Hey guys I'm Jon and welcome to Respect
Your Intellect. In this video, we're gonna
be talking about what is in my opinion
the single most impressive experiment
ever conceived: the Double Slit
Experiment. It's not just impressive for
how was first derived but also for all
the variations that were done in later
years that all give us a glimpse of how
wonderfully weird the quantum world is.
But as weird as it is, the quantum world
forms the building blocks of our reality
so it's important to understand how
things work. The Double Slit Experiment
is an experiment that demonstrates how
particles can display characteristics of
both waves and particles at the same
time. This is called the wave-particle
duality. This experiment also illustrates
the fundamental nature of quantum
mechanics which is all about uncertainty
and probability. The first person to
conduct this experiment was Thomas Young
in 1801. He performed it with light and
was able to show that the theory at the
time of light being a wave was correct.
The Double Slit Experiment is sometimes
referred to as Young's experiment or
Young's slits, in his honor. This
experiment is so easy to perform that
anyone can make it at home with a laser
and a sheet of paper or aluminum. I'll
leave a link in the description of
step-by-step instructions, if you're
interested. Here's how the experiment is
set up. First we need a light source that
outputs coherent light for best results
so it's best to use a laser. You can also
do it with sunlight but the results are
not as well-defined due to the different
wavelengths that are present in sunlight.
Next we have a plate in the middle with
two very narrow slits in it which we'll
be shining the laser on. And lastly, we
have a screen at the end that will
capture or detect the light so that we
can see the resulting pattern. Now let's
start with a quick question. What's the
pattern that you would expect to see on
the screen if we shine the laser through
the two slits? Feel free to pause the
video here and think about it.
If you guessed that you would see two
lines of light on the screen that
corresponds to the slits themselves, then
you would have the same expectation as
most people do. This is mostly due to our
experiencing the world in classical
physics so we have a tendency to think
along those lines. But the reason why
you're watching this video is because
weirdly enough it is "not" two lines of
light corresponding to the slits that we
see on the end screen. What we see is a
pattern like this which initially seems
to go against logic "completely". How can
we get this pattern if there are only
two slits where light can pass through.
And what about if we block one of the
slits and only let light go through a
single one; what do we get then? Feel free
to pause here again and think about what
you would expect to see in this case.
Well if you guessed that you would see a
single line corresponding to the slit
then you would be "mostly" correct. We
actually see more intensity in the
center that corresponds to the slit but
it does fade away a little bit towards
the sides. This, at least, seems to be a
lot more in line with what we would
expect. So how is it possible to see this
pattern when there are two slits
available when there's no such pattern
with only one slit? Well this is due to
what we call diffraction and it's what
happens when a wave encounters an
obstacle; in our case, the walls around
the slits. Here's an example with water
where the large waves have to go through
a narrow entrance and diffract into nice
circular waves as they enter. It's worth
noting that if the slits are not narrow
enough you won't get nice circular waves
so we'll just assume for the rest of
this video that the slits are narrow and
that we get circular waves coming out of
the slits. In our experiment, the slits
will create two circular waves almost as
if they were a completely new source.
These two circular waves will start to
interfere with each other and cause
different peaks and troughs in the
pattern. When two peaks meet up they're
amplified and that's called "constructive"
interference. When a peak meets a trough
though the peak is weakened, and can even
get canceled out completely, and that's
called "destructive" interference. This
results in a series of alternating
strong and weak lines that form this
interference pattern on the screen at
the end. I'll link to a video from
Veritasium here that has one of the best
demonstrations of this interference
shown in a pond. Before we get to how
this pattern emerges when using light,
let's talk about what happens when we
send out single photons at a time
instead of a stream of them. If we send
only a single photon at a time through
the slits, what do you think will happen
now? We already know we were getting an
interference pattern with a constant
stream so maybe the particles are simply
interfering with each other to create
the interference pattern. But will you
still see the interference pattern on
the screen if you don't have multiple
photons to interact with each other or
will it go back to only seeing two lines
on the screen? Feel free to pause the
video here again and think about it.
Well the answer here is that even if
you send only one photon at a time and
accumulate the impacts on the screen at
the end over time you'll still see the
interference pattern emerge. Now this is
crazy isn't it? How can a single particle
create an interference pattern if it has
nothing else to interact with? Well the
answer to that is that while a single
particle is behaving like a wave, it's
actually able to interfere with itself.
