As the night wears on at our programs at Perkins Observatory, the conversation often wanders to the weirder aspects of astronomy. You know. Science-fiction stuff. One such is the possibility of parallel “dimensions” and travel between and among them.
And I like to say, “It all began with a cat,” but that’s not really true, I suppose. I’m not sure whether the cat exists or not, and therein lies the possibility of parallel universes, or parallel dimensions as many science-fiction novels refer to them.
In fact, it all begins with quantum uncertainty. It’s a long road from there to parallel universes, so I’ll be taking two weeks to travel it.
For starters, let’s discuss that mysterious word “quantum.”
Physics can be divided into two types: classical physics and quantum mechanics. Classical physics explains most physical interactions, like why a ball bounces when it drops and why it dropped in the first place — in short, gravity. It can also be used to predict physical interactions, like what will happen when you drop a ball. All of those things happen on the level of the very large, in other words, the world of human experience, on a daily basis.
However, there are some physical interactions that it does not explain; for instance, how light can be turned into electricity and what that mysterious term “gravity” actually means.
For those explanations, quantum physicists dive into the level of the very small. They study the nature and interaction of subatomic particles, called quanta. Quantum mechanics thus provides a way for physicists to explain on the deepest level of the very small why those things happen on the level of the very large.
Bringing the two sets of explanations together has become the grand obsession of many people who study the two forms of physics, and with good reason.
The laws of classical physics can be tested and quantified. In that regard, Einstein’s relativistic theories simply explain what happens to the laws of physics at high velocities and masses.
On the level of the very small where quantum physics rule, that act of observation affects the result and makes accurate measurement quite difficult. That principle is known as quantum uncertainty.
Let’s say one subatomic particle is moving in such a way that its motion will carry it around another particle. Now imagine that all the other forces involved (the presence of other subatomic particles, etc.) balance out in such a way that the first particle has an equal chance to pass to the left or to the right of the second particle.
As long as it remains unobserved, the first particle could pass on either side. Where it actually goes will determine its interaction with other particles. The process continues ad infinitum with an increasing number of interactions and possibilities.
If we try to observe the interaction of the two particles, we would have to send photons or some other particles streaming in its direction. We would thus influence the motion of the particles and, in effect, choose one of the possibilities. If we don’t observe it, then all the motions remain possible.
It’s as if we threw a bowling ball down an alley and the only way to detect its motion toward the pins was to throw another bowling ball at it and listen for the sound of the impact. Before we threw the second ball, all the possible paths of the bowling ball remained possible, in potential at least. Throwing the second ball both discovered its path and changed the path at the same time.
What makes sense on the mathematical level in quantum physics seems patently absurd on the level of the gross physical world where classical physics rules. The ball goes where it goes. It knocks down 10 pins or nine.
On the level of the large, where we live, speculations about all the possible motions of the bowling ball seems silly. We do our best to understand why the bowling ball goes where it goes. We factor in the quality of our bowling shoes. We practice throwing the ball. We examine our strides and our release. But we also understand that we cannot consider all the factors involved. We do our best, but then we let ‘er rip and expect that the result is the only one that can happen even if we don’t understand everything that’s going on.
Thus, on the level of the large, imagining all the potential paths of the bowling ball seems pretty useless at best. Imagining that they actually happen in some parallel universe seems to violate both the laws of physics and the rules of logic. But that’s exactly what quantum physicists do. In other words, when we project the behavior of subatomic particles on the larger material world, weirdness prevails.
So how does quantum uncertainty relate to parallel universes and all that jazz? That’s where the cat comes in, but I’m sorry to say that Tabby will have to wait until next week.
Tom Burns is director of the Perkins Observatory in Delaware.