Scientists can levitate stuff and make it fly around using sound

Japanese scientists have made hundreds of tiny plastic balls float around like miniature spaceships.

Today’s news from the world of Awesome Science comes from the University of Tokyo, where a team has been levitating and controlling objects using sound. Here’s the video:

As the video points out at the beginning, levitation of objects using sound has been around for a few years. If you’ve ever stood in front of a large speaker you’ll know that they can pump out what feels like quite a forceful blast of air as they vibrate - but, somewhat deceptively, that’s not quite the whole story.

Rather than physically push air out from the speaker, what you’re experiencing is a wave of compression moving through the air. The speaker compresses a packet of air, which then “rolls” through the room, with the size of the compressed air corresponding the wavelength of the sound wave. And, just like sound waves, waves that overlap each other create new waves.

To levitate something just requires creating a standing wave. Think of it like this - if you’re watching a sound wave plotted out on a graph, it’ll be rolling along, going up and down as it oscillates. A standing wave occurs when two or more waves combine to create a new wave where, as the wave oscillates, there are points where there’s no movement. They’re called nodes.

Here’s a gif to illustrate how that works. The blue and green waves are combining to create the red wave, which has those points on the central axis that aren’t moving:

(Image: Wikimedia Commons)

If a speaker outputs a standing wave, in the most basic sense it means that it won’t feel like the areas of compression - those blasts of air - are moving. The gaps between those blasts of air will be positions of neutral force, with air pressure pushing in on it from both directions. If you stick an object in there that’s light enough, and smaller that the size the gap (which will be the sound’s wavelength), the force of the air should keep it floating in a stable position.

What the Tokyo University team has done is build upon that idea, by combining sound waves in three dimensions. The video shows not just tiny little plastic balls being levitated and controlled, but also resistors, LEDs, screws, bolts, and other small items. Rhett Allain at Wired worked out that you could levitate anything both smaller than 8mm and less dense than 1,000kg/m3, which is tiny - but it does have practical applications, particularly when people are working with sterile things they want to move but can't touch, like spaceship parts or medicines.

Ian Steadman is a staff science and technology writer at the New Statesman. He is on Twitter as @iansteadman.

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How the Night Tube could give London’s mice that Friday feeling

London Underground’s smaller inhabitants might be affected by the off-squeak service – and learn when the weekend’s coming up.

What will the mice who live in the Tube network make of the new all-night service? Half a million of them are thought to have made the London Underground their home – and will be in for a surprise when the Victoria and Central lines keep running this weekend.

The Londonist is concerned the mice “are unlikely to get any sleep” with the new Night Tube, and may move to the District line instead. Yet a number of scientists point out to the New Statesman that mice are nocturnal creatures, most likely to sleep while the lights are on and the trains are running.

So will they get on board with the change – or make a run for different platforms on other lines?

The bad news:

“When the Tube’s away, the mice will play,” is how the rhyme (almost) goes. 

Many have come to know  and even love  the mischiefs of the mice who stream off the tracks and out of the tunnels as the stations close at night, in search of discarded morsels of Maccy D. And until now, they’ve had a good few hours to conduct such galavanting in peace. But the new system means they will have to re-structure their sleep and foraging cycles, or “circadian rhythms”. 

“The presence of night trains should upset several of these entrainment factors (or zeitgebers = time givers) leading to disturbances in their behaviours,” explains Professor Patrick Nolan, from MRC Harwell, an international centre for mouse genetics. 

“When you fly across the Atlantic, for example, it takes a few days to adapt, you feel a bit groggy, don't perform as well as you usually do, don't eat well, etc. You soon adapt to the change. But if there are constant disruptions like this, the effect may be more severe and long-lasting. And this is how the schedule changes in the Underground might affect the resident mice.” 

So it's the constant switching between the week and weekend schedules that could leave the mice  and Tube drivers  most cheesed off. Agoraphobia (fear of open spaces) and photophobia (sensitivity to light) are two possible effects of the resulting anxiety, and their mating patterns and liver functions are also likely to be disturbed.

The good news:

Yet it is unlikely mice will be leaving the Night Tubes for good. 

The more time we humans have to drop our dinners, the larger the menu becomes for the mice (researchers tell me that strawberry milk and Wheetos are particularly favoured fare).

“Mice are active most of the time – so more trains at night hours will not make such a difference to them,” say the RSPCA’s wildlife officers. “In fact, it may help as it may provide more foraging opportunities.”

They’ve also faced worse before. The London Transport Museum reminds us that, during the Second World War, cats were employed to counter vermin on the network (spot the cat in the 1940s TfL workers' canteen below).


Credit: London Transport Museum

For Dr Samuel Solomon at UCL, there is plenty to suggest the mice will successfully adapt. His study of mouse reflexes shows how they respond to various visual stimuli – and can start running within one-tenth of a second. “There might be cues they pick up – if people clean the station differently on Fridays, for instance.”

The tracks’ electric current may no longer be entirely switched off (if it ever was), but their whiskers’ sensitivity to vibrations could help them juggle their escapades to fit around the Night Tube’s less frequent service.

What Dr Soloman can’t yet predict is whether the mice will start to anticipate that Friday feeling: “It will be interesting to see whether they can learn that Friday is Friday”.

All in all, the Tube mice seem well set for the Night Tube’s new challenge. Who knows, they may soon gain the confidence of their 24/7 brothers in New York – and start ordering take-out...

India Bourke is the New Statesman's editorial assistant.