Daily Archives: 20 December, 2011

The Uncertainty Principle

Today’s Music: They Might Be Giants – Particle Man

The Heisenberg Uncertainty Principle refers to measuring the position and speed of tiny particles in the quantum physics world. It states that the more precisely one property is measured, the less precisely the other can be controlled, determined, or known.
For those of you expecting a waggish post relating to why I always lose socks in the laundry, or why I can never find my keys, sorry to disappoint. While those are both interesting topics deserving of further attention, I’m actually going to stick with physics here.

I’m not a scientist. My math skills are adequate but unimpressive. But science, and quantum physics in particular fascinate me. The search for an explanation of the universe, from it’s largest phenomena to it’s smallest appeals to the tinkerer in me.
And while I knew what the Uncertainty Principle said, I never quite grasped the meaning.
Finally, someone explained it to me in a way I could understand.

It's not really there...

Right now, you’re able to see this because photons (light particles) are bouncing off whatever screen you’re looking at and reflecting onto your eyes.

Now think about the size of a photon vs. the size of your screen. there’s no comparison. Your screen is bombarded by photons, but because of the size difference, the constant barrage makes no difference.

Now shrink it down. Say to the size of an electron.

Objects on your monitor are MUCH smaller than they appear.

The photon and the electron are not that dramatically different in size. In fact, they are close enough, that the momentum of the photon that bounces off the electron is enough to move the electron, like you being bumped in a crowd during, say, a rush hour train commute.

So, the Uncertainty Principle (simplified) says you can either know the Speed of the electron OR it’s Position.
If I figure out the speed of the electron, I’ve done this by bombarding it with photons. How fast the electron passes through the photons tells me how fast it’s moving.
But since each photon that hits the electron changes it’s position a bit, I don’t know exactly where it is. To note it’s speed more accurately, I have to hit it with more photons, bouncing it around even more.

Going the other way, say I want to know where the electron is. So I tap it with just one photon. That will give the least bounce against the electron. So now I know where it is.
But because there is only the one (or other very small amount) of photons, I can’t track the speed of the photon.
I can only know one or the other.
Still with me? No shame if you’re not. Honestly, I first heard of the Uncertainty Principle in high school, and it’s taken me to the start of middle-age before I understood what it meant.

The explanation that cleared it up for me came from Dr. Brian Greene’s eminently readable The Elegant Universe. Seriously, there is very little math in the book, and some of the analogies are hilarious.

So, back to the Principle.
Since only one condition (speed or position) can be known at a time, only the probability of the other can be measured. If the speed is known, then I can only say that the electron is between a probably range here and there.
If I know where the electron is, I can only say it’s going between a probable range of this fast and that fast.

Albert Einstein (the guy with the hair) famously said “God does not play dice with the universe”. He never agreed with the uncertainty principle because it relied on probability instead of a solid “it’s going this fast, and it’s here”.

Do I look uncertain to you?!?

Now, all these years later, I finally have some idea of what the hell he’s talking about.
How cool is that?

****EDIT – If anyone sees anything wrong in the above, please correct me in the comments and I’ll update the post!!!  Thanks.