Pointer state

Eigenstates 3

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In quantum Darwinism and similar theories, pointer states are quantum states that are less perturbed by decoherence than other states, and are the quantum equivalents of the classical states of the system after decoherence has occurred through interaction with the environment.

— Wikipedia on Pointer state

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In calculation, if a quantum state is in a superposition, that superposition is a superposition of eigenstates.

However, real superposition does not just includes states that make macroscopic senses.

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That is the major mistake of the many-worlds interpretation of quantum mechanics.

— Me@2017-12-30 10:24 AM

— Me@2018-07-03 07:24 PM

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2018.07.03 Tuesday (c) All rights reserved by ACHK

Block spacetime, 9

motohagiography 42 days ago [-]

I once saw a fridge magnet that said “time is natures way of making sure everything doesn’t happen all at once,” and it’s stuck with me.

The concept of time not being “real,” can be useful as an exercise for modelling problems where to fully explore the problem space, you need to decouple your solutions from needing them to occur in an order or sequence.

From an engineering perspective, “removing” time means you can model problems abstractly by stepping back from a problem and asking, what are all possible states of the mechanism, then which ones are we implementing, and finally, in what order. This is different from the relatively stochastic approach most people take of “given X, what is the necessary next step to get to desired endstate.”

More simply, as a tool, time helps us apprehend the states of a system by reducing the scope of our perception of them to sets of serial, ordered phenomena.

Whether it is “real,” or an artifact of our perception is sort of immaterial when you can choose to reason about things with it, or without it. A friend once joked that math is what you get when you remove time from physics.

I look forward to the author’s new book.

— Gödel and the unreality of time

— Hacker News

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2018.06.26 Tuesday ACHK

Eigenstates 2.3.2

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eigenstates

~ classical states

~ definite states

— Me@2012-04-15 11:42:10 PM

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The concept of eigenstate is relative.

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First, you have to specify the eigenstate is of which physical observable.

A physical system can be at an eigenstate of one observable but at a superposition state of another observable.

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Second, you have to specify the state of that observable is eigen with respect to which observer.

— Me@2018-06-16 7:27 AM

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eigenstates

~ of which observable?

~ with respect to which observer?

— Me@2018-06-19 10:54:54 AM

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2018.06.19 Tuesday (c) All rights reserved by ACHK

Quantum Computing, 2

stcredzero 3 months ago

A note for the savvy: A quantum computer is not a magic bit-string that mysteriously flips to the correct answer. A n-qubit quantum computer is not like 2^n phantom computers running at the same time in some quantum superposition phantom-zone. That’s the popular misconception, but it’s effectively ignorant techno-woo.

Here’s what really happens. If you have a string of n-qubits, when you measure them, they might end up randomly in [one] of the 2^n possible configurations. However, if you apply some operations to your string of n-qubits using quantum gates, you can usefully bias their wave equations, such that the probabilities of certain configurations are much more likely to appear. (You can’t have too many of these operations, however, as that runs the risk of decoherence.) Hopefully, you can do this in such a way, that the biased configurations are the answer to a problem you want to solve.

So then, if you have a quantum computer in such a setup, you can run it a bunch of times, and if everything goes well after enough iterations, you will be able to notice a bias towards certain configurations of the string of bits. If you can do this often enough to get statistical significance, then you can be pretty confident you’ve found your answers.

— An Argument Against Quantum Computers

— Hacker News

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2018.05.17 Thursday ACHK

The Sixth Sense, 3

Mirror selves, 2 | Anatta 3.2 | 無我 3.2

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You cannot feel your own existence or non-existence. You can feel the existence or non-existence of (such as) your hair, your hands, etc.

But you cannot feel the existence or non-existence of _you_.

— Me@2018-03-17 5:12 PM

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Only OTHER people or beings can feel your existence or non-existence.

— Me@2018-04-30 11:29:08 AM

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2018.04.30 Monday (c) All rights reserved by ACHK

Quantum decoherence 8

12. On the other hand, consistent histories are just a particular convenient framework to formulate physical questions in a certain way; the only completely invariant consequence of this formalism is the Copenhagen school’s postulate that physics can only calculate the probabilities, they follow the laws of quantum mechanics, and when decoherence is taken into account, to find both the quantum/classical boundary as well as the embedding of the classical limit within the full quantum theory, some questions about quantum systems follow the laws of classical probability theory (and may be legitimately asked) while others don’t (and can’t be asked)[.]

— Decoherence is a settled subject

— Lubos Motl

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2018.04.24 Tuesday ACHK

Logical arrow of time, 6.3

“Time’s arrow” is only meaningful when considering with respect to an observer.

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c.f. the second law of thermodynamics

The direction of time is direction of losing microscopic information… by whom?

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“Time’s arrow” is only meaningful when considering with respect to an observer.

— Me@2018-01-01 6:14 PM

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2018.04.09 Monday (c) All rights reserved by ACHK

Logical arrow of time, 6.2

Source of time asymmetry in macroscopic physical systems

Second law of thermodynamics

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Physics is not about reality, but about what one can say about reality.

— Bohr

— paraphrased

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Physics should deduce what an observer would observe,

not what it really is, for that would be impossible.

— Me@2018-02-02 12:15:38 AM

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1. Physics is about what an observer can observe about reality.

2. Whatever an observer can observe is a consistent history.

observer ~ a consistent story

observing ~ gathering a consistent story from the quantum reality

3. Physics [relativity and quantum mechanics] is also about the consistency of results of any two observers _when_, but not before, they compare those results, observational or experimental.

