# Causal diamonds in time travel

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.

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).

— Raphael Bousso is right about firewalls

— Lubos Motl

.

There is no “god’s eye view” in physics.

— Me@2021-04-17 03:12:58 PM

.

Macroscopic time is with respect to an observer. Actually, physics is with respect to an observer.

In the real universe, any observer’s observations must be consistent. When two observers compare their observations, their results must be consistent, because the comparison itself is an observation of an observer.

Time travel in the absolute sense is logically impossible. Let’s assume that it is logically possible.

If a time travel story follows the principle of “an observer’s observations must be consistent”, each character in that story must see a consistent timeline, even if different characters’ timelines may be inconsistent. That is fine as long as such inconsistent observers never meet to compare their results.

If two of such observers choose to meet to compare their results, the action to “meet to compare” itself will render the results consistent. It is similar to the resolution of the twin paradox in special relativity.

There is no “god’s eye view” in physics. Every physical event must be described with respect to an observer. Every physical event, even if the event is “to compare observation results”, must be described with respect to an observer.

— Me@2017-05-10 07:45:36 AM

.

.

# 龍珠 3

On knowledge independent vectors:

When you already have a complete set of vectors in $n$ dimensional space, you have to go up one extra dimension in order to find a new independent vector.

— Me@2011.08.27

.

.

# Universal wave function, 21

For all, 9

.

A problem of universal wave function (universe) is that universe is a relative concept.

Another problem is that wave function is also.

— Me@2017-05-10 05:46:44 PM

— Me@2021-04-09 06:25:07 PM

.

universe ~ 100%

But 100% of what?

— Me@2021-04-09 05:20:23 PM

.

The wave function is expressed in terms of basis state vectors.

So it will have a different form if you choose a different basis.

— Me@2021-04-09 06:29:20 PM

.

.

# Logical arrow of time, 9.1

The source of asymmetry:

For prediction of the future, the result is what the observer/calculator can actually see.

For retrodiction of the past, the result is not.

— Me@2017-07-09 12:03:27 PM

— Me@2021-04-04 12:28:34 PM

.

.

# Conscious time, 2

If no one has any kind of date, records, memories, or evidence about the past, retro-diction MAY be the same as prediction. But in such a case, it is by definition not our “past” any more.

— Me@2013-08-08 3:11 PM

.

If no one has any kind of date, records, memories, or evidence about the past, then consciousness ceases to exist.

We, as conscious beings, cannot exist anymore.

— Me@2021-03-30 4:08 PM

.

.

# Meta-time 7

Two dimensional time, 6

.

y is meta x

~ y is the next dimension of x

.

y is meta-time

~ y is the second dimension of time

— Me@2017-07-10 06:12:08 PM

— Me@2021-03-26 06:00:45 PM

.

.

# Unfulfilled potential

What are you most scared of?

The Circle 2017 film

— Me@2017-02-02 07:22:47 AM

.

.

2021.03.26 Friday ACHK

# Particle indistinguishability is the major source of quantum effects, 1.05

If there is no particle indistinguishability, every particle has a well-defined identity, then every particle has a well-defined trajectory.

Then even if no detector is installed, there is a well-defined difference between go-left and go-right in the double-slit experiment.

Then there will be no indistinguishability of cases, aka quantum superposition states.

— Me@2021-02-04 7:04 AM

.

.

# Single-world interpretation, 11

Brian T. Johnston

All well and fine, but tunnel diodes work and they can’t unless there is MW

Luboš Motl

LOL, such a connection is at most a fairy-tale for kindergarten-age children

.

You need multiple worlds to explain one single world?

Are you stupid?

— Me@2017-07-17 02:58:25 PM

.

.

2021.03.16 Tuesday ACHK

# Energy 6

time ~ change

energy ~ the ability of causing change

.

Each system tends to reach its own lowest potential energy state because “lowest potential energy state” means “least ability to change”.

— Me@2013.06.29

— Me@2021-03-12

.

.

# Single-world interpretation, 10

If the operators corresponding to two observables do not commute, they have no simultaneous eigenstates and they obey the uncertainty principle. A state where one observable has a definite value corresponds to a superposition of many states for [another] observable.

