Jetsam

In his seminal 1964 paper titled On the Einstein Podolsky Rosen Paradox, Irish physicist John Stewart Bell presented a theorem that has been called "the most profound in science". It has also been called “one of the most profound discoveries since relativity”. Certainly, it is one of the most important theorems in quantum mechanics. It is called Bell’s theorem (a.k.a. Bell’s inequality).

Let’s say you and I have a hat (upside down) on a table before us. The hat contains two marbles, a red one and a blue one. I reach into the hat and remove a marble, taking care to not look at it. You remove the other marble, also taking care to not look at it. If I have the red marble, you must have the blue marble. If I have the blue marble, you must have the red marble. This would seem to be common sense. Classical physics says that my marble is either red or blue, even though I haven’t looked at it yet to determine its color. In other words, classical physics says that reality is independent of observation. But quantum mechanics says that my marble is both red and blue until I look at it. Quantum mechanics says my marble (and yours, too) exists in a state of superposition in which it has a 50% probability of being red and a 50% probability of being blue. Observing the marble is an act of measurement. When a measurement (observation) is made of the marble, the probability for one color instantly goes to 100% while the probability for the other color goes to 0%. So I look at my marble and in that instant my red/blue marble becomes one color and in that same instant your marble becomes the other color, even if we are trillions of miles apart. Which begs the question: if your marble was, like mine, both red and blue until I looked at my marble, how did your marble know, instantly, what was happening to my marble? We don’t know how this happens but we have a name for it. It’s called quantum entanglement.

Being a reasonable and rational person you probably object to the notion that my marble (and yours) is really both red and blue until one of us looks at it. You are probably thinking it really does have one color or the other, but we don’t know which color it has until we look.

For actual marbles, you may be right. But for atomic-scale things like protons, electrons, photons, and such, it’s not so clear whether their properties exist before we measure them or if they are created by the act of measurement. John Bell’s brilliance was that he devised a way for us to experimentally determine whether reality is observer-independent on an atomic scale. His theorem makes two assumptions about our world. It assumes that reality exists independently of an observer, and it assumes that no signal can travel faster than the speed of light. Numerous experiments have tested Bell’s theorem and they all confirm this simple truth: at the atomic scale, either reality does not exist independently from an observer, or faster than light communication is possible (or both). There are good reasons why faster than light communication is impossible, so we’re left pondering this thought: at the level of atoms, particles don’t have properties until someone or something measures those properties. But that begs the question: exactly what is a measurement.

Let’s go back to the marble experiment. Suppose I close my eyes and open my hand with the marble in it and take a photo of the marble. Later I look at the photo and see the marble is blue. Does the marble become blue when I look at the photo, or did it become blue when I took the photo? Quantum mechanics doesn’t tell us.

And there’s another obvious question: how big does a particle have to be before this red/blue superposition quits happening? Quantum mechanics doesn’t tell us that, either.

So just to be clear, quantum mechanics, a body of scientific principles discovered and added to since the early twentieth century, and which has been tested and experimentally verified again and again and again, and which has become the underlying mathematical framework of many fields of physics and chemistry, including condensed matter physics, solid-state physics, atomic physics, molecular physics, computational physics, computational chemistry, quantum chemistry, particle physics, nuclear chemistry, and nuclear physics – quantum mechanics tells us the moon doesn’t exist when no one is looking at it.

If you want to jettison that thought from your brain, then go ahead and lighten the load.