top of page

How to understand Quantum Mechanics?

 

Even the most famous scientists have struggled to understand Quantum Mechanics:

“Everything we call real is made of things that cannot be regarded as real” - Niels Bohr

“Those who are not shocked when they first come across quantum theory cannot possibly have understood it” - Niels Bohr

“If you are not completely confused by quantum mechanics, you do not understand it” - John Wheeler

“If [quantum theory] is correct, it signifies the end of physics as a science” - Albert Einstein

“I do not like [quantum mechanics], and I am sorry I ever had anything to do with it - Erwin Schrödinger

“Quantum mechanics makes absolutely no sense” - Roger Penrose

“It is safe to say that nobody understands quantum mechanics” - Richard Feynman

“Everyday Quantum Mechanics is just fine, for all practical purposes (FAPP)” – John S. Bell

“Just shut up and calculate” – David Mermin (on trying to understand the Copenhagen Interpretation of Quantum Mechanics).

 

 

Quantum Mechanics is just a mathematical theory

Quantum Mechanics (QM) is a mathematical theory (definitions, axioms, rules) applied to explain certain results of laboratory experiments, e.g. the double-slit experiment. It does not attempt to explain how Nature works (nobody knows that), it just aims to predict the outcome of certain (statistical) experiments and has been very successful in doing this. Conventional arithmetic is also a mathematical theory, but few people try to “understand” arithmetic, and no-one can “prove” that 1+1 = 2. We just learn our "times tables".

 

The main problem we face in applying QM in quantum computing and communications is that the laboratory experiments are hugely complex with equipment, laboratory, observers, and external forces (e.g. radiation and gravity). Qubits do not behave as we expect them to do from QM theory since we cannot account for all the external factors. Hence, there is decoherence and errors. The success of QM with technologies such as lasers is simple to explain. Lasers manipulate millions of photons and, if a few decohere, it’s not a problem. Manipulating individual photons, atoms and ions as qubits is a much more challenging task, and it is a great tribute to the scientists and engineers doing it today.

 

The many “interpretations” of Quantum Mechanics

There have been so many, it is difficult to count. New ones continue to pop up on the “arXiv” (e.g. by Inge Helland). The Copenhagen Interpretation is still by far the most widely accepted and is the version taught in most universities. However, among the global community of physicists, mathematicians and philosophers, there is no agreement on this. Some of the most notable alternative interpretations are as follows:

  • Pilot Wave/Hidden Variable theories – De Broglie/Bohm

  • Many Worlds – Hugh Everett

  • QBism – Fuchs/Schack

  • Cellular Automata – Gerard t’Hooft

  • Relational Quantum Mechanics – Carlo Rovelli

 

How to understand Quantum Mechanics?

For 99.9% of the population, the answer is simple – DON’T! Just shut up and calculate. It’s just a mathematical game that seems to work in practice. Learn the rules and play the game. Repeating John Bell: “Everyday (Copenhagen) quantum mechanics is just fine for all practical purposes (FAPP)”.

For the remaining 0.1% of population (physicists, mathematicians, philosophers) the search for new alternatives to QM must continue in order to advance the progress of science. As Einstein famously stated in 1935 (the “EPR” paper) QM is not a complete theory. So far, there has been no success in defining a quantum theory of gravity, which indicates that something is wrong with either QM or General Relativity, or we have not found a way to combine them. Neither String Theory nor Loop Quantum Gravity have yet solved this problem. We should anticipate changes to QM in the future which might impact Quantum Information Science.

Meanwhile, education in QM should be revisited. Teaching Copenhagen/FAPP as the only method is a failure to students. They must understand the broader aspects of QM to advance our knowledge in this field.

 

Suggested Reading

For those interested in digging further, the book “Speakable and unspeakable in quantum mechanics” by John S. Bell is invaluable. It is a collection of his papers on QM, including the amusing article on “Bertlmann’s socks” that discusses the nature of entanglement. Cambridge University Press, 1989

bottom of page