Understanding Quantum Theory: A Beginner's Guide to Particle Physics

[DevilsAvocado]

... B1/B2 would still be coherent with A that sent the single signal, and so both B1/B2 would see the same signal from A.

To make things even 'worse', consider this:
This is a theoretical thought experiment; therefore A & B has built an optical fiber cable to send the radio signal digitally...

To me this fiber cable doesn’t fit my understanding of 'reality', since it must exist in duplicate versions, transmitting physically the same information...?

How does this work in MWI??

 

[Chalnoth]

Well, if the interaction with the light signal is significant enough, it would progressively decohere as a decohered signal passes through the cable.

This really isn't that tough: interaction of an object with a signal that has decohered causes further decoherence.

 

[DevilsAvocado]

Well, if the interaction with the light signal is significant enough, it would progressively decohere as a decohered signal passes through the cable.

Okay Chalnoth, A and B1/B2 sends their digital signals in opposite direction.

At B1/B2 we now have 2 physical fiber cables, a start.
At A there’s only 1 physical fiber cable, a start.

This results in 3 digital signals, on their way in 3 physical separate fiber cables A/B1/B2.

After 1 light-year signal A 'meets' signal B1 & B2 halfway. The question is, in what cable? Or are the cables 'merged/split' by MWI, so that all works out at the endpoints? If so why? No one is doing observations at halfway? What kind of 'merge/split' would that be??


If you can answer the questions above, I’m going to make things much worse²:
A & B has planned for A not to observe the digital signal from B right away. A is saving the digital signal on a USB stick, and starts travel towards B (with a bottle of champagne ). The idea is that they are going to play the digital radio signal, with their respectively result, simultaneously at same location, and celebrate.

A goofed his measurement. A did not listen to the digital radio signal from B. A must be in the same 'old' single branch, when starting his journey towards B.

Questions:
Where do A go? B doesn’t exist anymore; he is split into B1/B2??

If A is split anyway (by some function in MWI), Ax and Ay does not know what’s on the USB stick.
How do MWI couple Ax/Ay with the right B1/B2??

 

[Chalnoth]

Okay Chalnoth, A and B1/B2 sends their digital signals in opposite direction.

At B1/B2 we now have 2 physical fiber cables, a start.
At A there’s only 1 physical fiber cable, a start.

This results in 3 digital signals, on their way in 3 physical separate fiber cables A/B1/B2.

You're making the mistake of assuming that the entire fiber cable must decohere at the exact same time. This isn't the case. You're also making the mistake of thinking of A/B1/B2 as being different cables: this is not the case, they are just different components of the same wavefunction. Furthermore, since A2=B1 and A1=B2, A is just a superposition of B1 and B2, and the only thing that changes as the B signal passes through is that the coherence is lost between A1 and A2.

 

[DevilsAvocado]

... You're also making the mistake of thinking of A/B1/B2 as being different cables: this is not the case, ...

When P2, which is in a superposition of states, is measured by B, what happens to B?

Is B physically split into B1 and B2, seeing different outcome?

Or, is B put in some other 'superposition of states', leading to the same physical person seeing two contradictive outcomes, parallel?? (...unreal...)

(This is so weird... Wikipedia: "decoherence refers to the untangling of quantum states to produce a single macroscopic reality"...)


P.S. I have no problem accepting microscopic superposition of states, but - macroscopic superposition of states? Has this ever been observed??

 

[Chalnoth]

At this point, I don't know how to make things more clear. I think you're just confusing yourself by considering more and more convoluted scenarios. The mechanism itself is exceedingly simple.

First, you start with a system in a superposition of states. Second, you interact this system with another (perhaps to make an observation, perhaps not). The interaction causes the superposition of states to lose coherence, meaning that the two states can no longer interact.

That's all there is to it.

 

[DevilsAvocado]

... I think you're just confusing yourself by considering more and more convoluted scenarios ...

You’re right Chalnoth.

I just have this final simple question:
Does MWI stipulate that a macroscopic object (like a human observer) can be in a superposition of states?

Yes or No?

