QUESTION: Hi James
simple questions...sort of!
Can quantum theory make any prediction on a SINGLE event?
The 2 slit experiment- quantum theory can only give an accurate prediction over many events? As a probalistic theory, it requires numerous events to realise its probabilities?
If this is the case....how is it science?
A theory must predict outcomes that are unfailingly correct for each event?
Thanks very much for your time
ANSWER: Hi Richard,
First, I apologize for the late answer, but I was out of town.
To answer your question, it seems that a theory does not have to predict EXACT outcomes. In fact, at the micro level, exact outcomes are often impossible to know. One of the most successful theories in physics is Quantum Electrodynamics (QED), for which its creators won the Nobel Prize in 1965. QED predicts the magnetic moment of the electron to within 10^(-14) of the experimentally measured value. It also predicts numbers of similar accuracy for other physical quantities, making it the most successful theory in history. But it is a "probabilistic" theory. Curiously, QED predicts that a single photon can take ANY path from A to B, at any velocity (not just 'c'). We just don't know what path any particular photon will take. But when we add up all possible paths, we get a straight line and total velocity 'c'! Please don't take my word for it. Please pick up a copy of Richard Feynman's little book "QED: The Strange Theory of Light and Matter" - Feynman was a co-creator of QED, but the book is very readable by the lay person.
It doesn't make much sense when we compare it to everyday life, but it appears that Nature operates (at the smallest level) in a probabilistic way. Einstein himself never agreed with it, saying that "God does not play dice". But it seems that God does! At least our best theories say so.
Hope that helps.
Prof. James Gort
---------- FOLLOW-UP ----------
no apology needed, I appreciate your time.
I have read and watched Richard Feynman and others a LOT!! He was a truly great man, not just for his science.
Most of what you said I already knew.
I just wanted to satisfy myself that quantum theory is totally probalistic.
I understand the success of QED and that it works.
It just seems to me that it fails on the most basic tenet of theoretical physics-to make testable prediction.
I understand some physicists accept quantum physics for what it is better than some others...
There seem to be many 'fringe' theorys around now days-extra dimensional etc.
ALWAYS, I hear these theories can't be treated seriously unless they make testable prediction.....yet there is the most successful, accurate and accepted theory that cannot make a prediction!! Seems kind of ironic.
I find myself wondering how you define a SINGLE event.
Interesting stuff anyway!!
Yes, physics must predict experimental results. That's exactly why I have problems with many cosmological theories or even string theory (see Peter Woit's excellent book - "Not Even Wrong"). To me, many of these "theories" are not science - they're untestable and therefore pure conjecture.
But that's what makes quantum theory so accepted! The predictions made by QED and others do agree with observation - but is that a result of the limitations to our observations or the limitations to the theory? It might be a moot point (like the Schrodinger's cat question).
I don't think physics can claim to describe "reality". The best physics can do is to create models of reality which can be used to make predictions. If the predictions are close to observation, it's a good model.
Is the world really a giant Schrodinger's wave equation? Is light itself a wave travelling through space? I would say "no" to both, but both can be used to accurately DESCRIBE reality.
The world "probably" isn't probabilistic, but the world is also not built on waves. Is it built on particles? We don't know - because we don't know what "particles" are! The point is - we just don't understand "true" reality (whatever that is) - we just try to model reality. And if quantum theory makes good predictions, it's a good model for that reality. But don't confuse it with reality.
As soon as quantum theory (or any of its branches) errs in its predictions of physical outcomes, we'll be searching for modifications or even totally new theories. But until then, it's the best model we have.
Prof. James Gort