“Well, Doc, You’re In”: Freeman Dyson’s Journey through the Universe is a biographical account of an epochal theoretical physicist with a mind that was, by any measure, delightful and diverse. It portrays Dyson, a self-described frog among birds, as a one-off synthesis of blitz-spirit Britishness with American space-age can-do. Of the elite cadre of theoretical physicists who ushered in the era of quantum field theory, which dominates theoretical physics to this day, who else would have devoted so much time and sincere scientific energy to the development of a gargantuan spacecraft, powered by nuclear bombs periodically dropped beneath it, that would take human civilisation beyond our solar system!
Written by colleagues, friends, family members and selected experts, each chapter is more of a self-contained monograph, a link in a chain, than it is a portion of the continuous thread that one would find for a more traditional single-author biography. What is lost as a result of this format, such as an occasional repetition of key life moments, is more than sufficiently compensated by richness of perspective and a certain ease of pick-up put-down that comes from the narrational independence of the various chapters. If it has been a while since the reader last had a moment to pick it up, not much will be lost when one delves back in.
The early years of Dyson-caliber 20th-century theoretical physicists and mathematicians of his cohort are often interwoven with events surrounding the development of nuclear weapons or codebreaking. Dyson’s story as told in “Well, Doc, You’re In” stands apart in this respect, as he spent the war years working in Bomber Command for the Royal Air Force in England. His reflections on aspects of his own experience mirror, in some ways, the sentiments of future colleagues involved in the Manhattan project, noting: “Through science and technology, evil is organised bureaucratically so that no individual is responsible for what happens.”
The following years spent wrestling with quantum electrodynamics (QED) at Cornell make for lighter reading. The scattered remarks from eminent theorists such as Bethe and Oppenheimer on Dyson and his work, as well as from Dyson on his eminent colleagues, bring a sense of reality to the unfolding developments that would ultimately become a momentous leap forward in the understanding of quantum field theory.
“The preservation and fostering of diversity is the great goal that I would like to see embodied in our ethical principles and in our political actions,” said Dyson. Following his deep contributions to QED, Dyson embraced this spirit of diversity and jumped from scientific pond to pond in search of progress, be it the stability of matter or the properties of random matrices. It is interesting to learn, with hindsight, of the questions that gripped Dyson’s imagination at a time when particle physics was entering a golden era. As a reader one almost feels the contrarian spirit, or rebellion, in these choices as they are laid out against this backdrop.
Although scientifically Dyson may have been a frog, jumping from pond to pond, professionally he was anything but. Aged 29 he moved to the Institute for Advanced Study at Princeton and he stayed there to the end. In around 1960 Dyson joined the JASON defence advisory group, a group of scientists advising the US government on scientific matters. He remained a member until his passing in 2020. This consistent backdrop makes for a biographical story, which is essentially free from the distractions of the professional manoeuvring that typically punctuates biographies of great scientists. A positive consequence is that the various authors, and the reader, may focus that bit more keenly on the workings of Dyson’s mind.
For as long as graduate students learn quantum field theory, they will encounter Dyson. Sci-fi fans will recognise the Dyson Sphere (a structure surrounding a star to allow advanced civilisations to harvest more energy) featured in Star Trek, or note the name of the Orion III Spaceplane in 2001: A Space Odyssey. Dyson’s legacy is as vast and diverse as the world his mind explored and “Well, Doc, You’re In” is a fascinating glimpse within.
Stanley G Wojcicki, a long-time leader in experimental particle physics, died on 31 May at the age of 86. Stan made a number of seminal contributions to the field, beginning with the discovery of many short-lived particles as a graduate student at Berkeley. He quickly rose to prominence, becoming an expert on K-meson physics, where he made a series of investigations and discoveries that played an important role in understanding the structure of the Standard Model.
Stan hardly had a typical childhood. Born in Warsaw, Poland, his youth was dominated by World War II, which caused great hardships, including the separation of his family for several years, followed by a difficult life under the communist regime. Finally, his mother, brother and he managed to escape to Sweden. There, they were refugees for eight months, before they were finally able to move to the US. Stan’s father remained in Poland, where he was jailed for five years, and never received a visa to rejoin his family.
From a very young age, Stan was an exceptional student who loved and excelled at mathematics. He continued to stand out in school in his new country and gained admission to Harvard University as an undergraduate, majoring in physics. He went on to Berkeley as a graduate student in physics, which is where he and I met and became lifelong friends, colleagues and sometimes collaborators.
Upon receiving his PhD in 1962, Stan spent a year at CERN and Collège de France, Paris (1964–1965). He returned frequently to CERN, including for a period supported through a John Simon Guggenheim Fellowship in 1973–1974. During that year, Stan continued his research on the excited states of hadrons made from combinations of quarks. He continued his close association with CERN, once again as a scientific associate in 1980–1981, and for shorter periods throughout his career.
