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.
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.
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,
“ 我们试试光是否适用这个原理 ”
“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 lighthad 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 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
Knowing where the previous photon appeared,
in no way allows you to predict,
where the next one will show up.
但是 把它们看成一个整体 它们好像
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?
不 不 不 这不可能
No, no, no, this is magic.
This is magic!
– 喵- 好 佐伊我听话
– Meow- No, you’re right Zoey.
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
量子计算 - 万物的基础1