The 2023 Nobel Prize For Physics was shared by three scientists who pioneered the use of ultrashort, attosecond laser pulses for studying the behaviour electrons in matter.
In this episode of the Physics World Weekly podcast, I chat with three people involved with the IOPP-ZJU International Symposium on Progress in Attosecond Science. The event will be held on 23 May at China’s Zhejiang University and can also be attended online via Zoom. It is organized by IOP Publishing (which brings you Physics World) and Zhejiang University.
Joining me in a lively discussion of attosecond science are Haiqing Lin of Zhejiang University, Caterina Vozzi of Italy’s Institute for Photonics and Nanotechnologies and David Gevaux of the IOPP journal Reports on Progress in Physics, which is supporting the symposium.
This week’s episode also features an interview with Anthony Quinlan, who was a two-time contestant in the PLANCKS international theoretical physics competition for students. He now helps organize the event, the finals of which will be held in Dublin next week.
Quinlan chats with Physics World’s Katherine Skipper about competition, which involves teams of undergraduate and masters’ students solving “fun” physics problems. Quinlan explains that contestants are encouraged to come up with creative solutions – which sometimes leads to unexpected paths to the correct answer.
Artificial intelligence (AI) is used just about everywhere these days and scientific research is no exception. But how can physicists best use the rapidly-changing technology – and how can they be confident in the results AI delivers?
This episode of the Physics World Weekly podcast features a conversation with Rick Stevens, who is a cofounder of the Trillion Parameter Consortium, which is developing AI systems for use in science, engineering, medicine and other fields.
Stevens is a computer scientist at the Argonne National Laboratory and the University of Chicago in the US and he explains how AI can help with a wide range of tasks done by scientific researchers.
European Spallation Source Cryogenics engineer and test leader Jianqin Zhang inspects the first medium beta elliptical cryomodule to be installed at the ESS superconducting linear accelerator. Each cryomodule contains several superconducting radio-frequency cavities. (Courtesy: Ulrika Hammarlund/ESS)
The largest use of helium II is currently in particle accelerators, how is it used at these facilities?
Helium II has two main uses in particle accelerators. One is to cool superconducting electromagnets to temperatures below 2.2 K. These create the large magnetic fields that bend and focus particle beams. The conducting wires in these magnets are usually made from niobium–titanium, which becomes a superconductor below about 9 K. However, further cooling allows the magnets to support higher current densities and higher field strengths. As a result, almost all the magnets on the Large Hadron Collider (LHC) at CERN are cooled by helium II.
The second main use of helium II at accelerators is to cool superconducting radio-frequency (SRF) cavities, which are used to accelerate particles. These are made from niobium, which is a superconductor at temperatures below about 9 K. Again, these cavities perform much better at superfluid temperatures, where they use less energy to achieve the same acceleration.
An important benefit of using helium II to cool magnets and SRFs is the superfluid’s very high effective thermal conductivity. As well as making it very efficient at removing heat, the high effective conductivity means that helium does not boil in the bulk – unlike normal liquid helium. This confers great advantage in cooling, particularly when it comes to SRF cavities. This is because the cavities are resonant devices and can be detuned by mechanical vibrations caused by boiling.
While CERN is currently the biggest user of helium II, it is also used at other accelerators worldwide. How will it be used at your institute, the European Spallation Source (ESS), which will be up and running next year?
Like existing spallation sources in the UK, US, Switzerland and Japan, the ESS will accelerate protons to very high energies in a linear accelerator. These protons will then strike a tungsten target, where neutrons will be created by the spallation (fragmentation) of the target nuclei. These neutrons with then be slowed down so that their de Broglie wavelengths are on par with the separations of atoms in solids and molecules. Such neutrons are ideal for experiments that explore the properties of matter.
The ESS accelerator is about 400 m in length and 90% of the acceleration will be done by SRF cavities operating at 2 K. The superfluid is created by a helium refrigerator providing up to 3 kW of cooling at 2 K.
Other accelerator facilities that use superfluid cooling include the Thomas Jefferson Laboratory in the US and the European X-ray Free Electron Laser in Germany. A future International Linear Collider – a possible successor to the LHC – would also employ superfluid-cooled SRFs.
While superfluid-cooled magnets are used in particle accelerators, that was not their first application.
That’s right. They were first designed for use in the Tore Supra tokamak, which began operation in 1988 in France. It has since been upgraded and called WEST, which operates today. Tore Supra, like other tokamaks, used magnetic fields to confine a hot hydrogen plasma. The ultimate goal of researchers working on tokamaks is to develop a practical way to harness nuclear fusion as a source of energy.
John Weisend Accelerator engineer and author of a book that outlines the history of how helium II has revolutionized science. (Courtesy: ESS)
Tore Supra’s designers wanted to create longer-lasting plasma pulses and realized that this would not be possible using conventional magnets. They saw superfluid-cooled superconducting magnets as the way forward. The Tore Supra team worked out how to handle liquid helium and also they also developed a piece of technology called a cold compressor that would allow them to efficiently and reliably get down to 2 K. These two developments showed that it was possible operate superfluid-cooled magnets.
