Waves - PHYSICS TODAY - Cooling protons with lasers Philosophy of thermodynamics Doing science sustainably - Literary Theory and Criticism
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pt_cover1121_Cover 10/14/2021 1:03 PM Page 1
PHYSICS TODAY
November 2021 • volume 74, number 11 A publication of the American Institute of Physics
waves
How wind makes
Cooling protons
with lasers
Philosophy of
thermodynamics
Doing science
sustainably
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pt_contents1121_Contents 10/15/2021 4:13 PM Page 5
PHYSICS TODAY November 2021 | volume 74 number 11
FEATURES
30 Illuminating the darkest galaxies
30 Joshua D. Simon and Marla Geha
The behavior of extremely dim galaxies provides stringent constraints on the
nature of dark matter. Establishing those constraints depends on precise
stellar-motion measurements.
38 How does the wind generate waves?
Nick Pizzo, Luc Deike, and Alex Ayet
Although the question is a classical problem, the details of how wind
transfers energy to waves at the ocean surface remain elusive.
44 The demons haunting thermodynamics
38 Katie Robertson
The contradictory implications of statistical mechanics have worried
physicists and philosophers for centuries. Does our present-day knowledge
allow us to exorcize those philosophical demons?
ON THE COVER: Waves are born at the interface of air and water, where
the flow is generally turbulent over a broad range of spatial and temporal
scales. Scientists have been struggling with that complexity for more than a
century. For a review of historical approaches and recent experimental and
numerical efforts to resolve how momentum is transferred from winds
to waves, turn to the article by Nick Pizzo, Luc Deike, and Alex Ayet on
44 page 38. (Image by iStock.com/Anna_Om.)
PHYSICS TODAY (ISSN 0031-9228, coden PHTOAD) volume 74,
Recently on number 11. Published monthly by the American Institute of
Physics, 1305 Walt Whitman Rd, Suite 300, Melville, NY
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New surveys of under- Amid the unrelenting heat On page 26 of the Copyright © 2021, American Institute of Physics. Single
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NOVEMBER 2021 | PHYSICS TODAY 5
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PHYSICS TODAY
Editor-in-chief
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Managing editor
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Editors
10 Readers’ forum Ryan Dahn rdahn@aip.org
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Letters Heather M. Hill hhill@aip.org
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14 Search & discovery David Kramer dk@aip.org
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A cryogenic circuit cools from afar • A new undersea volcano Christine Middleton cmiddleton@aip.org
10 is born east of Africa • An unusual material hosts both even
and odd superconducting phases
Johanna L. Miller jlm@aip.org
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Online
22 Issues & events Paul K. Guinnessy, director pkg@aip.org
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Angela Dombroski atd@aip.org
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advanced reactors Assistant editor
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52 Books Editorial assistant
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• Unifying two fields — Nicholas B. Suntzeff • New books Contributing editor
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FROM THE EDITOR
Aga sagas
Charles Day
M artin Amis, a former bad boy of British literature, was
interviewed last year by the New York Times about his
reading habits. Asked which genres he enjoys and which
he avoids, he replied, “I confess I have never read an Aga saga or
a bodice ripper—or indeed a western (though Hitler, incidentally,
read nothing else).”
What, you might ask, is an Aga saga? The term was coined But all is not well in Agashire. Since Trollope wrote her first
in 1992 to describe the novels of Joanna Trollope, which, Aga saga in the late 1980s, climate change has nudged up the
being set amid middle-class society in the countryside of mean temperature in the UK and led to more frequent and in-
southern England, could be pre- tense heat waves. AGAs run too
sumed to feature a type of stored- hot for some parts of the country.
heat cooker called an AGA. By hap- Indeed, without much difficulty, I
penstance, my home library includes found a news story about a resi-
the epitome of Aga sagas, Trollope’s dent of Aldeburgh in the east of
A Village Affair (1989). Motivated to England who complained that her
one-up Amis, I read the book. No AGA made her kitchen “unbear-
AGAs appear within its pages, ably hot” during a 2019 heat wave.
but thanks to YouTube, I spotted If the Aga saga endures, the genre’s
one in the trailer for the 1995 TV name will become vestigial.
adaptation. A changed climate—invariably
The acronym AGA stands for a worse one—has been a setting for
Aktiebolaget Svenska Gasaccumu- science fiction for decades. Notable
lator (Swedish Gas Accumulator novels that feature warmer worlds
Limited). The company has a con- include Octavia Butler’s Parable of
nection with physics. Its chief en- the Sower (1993), Peter F. Hamil-
gineer and inventor of the AGA ton’s Mindstar Rising (1993), Kim
cooker, Gustaf Dalén, was awarded Stanley Robinson’s Forty Signs of
the 1912 Nobel Prize in Physics “for Rain (2004), and Sarah Hall’s The
his invention of automatic regula- Carhullan Army (2007).
tors for use in conjunction with gas Climate change is also show-
accumulators for illuminating light- ing up in literary fiction. In Iain
houses and buoys.” Banks’s The Steep Approach to Gar-
Dalén was inspired to invent a badale (2007), a gamekeeper on an
cooker after being blinded in 1912 estate in the Scottish Highlands
in an acetylene explosion. Spend- laments, “Ah, it’s all changing. We
ing more time at home, he noticed can see it here. The salmon and
how much work his wife, Elma, put brown trout, they’re mostly gone.
into fueling and running the fam- And we don’t get the winters we
ily’s stove. His solution was to cre- used to. I’ve got clothes and win-
ate one that ran continuously. When ter gear I just never wear—well,
introduced to the UK in 1929, the maybe a day a year or some-
AGA stove became popular among the rich owners of large thing—because it’s milder all the time.”
