The year is 1927
29 people gather in Brussels to discuss physics
17 of those people will eventually win a Nobel Prize
and for a few short days
in the middle of Leopold Park
They will wrestle with the smallest question
or perhaps the biggest one
to ever face mankind
The question
at the foundation of everything.
[intro song]
For those few days
those 29 physicists
wrestled with the question
of the quantum determinancy
and whether our world at the minutest level
operates as a fixed system
or merely as a group of probabilities
Their question stemmed from one of the oldest problems in modern physics,
the problem of light
For nearly three centuries
since Newton wrote his famous treatise on optics
Physicists had debated whether light was a particle
or a wave
in 1803
this argument was thought to be put to rest
by one of the most beautiful and simple experiments ever created
the double slit experiment
Okay,
think of two buoys bobbing up and down in the water
as the waves spreading out from these buoys hit each other and overlap
They interfere with each other
if the peak of one wave
hits the peak of another
they'll amplify and become a bigger wave
Same with the troughs
but if a peak of one wave hits the trough of the other
they'll just flatten out
they'll combined back down to nothing.
A man named Thomas Young said,
Let's take that principle and apply it to light
And so he did the simplest thing imaginable
he took a monochromatic light,
to make sure that all the light had the same wavelength
and he shone it on a partition with two small slits cut into it
If light acted like a particle
he should simply see two columns of light on the wall on the other side
But
if light was a wave
then the waves coming through each of the slits
should interfere with each other
amplifying and cancelling each other out in places
And he would instead see a weird pattern of bright and dark lines as a result
And as he expected
he did indeed see that funky pattern
and that was that.
The particle theory of light was done and dusted
he'd solved the dang thing
Now everyone could finally move on to talking about just how smart he was
But then physicists in other labs
found something strange in their own experiments
They found that
when light strikes a material
it can force electrons to spew out of it
this wasn't that startling
but the way it happened was all wrong
and definitely not how it should have happened if light was the continuous wave they'd believed it to be
Then in 1900 a man named Max Planck
came up with an equation that fit
it made sense of what was happening
But as Planck himself would later say
it was an act of desperation
It went against everything he thought he knew
The only way he could get all of the math to work
was by treating energy as something that could only be absorbed or released in discrete units
How could this be? He thought.
How could energy not be continuous?
How could it not be a flow?
He had
no idea,
but then this fellow named Einstein took Planck's act of desperation and ran with it
He declared that light itself was quantized
That in many ways we
can think of it as a particle of zero mass
always moving at
well, the speed of light
And it is for this theory,
not for special or general relativity,
that Einstein was awarded his Nobel Prize
Because this concept, which we now call the photon
solved a number of lingering issues
with how light interacted with the world
But the photon brings us right back to the problem of Thomas Young's double-slit experiment
Because if light has both the properties
of a wave
and a particle
What happens if you fire those particles through the slits
one at a time?
Well, here is where this becomes the most astonishing and humble experiment
ever devised
Because if you shoot one photon at the slits and detect where it hits on the other side
You'll find that it impacts
some arbitrary point
just the way you think it should
And if you fire a second photon through,
you'll find that it too shows up at some other arbitrary point on the other side
But
if you do this enough times,
you'll eventually see the same interference pattern build up
that we got back in Thomas's original experiments.
That is madness
each individual photon, which should be completely independent of the rest
shows up at some seemingly random point on your wall
And exactly where they show up
will be different each time you run the experiments
And!
Knowing where the previous photon appeared in no way allows you to predict
where the next one will show up
yet when taken as a group
it's as if they're affected by how they
Should interfere with each other
This feels impossible
and yet it is experimental fact
And the reason for this phenomenon is one of the most hotly debated mysteries in physics
Because
the only way to conceptualize this
is that each photon passes through both slits as a wave,
interferes with itself
and then resolves down to a photon when it actually hits the wall
What is going on here?
What is this?
No, no, no,
this is magic.
This is magic !
[sine wave]
[meow]
No, you're right Zoe.
I should calm down because we are not done yet
because here
is where it gets
really freaky
Remember how when Thomas was first doing his experiment?
We said that if light were really a particle
we should just see two columns of light on the other side of his double slit paper?
Well,
if you put a detector on the slits,
so that you can determine which slit the photon you fired passes through
That is exactly what you get.
That's all you have to do
You don't have to change the experiment in any way or interfere directly with the photon
You simply have to measure which slit the photon
passes through.
Why does it do this?
Because a photon is a particle
and a wave
but it can't be both
simultaneously
The mere act of measuring which path the photon took
Forces it to resolve the wave-like nature of the photon into a particle
And this may be
the hardest thing to wrap your head around
in all of quantum physics
because the most common way to view this
is that the photon when acting like a wave
isn't a real
wave at all
but rather a wave of possibilities
That wave represents where the photon
could be but not where it is
It's only when something acts to detect the photon
whether it be your measuring device
or the wall on the opposite
side of your double slit experiment
That the photon is forced to,
for lack of a better term,
decide
on where it will actually be
and in doing so
becomes a particle
More unsettling still,
is the fact that these waves of possibility
interfere with each other just like normal waves
The interference pattern we see from firing particles
one at a time
through the double slit experiment
is caused by peaks and troughs of possibility
cancelling each other out
When you fire that photon and the wave of possibility hits your double slit paper
It is funneled through as two possibility waves
Just in the way
that any regular physical wave would be
and just like those regular waves
waves of possibility
interfere with each other
Essentially making it so there are places where it is more or less likely for a photon to land when detected
Thus
when you fire a lot of photons
one at a time through your double slit experiment
The bands you see are simply the high probability lines
playing out
But if you think we're done getting weird,
think again,
we're only on episode one
so join us next time as we get serious about this idea that energy only comes in discrete packets
and begin our journey on what this means
for the future of quantum computing.
We'll see you next time
or will we just
perceive you next time because would that mean we'd have to
watch you watching us
to know the
oh boy
[outro song]