Stan was appointed assistant professor in the physics department at Stanford in 1966, advanced to full professor in 1974, served as chair from 1982–1985 and stayed on the faculty until his retirement in 2015. He characteristically became interested in the newest and most exciting areas in the field, and was quick to join the design effort for the Superconducting Super Collider (SSC). He served as deputy director of the SSC central design group in Berkeley and was deeply involved in proposing and obtaining approval for the construction of the SSC in Texas. He continued to be active in many aspects of the SSC until it was cancelled by Congress in 1993, and wrote an insightful two-volume history of the project.
After the SSC disappointment, Stan characteristically bounced back to take on a new emerging area of particle physics: neutrino masses and oscillations. He proposed and led the MINOS experiment, a key element of a long-baseline neutrino experiment that sent a beam of neutrinos through a near detector at Fermilab and to a second detector, 735 km away, in a deep mine in Minnesota. MINOS was very important in providing evidence confirming the observations of atmospheric neutrino oscillations from Super-Kamiokande in Japan.
Stan received many honours, including the Pontecorvo Prize in 2011 and the APS Panofsky Prize in 2015 for his neutrino work. He met his wife, Esther, while he was a PhD student at Berkeley. They married in 1961 and had three daughters of whom he was very proud, Susan (CEO of YouTube), Janet (professor of paediatrics at UCSF Medical School) and Anne (founder and CEO of 23andMe). He will be very much missed by his many long-time friends and colleagues.
The existence of dark matter in the universe is one of the most important puzzles in fundamental physics. It is inferred solely by means of its gravitational effects, such as on stellar motions in galaxies or on the expansion history of the universe. Meanwhile, non-gravitational interactions between dark matter and the known particles described by the Standard Model have not been detected, despite strenuous and advanced experimental efforts.
Such a situation suggests that new particles and fields, possibly similar to those of the Standard Model, may have been similarly present across the entire cosmological history of our universe, but with only very tiny interactions with visible matter. This intriguing idea is often referred to as the paradigm of dark sectors and is made even more compelling by the lack of new particles seen at the LHC and laboratory experiments so far.
Dark universe
Cosmological observations, above all those of the cosmic microwave background (CMB), currently represent the main tool to test such a paradigm. The primary example is that of dark radiation, i.e. putative new dark particles that, unlike dark matter, behave as relativistic species at the energy scales probed by the CMB. The most recent data collected by the Planck satellite constrain such dark particles to make at most around 30% of the energy of a single neutrino species at the recombination epoch (when atoms formed and the universe became transparent, around 380,000 years after the Big Bang).
While such observations represent a significant advance, the early universe was characterised by temperatures in the MeV range and above (enabling nucleosynthesis), possibly as large as 1016 GeV. Some of these temperatures correspond to energy scales that cannot be probed via the CMB, nor directly with current or prospective particle colliders. Even if new particles had significant interactions with SM particles at such high temperatures, any electromagnetic radiation in the hot universe was continuously scattered off matter (electrons), making it impossible for any light from such early epochs to reach our detectors today. The question then arises: is there another channel to probe the existence of dark sectors in the early universe?
We are entering a golden era of GW observations across the frequency spectrum
For more than a century, a different signature of gravitational interactions has been known to be possible: waves, analogous to those of the electromagnetic field, carrying fluctuations of gravitational fields. The experimental effort to detect gravitational waves (GWs) had a first amazing success in 2015, when waves generated by the merger of two black holes were first detected by the LIGO and Virgo interferometers in the US and Italy.
Now, the GW community is on the cusp of another incredible milestone: the detection of a GW background, generated by all sources of GWs across the history of our universe. Recently, based on more than a decade of observations, several networks of radio telescopes called pulsar timing arrays (PTAs) – NANOGrav in North America, EPTA in Europe, PPTA in Australia and CPTA in China – produced tentative evidence for such a stochastic GW background based on the influence of GWs on pulsars (see “Hints of low-frequency gravitational waves found” and “Clocking gravity” image). Together with next-generation interferometer-based GW detectors such as LISA and the Einstein Telescope, and new theoretical ideas from particle physics, the observations suggest that we are entering an exciting new era of observational cosmology that connects the smallest and largest scales.
Particle physics and the GW background
Once produced, GWs interact only very weakly with any other component of the universe, even at the high temperatures present at the earliest times. Therefore, whereas photons can tell us about the state of the universe at recombination, the GW background is potentially a direct probe of high-energy processes in the very early universe. Unlike GWs that reach Earth from the locations of binary systems of compact objects, the GW background is expected to be mostly isotropic in the sky, very much like the CMB. Furthermore, rather than being a transient signal, it should persist in the sensitivity bands of GW detectors, similar to a noise component but with peculiarities that are expected to make a detection possible.
As early as 1918, Einstein quantified the power emitted in GWs by a generic source. Compared to electromagnetic radiation, which is sourced by the dipole moment of a charge distribution, the power emitted in GWs is proportional to the third time derivative of the quadrupole moment of the mass-energy distribution of the source. Therefore, the two essential conditions for a source to emit GWs are that it should be sufficiently far from spherical symmetry and that its distribution should change sufficiently quickly with time.
