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I’ve said it before and I’ll say it again:
quantum mechanics is weird.
It also hard to make videos about, but we’re
doing it!
It’s the physics of the super-tiny, and
it’s built around the idea that energy isn’t
a smooth spectrum — it can only come in
set amounts.
But as weird as the implications of that are,
it’s stood up to every experimental test thrown at it.
And as we’ve learned more about it, we’ve
got better at using it to solve problems and
invent all kinds of useful things, like lasers
and semiconductors.
Although we understand how to use the math
behind the theory, understanding what it means
has proven to be a difficult challenge.
There are a number of different interpretations
of quantum mechanics out there, each of which
looks at the theory differently.
They all make the same predictions about what
quantum mechanics looks like on the surface
— so, what will actually happen in experiments
in the labs — but the math, and the meaning
of the math, can look very different.
There’s a lot going on under the hood.
One of the more controversial interpretations
is called pilot wave theory.
At first glance it looks appealing: it allows
you to get around the uncertainty and randomness
that quantum mechanics is famous for.
But there’s a catch: getting rid of the
randomness involves breaking reality in other ways.
The conventional interpretation of quantum
mechanics is called the Copenhagen interpretation,
after the institute where it was devised in
the 1920s.
It includes a lot of the more well-known ideas
around quantum mechanics, like that something
can be a particle and a wave at the same time.
It also says that certain things, like which
way an electron is spinning, aren’t really
set until you observe them — and until you
do, the electron is spinning in both directions at once.
This concept, that particles are in multiple
states at the same time, is known as superposition,
and it’s what inspired Erwin Schrödinger’s
famous dead-and-alive cat.
Schrödinger was one of the pioneers of quantum
mechanics, and his cat-in-a-box thought experiment
was actually meant to show that some implications
of the theory were just … ridiculous.
But it turns out that poor cat was actually
a good illustration of superposition — and
that, yes, a lot of quantum mechanics makes
no sense when you try to apply it to the larger
world we’re more familiar with.
In the thought experiment, you hide a cat
in a box with a flask of deadly poison.
But the poison will only be released if a
radioactive atom decays, which has, say, a 50% chance of happening.
Radioactive decay is a quantum mechanical
process.
Whether or not it happens to an individual
atom is exactly the kind of random event that
the mainstream interpretation of quantum mechanics
says is entirely unpredictable.
You cannot predict it.
Before you open the box, you have no idea
whether or not the atom decayed, and therefore
no idea whether the poison was released or
not.
Normally, if there was a 50/50 chance a cat
was alive, you’d say the cat was either
alive or dead — you just wouldn’t know
which.
But here’s where the Copenhagen interpretation
is different from regular probability.
Before it’s observed, the atom is in a superposition
of decayed and not-decayed — meaning it’s both at the same time.
So the cat would be both alive and dead.
Once you open the box, you turn the superposition
into one state you actually observe — let’s say alive.
No kitties were harmed in the making of this
episode.
But opening the box doesn’t tell you that
the cat was always alive that whole time.
Before you opened the box, its true state
was the superposition of alive and dead.
Now, superpositions don’t appear in everyday
life, but according to the Copenhagen interpretation,
on the tiny scale of particles, they’re
everywhere, which is obviously very weird.
But the idea fits every experiment we’ve
ever done, and over time, physicists have
come to accept that reality is sometimes … kind
of blurred.
That’s what pilot wave theory tries to fix.
The theory was first proposed in 1927 by another
pioneer of quantum mechanics: Louis de Broglie.
It was shelved until the 1950s, when David
Bohm rediscovered and improved it.
Today it’s also known as the de Broglie-Bohm
theory.
It works by distinguishing between particles
and waves, instead of treating them as the
same thing like the Copenhagen interpretation
does.
In pilot wave theory, there are still particles
and waves, but they exist separately: there’s
an equation that gives you a particle’s
velocity, and that equation depends on the wave.
The wave interacts with the particle by guiding
the way it moves — or pilots it, in other words.
That’s where the name “pilot wave” comes
from.
This wave spans the entire system you’re
looking at, whether that’s just a few electrons
or the whole universe.
Because of this central wave, the properties
of matter are set before you observe them,
instead of being superimposed.
You may not have all the information you need
to figure out what those properties are in
advance, but the information is out there.
It’s a little bit like flipping a coin,
then covering it with your hand.
You know it’s either heads or tails, even
before you look at it, because it spun a set
number of times in the air before it landed.
