The Chinese particle physicist Chen-Ning Yang died on 18 October at the age of 103. Yang shared half of the 1957 Nobel Prize for Physics with Tsung-Dao Lee for their theoretical work that overturned the notion that parity is conserved in the weak force – one of the four fundamental forces of nature.
Born on 22 September 1922 in Hefei, China, Yang competed a BSc at the National Southwest Associated University in Kunming in 1942. After finishing an MSc in statistical physics at Tsinghua University two years later, in 1945 he moved to the University of Chicago in the US as part of a government-sponsored programme. He received his PhD in physics in 1948 working under the guidance of Edward Teller.
In 1949 Yang moved to the Institute for Advanced Study in Princeton, where he made pioneering contributions to quantum field theory, wotrking together with Robert Mills. In 1953 they proposed the Yang-Mills theory, which became a cornerstone of the Standard Model of particle physics.
The ‘Wu experiment’
It was also at Princeton where Yang began a fruitful collaboration with Lee, who died last year aged 97. Their work on parity – a property of elementary particles that expresses their behaviour upon reflection in a mirror – led to the duo winning the Nobel prize.
In the early 1950s, physicists had been puzzled by the decays of two subatomic particles, known as tau and theta, which are identical except that the tau decays into three pions with a net parity of -1, while a theta particle decays into two pions with a net parity of +1.
There were two possible explanations: either the tau and theta are different particles or that parity in the weak interaction is not conserved with Yang and Lee proposing various ways to test their ideas (Phys. Rev.104 254).
This “parity violation” was later proved experimentally by, among others, Chien-Shiung Wu at Columbia University. She carried out an experiment based on the radioactive decay of unstable cobalt-60 nuclei into nickel-60 – what became known as the “Wu experiment”. For their work, Yang, who was 35 at the time, shared the 1957 Nobel Prize for Physics with Lee.
Influential physicist
In 1965 Yang moved to Stony Brook University, becoming the first director of the newly founded Institute for Theoretical Physics, which is now known as the C N Yang Institute for Theoretical Physics. During this time he also contributed to advancing science and education in China, setting up the Committee on Educational Exchange with China – a programme that has sponsored some 100 Chinese scholars to study in the US.
In 1997, Yang returned to Beijing where he became an honorary director of the Centre for Advanced Study at Tsinghua University. He then retired from Stony Brook in 1999, becoming a professor at Tsinghua University. During his time in the US, Yang obtained US citizenship, but renounced it in 2015.
More recently, Yang was involved in debates over whether China should build the Circular Electron Positron Collider (CEPC) – a huge 100 km circumference underground collider that would study the Higgs boson in unprecented detail and be a successor to CERN’s Large Hadron Collider. Yang took a sceptical view calling it “inappropriate” for a developing country that is still struggling with “more acute issues like economic development and environment protection”.
Yang also expressed concern that the science performed on the CEPC is just “guess” work and without guaranteed results. “I am not against the future of high-energy physics, but the timing is really bad for China to build such a super collider,” he noted in 2016. “Even if they see something with the machine, it’s not going to benefit the life of Chinese people any sooner.”
Lasting legacy
As well as the Nobel prize, Yang won many other awards such as the US National Medal of Science in 1986, the Einstein Medal in 1995, which is presented by the Albert Einstein Society in Bern, and the American Physical Society’s Lars Onsager Prize in 1990.
“The world has lost one of the most influential physicists of the modern era,” noted Stony Brook president Andrea Goldsmith in a statement. “His legacy will continue through his transformational impact on the field of physics and through the many colleagues and students influenced by his teaching, scholarship and mentorship.”
The Inuit of the far north helped solve the mystery of a doomed 19th-century expedition. Now Canada needs them to strengthen its claim to this newly contested region.
It originated in a rap song, then featured in South Park, and is now the bane of schoolteachers in the US and UK as pupils shout it out at random. How did it become such a thing?
Millions of young people risk missing out on new treatments for health conditions and having to use medicines that are unsafe, ineffective or inappropriate because so few take part in medical research. One of those bucking the trend explains why he signed up to a study and how it transformed his life.
Gulliver Waite was diagnosed with clinical depression at 19. For years, he struggled with extremely low mood, anxiety, frequent panic attacks and occasional paranoia.
Nine decades later, Álvarez’s love of the game has only increased. A little after 10am on Saturday, the 104-year-old madrileño – believed to be the oldest active registered chess player in the world – stepped off a bus in the south of the city and pushed his homemade walker towards the door of the cultural centre where he comes for his weekly matches.
Zohran Mamdani hosted a soccer tournament in Coney Island and he and Andrew Cuomo appeared at a forum at Queens College as the New York City mayoral campaign enters its final weeks.
Assemblyman Zohran Mamdani’s campaign hosted a soccer tournament at Maimonides Park in Brooklyn, where teams from different boroughs competed in color-coded jerseys.
Report found coroners raised concerns over suicide forums at least 65 times to three government departments since 2019
Bereaved families and survivors of a pro-suicide forum have called for a public inquiry into the government’s failure to prevent harm linked to the online platform.
The calls came as a report found that coroners had raised concerns regarding suicide forums at least 65 times to three government departments since 2019.
Viral trend sees (mostly) young people meet to ‘release pent-up energy’ and relieve stress in fun, social setting
On a cold Monday afternoon in Hyde Park, London, a small group of people gather by the Huntress fountain chatting softly among themselves. Nothing about the group would seem unusual to passing dog walkers and runners – until they huddle together and one starts a countdown.
On three, a collective scream cuts through the park. The outburst lasts only a few seconds before giving way to laughter. They were only meant to do it once, but end up screaming again – louder, the second time.
