Thursday, September 02, 2010

An Experimental Test Of String Theory?

The biggest and major criticism against string theory is that it is untestable, or make predictions that is beyond the capability of it being tested. For a theory that has existed for this long, and grown this popular, not being able to test any part of its development is a serious shortcoming. No other theory in the history of physics has gone this long without producing a shred of empirical verification. Because of that, string theory comes in multiple different flavors, each one vying for attention and being accepted or rejected simply based on a matter of taste.

However, now comes a rather interesting proposal. A team out of Imperial College and Stanford has proposed a way to test an aspect of string theory using 4-qubit entangled particles. The paper is reported to appear in Phys. Rev. Lett. this week, but you can get a preprint at the link above.

Professor Duff recalled sitting in a conference in Tasmania where a colleague was presenting the mathematical formulae that describe quantum entanglement: “I suddenly recognised his formulae as similar to some I had developed a few years earlier while using string theory to describe black holes. When I returned to the UK I checked my notebooks and confirmed that the maths from these very different areas was indeed identical.”

The discovery that string theory seems to make predictions about quantum entanglement is completely unexpected, but because quantum entanglement can be measured in the lab, it does mean that at last researchers can test predictions based on string theory. There is no obvious connection to explain why a theory that is being developed to describe the fundamental workings of our universe is useful for predicting the behaviour of entangled quantum systems. “This may be telling us something very deep about the world we live in, or it may be no more than a quirky coincidence”, concluded Professor Duff. “Either way, it’s useful."

Hum... let's see if the many different labs that are doing Bell-type experiments might jump into this.

Edit: This paper has now appeared in print. The exact reference is:

L. Borsten et al., Phys. Rev. Lett. v.105, p.100507 (2010).



HM said...

I've just took a very quick look at the paper and to me it doesn't look like they have found a falsifiable prediction of string theory.

Apparently, there is a one-to-one correspondence between the the classification three-qubit entangelment and extremal black holes in some particular supersymmetric string theories. The task is now to classify four-qubits since experimentalists are able to control these (the paper claims), but the theoretical classification is not complete. What they show is that the entangelment<->black hole correspondence can be extended to four-qubits and this makes them able to classify four-qubit entangelment.

In other words, string theory is being used as a MATHEMATICAL TOOL to solve a theoretical problem in Quantum Information. This is therefore NOT a falsifiable prediction of string theory, it is not directly related to the physical part of string theory.

Things like this are not new. For example, Topological Insulators are very hot these days in condensed matter physics and Alexei Kitaev
has shown that there exists a very beautiful classification of these states of matter using K-theory (one can construct a "periodic table" of topological insulators and superconductors). This classification is mathematically related to the classification of D-brane charges in string theory, and string theory can be used as a tool to understand topological insulators/superconductors (Link). Although these authors use string theory in the same way as Borsten et al, they don't claim that they have found a falsifiable prediction of string theory.

What I am trying to say is that the physics in the paper is nothing but usual Quantum Information, they just use mathematical techniques from string theory. A four-qubit experiment confirming/falsifying the predictions, does not confirm/falsify string theory.

WARNING: what I say is based on a shallow look at the paper.

Emil said...

Hmm.. as far as I can tell from the news story, what they are saying is that they can test whether some of the mathematical derivations made in some corner of string theory are correct - because the same math shows up in quantum information. Right??