Probability: Schrodinger's Cat

Quantum physics is probably one of my favorite things in the world. It is the point where Science begins to blend with Philosophy and Religion, examining the very fabric of the universe. You have to ask yourself questions about Time (What happened before the Big Bang?), Space (What would happen if you fell into that Black Hole?), and even the particles and atoms that make up your own body. I'm making it sound really out there, like it is a bunch of old scientists sitting around and talking about life, but really it is Math-based and requires lots of experiments.

Before our time, Physicists (the scientists who work on this stuff) used to study the world on a level that we could see and touch. For example, you may have heard the story of Isaac Newton coming up with his theory of gravity when an apple dropped from a tree and hit him on the head. Well, that was in the year 1666, and we've come a long way since then.

[Extra: In the 1800s, Physicists were all about electricity and magnetism (which were discovered to be related when a teacher, setting up for class accidentally set some electric wires near a magnetic compass).]

As technology developed in the early part of the 1900s, people could make more and more exact observations about smaller and smaller objects. Physicists started to find that when they looked at the very smallest things, they behave in strange and surprising ways. I'm not talking about small like an ant or even small like the cells you learn about in Biology class. Smaller. I'm talking about atoms and the things that make up atoms, called subatomic particles. (That prefix sub- just means smaller than; and particle is a fancy word for object or thing.

[Extra: For those of you who have taken Biology, there are about 100 trillion atoms in a single cell -- and coincidentally there are about 100 trillion cells in a human body.)]

A picture of a Helium atom: two Protons and two Neutrons stuck in the middle (called the Nucleus), circled by two floating electrons.

You may already know about some subatomic particles from your science class: The most common ones are Protons, Neutrons, and Electrons. In addition to being subatomic particles, these can also be called quantum particles -- this means that they are particles that act in crazy and unpredictable ways because they are so small (we'll talk about some of that unpredictability in a minute).

Now, you may be wondering why a person would be interested in looking at something so small. Einstein got interested because he liked to think about Philosophy. The United States government got interested in the 1930s and 40s, when it was discovered that atoms hold huge amounts of energy for being so small and that if you blew up a lot of them at the same time you would have a bomb much more powerful than dynamite -- This led to the atomic bomb. Computer companies got interested in the 1980s when they started making electrical circuits so small that they acted in ways that the scientists couldn't understand -- Those circuits and chips now power your computers and cell phones.

One of the big things that makes the subatomic world so hard for us to understand is that opposite situations can be true at the same time. Remember how I mentioned that a coin when it's flipped can only be heads or tails when it lands? Not so for quantum particles.

These particles are rarely found alone (they are usually part of a larger atom, and atoms are usually clustered together), but let's imagine for a moment that we could flip an individual Neutron as if it were a coin. You toss it up in the air and when it lands, you cover it with your hand. If this were a quarter, you would know that it was Heads or Tails under there, and you were just waiting to find out. But for the Neutron, it would actually be both Heads and Tails -- until you looked at it. Only when you lifted your hand would the Neutron decide to be Heads or Tails. The situation in which the Neutron is both Heads and Tails -- or any time that opposites are true -- is called "Quantum Superposition" (This is part of the joke from the LOLCats picture at the beginning of this post: "kwantumz sooperpozishin.")

If you feel confused right now, it's perfectly natural; this idea makes no sense to us. Here's an example of how it would be if Quantum Superposition were possible for things that are our size: You could be watching a basketball game on TV, and let's say that a player is shooting at the buzzer to win the game. The ball is just leaving his hand, and suddenly your house's power goes out. You now do not know whether the ball goes in or not, whether the player's team wins or loses. Therefore, the team both wins and loses; the ball went through the net and it bounced off the rim. The team might even be both moving forward in a tournament and eliminated from it. ...That is, until you check the scores the next day to find out what happened (like lifting your hand off the Neutron). It turns out that the ball went in, and the team only remembers having won the game (not both possible outcomes). Crazy, huh?

The question that most people ask at this point is: how do we know that the Neutron wasn't Heads the whole time it was under our hand -- the way that a quarter would be -- and we only just found out when we checked it? That's the thing about the Quantum world: until a particle is observed (by another particle or by humans with a microscope), it doesn't have to decide what any of its characteristics are. A Neutron is not like a baseball or a watermelon that is solid and well-defined -- it's more fluid than that. A sub-atomic particle does not even have to choose where it is until an observer forces it to decide.

A Physicist friend of Einstein's, Erwin Schrodinger (SHRO-din-jer), took these issues a step further with the thought experiment (an experiment that you just imagine), that people usually now call Schrodinger's Cat. In the experiment, there is a certain atom that has a 50% chance of "decaying" (which means that it shoots out some of its energy as a tiny amount of light) in the course of one hour. The atom is inside a box with a detector that will see if light is released from the atom. That detector is hooked up to a device that, if some light is observed, would release poison gas into another box that has a cat inside. Once an hour has gone by (since it was the time period for the 50% chance that the atom would decay) so the experimenter turns off the detector.

PETA had a field day with this one.

This situation is different from the examples of "flipping" a Neutron and the Quantum basketball game, because it mixes the subatomic world with our very large one. A Neutron, by itself, has pretty much no impact on anything, so who cares if it's Heads or Tails. The quantum superposition of the basketball game simply does not happen in our world. But there is a quantum superposition for an atom's decay, and a cat's life hangs in the balance. (It could have been any animal or living thing in that box, just so long as it was something large enough to see and touch.)

The question that Schrodinger asked was: Could the cat be both dead and alive? Remember, we said that the 50% chance of the atom decaying and shooting off light is like the probability that a coin will land Heads or Tails, except that the coin is both Heads and Tails until we check -- the atom both has decayed and has not decayed. Can the cat in the box be in a quantum superposition of "Alive" and "Dead"? [There you go, that's the punch-line to the picture at the beginning of the post.] If the experimenter opened the box and found that the cat was alive, the cat would only remember having been alive. And if it were dead, then when did it die? When the experimenter opened the box?

Schrodinger first asked these questions in 1935. The best answer that anyone could give for a long time was: Yes, there is in fact a period of time in which the cat was both dead and alive (not like a zombie, but both fully dead and fully alive), until the experimenter opened the box. However, this is not true. Physicists have since figured out that there is no Quantum Superposition for very large things. If the experimenter opened the box and found that the cat was dead, then the cat died at some definite point before the experimenter opened the box and even before having turned off the detector. Remember how we said that the Neutron was both Heads and Tails, until we observed it? Well, the observer doesn't have to be a human; the detector in the box counts too. So there was in fact a definite time (let's say 35 minutes into the hour of the experiment) when the light was released from the atom and the cat died. There was no time when the cat was both dead and alive. [Note that if the atom had been all alone, far away from any other particles, then it would have been in a superposition of both decayed and not decayed -- but this is not the case in our experiment.]

This is all to give a simple explanation of an advanced concept in Probability. I'd said in a previous post that even though there is a 50% chance of getting Heads or Tails on any given flip, you couldn't have a coin flip that was 50% Heads and 50% Tails; it is always forced to be one or the other. Well, with (unobserved) subatomic particles, both Heads and Tails happen all the time. You can flip that Neutron as many times as you want, but as long as you don't look at it, then it is both Heads Up and Tails Up, every time.