- 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.