The most popular Physics Today articles of 2019

It’s not every day that Physics Today goes viral. The trick, apparently, is trying to convince people that Mercury is closer to Earth than Venus is. Here are the 10 stories that drew the most people to Physics Today‘s website in 2019.

1. Venus is not Earth’s closest neighbor (published online 12 March)

During a road trip, Tom Stockman started listening to a radio station named Venus. Its tagline: “Earth’s closest neighbor.” As an engineering PhD student based in the NASA hotbed of Huntsville, Alabama, Stockman wondered whether that slogan was scientifically accurate. His investigation culminated in an article, authored with Gabriel Monroe and Samuel Cordner, that evaluated the distances between Earth and each of the other planets. Although Venus comes closer to Earth than does any other planet, Mercury is our nearest neighbor most frequently and when averaged over time. (See which planet is currently closest .) Through the mathematical method they developed, the three authors found that not only is Mercury the closest neighbor to Earth, but it’s also nearest, on average, to all seven of its solar-system siblings.

The article shot up in popularity once it was upvoted on social media sites and chronicled by outlets including Gizmodo , Popular Mechanics , and even the UK tabloid Daily Mirror . In fact, 13 March was the single highest day of traffic in the 20-year history of Physics Today‘s website. Many readers seemed to enjoy the surprising yet intuitive argument; others expressed displeasure with some of the semantics (“When people say closest planet, they do mean at closest approach, not average,” wrote one commenter). For more information, check out these two videos from YouTube educator CGP Grey about identifying the “mostest closest planet.”

2. Tracking the journey of a uranium cube (May issue, page 36)

The cover story of Physics Today‘s May issue included Hitler, Heisenberg, and a missing haul of uranium that was supposed to fuel a nuclear reactor for Nazi Germany. The intrigue began in 2013 when University of Maryland physicist Timothy Koeth received a small, heavy cube along with the message “Taken from the reactor that Hitler tried to build. Gift of Ninninger.” Since then Koeth and colleague Miriam Hiebert have investigated the object—a five-pound block of uranium—and its origins. They learned that the cube was one of 664 produced by three groups of nuclear physicists in Hitler’s Germany, including one led by Werner Heisenberg. The plan was to develop a self-sustaining nuclear reactor—and the plan might have succeeded had the three groups better pooled their resources.

After the war, American officials with the Alsos mission recovered some of the cubes, which the German researchers had hastily buried in a field, and brought them back to the US. Today the whereabouts of most of those cubes is a mystery. Physics Today editors consulted with Koeth and Hiebert to track down some of them . If you happen to rummage through your attic and find an unusually heavy cube measuring two inches on a side, consider giving Koeth and Hiebert a call.

For more on the cubes and their history, listen to NPR’s coverage (a radio segment and a podcast episode ) of our article. And if you enjoy unsolved atomic-age mysteries, make sure you read about the time physicist John Wheeler misplaced secret plans for the hydrogen bomb on a train.

3. The mysterious crystal that melts at two different temperatures (published online 6 June)

Unlike the current location of the Nazi-era uranium, the mystery of the peculiar solid that melts at two different temperatures has been solved. Science writer David Adam tells the story of acetaldehyde phenylhydrazone (APH), the solid form of which was found in the late 19th century to sometimes melt at around 65 °C, other times at 100 °C. After finding the mostly forgotten 1896 study in a German-language journal, chemist Terry Threlfall of the University of Southampton in the UK set out to determine how multiple samples of the same crystalline solid could melt at different temperatures. A decade-long investigation with multiple collaborators culminated in the discovery of the mechanism behind APH’s quirky behavior.

4. Author Q&A: Audra Wolfe on the Cold War’s scientific and political legacy (published online 25 January)

In her recent book, Freedom’s Laboratory: The Cold War Struggle for the Soul of Science , historian Audra Wolfe argues that the Cold War was partly responsible for today’s widespread view that science is inherently apolitical. She talked to Physics Today about US government propaganda surrounding science during those years and its enduring impact. Wolfe said that claiming science completely transcends politics actually increases distrust of science, “because the claim is so obviously not true.”

For another take on how the science enterprise has changed since the Cold War, read Sarah Scoles’s report on what it’s like to work as a national security scientist at Los Alamos National Laboratory. The job offers the chance to pursue interdisciplinary research and to take up tangential projects, she wrote. “But that kind of science, and the secrecy around it, can get complicated.”