What possible particle-physics sources would satisfy these conditions? One of the most thoroughly studied phenomena as a source of GWs is the occurrence of a phase transition, typically associated with the breaking of a fundamental symmetry. Specifically, only those phase transitions that proceed via the nucleation, expansion and collision of cosmic bubbles (analogous to the phase transition of liquid water to vapour) can generate a significant amount of GWs (see “Ringing out” image). Inside any such bubble the universe is already in the broken-symmetry phase, whereas beyond the bubble walls the symmetry is still unbroken. Eventually, the state of lowest energy inside the bubbles prevails via their rapid expansion and collisions, which fill up the universe. Even though such bubbles may initially be highly spherical, once they collide the energy distribution is far from being so, while their rapid expansion provides a time variation.
The occurrence of two phase transitions is in fact predicted by the Standard Model (SM): one related to the spontaneous breaking of the electroweak SU(2) × U(1) symmetry, the other associated with colour confinement and thus the formation of hadronic states. However, dedicated analytical and numerical studies in the 1990s and 2000s concluded that the SM phase transitions are not expected to be of first order in the early universe. Rather, they are expected to proceed smoothly, without any violent release of energy to source GWs.
This leads to a striking conclusion: a detection of the GW background would provide evidence for physics beyond the SM – that is, if its origin can be attributed to processes occurring in the early universe. This caveat is crucial, since astrophysical processes in the late universe also contribute to a stochastic GW background.
In order to claim a particle-physics interpretation for any stochastic GW background, it is thus necessary to appropriately account for astrophysical sources and characterise the expected (spectral) shape of the GW signal from early-universe sources of interest. These tasks are being undertaken by a diverse community of cosmologists, particle physicists and astrophysicists at research institutions all around the world, including in the cosmology group in the CERN TH department.
Precise probing
For particle physicists and cosmologists, it is customary to express the strength of a given stochastic GW signal in terms of the fraction of the energy (density) of the universe today carried by those GWs. The CMB already constraints this “relic abundance” to be less than roughly 10% of ordinary radiation, or about one millionth of that of the dominant component of the universe today, dark energy. Remarkably, current GW detectors are already able to probe stochastic GWs that produce only one billionth of the energy density of the universe.
Generally, the stochastic GW signal from a given source extends over a broad frequency range. The spectrum from many early-universe sources typically peaks at a frequency linked to the expansion rate at the time the source was active, redshifted to today. Under standard assumptions, the early universe was dominated by radiation and the peak frequency of the GW signal increases linearly with the temperature. For instance, the GW frequency range in which LIGO/Virgo/KAGRA are most sensitive (10–100 Hz) corresponds to sources that were active when the universe was as hot as 108 GeV – six orders of magnitude higher than the LHC. The other currently operating GW observatories, PTAs, are sensitive to GWs of much smaller frequencies, around 10–9–10–7 Hz, which correspond to temperatures around 10 MeV to 1 GeV (see “Broadband” figure). These are the temperatures at which the QCD phase transition occurred. While, as mentioned above, a signal from the latter is not expected, dark sectors may be active at those temperatures and source a GW signal. In the near (and long-term) future, it is conceivable that new GW observatories will allow us to probe the stochastic GW background across the entire range of frequencies from nHz to 100 Hz.
Together with bubble collisions, another source of peaked GW spectra due to symmetry breaking in the early universe is the annihilation of topological defects, such as domain walls separating different regions of the universe (in this case the corresponding symmetry is a discrete symmetry). Violent (so-called resonant) decays of new particles, such as is predicted by some early-universe scenarios, may also strongly contribute to the GW background (albeit possibly only at very large frequencies, beyond the sensitivity reach of current and forecasted detectors). Yet another discoverable phenomenon is the collapse of large energy (density) fluctuations in the early universe, such as is predicted to occur in scenarios where the dark matter is made of primordial black holes.
On the other hand, particle-physics sources can also be characterised by very broad GW spectra without large peaks. The most important such source is the inflationary mechanism: during this putative phase of exponential expansion of the universe, GWs would be produced from quantum fluctuations of space–time, stretched by inflation and continuously re-entering the Hubble horizon (i.e. the causally connected part of the universe at any given time) throughout the cosmological evolution. The amount of such primordial GWs is expected to be small. Nonetheless, a broad class of inflationary models predicts GWs with frequencies and amplitudes such that they can be discovered by future measurements of the CMB. In fact, it is precisely via these measurements that Planck and BICEP/Keck Array have been able to strongly constrain the simplest models of inflation. The GWs that can be discovered via the CMB would have very small frequencies (around 10–17 Hz, corresponding to ~eV temperatures). The full spectrum would nonetheless extend to large frequencies, only with such a small amplitude that detection by GW observatories would be unfeasible (except perhaps for the futuristic Big Bang Observer – a proposed successor to the Laser Interferometer Space Antenna, LISA, currently being prepared by the European Space Agency).
