## Quantum Theory and the Idea that Particles Can Be in Two Places at Once

The** quantum** world is a strange place. If you look at an** object**, it changes. If you know **how fast it’s moving**, you can’t know** where it is**. Measurements that happened in the past can seemingly be erased later. **Particles** are sometimes **waves** and can be in **two places at once**.** Cats** may be **both dead and alive**. These are things we say when talking about the** quantum world**, but is this really what is going on?

**Quantum mechanics** is an incredibly well-established theory. It has passed every test it’s ever been subjected to. It underlies much of the technological progress we have seen in the past century, for what would **electronics be without discrete energy levels**, which came to us courtesy of** quantum mechanics**? We have the mathematics and we know how to work it, yet even after a century of debate, we don’t know what the mathematics of quantum mechanics means.

Let’s take an example: the idea that** particles can be in two places at once**. We are familiar with** particles** that are in** one place at a tim**e – an** electron**, say, that hits a screen and leaves a dot. These particles make an appearance in quantum mechanics as a possible solution to the equations, as we expect.

But** quantum mechanics** is a** linear theory**, which means if **particles in particular places exist**, then so do** sums of those particles**. We call those sums “**superpositions**”. And what is a** particle in one place plus the same particle in another place**? It’s no**t two particles** – that would be described by a product, not a **sum**. Could you say that if we have a **sum**, then that’s a **particle** which is in **both places**? Well, it’s been said many times, so arguably one can.

However, I don’t know what a **superposition** is, other than a piece of** mathematics** that we need in order to explain what we observe. We **need superpositions because they give particles their wave-like properties**. When we see **waves interfering in water** – **cancelling out where a crest meets a trough** – this is a **non-quantum effec**t, a “**classical**” effect as physicists say. But it turns out that **single particles** can **interfere** with **themselves**. When we** send** an individual** particle of light**, or **photon**, through **two thin slits** in a plate – a **double-slit** – we see, as expected,** a dot on the screen** behind the plate. But if we continue doing this for** many photon**s, we see an interference pattern built up from individual dots.

The only way we can explain this pattern is that **each particle is a sum** – a **superposition** – of **two paths**, one going **through the left slit** and one **through the right**. So why not just say that the pa**rticle goes both ways**?

Source: https://www.newscientist.com/

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