The Heisenberg Uncertainty Principle is one of those weird predictions of quantum mechanics which have somehow managed to bleed into popular culture. Indeed, I have found that, when I speak to a nonscientist, and inform them that I am a theoretical physicist, the common reaction is for them to crack some lame joke, either about the uncertainty principle or about Schrodinger’s cat, and then immediately change the subject*.
The principle, for those who have not heard it, is simply this: it is impossible to know both the position and momentum** of a particle beyond some minimum value of uncertainty. Thus, it’s possible to speak only in terms of ranges of values that position and momentum may take on. The more you know about position, the less you know about momentum and vice versa.
It’s a thing I’ve always just taken for granted; it falls so beautifully out of the mathematics. But, to my great shame, I must admit that, for whatever reason, it never actually occurred to me to ask whether this prediction had been experimentally confirmed or not.
It turns out that, while a number of experiments have been devised to indirectly test it, no one has ever actually simultaneously measured position and momentum. Indeed, this have even been commented on before: in The Philosophy of Quantum Mechanics (1974), the author, Max Jammer cheerfully notes:
“Turning now to the question of the empirical support [for the uncertainty
principle], we unhesitatingly declare that rarely in the
history of physics has there been a principle of such universal importance
with so few credentials of experimental tests.”
Writing in 2007, Busch, Heinonen and Lahti note that little has changed in the intervening decades.
Now, it seems that a team of researchers at the University of Toronto have conducted an experiment which suggests that this Uncertainty Principle might not actually be as certain as it seemed (here’s the actual paper).
Now, of course, obviously this is not to say that the UP is completely wrong and that the last hundred years of physics are garbage, but it does show that no matter how good your math is, ultimately science requires experimental evidence.
And considering just how ridiculously far removed from the bare possibility of experimental verification that so much of theoretical physics has become, I find it more than a little worrisome.
*When I tell them that I’m a mediaeval historian, I can almost manage to strike-up a conversation.
**Momentum, mind you; not speed. This is a common misconception.