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Quantum mechanics describes the weird world of sub-atomic particles (Source: Tokarsky/iStockphoto)

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Quantum mechanics 101

Tuesday 26th May 2015 1:43 pm

The word “quantum” is bandied around an awful lot — especially when it comes to the hard sell. It’s very frequently mentioned in connection with alternative medicine – such as ‘quantum therapy’, ‘quantum DNA’, ‘quantum entanglement in psychic abilities’ and the old standby, ‘quantum healing’.

In reality, ‘quantum mechanics’ refers to very tiny processes that happen down at a scale smaller than atoms. So it’s hard to take it seriously when an ad tries to sell me a fridge magnet that is claimed to use quantum magnetic effects to heal everything from arthritis to varicose veins.

But amazingly, it seems that quantum mechanics can have effects not just on itty-bitty particles — but on big things like trees and eyeballs.

But let’s dive into quantum land, down at the sub-atomic scale, to see what it’s really like. It’s very different from where we live.

First, stuff is ‘quantized’ — in other words, it can exist only in packets of fixed sizes. By way of comparison, in our world, a sound can have virtually an infinite number of loudness levels, and the same goes for the brightness of a light, and so on. But in quantum land, physical quantities (such as light, magnetism, or location) can change only by fixed and discrete amounts. You can have one photon, or you can have two photons — but you can’t have one-and-a-half photons.

Second, there is the strange concept of ‘wave-particle duality”. You might have heard that energy (such as light) depending on the circumstances, can be either a wave or a particle. But did you know that matter, again depending on the circumstances, can also switch between being either a wave or a particle?

Third, the famous uncertainty principle. Up here, in the big non-quantum world, a motorist can be busted for speeding. The police officer can be quite definite when they say that the car was at a certain location, and travelling at a certain speed. Not so down in the sub-atomic quantum world. You can know quite precisely either a particle’s ‘location’, or its ‘momentum’ — but you can never know both at the same time. The same uncertainty holds for ‘energy’ and ‘time’.

Fourth, measurement. In our big world, you can measure the speed of a car, or the length of a kitchen table — and you don’t change either the speed of the car, or the length of the table. But in quantum land, the mere act of measuring changes what you just measured. It’s now different from what it was before you measured it.

Fifth, quantum entanglement. This confusing phenomenon happens when (say) a sub-atomic particle might decay into an ‘entangled’ pair of smaller particles. Let me explain.

At the beginning, the original single particle had zero spin. To keep things balanced, the two daughter particles have opposite spins, which adds up to zero. So you can’t consider one particle without considering the other particle — they are ‘entangled’. I hope this makes sense, so far.

By the way, while you know that the spins have to be different, you don’t know which particle has which spin. In fact, neither of them has any spin — until you actually measure them. (Whoa, now this is beginning to get weird.)

So let’s say the two daughter particles get a huge distance apart. Measure one of them to find its spin. The act of measuring one forces the other particle — no matter how far away it is — to collapse into the other spin. And this happens instantly — with no delay. It happens faster than the speed of light. This is so weird that even Einstein was worried by it.

At this stage, you are probably saying (quite reasonably), “OK, quantum is crazy. So why bother with it?”

It’s a two-part answer.

First, quantum mechanics successfully explains many details of the Universe around us.

Quantum mechanics is the only tool that can explain the behaviour of that zoo of sub-atomic particles that makes up atoms and energy. This zoo includes protons and neutrons and electrons, as well as photons.

Quantum mechanics is also essential for understanding how separate atoms combine to make molecules. We can’t really understand covalent and ionic bonding unless we use quantum mechanics — except now we call it quantum chemistry.

And the second part of why bother with quantum mechanics?

Quantum mechanics is essential in our modern society. The laser, the memory stick, the simple transistor and the complex microchip, your smart phone and even the dumb light switch on the wall — they all depend on quantum mechanics.

But what does quantum mechanics have to do with that magical thing we call life? I’ll leap into that question, next time …

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This blog first appeared on Dr Karl's Great Moments in Science

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  1. Joe Sands says:

    Hi Dr Karl,

    As an amateur scientist I have always been interested in physics. Unfortunately, current theories including quantum mechanics appear irrational. I have come across a website http://www.youstupidrelativist.com/RopeLite.html which explains many of the issues I have with modern physics i.e. defines time, light, magnetism, debunks quantum theories etc

    I don’t have the expertise to mathematically look into these theories and was hoping your influence would lead to someone with more mathematical skills to explore the rope hypothesis and its consequences to understanding our universe.

    Looking forward to your response.

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