Feeling blue
Certain classes of inflationary models could also lead to “blue-tilted” (i.e. rising with frequency) spectra, which may then be observable at GW observatories. For instance, this can occur in models where the inflaton is a so-called axion field (a generalisation of the predicted Peccei–Quinn axion in QCD). Such scenarios naturally produce gauge fields during inflation, which can themselves act as sources of GWs, with possible peculiar properties such as circular polarisation and non-gaussianities. A final phenomenon that would generate a very broad GW spectrum, unrelated to inflation, is the existence of cosmic strings. These one-dimensional defects can originate, for instance, from the breaking of a global (or gauge) rotation symmetry and persist through cosmological history, analogous to cracks that appear in an ice crystal after a phase transition from water.
Astrophysical contributions to the stochastic GW background are certainly expected from binary black-hole systems. At the frequencies relevant for LIGO/Virgo/KAGRA, such background would be due to black holes with masses of tens of solar masses, whereas in the PTA sensitivity range the background is sourced by binaries of supermassive black holes (with masses up to millions of solar masses), such as those that are believed to exist at the centres of galaxies. The current PTA indications of a stochastic GW background require detailed analyses to understand whether the signal is due to a particle physics or an astrophysics source. A smoking gun for the latter origin would be the observation of significant anisotropies in the signal, as it would come from regions where more binary black holes are clustered.
We are entering a golden era of GW observations across the frequency spectrum, and thus in exploring particle physics beyond the reach of colliders and astrophysical phenomena at unprecedented energies. The first direct detection of GWs by LIGO in September 2015 was one of the greatest scientific achievements of the 21st century. The first generation of laser interferometric detectors (GEO600, LIGO, Virgo and TAMA) did not detect any signal and only constrained the gravitational-wave emission from several sources. The second generation (Advanced LIGO and Advanced Virgo) made the first direct detection and has observed almost 100 GW signals to date. The underground Kamioka Gravitational Wave Detector (KAGRA) in Japan joined the LIGO–VIRGO observations in 2020. As of 2021, the LIGO–Virgo–KAGRA collaboration is working to establish the International Gravitational Wave Network, to facilitate coordination among ground-based GW observatories across the globe. In the near future, LIGO India (IndIGO) will also join the network of terrestrial detectors.
Despite being sensitive to changes in the arm length of the order of 10–18 m, the LIGO, Virgo and KAGRA detectors are not sensitive enough for precise astronomical studies of GW sources. This has motivated the new generation of detectors. The Einstein Telescope (ET) is a proposed design concept for a European third-generation GW detector underground, which will be 10 times more sensitive than the current advanced instruments (see “Joined-up thinking in vacuum science”). On Earth, however, gravitational waves with frequencies lower than 1 Hz are inaccessible due to terrestrial gravity gradient noise and limitations to the size of the device. Space-based detectors, on the other hand, can access frequencies as low as 10–4 Hz. Several space-based GW observatories are proposed that will ultimately form a network of laser interferometers in space. They include LISA (planned to launch around 2035), the Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) led by the Japan Aerospace Exploration Agency and two Chinese detectors, TianQin and Taiji (see “In synch” figure).
Precision detection of the gravitational-wave spectrum is essential to explore particle physics beyond the reach of particle colliders
A new kid on the block, atom interferometry, offers a complementary approach to laser interferometry for the detection of GWs. Two atom interferometers coherently manipulated by the same light field can be used as a differential phase meter tracking the distance traversed by the light field. Several terrestrial cold-atom experiments are under preparation, such as MIGA, ZAIGA and MAGIS, or being proposed, such as ELGAR and AION. These experiments will provide measurements in the mid-frequency range between 10–2–1 Hz. Moreover, a space-based cold-atom GW detector called the Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE) is expected to probe GWs in a much broader frequency range (10–7–10 Hz) compared to LISA.
Astrometry provides yet another powerful way to explore GWs that is not accessible to other probes, i.e. ultra-low frequencies of 10 nHz or less. Here, the passage of a GW over the Earth-star system induces a deflection in the apparent position of a star, which makes it possible to turn astrometric data into a nHz GW observatory. Finally, CMB missions have a key role to play in searching for possible imprints on the polarisation of CMB photons caused by a stochastic background of primordial GWs (see “Acoustic imprints” image). The wavelength of such primordial GWs can be as large as the size of our horizon today, associated with frequencies as low as 10–17 Hz. Whereas current CMB missions allow upper bounds on GWs, future missions such as the ground-based CMB-S4 (CERN Courier March/April 2022 p34) and space-based LiteBIRD observatories will improve this measurement to either detect primordial GWs or place yet stronger upper bounds on their existence.
Outlook
Precision detection of the gravitational-wave spectrum is essential to explore particle physics beyond the reach of particle colliders, as well as for understanding astrophysical phenomena in extreme regimes. Several projects are planned and proposed to detect GWs across more than 20 decades of frequency. Such a wealth of data will provide a great opportunity to explore the universe in new ways during the next decades and open a wide window on possible physics beyond the SM.
Researchers from CERN, DESY, IBM Quantum and more than 30 other organisations have published a white paper identifying activities in particle physics that could benefit from quantum-computing technologies. Posted on arXiv on 6 July, the 40 page-long paper is the outcome of a working group set up at the QT4HEP conference held at CERN last November, which identified topics in theoretical and experimental high-energy physics where quantum algorithms may produce significant insights and results that are very hard or even not accessible by classical computers.
