding neutrinos from beyond the Solar System. Henri Kagan and Kenso Soai share the chemistry prize for reactions that amplify a slight preference for one mirror-image form of a molecule, an echo of the one-handedness of life’s chemistry. This is what each team found, how they found it, and what has followed.

Three prizes in three days

The Nobel Assembly at Karolinska Institutet announced the prize in physiology or medicine on Monday 5 October 2026. It went to Karl Deisseroth of the Howard Hughes Medical Institute and Stanford University, Peter Hegemann of Humboldt University of Berlin and Georg Nagel of the University of Würzburg, “for their discoveries concerning light-gated ion channels and optogenetics”^[1]. On Tuesday the Royal Swedish Academy of Sciences gave the physics prize to Francis Halzen of the University of Wisconsin–Madison^[2], and on Wednesday it gave the chemistry prize to Henri B. Kagan, professor emeritus at what was then Université Paris-Sud, and Kenso Soai, professor emeritus at Tokyo University of Science^[3]. Each prize is worth 12 million Swedish kronor, divided equally between the laureates in that field. The prizes will be presented in Stockholm on 10 December, the anniversary of Alfred Nobel’s death^[12].

Prize	Laureate	Born	Affiliation	Share	Announced
Physiology or Medicine	Karl Deisseroth	1971	Howard Hughes Medical Institute and Stanford University, USA	1/3	5 October
Physiology or Medicine	Peter Hegemann	1954	Humboldt University of Berlin, Germany (prize work at the MPI of Biochemistry, Martinsried)	1/3	5 October
Physiology or Medicine	Georg Nagel	1953	University of Würzburg, Germany (prize work at the MPI of Biophysics, Frankfurt)	1/3	5 October
Physics	Francis Halzen	1944, Tienen, Belgium	University of Wisconsin–Madison, USA	1/1	6 October
Chemistry	Henri B. Kagan	1930, Boulogne-Billancourt, France	Professor Emeritus, then Université Paris-Sud, France	1/2	7 October
Chemistry	Kenso Soai	1950, Hiroshima, Japan	Professor Emeritus, Tokyo University of Science, Japan	1/2	7 October

A protein from pond algae, a detector buried at the South Pole and a reaction in a flask have little in common on the surface. But each discovery began as a basic question that looked like a long shot, took decades to pay off, and handed other scientists a tool they now use routinely.

A switch borrowed from algae

Neuroscientists have long been able to record what the brain does far more easily than they could change it. Electrodes pick up activity, and drugs or lesions can dampen whole regions, but neither can single out one type of cell among the many that are intermingled in a small patch of tissue, still less turn it on and off at the speed at which neurons signal. In a 1999 essay Francis Crick suggested that one of the tools neuroscience most needed was a way to control one cell type while leaving the others untouched, and that light, delivered with molecular tricks, might be the means^{[16, 17]}.

The answer came from an unexpected direction. In the early 1990s Hegemann, then at the Max Planck Institute of Biochemistry in Martinsried, wanted to understand how the single-celled green alga Chlamydomonas reinhardtii steers towards light. The alga senses light with an eyespot, a small orange patch that contains retinal, the same light-absorbing molecule used in animal eyes. Using fine electrodes, Hegemann and his colleagues measured electrical currents that flowed about half a millisecond after the eyespot was lit, more than 20 times faster than the chain of chemical steps that turns light into a nerve signal in the human eye^{[13, 18, 19]}.

That speed suggested something unusual. Hegemann proposed that a single protein might both capture light and open a pore through the cell membrane, acting as its own ion channel. At the time no known ion channel responded directly to light, and the idea met with scepticism. The breakthrough came around the turn of the millennium, when a Japanese sequencing effort made thousands of Chlamydomonas genes public. Hegemann’s group spotted two that resembled known genes for rhodopsins, the family of light-sensing proteins to which the visual pigment of the eye belongs^{[13, 17]}.

To find out what they encoded, Hegemann turned to Georg Nagel at the Max Planck Institute of Biophysics in Frankfurt. Nagel was expert in a technique that uses the large egg cells of the African clawed frog as living test tubes: inject the genetic instructions for a protein, and the egg builds the protein and places it in its membrane, where its electrical behaviour can be measured. With colleagues including Ernst Bamberg, Nagel and Hegemann showed in 2002 that the first gene encoded a light-gated channel for protons, which they named channelrhodopsin-1^[20]. A year later they reported that the second, channelrhodopsin-2, was a directly light-gated channel for positively charged ions more generally, opening within about 0.2 milliseconds of a flash. When they put the gene into human and hamster kidney cells, those cells too became light-sensitive. The 2003 paper proposed it as a tool for depolarising cells with light^{[4, 13]}.

