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Schrödinger's cat for a 6th grader

The following is my response to a 6th grade student's questions about Schrödinger's cat. It involves a limited discussion of some of the issues in quantum mechanics, which led to Schrödinger's famous cat.

(To read Schrödinger's brief description of this thought experiments click here.)

There are partial answers to your questions based on the Copenhagen Interpretation of quantum mechanics. Schrödinger was referring to this interpretation in his description of the cat experiment. There are currently no scientific answers to your questions. The Copenhagen Interpretation is not a scientific theory and is not universally, or even widely, accepted by scientists.

First, if a person was in the box, then he or she would observe their own demise or continued existence. This would be the same as looking in the box. You second question is similar to the first. If any person looks in the box and sees what happened to the cat, then it has happened, and everyone else will see the same thing. I suspect your friend, that talked about people in different realities, has heard about the Many Worlds Interpretation of quantum mechanics. This is an interpretation that competes with the Copenhagen Interpretation, and did not exist at the time that Schrödinger described the cat experiment.

Schrödinger invented this fiendish experiment, not to explain what happens to the cat, but to show that the existing theory had confusing aspects to it. Scientists understand a great deal, but the more they learn the more new unanswered questions occur to them. Although the cat example is over 70 years old, it is left for future generations, perhaps for you and your friends, to figure out a good answer to your questions.

I do not usually have time to answer questions like yours, but I thought it might be useful and fun to write an simple explanation of Schrödinger's cat and post it on my web site.

To understand Schrödinger's cat, one must understand what a strange theory quantum mechanics is. In all other scientific theories, we have models of how we think things work. For example, we know that distance traveled equals time traveled multiplied by velocity. If you travel for two hours at 50 miles an hour, you will go one hundred miles. We can measure the time with a clock and the distance with the odometer on your car. Quantum mechanics is not like that. What we measure in experiments is not described by quantum mechanics. Instead quantum mechanics gives the probability that we will make a given measurement.

Probabilities occur all the time in science, because we almost never know everything we need to make a completely accurate prediction. For example, if you want to make a trip of a hundred miles, you can not know ahead of time exactly how long it will take. You might run into a traffic jam. You can only give an estimated time. In quantum mechanics probabilities are different. They are not considered to result from our limited understanding of the universe, but to be fundamental. Of course Einstein thought this was mistaken, but most physicists do not agree with him.

Quantum mechanics only describes how probabilities change with time. For example, if the particle in the cat example has a 50% probability of decaying in one hour, then in one minute it will have only a slight chance of having decayed. After 10 hours it will have a probability very close to one of having decayed. Quantum mechanics gives an exact model of how that probability changes over time. It says nothing at all about the state of the cat as these probabilities change. Science tells us what the probabilities are, but is completely silent on what (if anything) happens between observations. That is why there is no scientific answer to your questions. There is not even a scientific definition of what an observation is.

In the quantum mechanical model nothing ever happens! The particle never has to decay. The probability just keeps getting closer and closer to one. There is nothing to force a real event to happen. This is very confusing, because what we observe is always real events. We see the particle decay at some particular time.

Niels Bohr proposed a solution to create the events we all observe. He assumed that conscious observation caused events. He thought there was an aspect of the world described by the evolution of probabilities in quantum mechanics, and an aspect of the world that we observe. Whenever we look in the box or make a measurement, we get some definite result and not just a probability. But quantum mechanics never models these events. If we do not look, there are no events. There are just probabilities changing.

Bohr's idea is called the Copenhagen Interpretation of quantum mechanics. It is not science. For something to be part of science, you must be able to test it experimentally. Interpretations do not make predictions that differ from the probability evolution described by quantum mechanics. They are an attempt to give a philosophical explanation of how specific measurements come about. Because one cannot test them experimentally, physicists cannot reach agreement about them. Thus there are several different interpretations, and there is no way to know if any of them are correct.

If you are confused by this you are not alone. I do not think anyone has a good understanding of what is going on here although many physicists are firmly convinced of the correctness of the interpretation they favor. My own inclination is to think that Einstein was correct, and we need a deeper theory to explain events, like the decay of a particle, that will dispatch Schrödinger's poor cat.


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