For the most part, physics is a pretty straightforward
discipline.
While the math might be complicated, physics
is often about quantifying such things as
“what goes up, must come down” and “don’t
play chicken with a train.”
Statements like those are intuitive and pretty
much everyone can get their head around them.
But, over the last century or so, physics
has explored some very unfamiliar realms,
like what happens when objects go incredibly
quickly or what rules govern the universe
for objects the size of an atom or smaller?
That last one, the one involving the super
small, has a generated an entire field of
physics called quantum mechanics.
Now if you have heard anything at all about
quantum mechanics, there is a good chance
that you’ve heard about such things as how
it means that cats are alive and dead at the
same time, or that nothing is real until we
see it because we generate reality using only
our mind.
And, of course, none of these things are even
remotely true.
They're a series of outdated ideas that were
invoked in the early years of quantum mechanics
in attempts to explain the subject in simple
ways.
Each idea has burrowed its way into the subconscious
of people who are interested in science, but
aren’t quantum experts.
So, what is quantum mechanics all about really?
To understand that in any accurate detail
takes a ton of study, but I thought I’d
try to boil it down to its essence in a single
video.
Perhaps we should start with the name.
The word quantum implies a discrete quantity
of something.
For instance, the Sahara Desert can go on,
seemingly forever, with dune following dune.
But, as infinite as the sea of sand seems,
it has a quantum side.
The quantum of a desert is a single grain
of sand.
Alternatively, the ocean is majestic, liquid,
and in constant motion.
There, the quantum is a single molecule of
water.
So that’s what quantum means.
It means that we’re talking about the smallest
bit of something.
The term mechanics is much easier.
It just means the behavior and motion of something.
For instance, Celestial Mechanics means the
motion of the planets and other heavenly bodies.
Thus, quantum mechanics is the motion and
interaction of individual objects of a bigger
collection.
By that definition, the motion of an individual
car on a freeway could be described as being
governed by quantum mechanics, and of course
that’s not quite what we mean.
In order to experience the counterintuitive
bits of quantum mechanics, we have to look
at individual things that are very small-
generally smaller than an atom.
This is because quantum theory uses the Planck
constant, which is a tiny number.
It’s this tininess that keeps the quantum
weirdness constrained to the subatomic realm.
When physicists first learn quantum mechanics
as part of our training, we are studying the
behavior of individual electrons surrounding
atoms, or individual photons of light.
By and large, that is the realm of academic
quantum mechanics.
In a subsequent video, I’ll talk about how
the weirdnesses of quantum mechanics can manifest
themselves on larger scales.
But, for now, let’s talk about this world
of the ultra-small, where we’re talking
about individual subatomic objects.
Okay, so now I need to introduce a key idea,
called the wave function.
At the subatomic level, a wave function is
a mathematical function that describes what
is going on.
It is called a wave function, in part, because
for individual photons or electrons that aren’t
connected to an atom, the wave function looks
like a familiar sine wave.
There are other wave function shapes when
electrons are near a nucleus, but the big
ideas aren’t dependent on the details of
their shape.
For simplicity, you can simply think of wave
functions as sine waves.
Now that is the first curiosity.
The sand grain or water molecule is a distinct
and discrete object– what scientists call
a particle.
Waves don’t seem to be relevant.
However, in the world of the super tiny, the
wave function is the only way to explain what
is going on.
Now exactly what the wave function actually
means is a lot trickier than you’d think.
I’ll get to that in a little bit.
Answering that question is a big part of the
reason why conversations about quantum mechanics
are so mind blowing.
However, there are ways we can think about
the wave function that are easier to understand.
If we take the wave function and square it,
which means to multiply it by itself, the
squared wave function tells you where a particle
can be found.
Where the wave function is large, there is
a large probability of finding the particle
there, and where it is zero, the probability
of finding the particle there is zero.
So that’s not too bad.
The wave function squared gives you the probability
of where you can find the particle the wave
function is describing.
That’s the first big quantum idea.
Now when we detect the particle, say the electron
or photon, we don’t find it everywhere.
We find it in one location.
At the moment of detection, the location of
the particle is determined.
Scientists call this moment of detection the
“collapse of the wave function.”
There’s a reason for that particular phrase
and it is because of a quantum weirdness.
When you roll a fair die, there is a probability
that each side will come up.
It's difficult to predict which side we’ll
see when it starts rolling, but if we knew
the cube’s original velocity and the details
of the surface it’s rolling on, we could
reliably predict the top face of the die.
In quantum mechanics, it’s different.
In quantum mechanics, the electron or photon
is actually in all locations allowed by the
square of the wave function and it is at the
moment at which the particle’s location
is detected that the wave function changes
from being in many locations to the one location
where the particle is found.
The wave function collapsed from being in
many locations to just one.
So those are the two key components of quantum
mechanics.
Now, before we get into just why quantum mechanics
still seems so mysterious, it’s worth stating
the things on which most scientists agree.
There is a mathematical function called a
wave function that, when you square it, tells
you the locations that you can find the subatomic
particle and with what probability.
The subatomic particle is simultaneously everywhere
the wave function says it can be, kind of
like the electron or photon being spread out.
And then, when the particle is detected, the
wave function collapses and then we know with
100% certainty where the subatomic particle
is.
There's nearly universal agreement that this
way of thinking about things gives accurate
predictions.
So just what is so confusing about quantum
mechanics?
Well it boils down to the very simple fact
that nobody really knows what the wave function
really is.
I mean, we know how to use it to make predictions-
that’s been called the “shut up and calculate”
school of thought- but the true nature of
the wave function is perplexing.
The simple and honest fact is that, despite
a century of thinking about it, nobody knows
what it is.
There are philosophers who argue about whether
the wave function is an ontological or epistemological
construct.
And those are big words, because- you know-
philosophers.
Then, there are some people who claim that
the waves are actually things that just push
other things around.
They’d say that the electrons are particles
getting pushed by the waves like surfers on
a gnarly curl at Oahu.
The technical term in quantum mechanics for
this idea is “pilot waves.”
Another school of thought says that the wave
function predicts what can happen and when
you collapse the wave function, you literally
create several universes, in which each one,
one of the possible outcomes occurs.
This is called the many worlds interpretation.
So which one of those ideas is right?
Nobody knows.
It’s truly a head scratcher.
And because science hasn’t figured it out
yet, that opens up the conversation to weird
ideas, like the mind/body problem of traditional
philosophy because it’s been said that nothing
is real until it's observed.
Those things really aren’t connected to
quantum mechanics at all.
This is the bottom line.
We know how to use the wave function to make
accurate predictions of the subatomic world
and yet we don’t have a fundamental understanding
of the wave function and what happens when
it collapses.
This is a not-so-secret secret of physics.
And it’s not for lack of trying.
Many people have mulled things over and generated
tons of ideas, but none of them have resulted
in a definitive conclusion.
It’s a dissatisfying state of affairs to
be sure, but it is what it is and it’s going
to take a genius to move us forward.
For the time being, we’ll just have to wait
and see.
And hopefully, you now know enough to dismiss
any explanation that invokes zombie cats that
generate reality by looking at it.
So- I hope that this video was eye opening
in that it kind of gave you a sense of the
key ideas of quantum mechanics.
And hopefully you can see why books written
for the public about quantum mechanics can
be kind of sketchy.
Physicists can use quantum mechanics, but
don’t understand its underpinnings very
well.
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