From algae to neurons

Deisseroth came to the problem from medicine. During clinical training at Stanford University, according to the Nobel committee’s account, he was struck by how little psychiatric treatments could do for patients^[13]. When he started his own laboratory he looked for a protein that could make neurons fire on command. Having read about channelrhodopsin-2, he wrote to Nagel to ask for the DNA. In a paper published in 2005 with Edward Boyden, Feng Zhang, Bamberg and Nagel, his group showed that cultured rat neurons carrying the gene produced action potentials, the electrical spikes neurons use to communicate, in response to brief pulses of blue light, with millisecond precision^[5].

Deisseroth’s group then turned the method into a general-purpose toolkit. Viruses and genetic promoters could deliver the gene only to a chosen cell type. In 2006 the name optogenetics was coined for the approach^[21]. In 2007 the group fed a thin optical fibre through a small hole in a mouse’s skull and used light to stimulate cells in the motor cortex, controlling movements of the whiskers^[22]. The same year a light-driven chloride pump, halorhodopsin, was added to the kit, so that yellow light could silence neurons as well as blue light could activate them^[23].

The Nobel citation is limited by statute to three people, and optogenetics had more than three parents. When the Brain Prize was awarded for the field in 2013, its recipients also included Boyden, Gero Miesenböck of the University of Oxford, who had earlier used other genetically targeted, light-triggered systems to control neurons, including in fruit flies, and Bamberg^{[17, 24]}. Others welcomed the award; Michael Häusser of University College London told Nature: “We’ve been waiting for this for a long time… What took them so long?”^[24]

What light has revealed

In 2007 a team including Deisseroth and Luis de Lecea introduced channelrhodopsin-2 into neurons in the lateral hypothalamus that produce the signalling molecule orexin, also known as hypocretin. Stimulating these cells with light made sleeping mice more likely to wake up, showing that their activity could cause awakening^[25]. In 2011 Kay Tye, Deisseroth and colleagues showed that activating one specific pathway inside the amygdala reduced anxiety-like behaviour in mice, while inhibiting it increased it^[26].

In 2012 a team in the laboratory of Susumu Tonegawa, a Nobel laureate for work in immunology, used optogenetics to tackle one of the oldest questions in brain science: where a memory is physically stored. They labelled the neurons in the hippocampus that were active while mice learned to fear a particular box, then reactivated just those neurons with light in a different setting. The mice froze as if afraid^[27]. The following year the same group used the method to implant a false fear memory^[28]. The engram, the physical trace of a memory, had long eluded researchers; optogenetics offered the first direct test of which cells are sufficient to recall it^{[13, 29]}.

Since then the technique has been used to find circuits that govern pain, thirst, appetite, reward, attention and parental care, as well as interactions between the brain and the heart and gut^[13]. Per Svenningsson, chair of the Nobel Committee for Physiology or Medicine, said in the press release: “Optogenetics provides opportunities for mapping the brain in a way that we could once only dream of”^[1].

Medical uses have been slower and remain experimental. The most advanced concerns retinitis pigmentosa, an inherited disease that destroys the eye’s light-sensing rods and cones while leaving other retinal cells intact. Work in mice in 2006 showed that making those surviving cells light-sensitive with channelrhodopsin could restore some visual responses^[30]. In 2021 a team led by José-Alain Sahel and Botond Roska reported that a patient with advanced retinitis pigmentosa, treated with a light-sensitive channel protein delivered to the retina and wearing light-stimulating goggles, could perceive, locate and count objects in a laboratory setting^[6].

The shyest particle

The physics prize rewards a different kind of light detector. Neutrinos are electrically neutral particles with a tiny mass that interact with matter only through the weak force. Wolfgang Pauli proposed them in 1930 to balance the energy books in radioactive decay, and Clyde Cowan and Frederick Reines first detected them in 1956, using a nuclear reactor as a source^{[31, 32]}. They are so reluctant to interact that, according to the Nobel committee, about 65 billion neutrinos from the Sun pass through a human fingernail every second without leaving a trace^[33].

