Question about Quantum Physics

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zenten
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Question about Quantum Physics

Postby zenten » Sun Jun 24, 2007 11:01 pm UTC

Ok, I keep on hearing about how Quantum physics means that there are multiple possible futures for any given history. Thing is, from stuff like the double slit experiment and whatnot it seems more like it is saying there are multiple possible histories for any given future.

Am I right?

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Postby Herman » Mon Jun 25, 2007 12:25 am UTC

That's one interpretation, albeit not the orthodox one.

The multiple futures argument arises because of what's called quantum indeterimancy. According to quantum theory, every particle has a function, called the wave function. The wave function has no intuitive physical meaning, but if you square the WF, the resulting curve gives a "probability density function," which essentially describes the likelihood of a particle being observed inside a certain area.

This is very unlike what you're used to -- how could a particle not be in a certain place? This bothered the physicists at the beginning of last century who were starting to formulate quantum physics, because every physical theory up until then was deterministic -- positions and velocities and so on could be predicted precisely.

But experiments (such as the double-slit experiment) strongly indicated that quantum mechanics was correct. So the physicists came up with a few different interpretations. They were, among others:

1. The uncertainty is in our knowlege, not in nature. That is, our theories are incomplete if all they can do is give a statistical "guess" as to where a particle is. In reality, particles *are* precisely located. (This is the one accepted by Einstein, but it was shown to be definitively false in 1964).

2. The uncertainty is in nature. Before we observe a particle, it is non-localized. The act of observing a particle forces it to be in a particular place. But our theories cannot tell us where the particle was before we observe it because it was truly not precisely anywhere. (This is known as the Copenhagen Interpretation, and it is the one most physicists accept).

3. There is no uncertainty. A particle doesn't have to "choose" where to be when observed -- it takes all possible positions. This is possible because every random event causes reality to "fork" into as many parallel universes as there are possible states. So if I flip a coin, it lands heads in one universe and tails in another. This seems like uncertainty because I only live in one universe at a time, and it is impossible to see into different forks. This, I think, is the interpretation you were talking about. It predicts the exact same experimental results as #2.

But like I said, most physicists accept #2. Anyway, the question is really philosophical rather than scientific, because it is by definition impossible to see if the other "forks" exist.

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Postby zenten » Mon Jun 25, 2007 12:30 am UTC

Ah, ok, so mine interpretation is very much like 3, just sort of flipped.

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Postby Herman » Mon Jun 25, 2007 2:21 am UTC

"Flipped?" How do you mean? I thought the "many worlds" interpretation was what you were talking about. Maybe explain a little more.

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Postby Yakk » Mon Jun 25, 2007 3:35 am UTC

Yes, under the multiple universes interpretation, there are both multiple possible histories for any given present, and multiple possible futures for any given present.

In fact, under M-U interpretation, the various histories of a particle interfere with each other in a wave-like fashion.

An observation event is actually the observer interacting with the multiple histories in a way that makes certain histories inconsistent with each other. Instead of the state of the universe collapsing upon observation, the observer rotates in such a manner that it can only see the universes consistent with it's observation (and a different history of the observer rotates in a different fashion so that it sees the other possible universes).

Fun, eh?

Basically, Q-M boils down to "should the entire universe change state on every observation" or "should only the observer change state when it observes a particle". The first is non-locality, the second is multiple-universes. And as far as I know, Q-M implies either non-locality or multiple-universes.

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Postby ArmonSore » Mon Jun 25, 2007 4:23 am UTC

I always feel a little dizzy when people envoke things like multiple universes, or extra dimensions, or etcetera, to explain away some pretty complicated ideas. Why would we want to envoke constructs that we have no way of testing experimentally? I wish there were better ways to interpret the problems.
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Postby Gelsamel » Mon Jun 25, 2007 4:25 am UTC

I hate the use of the word "Observation" in Quantum Mechanics.
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Postby Herman » Mon Jun 25, 2007 6:16 am UTC

@ Yakk:

Ah, cool. I learned something. :)

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Postby Bondolon » Mon Jun 25, 2007 7:50 am UTC

Gelsamel wrote:I hate the use of the word "Observation" in Quantum Mechanics.


