## Can single-world interpretations of quantum theory be self-consistent?

I saw a provocative-looking paper on the arXiv the other day (by Daniela Frauchiger and Renato Renner) with the title *Single-world interpretations of quantum theory cannot be self-consistent.* No doubting what the authors think!

Here’s the abstract:

According to quantum theory, a measurement may have multiple possible outcomes. Single-world interpretations assert that, nevertheless, only one of them “really” occurs. Here we propose a gedankenexperiment where quantum theory is applied to model an experimenter who herself uses quantum theory. We find that, in such a scenario, no single-world interpretation can be logically consistent. This conclusion extends to deterministic hidden-variable theories, such as Bohmian mechanics, for they impose a single-world interpretation.

Since this is a subject we’ve had interesting debates about on this blog I thought I’d post a link to it here and see if anyone would like to respond through the comments. I haven’t had time to read it thoroughly yet, but I do have a bit of train travel to do tomorrow…

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May 5, 2016 at 1:03 pm

It is impossible ever to rule out a one-world deterministic hidden variable theory. All you can do is show that such a theory is inconsistent with various other things. Locality and causality have gone in that way, and I am wondering whether anything else has gone as a result of this work or if there is a subtle confusion concealed in it, as in much similar work.

A lot is concealed in that word “may” in the first sentence of the Abstract. Quantum theory does not specify which outcome will occur, but one of them will and the others won’t; that is simply the definition of “measurement” in quantum theory, the way you get a result written down on a piece of paper. If, however, you wish to treat the observer and the system observed as a single joint system to which the laws of quantum physics apply, you get a different description that knows nothing of ‘measurement’. (You also get a regression issue: how to interact with and learn about the observer-plus-system?) This is all about the classical limit. Ingenious applications of quantum mechanics such as quantum computing, in which parallelism is implemented in the physics, do not invalidate these statements.

Schrodinger’s cat was a regress problem on the classical side of the mental transition from the quantum world to the classical world. The regression terminates where the system is

declaredto have a classical description.May 5, 2016 at 8:52 pm

I haven’t had a chance to read it yet, but I’m naturally sceptical of their claim! Lots of proofs have been made about quantum mechanics, and lots of them have ‘fallen apart’ as Bell put it, when examined more closely.

The idea that one cannot formulate self-consistent ‘stories’ of quantum behaviour isn’t too surprising, though. The double-slit experiment hints at the idea that it’s not possible to say which particular slot a particle went through (without tampering so much with the experiment that you destroy the quantum behaviour).

May 5, 2016 at 8:54 pm

Googling around, there is a thoughtful blogpost here on the paper, that might be of interest:

http://www.unitaryflow.com/2016/05/are-single-world-interpretations-of-quantum-theory-inconsistent.html

May 10, 2016 at 2:51 am

The classicle interpretation of the double-slit experiment always ignore the fact that the collapse of the wave function is a physical arrest of the particle at the screen, not your observation that comes afterwards. This is much more than just a measurement. Before it is stopped, it is free to use the space as it will. Afterwards it is in a captive state by forces from the screen. And also after the capture, there will be a wave function, but much more spatially restricted. Also the passing through the slit zone is not well enough scrutinized. For example may the particle go straight through the area berween the slits without enough physical interaction to stop it, but only with some disturbance that will be different with one or two open slits.