Quantum mechanics, the theory which rules the
microworld of atoms and particles, certainly has the X factor. Unlike many
other areas of physics, it is bizarre and counter-intuitive, which makes it
dazzling and intriguing. When the 2022 Nobel prize in physics was awarded to
Alain Aspect, John Clauser and Anton Zeilinger for research shedding light on
quantum mechanics, it sparked excitement and discussion.

But debates about quantum mechanics—be they on chat
forums, in the media or in science fiction—can often get muddled thanks to a
number of persistent myths and misconceptions. Here are four.

## 1. A cat can be dead and alive

Erwin Schrödinger could probably never have
predicted that his thought experiment, Schrödinger's cat, would attain internet
meme status in the 21st century.

It suggests that an unlucky feline stuck in a box
with a kill switch triggered by a random quantum event—radioactive decay, for
example—could be alive and dead at the same time, as long as we don't open the
box to check.

We've long known that quantum particles can be in two
states—for example in two locations—at the same time. We call this a
superposition.

Scientists have been able to show this in the famous
double-slit experiment, where a single quantum particle, such as a photon or
electron, can go through two different slits in a wall simultaneously. How do
we know that?

In quantum physics, each particle's state is also a
wave. But when we send a stream of photons—one by one—through the slits, it
creates a pattern of two waves interfering with each other on a screen behind
the slit. As each photon didn't have any other photons to interfere with when
it went through the slits, it means it must simultaneously have gone through
both slits—interfering with itself.

For this to work, however, the states (waves) in the
superposition of the particle going through both slits need to be
"coherent"—having a well defined relationship with each other.

These superposition experiments can be done with
objects of ever increasing size and complexity. One famous experiment by Anton
Zeilinger in 1999 demonstrated quantum superposition with large molecules of
Carbon-60 known as "buckyballs".

So what does this mean for our poor cat? Is it
really both alive and dead as long as we don't open the box? Obviously, a cat
is nothing like an individual photon in a controlled lab environment, it is
much bigger and more complex. Any coherence that the trillions upon trillions
of atoms that make up the cat might have with each other is extremely
shortlived.

This does not mean that quantum coherence is
impossible in biological systems, just that it generally won't apply to big
creatures such as cats or a human.

## 2. Simple analogies can explain entanglement

Entanglement is a quantum property which links two
different particles so that if you measure one, you automatically and instantly
know the state of the other—no matter how far apart they are.

Common explanations for it typically involve
everyday objects from our classical macroscopic world, such as dice, cards or
even pairs of odd-colored socks. For example, imagine you tell your friend you
have placed a blue card in one envelope and an orange card in another. If your
friend takes away and opens one of the envelopes and finds the blue card, they
will know you have the orange card.

But to understand quantum mechanics, you have to
imagine the two cards inside the envelopes are in a joint superposition,
meaning they are both orange and blue at the same time (specifically
orange/blue and blue/orange). Opening one envelope reveals one color determined
at random. But opening the second still always reveals the opposite color
because it is "spookily" linked to the first card.

One could force the cards to appear in a different
set of colors, akin to doing another type of measurement. We could open an
envelope asking the question: "Are you a green or a red card?". The
answer would again be random: green or red. But crucially, if the cards were
entangled, the other card would still always yield the opposite outcome when
asked the same question.

Albert Einstein attempted to explain this with
classical intuition, suggesting the cards could have been provided with a
hidden, internal instruction set which told them in what color to appear given
a certain question. He also rejected the apparent "spooky" action
between the cards that seemingly allows them to instantly influence each other,
which would mean communication faster than the speed of light, something
forbidden by Einstein's theories.

However, Einstein's explanation was subsequently
ruled out by Bell's theorem (a theoretical test created by the physicist John
Stewart Bell) and experiments by 2022's Nobel laureates. The idea that
measuring one entangled card changes the state of the other is not true.
Quantum particles are just mysteriously correlated in ways we can't describe
with everyday logic or language—they don't communicate while also containing a
hidden code, as Einstein had thought. So forget about everyday objects when you
think about entanglement.

3. Nature is unreal and 'non-local'

Bell's theorem is often said to prove that nature
isn't "local", that an object isn't just directly influenced by its
immediate surroundings. Another common interpretation is that it implies
properties of quantum objects aren't "real", that they do not exist
prior to measurement.

But Bell's theorem only allows us to say that
quantum physics means nature isn't both real and local if we assume a few other
things at the same time. These assumptions include the idea that measurements
only have a single outcome (and not multiple, perhaps in parallel worlds), that
cause and effect flow forward in time and that we do not live in a
"clockwork universe" in which everything has been predetermined since
the dawn of time.

Despite Bell's theorem, nature may well be real and
local, if you allowed for breaking some other things we consider common sense,
such as time moving forward. And further research will hopefully narrow down
the great number of potential interpretations of quantum mechanics. However,
most options on the table—for example, time flowing backwards, or the absence
of free will—are at least as absurd as giving up on the concept of local
reality.

4. Nobody understands quantum mechanics

A classic quote (attributed to physicist Richard
Feynman, but in this form also paraphrasing Niels Bohr) surmises: "If you
think you understand quantum mechanics, you don't understand it."

This view is widely held in public. Quantum physics
is supposedly impossible to understand, including by physicists. But from a
21st-century perspective, quantum physics is neither mathematically nor
conceptually particularly difficult for scientists. We understand it extremely
well, to a point where we can predict quantum phenomena with high precision,
simulate highly complex quantum systems and even start to build quantum
computers.

Superposition and entanglement, when explained in
the language of quantum information, requires no more than high-school
mathematics. Bell's theorem doesn't require any quantum physics at all. It can
be derived in a few lines using probability theory and linear algebra.

Where the true difficulty lies, perhaps, is in how
to reconcile quantum physics with our intuitive reality. Not having all the
answers won't stop us from making further progress with quantum technology. We
can simply just shut up and calculate.

Fortunately for humanity, Nobel winners Aspect,
Clauser, and Zeilinger refused to shut up and kept asking why. Others like them
may one day help reconcile quantum weirdness with our experience of reality.

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