Science In Real Life

Welcome, welcome, one and all to the second helping of quantum weirdness, in case you didn’t get enough the first time around. We’ll be continuing along the same basic theme; see if you can guess what it is before I reveal it at the end, and tie this whole thing up in a pretty little bow with flowers on it.

Next up, quantum tunneling, which is in all honesty one of the coolest things ever. This is not to be confused with quantum Chunneling, which is when you suddenly find yourself transported from England to France. If you have a wavefunction approaching a barrier, which is to say a region where the particle is not allowed to be, there is a small probability that it will simply appear on the other side, like so:

This graph, again, is a wavefunction, a probability wave, so don’t think of this reduced amplitude as a less energetic particle. The size of the wave indicates only the relative likelihood of finding a particle when you do a measurement. This process has given us the Scanning Tunneling Microscope, and subatomic particles do this on a daily basis; it has effects in scenarios cosmological and biological.

So why can’t we walk through walls? What gives? Clearly not the walls. As you can see here, the thickness of the barrier is a very important factor. To us, a wall is sturdy, but only about as thick as we are. To one of your protons … you might as well try to launch an iPod unscathed through the thronging crowds of Macworld. It’s not going to end well. And for a macroscopic object like us to do it, all of our particles have to do this simultaneously and in perfect unison, otherwise you end up as a fine paste smeared across the floor. So things on our scale stay where they are, mostly.

There is one last bit of weirdness I want to discuss, and this is the weirdest of all. Just like the position of a quantum particle is fundamentally indeterminate, so too is the amount of energy of the entire Universe. It simply has no definite value, and we are sadly lacking in interactions with other Universes to fix it. What that means is, at any moment, there is a chance that a pair of virtual particles will pop into existence and then immediately annihilate again. For small particles, it’s a rather good chance. In fact, this is happening all the time, everywhere. Yes, even there. Please nobody be alarmed! I promise it’s harmless. Well, mostly harmless.

You see, this process is governed by the energy-time uncertainty principle, younger and more mysterious brother to the position-momentum version. Essentially, the shorter something is around, the bigger the variation in its energy. So on very very short timescales, you can get things bigger than particles. Like pogo sticks and ponies and Christopher Walken.

These are fun to think about, but I cannot stress enough how unlikely it would be for even a single atom to begin existence in this fashion. And so it is that the realm of zero point energy production, like so much else from quantum mechanics, remains hidden from our normal view.

The underlying concept behind all of these is something called superposition of states, and I don’t mean some mutant abomination hybrid of California and Texas. First, in the language of quantum mechanics the state of the system is whatever we need to know about it to describe it as distinct from other states. In classical physics (think Baroque) everything is in one state, and we can predict what state it will go to next with absolute certainty. This turns out to be very wrong. Objects can be in more than one state, this is that superposition business. It is only an interaction that forces it into one state or another, as dictated by the probabilistic laws that govern its behavior.

Into the very general framework I have just sketched you can color in just about any situation you want, from Schrodinger’s cat to the electrons in an atom to quantum tunneling and indeed the energy of the entire Universe. One need only change what is the possible selection of states to which one refers. So where does this weirdness go? We don’t see the delicate selection process. Our brains, accustomed to having evolved from a need to react quickly, need to make assumptions about where things are. Our brains are expert spatiotemporal librarians, quickly filing everything into a sequence of locations and making snap judgements about how to react. Any errors in this are small enough to not kill us, so we don’t notice them.

And so it is that quantum effects remain shielded from our view. But I hope that you will leave here tonight having made a quantum leap in perspective, the better to appreciate the magnitude of weirdness permeating our existence.