Helium II has also been used in space, what was the first mission to be superfluid cooled?
The first real use of helium II in space was to cool a space telescope called the Infrared Interferometer Spectrometer and Radiometer (IRAS). This mission was launched in 1983 by the US, the Netherlands and the UK and it surveyed the entire sky at infrared wavelengths. The atmosphere absorbs infrared light, which is why the telescope was launched into space. Once in orbit, its sensors must be kept as cold as possible to detect low levels of infrared light.
This cooling was done using helium II and mission designers had to overcome significant challenges such as how to vent helium vapour when it is mixed in with blobs of liquid in a low-gravity environment.
IRAS was a watershed mission in astronomy because nobody had so extensively observed the universe in these infrared wavelengths before. Astronomers could peer through dust clouds and see objects that had been invisible to other telescopes.
IRAS observed the universe for 300 days before its superfluid ran out, but a decade later NASA was able to transfer liquid helium in space. How was that done?
Yes, that was a project called Superfluid Helium On-Orbit Transfer (SHOOT), which carried superfluid helium onboard a Space Shuttle. The demonstration involved transferring superfluid from a full dewar to an empty dewar in microgravity. This was done using a pump that made use of the “fountain effect” in helium II.
How does the fountain effect work?
The effect can be understood in terms of the two fluid model, which describes helium II as having a superfluid component and a normal fluid component. These aren’t real physical phases within helium II, but rather provide a convenient way of understanding many of its mechanical and thermal properties.
The effect occurs when two regions of helium II are separated by a porous plug with micron-sized channels. If the helium II in one region is heated and the other region is cold, the superfluid component will move through the porous media towards the heater. This is possible because the superfluid component has zero viscosity and can move without resistance through the tiny channels – something that the normal fluid component cannot do.
Superfluid superuser The Large Hadron Collider at CERN is the world’s largest user of helium II. (Courtesy: Maximilien Brice/CERN)
In the heated region, some of the superfluid component will become normal. However, the normal component is viscous and cannot exit the warm region via the porous plug, so pressure builds up. This pressure can be used to pump helium II without the need for mechanical components.
SHOOT was an important demonstration of how helium II could be transferred in space. However, researchers realized that it is more cost efficient to launch experiments with larger dewars and lower heat loads, than to refill a dewar during a mission.
Helium II also has the ability to flow up the wall of a dewar, but despite its exotic properties a superfluid is relatively easy to handle in bulk. Why is that?
Research done in the 1970s and 80s showed that bulk helium II has essentially the same fluid mechanical properties as a conventional fluid – something that can also be explained by the two fluid model. When helium II flows, quantized vortices in the superfluid component interact with the viscosity of the normal fluid component. The result is that the bulk properties are the same as a conventional fluid.
This is tremendously helpful to engineers like me, I suppose we can be thankful that sometimes the universe is kind. The standard engineering rules that are used to design fluid-handling systems also apply to helium II – rules that help us chose components such as pipes, pumps and valves for a given system. The only instances when we need to consider the special properties of helium II are when we are transferring heat, using porous media or creating thin films of the superfluid.
There are several Nobel Prizes for Physics that were made possible by helium II cooling. Do you have a favourite?
For me it’s the 1996 prize, which went to David Lee, Douglas Osheroff and Robert Richardson for their discovery of superfluidity in helium-3. The superfluid that we have been talking about so far in this interview is helium-4, which is by far the most abundant isotope of the element. Helium-4 is a boson and bosonic atoms are able to condense into the lowest quantum energy state of the system, creating a superfluid.
Helium-3 atoms are not bosons, but are fermions. These atoms cannot undergo this Bose–Einstein condensation directly to create a superfluid. However, in the early 1970s Lee, Osheroff and Richardson showed that helium-3 can condense into a superfluid at the much lower temperature of 2.7 mK. The physical mechanism for this is similar to what occurs in superconductors, where at low temperatures, fermionic electrons pair up. These “Cooper pairs” are bosons, so they can condense to create a superconductor in which the electrons can flow without resistance.
Because of its magnetic properties, superfluid helium-3 is a much more complicated substance than superfluid helium-4. It has three different superfluid phases, rather than the one phase of helium-4.
What I like about this discovery is that the trio weren’t searching for superfluity in their experiment. Instead, they were studying the properties of solid helium-3 at very low temperatures and high pressure. I really like the fact that they were looking for one thing and found something entirely different. Often, the most exciting scientific discoveries are made this way.
How it works: the bottom animation shows how the addition of an adjustable needle valve between the refrigerator and helium reservoir prevents the relief valve from being used. (Courtesy: S. Kelley/NIST)
A simple modification to a popular type of cryogenic cooler could save $30 million in global electricity consumption and enough cooling water to fill 5000 Olympic swimming pools. That is the claim of researchers at the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder who describe their energy-efficient design in Nature Communications.