— Wikipedia on Quantum superposition

.

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

— Me@2021-03-07 06:11:22 PM

.

.

# Trajectory

It is not possible to derive Schrödinger’s equation from “anything we know”.

— R. P. Feynman

.

The most confusing part in the Quantum Mechanics is [the concept of] Trajectory.

There exist[s] no fixed path for a particle to go from Point A to Point B. This is clearly visible from [the] Interference Experiment.

So, the approach here is to work with deductive reasoning. We eliminate the possible region/paths which [are] impossible to be followed.

To do this we assume that Energy Conservation Relation is valid for Quantum Mechanics too. So, those regions where particle[s] [violate] this law automatically [get] eliminated.

Then, we guess [the] State Function[s] for certain conditions i.e. how it should be in certain cases, then build an energy conservation equation with that. We will shortly demonstrate how Schrodinger itself reached the conclusion.

— Why can’t the Schrödinger equation be derived?

— Abhas Kumar Sinha

.

.

2021.02.28 Sunday ACHK

# Particle indistinguishability is the major source of quantum effects, 1.2

However, this definition of “every trajectory is well-defined” has a problem.

If the trajectory concept cannot predict correct experiment results, “the trajectory concept is broken” is only one of the possible causes.

In other words, how can you know the non-classical results (aka quantum effects) are not due to other factors?

— Me@2021-02-15 05:03:20 PM

.

This question is exactly what the Bell tests designed for.

No. It is not correct. A Bell test can check whether the trajectory concept is well-defined, but not whether “the trajectory concept is broken” is the major source of quantum randomness.

However, it is the undefinable trajectory concept that makes the superposition, which is a unique and major feature of quantum mechanics.

— Me@2021-02-07 06:03:53 PM

— Me@2021-02-15 10:24:17 PM

— Me@2021-02-21 05:14:55 PM

.

To date, all Bell tests have found that the hypothesis of local hidden variables is inconsistent with the way that physical systems behave.

— Wikipedia on Bell test

.

Source of quantumness

~ the indistinguishability of cases

~ the individual trajectory of individual particles cannot be well-defined

~ the indistinguishability of particles

.

~ “individual” particle has no individuality

~ “individual” particle has no individual identity

— Me@2021-02-06 4:03 PM

— Me@2021-02-15 9:14 PM

.

.

# Particle indistinguishability is the major source of quantum effects, 1.1

.

If particles are distinguishable, there is no quantum-ness.

Why?

— Me@2021-02-06 4:00 PM

.

If there is no particle indistinguishability, all trajectories are distinguishable, then there is no case indistinguishability.

In other words, if every trajectory is well-defined, there is no indistinguishability of cases, even when no detector is installed.

Why?

— Me@2021-02-06 4:01 PM

.

In other words, how to define “every trajectory is well-defined” when no detector is installed?

— Me@2021-02-15 5:00 PM

.

Thought experiment:

In the double-slit experiment, turn on the detector. Then observe the pattern on the final screen.

Next, tune down the detector’s accuracy/resolution a little bit. Repeat the experiment. Observing the pattern again.

Keep repeating the experiment with a little bit lower detector accuracy/resolution at each iteration.

.

If a non-classical pattern never appears on the final screen, we can say that each trajectory is well-defined.

In other words, if the trajectory concept can predict correct experiment results, we say that the trajectory concept is well-defined.

— Me@2021-02-06 4:02 PM

.

It is because the final screen itself is a kind of detector, although not a position detector.

— Me@2021-02-06 05:07:21 PM

.

So it is a kind of Bell-type experiment.

— Me@2021-02-07 06:03:53 PM

.

However, this definition of “every trajectory is well-defined” has a problem.

If the trajectory concept cannot predict correct experiment results, “the trajectory concept is broken” is only one of the possible causes.

In other words, how can you know the non-classical results (aka quantum effects) are not due to other factors?

— Me@2021-02-15 05:03:20 PM

.

.

# Quantum information makes classical information consistent, 1.2

Consistent histories, 10.2 | Cosmic computer, 2.2

.

Quantum mechanics is a theory of classical information.

