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  • Check this out

  • 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