Ryan Snodgrass and colleagues in the US have designed a new way to operate pulse tube refrigerators (PTRs), which compress and expand helium gas in cooling cycle that is similar to that used in a household refrigerator. Developed in the 1980s, PTRs can now reach temperatures of just a few Kelvin, which is below the temperature that helium becomes a liquid (4.2 K).
While PTRs are reliable and used widely in research and industry, they are very power hungry. When Snodgrass and team looked at why commercial PTRs consume so much energy, they found that the devices were designed to be efficient at their final operating temperature of about 4 K. At higher temperatures, the PTRs are much less efficient – and this is a problem because the cooling process begins at room temperature.
Easier repairs
As well as using lots of electricity to cool down, this inefficiency means that it can take a very long time to cool objects. For example, the Cryogenic Underground Observatory for Rare Events (CUORE) – which is looking for neutrinoless double beta decay deep under a mountain in Italy – is cooled to a preliminary 4 K by five PTRs in a process that takes 20 days. Reducing such long cooling times would make it easier and less costly to modify or repair cryogenic systems.
A careful study of the room-temperature operation of PTRs revealed that the helium gas is compressed to a very high pressure. This causes a relief valve to open, sending some of the helium back to the compressor. Less helium is therefore used for cooling, reducing the efficiency of the PTR.
Snodgrass and colleagues solved this problem by replacing the manufacturer-supplied needle valves in a PTR with customized needle valves that can be adjusted constantly. These needle valves control the flow of gas between the refrigerator and its helium reservoirs. They are normally set to optimize the operation of the PTR at cryogenic temperatures.
In the new operating protocol developed at NIST, the needle valves are open at room temperature. This allows gas to flow in and out of the reservoir, which moderates the pressure in the refrigerator. As the temperature drops, the valves are slowly closed – keeping the system at an ideal pressure throughout its operation.
The team found that the modification can boost the cooling rate of PTRs by 1.7–3.5 times. As well as making cooling quicker and more energy efficient, the new design could also be used to reduce the size or number PTRs needed for specific applications. This could be very important for applications in space, where PTRs are already used to cool infrared telescopes such as MIRI on the James Webb Space Telescope.
Many physicists work for small-to-medium-sized companies that provide scientific instrumentation and services – and some have founded companies of their own. Such businesses can have limited resources for marketing and customer service, so using social media can be an efficient way to connect with existing users and attract new customers.
From their base in Durham, North Carolina, the duo also share their top tips for getting the most out of social media.
This podcast is sponsored by Thyracont Vacuum Instruments, which provides all types of vacuum metrology for a broad variety of applications ranging from laboratory research to coating and the semiconductor industry. Explore their sensors, handheld vacuum meters, digital and analogue transducers as well as vacuum accessories and components at thyracont-vacuum.com.
Billed as “Germany’s first quantum physics escape room,” the Kitty Q Escape Room has been unveiled by the Dresden-Würzburg Cluster of Excellence for Complexity and Topology in Quantum Matter (ct.qmat).
The room is located at the Technische Sammlungen Dresden science museum and is described as being, “perfect for family outings, children’s birthday parties, and school field trips”.
The installation has four separate rooms and 17 puzzles that offer visitors a multisensory experience that explores the quirky world of quantum mechanics. The goal for the participant is to discover the fate of Kitty Q (is she dead or alive?), an imaginary being that embodies the spirit of Schödinger’s cat.
Kitty Q might sound familiar to Physics World readers because we wrote about the imaginary cat in 2021, when ct.qmat launched a mobile phone app that teaches children about quantum mechanics. That app is an escape game, and it has now come to life in Dresden.
The app and escape room were designed in collaboration with Philipp Stollenmayer, who is founder of the independent games designer Kamibox.
Physicist and ct.qmat’s Dresden spokesperson Matthias Vojta says, “By embracing modern gamification techniques, we ensure that learning happens in an engaging and subtle way. The best part [is] you don’t need to be a math or physics expert to enjoy the game!”
This episode of the Physics World Weekly podcast explores how the medical physics community is embracing environmental sustainability. Our guests are the medical physicists Rob Chuter of the Christie NHS Foundation Trust in the UK and Kari Tanderup of Aarhus University in Denmark.
They chat with Physics World’s Tami Freeman about the environmental impact of healthcare provision – and how the community can reduce its carbon footprint without having negative impacts on health outcomes.
This podcast was created in collaboration with IPEM, the Institute of Physics and Engineering in Medicine. IPEM owns the journal Physics in Medicine & Biology.
Today’s measurement systems can get very complicated very quickly. Scientists working at the cutting edge of research will often have to cobble together instruments from a number of different suppliers. This is problematic because mixed-vendor systems can be difficult to operate and this can seriously compromise the accuracy and repeatability of the measurements being made.
Lake Shore Cryotronics has addressed these uncertainties with its MeasureReady M81-SSM (Synchronous Source and Measure) system, which allows up to three source and three measurement modules to be simultaneously operated by a single central control instrument. The M81-SSM uses Lake Shore’s proprietary MeasureSync technology to ensure that all connected source and measure modules are synchronously updated and sampled well within +/-10 ns of each other at a 375 kHz sampling rate.