(Promise, no more questions after this)

 

[Chalnoth]

You’re right Chalnoth.

I just have this final simple question:
Does MWI stipulate that a macroscopic object (like a human observer) can be in a superposition of states?

Yes or No?

(Promise, no more questions after this)

Not a coherent superposition. The continual interactions of large objects with their environment tends to make coherent superpositions very difficult to maintain.

 

[DevilsAvocado]

Not a coherent superposition.

Chalnoth, thanks a lot for taking the time and having patience with my "entangled-brain-activities".

This is a very interesting and amazing discussion for me. We are talking in rather basic words, about a rather basic setup A/B+P1/P2. To you; this is perfectly clear and, as I understand, almost as basic as 1+1=2.

To me; the 'red lights' are flashing all over the place – it’s a mystery and complete contradiction.

I now think I know where my understanding 'breaks down'. I have a slight feeling it lays in understanding the difference between "wavefunction" and "superposition"... but I’m definitely not sure (about that either).

Therefore I shall read the arXiv paper http://arxiv.org/abs/quant-ph/0312059" by Maximilian Schlosshauer.

I’ll be back in a couple of days; either expressing my frustration, or exclaiming – The 'Red Lights' are out!

Thanks again.

 

[DevilsAvocado]

Thrilling Update:

It looks like my question in https://www.physicsforums.com/showpost.php?p=2555746&postcount=21" about macroscopic EPR is just on the edge of being realized:

http://www.sciencenews.org/view/generic/id/57385/title/Physicists_observe_quantum_properties_in_the_world_of_objects"
Potential applications, he says, include using arrays of these resonators to control multiple quantum systems in information processing or to test predictions about “Schrödinger cat” states — named for a hypothetical feline simultaneously alive and dead — in which a system exists in a mix of states known as a superposition. Cleland’s team showed, somewhat indirectly, that a form of superposition existed inside their resonator. If the researchers could make a resonator with longer-lasting vibrations, scientists might be able to test superposition on the macroscopic scale.

A 60µm resonator isn’t directly a "human observer", but it is definitely macroscopic!

http://www.sciencenews.org/view/download/id/57383/name/Quantum_object.jpg
Amazing!

 

[yoda jedi]

Thrilling Update:

It looks like my question in https://www.physicsforums.com/showpost.php?p=2555746&postcount=21" about macroscopic EPR is just on the edge of being realized:


A 60µm resonator isn’t directly a "human observer", but it is definitely macroscopic!

http://www.sciencenews.org/view/download/id/57383/name/Quantum_object.jpg
Amazing!

posted time ago:
Mar18-10
Micron sized object observed in quantum state

https://www.physicsforums.com/showthread.php?t=387688

Thanks again.
Maybe it’s luck that I don’t work with these things (as it probably would drive me crazy ).

This is so extremely weird... almost as if there are two separate worlds, the macroscopic and the microscopic...

Hasn’t anyone tried to build a 'bridge' between the two, to see what happens? Like extremely large fullerenes, or something else? How about running the EPR paradox with objects on the 'border' to the QM world...

Or is it just impossible...

there is no border, is continous...
the microscopic and the macroscopic

 

[DevilsAvocado]

posted time ago:

Thanks for the info yoda jedi. Well, there seems to be 'some' discussion whether:
https://www.physicsforums.com/showpost.php?p=2630488&postcount=4"

As long as no one has showed me a cat that is simultaneously dead and alive, I regard this as an "open question"...

But macroscopic EPR, if possible, would be thrilling.

"If a Boeing 747 is filled with petite ballet dancers, it doesn’t automatically mean the Boeing 747 also can dance." -- DevilsAvocado

 

[Chalnoth]

The fundamental problem is that to get something in a coherent superposition, you have to ensure that it's not doing much of any interacting with anything else. The larger an object is, however, the more likely you'll end up with that coherence destroyed by interactions with surrounding photons (which are always present to some degree at temperatures above absolute zero). So to get a coherent superposition for a macroscopic object, you it to be both very cold and very isolated. And that's for stuff in the micrometer size range.

