Thanks for writing this. I want to really dig into this paper and make sure I understand it, but it certainly seems like an interesting approach. I’m curious why you say this, though:
Evidently, this approach suffers from a number of limits: the first and the most evident is that it works only in a situation where the system to be observed has already decohered with an environment. It is not applicable to, say, a situation where a detector reads a quantum superposition directly, e.g. in a Stern-Gerlach experiment.
Maybe I’m misunderstanding you, but I thought they addressed this issue:
(from the longer companion paper)
Actually, self-locating uncertainty is generic in quantum measurement.
In Everettian quantum mechanics the wave function branches when the system becomes
suciently entangled with the environment to produce decoherence. The normal case is one in
which the quantum system interacts with an experimental apparatus (cloud chamber, Geiger
counter, electron microscope, or what have you) and then the observer sees what the apparatus
has recorded. For any realistic room-temperature experimental apparatus, the decoherence time
is extremely short: less than 10^20 seconds. Even if a human observer looks at the quantum system
directly, the state of the observer’s eyeballs will decohere in a comparable time. In contrast,
the time it takes a brain to process a thought is measured in tens of milliseconds. No matter
what we do, real observers will nd themselves in a situation of self-locating uncertainty (after
decoherence, before the measurement outcome has been registered).
As long as there is macroscopic decoherence before the observer has time to register any thoughts, the approach seems to hold, and that’s certainly the case for Stern-Gerlach experiments.
Let me begin by saying that I’ve only glanced the companion paper very briefly and, although I have noticed the paragraph you quote, I may be unaware of other parts that directly address my response.
My remark that the approach wouldn’t work in a Stern-Gerlach experiment was aimed at the three steps structure of the experiment, not at the decoherence happening. If we consider the Stern-Gerlach apparatus as the observer, sure it decoheres, but there’s no middle environment upon which to distribute the measure of the system observed.
To make Carroll-Sebens procedure to work, you need both a three steps experiment and a wide middle enviroment, so it won’t work in any case where one of the element is missing.
Thanks for writing this. I want to really dig into this paper and make sure I understand it, but it certainly seems like an interesting approach. I’m curious why you say this, though:
Maybe I’m misunderstanding you, but I thought they addressed this issue:
(from the longer companion paper)
As long as there is macroscopic decoherence before the observer has time to register any thoughts, the approach seems to hold, and that’s certainly the case for Stern-Gerlach experiments.
I take it that’s supposed to be 10^-20 seconds?
Let me begin by saying that I’ve only glanced the companion paper very briefly and, although I have noticed the paragraph you quote, I may be unaware of other parts that directly address my response.
My remark that the approach wouldn’t work in a Stern-Gerlach experiment was aimed at the three steps structure of the experiment, not at the decoherence happening. If we consider the Stern-Gerlach apparatus as the observer, sure it decoheres, but there’s no middle environment upon which to distribute the measure of the system observed.
To make Carroll-Sebens procedure to work, you need both a three steps experiment and a wide middle enviroment, so it won’t work in any case where one of the element is missing.