# How I Learned to Stop Worrying About Schrodinger's Cat

### Tuesday, Jan 24, 2006 11:00 - 12:00 AM Hans Christian von Baeyer, winner of the Gemant Award

In the recent past, it has been shown that atoms are real (they can be photographed and manipulated), special relativity is correct (as shown in the GPS system), but light quanta are still controversial.

- The Heroic Age of Quantum Mechanics was 1925-1965: Heisenberg, Schrodinger, Born, Dirac and Bohr.
- The Muddle Ages were 1965-2005.
- The Information Age is 2005-?

What happened in 1965? Bohr had died in 1962. John Bell ruled out the local hidden variable theory, i.e., he showed that quantum mechanics is not a result of some underlying undiscovered classical mechanics. Feynman published his lectures in physics. Experiments confirmed the Bell inequality. Experiments could be done on single particles. Gedanken experiments became real. Quantum computation was in its infancy.

### There were competing interpretations of quantum mechanics:

- Copenhagen
- guiding wave
- many worlds
- spontaneous collapse
- consistent histories
- Ithaca
- Modal

### Some persistent problems were

- collapse of the wave function
- randomness
- interference (Feynman)
- entanglement (Schrodinger)
- emerging classical world

Schrodinger: "The wave picture does not describe observable facts, or what nature is really like"it is believed to give us information" Quantum computing uses qubits which can be described in terms of ket vectors: |q>=a|1>+b|2>, where a and b are complex numbers describing probability. As a result, a qubit can store an infinite amount of information. However, any attempt to retrieve the data results in only 1 bit of classical data. Probabilities are not frequencies, but rational estimates of betting odds. When new data is added, probabilities are updated. Examples: the probability of snow is 40%. We cannot get solid information from this probability. We must be careful not to make two grave mistakes: a) ignore the prior information. b) ignore new information. Example: There is a 1% incidence of a type of cancer. The test for the cancer is 99% accurate. You test positive. What is the probability that you have the cancer? Answer: 50%.