They are peaceful, female-led and use sex in everyday interactions. Now a new conservation scheme could offer a lifeline to our critically endangered close relatives living on the Congo river
A few dozen large nests appear in the mist of equatorial dawn, half-hidden behind a tangle of vines and leaves. That is where the bonobos sleep, 12 metres above the ground. But it has rained all night, and the primates are in no hurry to get up. It is 6.30am when the first head emerges. It gives a cry, a sharp bark, and another silhouette unfolds from its cocoon of branches. And then another. Within five minutes, the whole group is awake – yawning, stretching, straightening. Their features are fine, their limbs long and delicate, their build less stocky than that of chimpanzees, their closest cousins.
With 97% of schools destroyed or damaged, 600,000 children have just begun their third year out of formal education. Three students and a teacher share their stories – and their hopes
Unsportsmanlike conduct in grassroots football and on the sidelines at school events is on the rise. How can parents support their child in the right way?
Pushy and shouty parents are the “biggest problem in sports performance”, sports psychologists have said, amid growing concern that pressure and abuse is hampering competitive sport in the UK.
This week parents were banned from attending sports events at a number of south London primary schools due to “concerning behaviours”, including abuse towards officials and children and creating “too much pressure around performance and winning at all costs”.
Measures come amid concern generative AI characters are having inappropriate conversations with under-18s
Parents will be able to block their children’s interactions with Meta’s AI character chatbots, as the tech company addresses concerns over inappropriate conversations.
The social media company is adding new safeguards to its “teen accounts”, which are a default setting for under-18 users, by letting parents turn off their children’s chats with AI characters. These chatbots, which are created by users, are available on Facebook, Instagram and the Meta AI app.
It’s four decades since the travel writer last ventured on to the slopes. A resort in the Tirol is the perfect place to rediscover the joys of skiing
‘You’re mad!” Caroline the greengrocer said cheerfully when I told her I was going skiing. A reasonable reaction since not so long ago I was shopping on crutches following a hip replacement. My sister’s friends were more concerned: “How old are you? 80? I don’t think this is a good idea. You’ll fall and break something.” My brother, Andrew, 86, decided it was better not to tell anyone.
For at least two decades I’d had a half-buried wish to experience one more ski trip. A final fix of blue sky, frosty air and the exhilaration that comes with finding yourself still intact at the bottom of a snow-covered slope. I was never much good, and hadn’t skied for decades, but that wasn’t the point. At 83, I needed to see if I could still do it. And if I could do it, how about inviting my sister, Kate, one-third of our Old Crones group who encourage each other to do parkrun each week? Then I remembered that, as teenagers, Andrew had joined me on my first ski holiday. That was 67 years ago, but Andrew used to be quite good, so I invited him too. My friend Penny, who is so absurdly young (67, so she says) that she doesn’t really count, was also allowed to come and try her luck with the oldies and practise her German. We all made an effort to get as fit as possible, but none of us had skied for at least 40 years.
From Interrailing around Europe to trekking in the Himalayas, our tipsters share their memorable trips made later in life • Tell us about a great winter mountain holiday – the best tip wins a £200 holiday voucher
I went Interrailing at 16 – so decided to do it again at 61! My wife and I bought our passes for all of Europe (under £500 for one-month unlimited rail trips) and it was great to rediscover the sense of freedom and adventure travelling by train gave. Having a romantic dinner in Paris, getting on the night train and having coffee and croissants for breakfast in Nice on the Côte d’Azur for example. I corrected the teenage mistake of trying to do too much and see too many places so we lingered longer in places such as Poland and Romania, soaking up the atmosphere in Wrocław and Bucharest. It was interesting to compare the speed, quality and comfort of train services too. We found that sometimes slow travel was better – like when we got on the wrong train from Rome to Naples, allowing us to appreciate the scenery, locals and way of life of people who were not in a hurry. The trip was a learning experience at 61 as much as it had been at 16. Peter
Plaintiffs had ‘overwhelming evidence’ of climate crisis but a court injunction would be ‘unworkable’, ruling says
A federal judge has dismissed a lawsuit filed by young climate activists that aimed to halt Donald Trump’s pro-fossil fuel executive orders.
The dismissal by US district judge Dana Christensen on Wednesday came after 22 plaintiffs, ages seven to 25 and from five states, sought to block three of the president’s executive orders, including those declaring a “national energy emergency” and seeking to “unleash American energy” – as well as one aimed at “reinvigorating” the US’s production of coal.
Active pension scheme expected to start in January is part of chancellor’s ‘autumn of reforms’ to tackle economic stagnation
Germans who continue in the labour market beyond retirement age will be able to earn up to €2,000 (£1,750) a month tax-free on top of their pension under a scheme aimed at boosting economic growth and labour force participation rates.
The “Aktivrente”, or active pension scheme, due to come into force in January, was promised on the campaign trail by the chancellor, Friedrich Merz, before he came into office five months ago.
Quantum pioneer: Michael Berry is best known for his work in the 1980s on the Berry Phase. (Courtesy: Michael Berry)
The theoretical physicist Michael Berry from the University of Bristol has won the 2025 Isaac Newton Medal and Prize for his “profound contributions across mathematical and theoretical physics in a career spanning over 60 years”. Presented by the Institute of Physics (IOP), which publishes Physics World, the international award is given annually for “world-leading contributions to physics by an individual of any nationality”.
Born in 1941 in Surrey, UK, Berry earned a BSc in physics from the University of Exeter in 1962 and a PhD from the University of St Andrews in 1965. He then moved to Bristol, where he has remained for the rest of his career.
Berry is best known for his work in the 1980s in which he showed that, under certain conditions, quantum systems can acquire what is known as a geometric phase. He was studying quantum systems in which the Hamiltonian describing the system is slowly changed so that it eventually returns to its initial form.
