- How beautiful is this? This is an experiment in quantum mechanics called the double slit experiment but I really didn't expect to
see this when I did it myself. And seeing this changed my
mind on an important question in physics, what is light? But before all that, here's a simple experiment
that you can do. All you'll need is a laser
pointer and a strand of hair. So if I just take this
laser and I shine it at this whiteboard here, you can see that you just get a dot. But what do you reckon will happen if I put this hair in the way? - You might expect the dot
to get split into two dots or for there to be a
dark line through the dot or nothing at all to happen
since the hair is so small. But let me show you what
actually happens in the dark because it looks really impressive. Surprising, right? And it's nothing to do
with the hair itself. You can do the exact same
experiment with anything thin. This is a variation on
the double slit experiment which is an experiment
that's so important, you learn about it of pretty much day one of any quantum 101 class. The normal way to do the
experiment is to shine a light on two very narrow slits. Then the light that goes through, goes through the left or to the right. This variation is pretty much the same. The light goes through
one way or the other. But besides being easier to do this way, the other advantage is
the pattern you get. You can see that they both
have the same sort of flavor but they're different. For example, the dashes of
light for the hair version are all evenly spaced
and get lighter as we go but the dots of the double
slit are much more patchy. That's because of something
called single slit interference which is basically a
distraction in this scenario. So yeah, I kinda prefer the hair version. Plus check this out. I'm using this smoke in a can
stuff to see the actual beam but look really closely at the beam and you can actually see
where the hair splits the light into two. How crazy is that? And looking at this, I think
it's really hard to understand why you get the crazy double slip pattern when it seems like obviously
you should just get a dot with a line through it. So I've done my PhD in quantum stuff and I thought I understood
the theory of this experiment really well and so my plan
was just to explain it to you in a cute little video about how to do the experiment at home. Instead, I got sucked into an odyssey for the last few months where
I did all kinds of variations on the experiment and I
ended up like questioning pretty much everything I
thought I knew about light. It turns out this experiment
is just way more interesting than I thought. There's always been this debate in physics about whether light is made
out of a stream of particles or light is some sort of
excitation like a wave. The double slit experiment
was meant to definitively prove that light is a wave
except later experiments seem to show that light has particle light properties as well. I used to hate this
whole wave particle thing because light is clearly neither. A great analogy I heard for it is that light is like a platypus, it doesn't make sense to
say that it's both a duck and a beaver at the same time
because it is its own thing. But actually doing this
experiment finally made me see the value in having good analogies to help you think about light. And in particular, it really
sold me on thinking about light in terms of waves. It also made me see how
misleading the particle analogy can be if you're not
super careful about it. I think many of us imagine
photons which are what physicists now call particles of
light in a similar way to how Newton thought of light particles. But the double slit experiment shows why Newton's particle view
is definitely wrong. Newton imagined light in the
same way we now think of water. Inside it's just made of a
stream of light particles in the same way that water is
made out of water particles. I'm gonna call these particles that Newton thought light was made out of localized particles because
at every point in time they're just in one spot in space. They're not in multiple places or very spread out or anything like that. If you turn down the laser enough, Newton would've expected
that light comes out in these individual particles. Instead, people like Thomas
Young imagine light coming out as some sort of wave. In this case, no matter how
much you turn down the laser, the light would still be a wave, it would just be less and less wavy. The light travels forward not in lines but as peaks of a wave
progressing forward. In the case of just
turning on a flashlight, both Newton's picture and
Thomas Young's wave picture make exactly the same predictions. But Newton's view just feels right like you have these particles just traveling in straight
lines which is so much simpler than this strange abstract wave thing which is why it's so tempting
to think of light this way. In fact, I've seen
pictures just like this one to illustrate photons
exiting a light source in many reputable textbooks and I get why, that's kind of how I imagine photons too. So how did Thomas Young
go about disproving it? He needed to find an experiment
that could only be explained with waves and not with
Newton's particles. Well, one unique thing that waves do is that they bend and spread out. You may have seen this
before but for me at least, it was kinda surprising. Let me show you with this high
tech ripple tank I made here. You can see that when I bob
this plastic in the water, it makes fairly straight waves. But what do you expect will happen when I put this obstruction in the way? I would've guessed that
the bit of the wave that makes it past just
goes straight ahead and that's what the
majority of the wave does do but the rest of it bends and spreads out. You can see that in action
here but why does it happen? I don't really know. I would love to understand it better and make a video about it but until then, you can see that this is
something waves actually do. They spread out when
they meet an obstacle. That definitely feels a lot different from how I'd expect particles to behave. So why don't we do a similar
experiment with light and see what happens? Will light bend and spread
out or will it go straight? I'm gonna use a piece of paper as the obstruction to find out. And there you go. The laser light does actually spread out. Case closed, light is a wave, right? Well, maybe we should give
Newton a chance to rebut this. I guess he could try and
explain this with particles. Perhaps the particles are
colliding into the paper and then ricocheting and that's why they spread out like this. Okay, fine. Maybe you can explain that. But there is another wave phenomenon that particles definitely can't mimic and that's canceling out. One way to think of the last experiment is that it's like we
took the hair experiment and then just blocked one path. We found that at least
some of the light ends up in all of these places
including in this spot. If we had blocked the right path instead, we'd have gotten a very similar result. And again, notice that at
least some of the light falls on this spot. Now that's open both paths simultaneously and the light just
disappears from this spot. There is no way for nu to explain this. Think about it. If you believe in the kind
of particles that Newton did, then when just the left path is open, some of your particles