That elusiveness is what makes them attractive to astronomers. Light from distant objects is absorbed by dust or gas, and charged cosmic rays, the high-energy protons and nuclei that rain onto Earth, are bent by magnetic fields so that they no longer point back to where they came from. Neutrinos are not deflected and pass through almost anything. Cosmic rays reach energies around 10²⁰ electronvolts, far beyond the roughly 7 TeV (7 × 10¹² electronvolts) per beam of the Large Hadron Collider, and where such particles are accelerated they should collide with surrounding matter and radiation to produce pions, which decay into neutrinos and gamma rays^[31]. High-energy neutrinos would therefore mark the places where nature’s most powerful particle accelerators operate.

Earlier neutrino physics had already earned prizes. Raymond Davis Jr. and Masatoshi Koshiba shared half of the 2002 physics prize for detecting neutrinos from the Sun and from supernova SN 1987A in the Large Magellanic Cloud^[34]. Takaaki Kajita and Arthur B. McDonald shared the 2015 prize for showing that neutrinos change type in flight, the phenomenon called neutrino oscillation, which requires them to have mass^[35]. But those neutrinos had energies of millions of electronvolts. Catching the far rarer neutrinos at millions of times higher energy needed a detector of an entirely different scale.

A telescope made of ice

The basic principle had been proposed around 1960 by Moisei Markov, Reines and Kenneth Greisen: when a neutrino occasionally strikes an atomic nucleus in water, it produces charged particles that travel faster than light does in that medium and emit a faint blue glow, Cherenkov radiation, which sensitive light detectors can record^[31]. Because the interactions are so rare, the detector must be enormous, and it must be shielded from the background of particles produced by cosmic rays in the atmosphere. An early attempt, DUMAND, aimed to string detectors 4,800 metres down in the Pacific off Hawaii. A first string was deployed in 1993 but failed, and the project was cancelled in 1995^[31].

Halzen, born in 1944 in Tienen, Belgium, and educated at KU Leuven, was a particle theorist at Wisconsin when he took up the problem^{[14, 33]}. According to the committee’s scientific background, after a talk at the University of Kansas in the autumn of 1987 a glaciologist in the audience told him about Russian plans to detect radio signals from neutrino interactions in Antarctic ice. Halzen and colleagues concluded that such a detector would miss the neutrinos they cared about, and his thoughts turned instead to Cherenkov light in the ice itself. In June 1988 he and John Learned, then spokesperson for DUMAND, set out the idea of ice serving as both target and detector medium^{[31, 33]}. Halzen later wrote that others had probably considered the idea and given it up, and that “had I not been completely ignorant of what was then known about the optical properties of natural ice, I would probably have done the same”^[31]. Asked after the announcement about the ice, he described it as “ancient snow that fell on Antarctica 50,000 years ago”^[36].

The first tests were done in boreholes in the Greenland ice sheet, where photomultiplier tubes recorded muons produced by cosmic rays^[37]. Then came AMANDA, built at the Amundsen–Scott South Pole Station from 1993 to 2000. The early results were discouraging: at 800 to 1,000 metres, tiny air bubbles scattered the light within less than 50 centimetres, blurring the tracks. But below about 1,400 metres the bubbles disappeared and the ice proved clearer than anyone had expected, letting light travel hundreds of metres^{[31, 33]}. In 2001 the AMANDA team reported neutrinos that had passed through the Earth and interacted in the ice, showing that the approach worked^[38].

The full-scale IceCube was proposed to the National Science Foundation in 1999 and approved in 2002, with Halzen as principal investigator^[31]. Between 2005 and 2010, crews used hot water to melt 86 holes about 2.5 kilometres deep and lowered a cable into each before it froze. Each cable carries 60 digital optical modules, glass spheres holding a 25-centimetre photomultiplier and its electronics, spaced 17 metres apart between 1,450 and 2,450 metres below the surface. The 5,160 sensors instrument roughly a billion tonnes of ice and have been recording more than 99% of the time since completion in 2011^{[7, 31]}. The collaboration that runs it now includes about 450 people from 58 institutions in 14 countries^[31]. Halzen has thanked the National Science Foundation for taking a chance on the project, “which in retrospect amazes me”^[39].

First light from deep space

The payoff arrived in 2013. IceCube reported two neutrinos with energies above a petaelectronvolt (10¹⁵ electronvolts), hard to explain as neutrinos made in the atmosphere^[40]. A search for events starting inside the detector then found 28 events between about 30 TeV and 1.1 PeV, published in Science later that year as the first evidence for high-energy neutrinos from outside the Solar System^[41]. With three years of data the signal reached 5.7 standard deviations, the level physicists treat as a discovery^[42].

