Tuesday, August 18th, 2009

Quantum Possibility

Nothing is impossible.

How many times have we heard this? Probably enough to make it less of a truth statement and more of an encouraging gesture.

But what if I told you that Quantum Mechanics – a scientific field which has never made a prediction that observable evidence has contradicted – actually proved that nothing is impossible? What if our universal laws were such that instead of defining exactly what was possible, they defined only how possible events were.

I wish to introduce to you the science of Quantum Possibility.

1. Introduction

Quantum Theory has been around since the 1920’s, and has yet to yield a prediction that disagrees with observation. It was engineered in order to describe the newly made observations of the early twentieth century in the realm of atoms. No other scientific theory correctly models electrons in non-decaying orbits around atoms. Additionally, no other scientific theory explains blackbody radiation.

Classical mechanics failed to explain how electrons maintained their energy orbits. The idea of quantized energy solved this problem.

Classical mechanics failed to explain how electrons maintained their energy orbits. The idea of quantized energy solved this problem.

In grade school, I learned that an atom was comprised of a nucleus (protons and neutrons) and some orbiting electrons. This is the Rutherford model of the atom. This model failed in a few ways, specifically because it demanded (with classical, or pre-quantum mechanics) that any electrically charged particle moving in a circular orbit (any path not straight) would lose energy and spiral down into the nucleus. This isn’t what happens.

Neils Bohr solved this problem by observing that electrons only lose energy in specific amounts, in quanta. Therefore an electron orbiting a nucleus doesn’t slowly spiral in towards the nucleus because it isn’t physically possible to lose the small amounts of energy required for this decay.

Electrons in an atom therefore have specific energies which depend on the electromagnetic configuration of the whole atom. These are known as the orbitals or quantum states of an electron.

2. Probability

The idea of quantized energy is central to quantum mechanics, but many are better acquainted with the idea of probability which arises out of quantum mechanics. So where does this come from?

We can illustrate it well using an example of measurement.

Consider a completely dark room in which a billiard ball is floating around. In order to determine where the ball is, you throw some ping-pong balls around the room. If you write down how fast and in which direction you throw each ping-pong ball, and you also detect where each ping-pong ball strikes the walls of the room, you can figure out which ones hit the billiard ball, and use some trigonometry to find out where the billiard ball was when it was struck.

However, since you don’t know how the billiard ball was moving before the collision, you actually can’t tell anything about it’s movement after the collision. You could throw a heck of a lot of ping-pongs around, and determine the billiard ball’s path in the room, but you still don’t know how the ping-pong balls strike the billiard ball – head on or glancing impact?

This is how it is in the quantum world, if say the ping-pong ball represents a photon (light wave) and the billiard ball represents an electron.

Now if we knew some more about the layout of the dark room, we might be able to draw some conclusions without measuring anything, such as the probability that the billiard ball would be somewhere or moving somehow at some time. We could construct a probability, but not a definitive conclusion of the ball’s properties.

3. Scope

Cue ball on a billiard table. Macro shot. Credit: Christopher X. Gerber, 2009.

In order for quantum physics to really work, it also needs to describe how regular things work. For instance, on a well lit billiard table, I can measure both the position of a billiard ball and it’s momentum at the same time. How is that possible?

We can explain this phenomenon by further understanding probability. In our previous analogy, we were using ping-pong balls instead of light waves to see. In our real example, we are using actual light waves, which together have such a small impact on the billiard ball’s movement that the uncertainty “disappears”.

4. Possibility

However, even in the real world, there still is an uncertainty in the billiard ball’s momentum and position, despite it being incredibly small in relation to the ball’s size.

Imagine you are looking at the billiard ball on the green table. Did you know that it is possible for the ball to suddenly disappear and reappear on Jupiter? Mathematically speaking, the probability that this will happen is really really small, but absolutely not zero.

Quantum tunnelling is the process by which a particle/wave passes through a barrier. The probability of the particle travelling beyond the barrier is less (as seen above) than bouncing back.

Quantum tunnelling is the process by which a particle/wave passes through a barrier. The probability of the particle travelling beyond the barrier is less (as seen above) than bouncing back.

We actually live in a universe where nothing is impossible – over all time. What I mean is that something strange, like suddenly finding yourself a hundred kilometers away, will absolutely happen if you have an infinity of time to wait. That said, there’s nothing to say (mathematically) that there is a difference between waiting a trillion years and waiting a few seconds for it to happen.

I should point out however, that there are no particles as we are used to thinking about them. Every particle in your body, for instance, is a probability wave that stretches out past Jupiter. In a way, the probability wave that you are already exists on Jupiter. However the size of your probability wave there is really really small, and where you are it is really really big. That’s why you are where you are most of the time.

At any rate, we can conclude with quantum mechanics that anything is possible, at any time.

Since there is nothing to distinguish between right now and the distant future, is there any reason to take the incredibly small probabilities as essentially zero? Is there any reason to conclude based on what we usually see in the world that there are things that are essentially impossible (incredibly improbable)?

Chris Gerber is 33 and lives in Vancouver, Canada.

After graduating from a B.Sc (Physics major) program in 2007, he spent seventy days in Southeast Asia with the goal of discovery, on both personal and cultural levels.

Since his return, he has resisted automatic entry into the work-force, spending his time instead wondering how best to serve the world.