Quantum mechanics, the theory that governs 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 their research shedding light on quantum mechanics, this generated enthusiasm and discussions.

But debates about quantum mechanics – whether on discussion forums, in the media, or in science fiction – can often be muddled by a number of persistent myths and misconceptions. Here are four.

## 1. A cat can be dead and alive

Erwin Schrödinger probably could never have predicted that his thought experiment, Schrödinger’s Cat, would achieve internet meme status in the 21st century.

This suggests that an unlucky feline stuck in a box with an off 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 have known for a long time that quantum particles can be in two states – for example in two places – at the same time. We call this an overlay.

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

In quantum physics, the state of each particle is also a wave. But when we send a stream of photons – one at a time – through the slits, it creates a pattern of two waves interfering with each other on a screen behind the slit.

Since each photon had no other photons to interfere with as it passed through the slits, this means that it must have passed through both slits simultaneously – interfering with itself (image below).

For this to work, however, the states (waves) in the superposition of the particle passing through the two slits must be “coherent” – have a well-defined relationship to each other.

These layering experiments can be done with objects of ever-increasing size and complexity.

A famous experiment by Anton Zeilinger in 1999 demonstrated quantum superposition with large carbon-60 molecules called “buckyballs”.

So what does this mean for our poor cat? Is he really both alive and dead until you open the box?

Obviously, a cat is nothing like an individual photon in a controlled lab environment, it’s much bigger and more complex.

Any coherence that the trillions and trillions of atoms that make up the cat might have with each other is extremely fleeting.

This does not mean that quantum coherence is impossible in biological systems, just that it generally does not apply to large creatures such as cats or humans.

## 2. Simple analogies can explain entanglement

Entanglement is a quantum property that connects two different particles so that if you measure one, you automatically and instantly know the state of the other, regardless of their distance.

Common explanations usually involve everyday objects from our classic macroscopic world, such as dice, cards, or even odd-colored pairs of socks.

For example, imagine telling your friend that you placed a blue card in one envelope and an orange card in another. If your friend takes 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 that the two cards inside the envelopes are in a conjoined superposition, which means they are both orange and blue (specifically orange/blue and blue/orange) .

Opening an envelope reveals a random color. But opening the second always reveals the opposite color because it is “frightening” related to the first card.

One could force the cards to appear in a different set of suits, which is like doing another type of measurement. We could open an envelope by asking the question: “Are you a green card or a red card?”.

The answer would again be random: green or red. Importantly, if the cards were entangled, the other card would always give the opposite result when asked the same question.

Albert Einstein attempted to explain this with classical intuition, suggesting that cards could have had a set of hidden internal instructions that told them which color to appear for a certain question.

He also dismissed the apparent “frightening” action between the cards that apparently allows them to influence each other instantly, which would mean communication faster than the speed of light, which is forbidden by Einstein’s theories.

However, Einstein’s explanation was later ruled out by Bell’s Theorem (a theoretical test created by physicist John Stewart Bell) and the experiments of the 2022 Nobel laureates. The idea that measuring a map entangled 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 containing hidden code, as Einstein had thought.

So forget everyday objects when you think of entanglement.

## 3. Nature is unreal and “non-local”

It is often said that Bell’s theorem proves that nature is not “local”, that an object is not only directly influenced by its immediate environment. Another common interpretation is that it implies that the properties of quantum objects are not “real”, that they do not exist before measurement.

But Bell’s theorem only allows us to say that quantum physics means that nature is not both real and local if we assume a few other things at the same time.

These assumptions include the idea that measurements have only one outcome (and not many, perhaps in parallel worlds), that cause and effect move forward in time, and that we don’t live in a “world of watchmaking” in which everything has been predetermined since the dawn of time.

Despite Bell’s theorem, nature could well be real and local, if you allowed to break other things that we consider common sense, like the passing of time. And further research will hopefully narrow down the vast number of potential interpretations of quantum mechanics.

However, most of the options on the table – for example, time running backwards or the absence of free will – are at least as absurd as abandoning the concept of local reality.

## 4. No one understands quantum mechanics

A classic quote (attributed to physicist Richard Feynman, but in this form also paraphrasing Niels Bohr) presumes: “If you think you understand quantum mechanics, you don’t.”

This opinion is widely held in the public. Quantum physics is supposed to be impossible to understand, even by physicists. But from a 21st century perspective, quantum physics is neither mathematically nor conceptually particularly challenging for scientists.

We understand it extremely well, to the point where we can predict quantum phenomena with great accuracy, simulate very complex quantum systems, and even begin to build quantum computers.

Superposition and entanglement, when explained in the language of quantum information, requires no more than high school math. Bell’s theorem does not require any quantum physics. It can be derived in a few lines using probability theory and linear algebra.

The real difficulty may lie 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 just shut up and calculate.

Luckily for humanity, Nobel laureates 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.

*Alessandro Fedrizzi, Professor of Physics, Heriot-Watt University and Mehul Malik, Professor of Physics, Heriot-Watt University*

**This article is republished from The Conversation under a Creative Commons license. Read the original article.**

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