“We purposely put synchronous source and measure capabilities together,” explains Chuck Cimino, who is senior product manager at Lake Shore. “This also enables use of a common accuracy reference to the sample being characterized and ensures consistently minimal noise performance.”
The company is headquartered in Westerville, Ohio and has been developing measurement and control solutions for 56 years. “We’ve got multiple patented technologies in the M81-SSM that very much enable superior synchronization, mixed DC and AC sourcing and measuring, and smoother/faster voltage measurement range changing,” he adds.
At the heart of the M81-SSM is a controller instrument that currently supports four different types of source and measurement modules: a constant voltage source module; a balanced or differential constant current source module; a greater than 1 TΩ input impedance voltage measurement module; and a zero-offset voltage type current measurement module with programmable DC bias built in. Cimino says that more application-specific type modules are being developed to expand the system’s capabilities.
Extremely low-noise operation
All M81-SSM modules contain linear amplifier electronics that are powered by highly-isolated linear power supplies. Cimino says the result is extremely low-noise operation that exceeds many of the best conventionally-built single-box source and measure instruments, including a number of commonly used lock-in amplifiers.
The M81-SSM was designed from the start to cover the widest possible range of voltage versus current characterization applications with lowest possible noise and quickest configuration and setup for measuring samples in cryogenic and/or high field experiment environments. Lake Shore has extensive experience characterizing materials and devices in these and other extreme environments and has also fully leveraged proprietary signal conditioning and measurement technologies, as well as the company’s application scientists’ expertise, in the design of the M81-SSM.
This low-power and low-noise signal capability makes the M81-SSM very useful for measuring magnetic field effects using various Hall bar structures and magnetic field sensors. These devices are used in a number of applications, including spin transport experiments and the study of superconducting materials at cryogenic temperatures. As well as making measurements at very low temperatures while minimizing self-heating, the M81-SSM can also characterize materials in room and very high-temperature environments, avoiding thermal offsets via AC sourcing with lock-in detection.
Hall bar measurements, which are used to make very precise measurements of a sample’s electrical resistance, can be done very effectively using the M81-SSM’s differential current source and voltage measurement module combination. General four-wire resistance measurement applications also benefit from these low-noise, low-power fully differential connected modules.
The modular nature of the M81-SSM and the ability of the modules to switch easily between AC (up to 100 kHz), DC and lock-in detection modes gives users great flexibility in the types of measurements that can be done without swapping between or modifying dedicated DC and AC only instrumentation. This modularity and flexibility also means that the M81-SSM can be used to test multiple devices under identical conditions to deliver consistent results.
The VM-10 voltage measurement module can detect signals from the low nanovolt range up to 10 V. It operates from DC up to 100 kHz and can detect amplitude, phase and harmonics. The CM-10 current measurement module can detect currents in the femtoampere to 100 mA range. Current measurements can be made from DC to 100 kHz and include amplitude, phase and harmonic detection.
The BCS-10 balanced current source module is programmable from 100 fA to 100 mA from DC to 100 kHz sinusoidal output, while the VS-10 voltage source module provides programmable voltages from 1 µV DC/100 nV AC up to ± 10 V with DC to 100 kHz sinusoidal output.
The controller instrument offers a range of digital connectivity including USB, GPIB and Ethernet as well as interfacing with external reference sources or detectors. The controller and modules are compact benchtop instruments that can also be rack mounted.
Synchronization and integration
Thanks to its high degree of synchronization and integration of sourcing and measuring capabilities, the M81-SSM can reduce the number of separate instruments, in many cases to just the M81-SSM system, required to make precision measurements. This level of integration also minimizes the number and length of cables typically used to connect separate sources, measuring instruments and samples. This integration avoids the introduction of parasitic effects – such as leakage, noise, resistance and reactance – all of which can significantly degrade measurements.
The remote modules are connected to the main instrument via standard 2 m noise-immune power and signal cables, which can be optionally extended to 6 m total between instrument and any one module. This means that the modules can be placed very close to where the measurements are being made, such as in a cryogenic probe station. “The name of the game in low-level measurements is to minimize the length of the signal level cables,” says Cimino. “With the M81-SSM you can put the amplifier modules right next to the sample if desired.”
The modular nature of the system means that a wide range of configurations can be created by simply swapping connected modules. This makes the M81-SSM an extremely flexible system and its performance is more predictable than setups built of multiple separate instruments and instruments from different vendors. Furthermore, the entire system is supported by one supplier, making customer service and technical assistance simpler and more streamlined.
The M81-SSM uses a patented analogue system to transmit signals between the controller and its modules. Cimino explains that this keeps noisy digital circuits far away from the sensitive analogue circuits in the modules. This also minimizes ground errors and ensures the tight synchronization of all modules.
Dedicated ADCs and DACs
Signals from up to three connected measurement modules are digitized in the controller by dedicated analogue-to-digital (ADC) converters. Output signals from up to three source modules are defined by the controller by dedicated digital-to-analogue converters (DACs).
Accurate timing Up to three measure modules and three source modules can be synchronized by the M81-SSM using Lake Shore’s MeasureSync technology. (Courtesy: Lake Shore Cryotronics)
The ADCs and DACs are triggered by the rising edge of a shared MeasureSync clock signal. MeasureSync is Lake Shore’s patent-pending signal synchronization system that uses a common 375 kHz clock signal for updating and reading all modules – enabling continuous data sampling on every connected channel rather than the typical multiplexed multi-channel alternatives.
During the gaps between sampling clock edges, ADC data are read by the controller, and the DACs are set to provide the next output values. The result is the complete synchronization and continuous sampling of up to six connected amplifier channels – which means that several synchronized measurements can be made in parallel. Each channel can be set to perform AC, DC or lock-in measurements. Raw samples are acquired and processed at 375 kilosamples per second (kSa/s) and completed measurements are transmitted via LAN, USB or GPIB to a host PC at up to 5k records per second or an aggregate rate for 3 measure channels of 15k measurements per second.
This high degree of synchronization between source and measurement means that the M81-SSM can be used to make lock-in measurements that can extract very weak signals from noisy backgrounds. This is a significant benefit for users because lock-in measurements are usually done using a dedicated AC measurements-only lock-in amplifier instrument.
Lock-in at the touch of a button
“I’ve demonstrated for some interested customers the M81-SSM main capabilities and they ask ‘so where is the lock-in amplifier?’,” says Cimino. “Initially, they just see the compact modules and controller elements, and I have to explain that the lock-in is implemented digitally. ‘It’s a lock-in at the touch of a button’ is the enthusiastic response I got from multiple potential users.”
Cimino adds that novices and experts alike appreciate how simply and intuitively Lake Shore has configured the user interface. At the expert user end of the spectrum, Cimino says that “one M81-SSM user has adopted our Python driver as his group’s driver standard across all of his equipment. He just liked the way we abstracted the M81-SSM controls in our Python driver.”
“Or if you don’t want to program at all, our MeasureLINK software allows you to just drag and drop high-level source and measurement commands to stream data or to do long-duration testing,” says Cimino. “If you want to manipulate a magnetic field or a sample temperature while making electrical measurements, you can do that with no programming.”
Cimino describes the M81-SSM’s user interface as “discoverable” and that any smartphone user (i.e., “everyone”) would be comfortable using it. “Each module is represented in the interface and when you click on a module, you see a virtual front panel for that instrument,” he explains. The default settings on the interface correspond to the most common measurements, but users can also easily navigate the interface to control the M81-SSM in a way that matches their skill level and measurement requirements. And for the novice and expert user alike, Lake Shore provides support from its team of PhD-level application engineers.
“The M81-SSM is the result of five years of really hard work by Lake Shore’s engineering and application teams,” says Cimino, adding that positive responses from the user community suggests that it was well worth the effort.
The coalition launched earlier this year, and its members have promised that they will continue to reinvest 100% of their funds back into science. Members have also pledged to “publish only the content that genuinely adds to scientific knowledge,” and have also promised to “put research integrity ahead of profit”.
This episode of the Physics World Weekly podcast features an interview with Antonia Seymour, who is chief executive of IOP Publishing. She played an important role in the creation of Purpose-Led Publishing and argues that scientists, science and society all benefit when physicists publish in not-for-profit journals.
Audio engagement
Also in this episode, we meet Corragh-May White who is surveying podcast listeners to try to work out the best ways for using audio to get people engaged in science. She is doing a master’s degree in science communication at the University of the West of England and is making short science podcasts in different styles for her subjects to listen to.
If you would like to take part in the 20-minute survey, you can contact White at Corragh2.White@live.uwe.ac.uk for more information.
Happy World Quantum Day . To celebrate, Physics World and our colleagues at IOP Publishing have put together a package of quantum-related content that we hope you will enjoy.
From today, IOPP is offering a special 20% discount on a selection of its quantum-related ebooks. You can find the eligible books here, and the offer expires on 31 May, 2024. Use the code WQD_2024 when you make your purchase.
Meanwhile, over at Physics World you can read about all things quantum on our quantum channel. There you will find an eclectic mix of news, feature articles and opinion that is focused on quantum science and technology.
And stay tuned to Physics World because on 2 July, the first episode of Physics World Livewill explore the burgeoning field of quantum sensors. This live online panel debate will feature leading experts in the field and you can register here to take part and put your questions to the panellists.
And to find out why Physics World’s Matin Durrani is “irked” by the story behind the choice of 14 April, check out his blog: “Get set for World Quantum Day 2024”.
This episode of the Physics World Weekly podcast features an interview with Tannie Liverpool, who uses statistical physics to explore outstanding questions in biology. Based at the UK’s University of Bristol, where he is professor of theoretical physics, Liverpool explains how complex biological behaviours can be described at a very fundamental level using statistical physics.
He chats with Physics World’s Katherine Skipper about own research into cells and tissues, including the mathematics of wound healing. Liverpool also explains how physicists, materials scientists and mathematicians working in other fields are being inspired by the statistical physics of life.
