On 6/30/2014 9:24 PM, Henry Baker wrote:
Cramer's "transactional" interpretation of QM is very computer sciencey, at least to my eyes.
Consider a thought experiment in which you're going to construct a parallel computer simulation of the double slit experiment, and you compute the probability density of photons at a uniform sampling of points in space.
The problem comes when you have to flip a probability-weighted coin at each point of the photon absorber in order to decide if a photon will be absorbed at that point. The problem is that you can't decide _locally_ and _independently_ at each absorbing point whether to absorb a photon at that point, because you have to guarantee the conservation of energy. The problem is that energy isn't just conserved _probabilistically_, but _exactly_, so if you choose badly, you can have more photons being absorbed than were emitted -- a clear violation of the conservation of energy.
But why not just decide, probabilistically, for each photon, where it will be absorbed? That guarantees conservation of photon number. The "interpretation" problem of QM arises where there is a transition to classical physics notions like "absorbed at a point". It's absorption isn't something we can see directly. The theory says something like: The photon strikes one of many silver halide molecules, with different probabilities and causes them to enter a superposition of states with different probability weights for each molecule. Then, somehow, we see only one spot of silver on the film. Brent
What a computer simulation program would do would be to utilize _transactions_, which would guarantee that energy was exactly conserved on a global basis; such transactions would certainly slow down the parallel simulation program due to contention of the locking mechanisms on the shared resource which keeps the energy in exact balance. For a computer scientist, who expects the universe to be "embarrassingly parallel", this transaction/locking mechanism is extremely inelegant. Yet the possibility of _entanglement_ virtually guarantees that some sort of transaction mechanism will be required to guarantee consistency of the simulation.
There are several ways to look at this problem. One is to assume that it is a wart, and attempt to remove it with Bohm-like and pilot wave-like models. The other is to turn this bug into a feature, and attempt to utilize QM itself to do the dirty work in computer simulations by having QM entanglement handle whatever transactional guarantees are required (I'm not sure exactly how this might be done, but there are lots of computer scientists looking into the usefulness of quantum computers).
BTW, there are several different flavors of computer transaction implementations, which are also mirrored in discussions about the philosophy of QM. "Conservative" transaction implementations refuse to do any work until a process has _exclusive_ access to a shared resource, while "speculative" transaction implementations are willing to do quite a lot of work, so long as they are also willing to throw away work that proves not to be consistent when the transaction is "closed". QM seems to be quite content to do its thing obliviously until a "measurement" is made, at which point all transactions must be closed, so that the observer sees a consistent picture.
At 02:12 PM 6/30/2014, Jeff Caldwell wrote:
I hope to better understand the curious case of the photon.
It has no frame of reference and, were it conscious, would perceive itself to be emitted and absorbed simultaneously, i.e. no time passing between the events.
With zero time between emission and absorption, whimsy allows me to think of emitter and absorber as in some sense touching, albeit one is an ancient star and the other a cone in my living eye.
Zero time means zero distance, in my book, although applying that rule to the no-frame-of-reference photon is probably a category error.
Cramer's transactional interpretation, inspired by Wheeler-Feynman time-symmetric theory, has both forward and backward-in-time waves between emitter and absorber, agreeing upon the transaction before (as? timey-wimey words ...) it takes place, which leaves precious little room for the free will electrons have if you or I do, John Conway and Simon Kocken say, and leaving no room at all for deciding whether or not to slide a detector into a photon's path after the photon has been emitted.
Emitters, detectors and photons have united, and their agreements will be kept!
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