I'm using this speaker to vibrate a petri dish containing silicon oil

Now if I take this toothpick and make a little droplet on the surface

the droplet will stay there, hovering above the surface

The droplet is actually bouncing

and it will keep bouncing for a very long time

Now the reason for this is a little layer of air between the droplet and the surface

And the droplets bouncing so rapidly that that layer never shrinks to about 100 nanometers

Which is what it would take for the droplet to recombine with the oil

Now, every time the droplet lands on the surface, it creates a wave

But this is a special type of wave

Driven by the vibration of the oil bath

It is a standing wave

Meaning that it is not traveling out

It's just oscillating up and down

So the droplet makes the wave

And then it interacts with that wave on its next bounce

If the drop lands on one side of the wave, it is pushed forwards

And as long as the bounce of the droplet remains synchronized with the wave

It will keep landing on the front side of the wave getting pushed farther forwards

Droplets like these are known as "Walkers"

The bouncing oil drops has been known about since the 1970s

But only recently has it been discovered that you can use these little droplets

to replicate many of the strange phenomena of quantum mechanics

Now obviously this is not a quantum system, the droplets are about a millimeter in diameter

But you can think of the droplets like, uh, quantum particles, say electrons

One experiment that captures the key features of quantum mechanics is the Double-Slit Experiment

If you send a beam of electrons at two narrow slits

Well, the electrons, rather than behaving like particles and ending up in two clumps behind the slits

They produce an interference pattern

Even when you send each electron through one at a time

With Walking droplets, the pilot wave goes through both slits

Interfering with itself, while the droplet only goes through one slit

The droplet does move in a straight line though

It's deflected by its interaction with the wave

The resulting distribution of where the droplets end up

Looks very similar to quantum double-slit interference patterns

Or take tunneling

In quantum mechanics, it's possible for a particle to get through a barrier

that it wouldn't classically have enough energy to get over

This has been demonstrated with Walkers by

creating a shallow barrier under the surface of the oil

Usually the barrier reflects the pilot wave and its bouncing droplet

But in rare cases, the droplet does cross the boundary

And the probability of the droplet crossing the barrier

Decreases exponentially with increasing width of the barrier, just as in quantum tunneling

Perhaps the most surprising thing about these Walkers is they exhibit quantization, just like electrons bound to atoms

Here the Walker is confined to a circular corral

The droplet seems to move around randomly as it interacts with its pilot wave

The complex interaction between the droplet and the wave leads to chaotic motion of the droplet

But over time, a pattern builds up

This is the probability density of finding the droplet at any point within

the corral and it looks very similar to

the probability density of electrons

confined in a quantum corral

all of these similarities are no coincidence

the walking droplets actually create a

remarkable physical realization of a

theory proposed by de Broglie nearly a

hundred years ago in the early days of

quantum mechanics he postulated that all

particles have a wave that accompanies

them and guides their motion and that

wave is actually created by tiny

oscillations of the particle

Now this pilot wave theory was marginalized when

the standard Copenhagen interpretation

became widely adopted

the Copenhagen interpretation excludes anything that

cannot be directly observed and it says

everything that can be known about a

particle is contained in its so-called

"Wave Function" but adopting this view

forces you to give up on some common

sense notions like the idea that

particles have a definite position and

momentum even when they're not being measured

and it also meant that the

universe was

no longer deterministic

randomness is built into standard quantum mechanics

for example take the double-slit experiment

according to quantum mechanics the wave

function of the electron is a

superposition of the electron going

through one slit and the other slip simultaneously

using this wave function you can calculate the probability of

where the electron is likely to be and

then when you detected at the screen the

electron pops up at one point at random

that was in that distribution we say

that its wave function collapses

instantaneously at the moment of

measurement you can't say that the

electron was there before you measured

it and you can't even say that the

electron must have gone through one slit

or the other

compare that with the picture provided

by the bouncing droplets in this case

the pilot wave goes through both slits

but the droplet only goes through one

the droplet is pushed around by its

interaction with the wave so that the

resulting statistical distribution is

the same the droplet never exists in two

places at once and there's no randomness

if there is any uncertainty it's just

due to our ignorance of what's going on

it's not that it doesn't exist so pilot

wave dynamics can produce many of the

same results as quantum mechanics does

this mean that this is really what

quantum particles are doing

no but I think it'll at least suggest

that this is possible these are possible

dynamics that could lead to the

statistics which are captured in the

quantum mechanical theory and what's

appealing about this is it gives you a

clear idea of what's going on you don't

have to abandon the idea that the

universe is deterministic and you get

particles with definite position and momenta.

I think it's great that we have

two competing theories for the same

experiments and they both asked you to

accept odd things just different odd

things and it comes down to what you're

comfortable with really whether you

prefer the Copenhagen interpretation is standard quantum mechanics

or a pilot wave theory

let me know what you think in the

comments do you like the pilot waves I

mean it's definitely a very appealing

picture whether or not correspond to

reality that remains to be seen

Hey this episode of Veritasium was supported in part by viewers like you on Patreon

and by Google's Making & Science Initiative which seeks to

inspire people to learn more about

science and pursue their science goals

now I know someone else who is pursuing

their science goals this weekend that is

Destin over it Smarter Every Day he

and I were looking at basically the same

phenomenon but he was looking at water

droplets and why they don't coalesce so

if you want to see how that works and

how it works in space go check it out on

his channel over at Smarter Every Day

and as always thanks for watching

looking at only one frame per bounce you

can see how the droplets motion is

guided by the wave it's effectively

surfing on and the wave remains even if

the droplet disappears has happened

sometimes if it encounters a little bit

of dirt

what's really cool about this is the

wave actually stores information about

where the droplet has been.

This is because every time the droplet bounces

it creates a new circular wave centered

on its present location and that wave

adds to the existing wavefield on the

surface so as the droplet moves the

waves it makes keep adding up, storing

the information of where it's been

in fact you can actually get the droplet

to land on the backside of the wave so

now it's pushed backwards and it

retraces it steps erasing each way that

made previously one-at-a-time