The theory of quantum mechanics governs the world of atoms and particles.
It is strange and counter-intuitive which makes it fascinating.
It sparked excitement and discussion when the prize for research on quantum mechanics was given to three people.
There are a lot of myths and misconceptions about quantum mechanics that can get in the way of discussions. Four are here.
The internet meme status of Schrdinger's cat could not have been predicted.
It suggests that a cat stuck in a box with a kill switch could be alive and dead at the same time if we didn't open the box.
For example, quantum particles can be in two states at the same time. This position is called a superposition by us.
In the famous double-slit experiment, a single quantum particle, such as a photon or electron, can go through two different slit in a wall at the same time. We don't know how to know that.
Each particle is a wave. When we send a stream of light through the slit, it creates a pattern of two waves that interfere with each other on a screen behind it.
As each photon didn't have any other photon to interfere with when it went through the slit, it means it must have gone through both slit and interfered with itself.
The states of the particle going through the two slit need to be coherent in order to work.
These experiments can be done with larger objects.
One famous experiment in 1999 demonstrated quantum superposition with large Molecules of Carbon-60.
What is it that this means for our cat? Is it still alive if we don't open the box?
A cat is larger and more complex than an individual photon in a lab environment.
The trillions of atoms that make up the cat are not going to last long.
This doesn't mean that quantum coherence isn't possible in biological systems, just that it won't apply to cats or humans.
If you measure one of the particles, you will instantly know the state of the other, even if you don't know it yet.
It can be explained by everyday objects from our classical world, such as dice, cards, or even pairs of odd colored socks.
Imagine if you told your friend that you put a blue card in one envelope and an orange card in the other. If your friend opens one of the envelopes and finds the blue card, they will know you have the orange card, even if they don't know it.
To understand quantum mechanics, you have to imagine that the two cards inside the envelopes are both orange and blue at the same time.
One color is determined by opening an envelope. The color of the second card is linked to the first one.
It is possible to force the cards to appear in a different color. We could ask if you are a green or red card.
The answer could be either green or red. The other card would always give a different result when asked the same question.
Albert Einstein tried to explain it with classical intuition, suggesting that the cards could have been given with an internal instruction set which told them what to look like.
The action between the cards that seems to allow them to instantly influence each other, which is forbidden by Einstein's theories, was rejected by him.
Einstein's explanation was ruled out by John Stewart Bell's Bell's theorem and experiments done by the Laureates of the Prize. It's not true that measuring one card changes the other's state.
Einstein had thought that quantum particles contained a hidden code, but they don't communicate and are just mysteriously correlated.
Don't think about everyday objects when you think aboutentanglement.
Bell's theorem is said to show that nature isn't "local" and that an object isn't just influenced by its surroundings. One interpretation is that it means that quantum objects don't exist before being measured.
If we assume a few other things at the same time, we can say that quantum physics means nature is not real and local.
The idea that we don't live in a "clockwork universe" in which everything has been decided since the dawn of time is included.
If you allow for breaking some other things we consider common sense, nature may be real and local. There are many possible interpretations of quantum mechanics.
Time flowing backward, the absence of free will, and other options on the table are just as ridiculous as giving up on local reality.
"If you think you know quantum mechanics, you don't" is a classic quote attributed to physicist Richard Feynman.
Many people hold this view in public. Physicists are said to be unable to understand quantum physics. From a 21st century perspective, quantum physics is not difficult for scientists.
To a point where we can predict quantum phenomena with high precision and even build quantum computers, we understand it incredibly well.
When explained in the language of quantum information, superposition and entanglement don't require much more than high school math. Bell's Theorem does not require quantum physics. A few lines can be used to derive it.
How to reconcile quantum physics with our intuitive reality is the real difficulty. We will continue to make progress with quantum technology even though we don't have all the answers. Shut up and calculate, we can do that.
Aspect, Clauser, and Zeilinger were able to keep asking why. It is possible that others like them will help reconcile quantum weirdness with reality in the future.
Professor of physics, Heriot-Watt University, and Professor of physics, Heriot-Watt University, are both related to one another.
Under a Creative Commons license, this article is re-posted. The original article is worth a read.
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