country houses. Banks himself became increasingly alarmed about climate
AGAs not only serve as cookers, they also heat water. The change. In 2007 he sold his fleet of cars (BMW M5, Land Rover
stove is hot enough that it warms a kitchen. In a drafty old Discovery, Porsche 911 Turbo, and Porsche Boxter S) and re-
house, the kitchen is the coziest room because of the AGA. placed them with a Lexus RX 400 hybrid. PT
8 PHYSICS TODAY | NOVEMBER 2021PT_Nov21_p9_Blank-Ad-Page.qxd 10/15/2021 3:27 PM Page 9
DEPARTMENT HEAD/PROFESSOR OF
PHYSICS & ASTRONOMY
Purdue University seeks a new Head of the Department of Physics & Astronomy to lead a large and dynamic faculty
towards its full potential for forefront scientific research, state-of-the-art teaching, and outstanding service to the
community. Unprecedented enrollment increases and substantial growth in the campus combined with the nation-
ally recognized quality of life of the surrounding community provide a strong foundation for a successful candidate to
achieve an ambitious strategic vision.
Recent faculty initiatives provide a solid basis for growth, including a founding role in the DOE Quantum Science
Center (at Oak Ridge), the preeminent Microsoft Quantum Lab Purdue for topological quantum computing, a key role
in the NSF Harnessing the Data Revolution Institute (A3D3) for artificial intelligence, establishing a highly rec*gnized
new group in atomic, molecular & optical (AMO) physics, and ambitious projects in astrophysics spanning multi-
messenger astronomy and utilizing the upcoming Vera Rubin Observatory and James Webb Space Telescope.
Internationally recognized research in biophysics, condensed matter physics, high energy physics (including dark
matter searches), and high energy nuclear physics is growing through the contribution of a young faculty, with one
third of the 60-member faculty having joined since 2015 as new faculty continue to be recruited.
The Department of Physics & Astronomy is in the College of Science, which has spear-headed key initiatives in data
& computational sciences and quantum science & engineering as well as recently making substantial investments in
the geosciences, life sciences, environmental & atmospheric science, and space sciences & engineering.
Qualifications: The successful candidate will have a Ph.D. in Physics, Astronomy, or related discipline, an outstand-
ing record of scholarly achievement, and a history of extramurally funded research commensurate with the rank of full
professor at Purdue. The candidate must have a clear vision for the future of the Department in the University, the
State of Indiana, and the Nation. Requirements include exceptional leadership abilities, a commitment to excellence
in education, a record of teaching achievement, an enthusiasm for engagement, and a dedication to championing
diversity, equity, and inclusion. Outstanding candidates are eligible for the rank of Distinguished Professor and may
relocate an existing research program to Purdue. The Dual Career Assistance Program at Purdue will assist with find-
ing appointments for a faculty spouse or partner.
Applications: Candidates should submit (1) a cover letter including the names of three references, (2) a complete
curriculum vitae, (3) a statement of research interests and accomplishments, (4) a statement of teaching interests
and accomplishments, and a (5) vision statement for the Department’s future. Purdue University is committed to
advancing diversity in all areas of faculty effort, including scholarship, instruction, and engagement. Candidates
should address at least one of these areas in (6) a separate Diversity and Inclusion Statement, indicating past
experiences, current activities, and/or future goals to promote a climate that values diversity and inclusion.
Applications should be submitted to:
https://career8.successfactors.com/sfcareer/jobreqcareer?jobId=16418&company=purdueuniv
Inquiries can be directed to Steve Durbin, chair of the head search committee (durbin@purdue.edu). Review of
applications will begin after Jan 1, 2022 and will continue until the position is filled. A background check is required for
employment at Purdue University.
Purdue University is an EEO/AA employer. All individuals, including minorities,
women, individuals with disabilities, and veterans are encouraged to apply.pt_readersforum1121_Letters 10/18/2021 5:16 PM Page 10
READERS’ FORUM
Rethinking the Nebra Sky Disk
T
he June 2020 issue of Physics Today arc with two distinct lines along its sky and to the north. i propose that it
features an image of the Nebra sky length and many shorter ones radiating was intended as a representation of the
disk as the illustration for a book re- from its sides. some have interpreted the aurora borealis, which would have made
view by Bernie Taylor (page 53). The sky object to be a mythical boat that ferries periodic appearances in Nebra. That
disk is one of the earliest depictions of the sun across the sky, with the short en- would agree with the observational na-
recognizable astronomical objects and graved strokes representing the oars.2 ture of the rest of the disk.
relationships. Researchers have ana- images of the disk are often oriented
lyzed the disk extensively and debated so that the third arc is on the bottom, em-
about when it was created and where it phasizing the possibility that the symbol 1. B. Ferreira, “a bitter archaeological feud
was found. The disk had been used for represents a boat. But if the two side over an ancient vision of the cosmos,”
New York Times, 19 January 2021.
generations, and elements were added to pieces indeed represent the extent of sun-
2. “The Nebra sky disc (Germany),” nomi-
it over the years.1 rises and sunsets throughout the year, nation form for the international Memory
The 30-centimeter disk originally fea- then the disk is meant to be viewed as of the World Register (2012), https://en
tured elements that appear to be stars, the though the edge represents the horizon, .unesco.org/sites/default/files/germany_
sun or a full Moon, and a crescent Moon. as with modern overhead sky charts. if nebra_sky_disc.pdf.
