Q&A: Quantum Theory and Laplace's Demon
Quantum Theory and Laplace's Demon
Question
Hello Rabbi
There is a deterministic saying known as "Laplace's demon," according to which an intellect that, at a certain moment, knew all the forces and laws operating in nature and all the positions of all the items of which the universe is composed—if that intellect were powerful enough to perform the appropriate analysis on that information—would compress into a single formula the motions of the largest bodies in the universe as well as those of the tiniest atom. For such an intellect, nothing would be uncertain, and the future, just like the past, would be present before its eyes.
As a believing Jew, I think there are two factors that disrupt this calculability, and they are:
'Divine providence,' which is capable of intervening and changing the laws or states of nature miraculously,
and 'human free choice,' which admittedly cannot change the laws of nature but can disrupt the deterministic natural state and can change the picture of the universe in accordance with the limits of human ability.
My question is: does quantum theory contradict the above?
Does superposition at the micro level mean that even without the two variables I mentioned (divine providence and human choice), it is impossible to predict the future state of the universe in light of its current state?
Thank you
Answer
Hello Tzvia.
First, Laplace did not claim that this would be a single formula. That was Einstein's hope in seeking a unified field theory. But it is not clear that such a thing exists, and I do not think Laplace assumed that it does. What he claimed was that it would be possible to describe everything that will happen, but using several formulas and not necessarily one.
Second, indeed, quantum theory refutes Laplace's claim, and the classical notion of causality in general. Unless a complete theory of hidden variables is found, which at the moment does not seem likely.
Third, if you believe in free will, and certainly in divine involvement, that does not fit with Laplace's view. He was a determinist. But these are beliefs and not findings, so it is hard to see them as a refutation of his view. The question remains open: someone who is a determinist can accept Laplace's claim (aside from the reservations arising from quantum theory), and someone who is not cannot accept it.
Discussion on Answer
Blessed is He who revives the dead. It's a bit hard to conduct a discussion like this with two-month gaps between one message and the next.
At the macro level that is accessible to the senses—yes; at the micro level in quantum theory—no.
Isn't this what our whole understanding of the concept of prayer is based on—that if a Jew can change or influence events in the world through his prayer, insofar as that is God's will, then nothing is predictable? Because if not, what value does prayer have?
My question was, of course, about the possibility of prediction without divine or human involvement.
Is the concept of causality refuted even at the micro level? After all, we see that causality works very well in all areas of life, in what appears to the eye and the senses.
Thank you