Feynman on probability and uncertainty

Watch the presentation (thanks Bill).

Feynman gives a captivating talk using the double slit experiment which shows that electrons and photons have both lump behaviors (like bullets) and interference (like waves), and that observation can destroy the interference behavior. Quantum physics is not analogous to anything we know from our daily experience.


Observation messes up interference:

The probability of any event in an ideal experiment (that just means when everything is specified as well as it can be), the probability of an event is the square of something, which is called the probability amplitude. And when an event can occur in several alternative ways, the probabilty amplitude ‘A’ is the the sum of the of the A’s for each of the various alternatives. And finally, if an experiment is performed which is capable of determining which alternative is taken, the probability of the event is is the sum of the probabilities for each alternative – that is, you lose the interference.

The probability amplitude is an complex number whose absolute value squared represents a probability (real positive numbers).


Limits of predictability:

The uncertainty of quantum physics is a feature of the fundamental law. It is not due to our lack of detailed knowledge that we cannot make a prediction. We know that there is no hidden variable or machinery for us to discover and understand.

Someone said it this way: “nature herself doesn’t know which way the electron is going to go”.


How does science work in presence of the unknowable?

A philosopher once said (a pompous one): “it is necessary for the very existence of science that the same conditions always produce the same results”.

Well, they don’t. (…)

They don’t, and yet the science goes on in spite of him.

What is necessary for the very existence of science is just the ability to experiment, the honesty in reporting results (…) and finally, an important thing, is the intelligence to interpret the results. But the important point about this intelligence is: that it must not – it should not – be sure ahead of time about what must be. (…)

I don’t mean absolute prejudice, just bias. But not strict bias, not complete prejudice.

As long as you’re biased, it doesn’t make any difference because if the fact is true, there will be a perpetual accumulation of experiments to perpetually annoy you, until they cannot be disregarded any longer.



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