Quanta Physics World  October 2020

Seen and heard

Weird and wonderful stories from the world of physics

Lunar dust buster

(Nasa)

Any astronauts thinking of living on the Moon won’t only have to contend with an arduous journey, weird food and space suits. They’ll also face the problem of lunar dust, which is sticky because it acquires electric charge when bombarded by radiation from the Sun. The dust will cling to their spacesuits, solar panels and other equipment – and any lunar colonizers won’t just be able to brush it off with a large feather duster. Xu Wang, a research associate in the Laboratory for Atmospheric and Space Physics at Colorado University Boulder, and colleagues have now developed a new solution in the form of an electron gun. Firing electrons from their device at the dust particles will give them even more charge, causing the particles to repel each other and disperse. The team has already tested its lunar “dust buster” in a vacuum chamber using dust particles similar to those found on the Moon. “It literally jumps off,” says Benjamin Farr, who completed the work as an undergraduate physics student at Boulder. The researchers admit that their technology isn’t ready for lunar spring cleaning quite yet, but hope that it may become a fixture on a future Moon base.

  • Wang and Farr discuss their lunar dust buster on the 10 September Physics World Weekly podcast (bit.ly/3hmRwyF)

Borderline collider

In 1977 the Nobel-prize-winning physicist Leon Lederman published a tongue-in-cheek proposal to build a collider using existing subway tunnels in New York. The city was then in financial crisis and Lederman, who died in 2018, reckoned that physicists could acquire the tunnels for a knock-down price. It never happened, of course, but his proposal has now inspired Caltech physicist David Hitlin to propose building another collider to address the politically controversial wall along the US–Mexican border. In a preprint paper (arXiv: 2009.01023), Hitlin describes how long, straight sections of the border between the states of Sonora and Arizona could be blocked by a huge linear particle collider. Stretching some 300 km long, he reckons the machine could achieve a centre-of-mass energy of 5 TeV. The proposed International Linear Collider (ILC) in Japan, in contrast, would be a mere 20 km long and have an initial energy of just 250 GeV. “A few of my colleagues have been joking about this,” says accelerator physicist Ryan Bodenstein from the Institute for Advanced Nuclear System in Möl, Belgium. “We never expected anyone to write it down.” And what would Hitlin call the facility if it was built? The TrumpILC, of course.

Masked debate

It’s a debate that has been raging since the COVID-19 pandemic began – what mask is best for curbing the spread of COVID-19? Researchers in Japan have used Fugaku – the world’s most powerful supercomputer – to compare the efficacy of disposable, medical-style masks with reusable masks made from woven fabrics, specifically cotton and polyester. The calculations suggest that disposable masks, which are made from a non-woven polypropylene fabric, stop more cough droplets than woven masks. The disposable masks are particularly good at stopping smaller droplets the researchers say. As for the problem of people discarding their disposable masks on the street, not even a supercomputer can solve that.

 

(Timoteo Carletti)

Fair slices

What’s the fairest way to slice up a watermelon? Well, physicists in Belgium, France and Italy have now tackled the problem using geometry and calculus. After cutting the whole watermelon in half along its length and then in the middle to yield four equal quarters, the researchers discovered that this “half rule” fell away when then trying to slice the watermelon up into equal thin portions. Instead, they found a “2/3 rule”. So for a spheroid of length 10 cm, for two slices, the first slice should be made at 3.5 cm along the length but for three slices, the first slice should be 2.1 cm and the second 4.2 cm (arXiv:2009.02325). After doing the calculations, the researchers tested them on a 4 kg watermelon and used Archimedes’ principle to confirm that the slice volumes were equal. Eureka!