If there is some thing, and you notice that thing (in really any way), you are observing it. Why do you hate it?

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Postby SpitValve » Mon Jun 25, 2007 8:29 am UTC

Bondolon wrote:
Gelsamel wrote:I hate the use of the word "Observation" in Quantum Mechanics.


If there is some thing, and you notice that thing (in really any way), you are observing it. Why do you hate it?


Well, you don't even need to "notice" it really... you're making it "knowable", which is a slightly different thing.

e.g. when scattering through a lattice where the atoms that the particle scatters off are spin-flipped. I don't know if there's a way to actually measure which atom is spin-flipped, but because you could in principle determine which atom spin-flipped and hence determine what happened to the particle that shot through the lattice, that particle's wavefunction actually collapses.

(or something like that... [tries to remember 3rd year quantum])

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Postby ludwig_van » Mon Jun 25, 2007 8:38 am UTC

just an afterthought, so I can go off on a wikipedia tangent-- what was the name/nature of the experiment that proved Einstein wrong about scientific determinism? because frankly that's the most interesting thing I've heard in my life.

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Postby Bondolon » Mon Jun 25, 2007 8:39 am UTC

SpitValve wrote:
Bondolon wrote:
Gelsamel wrote:I hate the use of the word "Observation" in Quantum Mechanics.


If there is some thing, and you notice that thing (in really any way), you are observing it. Why do you hate it?


Well, you don't even need to "notice" it really... you're making it "knowable", which is a slightly different thing.

e.g. when scattering through a lattice where the atoms that the particle scatters off are spin-flipped. I don't know if there's a way to actually measure which atom is spin-flipped, but because you could in principle determine which atom spin-flipped and hence determine what happened to the particle that shot through the lattice, that particle's wavefunction actually collapses.

(or something like that... [tries to remember 3rd year quantum])


I'll concede that, but it falls under what I meant for notice. You are discussing a particle that is "there", and doing other stuff to figure out what's up with the particle. If one were talking about some particle, based purely on virtue, one just shifted from observation (the concrete) to theory (the abstract).

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Postby Gelsamel » Mon Jun 25, 2007 1:02 pm UTC

When explaining to laymen using words like "observe" in quantum mechanics you end up with them thinking about stuff like This and This.

Of course, they may not - but when ever I hear 'observe' being used like this it reminds me of that stuff and it makes me cringe.
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Postby aoanla » Mon Jun 25, 2007 3:38 pm UTC

ludwig_van wrote:just an afterthought, so I can go off on a wikipedia tangent-- what was the name/nature of the experiment that proved Einstein wrong about scientific determinism? because frankly that's the most interesting thing I've heard in my life.


http://en.wikipedia.org/wiki/Bell's_theorem is what you're looking for, although there's an even more restrictive form which has also been confirmed (but very recently).

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Postby ehiunno » Mon Jun 25, 2007 4:07 pm UTC

Oh dear God, we had to watch What the Bleep Do We Know in my Theory of Knowledge class last year. It was really interesting... for the first 15 minutes. It was all downhill from there. I thought it would be something like The Elegant Universe, but it wasn't... not at all. Eventually I got so infuriated I just made a rebuttle to every retarded thing the psuedo scientists in the movie said. Then my ToK teach just started talking about how I just didn't get it, implying that I was too immature to understand what they were talking about or something.

No no, I got it, it was just wrong.

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Postby Yakk » Mon Jun 25, 2007 5:45 pm UTC

The reason one throws around untestable (or unreproducibly testable) models for the math is that it provides a context to model what else could be true.

The various interpritations of Q-M can help light up what might happen in Q-M border cases. It is true that the raw math has pretty much the same predictive properties -- but the interpritation can provide a cognative shortcut.

"Observe" under M-U means "perform an operation that rotates your state in a way that you are orthoginal to some of the possible states of the thing you are observing" -- ie, it is an operation on the thing doing the observing.

"Observe" under Copenhagen means "perform an operation on the entire universe so that only those states which are consistent with whichever collapsed state you reveal occur".

As many philosophers have noted, there is a duality between "operations on oneself" and "operations on the entire universe" -- for every operation you perform on oneself (say, move) there is an equivilent operation you can perform on the rest of the universe (move it the other way!) that you cannot distinguish between.