Combining quantum and information theory, quantum computing is natively aligned with the underlying physics of the Standard Model. Quantum bits, or qubits, are the computational representation of a state that can be entangled and brought into superposition. Once measured, qubits do not represent discrete numbers 0 and 1 as their classical counterparts, but a probability ranging from 0 to 1. Hence quantum-computing algorithms can be exploited to achieve computational advantages in terms of speed and accuracy, especially for processes that are yet to be understood.
“Quantum computing is very promising, but not every problem in particle physics is suited to this model of computing,” says Alberto Di Meglio, head of IT Innovation at CERN and one of the white paper’s lead authors alongside Karl Jansen of DESY and Ivano Tavernelli of IBM Quantum. “It’s important to ensure that we are ready and that we can accurately identify the areas where these technologies have the potential to be most useful.”
Neutrino oscillations in extreme environments, such as supernovae, are one promising example given. In the context of quantum computing, neutrino oscillations can be considered strongly coupled many-body systems that are driven by the weak interaction. Even a two-flavour model of oscillating neutrinos is almost impossible to simulate exactly for classical computers, making this problem well suited for quantum computing. The report also identifies lattice-gauge theory and quantum field theory in general as candidates that could enjoy a quantum advantage. The considered applications include quantum dynamics, hybrid quantum/classical algorithms for static problems in lattice gauge theory, optimisation and classification problems.
With quantum computing we address problems in those areas that are very hard to tackle with classical methods
In experimental physics, potential applications range from simulations to data analysis and include jet physics, track reconstruction and algorithms used to simulate the detector performance. One key advantage here is the speed up in processing time compared to classical algorithms. Quantum-computing algorithms might also be better at finding correlations in data, while Monte Carlo simulations could benefit from random numbers generated by a quantum computer.
“With quantum computing we address problems in those areas that are very hard – or even impossible – to tackle with classical methods,” says Karl Jansen (DESY). “We can now explore physical systems to which we still do not have access.”
The working group will meet again at CERN for a special workshop on 16 and 17 November, immediately before the Quantum Techniques in Machine Learning conference from 19 to 24 November.
Since their direct discovery in 2015 by the LIGO and Virgo detectors, gravitational waves (GWs) have opened a new view on extreme cosmic events such as the merging of black holes. These events typically generate gravitational waves with frequencies of a few tens to a few thousand hertz, within reach of ground-based detectors. But the universe is also expected to be pervaded by low-frequency GWs in the nHz range, produced by the superposition of astrophysical sources and possibly by high-energy processes at the very earliest times (see “Gravitational waves: a golden era”).
Announced in late June, news that pulsar timing arrays (PTAs), which infer the presence of GWs via detailed measurements of the radio emission from pulsars, had seen the first evidence for such a stochastic GW background was therefore met with delight by particle physicists and cosmologists alike. “For me it feels that the first gravitational wave observed by LIGO is like seeing a star for the first time, and now it’s like seeing the cosmic microwave background for the first time,” says CERN theorist Valerie Domcke.
Clocking signals
Whereas the laser interferometers LIGO and Virgo detect relative length changes in two perpendicular arms, PTAs clock the highly periodic signals from millisecond pulsars (rapidly rotating neutron stars), some of which are in Earth’s line of sight. A passing GW perturbs spacetime and induces a small delay in the observed arrival time of the pulses. By observing a large sample of pulsars over a long period and correlating the signals, PTAs effectively turn the galaxy into a low-frequency GW observatory. The challenge is to pick out the characteristic signature of this stochastic background, which is expected to induce “red noise” (meaning there should be greater power at lower fluctuation frequencies) in the differences between the measured arrival times of the pulsars and the timing-model predictions.
The smoking gun of a nHz GW detection is a measurement of the so-called Hellings–Downs (HD) curve based on general relativity. This curve predicts the arrival-time correlations as a function of angular separation for pairs of pulsars, which vary because the quadrupolar nature of GWs introduces directionally dependent changes.
Following its first hints of these elusive correlations in 2020, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has released the results of its 15-year dataset. Based on observations of 68 millisecond-pulsars distributed over half the galaxy (21 more than in the last release) by the Arecibo Observatory, the Green Bank Telescope and the Very Large Array, the team finds 4σ evidence for HD correlations in both frequentist and Bayesian analyses.
We are opening a new window in the GW universe, where we can observe unique sources and phenomena
A similar signal is seen by the independent European PTA, and the results are also supported by data from the Parkes PTA and others. “Once the partner collaborations of the International Pulsar Timing Array (which includes NANOGrav, the European, Parkes and Indian PTAs) combine these newest datasets, this may put us over the 5σ threshold,” says NANOGrav spokesperson Stephen Taylor. “We expect that it will take us about a year to 18 months to finalise.”
