Sunday, April 4, 2010

Quantum Confusion

I’m going to let you in on a secret that I’ve learned as an undergraduate physicist at MIT.

No one understands quantum mechanics.

That seems odd, considering that most all of modern technology relies on it. Sure, physicists do the calculations (and do them spectacularly), but turning the crank on a machine doesn’t tell you why it does what it does. The problem is, science is really good at answering the question of “what happens,” but not so skilled at “why it does.” This leads physicists to uncomfortable situations when trying to tell people what in the blazes they’re talking about.

The mathematical foundations of quantum physics are rock solid; indeed they are the most statistically accurate theory we’ve ever created when it comes to testing predictions. The odd part is, the math involved is incredibly different then anything we had used before. A series of rules, called “axioms,” are followed. This is an idea descended from the Greeks, who realized that if you want to prove anything, you have to start from some basic unprovable assumptions (e.g., parallel lines will never touch).

It’s within these axioms that the weirdness of quantum physics hides. We can’t test why the weirdness exists, because we had to assume its existence to get anywhere.

For example, you may have heard of the famous example of “Schrödinger’s Cat.” Often told, this story is a way that physicists try to explain one particular weirdness about quantum mechanics. The story goes something like this:

“A mechanism is set up containing a radioactive material that could randomly decay at any moment, and when it does, a poisonous gas is released. If a cat is sealed in a box with this device and isolated from the outside world, quantum mechanics tells us that at a given moment, instead of a cat who is definitely alive or dead, we’d have a quantum cat who is alive and dead at the same time. Then, when the box is opened and observed there is a 50% chance of us seeing a dead or live cat.”

This is pretty confusing. The thing is, quantum mechanics really deals with insanely tiny object, like atoms or electrons. Connecting a tiny object (random decay of a radioactive material) to a large object (Schrödinger’s Cat) is a way of making us see how weird are the things quantum mechanics says go on inside tiny objects.

One of the axioms of quantum mechanics is that we cannot say an electron is doing one particular thing at a given time, and that we have to account for numerous possibilities instead, until an observation happens. The math then tells, incredibly accurately, the probabilities for different outcomes.

What the math cannot tell us, though, is what “exists”; in fact, the math doesn’t care about our philosophical notions of existence. The reality is that when an electron is flying around, we have to take various possibilities into account at once. From this uncomfortable situation, we run into the issue that an electron can “be” in two places at the same time; and even weirder, that these two possible electrons can “bump into” and affect each other! It’s like the live cat being able to sniff the dead cat’s body!

We’ve done countless experiments, and there’s no way around it. In the real world, a single electron can and does bump into itself. And no one knows why.

In my next post, I’ll be telling you about something even weirder: the work of a scientist, Alan Guth, who thinks the universe might have spawned from pure nothing. I’ll only be able to scratch the surface, but you’ll be able to hear more from Professor Guth himself at an incredible event happening during the Festival called “Big Ideas for Busy People,” which Meghan has already blogged about. There, you can hear prominent scientists from a wide variety of fields talking about the beautiful, and often counterintuitive, truths about our world. Its a great opportunity to hear revolutionary thoughts from the pioneers themselves, and I am really excited to see it happen!

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