4. That consistency is guaranteed because the comparison of results itself can be regarded as a physical event, which can be observed by a third observer, aka a meta observer.

Since whenever an observer can observe is consistent, the meta-observer would see that the two observers have consistent observational results.

5. Either original observers is one of the possible meta-observers, since it certainly would be witnessing the comparison process of the observation data.

— Me@2018-02-02 10:25:05 PM

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2018.02.03 Saturday (c) All rights reserved by ACHK

Superposition always exist

A Non-classical Feature, 2

superposition ~ linear overlapping

~ f(ax + by) = a f(x) + b f(y)

Reality is a linear overlapping of potential realities, although different components may have different weightings.

Superposition always exists, if it exists at the beginning of a process.

So the expression “the wave function collapses and the superposition ceases to exist” does not make sense. 

Superposition always exist; interference (pattern) does not.

For a superposition to have an interference pattern, the two (for example) component eigenstates need to have a constant phase difference.

In other words, they have to be coherent.

superposition without an interference pattern

~ microscopically decoherent component states

~ macroscopically a classical state

— Me@2016-09-01 4:42 AM

2016.11.27 Sunday (c) All rights reserved by ACHK

Black hole complementarity 3

Raphael nicely avoids many of the confusions by introducing a refined version of the complementarity principle, the so-called observer complementarity… If I add some “foundations of quantum mechanics” flavor to the principle, it says:

Quantum mechanics is a set of rules that allows an observer to predict, explain, and/or verify observations (and especially their mutual relationships) that he has access to.

An observer has access to a causal diamond – the intersection of the future light cone of the initial moment of his world line and the past light cone of the final moment of his world line (the latter, the final moment before which one must be able to collect the data, is more important in this discussion).

No observer can detect inconsistencies within the causal diamonds. However, inconsistencies between “stories” as told by different observers with different causal diamonds are allowed (and mildly encouraged) in general (as long as there is no observer who could incorporate all the data needed to see an inconsistency).

Bohr has said that physics is about the right things we can say about the real world, not about objective reality, and it has to be internally consistent. However, in the context of general relativity, the internal consistency doesn’t imply that there has to be a “global viewpoint” or “objective reality” that is valid for everyone.

— Raphael Bousso is right about firewalls

— Lubos Motl

2016.07.27 Wednesday ACHK

Logical Fatalism

Logical Fatalism and the Argument from Bivalence

Another famous argument for fatalism that goes back to antiquity is one that depends not on causation or physical circumstances but rather is based on presumed logical truths.

The key idea of logical fatalism is that there is a body of true propositions (statements) about what is going to happen, and these are true regardless of when they are made. So, for example, if it is true today that tomorrow there will be a sea battle, then there cannot fail to be a sea battle tomorrow, since otherwise it would not be true today that such a battle will take place tomorrow.

The argument relies heavily on the principle of bivalence: the idea that any proposition is either true or false. As a result of this principle, if it is not false that there will be a sea battle, then it is true; there is no in-between. However, rejecting the principle of bivalence—perhaps by saying that the truth of a proposition regarding the future is indeterminate—is a controversial view since the principle is an accepted part of classical logic.

— Wikipedia on Fatalism

Quantum superposition can solve logical fatalism:

Macroscopic time is due to quantum decoherence.

The future is a coherent (constant phase difference) superposition of eigenstates.

That’s why classical probability can be regarded as part of quantum theory.

Quantum decoherence gives classically consistent histories.

— Me@2012.04.08

— Me@2015.03.26

2015.03.27 Friday (c) All rights reserved by ACHK

Quantum decoherence 7

There is no room for a physical collapse or, on the contrary, for an ad hoc privileged role of conscious observers; the wave functions only predict the probabilities but they can be calculated for any set of consistent histories, regardless of whether the systems look conscious, unconscious, macroscopic, or microscopic; the only “collapse” that occurs is the rapid diagonalization of the density matrix in the preferred basis by the interactions with the environment; however, the “unrealized” diagonal entries of the matrix (probabilities of outcomes that won’t come true) are never “physically” set to zero because their interpretation always remains probabilistic, even when the classical approximation becomes acceptably accurate[.]

— Decoherence is a settled subject

— Lubos Motl

2015.02.19 Thursday ACHK

Logical arrow of time, 6

The source of the macroscopic time asymmetry, aka the second law of thermodynamics, is the difference of prediction and retrodiction.

In a prediction, the deduction direction is the same as the physical/observer time direction.

In a retrodiction, the deduction direction is opposite to the physical/observer time direction.

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— guess —

If a retrodiction is done by a time-opposite observer, he will see the entropy increasing. For him, he is really doing a prediction.

However, it may not be possible for such an observer to exist. Me@2018-02-02 09:37:48 PM

— guess —

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— Me@2013-10-25 3:33 AM

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2013.10.29 Tuesday (c) All rights reserved by ACHK

T-symmetry 10

Uncertainty principle, 5.2 | Universal wave function, 12.2 | Reductionism 5

The uncertainty principle states the limit of reductionism. Science is based on reductionism, which assumes we can investigate part of the universe. So the uncertainty principle, in effect, states the limit of science.

— Me@2011.11.29

You need to be a meta observer to get all the information of the universe in order to see the macroscopic time symmetry. However, by definition, the universe cannot have any meta. 

— Me@2013-08-17 6:52 PM

The arrow of time is due to macroscopic states, aka incomplete pieces of information about the microstates. The microscopic state information keeps losing to the environment.

— Me@2013-08-14 6:58 PM

2013.08.18 Sunday (c) All rights reserved by ACHK