Looking further into the future, on 2 July the first instalment of Physics World Live will look at the burgeoning field of quantum sensors. This live online panel debate will feature leading experts in quantum sensors. Register here to take part and put your questions to the panellists.
Preliminary observations made by the Dark Energy Spectroscopic Instrument (DESI) hint that the acceleration of the expansion of the universe has not been constant – in other words, dark energy has changed over the history of the universe.
At the turn of the millennium, astronomers discovered that the universe has been expanding at an ever increasing rate. This came as a shock to most cosmologists who had assumed that the pull of gravity was slowing the expansion of the universe after the Big Bang.
In 1998 and 1999, two independent teams discovered the acceleration by measuring the distances to supernovae and the speeds that they are receding from Earth. Dark energy – a term that was coined in 1998 – was invoked to provide the vast amount of energy required for this constant acceleration. Three leaders of those teams shared the 2011 Nobel Prize for Physics for the discovery, and in the past quarter century a variety of observations have backed the inclusion of dark energy in the Standard Model of cosmology.
Now, another shock could be coming thanks to DESI, which was designed to study the expansion of the universe.
Robot-controlled optical fibres
DESI is located on the Nicholas U Mayall Telescope at the Kitt Peak National Observatory in Arizona. In comprises thousands of robot-controlled optical fibres that send light to an array of spectrographs. This allowed DESI to make an extensive map of galaxies and quasars in the universe. The spectroscopic data provide a measure of how fast a galaxy is moving away from us, which is determined by a galaxy’s redshift.
Key to DESI’s dark-energy study is that galaxies are not uniformly distributed throughout the universe, but rather are concentrated in bubble-like regions that are surrounded by emptier space. This is a result of how the early universe expanded and cooled. The process started with a hot plasma through which sound waves propagated, creating areas of high and low density called baryon acoustic oscillations (BAO).
Eventually this plasma “froze” to create the gas that would go on to form the earliest stars and galaxies. Bubbles of galaxies tended to form in the dense regions created by BAO, and these expanded along with the universe. Therefore, the size of a galaxy bubble tells astronomers how old it was when it sent us its light. The team also used the light from ancient quasars to illuminate the BAO, allowing them to probe further back in time than was possible with the galaxy measurements.
Tantalizing hints
Putting the spectroscopic and BAO information together, DESI team could determine the expansion rate of the universe at seven different points in time over the past 11 billion years. While their observations are broadly in line with a constant value of dark energy, DESI scientists have reported tantalizing hints of some deviation.
“So far, we’re seeing basic agreement with our best model of the universe, but we’re also seeing some potentially interesting differences that could indicate that dark energy is evolving with time,” explains DESI’s director Michael Levi, who is based at the Lawrence Berkeley National Laboratory in the US. “Those may or may not go away with more data, so we’re excited to start analysing our three-year dataset soon.”
While the team found that its observations are consistent with dark energy varying with time, the statistical significance of the deviation is only about 3σ. This means that there is about a 0.2% chance that the observation is a statistical fluke. In cosmology and some other fields of physics, a significance of 5σ is required for a discovery.
These observations were made in the first year of operation of DESI, which is expected to survey the universe for at least five years.
“It’s astonishing that with only our first year of data, we can already measure the expansion history of our universe at seven different slices of cosmic time, each with a precision of 1 to 3%,” says Berkeley’s Nathalie Palanque-Delabrouille. “The team put in a tremendous amount of work to account for instrumental and theoretical modelling intricacies, which gives us confidence in the robustness of our first results.”
As well as shedding new light on the expansion of the universe, DESI has also provided new information about the mass of the neutrino.
The BAO observations are described in a preprint on arXiv. Related publications can be found here.
In this episode of the Physics World Weekly podcast I am in conversation with Frederic Bertley – who is president and CEO of COSI (Center of Science and Industry) in Columbus, Ohio. Bertley explains how science centres like COSI can boost scientific literacy and talks about the Color of Science initiative, which he founded to highlight and promote diversity in science, technology, engineering, arts and mathematics.
Bertley also talks about his life-long love of ice hockey and how sports can be used to get people interested in science. Indeed, he explains in detail the physics of baseball pitches and the hockey slapshot.
He also talks about how COSI is encouraging Ohioans to observe and understand the total eclipse of the Sun, which will occur in a significant portion of the state on 8 April. He explains how COSI will engage with the public in venues as diverse as libraries and bars to share the science surrounding the eclipse.
The effects of quantum mechanics are all around us, but the quantum properties of matter are generally only apparent at the microscopic level. Superfluidity is an exception, and some of its bizarre characteristics can be seen with the naked eye. What is more, superfluid helium II has found several important applications in science and technology – and is used multi-tonne quantities today at facilities like the Large Hadron Collider.
My guest in this episode of the Physics World Weekly podcast is John Weisend who is senior accelerator engineer at the European Spallation Source and adjunct professor at Lund University in Sweden. He is a specialist in cryogenic engineering, and has written the book Superfluid: How a Quantum Fluid Revolutionized Modern Science.