Two golden arcs (one of which is now that is the case, the disk’s iconography Richard Mentock
missing) were later added to the rim, might be interpreted differently. Cary Academy
which may have changed the disk’s func- The disk could be depicting the star Cary, North Carolina
tion. it has been posited that those arcs cluster nearly midway between the side
3-2-1 Contact:
represent the distances the sunrise and arcs—often thought to be the Pleiades—
sunset travel between solstices at the lat- in the sky to the south, as it would some-
itude where the disk was purportedly times appear from the area around Nebra.
found in 1999 near Nebra, Germany.
added later to the disk was another
The object often thought to be a sun boat
then would be a fuzzy swath, low in the Scientists at the
writers’ table
I
ngrid ockert’s article “s is for science:
The making of 3-2-1 Contact” (Physics
Today, January 2021, page 26) ably
portrays the pioneering roles viewer re-
search and innovative programming
played in the landmark series. But as a
former curriculum developer at the chil-
dren’s Television Workshop, which pro-
duced 3-2-1 Contact, i’d like to point out
that the article omits mention of a less
obvious way in which the show broke
new ground.
Rather than relying solely on outside
content advisers, the children’s Televi-
sion Workshop adopted the novel strat-
egy of also bringing people with scien-
tific expertise in-house to collaborate on
the day-to-day making of 3-2-1 Contact.
ANAGORIA/WIKIMEDIA COMMONS/CC BY 3.0
after the show’s maiden year, that re-
sponsibility fell to the biologist Ed
atkins, who went on to oversee all the
science in the series. he was perhaps the
first scientist invited into the intricacies
of television production on this scale.
he was an inexhaustible source of
The Nebra Sky Disk ideas for episodes and a conduit for the
many other scientists who contributed to
10 PHYSICS TODAY | NOVEMBER 2021pt_readersforum1121_Letters 10/18/2021 3:04 PM Page 11
the show in front of the camera or behind issue of PHYSICS ToDAY (page 14). She de- A second test is related to the other
the scenes. In his work with writers, an- scribes the SeaQuest experiment, which explanation Miller mentions, the pion
imators, editors, and producers, Atkins found that there are about 50% more d ‾ cloud. While pions can indeed generate
touched every science-related image and antiquarks than u ‾ antiquarks. The result asymmetry in the isospin of the sea, they
idea that appeared on screen and essen- is surprising, since the traditional mech- will not do so for the spin, since pions
tially established the role of content di- anism generating the sea was commonly have spin zero. The ’t Hooft Lagrangian,
rector in science television thereafter. expected to be mediated by gluons, on the other hand, leads to strict predic-
That so many in the scientific commu- which are “flavor blind” and cannot tell tions for the quark polarizations. For ex-
nity today think back to 3-2-1 Contact and u
‾ from d ‾. ample, a left-handed up quark u L can
smile is a wonderful testament both to Miller’s report mentions two theoret- produce only a 100% polarized d ‾ Rd L pair.
the soundness of the Children’s Televi- ical ideas proposed to explain the asym- Therefore, a key to the sea’s antiquark
sion Workshop’s vision of melding con- metry. one is that the presence of two u asymmetry should come from future
tent with production in the development valence quarks leads to “Pauli blocking” theoretical and experimental investiga-
of the series and to Atkins’s unique gifts of sea u quarks, the twin brothers of u ‾ tions that relate isospin and the spin
in that arena. antiquarks. But quarks have six states asymmetry.
Ralph Smallberg available: three colors × two spin orienta-
(rsmallberg49@gmail.com) tions. In addition, valence and sea quarks
New York City overlap little in momentum space. Pauli 1. A. E. Dorokhov, N. I. Kochelev, Phys. Lett.
blocking is therefore way too small to ex- B 304, 167 (1993).
2. G. ’t Hooft, Phys. Rev. D 14, 3432 (1976).
‣ Ockert replies: Ralph Smallberg plain the data. (I’ll turn to the second
3. E. Shuryak, Phys. Rev. D 100, 114018
brings up an excellent point. While some idea—the contribution of the pion (2019).
1970s shows had science advisers, the cloud—at the end.) Edward Shuryak
Children’s Television Workshop was one Unfortunately, Miller does not men- (edward.shuryak@stonybrook.edu)
of the first production companies to give tion a third idea that has been put forth, Stony Brook University
them a seat at the writers’ table. In 1977 which is more nontrivial and seems like- Stony Brook, New York
the company launched a three-day lier to explain the puzzling asymmetry.
It started with an observation by Alexan-
Rare earths in space
workshop in Glen Cove, New York,
bringing together leading scientists and der Dorokhov and Nikolai Kochelev1
educators—including MIT physicist that the so-called ’t Hooft effective four-
communications
Philip Morrison—to brainstorm topics quark Lagrangian2 is “flavor nondiago-
for what became 3-2-1 Contact. From nal,” leading to processes u → u(d ‾d) and
d → d(u ‾u) but not to u → u(u ‾u) and
I
there, the Children’s Television Work-
d → d(d ‾d). have enjoyed reading several items on
shop formed a formal science advisory
In a way, the effect is also due to the the rare-earth elements in PHYSICS
committee for the show, intentionally in-
Pauli exclusion principle, but at a differ- ToDAY over the past few years. David
cluding scientists from Black, Hispanic,
ent level. Topological tunneling events, Kramer’s most recent piece focused on the
and Asian American communities.
known as instantons, create fields so topic (February 2021, page 20) was about
As I note in the article, the first con-
strong that they fix the color and spin neodymium-based rare-earth magnets,
tent director of 3-2-1 Contact was Charles
states of participating quarks uniquely. but readers might be interested to learn
Walcott, a biologist at the State Univer-
Instead of six possibilities, there remains of another class of rare-earth magnets
sity of New York at Stony Brook, who
only one, thus a complete blocking. Since based on samarium. Among their appli-
did a wonderful job of facilitating the
the proton has two valence u quarks and cations are traveling wave tubes (TWTs),
collaborations between the scientists and
only one valence d quark, that mecha- which form the backbone of the world’s
the production staff. Likewise, Ted Ducas,
nism would suggest d ‾/ u
‾ = 2 rather than 1. entire space communications system.