Finding where the neutrinos came from took longer. On 22 September 2017 IceCube detected a neutrino of about 290 TeV and sent an automated alert to telescopes within a minute. Its direction was consistent with TXS 0506+056, a blazar (a galaxy whose central black hole fires a jet towards Earth) that gamma-ray telescopes then found to be flaring^[8]. Searching its archive, IceCube found an excess of neutrinos from the same direction in 2014 and 2015^[43]. In 2022 the collaboration reported evidence, at 4.2 standard deviations, for 79 neutrinos from Messier 77, also known as NGC 1068, an active galaxy about 46 million light years away whose core is hidden from ordinary telescopes by dust^[9]. In 2023 it reported neutrinos from the plane of the Milky Way at 4.5 standard deviations^[44], and an analysis of 12 years of data posted in 2026 put that signal at 5.7, the first neutrino source to pass the five-sigma threshold^{[10, 31]}.

The detector has also tested particle physics. In 2024 IceCube reported seven candidate tau neutrinos with energies between 20 TeV and 1 PeV, too energetic to have been produced by oscillation in the atmosphere^[45]. Its highest-energy published neutrino so far, about 11.4 PeV, was recorded on 31 March 2019 and described in March 2026^[31]. Measurements of the overall energy spectrum from 5 TeV to 10 PeV now show that it is not described by a single power law, a feature that models of the sources will need to explain^[46]. Other telescopes are following, among them KM3NeT in the Mediterranean, which in 2025 reported a single event, recorded on 13 February 2023, attributed to a neutrino with a median estimated energy of about 220 PeV^{[31, 47]}.

Mark Pearce, chair of the Nobel Committee for Physics, said that Halzen’s “tenacity and scientific vision has paved the way for a new kind of astronomy”^[2]. An upgrade adding more densely spaced sensors was installed in 2025–26, and the collaboration has proposed IceCube-Gen2, with about eight times the instrumented volume^{[31, 48]}. Halzen, who is 82, said the award was “a great relief for me to finally deliver the recognition that this great collaboration deserves”^[48].

Life’s one-handed chemistry

The chemistry prize concerns a property best illustrated by a pair of hands. Many molecules are chiral: they come in two forms, called enantiomers, that are mirror images of each other but cannot be superimposed, like a left and a right glove. The two forms have identical physical properties in most respects, but they rotate polarised light in opposite directions, and they can behave very differently inside living things, whose receptors and enzymes are themselves chiral^[15].

In the mid-19th century Louis Pasteur found that crystals from tartaric acid came in two mirror-image forms, which he separated with tweezers; dissolved separately, they rotated polarised light in opposite directions. In 1857 he found that bacteria fermented only one of the two forms^[15]. Life turned out to be consistently one-handed. The amino acids in proteins are almost all of one form, and the sugars in DNA and RNA are likewise of a single form, a property called homochirality. Yet ordinary laboratory reactions that create a chiral centre from non-chiral starting materials give a 50:50 racemic mixture^{[15, 49]}. How life came to use only one hand has been debated for more than a century, and Heiner Linke, chair of the Nobel Committee for Chemistry, described the laureates’ work as addressing “a chemical mystery that is over a century old: how homochirality can emerge spontaneously”^[3].

In the early 1900s the German chemist Willy Marckwald carried out what the committee describes as the first successful asymmetric reaction, producing slightly more of one form than the other^[15]. In 1953 the physicist Charles Frank of the University of Bristol published a short theoretical paper describing how one form could come to dominate. He imagined a molecule that catalyses its own formation, a process known as autocatalysis, and also suppresses production of its mirror image. Any small chance excess of one form would then be amplified until it took over. “A laboratory demonstration may not be impossible,” he wrote^{[49, 50]}. It took more than four decades for chemists to provide one.

Kagan’s non-linear effect

Meanwhile chemists learned to make chiral catalysts that favour one enantiomer, which matters because the two forms of a drug, flavour or pesticide can act differently in the body. William Knowles, Ryoji Noyori and Barry Sharpless shared the 2001 chemistry prize for such catalysts, and Benjamin List and David MacMillan the 2021 prize for small organic ones^{[51, 52]}. The usual assumption was simple: the purity of the product, measured as enantiomeric excess, should be proportional to the purity of the catalyst.

