Veritasium/YouTube

These levitating droplets behave strangely like quantum particles

Is this what quantum mechanics looks like?

FIONA MACDONALD
4 NOV 2016
 

There's something strange about these droplets of silicon oil - and it's not just the way they're bouncing above the petri dish. They actually replicate a lot of the weird phenomena of quantum mechanics, but on a scale we can actually see. And this episode of Veritasium explains how that can help us wrap our minds around some of the stranger hypotheses surrounding quantum particles.

 

But to start with - why are these oil droplets bouncing in the first place? In the experiment above, Derek from Veritasium has set up a petri dish full of silicon oil on top of a speaker that he's using to vibrate the dish.

By creating droplets with a toothpick, he can cause them to hover along the surface, bouncing above a tiny layer of air between the oil and the droplet that never gets small enough for the oil to recombine.

Every time that droplet bounces, it creates a standing wave in the dish that oscillates up and down.

Not only does the droplet create the standing wave, it also interacts with it on its next bounce. And if it keeps landing on the same side of the wave, the droplet gets pushed forward, which scientists call a 'walker'.

But what's really interesting is how these droplets behave. They're way too big to be quantum - they're around 1 millimetre across - but scientists have recently discovered that they can use these little droplets to replicate many of the strange phenomena of quantum mechanics.

Take, for example, the classic double-slit experiment. In traditional quantum mechanics, the double-slit experiment involves firing a beam of particles, such as electrons, at two narrow slits. 

 

On the other side, rather than the electrons ending up in two distinct clumps behind the slits, as you'd expect, they produce an interference pattern - a pretty even spattering of electrons across area behind the slits.

That even happens when you send the electrons through one at a time, and it's just one of the many baffling phenomena in quantum mechanics. 

With these bouncing oil droplets, you can recreate the double-slit experiment, and watch as the standing wave (or pilot wave) travels through both slits, while the droplet itself only travels through one.

But the droplets don't always move in a straight line, because they're jostled around and guided by these standing waves. So you end up with the drops scattered on the other side in a very similar pattern to a quantum interference pattern.

(Side note: this is probably the best and simplest demonstration of the double-slit experiment we've ever seen.)

That's weird enough, but the droplets also display the same phenomenon of quantum tunnelling - where it's possible for a particle to get through a barrier that it wouldn't classically have enough energy to get over.

And if you look at a 'walker' oil droplet confined to a circular corral, such as a petri dish, and track its chaotic movement over time as it bounces off its standing waves, you can create a probability density that shows the likelihood of the droplet being found at any point within the petri dish at any one time.

It turns out, that pattern will look a lot like the probability density of electrons confined in a quantum corral. Which is pretty freaky, but definitely not a coincidence.

So why are these droplets so similar to quantum particles?

We'll let Derek explain that in the video above, but let's just say that observing these little bouncing droplets can help physicists (and the rest of us) wrap our tiny human minds around some of the competing hypotheses out there about quantum mechanics, such as the Copenhagen interpretation and pilot wave theory.

Check it out and see which you think makes more sense - or just marvel at strange bouncing oil droplets that seem to defy physics. You do you.

And if you want to find out more about how oil droplets - and even water - levitate in the first place, check out the incredible episode of Smarter Every Day below... he even makes them happen in space:

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