a physicist at Wellesley College, deserves
Recently I made the first attempt to The core feature of most TWTs is a
credit for his role in cowriting the show’s
evaluate that effect quantitatively, by stack of samarium–cobalt (SmCo5 or
excellent first season.
calculating the wavefunction of the five- Sm2Co17) magnetic rings, each magne-
Ingrid Ockert
quark uuduu ‾ and uuddd ‾ sectors of the tized in opposition to its neighbor. one
Lawrence Berkeley National Laboratory
proton induced by the ’t Hooft La- design uses a 25 cm stack of 16 rings that
Berkeley, California
grangian.3 The results approximately are 4 cm in diameter. The tubes can am-
match the data, in magnitude and mo- plify and transmit millimeter waves in
Another look at the mentum dependence.
How can one test that idea further? If
frequency ranges of 300 MHz to 50 GHz.
They have bandwidths as high as two oc-
proton sea
that explanation is true, the sea of Δ++ taves, power gains of 40–70 dB, and out-
baryons, which have three up quarks, put powers of a few watts to megawatts.
would have only d ‾ antiquarks (at corre- TWTs also exhibit excellent reliability.
J
ohanna Miller nicely summarizes the sponding momentum fraction x). It is Voyager 1, launched in 1977, has a SmCo
current experimental situation with hardly possible to check that experimen- TWT produced by Watkins-Johnson
the puzzling asymmetry of the pro- tally, but it can be tested numerically, via that is still broadcasting from more than
ton's antiquark "sea" in the May 2021 lattice gauge theory. 23 billion kilometers away from Earth!
NOVEMBER 2021 | PHYSICS TODAY 11pt_readersforum1121_Letters 10/18/2021 3:04 PM Page 12
READERS’ FORUM
The large communications satellites SmCo magnets have an extremely
in geosynchronous orbit have around large coercive force, a measure of their
20–50 TWTs that provide many essential ability to resist demagnetization, and an
services. Complete world coverage also extremely large energy product (the
requires satellites in polar orbits. Fortu- maximum product of the B and H fields
nately, Iridium Communications now during demagnetization), a measure of
PLAN TO maintains a constellation of 66 low- their ability to do work. The maximum
PARTICIPATE Earth-orbit communications satellites
that are in polar orbits. The geosynchro-
energy products for SmCo5 and Sm2Co17
are around 20 megagauss oersteds
nous-orbit and low-Earth-orbit satellites, (160 kJ/m3) and 32 MGOe (250 kJ/m3),
taken together, provide the world with respectively. They are appropriate for
access to space-based communications operating at temperatures from absolute
because of the discovery and develop- zero (−273 °C) to 300 °C for SmCo5 and
ment of SmCo5 and Sm2Co17 magnets in to 350 °C for Sm2Co17.
the late 1960s. Importantly, the magnetic field SmCo
In 1966 Karl Strnat and Gary Hoffer magnets produce is parallel to the c-axis
reported finding promising magnetic of their hexagonal unit cell and never
properties in the yttrium–cobalt com- flips to the easy basal plane at any tem-
pound YCo5. The following year, they perature. The phenomenon, known as
and their colleagues reported the dis- magnetocrystalline anisotropy, gives ap-
covery of a new family of cobalt-based plication designers great flexibility in
permanent magnet materials.1 The re- magnet shape.
searchers substituted other rare earths Neodymium-iron-boron magnets,
The world’s for Y and determined that SmCo5 was the
optimal choice for practical applications.
discovered in 1984, have far better mag-
netic properties than SmCo5 or Sm2Co17
premier lasers, Strnat and Alden Ray continued that line at ordinary temperatures. Their maxi-
biomedical of study and ultimately discovered
Sm2Co17, which has even more impres-
mum energy product is about 55 MGOe
(440 kJ/m3), and their useful temperature
optics, and sive magnetic properties. That research range is between −138 °C and 150 °C.
was possible only because a separation Therefore, NdFeB magnets are the only
optoelectronics process developed by the Department of choice for moderate-temperature appli-
event Energy’s Ames Laboratory for the Man-
hattan Project made pure rare-earth ele-
cations such as electric car motors or MRI
devices. They have already replaced the
ments available for the first time. superconducting magnets used in older
SmCo5 and Sm2Co17 magnets are supe- MRIs, mitigating claustrophobia and
rior to the platinum–cobalt magnets they size issues. SmCo5 and Sm2Co17 magnets,
replaced in terms of magnetic properties, however, have far better magnetic prop-
cost, size, and weight. At present, they re- erties than neodymium-iron-boron mag-
main the only choices for many applica- nets at low temperatures and are the
tions, particularly those that require very only choice for most space applications,
low or very high operating tempera- such as TWTs. Thus, SmCo and NdFeB
tures. Uses include gyros for space launch magnet technologies will coexist peace-
22–27 January 2022 vehicles, brushless high-torque motors fully because their application areas do
for dental and medical power tools, air- not overlap.
San Francisco, craft radar, and computer disk drives.
California, USA
1. K. Strnat et al., J. Appl. Phys. 38, 1001
CONTACT
Letters and commentary are (1967).
