Don’t get caught up in these 4 misconceptions about quantum mechanics: ScienceAlert

Quantum mechanics, the theory that governs the microworld of atoms and particles, certainly has the X factor.

Unlike many other areas of physics, it’s bizarre and counterintuitive, which makes it dazzling and intriguing.

When the Nobel Prize in Physics was awarded in 2022 awarded to Alain Aspect, John Clauser and Anton Zeilinger for research that sheds light on quantum mechanics, it sparked excitement and discussion.

But debates about quantum mechanics — whether on chat forums, in the media, or in science fiction — can often become confused thanks to some persistent myths and misconceptions. Here are four.

## 1. A cat can be dead and alive

Erwin Schrödinger probably could never have predicted that his… mind experimentSchrödinger’s cat, would reach internet meme status in the 21st century.

It suggests that an unlucky cat trapped 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 have known for a long time that quantum particles can be in two states at the same time, for example in two locations. We call this a superposition.

Scientists have demonstrated this in the famous double slit experiment, in which a single quantum particle, such as a photon or electron, can simultaneously pass through two different slits in a wall. How do we know?

In quantum physics, the state of any particle is also a wave. But if we send a stream of photons through the slits one by one, it creates a pattern of two waves that interfere 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 it must have passed through both slits at the same time — with itself in the way (image below).

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

These superposition experiments can be done with objects of increasing size and complexity.

A famous experiment by Anton Zeilinger in 1999 demonstrated quantum superposition with large molecules of Carbon-60 known as “bucky balls”.

So what does this mean for our poor kitty? 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’s much larger and more complex.

Any connection that the trillions upon trillions of atoms that make up the cat could have with each other is extremely short-lived.

This doesn’t mean that quantum coherence is impossible in biological systems, just that it generally doesn’t apply to large creatures like cats or a human.

## 2. Simple analogies can explain entanglement

entanglement is a quantum property that connects two different particles, so that when you measure one, you automatically and immediately know the state of the other – no matter how far apart they are.

Common explanations for it usually refer to everyday objects from our classic macroscopic world, such as dice, cards or even pairs of odd colored socks.

For example, imagine you tell your friend that you put a blue card in one envelope and an orange card in another. If your friend takes one of the envelopes and opens it and finds the blue card, they will know you have the orange card.

But to understand quantum mechanics, imagine that the two cards in the envelopes are in a joint superposition, meaning they are orange and blue at the same time (specifically orange/blue and blue/orange).

Opening an envelope reveals a randomly determined color. But opening the second still always reveals the opposite suit because it’s “ghostly” linked to the first card.

You could force the cards to appear in a different suit of suits, similar to taking a different type of measurement. We could open an envelope and ask, “Are you a green or a red card?”.

The answer would again be random: green or red. But crucially, if the cards were intertwined, the other card would still have the opposite result if the same question was asked.

Albert Einstein tried to explain this with classical intuition and suggested that the cards could have a hidden, internal instruction set who told them what color to appear in for a particular question.

He also rejected the apparent “ghost-like” action between the cards that allows them to seemingly instantaneously affect each other, meaning communication is faster than the speed of light, something forbidden by Einstein’s theories.

However, Einstein’s statement was subsequently ruled out by: Bello’s theorem (a theoretical test created by the physicist John Stewart Bell) and experiments by the 2022 Nobel laureates. The idea that measuring one entangled map 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 some hidden code, as Einstein had imagined.

So forget everyday objects when you think about 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 properties of quantum objects are not “real”, that they do not exist prior to 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 several, perhaps in parallel worlds), that cause and effect flow over time, and that we don’t live in a “clockwork universe” where everything has been predetermined since the dawn of time. times.

Despite Bell’s thesis, nature may be real and local, if you allowed to break some other things we consider common sense, like time moving forward. And further research will hopefully narrow the vast number of possible interpretations of quantum mechanics.

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

## 4. Nobody understands quantum mechanics

A classic quote (attributed to physicist) Richard Feynmanbut also paraphrase in this form Niels Bohr) surmises, “If you think you understand quantum mechanics, you don’t understand it.”

This opinion is widely supported in public. Quantum physics is supposedly impossible to understand, not even by physicists. But from a 21st-century perspective, quantum physics is neither mathematically nor conceptually difficult for scientists.

We understand it extremely well, to a point where we can predict quantum phenomena with high precision, simulate very complex quantum systems, and even start with building quantum computers.

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

Where the real difficulty lies, perhaps, is 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.

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

*Alessandro Fedrizziprofessor of physics, Heriot Watt University and Mehul Malikprofessor of physics, Heriot Watt University*

**This article was republished from The conversation under a Creative Commons license. Read the original article.**

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