check this out I'm using this speaker to
vibrate a petri dish containing silicon
oil now if I take this toothpick and I
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 there's a
little layer of air between the droplet
and the surface and the droplets
bouncing so rapidly that that layer
never shrinks to about a hundred
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's 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'll keep landing on the front side of
the wave and getting pushed farther
forwards droplets like these are known
as rockers the bouncing oil drops have
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 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 doesn't
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 it's bouncing droplet but in
rare cases the droplet does cross the
boundary and the probability of the
droplet crossing the barrier decreases
exponentially with the 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 they're walking droplets
actually create a remarkable physical
realization of a theory proposed by
debroglie nearly a hundred years ago in
the early days of quantum mechanics he
postulated that all particles have a
wave that a company is 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 this 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
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 slit
simultaneously using this wave function
you can calculate the probability of
where the electron is likely to be and
then when you detect it at the screen
the electron pops up at one point at
random that was in that distribution we
say that its wavefunction 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 clearer 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 momentum I think it's great
that we have two competing theories for
the same experiments and they both ask
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
of 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 it
corresponds 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 in
science initiative which seeks to
inspire people to learn more about
science and pursue their science goals
now I know someone else who was pursuing
their science goals this week and that
is Destin over at 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 as happens
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 wave field 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
its steps erasing each wave it made
previously one at a time