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right to edit submissions. dionuclide is iridium-192. PT
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INDUSTRY VIEWPOINT:
COMSOL THE INSIDE VIEW FROM
OUR INDUSTRY PARTNERS
Multiphysics Modeling Is Quickly Becoming Standard Practice in All
Engineering Simulations
For all fields of engineering, simulation has been an important step in workflows
from product design to their manufacture. As part of this, design engineers and
modeling specialists have wrestled with the computing and solving capabilities
of the day to produce more accurate and complex models. The complexity arises
in part from the need to consider more details in a design’s geometry, as these
are integral to a better, more compact, and optimized design. In addition, taking
more details of a design’s physical behavior into consideration is usually required
to increase its model’s accuracy. Subsequently, in order to accurately simulate and
optimize a design, the model needs to include all or many of the physics associated
with the design’s function, which includes its interaction with processes and its
Phil Kinnane surrounding environment.
VP of Sales, COMSOL
Coupling supplementary physics to engineering problems has been difficult
because of the limitations imposed by traditional software. The software may
be hardwired to simulate certain types of applications; it may be based on a set of questions used to define a single-
physics phenomenon, such as electromagnetic fields around an antenna; or it may focus on a predefined coupled-
physics phenomenon, such as thermal stresses in structural analysis. In some cases, the numerical method that is
the basis for the software is not suitable for modeling the additional physics. Therefore, many simulations rely on
assumptions, the exclusion of certain behavior, or the use of empirical data in order to consider the additional physics.
This is not to say that software strategies that are hardwired for such modeling are to be discounted. Much design
work is adequately satisfied by the information such software provides. Yet multiphysics modeling is now opening
up modeling possibilities, improving on current practices, and forging substantial inroads into the market once
dominated by hardwired solutions.
Two reasons account for this growing dominance. The first is that certain applications are too difficult to simulate
with hardwired software, because it does not allow the coupling of any arbitrary physics to any other. Thermal
runaway in batteries is a great example of this, where the design engineer needs to couple most of the behaviors of
a battery, which include electrochemical reaction kinetics, electric fields, chemical deposition, ionic transfer and
electrical heat production, thermal stresses, and conjugate heat transfer within cooling. COMSOL Multiphysics®, on
the other hand, satisfies this need quickly and effectively by providing flexibility in coupling these physics. It even has
an add-on module specific for this type of modeling.
The second reason is that even in applications that were rather well served by hardwired simulation software,
multiphysics modeling often provides greater accuracy and optimization as well as new knowledge about a
product’s design and manufacture. For example, electric-motor simulation software has primarily modeled just the
electromagnetic field within motors. However, as high-power-density motors come into greater use, induction
heating and thermal and other stresses can no longer be ignored; the design as a whole must be considered. In this
case, multiphysics modeling with COMSOL Multiphysics® provides flexibility and expands the usefulness of these
simulations by allowing the addition of arbitrary physics to existing analyses.
As the required complexity of engineering simulations continues to increase due to improvements in hardware
capabilities and the numerical methods used to solve them, multiphysics modeling in product design and
manufacture is quickly becoming established practice in engineering simulations.
Industry ViewPoint shares the perspectives of PHYSICS TODAY’s advertising partners, who supply the content. It is written by the listed company and copyedited
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NOVEMBER 2021 | PHYSICS TODAY 13pt_search1121_search 10/14/2021 2:13 PM Page 14
SEARCH & DISCOVERY
A cryogenic circuit cools from afar
A cloud of ultracold ions can lower the temperature
of a trapped proton 9 cm away.
bers in Mainz work on measur-
M
uch of modern experimental atomic
physics relies on a counterintuitive ing the proton g factor, while
principle: Under the right circum- collaborators at CERN work on
stances, zapping matter with a laser antiprotons.
doesn’t inject energy into the system; The experiments require
rather, it sucks energy out. By cooling the particles cooled to between 100
system to a fraction of a degree above ab- and 500 mK. “Warmer than
solute zero, one can observe quantum ef- that, and the spin states get
fects that are otherwise hidden. muddled together,” explains
Laser cooling works like a charm, but Christian Smorra, leader of the
only when a system’s ladder of quantum Mainz group. So far, the re-
states contains a transition that the lasers searchers have achieved those
can repeatedly and reliably cycle to leave temperatures by a technique
the system with a bit less energy each called resistive cooling. As the
time. Atoms of alkali metals and a few proton oscillates back and forth
other elements are ideal. Molecules are in its trap, the moving charge
much more challenging: Their vibrational induces a tiny current, called an
and rotational degrees of freedom create image current, in the trap elec-
a multitude of low-lying quantum states trodes. The current is allowed
that can disrupt a cooling cycle. And to flow into an external circuit,
fundamental particles such as protons, where its energy is dissipated,
which lack internal states altogether, can’t and the proton energy steadily
be laser cooled at all. decreases.
Nevertheless, there’s a lot of interest The energy flow, however,
in experimenting on fundamental parti- goes both ways. Thermal noise
cles at low temperature. Toward that end, in the circuit creates voltage
researchers from the Baryon Antibaryon fluctuations in the trap electrodes; the FIGURE 1. MATTHEW BOHMAN examines
Symmetry Experiment (BASE) collabo- fluctuating electric potential jostles and the core of the BASE collaboration’s new
ration have now demonstrated a method warms the proton. The circuit is cooled cooling apparatus. The central gold-colored
cylinder contains two electromagnetic
for using a cloud of laser-cooled ions to with liquid helium to 4 K, so it might seem
Penning traps: one to hold laser-cooled
cool a single proton, even when the pro- that the researchers can’t resistively cool
beryllium atoms, the other to hold a single
ton and ions are too far apart to directly their proton any further than that.
proton. (Photo by Stefan F. Sämmer,
interact.1 Figure 1 shows team member But they can—using a trick that capi-
Johannes Gutenberg University Mainz.)