It will take longer to decipher the precise origin of the low-frequency PTA signals. If the background is anisotropic, astrophysical sources such as supermassive black-hole binaries would be the likely origin and one could therefore learn about their environment, population and how galaxies merge. Phase transitions or other cosmological sources tend to lead to an isotropic background. Since the shape of the GW spectrum encodes information about the source, with more data it should become possible to disentangle the signatures of the two potential sources. PTAs and current, as well as next-generation, GW detectors such as LISA and the Einstein Telescope complement each other as they cover different frequency ranges. For instance, LISA could detect the same supermassive black-hole binaries as PTAs but at different times during and after their merger.
“We are opening a new window in the gravitational-wave universe in the nanohertz regime, where we can observe unique sources and phenomena,” says European PTA collaborator Caterina Tiburzi of the Cagliari Observatory in Sardinia.
On 10 August, the Muon g-2 collaboration at Fermilab presented its latest measurement of the anomalous magnetic moment of the muon aμ. Combining data from Run 1 to Run 3, the collaboration found aμ = 116 592 055 (24) × 10–11, representing a factor-of-two improvement on the precision of its initial 2021 result. The experimental world average for aμ now stands more than 5σ above the Standard Model (SM) prediction published by the Muon g-2 Theory Initiative in 2020. However, calculations based on a different theoretical approach (lattice QCD) and a recent analysis of e+e– data that feeds into the prediction are in tension with the 2020 calculation, and more work is needed before the discrepancy is understood.
The anomalous magnetic moment of the muon aμ = (g-2)/2 (where g is the muon’s gyromagnetic ratio) is the difference between the observed value of the muon’s magnetic moment and the Dirac prediction (g = 2) due to contributions of virtual particles. This makes measurements of aμ, which is one of the most precisely calculated and measured quantities in physics, an ideal testbed for physics beyond the SM. To measure it, a muon beam is sent into a superconducting storage ring reused from the former g-2 experiment at Brookhaven National Laboratory. Initially aligned, the muon spin axes precess as they interact with the magnetic field. Detectors located along the ring’s inner circumference allow the precession rate and thus aμ to be determined. Many improvements to the setup have been made since the first run, including better running conditions, more stable beams and an improved knowledge of the magnetic field.
The new result is based on data taken from 2019 and 2020, and has four times the statistics compared to the 2021 result. The collaboration also decreased the systematic uncertainty to levels beyond its initial goals. Currently, about 25% of the total data (Run 1–Run 6) has been analysed. The collaboration plans to publish its final results in 2025, targeting a precision of 0.14 ppm compared to the current 0.2 ppm. “We have moved the accuracy bar of this experiment one step further and now we are waiting for the theory to complete the calculations and cross-checks necessary to match the experimental accuracy,” explains collaboration co-spokesperson Graziano Venanzoni of INFN Pisa and the University of Liverpool. “A huge experimental and theoretical effort is going on, which makes us confident that theory prediction will be in time for the final experimental result from FNAL in a few years from now.”
The theoretical picture is foggy. The SM prediction for the anomalous magnetic moment receives contributions from the electromagnetic, electroweak and strong interactions. While the former two can be computed to high precision in perturbation theory, it is only possible to compute the latter analytically in certain kinematic regimes. Contributions from hadronic vacuum polarisation and hadronic light-by-light scattering dominate the overall theoretical uncertainty on aμ at 83% and 17%, respectively.
To date, the experimental results are confronted with two theory predictions: one by the Muon g-2 Theory Initiative based on the data-driven “R-ratio” method, which relies on hadronic cross-section measurements, and one by the Budapest–Marseille–Wuppertal (BMW) collaboration based on simulations of lattice QCD and QED. The latter significantly reduces the discrepancy between the theoretical and measured values. Adding a further puzzle, a recently published value of hadronic cross-section measurements by the CMD-3 collaboration that contrasts with all other experiments narrows the gap between the Muon g-2 Theory Initiative and the BMW predictions (see p19).
“This new result by the Fermilab Muon g-2 experiment is a true milestone in the precision study of the Standard Model,” says lattice gauge theorist Andreas Jüttner of CERN and the University of Southampton. “This is really exciting – we are now faced with getting to the roots of various tensions between experimental and theoretical findings.”
Milos Lokajicek, a long-time employee of the division of elementary particle physics of the Institute of Physics of the Czech Academy of Sciences, passed away in June at the age of 70. Milos was involved in almost all the key experiments in which the Czech particle-physics community participated, especially in the collection and processing of experimental data.
Milos began his career in the 1980s on an experiment at the Serpukhov accelerator in the former USSR, investigating proton–antiproton and later deuteron–antideuteron collisions in the Ludmila hydrogen bubble chamber. After obtaining his PhD in 1984, while still at JINR Dubna, he was also involved in the DELPHI experiment at LEP, which played a key role in the Czech Republic’s entry into CERN in 1993.
After returning to the Institute of Physics, he was at the origin of the participation of Czech physicists in the ATLAS experiment at the LHC, the construction of which was approved in 1994. Together with other staff of the Institute of Physics and colleagues from Charles University, he initiated the construction of the ATLAS TileCal hadron calorimeter and built a laboratory for the assembly and testing of the calorimeter submodules in the former garage of the Institute of Physics.
