On a couple of blogs (Not Even Wrong and Luboš Motl’s reference frame) a question has creeped up which is what role studies of the foundations of quantum theory have in a future theory of quantum gravity. At the Strings2005 conference in Toronto recently, this question was raised during a panel discussion. When someone claimed that foundations hasn’t contributed anything to physics, Lee Smolin apparently said something to the effect that study of foundations of quantum theory has given us quantum computing.

It is true that the original thinkers about quantum computers, and in particular I’m thinking about David Deutsch, where inspired by interpretation issues in quantum theory. But I think the relationship between foundational studies of quantum theory and what is now quantum information science is a bit different. I think that foundational studies have played the role of a “clarifier” in quantum information science. Instead of many results being motivated by foundational issues directly, studies in foundations have lead those in quantum computing to a better understanding of what quantum theory is and what it is not. I certainly think that banging my head up against the different interpretations of quantum theory has given me a very good grasp of the limits of what quantum theory can say and what it cannot say. Thus I think that quantum information science has benefited immensely by listening to the foundation crowd and learning just exactly what is so different about quantum theory. So, while the interpretations themselves don’t have any gigantic successes (one could argue for smaller ones!), I think they are an essential base which betters the field of quantum computing.

Now back to quantum gravity. Whether or not the foundations of quantum theory has anything to say about quantum gravity, I think, is a question we can debate until the cows come in. There are certainly very good philosophical points of view that the strain between gravity and quantum theory must be broken in some nontrivial manner, and whether we break quantum theory or gravity is, I think, an intriguing question. But if we take quantum computing as an example, the lesson that may be learned is that by careful study of quantum theory you can gain an appreciation for it which might allow you to make progress in forming a quantum theory of gravity. I must say that I am often shocked by the lack of understanding of the details of quantum theory among high energy physicists. Sure they use it every day. But do they really deeply get the theory and what it can and can not do? Of course this is a very prejudiced statement coming from a very prejudiced quantum computing dude. We hold quantum theory fairly sacred and hate to see it abused. I’m also sure that high energy physicists are greatly pained by quantum information scientists lack of understanding of “real” physics!

My person views on the relationship between foundations and quantum gravity are about as close as I get to pseudoscientific gobldy-gook. I gave a talk on those views once. It was supposed to be one hour and it ran to two. Sometimes I contemplate writing a book about these views… Penrose did it, so why can’t I? 😉

> Whether or not the foundations of quantum theory

> has anything to say about quantum gravity, I

> think, is a question we can debate until the cows

> come in.

I suspect it could also be the other way around.

As I mentioned on Peter’s blog, the wave length assigned to macroscopic bodies would typically be much smaller than the Planck length. Thus it seems plausible that QM needs to be modified for such cases. This also seems to be the reasoning of Penrose and others.

I have the opposite view of philosophical foundations. I think of quantum information and quantum computation as clearing the air so that there is no need to debate interpretations. Interpretations in science, like belief, are ultimately an aid to research. When you have substantive math and science in front of you, you can finally decide which interpretations are useful.

In the absence of substantive science, interpretations quickly become inane. After all, no one is silly enough to debate the many-worlds “interpretation” of classical probability distributions. (On the other hand, path summation, which is a lot like “many worlds”, is a fine framework for intuition and calculations, both classically and quantumly.)

Wolfgang: I suspect you might be right. But what do I know? At least you’ve written papers on quantum gravity…

Dave,

I can confirm that the aspects of QM that play a role in particle physics are quite different from the foundational ones, or the ones most important to quantum information. So, it is understandable that you find us particle folk a bit rusty when it comes to those things. (You may be too kind to refer to particle physics as “real” physics 🙂

I can remember telling one of my postdocs at Yale that it was a shame we were wasting so much time on some stupid supersymmetric gauge theory calculation, when neither of us had (admittedly) yet had time to read Everett’s paper or think deeply about many worlds.

In any case, thanks again for your comments on my recent preprint!