Berry showed that the adiabatic theorem widely used to describe such systems was incomplete and that a system acquires a phase factor that depends on the path followed, but not on the rate at which the Hamiltonian is changed. This geometric phase factor is now known as the Berry phase.
Over his career Berry, has written some 500 papers across a wide number of topics. In physics, Berry’s ideas have applications in condensed matter, quantum information and high-energy physics, as well as optics, nonlinear dynamics, and atomic and molecular physics. In mathematics, meanwhile, his work forms the basis for research in analysis, geometry and number theory.
Berry told Physics World that the award is “unexpected recognition for six decades of obsessive scribbling…creating physics by seeking ‘claritons’ – elementary particles of sudden understanding – and evading ‘anticlaritons’ that annihilate them” as well as “getting insights into nature’s physics” such as studying tidal bores, tsunamis, rainbows and “polarised light in the blue sky”.
Over the years, Berry has won a wide number of other honours, including the IOP’s Dirac Medal and the Royal Medal from the Royal Society, both awarded in 1990. He was also given the Wolf Prize for Physics in 1998 and the 2014 Lorentz Medal from the Royal Netherlands Academy of Arts and Sciences. In 1996 he received a knighthood for his services to science.
Berry’s latest honour forms part of the IOP’s wider 2025 awards, which recognize everyone from early-career scientists and teachers to technicians and subject specialists. Other winners include Julia Yeomans, who receives the Dirac Medal and Prize for her work highlighting the relevance of active physics to living matter.
Lok Yiu Wu, meanwhile, receives Jocelyn Bell Burnell Medal and Prize for her work on the development of a novel magnetic radical filter device, and for ongoing support of women and underrepresented groups in physics.
In a statement, IOP president Michele Dougherty congratulated all the winners. “It is becoming more obvious that the opportunities generated by a career in physics are many and varied – and the potential our science has to transform our society and economy in the modern world is huge,” says Dougherty. “I hope our winners appreciate they are playing an important role in this community, and know how proud we are to celebrate their successes.”
The full list of 2025 award winners is available here.
John Clarke, Michel Devoret and John Martinis share the 2025 Nobel Prize for Physics “for the discovery of macroscopic quantum mechanical tunnelling and energy quantization in an electric circuit”.
The award includes a SEK 11m prize ($1.2m), which is shared equally by the winners. The prize will be presented at a ceremony in Stockholm on 10 December.
The prize was announced this morning by members of the Royal Swedish Academy of Science. Olle Eriksson of Uppsala University and chair of the Nobel Committee for Physics commented, “There is no advanced technology today that does not rely on quantum mechanics.”
Göran Johansson of Chalmers University of Technology explained that the three laureates took quantum tunnelling from the microscopic world and onto superconducting chips, allowing physicists to study quantum physics and ultimately create quantum computers.
Speaking on the telephone, John Clarke said of his win, “To put it mildly, it was the surprise of my life,” adding “I am completely stunned. It had never occurred to me that this might be the basis of a Nobel prize.” On the significance of the trio’s research, Clarke said, “The basis of quantum computing relies to quite an extent on our discovery.”
As well as acknowledging the contributions of Devoret and Martinis, Clarke also said that their work was made possible by the work of Anthony Leggett and Brian Josephson – who laid the groundwork for their work on tunnelling in superconducting circuits. Leggett and Josephson are previous Nobel winners.
As well as having scientific significance, the trio’s work has led to the development of nascent commercial quantum computers that employ superconducting circuits. Physicist and tech entrepreneur Ilana Wisby, who co-founded Oxford Quantum Circuits, told Physics World, “It’s such a brilliant and well-deserved recognition for the community”.
A life in science
Clarke was born in 1942 in Cambridge, UK. He received his BA in physics from the University of Cambridge in 1964 before carrying out a PhD at Cambridge in 1968. He then moved to the University of California, Berkeley, to carry out a postdoc before joining the physics faculty in 1969 where he has remained since.
Devoret was born in Paris, France in 1953. He graduated from Ecole Nationale Superieure des Telecommunications in Paris in 1975 before earning a PhD from the University of Paris, Orsay, in 1982. He then moved to the University of California, Berkeley, to work in Clarke’s group collaborating with Martinis who was a graduate student at the time. In 1984 Devoret returned to France to start his own research group at the Commissariat à l’Energie Atomique in Saclay (CEA-Saclay) before heading to the US to Yale University in 2002. In 2024 he moved to the University of California, Santa Barbara, and also became chief scientist at Google Quantum AI.
Martinis was born in the US in 1958. He received a BS in physics in 1980 and a PhD in physics both from the University of California, Berkeley. He then carried out postdocs at CEA-Saclay, France, and the National Institute of Standards and Technology in Boulder, Colorado, before moving to the University of California, Santa Barbara, in 2004. In 2014 Martinis and his team joined Google with the aim of building the first useful quantum computer before he moved to Australia in 2020 to join the start-up Silicon Quantum Computing. In 2022 he co-founded the company Qolab, of which he is currently the chief technology officer.
The trio did its prizewinning work in the mid-1980s at the University of California, Berkeley. At the time Devoret was a postdoctoral fellow and Martinis was a graduate student – both working for Clarke. They were looking for evidence of macroscopic quantum tunnelling (MQT) in a device called a Josephson junction. This comprises two pieces of superconductor that are separated by an insulating barrier. In 1962 the British physicist Brian Josephson predicted how the Cooper pairs of electrons that carry current in a superconductor can tunnel across the barrier unscathed. This Josephson effect was confirmed experimentally in 1963.
Single wavefunction
The lowest-energy (ground) state of a superconductor is a macroscopic quantum state in which all Cooper pairs are described by a single quantum-mechanical wavefunction. In the late 1970s, the British–American physicist Anthony Leggett proposed that the tunnelling of this entire macroscopic state could be observed in a Josephson junction.
