The Uncertainty Prinicple

[Suggestion Box]

How do we know that quantum mechanics display non-deterministic randomness?

Is it really simply because it's impossible to measure? That seems silly. Inability for humans to predict something doesn't mean there's no cause for it. Although leave it to physicists to make a 90-year mistake simply to cover their own butts.

That being said, i'm sure there's a real answer out there. anyone?

 

[pgp_protector]

Are you asking about the Heisenberg uncertainty principle ?

 

[Suggestion Box]

ya

 

[pgp_protector]

Well I could be wrong, but if I'm correct all the Heisenberg uncertainty principle states is that you the more exact you know something like the position , the less exact you can know the momentum.

 

[Suggestion Box]

the way i have heard this explained is in regards to measuring an electron. the act of measuring the electron's momentum changes its position, and vice versa, so you can't know exactly where it is at any given time.

that may as well be true for all modern practical uses, but i don't see why scientists have assumed that, because a thing cannot be measured, it is random.

 

[Jester4kicks]

the way i have heard this explained is in regards to measuring an electron. the act of measuring the electron's momentum changes its position, and vice versa, so you can't know exactly where it is at any given time.

that may as well be true for all modern practical uses, but i don't see why scientists have assumed that, because a thing cannot be measured, it is random.

To be more accurate, we (currently) cannot know where it is AND where it's going at the same time.

I'm not sure where this "random" part is coming in though. Where are you reading about random movement?

 

[Suggestion Box]

ok i talked to my roommate's girlfriend and she's a physics major. the current theory says that quantum particles/waves/whateveryoucallem move in a 'random' fashion.

the problem is that some people misinterpret the word 'random' to mean 'without cause' (objectively random). that is not at all the implication of Heisenberg's Uncertainty Principle. that's what Einstein meant by 'God doesn't play dice with the universe'. current quantum theory uses a weaker definition of 'random', as you say: something humans cannot possibly predict by current methods (subjectively random).

as for where i've read/heard about random movement, an old prof of mine was convinced that particles at a 'basic' level move around without defined cause. i've also read around on scienceforums.net and wikipedia, and generally heard this idea from colleagues for years.

in essence, it seems that whoever claims objective randominity to describe anything in our currently understood universe has not done their homework.

 

[Jester4kicks]

ok i talked to my roommate's girlfriend and she's a physics major. the current theory says that quantum particles/waves/whateveryoucallem move in a 'random' fashion.

the problem is that some people misinterpret the word 'random' to mean 'without cause' (objectively random). that is not at all the implication of Heisenberg's Uncertainty Principle. that's what Einstein meant by 'God doesn't play dice with the universe'. current quantum theory uses a weaker definition of 'random', as you say: something humans cannot possibly predict by current methods (subjectively random).

as for where i've read/heard about random movement, an old prof of mine was convinced that particles at a 'basic' level move around without defined cause. i've also read around on scienceforums.net and wikipedia, and generally heard this idea from colleagues for years.

in essence, it seems that whoever claims objective randominity to describe anything in our currently understood universe has not done their homework.

Psst.... "randominity" isn't a word. "Randomness" would have been appropriate.

Anyway, you got your answer, and you rephrased it well.

I like my pool table analogy for this. I came up with this in a discussion with Tansy, but the same principle applies.

If you were to look at a pool table right after the first break, you would see the balls going all over the place. Now, if you had enough time, you could calculate where the balls were going and where their paths would take them, accounting for all the little variables like drag, collisions, etc. With enough information, you could calculate where all the balls had come from, and what caused them to move the way they did... and where all the balls were going.

Now, shrink that pool table down to the subatomic level... and you essentially have the same thing. Everything is moving according to well established principles... but the more we figure out about where all the balls are, the less we know about where they are going... and the more we know about where they are going, the less we know about where they are.

The simple fact that we cannot take into account all of the principles simulataneously has nothing to do with the fact that all of it is still acting in accordance with natural laws.