So how are individual photons able to
create the interference pattern in this
Double Slit Experiment? Well let's start
by talking about what exactly happens on
the journey from the beginning to the
end. The first thing that happens is that
the photons are shot out towards the
slits. Since we haven't asked the photon
to be in a specific position yet by
performing an observation, it's
essentially everywhere at the same time
as a wave of probable locations. This
wave is not a physical thing but rather
a set of pure probabilities of where the
particle could be found "if" an
observation was made. Until we ask the
photon a question about where it is or
what path it took, it will continue to be
a wave and propagate like a wave in a
pond. As soon as we ask it any of those
two questions, it will immediately decide,
based on a probability of locations "on"
the wave, where it is and what path it
took to get there. This is what we call
the "collapse" of the wave function and it
happens for every observation regardless
of what the observation is. You can think
of an observation as any interaction at
all with the real world. Once the wave is
collapsed into a particle in a precise
location and we get its position, it will
return to a wave behavior from this new
source position after the observation. Ok
so that was a lot of information. So
let's try to get a better feel for how
the photons can interfere with
themselves with a nice little video. Here
we see single particles being shot out
and the wave is coherent before hitting
the slits. Since there hasn't been any
interaction yet with the real world, the
photon is still a wave when it reaches
the slits. This means that the wave is
actually going through both slits at the
same time. Once it's through the slits it
then diffracts and interferes with
itself, forming peaks and troughs. The
final step in this example is the end
screen which is the interaction with the
real world that counts as an observation
and collapses the wave
function. If you repeat this many times,
you eventually see the interference
pattern re-emerge, showing that the
photon was in fact a wave that went
through both slits at the same time. So
what happens when we add a detector to
perform an observation right before the
slits so that we know which slit the
particle is going through? This variation
of the experiment is called a "Which-way"
experiment. What we're essentially doing
here is that we're asking the particle
to choose its actual position so that we
can measure it. This interaction results
in the wave collapsing into a particle
and it can no longer go through both
slits at the same time. This means that
with simply having added an observation,
the interference pattern is destroyed
which shows that the photon did not go
through both slits at the same time. Now
if you leave the detector there and
quietly go unplug it to try and trick
the photons into thinking that they're
still being observed, what do you think
will happen? Feel free to pause the video
here again and think about it.
Well the answer is that since the
interaction between the particle and the
real world is removed, the particle can
continue to be a wave, go through both
slits simultaneously, and interfere with
itself. This restores the interference
pattern on the end screen. So if you
guessed that, you were correct. So
essentially just plugging and unplugging
the detector will change the pattern on
the end screen between the interference
pattern and the two lines corresponding
to the two slits. Is your mind blown yet?
But wait, there's even more. Physicists
are pretty clever so they've been trying
to figure out ways of measuring the
particles in different ways without
collapsing the wave function. One of these
variations of the experiment is by
changing the detectors position to be
after the slits and before the end
screen. This variation of the experiment
is called the "Delayed Choice" experiment.
By putting the detector after the slits,
the particle should be behaving like a
wave, going through both slits
simultaneously as we've seen before, and
have time to interfere with themselves
before the detector collapses the wave
function. Feel free to pause the video
here again and think about whether or
not you would see an interference
pattern on the end screen in this
version of the experiment.
Well the result here is that the interference pattern is
once again destroyed so if you guessed that,
you were right. What this Delayed Choice
experiment shows is that the particles
are somehow able to retroactively
choose which slit they went through; even
if there were still a wave when they
actually went through. This means that an
observation that collapses the
wave function doesn't just turn the wave
into a particle at that particular point,
it also determines the entire path that
was taken for the particle to end up
where it is; even if it wasn't yet a
particle at the time. I personally find
this simply "amazing".
There's also another variation of this
experiment called the "Quantum Eraser"
that is even more impressive but that
will have to wait to be done in a future
video dedicated to it. Now as if all this
wasn't weird enough, we've only been
talking about photons up to now but it
doesn't stop there. These experiments
were also later performed with electrons
and showed that even matter acted in the
exact same way. This was repeated with
particles of matter with higher and
higher masses until 2013 where we did it
with molecules comprised of 810 atoms of
over 10,000 atomic mass units. The
results showed that even if the matter
was a particle at the beginning where it
gets emitted, it still acts like a wave
until it hits the screen at the end or
an observation is made. This means that
all the matter we're made of is also
behaving as waves until it's necessary
for their locations to be determined.
Otherwise, they're occupying all the
possible points in their wave
simultaneously and interfering with
themselves and each other.
Oddly enough, this is also a technique
used in programming to avoid having to
compute everything if it's not necessary
to do so. So it's as if the universe is
trying to save resources by reducing
computations until it's absolutely
necessary to do so. Before we end, I want
to also address one of the most common
misconceptions about the collapse of the
wave function, which is that consciousness
is required to collapse it. This was
proven to be false when they stored the
information in atoms about which slit
the particles went through instead of
larger macroscopic devices. The result
was that the interference pattern was
still destroyed even though there was no
consciousness involved in the
measurement. You can read more about this
in a scientific paper called "Quantum
Mechanics Needs No Consciousness". I'll leave
a link in the description. The
Double Slit Experiment is one of the
best examples of why we need to use both
classical physics and quantum mechanics
at the same time. Classical physics is
all about smooth, orderly, and predictable
patterns while quantum mechanics is
completely unpredictable and random. The
collapse of the wave function, where a
wave of probabilities collapses into a
specific position, is only one example of
the fundamental unpredictability and
randomness of quantum mechanics. So what
do you think? Do you agree that this
experiment and its variations is one of
the most impressive you've ever heard
about? Let me know in the comments which
aspect of this experiment you liked the
most and how many of the questions you
answered correctly. If you liked this
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remember: Respect Your Intellect!