Quirks & Quarks

How big is too big for quantum mechanics?

The behaviour of both large and small objects can explained with quantum mechanics, it just gets really complex with bigger objects.

There is no theoretical size limit for quantum effects, but there is a practical one

A theoretical physicist takes notes on a chalkboard. (Dean Mouhtaropoulos/Getty Image)

This week's question comes from Peter Furlan from Oakville, Ont. He asks:

Classical physics describes how large objects act. Quantum mechanics describes how incredibly small matter acts. What can we learn at the cutoff point between "large" and "small"? Is this point even observable?

Stephanie Simmons, an assistant professor in the Department of Physics at Simon Fraser University in Burnaby, B.C., says quantum mechanics can be used to describe the behaviour of objects of all sizes but it's typically used only for small particles because it becomes very complex with larger objects.

Quantum effects like superposition, which allows objects to be in two places at once, are observed on small particles in laboratories all around the world. But there's nothing that says this can't be observed on larger objects - technically a blue whale could be in superposition in it's ocean habitat and your living room.

However when working with larger objects with lots and lots of particles, like blue whales, calculating the object's behaviour with quantum mechanics requires a lot of number crunching. If you were to do it, you would end up with an answer that is very close to the answer classical physics provides. So quantum mechanics works on any size of object but for convenience, classical physics is generally used for objects that are larger than a few particles.


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