The idea is to put the system into a metastable state in which electrical current flows without resistance across the junction – resulting in zero voltage across the junction. If the system is indeed a macroscopic quantum state, then it should be able to occasionally tunnel out of this metastable state, resulting in a voltage across the junction.
This tunnelling can be observed by increasing the current through the junction and measuring the current at which a voltage occurs – obtaining an average value over many such measurements. As the temperature of the device is reduced, this average current increases – something that is expected regardless of whether the system is in a macroscopic quantum state.
However, at very low temperatures the average current becomes independent of temperature, which is the signature of macroscopic quantum tunnelling that Martinis, Devoret and Clarke were seeking. Their challenge was to reduce the noise in their experimental apparatus, because noise has a similar effect as tunnelling on their measurements.
Multilevel system
As well as observing the signature of tunnelling, they were also able to show that the macroscopic quantum state exists in several different energy states. Such a multilevel system is essentially a macroscopic version of an atom or nucleus, with its own spectroscopic structure.
The noise-control techniques developed by the trio to observe MQT and the fact that a Josephson junction can function as a macroscopic multilevel quantum system have led to the development of superconducting quantum bits (qubits) that form the basis of some nascent quantum computers.
With the 2025 Nobel Prize for Physics due to be unveiled on Tuesday 7 October, Physics World has been getting in the mood by speculating who might win. It’s a prediction game we have fun with every year – and you can check out our infographic to make your own call.
Over the 125 years since the prize was first awarded, almost every seminal finding in physics has been honoured – from the discovery of the electron, neutrino and positron to the development of quantum mechanics and the observation of high-temperature superconductivity.
But what have been the most significant physics prizes of the 21st century? I’m including 2000 as part of this century (ignoring pedants who say it didn’t start till 1 January 2001). During that time, the Nobel Prize for Physics has been awarded 25 times and gone to 68 different people, averaging out at about 2.7 people per prize.
Now, my choice is entirely subjective, but I reckon the most signficant prizes are those that:
are simple to understand;
were an experimental or theoretical tour-de-force;
have long-term implications for science and open new paths;
expose deeper questions at their heart;
were on people’s bucket lists and/or have long, historical links;
were won by people we’d heard of at the time;
are of wider interest to non-physicists or those with only a passing interest in the subject.
So with that in mind, here’s my pick of the five top physics Nobel prizes of the 21st century. You’ll probably disagree violently with my choice so e-mail us with your thoughts.
5. Neutrino oscillation – 2015 prize
Tiny success The SuperKamiokande detector in Japan, where neutrino oscillations were first spotted, led to the 2015 Nobel Prize for Physics to Takaaki Kajita and Art McDonald. (Courtesy: Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo)
Coming in at number five in our list of top physics Nobels of the 21st century is the discovery of neutrino oscillation, which went to Takaaki Kajita and Art McDonald in 2015. The neutrino was first hypothesized by Wolfgang Pauli back in 1930 as “a desperate remedy” to the fact that energy didn’t seem to be conserved when a nucleus emits an electron via beta decay. Fred Reines and Clyde Cowan had won a Nobel prize in 1995 for the original discovery of neutrinos themselves, which are chargeless particles that interact with matter via the weak force and are fiendishly hard to detect.
But what Kajita (at the Super-Kamikande experiment in Japan) and McDonald (at the Sudbury Neutrino Observatory in Canada) had done is to see them switch, or “oscillate”, from one type to another. Their work proved that these particles, which physicists had assumed to be massless, do have mass after all. This was at odds with the Standard Model of particle physics – and isn’t it fun when physics upends conventional wisdom?
What’s more, the discovery of neutrino oscillation explained why Ray Davies and John Bahcall had seen only a third of the solar neutrinos predicted by theory in their famous experiment of 1964. This discrepancy arose because solar neutrinos are oscillating between flavours as they travel to the Earth – and their experiment had detected only a third as it was sensitive mainly to electron neutrinos, not the other types.
4. Bose–Einstein condensation – 2001 prize
Cool finding The first Bose–Einstein condensate (BEC) was created in 1995 from a cloud of cold rubidium atoms by Eric Cornell and Carl Wieman, with the “spike” in the density of atoms indicating many atoms occupying the same quantum state – the signature of a BEC. Cornell and Wieman won the 2001 Nobel Prize for Physics along with Wolfgang Ketterle, who made a BEC a few months later (Courtesy: NIST/JILA/CU-Boulder)
At number four in our list of the best physics Nobel prizes of the 21st century is the 2001 award, which went to Eric Cornell, Wolfgang Ketterle and Carl Wieman for creating the first Bose–Einstein condensates (BECs). I love the idea that Cornell and Wieman created a new state of matter – in which particles are locked together in their lowest quantum state – at exactly 10.54 a.m. on Monday 5 June 1995 at the JILA laboratory in Boulder, Colorado.
First envisaged by Satyendra Nath Bose and Albert Einstein in 1924, Cornell and Wieman created the first BEC by cooling 2000 rubidium-87 atoms to 170nK using the then new techniques of laser and evaporative cooling. Within a few months, Wolfgang Ketterle over at the Massachusetts Institute of Technology also made a BEC from 500,000 sodium-23 atoms at 2 μK.
Since then hundreds of groups around the world have created BECs, which have been used for everything from slowing light to making “atom lasers” and even modelling the behaviour of black holes. Moreover, the interactions between the atoms can be finely controlled, meaning BECs can be used to simulate properties of condensed-matter systems that are extremely difficult – or impossible – to probe in real materials.
3. Higgs boson – 2013 prize
Particle pioneers Peter Higgs (right) in the CERN auditorium with François Englert on 4 July 2012 when the discovery of the Higgs boson was announced, for which the pair won the 2013 Nobel Prize for Physics. (Courtesy: CERN/Maximilien Brice)
Higgs and Englert didn’t, of course, win for detecting the Higgs boson, although the Nobel citation credits the ATLAS and CMS teams in its citation. What they were being credited for was work done back in the early 1960s when they published papers independently of each other that provided a mechanism by which particles can have the masses we observe.
Higgs had been studying spontaneous symmetry breaking, which led to the notion of massless, force-carrying particles, known as Goldstone bosons. But what Higgs realized was that Goldstone bosons don’t necessarily occur when a symmetry is spontaneously broken – they could be reinterpreted as an additional quantum (polarization) state of a force-carrying particle.
The leftover terms in the equations represented a massive particle – the Higgs boson – avoiding the need for a massless unobserved particle. Writing in his now-famous 1964 paper (Phys. Rev. Lett.13 508), Higgs highlighted the possibility of a massive spin-zero boson, which is what was discovered at CERN in 2012.
That work probably got more media attention than all Nobel prizes this century, because who doesn’t love a huge international collaboration tracking down a particle on the biggest physics experiment of all time? Especially as the Standard Model doesn’t predict what its mass should be so it’s hard to know where to look. But it doesn’t take top slot in my book because it “only” confirmed what we had expected and we’re still on the look-out for “new physics” beyond the Standard Model.
2. Dark energy – 2011 prize
Cosmic discovery The universe has been expanding since the dawn of time, but instead of slowing down, in the last five or six billion years the expansion has sped up, bagging the 2011 Nobel prize for Saul Perlmutter, Adam Riess and Brian Schmidt. (Courtesy: NASA/WMAP Science Team)
Theirs was a pretty sensational finding that implied that about three-quarters of the mass–energy content of the universe must consist of some weird, gravitationally repulsive substance, dubbed “dark energy”, about which even now we still know virtually nothing. It had previously been assumed that the universe would – depending on how much matter it contains – either collapse eventually in a big crunch or go on expanding forever, albeit at an ever more gentle pace.
The teams had been studying type 1a supernovae, which always blow up in the same way when they reach the same mass, which means that they can be used as “standard candles” to accurately measure distance in the universe. Such supernovae are very rare and the two groups had to carry out painstaking surveys using ground-based telescopes and the Hubble Space Telescope to find enough of them.
The teams thought they’d find that the expansion of the universe is decelerating, but as more and more data piled up, the results only appeared to make sense if the universe has a force pushing matter apart. The Royal Swedish Academy of Sciences said the discovery was “as significant” as the 2006 prize, which had gone to John Mather and the late George Smoot for their discovery in 1992 of the minute temperature variations in the cosmic microwave background – the fossil remnants of the large-scale structures in today’s universe.
But to me, the accelerating expansion has the edge as the implications are even more profound, pointing as they do to the composition and fate of the cosmos.
1. Gravitational waves – 2017 prize
Space–time collision Artist’s impression of a black-hole binary system generating gravitational waves, the discovery of which led to the 2017 Nobel Prize for Physics for Barry Barish, Kip Thorne and Rainer Weiss, which is (so far) the top physics prize of the 21st century. (Courtesy: LIGO/T Pyle)
And finally, the winner of the greatest Nobel Prize for Physics of the 21st century is the 2017 award, which went to Barry Barish, Kip Thorne and the late Rainer Weiss for the discovery of gravitational waves. Not only is it the most recent prize on my list, it’s also memorable for being a genuine first – discovering the “ripples in space–time” originally predicted by Einstein. The two LIGO detectors in Livingston, Louisiana, and Hanford, Washington, are also astonishing feats of engineering, capable of detecting changes in distance tinier than the radius of the proton.
The story of how gravitational waves were first observed is now well known. It was in the early hours of the morning Monday 14 September 2015, just after staff who had been calibrating the LIGO detector in Livingston had gone to bed, when gravitational waves created from the collision of two black holes 1.3 billion light-years away hit the LIGO detectors in the US. The historic measurement dubbed GW150914 hit the headlines around the world.
More than 200 gravitational-wave events have so far been detected – and observing these ripples, which had long been on many physicists’ bucket lists, has over the last decade become almost routine. Most gravitational-wave detections have been binary black-hole mergers, though there have also been a few neutron-star/black-hole collisions and some binary neutron-star mergers too. Gravitational-wave astronomy is now a well-established field not just thanks to LIGO but also Virgo in Italy and KAGRA in Japan. There are also plans for an even more advanced Einstein Telescope, which could detect in a day what it took LIGO a decade to spot.
Gravitational waves also opened the whole new field of “multimessenger astronomy” – the idea that you observe a cosmic event with gravitational waves and then do follow-up studies using other instruments, measuring it with cosmic rays, neutrinos and photons. Each of these cosmic messengers is produced by distinct processes and so carries information about different mechanisms within its source.
The messengers also differ widely in how they carry this information to the astronomer: for example, gravitational waves and neutrinos can pass through matter and intergalactic magnetic fields, providing an unobstructed view of the universe at all wavelengths. Combining observations of different messengers will therefore let us see more and look further.
Think we’re right or spectacularly wrong with our pick of the top five Nobel physics prizes of the 21st century? Get in touch by e-mailing us with your thoughts.
In this episode of the Physics World Stories podcast, Rosemary Coogan offers a glimpse into life as one of the European Space Agency’s newest astronauts. Selected as part of ESA’s 2022 cohort, she received astronaut certification in 2024, and is now in line to visit the International Space Station within the next five years. One day, she may even walk on the Moon as part of the Artemis programme.
Coogan explains what astronaut training really entails: classroom sessions packed with technical knowledge, zero-gravity parabolic flights, and underwater practice in Houston’s neutral buoyancy pool. Born in Northern Ireland, Coogan reflects on her personal journey. From a child dreaming of space, she went on to study physics and astrophysics at Durham University, then completed a PhD on the evolution of distant galaxies.