Since his participation in the Ludmila and DELPHI experiments, Milos focused on data processing. Already in the mid-1990s, he had built a computer farm for data processing and modelling at the Institute of Physics, which today serves several large experiments.
In 1997, together with colleagues from Charles University and the Czech Technical University, he initiated the group’s participation in the D0 experiment at the Tevatron, Fermilab. Participation in this experiment was important for the training of young physicists in ATLAS, the construction of which was beginning at that time. After the Tevatron was decommissioned in 2011, Milos obtained funding for the Fermilab–CZ research infrastructure in 2016 with a gradual transition to the neutrino-physics programme. He worked on the NOvA experiment and also used his experience and contacts at CERN for the future DUNE experiment.
The reach of Milos’s work extends far beyond his home institute. Within the Czech Republic, it was the coordination of the activities of Czech institutions in Fermilab and the development of data processing. He was also a long-standing member of the Committee for Cooperation of the Czech Republic with CERN. His international reputation is documented by numerous memberships in steering committees of experiments and projects, and a number of conferences he co-organised. Among the most important are ACAT 2014, CHEP2009, DØ Week 2008 and ATLAS Week 2003.
Milos’s collegiality and friendship will be missed by all of us.
James Burkett Hartle passed away on 17 May in Zurich at the age of 83. Known as the father of quantum cosmology, Jim made landmark contributions to our understanding of the origin of the universe.
Born in Baltimore, Maryland, Jim obtained his undergraduate degree in physics at Princeton University, where he was mentored by John Wheeler. He attended graduate school at Caltech where he worked under Murray Gell-Mann, earning his PhD in 1964 with a dissertation entitled The complex angular momentum in three-particle potential scattering.
After graduating, Jim briefly taught at Princeton before joining the faculty at the University of California, Santa Barbara (UCSB) in 1966. Excited by the discoveries of pulsars, quasars and the cosmic microwave background radiation, Jim turned away from particle physics. In the late 1960s he wrote a series of influential papers, one with Kip Thorne, on the dynamics of rotating neutron stars. The pair organised regular gatherings between their research groups, which turned into the Pacific Coast Gravity Meetings that still run today.
In 1971 Jim used a Sloan Fellowship to go to the University of Cambridge, where he was immersed in the emerging fields of relativistic astrophysics and cosmology. There he met Stephen Hawking, with whom he developed a remarkable long-term collaboration. Two of their papers became classics: one, in 1976, introduced the Hartle–Hawking quantum state for matter outside a black hole, which is fundamental to black-hole thermodynamics and inspired the so-called Euclidean approach to quantum gravity; the other, in 1983, put forward the Hartle–Hawking “no-boundary” wave function of the universe, showing for the first time how the conditions at the Big Bang could be determined by physical theory.
Except for a brief appointment at the University of Chicago, Jim spent his entire career at UCSB, an environment he found congenial, supportive and inspiring. Jim was a wise and caring mentor to countless young scientists and, though reluctant to venture into the public arena, he also did much to forge a strong physics community. In 1979 he cofounded the Institute for Theoretical Physics (now the Kavli Institute for Theoretical Physics) at Santa Barbara, a mecca for physicists ever since.
The Hartle–Hawking wave function not only revolutionised quantum cosmology but also raised tantalising new questions. Jim began to think more deeply about what it entails to apply quantum mechanics to the universe as a whole. Throughout the 1990s, he and Gell-Mann developed the consistent-histories formulation of quantum mechanics, which clarified the physical nature of the branching process in Everettian quantum mechanics and was sufficiently general to describe single closed systems.
While part of some extraordinary collaborations, Jim was also an independent thinker. About one-third of his publications are beautifully written single-author papers often touching on seemingly intractable questions, far from current fashions and approached with enormous care and open-mindedness. In 2003 Jim published Gravity: An Introduction to Einstein’s General Relativity, a textbook gem with a minimum of new mathematics and a wealth of illustrations that made Einstein’s theory accessible to nearly all physics majors.
Jim retired in 2005, to focus on physics. In 2006 he became an external professor at the Santa Fe Institute, collaborating with Gell-Mann during summer visits. That year also marks the start of my own collaboration with Jim. We took up quantum cosmology again and became immersed in some of the field’s heated debates. Unperturbed, Jim set out the beacons. Often, we would be joined by Hawking, who by then had great difficulties communicating, to flesh out the predictions of the no-boundary wave function. Studying the role of the observer in a quantum universe, we were led to a top-down approach to cosmology in which quantum observations retroactively determine the outcome of the Big Bang, thereby realising an old vision of Wheeler’s.
Few scholars ventured as deeply into the fundamentals of physics as Jim did. A selection of his reflections on the deeper nature of physical theory were published in 2021 in The Quantum Universe: Essays on Quantum Mechanics, Quantum Cosmology, and Physics in General. With characteristic humility, however, Jim reminded us that he didn’t have a philosophical agenda.
Despite suffering the devastations of Alzheimer’s disease, physics remained the driving force in Jim’s life until the very end. Yet his intellectual curiosity stretched much further. He was a polymath and an eclectic reader whose interests ranged from Middle Eastern and Mayan archaeology, to American colonial history, Russian literature and eccentric 19th-century religious female figures. Above all, Jim was an exceptionally generous, wise, humble and gentle man.
