Because “state” is expressed in RQM as the correlation between two systems, there can be no meaning to “self-measurement”.

— Wikipedia on *Relational quantum mechanics*

2012.11.09 Friday ACHK

Because “state” is expressed in RQM as the correlation between two systems, there can be no meaning to “self-measurement”.

— Wikipedia on *Relational quantum mechanics*

2012.11.09 Friday ACHK

Instead, an observer can only detect the information at the horizon itself, or inside, but never both simultaneously. Complementarity is a feature of the quantum mechanics of noncommuting observables, and Susskind proposed that both stories are complementary in the quantum sense.

Interestingly enough, an infalling observer will see the point of entry of the information as being localized on the event horizon, while an external observer will notice the information being spread out uniformly over the entire stretched horizon before being re-radiated. To an infalling observer, information and entropy passes through the horizon with nothing strange happening. To an external observer, the information and entropy is absorbed into the stretched horizon which acts like a dissipative fluid with entropy, viscosity and electrical conductivity.

— Wikipedia on *Black hole complementarity*

2012.10.30 Tuesday ACHK

Leonard Susskind proposed a radical resolution to this problem by claiming that the information is both reflected at the event horizon *and* passes through the event horizon and can’t escape, with the catch being no observer can confirm both stories simultaneously.

— Wikipedia on *Black hole complementarity*

2012.10.28 Sunday ACHK

Single-world interpretation, 8

The interpretation based on *consistent histories* is used in combination with the insights about quantum decoherence. Quantum decoherence implies that irreversible macroscopic phenomena (hence, all classical measurements) render histories automatically consistent, which allows one to recover classical reasoning and “common sense” when applied to the outcomes of these measurements.

— Wikipedia on* Consistent histories*

2012.04.14 Saturday ACHK

It turns out that the usual rules for combining quantum mechanical and classical descriptions violate the principle of locality without violating causality.

Causality is preserved because there is no way for Alice to transmit messages (i.e. information) to Bob by manipulating her measurement axis. Whichever axis she uses, she has a 50% probability of obtaining “+” and 50% probability of obtaining “-“, completely at random; according to quantum mechanics, it is fundamentally impossible for her to influence what result she gets.

Furthermore, Bob is only able to perform his measurement once: there is a fundamental property of quantum mechanics, known as the “no cloning theorem”, which makes it impossible for him to make a million copies of the electron he receives, perform a spin measurement on each, and look at the statistical distribution of the results. Therefore, in the one measurement he is allowed to make, there is a 50% probability of getting “+” and 50% of getting “-“, regardless of whether or not his axis is aligned with Alice’s.

— Wikipedia on* EPR paradox*

In fact, a theorem proved by Phillippe Eberhard shows that if the accepted equations of relativistic quantum field theory are correct, it should never be possible to experimentally violate causality using quantum effects (see reference [6] for a treatment emphasizing the role of conditional probabilities).

— Wikipedia on *Delayed choice quantum eraser*

2012.04.08 Sunday ACHK

… the universe is the sum total of all that is in existence. Physically, a (physical) observer outside of the universe would require the breaking of gauge invariance, and a concomitant alteration in the mathematical structure of the theory. Similarly, RQM conceptually forbids the possibility of an external observer. Since the assignment of a quantum state requires at least two “objects” (system and observer), which must both be physical systems, there is no meaning in speaking of the “state” of the entire universe.

This is because this state would have to be ascribed to a correlation between the universe and some other physical observer, but this observer in turn would have to form part of the universe, and as was discussed above, it is impossible for an object to give a complete specification of itself. Following the idea of relational networks above, an RQM-oriented cosmology would have to account for the universe as a set of partial systems providing descriptions of one another. The exact nature of such a construction remains an open question.

— Wikipedia on *Relational quantum mechanics*

2012.03.27 Tuesday ACHK

Physicists including your humble correspondent always preferred to keep the normal logic. I really don’t understand how one could work with von Neumann’s strange rules. The consistent histories, the most complete framework to interpret quantum mechanics today, imply that exactly those quantum histories (sequences of projection operators at different times) for which the classical rules of logic work – the consistent histories themselves – are allowed.

— John von Neumann: 104th birthday

— Lubos Motl

2012.03.18 Sunday ACHK

Quantum decoherence 5.3

In the delayed choice experiment, the wave function of the system (the photons and the environment) is also in a superposition of eigenstates, not just the wave functions of the individual photons are.

All the past is there, but our present measurement “chooses” which part to see.

— Me@2011.10.21

The chosen part must be a consistent story, according the quantum mechanics.

The chosen part is what we called “an observer”.

— Me@2018-01-22 09:35:02 PM

2011.11.20 Sunday (c) All rights reserved by ACHK

Quantum decoherence 5.2 | Event Realism 5 | 事件實在論 5

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For a delayed choice quantum eraser, both interference patterns are there.

But since they overlap each other, you cannot see them individually.

— Me@2011.10.21

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One of the easiest ways of “making sense” of the delayed-choice paradox is to examine it using Bohmian mechanics. The surprising implications of the original delayed-choice experiment led Wheeler to the conclusion that “no phenomenon is a phenomenon until it is an observed phenomenon”, which is a very radical position. Wheeler famously said that the “**past has no existence except as recorded in the present**“, and that the Universe does not “exist, out there independent of all acts of observation”.

— Wikipedia on *Wheeler’s delayed choice experiment*

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What we do in the present does not change the past, but change we can see/say about the past.

— Wheeler on *Delayed choice quantum eraser*

— paraphrased

— Me@2018-02-04 03:40:27 PM

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2011.11.19 Saturday (c) All rights reserved by ACHK