We chat about the physics behind this amazing substance and how it is used in some of biggest physics experiments on the planet.
Pfeiffer Vacuum provides all types of vacuum equipment, including hybrid and magnetically-levitated turbopumps, leak detectors and analysis equipment, as well as vacuum chambers and systems. You can explore all of its products on the Pfeiffer Vacuum website.
Today, official time is kept by atomic clocks – and technologies such as the Internet, positioning systems and mobile-phone networks depend on the clocks’ extraordinarily accurate time signals.
These atomic clocks define the second in terms of the frequency of light that is involved in a specific transition in atomic caesium. The definition was chosen so that 86,400 atomic seconds corresponds very closely to the length of a day on Earth – which is the traditional definition of the second.
However, the correspondence is not exact. Between 1970 and 2020, the average length of a day on Earth (the period of Earth’s rotation) was about 1–2 ms longer than 86,400 s. This means that every few years, a second-long discrepancy builds up between time as measured by Earth’s rotation and time measured by an atomic clock.
Since 1972 this deviation has been corrected by the insertion of 27 leap seconds into co-ordinated universal time (UTC).
Complicated process
This correction process is complicated by the fact that various factors cause Earth’s period to vary on a number of different time scales. So leap seconds are inserted when needed – not according to a regular schedule like leap years. Nine leap seconds were inserted in 1972–1979, for example, but none have been inserted since 2016.
Indeed, since about 2020 Earth’s average period has dipped below 86,400 s. In other words, Earth’s rotation appears to be speeding up. This bucks the long-term trend of the rotation slowing, and is probably related to interactions deep within the Earth. As a result, metrologists face the unprecedented prospect of “negative leap seconds” – which could be even more disruptive to computer systems than leap seconds.
But now, Duncan Agnew of the Scripps Institution of Oceanography and the University of California, San Diego has identified a new process that may be countering this increase in rotational speed – something that could postpone the need for negative leap seconds.
Writing in Nature, he shows that the increased melting of ice in Greenland and Antarctica is decreasing the Earth’s angular velocity. This is because water from the poles is being redistributed throughout the oceans, thereby changing our planet’s moment of inertia. Because angular momentum is conserved, this change results in a decrease in angular velocity – think of a spinning ice skater who slows down by extending their arms.
Agnew reckons that this will postpone the need for a negative leap second by three years. A negative leap second could be needed in 2029, but it could be one of the last because metrologists have voted to get rid of the leap-second correction in 2035.
Before we had Instagram and TikTok to amuse us, we had an array of quirky desktop toys that captivated our imaginations. Some of these even demonstrated physical principles – including Newton’s cradle with its suspended spheres that clacked back and forth.
For me, the most mysterious of these toys is the drinking bird. Often called a dippy bird, it pivots about an axle, continuously dipping its beak in and out of a container of liquid with no obvious source of power.
Rather than being a perpetual motion machine, a dippy bird is a heat engine. It comprises two glass bulbs that are connected by a glass tube. The tube is attached to the axle so the dumbbell-like configuration can rotate. A “beak” made of absorbent material such as felt is attached to the bulb that forms the head of the bird (see figure).
Pressure drop
The interior of bird is partially filled with a liquid that is highly volatile such as methylene chloride. If the beak is dipped into a glass of water and then removed, evaporation will cause the upper bulb to cool, causing some of the vapour in it to condense and the pressure to drop.
This mismatch in pressure will cause some liquid from the lower bulb to flow up to the top bulb. The now top heavy bird will dip down, putting its beak into the water and starting the process all over again.
So, lots of idle fun watching the bird drink – but how can the effect be used to generate electricity?
Practical use
When working as a postdoc at The Hong Kong Polytechnic University, Hao Wu was trying to boost the voltage created by an evaporation energy generator. She thought of the dippy bird and realized that it could have practical uses beyond demonstrating thermodynamics.
Now a professor at the South China University of Technology, Wu and her colleagues began with a commercial dippy bird toy, which they modified by adding two triboelectric nanogenerator modules. These generate energy from the transfer of electric charge between two surfaces that move across each other. In this case the surfaces were fixed to the pivoting bird and the stationary stand.
The big challenge for the researchers was to minimize the friction at the interface, while still generating electricity. When they got it right, their dippy bird achieved an output voltage exceeding 100 V and could power 20 liquid-crystal displays.
The team is now exploring practical applications for their dippy device. You can read more about the research in an open access paper in Device.
As computing power continues to grow, theoretical physicists have been able to do larger and more complicated simulations. Running these models consumes a growing amount of energy, and for the time being, this results in more greenhouse-gas emissions that contribute to climate change. Indeed, doing an intensive supercomputer simulation can result in emissions that are on par with taking a long-haul flight.
In this episode of the Physics World Weekly podcast, Alejandro Gaita and Gerliz Gutiérrez of Spain’s University of Valencia tell Physics World’s Margaret Harris how the physics community can reduce its computing-related carbon emissions.