 

[Suggestion Box]

indeed. why is it that so many people misinterpret this!?

oh, and i looked up randominity. it's up for debate as to whether its a word. i think i got the point across though. here's what wiki has to say:

The term “randominity” is considered a neologism based on standardized Wiktionary criteria. Neologisms are newly acknowledged terms. They typically have not been in circulation long enough or widely enough for their social status to be determined. Neologisms can be nonces, slang terms, or even illiteracies.

for the record, i don't actually want to debate whether or not randominity is a word...

just sayin' is all.

 

[sfs]

The conventional physics wisdom is that quantum phenomena are consistent with a deterministic model. What they are not consistent with is a deterministic model that is also local. Non-local theories (look up "hidden variable theories" and Bell's inequalities if you want to read about them) include faster-than-light communication, which violates special relativity and which is not attractive to many physicists. But they can be consistent with observed phenomena, at least in principal; I don't think there is actually a full working version of such a theory.

(I also see that there are papers that claim to find a loophole that allows local deterministic theories, but I'm not a physicist anymore and I don't have to pretend to be interested enough to read them.)

 

[ragarth]

IANAP, but this is my opinion on the issue:

The Heisenberg Uncertainty Principle states that you can either know the position or the velocity of a particle, but not both at the same time. This rule describes the observational limitations of particles, not the reality of them. Therefore chaos theory is open to describing the movement of particles outside of our observational limits.

If we define a system as a chaotic system, then it's a system of such a high complexity that we cannot accurately measure all it's inputs, and/or cannot accurately model all it's internal relationships. In such a system our predictions become increasingly inaccurate over time due to the increasing effect of the unknown on it (ie, a butterfly in africa causing a hurricane in florida). A chaotic system is not by any means random, but it is unpredictable, and therefore if the interaction of subatomic particles is a chaotic system then there is no need for inherent randomness for it's unpredictability to be derived.

 

[chilehed]

How do we know that quantum mechanics display non-deterministic randomness?

Is it really simply because it's impossible to measure? That seems silly. Inability for humans to predict something doesn't mean there's no cause for it. Although leave it to physicists to make a 90-year mistake simply to cover their own butts.

That being said, i'm sure there's a real answer out there. anyone?

A number of times, in conversations about the existance of God, I've asked atheists pretty much this same question. Some variant of "do you have actual evidence that it's uncaused, or are you just assuming that all causes are observable?"

The conversation always ceases at that point.

 

[lawtonfogle]

How do we know that quantum mechanics display non-deterministic randomness?

Is it really simply because it's impossible to measure? That seems silly. Inability for humans to predict something doesn't mean there's no cause for it. Although leave it to physicists to make a 90-year mistake simply to cover their own butts.

That being said, i'm sure there's a real answer out there. anyone?

Two issues. One is that there is a deterministic model for quantum mechanics (I know next to nothing about it though). The other is that under the current HUP, it is not because we cannot measure it, but because it does not exist discretely. There is no one little dot which is the electron, it exist as a probability, something which seems nonsensical to us living in the macro world.

 

[sfs]

IANAP, but this is my opinion on the issue:

The Heisenberg Uncertainty Principle states that you can either know the position or the velocity of a particle, but not both at the same time. This rule describes the observational limitations of particles, not the reality of them. Therefore chaos theory is open to describing the movement of particles outside of our observational limits.

It's not just a question of what you can observe, and it has nothing to do with chaos. If there is a hidden but definite state of the particle, you get different predicted results than if the state is genuinely indeterminate, as long as the hidden state is local to the particle, that is, that is does not get to engage in faster-than-light communication.

This kind of problem is usually discussed using the example of particle spins, rather than momentum and position. Take a pair of electrons (which each have a fixed amount of spin), emitted back to back so that their spins have to be oriented in opposite directions, to conserve angular momentum. You can measure the spin of one electron in the vertical direction or in the horizontal direction, but whichever way you measure it, if you measure the other electron in the same axis, it will have the opposite direction (up vs down, or right vs left).

Quantum mechanically, the two axes (vertical and horizontal) have a Heisenberg Uncertainty Relation between them: if you measure along one axis, you cannot know anything about spin along the other axis. (If you measure the first electron vertically and the second horizontally, you have a 50% chance of getting a spin pointing right on the second measurement and a 50% chance of getting a spin pointing left.) In QM, the spin along the second axis is indeterminate (really random) once a measurement has been made along the first axis (even if the two measurements are made on different electrons in the pair).

