##### Physics I For Dummies

In the traditional interpretation of quantum physics, the wavefunction is seen as a representation of the probability that a particle will be in a given location. After a measurement is made, the wavefunction collapses, giving the particle a definite value for the measured quantity.

In the double slit experiments, the wavefunction splits between the two slits, and this wavefunction results in an interference of probabilities on the screen. When the measurements are made on the screen, the probabilities are distributed so that it’s more likely to find particles in some places and less likely to find them in other places, resulting in the light and dark interference bands.

The particle never splits, but the probability of where the particle will be does split. Until the measurement is made, the distribution of probabilities is all that exists.

This interpretation was developed by the physicist Max Born and grew to be the core of the Copenhagen interpretation of quantum mechanics. For this explanation, Born received (three decades later) the 1954 Nobel Prize in Physics.

Almost as soon as the explanation of probabilities was proposed, Erwin Schrödinger came up with a morbid thought experiment intended to show how absurd it was. It’s become one of the most important, and misunderstood, concepts in all of physics: the Schrödinger’s cat experiment.

In this experiment, Schrödinger hypothesized a radioactive particle that has a 50 percent chance of decaying within an hour. He proposed that you place the radioactive material within a closed box next to a Geiger counter that would detect the radiation. When the Geiger counter detects the radiation from the decay, it will break a glass of poison gas. Also inside the box is a cat. If the glass breaks, the cat dies.

Now, according to Born’s interpretation of the wavefunction, after an hour the atom is in a quantum state where it is both decayed and not decayed — 50 percent chance of each result. This means the Geiger counter is in a state where it’s both triggered and not triggered. The glass containing the poison gas is both broken and not broken. The cat is both dead and alive!

This may sound absurd, but it’s the logical extension of the particle being both decayed and not decayed. Schrödinger believed that quantum physics couldn’t describe such an insane world, but that the cat had to be either completely alive or completely dead even before the box is opened and observed.

After you open the box, according to this interpretation, the cat’s state becomes well defined one way or the other, but in the absence of a measurement, it’s in both states. Though Schrödinger’s cat experiment was created to oppose this interpretation of quantum mechanics, it has become the most dramatic example used to illustrate the strange quantum nature of reality.