Matthew Bohman in the lab at Johannes talizes on the definition of temperature
Gutenberg University Mainz in Germany. and the fact that the trap contains only
By coupling both the proton and the ions one proton. An ensemble of particles at a than its surroundings, just by connecting
to an inductor–capacitor (LC) circuit, given temperature is spread among states and disconnecting the trap enough times
they transport the effect of the laser cool- of different energies, according to the until the proton is caught in a low-energy
ing from one location to the other. Boltzmann distribution. So a single par- state. The process doesn’t take long—
ticle in a thermalized system hops among about an hour. But a full g-factor mea-
Try, try again states, and at any given instant its energy surement campaign requires 1000 cold-
BASE, as the full name suggests, seeks to might be higher or lower. When the BASE proton measurements, which means that
compare the properties of protons with researchers abruptly disconnect their a solid month or more of experiment
those of antiprotons to test how exact trap from the circuit, they lock the proton time might be spent just waiting for the
the symmetry between matter and anti- into whatever state it is in at the time. particle to find its way into a cold enough
matter really is. In particular, the goal That state, with a few percent chance, state.
is to measure the magnetic moments, or could be low enough in energy for a
g factors, of each particle by observing the g-factor measurement. Cooling from a distance
spin flips of a single particle held in an It’s possible, then, to “cool” the pro- To speed things up, the BASE team
electromagnetic Penning trap. Team mem- ton to a significantly lower temperature turned to an idea proposed 30 years ago
14 PHYSICS TODAY | NOVEMBER 2021pt_search1121_search 10/18/2021 10:09 AM Page 15
by David Wineland and Daniel Heinzen: both take bites out of the circuit-noise
using laser-cooled ions in one trap to bump as a sharp spike and a slightly
sympathetically cool a proton in another wider dip, respectively.
trap.2 The trap electrodes are wired to- The researchers use the spectrum to
gether, and as before, energy is ex- verify that the resonant frequencies all
changed via the induced image currents: match. They also use it to track the pro-
from the proton to the trap electrodes to ton’s temperature. Typically, in experi-
the ions, where it’s promptly laser cooled ments on trapped particles, it’s desirable
away. for the trap potential to be as close to per-
It’s essential that the laser-cooled par- fectly harmonic as possible. Then, the
ticles be ions, not atoms: Only charged particles oscillate at the same frequency
particles can be confined in a Penning trap no matter their energy. But here, the re-
and couple to the image currents. The re- searchers deliberately made the proton
searchers chose singly charged beryllium trap slightly anharmonic. As the proton
ions, which have the same electronic cools, its frequency shifts.
configuration as neutral lithium atoms The frequency-spectrum measure-
and are thus among the most easily laser ments revealed that the proton was cooled
cooled of all species. A single laser can to 2.6 K. Relative to the 100–500 mK
cool a cloud of Be+ ions to 0.5 mK, so in needed for a g-factor measurement, that
principle, the proton could reach a tem- number might seem unimpressive. But
perature almost as low. the fact that it’s more than 80% lower
The proton cooling is still slow. For than the LC circuit temperature of 15 K
particles in the two traps to exchange en- is an encouraging demonstration that
ergy, they need to oscillate in their traps image-current cooling is possible.
at identical frequency. The oscillation fre- The LC circuit was cooled with liquid
quency depends on each trap’s electric helium at 4 K; its temperature was so
potential, which is hard to keep perfectly much higher than that because it was
stable. The laser-cooled Be+ ions might heated by the amplifier that was used
therefore be cooling the proton for only to read out the noise spectrum. Under
a fraction of the time that the traps are the best of circumstances, that heating
connected. is limited to a couple of degrees. But as
The innovation in the new work was an amplifier ages, the heating problem
to also wire in the LC circuit, as shown in worsens.
figure 2a. When the circuit’s resonant fre- “It just happened that by the time we
quency matches the frequency of both got everything else working, the ampli-
traps, the current—and thus the rate of fier was old,” says Smorra. “That’s exper-
energy exchange—is amplified by the imental physics for you.” Swapping out
circuit’s quality factor Q. The BASE col- the amplifier for a new one will reduce the
laboration uses high Q-factor circuits for whole system’s temperature by a factor
its other experiments, and its researchers of 3—not quite low enough for a g-factor
are good at making them. The Q factor of measurement.
the circuit used in the proton-cooling ex- The BASE researchers expect that the
periment, 15 000, is typical of their work. essential improvement could come from
The drawback is that, once again, the tinkering with the trap frequencies. In the
thermal noise in the circuit leaks into the proof-of-principle experiment, both traps
traps and heats the particles. Still, because were tuned to the peak of the LC circuit’s
the laser-cooled Be+ ions are continu- resonance. That arrangement maximizes
ously cooling the proton at the same time the rate of energy exchange between the
as the circuit is heating it, the system can two traps, but it also maximizes the rate
settle into a steady state with the proton of heating from the circuit; detuning the
much colder than the circuit. frequencies slightly would decrease both
rates. Detuning too much, of course, will
Resonance upon resonance negate the advantage of using the circuit
To monitor the system, the researchers in the first place. But in between, there’s
read out the circuit’s noise spectrum, as a range where the sympathetic-cooling
shown in figure 2b. The widest bump is rate should have enough of an advan-
the whole LC circuit resonance—although tage to push the proton to the desired
it looks broad in the figure, it is needle- temperature.
