Thursday, June 6, 2013

Scott Aaronson's "The Ghost in the Quantum Turing Machine"

Scott Aaronson recently uploaded a mind-boggling paper, full of challenging ideas regarding free-will, quantum mechanics and computing, philosophical big questions, neuroscience, and many other hot topics. The title is The Ghost in the Quantum Turing Machine, and will be a chapter in the book The Once and Future Turing, edited by S. Barry Cooper and Andrew Hodges, 2013.

His paper is like a storm of puzzle pieces, which fit together perfectly in an amazing tapestry, centered around his idea of Knightian freedom.

Here is the abstract
In honor of Alan Turing's hundredth birthday, I unwisely set out some thoughts about one of Turing's obsessions throughout his life, the question of physics and free will. I focus relatively narrowly on a notion that I call "Knightian freedom": a certain kind of in-principle physical unpredictability that goes beyond probabilistic unpredictability. Other, more metaphysical aspects of free will I regard as possibly outside the scope of science. I examine a viewpoint, suggested independently by Carl Hoefer, Cristi Stoica, and even Turing himself, that tries to find scope for "freedom" in the universe's boundary conditions rather than in the dynamical laws. Taking this viewpoint seriously leads to many interesting conceptual problems. I investigate how far one can go toward solving those problems, and along the way, encounter (among other things) the No-Cloning Theorem, the measurement problem, decoherence, chaos, the arrow of time, the holographic principle, Newcomb's paradox, Boltzmann brains, algorithmic information theory, and the Common Prior Assumption. I also compare the viewpoint explored here to the more radical speculations of Roger Penrose. The result of all this is an unusual perspective on time, quantum mechanics, and causation, of which I myself remain skeptical, but which has several appealing features. Among other things, it suggests interesting empirical questions in neuroscience, physics, and cosmology; and takes a millennia-old philosophical debate into some underexplored territory.

A local explanation of entanglement by using wormholes

Recently, a new paper by Maldacena and Susskind appears, named Cool horizons for entangled black holes (arxiv:1306.0533). In the paper, the two authors propose that two entangled particles are connected by an Einsten-Rosen bridge, a wormhole. Their stake is in fact related to the black hole information paradox, the Maldacena correspondence, and the recent idea of black hole firewalls. It was covered, among others, by Sean Carroll.

This article reminded me of an example I gave at FQXi's blog, under an article by Florin Moldoveanu, (whose blog, Elliptic composability, I highly recommend)

Here is my comment from two years ago:
Cristi Stoica wrote on Aug. 5, 2011 @ 13:32 GMT


It is based on a non-trivial topology (wormholes).

Cut two spheres out of our space, and glue the two boundaries of the space together. This wormhole can be traversed by a source free electric field, and used to model a pair of electrically charged particles of opposite charges as its mouths (Einstein-Risen 1935, Misner-Wheeler's charge-without-charge 1957, Rainich 1925).

For EPR we need a wormhole which connects two electrons instead of an electron-positron pair. A wormhole having as mouths two equal charges can be obtained as follows: instead of just gluing together the two spherical boundaries, we first flip the orientation of one of them. Since the electric field is a bivector, the change in orientation changes the sign of the electric field, and the two topological charges have the same sign.

Now associate to the two electrons your favorite local classical description. The communication required to obtain the correlation can be done through the wormhole.


This may be the basis of a mathematically correct local hidden variable theory. Also, it seems to disprove, or rather circumvent, Bell's theorem. For Bohm's hidden variable theory, it provides a mechanism to get the correlation without faster than light signals. I proposed it here for theoretical purposes only, as an example. My favorite interpretation is another one.


I did not want to spend more time on this, only to break my neck proposing local models of entanglement, especially since I did not find the idea of hidden variables relevant.