Gaita and Gutiérrez are theoretical materials physicists and they argue that scientists should take a frugal approach to computer modelling, which can achieve scientifically relevant results while minimizing energy consumption.
Do the twist: schematic of the Einstein–de Haas experiment showing the mirror, solenoid and ferromagnetic cylinder. (Courtesy: Jasper Olbrich/CC BY-SA 3.0)
Albert Einstein is famous as a theoretical physicist, but he also did one significant experiment. This was the Einstein–de Haas experiment, which he did in 1915 with the Dutch physicist Wander de Haas. This work showed that the magnetization of ferromagnetic materials such as iron is related to the angular momentum of electrons.
Now, some of the apparatus used by Einstein and de Haas has been found languishing in the Ampère Museum near Lyon, which is one of France’s oldest science museums. The finding was made by Alfonso San Miguel of the Claude Bernard Lyon 1 University and Bernard Pallandre, who is a curator at the museum. They say that the provenance of the objects can be verified by documents associated with Geertruida de Haas-Lorentz. She was a physicist and the wife of de Haas. San Miguel and Pallandre say that she donated the equipment to the museum in the 1950s.
The Einstein–de Haas experiment involves a cylinder of ferromagnetic material that is suspended by a thread so that it can rotate about its axis of symmetry. A mirror is situated at top of the cylinder such that the rotation of the cylinder can be measured by reflecting a beam of light onto a screen (see figure).
Curious rotation
The cylinder is placed in the centre of a solenoid. When an electrical current is sent through solenoid, it creates a magnetic field that magnetizes the cylinder – which becomes a bar magnet. This results in the cylinder rotating slightly, which is observed in the deflection of the light beam. If the magnetic field is then reversed, the cylinder rotates in the opposite direction.
This rotation is not predicted by classical electromagnetic theory because the cylindrical symmetry of the experiment offers no way for the magnetic field to exert a torque on the ferromagnet.
Instead, the observed rotation supports the idea that magnetism is created by charged currents that flow in circles within a ferromagnetic material – an idea that was first put forth nearly a century earlier by the French physicist André-Marie Ampère.
As well as having magnetic moments, these orbiting electrons also have angular momentum. The magnetization of the cylinder involves the alignment of these magnetic moments. This results in changes in the directions of the angular momenta of the electrons when the magnetic field is applied. Because angular momentum must be conserved, the cylinder rotates in response to this change.
We now know that electrons have intrinsic angular momentum (spin) as well as orbital angular momentum. The Einstein–de Haas experiment can be used to study how both of these contribute to the magnetization of a material.
This episode of the Physics World Weekly podcast features a wide ranging interview with Keith Burnett, who is president of the Institute of Physics (IOP).
The IOP is the professional body and learned society for physics in the UK and Ireland. It represents 21,000 members and a key goal of the institute is to make physics accessible to people from all backgrounds.
Burnett, who is halfway through his two-year term in office, was knighted in 2013 for his services to science and higher education. He has served as vice chancellor of the University of Sheffield and is also an advocate for high-quality vocational education and technician training.
He talks to Physics World’s Matin Durrani about the challenges facing universities; physicists as entrepreneurs; supporting early-career physicists; and the need for the IOP to continue its drive to boost the diversity of the physics community.
The Institute of Physics owns IOP Publishing, which brings you Physics World
This morning some of the Physics World team set out from Bristol at 7:00 and by 8:30 we were standing on a muddy riverbank in the pouring rain. Along with a growing crowd of people, we were watching the River Severn rush towards the sea – swollen by this winter’s heavy rains.
While some were sharing flasks of coffee and tea while huddling under umbrellas, the braver in the crowd were launching surfboards and kayaks into the cold river. Most had wetsuits and specialist gear on, but one hardy paddler was out in a T-shirt and tracksuit bottoms. (No Physics World personnel got into the river, we watched safely from the bank).
Then, just after 9:00 and ahead of schedule, a huge wave came roaring up from the sea some 50 km away. This was the Severn’s tidal bore. I first spotted it as it rounded a bend in the river, picking up about half a dozen surfers and kayakers and launching them upstream. While most were just scattered by the wave, two managed to surf several hundred metres past us before being pushed into a tree that was leaning precariously from the opposite bank.
Extreme range
Today’s bore was rated a five-out-of-five, and that’s why we made the trek to watch it. The Severn has one of the highest tides in the world and this morning the tidal range in its estuary (at Avonmouth) was nearly 14 m. This extreme range was caused by the alignment of the Moon and Sun through Earth’s equator – which happens around the equinoxes.
The tidal bore is created when the incoming tide enters a shallow, narrowing river. When the rising tide over tops the river flow, a surge of water travels upstream as a series of waves. Indeed, another amazing aspect of this morning was how rapidly the tide rose as the bore passed. Before the event, the level of the river was constant but after the wave passed it had risen about 2 m in what seemed just a few minutes.
There are several other rivers around the world that have tidal bores, and you can read more about them – and the physics behind the phenomenon – in this article by the physicist Michael Berry: “Chasing the Silver Dragon: the physics of tidal bores”.
Connexion