When not preparing for lift off, Coogan counts sci-fi among her interests – she loves getting lost in the world of possibilities. She’s also candid about the psychological side of astronaut training, and how she’s learned to savour the learning process itself rather than obsess over launch dates. Hosted by Andrew Glester, this episode captures both the challenge and wonder of preparing for an imminent journey to space.
Rainer Weiss, who shared the Nobel Prize for Physics in 2017 for the discovery of gravitational waves, died on 25 August at the age of 92. Weiss came up with the idea of detecting gravitational waves by measuring changes in distance as tiny as 10–18 m via an interferometer several kilometres long. His proposal eventually led to the formation of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO), which first detected such waves in 2015.
Weiss was born in Berlin, Germany, on 29 September 1932 shortly before the Nazis rose to power. With a father who was Jewish and an ardent communist, Weiss and his family were forced to flee the country – first to Czechoslovakia and then to the US in 1939. Weiss was raised in New York, finishing his school days at the private Columbia Grammar School thanks to a scholarship from a refugee relief organization.
In 1950 Weiss began studying electrical engineering at Massachusetts Institute of Technology (MIT) before switching to physics, eventually earning a PhD in 1962, developing atomic clocks under the supervision of Jerrold Zacharias,. He then worked at Tufts University before moving to Princeton University, where he was a research associate with the astronomer and physicist Robert Dicke.
In 1964 Weiss returned to MIT, where he began developing his idea of using a large interferometer to measure gravitational waves. Teaming up with Kip Thorne at the California Institute of Technology (Caltech), Weiss drew up a feasibility study for a kilometre-scale laser interferometer. In 1979 the National Science Foundation funded Caltech and MIT to develop the proposal to build LIGO.
Construction of two LIGO detectors – one in Hanford, Washington and the other at Livingston, Louisiana, each of which featured arms 4 km long – began in 1990, with the facilities opening in 2002. After almost a decade of operation, however, no waves had been detected so in 2011 the two observatories were upgraded to make them 10 times more sensitive than before.
On 14 September 2015 – during the first observation run of what was known as Advanced LIGO, or aLIGO – the interferometer detected gravitational waves from two merging black holes some 1.3 billion light-years from Earth. The discovery was announced by those working on aLIGO in February 2016.
The following year, Weiss was awarded one half of the 2017 Nobel Prize for Physics “for decisive contributions to the LIGO detector and the observation of gravitational waves”. The other half was shared by Thorne and fellow Caltech physicist Barry Barish, who was LIGO project director.
‘An indelible mark’
As well as pioneering the detection of gravitational waves, Weiss also developed atomic clocks and led efforts to measure the spectrum of the cosmic microwave background via weather balloons. He co-founded NASA’s Cosmic Background Explorer project, measurements from which have helped support the Big Bang theory describing the expansion of the universe.
In addition to the Nobel prize, Weiss was awarded the Gruber Prize in Cosmology in 2006, the Einstein Prize from the American Physical Society in 2007 as well as the Shaw Prize and the Kavli Prize in Astrophysics, both in 2016.
MIT’s dean of science Nergis Mavalvala, who worked with Weiss to build an early prototype of a gravitational-wave detector as part of her PhD in the 1990s, says that every gravitational-wave event that is observed “will be a reminder of his legacy”.
“[Weiss] leaves an indelible mark on science and a gaping hole in our lives,” says Mavalvala. “I am heartbroken, but also so grateful for having him in my life, and for the incredible gifts he has given us – of passion for science and discovery, but most of all to always put people first.”
Entranced by quantum William Phillips. (Courtesy: NIST)
William Phillips is a pioneer in the world of quantum physics. After graduating from Juniata College in Pennsylvania in 1970, he did a PhD with Dan Kleppner at the Massachusetts Institute of Technology (MIT), where he measured the magnetic moment of the proton in water. In 1978 Phillips joined the National Bureau of Standards in Gaithersburg, Maryland, now known as the National Institute of Standards and Technology (NIST), where he is still based.
Phillips shared the 1997 Nobel Prize for Physics with Steven Chu and Claude Cohen-Tannoudji for their work on laser cooling. The technique uses light from precisely tuned laser beams to slow atoms down and cool them to just above absolute zero. As well as leading to more accurate atomic clocks, laser cooling proved vital for the creation of Bose–Einstein condensates – a form of matter where all constituent particles are in the same quantum state.
To mark the International Year of Quantum Science and Technology, Physics World online editor Margaret Harris sat down with Phillips in Gaithersburg to talk about his life and career in physics. The following is an edited extract of their conversation, which you can hear in full on the Physics World Weekly podcast.
How did you become interested in quantum physics?
As an undergraduate, I was invited by one of the professors at my college to participate in research he was doing on electron spin resonance. We were using the flipping of unpaired spins in a solid sample to investigate the structure and behaviour of a particular compound. Unlike a spinning top, electrons can spin only in two possible orientations, which is pretty weird and something I found really fascinating. So I was part of the quantum adventure even as an undergraduate.
What did you do after graduating?
I did a semester at Argonne National Laboratory outside Chicago, working on electron spin resonance with two physicists from Argentina. Then I was invited by Dan Kleppner – an amazing physicist – to do a PhD with him at the Massachusetts Institute of Technology. He really taught me how to think like a physicist. It was in his lab that I first encountered tuneable lasers, another wonderful tool for using the quantum properties of matter to explore what’s going on at the atomic level.
Chilling out William Phillips working on laser-cooling experiments in his laboratory circa 1986. (Courtesy: NIST)
Quantum mechanics is often viewed as being weird, counter-intuitive and strange. Is that also how you felt?
