What the Heck Is the Uncertainty Principle?

Originally published here: https://mcgill.ca/oss/article/did-you-know/uncertainty-principle

If you’ve watched the Big Bang theoryor taken some science classes you’ve probably heard of something called the Uncertainty Principle. This theory, which looks like this in formula form: ΔpΔx = h basically states that we cannot know both the speed and the position of a subatomic molecule. Now, at least to me, that has always sounded a little bit like witchcraft. It just doesn’t quite sound real- we can’t both know the position and the speed of a molecule? But recently, in the fourth year of my chemistry degree, I’ve finally had a textbook explain this principle in a way that makes sense. 

You see, to measure a particle’s anything- speed, momentum, position- we need to detect it, or see it, or sense it. In some way, with a machine or our eyes, we need to measure it. And this act of measuring changes the parameter it measures. To ‘see’ a molecule, light (or some other molecule) needs to interact with it.  The photon of light that allows us to see the subatomic particle hits it,and bounces back to our retinas, but some of the energy and momentum of the photon is transferred to the molecule, like when 2 cars collide. So by any means we have of measuring a particle’s position or speed, we influence that parameter. This means that if we want to measure the position of a molecule, we can do so, but the photon we use to do so will change that molecule’s speed, so we can’t ever know the exact speed and position of a molecule.

But this is only true of subatomic particles, right? Nope! This effect actually occurs with everything, from a baseball flying through the air at the Skydome to your dad’s van driving down the road. Why don’t we notice this effect then? Simply because it’s too small.

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