It was too fast for you to follow the spinning
with your eyes, but if you had a high-speed
camera or something you’d be able to figure
it out.
The Copenhagen interpretation says that type
of information doesn’t exist for something
like radioactive decay, so until you observe
it, an atom can be both decayed and not decayed
at the same time — and Schrodinger’s cat
can be both alive and dead.
Pilot wave theory says the information does
exist.
Like with the coin, you may not have access
to the knowledge that would tell you whether
the atom decayed — so on a practical level,
we can’t really use the math to figure it out.
But the information is out there, and the
atom is one or the other, not both.
That’s a little closer to how we perceive
reality in everyday life, so that is nice.
The problem is, there’s a tradeoff: in a
very intrinsic way, pilot wave theory breaks
a different, really big rule in physics: locality.
Locality is the idea that everything in the
universe can only ever affect its immediate surroundings.
You can’t interact with something far away
without sending some kind of signal to it,
and that signal needs to be transmitted through
the space between you and that thing.
Most importantly, this means that all signals
take time to travel.
That’s why you see lightning before you
hear thunder.
We also know that there should be an upper
limit to how fast these signals can move:
Einstein worked out that the universe’s
speed limit is the speed of light.
Now, the Copenhagen interpretation actually
does violate locality in certain situations.
For example, you can generate two electrons
in a way that means they must have opposite spins.
Until you actually observe their spins, though,
each electron is in superposition, spinning
in both directions at the same time.
But if you send the two electrons away from
each other, wait until they’re really far
apart, and observe one right after the other,
you’ll always find that they spin in opposite directions.
That’s true even if there’s no time for
a signal to pass from the first to the second
without moving faster than the speed of light.
Einstein really did not like this, but we’ve
tested it, and we know it happens.
If you’re going by the Copenhagen interpretation,
that means somehow the electrons are affecting
each other faster than the speed of light.
Still, at least in the Copenhagen interpretation
violating locality is the exception rather than the rule.
Pilot wave theory, on the other hand, is entirely
based on the idea that locality does not need
to be a thing, and particles can affect each
other instantaneously even if they’re light-years apart.
That’s the whole point of the pilot wave.
All particles in a system are tied to each
other through that one wave, so by extension,
all particles in the universe affect each
other — without the time delay you’d get
if there were signals traveling at the speed
of light.
In other words, if you say the information
that would tell you whether the cat is alive
or dead is embedded in the rest of the particles
in the universe, you’re also saying those
particles somehow affect each other faster
than the speed of light.
And unlike the Copenhagen interpretation,
it’s not just certain situations that have
this problem — it’s everything, everywhere,
always.
So, uh, some physicists take issue with that.
Still, pilot wave theory is a good reminder
that our theories of physics aren’t just
guided by what’s really happening, but also
by aesthetic and pragmatic choices.
If we go with the Copenhagen interpretation,
then we need to accept some really weird things,
like cats that are simultaneously dead and
alive.
We also need to accept that some things are
just unknowable.
But if we subscribe to pilot wave theory,
then we need to accept that some effects may be truly non-local.
Both choices have their pros and cons, and
for the most part, physicists have decided
that Copenhagen makes more sense.
Pilot wave theory has seen a bit of a renaissance
in recent years, though, thanks to computer simulations.
In the real world we don’t have access to
faster-than-light signals, but on a computer,
we can pretend that we do.
So physicists are using the math behind pilot
waves to do quantum mechanics simulations,
which could be an improvement on conventional
methods for some things.
And it’s not just Copenhagen or pilot waves
— there are other interpretations of quantum
mechanics out there, too.
The history of physics is full of people coming
up with weird, impossible-sounding ideas that
happen to be right.
It’s also full of ideas that happen to be
wrong, but are still useful in lots of ways.
So even if we never discover whether this
theory is actually how the universe works,
there’s a lot we can learn just by exploring
it to see what it can do.
And if you enjoy exploring theories like this,
I think you’ll like the Quantum Computing
course on Brilliant.org, where you’ll learn
about the laws of quantum mechanics by building
your own quantum circuit and racing a classical
computer in solving codebreaking puzzles.
You can check it out at Brilliant.org/SciShow,
and right now, the first 200 people to sign
up at that link will get 20% off of an annual
premium subscription to Brilliant.
So head to Brilliant.org/SciShow to learn
more, and know that when you do, you’re
also helping to support SciShow, so thanks!
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