— Me@2021-02-03 07:48:01 AM

.

Quantum mechanics is a theory of measurement results.

Quantum mechanics explains why measurement results are always consistent with each other.

— Me@2021-02-11 11:10:17 AM

.

.

# Quantum information makes classical information consistent

Consistent histories, 10 | Cosmic computer, 2

.

Wrong: Quantum information is inconsistent and classical information is consistent.

Source: Misunderstanding quantum superposition; regarding mathematical superposition as physical superposition, violating logic, such as a particle has gone through both the left slit and right slit at the same time.

Right: Quantum information is what makes sure that classical information is consistent even when there are indistinguishabilities of some classical cases.

.

a quantum superposition state

~ a state without classical equivalence because some classical cases are indistinguishable-even-in-principle that they are logically forced into one SINGLE physical state

— Me@2021-02-03 01:46:43 PM

.

quantum superposition

= mathematical superposition

= physical NON-superposition

= logically one SINGLE physical state

= go-left and go-right are logically indistinguishable due to the “experiment setup is without detector” part of the definition

= the SINGLE state of “both slits are open but no measuring device is installed; so for each photon, we have no which-way information; because there is no which-way DEFINITION”

.

The definition requirement means that you have to answer

Under what physical phenomenon/phenomena occur(s) that you will say that the photon has gone through the left slit?

In other words, you need to DEFINE “go-left” in terms of at least one potential observable or measurable physical phenomenon. Same for “go-right”.

.

In the double-slit experiment, if there is no meaning of “the difference between ‘go-left’ and ‘go-right’“, then there is no meaning of “go-left”. (Same for “go-right”.)

In that case, we have only the meaning of “go-through-the-slits (without distinguishing ‘go-left’ and ‘go-right’)“.

We still have that meaning because we can still define

the photon has gone through the board (that consists of those 2 slits)

as

there is a dot appearing on the final screen almost immediately after a photon is emitted from the source“.

— Me@2021-02-03 07:48:01 AM

.

quantum information = mathematical information

classical information = physical information

.

quantum entanglement

~ all measurement results will be consistent

~ all measurement results follow the three basic logic laws (i.e. identity, non-contradiction, excluded middle)

— Me@2021-01-30 09:46:13 AM

— Me@2021-02-03 12:27:07 AM

.

Quantum theory is:

The minimal mathematical formalism that correctly describes all physical interaction as classical information exchange and all classical information exchange as physical interaction.

Entanglement is:

The condition of interacting with the world through an imaginary interface on which classical information appears.

— What Is Entanglement Anyway?

— Chris Fields

.

Quantum mechanics is a theory of classical information.

Quantum mechanics explains why all the measurement results are always consistent in spite of the quantum effects, the effects due to the indistinguishabilities of some classical cases.

Quantum mechanics explains why all the measurement results are always consistent in spite of the indistinguishabilities of some classical cases.

— Me@2021-01-28 09:55:56 PM

— Me@2021-02-03 07:48:01 AM

.

.

# Cosmic computer

Consistent histories, 9

.

There is a cosmic computer there

which is responsible to make sure that

quantum mechanics (laws) will always give consistent measurement results,

such as the ones of the EPR entangled pairs.

.

NO. That is wrong.

.

Instead, quantum mechanics itself is THAT cosmic computer that renders all the measurement results consistent.

— Me@2021-01-27 3:54 PM

.

.

# Superposition always exists, 2.2.1

Decoherence and the Collapse, 2.1 | Quantum decoherence 7.2.1

.

But wait! Doesn’t this mean that the “consciousness causes collapse” theory is wrong? The spin bit was apparently able to cause collapse all by itself, so assuming that it isn’t a conscious system, it looks like consciousness isn’t necessary for collapse! Theory disproved!

No. As you might be expecting, things are not this simple. For one thing, notice that this ALSO would prove as false any other theory of wave function collapse that doesn’t allow single bits to cause collapse (including anything about complex systems or macroscopic systems or complex information processing). We should be suspicious of any simple argument that claims to conclusively prove a significant proportion of experts wrong.