5. When condensed-matter physics became king (January issue, page 30)

Today condensed matter is by far the largest discipline in physics. That’s pretty remarkable considering it wasn’t recognized as a field until after World War II. In Physics Today‘s January 2019 issue, historian of science Joseph Martin walked readers through the early Cold War rise of solid-state physics, a wide-ranging field of study that encompassed pure and applied research on semiconductors, ferromagnets, and other solid materials. The switch to the broader term condensed-matter physics occurred in the 1960s, when, as Martin put it, “Critical phenomena such as phase transitions, nonlinear dynamics of fluid systems, and liquid-helium research that had little or nothing to do with solids took center stage.”

Despite the discipline’s prominence in terms of people and funding, it often gets overlooked in science media coverage. In a commentary late last year, condensed-matter physicist Douglas Natelson explored his field’s image problem and offered suggestions for making it more accessible to a broad audience.

6. Do you believe in rectangles? (published online 18 October)

Parallel lines are everywhere equidistant. The angles of a triangle add up to 180°. Rectangles exist. Those three statements are logically equivalent to the parallel postulate, which was introduced by Euclid of Alexandria in his historic textbook, the Elements. Although mathematicians have had 2300 years to work on the problem, no one has been able to confirm the postulate’s validity with a proof. Must we accept that rectangles may not exist? To address that question, Physics Today editor Johanna Miller explained what we’ve learned in more recent history. The thought-provoking column was a follow-up to her examination of a fundamental difference between mathematics and physics .

Miller’s columns, which debuted in January and appear once or twice a month, have explored climate change , writer’s block , graduate-school mentorship , and more. You can read all of them here .

7. Machine learning meets quantum physics (March issue, page 48)

Seemingly every scientific field is experimenting with artificial intelligence, and this year Physics Today reported on several examples from the physical sciences. The most popular of those articles explored the potential for advances in machine learning and quantum physics that benefit both fields. Recent successes in machine learning “raise new possibilities . . . to solve open problems in quantum physics,” Sankar Das Sarma, Dong-Ling Deng, and Lu-Ming Duan wrote in their March 2019 article. At the same time, “new quantum algorithms may offer tantalizing prospects to enhance machine learning itself.” Researchers studying phase transitions, neural networks, quantum entanglement, and quantum many-body problems could benefit from such a synergy. Physics Today also explored the use of artificial intelligence to aid weather forecasters and to improve the resolution of fluorescence images .

8. The confounding magnetic readings of Voyager 1 (published online 15 February)

In 2012 Voyager 1 became the first human-made object to enter interstellar space. The probe’s exit from the solar bubble was marked by several indicators, including cosmic-ray flux, but not the one most researchers had expected: a shift in the direction of the ambient magnetic field. Seven years later, Voyager 1 is still recording magnetic readings similar to those within the heliosphere, and space scientists are trying to figure out why. The research has important implications for understanding the interstellar environment and how it interacts with the solar wind. Since the article appeared, mission scientists have published magnetic measurements from Voyager 2 , which got its first taste of interstellar space last year.

9. Superconductivity is found in a nickel oxide (November issue, page 19)

Georg Bednorz and Alex Müller discovered cuprate superconductors in 1986, yet no consensus has yet emerged as to what causes their behavior. In a new study, researchers at SLAC and Stanford University synthesized neodymium strontium nickel oxide, a cuprate analogue, and found that it superconducts below 15 K. “The differences of detail” between nickel and cuprate compounds “should shed light on the origins of superconductivity,” Physics Today‘s Mark Wilson wrote.

Other popular superconducting coverage included a review of the high-pressure hydrides that superconduct at nearly room temperature and a report of a uranium compound that maintains a superconducting state even when exposed to strong magnetic fields.

10. A physics master’s degree opens doors to myriad careers (April issue, page 22)

People who secure a master’s degree in physics end up in industry, government, education, and not-for-profit sectors, Physics Today‘s Toni Feder reported in her exploration of several degree-granting programs. For students who don’t want to perform research in a lab or compete for the limited number of jobs in academia, a master’s can make more sense than a PhD. Others pursue a master’s because they want to leverage science knowledge for starting their own businesses.

Feder’s article is one of many this year that focus on careers and education in the physical sciences. In October, Physics Today published its first annual careers issue, which included articles about the career paths of PhD physicists and the Cal-Bridge program , which aims to improve representation and inclusion in STEM PhD departments.

Andrew Grant is Physics Today‘s online editor. He encourages readers to email story tips to pteditors@aip.org .