Quantum Mechanics

What is quantum mechanics?

When you are trying to describe how a tiny particle will act (compared to an object you can see unaided in the real world) weird stuff starts to happen. This is the basic premise of quantum mechanics. 

Quantum mechanics is very different from classical mechanics. For example, Newton’s laws of motion explain how everything moves in our everyday, classical world. If you kick an object like a football, you expect it to move in a certain way. 

In the quantum world, footballers would be out of a job. If you kicked a quantum football, it wouldn’t travel in a straight and continuous line. It would move in a series of quantum leaps (random jumps) between different locations. 

This is a key principle of quantum mechanics. In the quantum world, properties like position, speed and colour are quantised (they have discrete quantities) and are not continuous. 

Another concept to get your head around is that quantum mechanics is based on probability. 

In other words, you can’t predict the exact position of the quantum football at a particular time. But you can predict its likely locations.

One of the core principles of quantum mechanics is the “Heisenberg Uncertainty Principle”. This states that the velocity of a particle and its position can never be precisely measured at the same time.

If you increase the precision of measuring one property (say, the position), then you cannot measure the other property (velocity) as precisely.

The equation for the Heisenberg Uncertainty Principle is this:

ΔxΔp ≥ h/4π

• Where Δx is the uncertainty in the exact position
• Δp is the uncertainty in the exact momentum (or velocity because momentum is simply mass times velocity)
• h is a constant called Planck’s constant 
• π is a constant called Pi

All this is saying is that the product (multiplication) of the uncertainties in the position and velocity must be greater than or equal to some number. 

If you increase the certainty of a particle’s position, you decrease the certainty of its velocity. Here’s a more thorough explanation of the Heisenberg Uncertainty Principle.

So what?

If quantum mechanics exists on a scale so small that we can’t see it, why do we need to understand it?

Well, the principles of quantum mechanics have guided some of our most important discoveries. For example, the transistor and diode (two important parts on a computer chip) are based on the laws of quantum mechanics. Without quantum mechanics, I wouldn’t have a computer to write this on.

​There is also a range of practical uses for one of the spookiest quantum mechanical phenomenon – entanglement. Quantum entanglement states that particles on opposite sides of the universe separated by billions of light-years are intrinsically linked, and this link lets them share information instantly. ​Einstein scoffed at the premise – but it was later proven by John Stewart Bell.

But that’s one of the core problems with quantum mechanics – its principles and ideas are so alien to our human brains that we can struggle to visualise and understand them.

In fact, physicist Richard Feymann once said: “I think I can safely say that, nobody understands quantum mechanics.”

What else?

Scientists have started to provide us with the first glimpses of quantum effects happening on a scale just big enough to be seen by the human eye. This is exciting as it means a new era of understanding and discoveries in the quantum world can, hopefully, begin.

Want more?

Jim Al-Khalili’s two-part BBC series “The Secrets of Quantum Physics” brings this perplexing scientific theory to the masses. Here’s the first episode: