# What are the practical applications of quantum foundations?

Many quantum foundation researchers keep emphasizing that For All Practical Purposes (FAPP), quantum foundations are irrelevant. They even invented an acronym for it! Does that mean that quantum foundations have no practical applications? If so, why bother working on quantum foundations?

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Why bother painting? (Here "painting" to be understood as a metaphor for the intrinsically rewarding activity of working on foundations for physics. Incidentally, it's also a metaphor for FAPPing.) – NikolajK Jan 7 '14 at 14:13
I second @NickKidman's comment, but would add this: if we truly had quantum foundations down pat so that they exactly matched reality, then it would be a fair bet that it would impossible for there not to be applications! Indeed, it might be a bit scary what humanity could do with such knowledge. – WetSavannaAnimal aka Rod Vance Jan 8 '14 at 14:32

I suppose there are two scientific reasons to look into the foundations of QM:

1. As part of checking in finer and finer detail that indeed the world is governed by standard quantum physics. The towering example here is Bell's theorem. From inspection of the foundations this makes some prediction which can be and has been checked by experiment.

2. As part of the general endeavour of understanding reality. This may have little practical relevance (for us, now), but is certainly part of what science is about. To appreciate this somewhat more subtle point, maybe think about it from the point of view of this nice quote from Feynman's book "The character of physical law" (1967):

"For those people who insist that the only thing that is important is that the theory agrees with experiment, I would like to imagine a discussion between a Mayan astronomer and his student. The Mayans were able to calculate with great precision predictions, for example, for eclipses and for the position of the moon in the sky, the position of Venus, etc. It was all done by arithmetic. They counted a certain number, and subtracted some numbers, and so on. There was no discussion of what the moon was. There was no discussion even of the idea that it went around. They just calculated the time when there would be an eclipse, or when the moon would rise at the full, and so on. Suppose that a young man went to the astronomer and said ’I have an idea. Maybe those things are going around, and there are balls of something like rocks out there, and we could calculate how they move in a completely different way from just calculating what time they appear in the sky’, ’Yes’, says the astronomer, ’and how accurately can you predict eclipses?’ He says, ’I haven’t developed the thing very far yet’, Then says the astronomer, ’Well, we can calculate eclipses more accurately than you can with your model, so you must not pay any attention to your idea because obviously the mathematical scheme is better’. There is a very strong tendency, when someone comes up with an idea and says, ’Let’s suppose that the world is this way’, for people to say to him, ’What would you get for the answer to such and such a problem?’ And he says ’I haven’t developed it far enough’. And they say, ’Well, we have already developed it much further, and we can get the answers very accurately’. So it is a problem whether or not to worry about philosophies behind ideas."

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Sometimes, Feynman was very anti-philosophical, and sometimes, he's pro-philosophical. Sometimes, he was against quantum foundations, and sometimes, he gave keynote speeches to computer scientists detailing his work on explaining the Bell inequalities using negative probabilities and his speculations on retrocausality and superdeterminism. – Just a lil kid Jan 9 '14 at 13:32
It's interesting to try to discuss the possibilities. I mentioned something about the possibility of time--of things being affected not just by the past, but also by the future, and therefore that our probabilities are in some sense "illusory." We only have the information from the past, and we try to predict the next step, but in reality it depends upon the near future which we can't get at, or something like that. Feynman quote – Just a lil kid Jan 9 '14 at 13:41
we have an illusion that we can do any experiment that we want. We all, however, come from the same universe, have evolved with it, and don't really have any "real" freedom. For we obey certain laws and have come from a certain past. Is it somehow that we are correlated to the experiments that we do, so that the apparent probabilities don't look like they ought to look if you assume that they are random. Feynman quote – Just a lil kid Jan 9 '14 at 13:43
Somebody mumbled something about a many-world picture, and that many-world picture says that the wave function is what's real, and damn the torpedos if there are so many variables, N^R. All these different worlds and every arrangement of configurations are all there just like our arrangement of configurations, we just happen to be sitting in this one. Feynman quote – Just a lil kid Jan 9 '14 at 13:45
The only difference between a probabilistic classical world and the equations of the quantum world is that somehow or other it appears as if the probabilities would have to go negative, and that we do not know, as far as I know, how to simulate. Okay, that's the fundameratal problem. Feynman quote – Just a lil kid Jan 9 '14 at 13:49

Every great physical theory has been proven wrong.

That's of course not (yet or ever) true, but if history is any guide, physical theories fail.

Newton's gravity is a prime example. Even Newton could see problems in its foundations, yet it is highly accurate and can be used to guide satellites and space probes. By questioning those foundations (and other things) Einstein found General Relativity, which works better than Newton's theory, and also solves the main problem with it.

Of course many (most?) physicists think that with QM 'this time its different'. It might be different this time, but likely not. Some hole could be found using thoughts on QM foundations. For instance de Broglie - Bohm theory gives one insight into possible experiments to perform that might show where QM 'ends'.

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