It’s time for our annual roundup of the most popular stories on Physics Today’s website. This year we’ve extended our roster to include not just the most-read pieces of 2021 but also the most shared (as measured by Altmetric score) and the most accessed from the Physics Today archives.

The most read

1. Lawrence Livermore claims a milestone in laser fusion (published online 17 August)

On 8 August researchers at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory came by far the closest yet to achieving their goal of getting a pellet of hydrogen fuel to release more energy than that of the lasers fired upon it. Their 177th attempt at ignition since 2009 released 1.3 megajoules, Physics Today’s David Kramer reported, which is nearly 70% of the laser energy. The achievement came nearly a decade after the initial deadline NIF officials had set for achieving the still-elusive goal of ignition. Reaching that goal is not a given- Three laser shots conducted in November failed to match the mark set over the summer.

2. A galactic fast radio burst finally reveals its origin (January issue, page 15)

After more than a decade of puzzling over short, intense pulses of radio waves, astronomers finally pinpointed the specific astronomical source of a fast radio burst. In January, Rachel Berkowitz chronicled the international effort that combined radio, x-ray, and gamma-ray observations to attribute a milliseconds-long burst to a magnetar within the Milky Way.

3. Analyzing Einstein’s handwriting (published online 26 August)

Inspired by the $1.2 million sale of a letter written by Albert Einstein, historian and Physics Today editor Ryan Dahn explored how and possibly why Einstein abruptly switched his handwriting from an old German script to Latin script. The switch, in his annus mirabilis year of 1905, serves as a microcosm of the famous physicist’s turbulent life.

4. What’s under the hood of a quantum computer? (published online 5 March)

How does a quantum computer work? Physics Today’s Christine Middleton tackled that complex question by breaking down the quantum computing stack—the components that lie between a user and the physical qubits.

5. Magnetic fields for modulating the nervous system (February issue, page 28)

Michael Christiansen and Polina Anikeeva’s February feature article focused on recent research to stimulate neurons with magnetic fields. Some of those efforts have produced controversy, which the authors explored head-on. ‘Work that garners the greatest attention is the kind that seeks to shift existing paradigms,’ the authors wrote, ‘so perhaps it should be unsurprising when controversy follows closely behind some of the most prominent claims.’

The most shared

1. Eunice Newton Foote’s nearly forgotten discovery (published online 23 August)

In 1856 the scientist, inventor, and women’s rights activist Eunice Newton Foote conducted an experiment that demonstrated the heat-absorbing properties of carbon dioxide. She correctly predicted that an increase of CO₂ in Earth’s atmosphere would lead to higher temperatures, and she did so three years before John Tyndall’s first paper on the greenhouse effect. Yet until very recently, Foote languished in obscurity. Maura Shapiro’s August story explored why both contemporaries and historians failed to recognize Foote’s accomplishments for so long.

2. Joanne Cohn and the email list that led to arXiv (published online 8 November)

‘Before there was arXiv,’ wrote Physics Today’s Toni Feder, ‘there was Joanne Cohn.’ Last month Feder told the story of Cohn, who started an informal email exchange of string theory manuscripts in the late 1980s. ‘I was motivated by a sense of community responsibility,’ Cohn said, and also by competitiveness- ‘There was this hunger, this interest in finding out what was going on and being able to use it and build on it.’ Openness and competition remain major driving forces of arXiv, which launched 30 years ago when Cohn’s colleague Paul Ginsparg converted her email-based service into a website where string theorists and other physicists could submit and share their preprints.

3. Why does biophysics attract a disproportionate number of women? (published online 7 June)

Biophysics is one of the few physics subfields that attract a disproportionate number of women. Feder spoke with several female biophysicists to explore why that is. Sarah Keller of the University of Washington put it most succinctly- ‘It is entirely because there are fewer assholes.’ Feder’s sources cited the chance to pursue research that spans multiple disciplines, the welcoming environment, and supportive mentors.

4. #BlackInPhysics Week 2021 essay series (published online 25–29 October)

For the second consecutive year, Physics Today and Physics World published a series of essays for #BlackInPhysics Week, an event dedicated to celebrating Black physicists and their contributions to the scientific community. This year’s essays focused on burnout, a critical topic for Black physicists confronting systemic racism both within and outside academia. ‘Burnout is not a personal issue,’ wrote Yale University physicist and dean Larry Gladney in his essay, ‘but rather a symptom of the humanity we lack in our hard science.’

5. Steven Weinberg obituary (published online 3 August)

The death of Nobel laureate Steven Weinberg in July inspired several of the theoretical physicist’s close colleagues to write remembrances. Contributors included Helen Quinn, Mark Raizen, and John Preskill. ‘Working with Steve taught me to think deeply and clearly to define the problem to be solved before launching into solutions,’ Quinn wrote. ‘He also gave me a sense of the history of physics, because, even then, he was conscious of it and cared about how the work we were doing would play a role in it.’ Raizen wrote the Weinberg obituary that appeared in the October issue.

The oldies but goodies

1. Helium shortage has ended, at least for now (published online June 2020)

In 2019 some scientists who work with superconducting magnets were forced to shut down their experiments due to a lack of helium. This year readers continued to reference David Kramer’s mid-2020 report on the rebounding helium supply. Kramer noted that despite reduced demand due to the pandemic, users were finding helium prices slow to drop from the highs they reached during the most recent shortage.

2. Tip of the iceberg (December 2019 issue, page 70)

In February glaciologist Megan Thompson-Munson tweeted a plea for scientists to draw icebergs accurately. Although the conventional wisdom that most of an iceberg’s volume lies underwater is true, she wrote, iceberg illustrations often depict impossible orientations. To set her followers straight, she pointed them to geophysicist Henry Pollack’s 2019 Quick Study about stable orientations of floating objects. Thompson-Munson’s tweet spurred not only new visitors to Physics Today but also the creation of a tool that allows users to see how an iceberg of arbitrary shape would float.

3. Venus is not Earth’s closest neighbor (published online March 2019)

As of this writing, Venus is about 51 million kilometers away, which is nearly as close as it, or any planet, ever gets to Earth. Over the past three years, some readers have been surprised to learn that despite Venus’s periodic close encounters, Mercury is the planet that is nearest to Earth both on average and most frequently. Tom Stockman, Gabriel Monroe, and Samuel Cordner laid out the evidence in a piece that has inspired articles, videos, and a fair share of passionate comments.

4. A more fundamental International System of Units (July 2014 issue, page 35)

The 2019 adoption of a revised International System of Units (SI), which included a new definition of the kilogram, attracted lots of media attention, including in Physics Today. Yet the gold-standard primer for the new SI continues to be NIST physicist David Newell’s 2014 article in which he explained how metrologists’ improved knowledge of fundamental constants had enabled novel ways to express physical measurements.

5. The formation of the heaviest elements (January 2018 issue, page 30)

Four years ago, astrophysicists Anna Frebel and Timothy Beers detailed fresh research regarding the r-process, which accounts for the cosmic production of about half of the isotopes heavier than iron. Analyses of the chemical composition of a faint Milky Way satellite galaxy and the electromagnetic and gravitational emissions of colliding neutron stars suggest that neutron-star mergers, not supernova explosions, are the primary means by which r-process elements are produced.