Henri Kagan, born in 1930 in Boulogne-Billancourt, worked at Université Paris-Sud^{[3, 15]}. In 1986 he and his colleagues tested that assumption by deliberately using catalysts made from mixtures of the two forms of a chiral ingredient. In three different reactions the product’s enantiomeric excess did not track the catalyst’s. In one of them, the Sharpless epoxidation of geraniol, the product was purer than the catalyst, which Kagan called a positive non-linear effect; in the other two it was less pure, a negative effect^{[49, 53]}. The explanation is that catalyst molecules can pair up. Pairs made of two identical forms and pairs made of one of each behave differently, and if the mixed pairs are less active, or tied up out of the way, the excess form does a disproportionate share of the work^{[49, 54]}.

Others soon found dramatic examples. Noyori’s group reported in 1989 that a zinc-based reaction run with a chiral amino alcohol of only 15% enantiomeric excess gave a product of 98%^[55]. Non-linear effects became a standard diagnostic: whether a reaction shows one, and of what kind, gives chemists clues about how many catalyst molecules take part in the key step^{[49, 54]}. The scientific background describes an industrial example from Merck: in a step towards a drug candidate, a reagent made from α-pinene of only 70% optical purity gave a product with 95% enantiomeric excess^[49]. When Chemistry World reported the prize it also noted that there had been anger in some quarters in France that Kagan was not included in the 2001 award^[56].

Soai’s self-copying molecule

Kagan’s effect satisfied one of Frank’s conditions, an amplification of the favoured form. The other was a molecule that makes copies of itself. Kenso Soai, born in 1950 in Hiroshima and based at Tokyo University of Science, worked on the addition of organozinc reagents to aldehydes, reactions with strong non-linear effects^{[3, 15]}. He noticed that the product of such a reaction resembled the amino alcohols used to catalyse it, and wondered whether the product could catalyse its own formation. In 1990 he showed that it could, although the product came out less pure than the starting catalyst^{[49, 57]}.

Changing the molecule solved that. In 1995 Soai’s group reported in Nature that a pyrimidine-based alcohol formed from an aldehyde and diisopropylzinc catalysed its own formation while increasing its enantiomeric excess. Starting from an excess of only a few per cent, successive rounds in which each batch of product seeded the next raised it to about 90%^{[15, 49, 58]}. It was the first asymmetric autocatalysis with amplification, and it fulfilled Frank’s scheme. In 2003 the group showed that a starting excess as small as 0.00005% could be amplified to more than 99.5% in three rounds, a factor of around 630,000^{[11, 49]}.

The most striking experiments used no chiral starting material at all. Running the reaction many times without any deliberately added chiral substance, Soai’s team found that each run nonetheless ended strongly enriched in one form or the other. In one report of 37 runs, 18 gave one enantiomer and 19 the other, a distribution consistent with chance fluctuations at the start being amplified^[59]. Daniel Singleton and L. K. Vo independently reported similar behaviour in 2002^[60]. The Nobel committee’s popular account calls this the first time chirality had been created from a non-chiral mixture of molecules since the dawn of life^[15]. Peter Somfai, a member of the committee, said at the announcement that the Soai reaction “is probably the coolest experiment in organic chemistry”^[56].

Its precise mechanism has been debated for many years. A 2020 study by Scott Denmark’s group combined crystal structures, kinetics and computation to identify the zinc-containing assemblies responsible^[61], and other groups have proposed alternatives^[49]. The committee is careful about what the reaction does not show: it uses organozinc reagents in an organic solvent and, in the words of the scientific background, is not relevant to how biological homochirality emerged in water-based systems^{[15, 49]}. What it does show is that the route Frank sketched is chemically possible, and it has encouraged searches for comparable behaviour in amino acids and sugars^[15].

Curiosity first

Each of this year’s science prizes went to work whose practical value was invisible at the start. Hegemann wanted to know how an alga swims towards light; Halzen wanted to see whether ice could do what seawater was meant to; Kagan and Soai were testing assumptions about how catalysts behave. Each relied on many people not named in the citations, from the drilling crews at the South Pole to the chemists who reproduced the results.

The outcomes have been tools. Optogenetic switches are now a standard method in neuroscience laboratories, with the first clinical trials under way. IceCube issues automated alerts that let telescopes around the world look for counterparts to unusual neutrinos, a practice known as multi-messenger astronomy. Non-linear effects are a routine way to probe how chiral catalysts work. The laureates will receive their medals in Stockholm in December.

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