Every year the ALICE, ATLAS, CMS and LHCb collaborations award outstanding PhD students, who worked on the experiments, with the thesis prizes. Over the past months 15 early-career researchers have been recognised for their contributions during the collaborations’ meeting weeks.
On 7 June, the LHCb collaboration honoured PhD students who made exceptional contributions to the collaboration with their theses. Saverio Mariani (Universita di Firenze; CERN) was awarded for his work on fixed-target physics with the LHCb experiment, using proton-helium collision data to understand antiproton production in cosmic rays. Peter Svihra (University of Manchester; CERN) was recognised for detector R&D towards a silicon-pixel detector for the upgraded LHCb detector.
Collision – Stories from the Science of CERN is a highly readable anthology built on the idea of teaming up great writers with great scientists. There are 13 stories in all, each accompanied by an afterword from a member of the particle physics community. The authors are a very diverse bunch, so there’s something for everyone – from exploring the nature of symmetry through the mirror of human interaction, to imagined historical encounters and, inevitably, the apocalyptic: we humans have always ventured into the unknown with trepidation.
Being of the same vintage as the BBC’s Dr Who, I was pleased to discover that the first story was penned by one of the programme’s most successful showrunners, Steven Moffat. Although I found myself doubting the direction of travel after the opening paragraphs, I enjoyed the destination. It was a good start, and it established a standard that the book maintains to the very last word.
In Adam Marek’s story, I found myself listening along to protagonist Brody Maitland’s selection of music for his appearance on BBC Radio 4’s Desert Island Disks, something of a national institution in the UK. This story also contains the wonderful line: “we live in a world where it is more impressive to have millions of followers than to lift the stone of the universe and reveal the deep mysteries scurrying beneath it.” How true that is in a world of diminishing attentions spans.
Broadcaster and journalist Bidisha Mamata provides a welcome commentary on contemporary global politics. An unscrupulous leader manipulates an ambitious individual in a bid to undermine the global order. Sound familiar? In this case, the individual concerned is a CERN scientist, the reputation at stake, CERN’s, and the tool to achieving that goal the creation of a locally apocalyptic event. Politically spot on. Scientifically wide of the mark.
Post-apocalyptic scenarios make other appearances, though in these cases it’s what happens next that’s important. Stephen Baxter’s AI protagonist guides us through millennia of human stupidity, while Lillian Weezer imagines what might happen if people unearthed the LHC in some post-apocalyptic world.
Prometheus and Frankenstein make their appearances in Margaret Drabble’s wonderfully erudite tale set at CERN in the 2050s. Desiree Reynolds imagines a delicious encounter that never happened between CERN’s first Director General, Felix Bloch, and the American writer and civil rights activist James Baldwin. Would they have gelled? I’d like to think so. There’s a cautionary tale from Courttia Newland about AI, which draws the conclusion that whatever form intelligence may take, life, of a kind, will go on and the laws of the universe will remain the same. Ian Watson’s joyous facility with words puts a smile on your face from the first line of his galaxy-skipping parable. You’ll have to read it for yourself to find out whether he leaves you smiling at the end.
A recurring theme is the parallel between life and physics: Poet Lisa luxx, for example, entwines forces at work in nature with those between people, while Lucy Caldwell examines notions of uncertainty in life and physics in a story set in her native city Belfast. Peter Kalu applies a similar principle to computer security, with a cautionary yet warming tale about a side-channel attack of sorts.
Enough of the stories, what about the afterwords? Peter Dong’s comment leaves you wanting to sit in on his physics classes, while Jens Vigen gives a thoughtful account of the origins of CERN. Kirstin Lohwasser does a fine job of bringing Bidisha’s science back to the realms of reality. Tessa Charles is bullish about the FCC, currently at the feasibility stage. Michael Davis gives a glimpse of the vast industry that is modern day computer security.
Anyone that has juggled particle physics and parenting will identify with Luan Goldie’s story, which is accompanied by a heartfelt paean to CERN by one who has done just that. “Life is work and work is life,” says Carole Weydert, concluding with the words: “CERN. Grey. But sparkling.”
Andrea Bersani introduces us to the speculations that distorted spacetime allow, while Andrea Giammanco does a similar job for the dark sector. Daniel Cervenkov discusses CP violation, while Joe Haley ponders the development of ideas over time: Newton subsumed by Einstein, the Standard Model by something yet to be found. Gino Isidori, for his part, takes us on a brief guided tour of a metastable universe. John Ellis’s pairing with Stephen Baxter is particularly successful. The writer’s central story, which spans millennia and civilisations resonates well with the theoretical physicist’s daily work of examining Gauguin’s questions: “D’où venons nous, Que sommes nous, Où allons nous.”
All in all, the book makes for a varied, thought provoking and engaging read. As with the Arts at CERN programme, it demonstrates that creativity is not the preserve of the arts or of science, and that great things can happen when the two collide.
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