I’m the kind of person entranced by everything in the natural world. But even in graduate school, I don’t think I understood just how strange entanglement is. If two particles are entangled in a particular way, and you measure one to be spin “up”, say, then the other particle will necessarily be spin “down” – even though there’s no connection between them. Not even a signal travelling at the speed of light could get from one particle to the other to tell it, “You’d better be ‘down’ because the first one was measured to be ‘up’.” As a graduate student I didn’t understand how deliciously weird nature is because of quantum mechanics.
Is entanglement the most challenging concept in quantum mechanics?
It’s not that hard to understand entanglement in a formal sense. But it’s hard to get your mind wrapped around it because it’s so weird and distinct from the kinds of things that we experience on a day-to-day basis. The thing that it violates – local realism – seems so reasonable. But experiments done first by John Clauser and then Alain Aspect and Anton Zeilinger, who shared the Nobel Prize for Physics in 2022, basically proved that it happens.
What quantum principle has had the biggest impact on your work?
Superposition has enabled the creation of atomic clocks of incredible precision. When I first came to NIST in 1978, when it was still called the National Bureau of Standards, the very best clock in the world was in our labs in Boulder, Colorado. It was good to one part in 1013.
Because of Einstein’s general relativity, clocks run slower if they’re deeper in a gravitational potential. The effect isn’t big: Boulder is about 1.5 km above sea level and a clock there would run faster than a sea level clock by about 1.5 parts in 1013. So if you had two such clocks – one at sea level and one in Boulder – you’d barely be able to resolve the difference. Now, at least in part because of the laser cooling and trapping ideas that my group and I have worked on, one can resolve a difference of less than 1 mm with the clocks that exist today. I just find that so amazing.
What research are you and your colleagues at NIST currently involved in?
Our laboratory has been a generator of ideas and techniques that could be used by people who make atomic clocks. Jun Ye, for example, is making clocks from atoms trapped in a so-called optical lattice of overlapping laser beams that are better than one part in 1018 – two orders of magnitude better than the caesium clocks that define the second. These newer types of clocks could help us to redefine the second.
We’re also working on quantum information. Ordinary digital information is stored and processed using bits that represent 0 or 1. But the beauty of qubits is that they can be in a superposition state, which is both 0 and 1. It might sound like a disaster because one of the great strengths of binary information is there’s no uncertainty; it’s one thing or another. But putting quantum bits into superpositions means you can do a problem in a lot fewer operations than using a classical device.
In 1994, for example, Peter Shor devised an algorithm that can factor numbers quantum mechanically much faster, or using far fewer operations, than with an ordinary classical computer. Factoring is a “hard problem”, meaning that the number of operations to solve it grows exponentially with the size of the number. But if you do it quantum mechanically, it doesn’t grow exponentially – it becomes an “easy” problem, which I find absolutely amazing. Changing the hardware on which you do the calculation changes the complexity class of a problem.
How might that change be useful in practical terms?
Shor’s algorithm is important because of public key encryption, which we use whenever we buy something online with a credit card. A company sends your computer a big integer number that they’ve generated by multiplying two smaller numbers together. That number is used to encrypt your credit card number. Somebody trying to intercept the transmission can’t get any useful information because it would take centuries to factor this big number. But if an evildoer had a quantum computer, they could factor the number, figure out your credit card and use it to buy TVs or whatever evildoers buy.
Now, we don’t have quantum computers that can do this yet – they can’t even do simple problems, let alone factor big numbers. But if somebody did do that, they could decrypt messages that do matter, such as diplomatic or military secrets. Fortunately, quantum mechanics comes to the rescue through something called the no-cloning theorem. These quantum forms of encryption prevent an eavesdropper from intercepting a message, duplicating it and using it – it’s not allowed by the laws of physics.
Sharing the excitement William Phillips performing a demo during a lecture at the Sigma Pi Sigma Congress in 2000. (Courtesy: AIP Emilio Segrè Visual Archives)
Quantum processors can be made from different qubits – not just cold atoms but trapped ions, superconducting circuits and others, too. Which do you think will turn out best?
My attitude is that it’s too early to settle on one particular platform. It may well be that the final quantum computer is a hybrid device, where computations are done on one platform and storage is done on another. Superconducting quantum computers are fast, but they can’t store information for long, whereas atoms and ions can store information for a really long time – they’re robust and isolated from the environment, but are slow at computing. So you might use the best features of different platforms in different parts of your quantum computer.
But what do I know? We’re a long way from having quantum computers that can do interesting problems faster than classical device. Sure, you might have heard somebody say they’ve used a quantum computer to solve a problem that would take a classical device a septillion years. But they’ve probably chosen a problem that was easy for a quantum computer and hard for a classical computer – and it was probably a problem nobody cares about.
When do you think we’ll see quantum computers solving practical problems?
People are definitely going to make money from factoring numbers and doing quantum chemistry. Learning how molecules behave could make a big difference to our lives. But none of this has happened yet, and we may still be pretty far away from it. In fact, I have proposed a bet with my colleague Carl Williams, who says that by 2045 we will have a quantum computer that can factor numbers that a classical computer of that time cannot. My view is we won’t. I expect to be dead by then. But I hope the bet will encourage people to solve the problems to make this work, like error correction. We’ll also put up money to fund a scholarship or a prize.
What do you think quantum computers will be most useful for in the nearer term?
What I want is a quantum computer that can tackle problems such as magnetism. Let’s say you have a 1D chain of atoms with spins that can point up or down. Quantum magnetism is a hard problem because with n spins there are 2n possible states and calculating the overall magnetism of a chain of more than a few tens of spins is impossible for a brute-force classical computer. But a quantum computer could do the job.