will reach that spot and when the right path is open, some of your particles
will reach that spot. But when both of them are
open, it's like the particles from the right side and the
particles from the left side just happened to cancel
each other out in this spot which just doesn't make sense. But why not? Why can't you just have
the particles cancel? Well, because in this
spot they need to cancel but just a little bit over, the particles coming from
the left and from the right should happily coincide. It's like they sometimes choose to cancel and sometimes they choose not to. So I'm sorry Newton,
but particles like yours really aren't good enough to explain how you could ever get canceling. But then how do waves manage to explain the double slit experiment? Let's imagine a wave is
coming out of the laser. On the right side, the
wave encounters the hair and because it's an obstacle, it bends and it spreads out
like we discussed before. On the other side, the
exact same thing happens. Back in our wave pool, the black obstruction in the
middle now represents the hair. And as you can see, the waves
on either side do spread out and they overlap. But something very surprising
is happening in that overlap and you can't really make it
out in this precision setup but it's gonna be a lot
more clear in the animation. Where are all these lines coming from? To understand what's going on, imagine you're on a boat in this spot. Then the wave coming from
the left will have its peak reach you at exactly the same time as the wave coming from the right. Together they tend to
push you up even higher. The troughs also meet you at the same time so this spot is just really choppy. But if we instead move our boat to here, then the peak of the
right wave hits exactly when the trough of the left wave hits you. The peak tries to move you upward while the trough tries to pull you down. And so in the end, they cancel out and the boat doesn't
really move up or down. So this spot is surprisingly calm. There are these choppy
and calm spots all over and if you map them out,
then you get these lines. What's this got to do with
the double slit experiment? Well, look at the waves across the wall. You have these alternating
parts of choppiness and calmness that lines up suspiciously well with the pattern of the double slit where you have alternating
spots of light and dark. Thomas Young's hypothesis was, when there's this waving, there's light and when the waves cancel
out and so there's no waving, there is no light. In other words, the amount
of light produced in a spot is proportional to the amount of waving. You know, I've given this
explanation to students for years but I've never taken the whole
wave picture very seriously. Like yes, I knew that to
make the right prediction for the double slit, you would have to use
waves in your calculation. But I thought they were
just that, you know? Like just calculation tools. I didn't think that they were real. I mean, like literally
in quantum mechanics, the waves are complex valued. So instead in my head, I thought
of the light as a particle when it exited the laser
and then a particle again when it got measured at
the wall but in between it just seemed like there's
nothing you could really say about light besides a bunch of equations. But this next experiment
is what changed my mind and made me finally feel that
the waves are actually real. So I've been playing with this for a while and I haven't been able to get
this thought out of my mind. There's a beam here that
leads to this middle dot, that's clearly where
the light for that dot is coming from, right? But what about all these other dots? Like surely that light is
coming from somewhere as well and it feels like you
can almost trace back exactly where that light's from. For this one, if I follow it, I can go all the way back. And same for these as well. So it feels like there should be beams kinda coming out here. I got obsessed with this idea
that you should be able to see these beams of light if you
just use the right method. I tried everything like doing
the experiment at midnight for maximum contrast and then
filling a box with smoke. Then I tried filling a
tub with water vapor. I even tried submerging my
laser into aga, nothing. In the end I caved and
I bought a smoke machine from a DJ shop and I also decided to use the proper double slit
so that the little bands are more intense and it worked. I would love if you actually
tried this experiment yourself because it's so easy to do but I need to warn you about something. Laser pointers are way less
safe than you probably think. I bought a bunch of lasers from Amazon that all claim to be
under five milliwatts. Anything over that limit
can permanently burn a hole in your retina. So let's compare these cheap lasers with a proper five milliwatt
laser to see if they're safe. As you can see, this green
one is definitely not okay. In general, green lasers are something I really don't recommend
you buy from Amazon. Red laser pointers are
better in my experience especially little ones like
this that are advertised as like key chains or cat toys
rather than like this type. But yeah, if you're going to
be using a laser for presenting or experiments or playing with your cat, just make sure it isn't
one that's going to burn someone's eyes out. Anyway, it was worth
buying the smoke machine. This experiment looks
just magical in real life and all I've done to
capture it is use my phone, nothing fancy. I set the double set up a few meters away and blocked all the lights and
turned on the smoke machine and all of these beams
suddenly became visible. Here you can see that
there are beams of light going to all of the dots
of the double slit pattern. But to see it even more clearly, let's turn around and
look toward the laser. And now so many beams become visible. The double slit doesn't split
the light up into two beams, it splits the light into many, many beams. So why was this experiment
such a revelation for me? Well, it felt like I was actually
seeing the light be a wave for a first time. Let me show you what I
mean with this laser. Here you can see the
narrow beam of the laser but if I put this paper in the way, a wave should spread out and yeah, that's exactly
what the laser does. But to see it more clearly,
let's add some smoke. This looks almost unreal. The laser beam is spread out
but only in this one plane just like we'd predict for a wave. If I remove the paper and put
in the double slit instead, you can see the two sides
of the beam interfere and cancel out again,
just like a wave should. Seeing this was the first
time that I actually felt in my gut that light
is somehow wavelike. Up until now, I just
thought that the waves of quantum mechanics were
abstract and they're not. They're actually there and you can see pretty
clear evidence of them. And so since then I've started
taking the whole wave picture much more seriously. On the other hand, I've
become more cautious about imagining light as particles. Photons are much more nuanced
clearly than I thought before and I wanna try and do
experiments with them as well to understand how I
should be thinking of them which is why quantum experiments at home is gonna be a series. I hope you have lots of
questions left about light and if you do, please
leave them in the comments and we'll try and sort them out
together in future episodes. See you then.