Sadly, we end up with two models, each with screwy things:

M-U has, well, multiple universes. Or, a universe that is much larger and growing insanly broader (in a very strange sense) every moment, of which we are seeing one shadow, and every operation changes exactly what angle of the shadow of the universe we are experiencing.

Copenhagen has spooky-action-at-a-distance -- what happens here, under Q-M math, impacts the state of the far side of the universe, because no quantum state can be localized -- the collapse of the state of a single electron has universe-global impact.

These are both sorta screwy.

There is the hope that Q-M is only an approximation to what is going on -- ie, maybe the universe-as-shadow-of-real-universe is right, but the breadth of the universe isn't quite as large: we see shadows of shadows, but what we see is what is "really" there.

Or maybe quantum state is more local than we think -- but we have already spread quantum state over macroscopic distances (photon entanglement splitting).

Or maybe I (and maybe we!) just can't grasp Q-M well enough yet, and there is a case I can't describe. :)

Note I'm not a professional -- these are just half-educated opinions, from exposure to people who are professionals and/or recreational reading.

I do notice a tendency for Feymnanism to hold amoung many breeds of Physicist -- "shut up and calculate". Sure, we don't know what the fuck is going on -- but it predicts stuff, so let's use it until it fails to predict what happens.

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Postby shill » Mon Jun 25, 2007 6:22 pm UTC

ehiunno wrote:Oh dear God, we had to watch What the Bleep Do We Know in my Theory of Knowledge class last year. It was really interesting... for the first 15 minutes. It was all downhill from there. I thought it would be something like The Elegant Universe, but it wasn't... not at all. Eventually I got so infuriated I just made a rebuttle to every retarded thing the psuedo scientists in the movie said. Then my ToK teach just started talking about how I just didn't get it, implying that I was too immature to understand what they were talking about or something.

No no, I got it, it was just wrong.

I'm not seeing what WtBDWK has to do with ToK, but then again, I've never taken a ToK class.

Here's an excellent review of the movie: http://skeptico.blogs.com/skeptico/2005 ... leep_.html

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Postby ehiunno » Mon Jun 25, 2007 7:21 pm UTC

shill wrote:
ehiunno wrote:Oh dear God, we had to watch What the Bleep Do We Know in my Theory of Knowledge class last year. It was really interesting... for the first 15 minutes. It was all downhill from there. I thought it would be something like The Elegant Universe, but it wasn't... not at all. Eventually I got so infuriated I just made a rebuttle to every retarded thing the psuedo scientists in the movie said. Then my ToK teach just started talking about how I just didn't get it, implying that I was too immature to understand what they were talking about or something.

No no, I got it, it was just wrong.

I'm not seeing what WtBDWK has to do with ToK, but then again, I've never taken a ToK class.

Here's an excellent review of the movie: http://skeptico.blogs.com/skeptico/2005 ... leep_.html


its probably because you have never taken a ToK class ;-). It is really not on the mainstream syllabus, but it can be worked in, and we had extra time.

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Postby gmalivuk » Mon Jun 25, 2007 7:33 pm UTC

Yakk wrote:for every operation you perform on oneself (say, move) there is an equivilent operation you can perform on the rest of the universe (move it the other way!) that you cannot distinguish between.


Unless you have 200% efficient engines that run on dark matter, in which case moving the rest of the universe the other way allows you to seem to be going faster than light. :-)
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Postby 3.14159265... » Tue Jun 26, 2007 3:49 am UTC

gmalivuk wrote:
Yakk wrote:for every operation you perform on oneself (say, move) there is an equivilent operation you can perform on the rest of the universe (move it the other way!) that you cannot distinguish between.


Unless you have 200% efficient engines that run on dark matter, in which case moving the rest of the universe the other way allows you to seem to be going faster than light. :-)
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Postby ArmonSore » Tue Jun 26, 2007 3:58 am UTC

Actually, it's not an apparent effect. Farnsworth claims that the speed of light was raised by scientists in 2508.

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Postby ehiunno » Tue Jun 26, 2007 12:43 pm UTC

haha, but engines can't run at 200% efficiency, thats impossible!


no its not


then explain it!

now thats impossible


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