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A-Z: Quantum theory

Stephen Blundell

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'I think I can safely say that nobody understands quantum mechanics,' said Richard Feynman. Coming from this giant of 20th-century physics, the rest of us might feel we should abandon all hope. Yet it's not that people don't understand how to describe the world with quantum mechanics (it's the most accurately tested physical theory and has led to our technological world of iPods and mobile phones), it's the fact that the picture of the universe it presents is so deeply counterintuitive.  

You learned at school about waves (like sound or light) and particles (lumps of matter)? Well quantum mechanics tells you that they are the same thing: particles are really a bit wave-like, waves are really a bit particle-like. More confusingly, there, inside a box, is Schrödinger's cat, famously both alive and dead. It exists in a kind of limbo until you open the box and observe which state it is in. Then there are the strange correlations that exist between particles separated by a large distance but which retain a 'spooky' connection, even though no physical information can be transmitted between them. What is one to make of this extraordinary and surprising set of ideas?

First, our picture of the world has altered dramatically from the 17th-century view in which the universe seemed like an intricate kind of clock. Science was about figuring out how the gears and wheels worked in the cosmic machine, and it was thought possible, at least in principle, to know the mechanism precisely. Quantum mechanics shows that there are things which can never be known and remain unknowable, codified by Heisen-berg's famous uncertainty principle. Most debates about the relationship between science and faith are built on a pre-quantum view of science, tiny balls bouncing off each other in a predetermined cosmic game of snooker, rather than the mysterious world of the quantum.

Second, in the last few years people have been developing new information technologies which exploit quantum mechanics, replacing the zeros and ones of conventional computer bits with the Schrödinger-cat-like quantum bit (nicknamed a 'qubit').  Moreover, physicists are realizing that the key ingredient of the universe is not fundamental particles, but instead the quantum information that those particles encode. In the beginning then was not the particle, but the Word.

Third, quantum mechanics revolutionizes our view of the universe and reminds us how far away from everyday intuition our existence is. It shows that the universe is more surprising, subtle and beautiful than one could ever have imagined.  If this doesn't inspire a measure of wonder and awe, I don't know what does.

Stephen Blundell