thin relative to the resonant frequency of “Once we optimize the setup, 100 mK
479 kHz. The proton and Be+ resonances certainly seems possible,” says Bohman,
NOVEMBER 2021 | PHYSICS TODAY 15pt_search1121_search 10/14/2021 2:13 PM Page 16
SEARCH & DISCOVERY
a Laser-cooled Be+ ions Single proton Cryogenic LC circuit
313 nm
cooling laser
5 mm 9 mm
Be+ trap 9 cm Proton trap
b Circuit resonance
Beryllium
−110 ions
AMPLITUDE (dBV) −115
−120 Proton
Δν
−125
−100 −50 0 50 100
FREQUENCY ν − ν0 (Hz)
FIGURE 2. AN INDUCTOR–CAPACITOR (LC) circuit (a) amplifies the rate of energy transfer
between a pair of Penning traps 9 cm apart, so the laser-cooled beryllium ions in the
first trap can sympathetically cool the proton in the second trap in just seconds. (b) The
circuit’s noise spectrum contains a bump at the circuit resonant frequency ν0 = 479 kHz,
with nested dips marking the Be+ and proton resonances. The proton trap is slightly
anharmonic, so as the proton cools, its resonant frequency shifts. (Adapted from ref. 1.)
“and even below that is maybe some- be to reverse the antiproton trap’s electric
thing we can do.” potential to account for the particle’s
negative charge.
Beyond protons But the possibilities don’t end there.
An LC circuit isn’t the only possible way Molecules of two or more atoms, when
to use one set of trapped particles to cool cooled into the quantum regime, offer a
another. The most straightforward ap- range of ways to test fundamental phys-
proach is to put both sets of particles in ical laws (see the article by Dave DeMille,
the same trap, where they can directly PhySiCS ToDAy, December 2015, page 34).
exchange energy and approach a com- Molecules are challenging to laser cool,
mon thermal equilibrium. Another pos- but molecular ions could lend themselves
sibility other researchers are exploring is to cooling through the LC circuit.
to keep the particles in separate traps but Another possible target is highly
bring those traps close enough together charged ions, of interest for testing the
that the particles can exchange energy theory of quantum electrodynamics (see
through their Coulomb interactions. PhySiCS ToDAy, December 2012, page 22).
The big advantage of the circuit- Most highly charged ions lack a conven-
mediated approach, though, is its adapt- ient laser-cooling transition. But, Smorra
ability. Because the particles in their re- predicts, they should be easy to cool with
spective traps never come close together, the circuit-mediated method. “in fact, it
it doesn’t matter if they attract or repel, would be even easier,” he says, “because
chemically react, or even annihilate each the image current, and therefore the in-
other. teraction strength, is enhanced by a fac-
The flexibility bodes well for the BASE tor of the charge.”
collaboration’s experiments of interest, Johanna Miller
in which they hope to cool antiprotons
just as easily as they cool protons. The
antiproton setup could use the same pos-
References
1. M. Bohman et al., Nature 596, 514 (2021).
itively charged Be ions as in the proton 2. D. J. heinzen, D. J. Wineland, Phys. Rev. A
setup; the only necessary change would 42, 2977 (1990).
16 PHYSICS TODAY | NOVEMBER 2021pt_search1121_search 10/14/2021 4:57 PM Page 17
A new undersea volcano is born east of Africa
Geophysical and
oceanographic
observations establish
how a deep magma
chamber fueled an
extraordinary eruption.
T
he largest submarine volcanic erup-
tion ever recorded began on 10 May
2018 off the eastern shore of Mayotte,
one of the Comoros group of islands east
of Mozambique and north of Madagas-
car. Then and there, a highly viscous and
ductile volume of molten rock from the
asthenosphere—Earth’s upper mantle—
pierced the cooler and brittler litho-
sphere above it and erupted lava onto
the seafloor.
That day, people in Mayotte felt a
magnitude 4.3 earthquake, the first of
many. The largest event, of magnitude
5.9, struck several days later, on 15 May.
Over the next few weeks, the moving
magma generated a few very low fre-
quency earthquakes in the crust and thou-
sands of deeper ones.1 The result of all
FIGURE 1. JÉRÉMY GOMEZ of the Paris Institute of Earth Physics (IPGP) helps retrieve
that geophysical activity was a new moun-
an ocean-bottom seismometer that was deployed off the eastern coast of Mayotte, an
tain on the seafloor. (To learn about other island north of Madagascar. He and several other scientists participated in a May 2019
submarine volcanoes, see PhysiCs TOday, research cruise to collect geochemical, seismological, and seafloor topographic data
august 2012, page 16.) after the region began experiencing volcanism and earthquakes in May 2018. (Courtesy
as the chief scientist of the May 2019 of Eric Jacques, IPGP.)
research cruise MayOBs 1, Nathalie
Feuillet of the Paris institute of Earth
Physics (iPGP) and the University of over the past 30 years geologists had cat- 2019.) By the end of 2018, the project
Paris led an effort to collect seismic and aloged just two small earthquakes near was finalized, and Feuillet and some col-
surface deformation data of the volcanic Mayotte, according to an earthquake leagues traveled to Mayotte in February
eruption and the ongoing earthquakes. database maintained by the Us Geologi- 2019 to deploy the OBss.
(Mayotte is an overseas department of cal survey. The BRGM recorded the ini- The discovery of the volcano came in
France.) she and her colleagues—from tial seismic activity associated with the May 2019 when the MayOBs 1 research
institutions including the French geolog- 2018 eruption using a single seismic sta- cruise recovered the OBss deployed in
ical survey BRGM, the CNRs, and the tion on Mayotte. With their sparse obser- February; one of the devices is shown in
French Research institute for Exploita- vations, the BRGM suspected the earth- figure 1. The researchers on the ship used
tion of the sea—found that the new un- quakes originated somewhere in the a multibeam echo sounder to bounce
dersea volcano now stands 820 m tall and ocean east of Mayotte but couldn’t pin- sound waves off the ocean floor across an
lies at the end of a 50 km ridge formed point the exact source of the tremors. area of 8600 km2, slightly smaller than
by a series of recent lava flows.2 The ge- shortly thereafter, Feuillet and other the size of Puerto Rico, to determine
ological feature is likely part of a tectonic volcanologists and seismologists made the seafloor elevation. an instrument
structure formed by fissures and faults plans to study the region more closely. deployed to more than 3000 m below
associated with the East african Rift to among other activities, they installed the ocean surface looked for some trace
the west. ocean-bottom seismometers (OBss), in- of volcanic activity by determining the
struments capable of picking up under- seawater’s conductivity, temperature, and
Seafloor anomaly sea earthquake activity. (For more on chemical composition as a function of
The occurrence of the 2018 eruption was those devices, see “deploying seismom- depth. absolute pressure gauges attached
unusual. No volcanic activity had ever eters where they’re needed most: Under- to the OBss measured the vertical defor-
been reported in the area before, and water,” PhysiCs TOday online, 24 May mation of the seafloor.
NOVEMBER 2021 | PHYSICS TODAY 17pt_search1121_search 10/14/2021 2:13 PM Page 18
SEARCH & DISCOVERY
“One evening we saw a big
anomaly on a polar echogram of the
water column,” says Feuillet, recall-
ing the cruise. “It was a 2000-meter-
high acoustic plume.” East of Ma-
yotte, the source of the anomaly
turned out to be a mixture of solid
particles, liquid droplets, and bub-
bles. The jet of materials had the tell-
tale characteristics of volcanism:
highly turbid, alkaline water and
elevated concentrations of molecu-
lar hydrogen, methane, and carbon
dioxide. “It was one of the biggest
acoustic plumes ever detected in the
water column,” says Feuillet. The
volcano, which had formed 10
months earlier, was erupting again.
A fuller picture of the volcano
and its surroundings emerged when
the seafloor topography data re-
vealed the Mayotte volcanic ridge.
A previous research cruise, led by
the French Naval Hydrographic and FIGURE 2. A NEW VOLCANO formed east of the East African Rift off the eastern shore
Oceanographic Service, had fortuitously of Mayotte after lava erupted through the seafloor in mid 2018. (a) This elevation map
mapped the same stretch of seafloor in from measurements collected during a 2014 oceanographic research cruise, shows a
2014. Figure 2a shows what was once relatively flat seafloor topography. (b) More recent data from a 2019 cruise and seismic
the relatively flat seafloor topography; stations (black triangles in inset) reveal the formation of an undersea volcanic ridge.
figure 2b, the new undersea mountain (Adapted from ref. 2; inset from ref. 3.)
and ridge.
Earth’s interior is rare. Many seismic As the region stretches, the litho-
Seismic swarm events caused by volcanic activity occur sphere is susceptible to fracturing, which
To learn more about the new volcano, in the brittler crust rather than the provides more pathways for the magma
which spewed about 5 km3 of lava dur- deeper, more ductile mantle. At Mayotte, to reach the surface. Once the magma
ing the 2018 eruption, coauthors Wayne the crust descends about 17 km below travels through the weakened crust and
Crawford and Jean-Marie Saurel of IPGP the surface. Below that brittle–ductile reaches the seafloor, it can instigate
and the University of Paris and other transition zone, molten rock more easily swarms of deep earthquakes. That inter-
seismologists on the team analyzed the deforms and is therefore less likely to pretation is supported by another re-
seismic-wave data collected by a net- crack and instigate tremors in response cently published paper by Océane Foix,
work of seismometers on land and on the to seismic energy. Feuillet, and their colleagues. They used
seafloor; most of them were installed or Still, Feuillet and her colleagues sus- a tomographic method to construct a more
deployed after the 2018 eruption. pect that magma activity may have detailed picture of the new volcano’s
From 25 February to 6 May 2019, the caused the deep earthquakes that were plumbing.3
network detected some 17 000 earth- observed below Mayotte. Most of them
quakes, 94% of which cluster on the were clustered beneath a caldera struc- Early warning
western segment of the Mayotte ridge ture, a large, low topographical region The new volcano is 50 km east of Ma-
and 25–50 km below the seafloor. An ad- that formed when an ancient volcano yotte. Feuillet and her colleagues sus-
ditional 84 earthquakes were identified by erupted and collapsed. The many faults pect that the main magma reservoir is
IPGP and University of Paris coauthors and fissures of the caldera form channels 5–10 km east of the island and about 70 km
Claudio Satriano, Angèle Laurent, and through which the magma could have below the seafloor. Another eruption from
Pascal Bernard as very low frequency easily navigated from a deep upper- the reservoir, if it’s closer to the island,
events lasting up to about 30 minutes, mantle reservoir to the seafloor. could be more dangerous than the last
with seismic-wave energy detected below The researchers infer that the Comoros one. The next goal is to develop a warn-
0.10 Hz. The very low frequency earth- archipelago, with the African continent ing system that would alert everyone in
quakes can be generated by a seismic to the west, is part of a tectonic zone the region, especially Mayotte’s 270 000
source that’s been repeatedly excited, pos- where the crust is pulling and sliding residents, of a future eruption as early as
sibly faults destabilized by magma from apart. Some earthquakes could be the re- possible.
the upper mantle that pressurize a large sult of the transfer of tectonic deforma- On land, GPS instruments and seis-
deep reservoir. tion from the East African Rift to an area mometers are collecting real-time data.
To have so many earthquakes deep in of Madagascar with rifting in the crust. But a permanent underwater observa-
18 PHYSICS TODAY | NOVEMBER 2021You can also read