There are quantum computers that already have lots of qubits but you’re not going to get a reliable answer from them. For that you have to do error correction by assembling physical qubits into what’s known as a logical qubit. They let you determine whether an error has happened and fix it, which is what people are just starting to do. It’s just so exciting right now.
What development in quantum physics should we most look out for?
The two main challenges are: how many logical qubits we can entangle with each other; and for how long they can maintain their coherence. I often say we need an “immortal” qubit, one that isn’t killed by the environment and lasts long enough to be used to do an interesting calculation. That’ll determine if you really have a competent quantum computer.
Reflecting on your career so far, what are you most proud of?
Back in around 1988, we were just fooling around in the lab trying to see if laser cooling was working the way it was supposed to. First indications were: everything’s great. But then we discovered that the temperature to which you could laser cool atoms was lower than everybody said was possible based on the theory at that time. This is called sub-Doppler laser cooling, and it was an accidental discovery; we weren’t looking for it.
People got excited and our friends in Paris at the École Normale came up with explanations for what was going on. Steve Chu, who was at that point at Stanford University, was also working on understanding the theory behind it, and that really changed things in an important way. In fact, all of today’s laser-cooled caesium atomic clocks use that feature that the temperature is lower than the original theory of laser cooling said it was.
Another thing that has been particularly important is Bose–Einstein condensation, which is an amazing process that happens because of a purely quantum-mechanical feature that makes atoms of the same kind fundamentally indistinguishable. It goes back to the work of Satyendra Nath Bose, who 100 years ago came up with the idea that photons are indistinguishable and therefore that the statistical mechanics of photons would be different from the usual statistical mechanics of Boltzmann or Maxwell.
Bose–Einstein condensates, where almost all the atoms are in the same quantum state, were facilitated by our discovery that the temperature could be so much lower. To get this state, you’ve got to cool the atoms to a very low temperature – and it helps if the atoms are colder to start with.
Did you make any other accidental discoveries?
We also accidentally discovered optical lattices. In 1968 a Russian physicist named Vladilen Letokhov came up with the idea of trapping atoms in a standing wave of light. This was 10 years before laser cooling arrived and made it possible to do such a thing, but it was a great idea because the atoms are trapped over such a small distance that a phenomenon called Dicke narrowing gets rid of the Doppler shift.
Everybody knew this was a possibility, but we weren’t looking for it. We were trying to measure the temperature of the atoms in the laser-cooling configuration, and the idea we came up with was to look at the Doppler shift of the scattered light. Light comes in, and if it bounces off an atom that’s moving, there’ll be a Doppler shift, and we can measure that Doppler shift and see the distribution of velocities.
So we did that, and the velocity distribution just floored us. It was so odd. Instead of being nice and smooth, there was a big sharp peak right in the middle. We didn’t know what it was. We thought briefly that we might have accidentally made a Bose–Einstein condensate, but then we realized, no, we’re trapping the atoms in an optical lattice so the Doppler shift goes away.
It wasn’t nearly as astounding as sub-Doppler laser cooling because it was expected, but it was certainly interesting, and it is now used for a number of applications, including the next generation of atomic clocks.
How important is serendipity in research?
Learning about things accidentally has been a recurring theme in our laboratory. In fact, I think it’s an important thing for people to understand about the way that science is done. Often, science is done not because people are working towards a particular goal but because they’re fooling around and see something unexpected. If all of our science activity is directed toward specific goals, we’ll miss a lot of really important stuff that allows us to get to those goals. Without this kind of curiosity-driven research, we won’t get where we need to go.
In a nutshell, what does quantum meant to you?
Quantum mechanics was the most important discovery of 20th-century physics. Wave–particle duality, which a lot of people would say was the “ordinary” part of quantum mechanics, has led to a technological revolution that has transformed our daily lives. We all walk around with mobile phones that wouldn’t exist were it not for quantum mechanics. So for me, quantum mechanics is this idea that waves are particles and particles are waves.
This episode features a wide-ranging interview with the astrochemist Ewine van Dishoeck, who is professor emeritus of molecular astrophysics at Leiden Observatory in the Netherlands. In 2018 she was awarded The Kavli Prize in Astrophysics and in this podcast she talks about her passion for astrochemistry and how her research combines astronomy, astrophysics, theoretical chemistry and laboratory experiments.
Van Dishoeck talks about some of the key unanswered questions in astrochemistry, including how complex molecules form on the tiny specks of dust in interstellar space. We chat about the recent growth in our understanding of exoplanets and protoplanetary discs and the prospect of observing signs of life on distant planets or moons.
The Atacama Large Millimetre Array radio telescope and the James Webb Space Telescope are two of the major facilities that Van Dishoeck has been involved with. She talks about the challenges of getting the astronomy community to agree on the parameters of a new observatory and explains the how collaborative nature of these projects ensures that instruments meet the needs of multiple research communities.
Van Dishoeck looks to the future of astrochemistry and what new observatories could bring to the field. The interview ends with a call for the next generation of scientists to pursue careers in astrochemistry.
This podcast is sponsored by The Kavli Prize.
The Kavli Prize honours scientists for basic research breakthroughs in astrophysics, nanoscience and neuroscience – transforming our understanding of the big, the small and the complex. One million dollars is awarded in each of the three fields. The Kavli Prize is a partnership among The Norwegian Academy of Science and Letters, the Norwegian Ministry of Education and Research, and The Kavli Foundation (USA).
The vision for The Kavli Prize comes from Fred Kavli, a Norwegian-American entrepreneur and philanthropist who turned his lifelong fascination with science into a lasting legacy for recognizing scientific breakthroughs and for supporting basic research.
The Kavli Prize follows a two-year cycle, with an open call for nominations between 1 July and 1 October in odd-numbered years, and an announcement and award ceremony during even-numbered years. The next Kavli Prize will be announced in June 2026. Visit kavliprize.org for more information.
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