 

[DevilsAvocado]

Hi Chalnoth, interesting info. I guessed it would imply some 'restrictions' on large macroscopic objects.

(I’ll continue my 'processing' of post #59)

P.S. Is there any such thing as a "decoherent superposition"...?

 

[Chalnoth]

P.S. Is there any such thing as a "decoherent superposition"...?

Well, if it's decohered, then we can only measure one of the decohered components of the superposition.

 

[DevilsAvocado]

Thanks!

 

[yoda jedi]

Thanks for the info yoda jedi. Well, there seems to be 'some' discussion whether:
https://www.physicsforums.com/showpost.php?p=2630488&postcount=4"

As long as no one has showed me a cat that is simultaneously dead and alive, I regard this as an "open question"...

But macroscopic EPR, if possible, would be thrilling.

"If a Boeing 747 is filled with petite ballet dancers, it doesn’t automatically mean the Boeing 747 also can dance." -- DevilsAvocado

I regard this as an "open question"...

of course, soon it will be known...
but the superposition it will last some time...



for a
"to observe the oscillations of a 2.7 kg pendulum mode at a level close to its quantum ground state"
http://www.sciencedaily.com/releases/2009/07/090716093526.htm

"Observation of a kilogram-scale oscillator near its quantum ground state"
http://iopscience.iop.org/1367-2630/11/7/073032/

..."cool the mirror oscillator to about 1 millionth of a degree above absolute zero"...





i think nonlinearity it will broke the superposition at some level (and/or some time) and NONLINEAR QUANTUM MECHANICS (NLQM) it will supersedes STANDARD QUANTUM MECHANICS (SQM) in the realm of physics..



..."If the nonlinearity idea is correct, this superposition will not last forever".....Singh.



nonlinearity collapses the superposition.

 

[DevilsAvocado]

... i think nonlinearity it will broke the superposition at some level (and/or some time) and NONLINEAR QUANTUM MECHANICS (NLQM) it will supersedes STANDARD QUANTUM MECHANICS (SQM) in the realm of physics..

..."If the nonlinearity idea is correct, this superposition will not last forever".....Singh.

nonlinearity collapses the superposition.

Wow! This is interesting!

Tejinder Singh
There is perhaps a more promising experimental test of the nonlinearity. The experiment being planned by the group of Aspelmeyer at Vienna (jointly with Kieth Schwab of Cornell) will attempt to create a superposed quantum state of micromirrors having a billion atoms.

If the nonlinearity idea is correct, this superposition will not last forever. It will have a finite lifetime, which decreases with the increasing number of atoms in the micro mirror. According to the calculations based on my paper

http://arxiv.org/abs/0711.3773

http://arxiv.org/abs/0711.3773"
...
Standard linear quantum mechanics is an approximation to this non-linear theory, valid at energy scales much smaller than the Planck scale. Using ideas based on noncommutative differential geometry, we develop such a reformulation and derive a non-linear Schrödinger equation, which can explain collapse of the wave-function. We also obtain an expression for the lifetime of a quantum superposition.

I shall definitely read this paper (as well as the one by Maximilian Schlosshauer).

This is so amazingly weird... logically one would think that we and the world is built by tiny little 'bricks', behaving 'unreal', and therefore we should also behave 'unreal'. But 'mostly' we don’t...

And at the same time: If we look at very large objects like galaxies they, behave in way that doesn’t work in a scale model on Earth (they rotate too fast to keep the outer stars, but DM 'fixes' the problem).

Maybe, just maybe, there are 'transitions', between different 'scales', that makes the nature work differently at different 'scales', that we are not fully aware of yet...?? 1+1=3...!?

Just a philosophical thought...

 

[yoda jedi]

and:


Singh
Quantum Theory, Gravity, and the Standard Model of Particle Physics.
http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.3391v1.pdf

Nonlinear Quantum Mechanics, the Superposition
Principle, and the Quantum Measurement Problem.
http://arxiv.org/PS_cache/arxiv/pdf/0912/0912.2845v2.pdf