To see what’s going on here, let’s look at what happens if we don’t assume that the spin bit causes the wave function to collapse. Instead, we’ll just model it as becoming fully entangled with the path of the particle, so that the state evolution over time looks like the following:

$\displaystyle{|O, \uparrow \rangle \to \frac{1}{\sqrt{2}} |A, \downarrow \rangle + \frac{1}{\sqrt{2}} |B, \uparrow \rangle \to \frac{1}{\sqrt{2}}\sum_i \left( \alpha_i | i, \downarrow \rangle + \beta_i |i, \uparrow \rangle \right) = | \Psi \rangle}$

The interference has vanished, even though we never assumed that the wave function collapsed!

And all that’s necessary for that is environmental decoherence, which is exactly what we had with the single spin bit!

A particle can be in a superposition of multiple states but still act as if it has collapsed!

You might be tempted to say at this point: “Well, then all the different theories of wave function collapse are empirically equivalent! At least, the set of theories that say ‘wave function collapse = total decoherence + other necessary conditions possibly’. Since total decoherence removes all interference effects, the results of all experiments will be indistinguishable from the results predicted by saying that the wave function collapsed at some point!”

But hold on! This is forgetting a crucial fact: decoherence is reversible, while wave function collapse is not!!!

Now the two branches of the wave function have “recohered,” meaning that what we’ll observe is back to the interference pattern!

— Decoherence is not wave function collapse

— MARCH 17, 2019

— SQUARISHBRACKET

— Rising Entropy

.

Decoherence is not wave function collapse

In case the original link does not work, use the Internet Archive version:

https://web.archive.org/web/20210124095054/https://risingentropy.com/decoherence-is-not-wave-function-collapse/

— Me@2021-01-24 07:14:50 PM

.

A particle can be in a superposition of …

Note that it is not that the particle is in a superposition. Instead, it is that the system is in a superposition.

— Me@2021-01-24 07:16:49 PM

.

.

2021.01.25 Monday ACHK

# Superposition always exists, 2.2.2

Decoherence and the Collapse, 2.2 | Quantum decoherence 7.2.2

.

superposition ~ indistinguishability

superposition state ~ logically indistinguishable states (forming one SINGLE quantum state)

logically indistinguishable ~ indistinguishable by definition ~ indistinguishable due to “the experiment setup is without detector” part of the definition

By the Leibniz’s Law (Identity of indiscernibles), logically indistinguishable cases are actually the same one SINGLE case, represented by one SINGLE quantum state.

Classically, there are no such logically indistinguishable cases because classically, all particles are distinguishable. So the probability distribution in the newly invented non-classical state should be completely different from any probability distributions provided by classical physics. Such cases of a new kind are called quantum states.

A quantum state’s probability distribution can be calculated from its wave function.

“Why that single quantum state is represented by a superposition of eigenstates and why its wave function is governed by the Schrödinger equation” is ANOTHER set of questions, whose correct answers may or may not be found in the Wikipedia article Theoretical and experimental justification for the Schrödinger equation.

Superpositions always exist. Logically indistinguishable cases are always there. You just trade some logically indistinguishable cases with some other logically indistinguishable cases.

The “superpositions” are superpositions in definition, in language, in logic, in calculation, and in mathematics, but not in physical reality, not in physical spacetime.

— Me@2021-01-24 09:29:13 PM

.

.

# … is in a superposition

Quantum decoherence 5.3.2 | Wheeler’s delayed choice experiment 1.2

.

For example, in the double-slit experiment, if no detector is installed, the system is in a quantum superposition state.

It is not that each individual photon is in a superposition. Instead, it is that the system of the whole experimental setup is in a superposition.

— Me@2021-01-23 12:57 AM

.

[guess]

However, “what ‘the whole experimental setup‘ is” is not 100% objective. In other words, it is a little bit subjective.

“The whole experimental setup”, although largely objective, is partially defined with respect to an observer.

— Me@2021-01-23 12:58 AM

.

So quantum probability/indistinguishability effect is partly observer-dependent, although the subjectivity is just tiny compared with that of the classical probability in a mixed state.

— Me@2021-01-23 12:59 AM

[guess]

.

.