Suppose there is actually a real, hidden spin direction that we cannot directly measure, and that this is why the two always point in opposite directions. This would explain the correlation between the two spin measurements, and give the same answers as the "really random" QM formulation -- as long as you only measure vertically and horizontally. If instead you measure one of the spins at 45 degrees, you will get a different predicted correlation between the two measurements for the hidden, definite spin than for QM. (This is the essence of Bell's Inequality.) So you can perform an experimental test to determine which prediction is correct. That's been done, and the QM prediction is correct and the hidden variable prediction wrong.

This conclusion is valid unless the first particle can communicate with the second (regardless of how far apart they are, and how close together the measurements are in time) to tell it what axis the experimenter chose to measure along, i.e. unless the hidden variable is non-local.

 

[sfs]

Two issues. One is that there is a deterministic model for quantum mechanics (I know next to nothing about it though).

As far as I know (and what I know is now a decade out of date), there is only a non-relativistic deterministic model of QM (Bohm's pilot wave theory); there is no relativistically valid theory.

 

[Suggestion Box]

perhaps i should have made this a poll. QM: deterministic or not?

honestly, from what i've heard from the physics people at my college, it sounds like QM is a lot of guess and check, and i think people are gonna have to figure out what the underlying concepts are before we can claim any real evidence for whether stuff is deterministic.

one theory says the hidden variable is consciousness. they actually have experiments out now that try to direct thought at a QM reaction to influence the flip of a coin. i read that somewhere... crazy mind powers *waves hands eerily*

 

[Tenka]

A number of times, in conversations about the existance of God, I've asked atheists pretty much this same question. Some variant of "do you have actual evidence that it's uncaused, or are you just assuming that all causes are observable?"

The conversation always ceases at that point.

Hey Chilehed, I enjoyed reading your thread on entropy in the origins theology section.

 

[Cabal]

It is inherent non-determinism, as far as I know. It is not random either, when collective behaviour is viewed.

 

[ragarth]

It's not just a question of what you can observe, and it has nothing to do with chaos. If there is a hidden but definite state of the particle, you get different predicted results than if the state is genuinely indeterminate, as long as the hidden state is local to the particle, that is, that is does not get to engage in faster-than-light communication.

This kind of problem is usually discussed using the example of particle spins, rather than momentum and position. Take a pair of electrons (which each have a fixed amount of spin), emitted back to back so that their spins have to be oriented in opposite directions, to conserve angular momentum. You can measure the spin of one electron in the vertical direction or in the horizontal direction, but whichever way you measure it, if you measure the other electron in the same axis, it will have the opposite direction (up vs down, or right vs left).

Quantum mechanically, the two axes (vertical and horizontal) have a Heisenberg Uncertainty Relation between them: if you measure along one axis, you cannot know anything about spin along the other axis. (If you measure the first electron vertically and the second horizontally, you have a 50% chance of getting a spin pointing right on the second measurement and a 50% chance of getting a spin pointing left.) In QM, the spin along the second axis is indeterminate (really random) once a measurement has been made along the first axis (even if the two measurements are made on different electrons in the pair).

Suppose there is actually a real, hidden spin direction that we cannot directly measure, and that this is why the two always point in opposite directions. This would explain the correlation between the two spin measurements, and give the same answers as the "really random" QM formulation -- as long as you only measure vertically and horizontally. If instead you measure one of the spins at 45 degrees, you will get a different predicted correlation between the two measurements for the hidden, definite spin than for QM. (This is the essence of Bell's Inequality.) So you can perform an experimental test to determine which prediction is correct. That's been done, and the QM prediction is correct and the hidden variable prediction wrong.

This conclusion is valid unless the first particle can communicate with the second (regardless of how far apart they are, and how close together the measurements are in time) to tell it what axis the experimenter chose to measure along, i.e. unless the hidden variable is non-local.

Thank you for that, but it seems to me that the conclusion drawn is invalidated in the presence of quantum entanglement, and spooky action at a distance. It has further been confirmed that that spook takes place at a speed faster than we can measure irregardless of distance. This has been proven to the point at which it violates the speed of light. Two entangled photons were generated and sent down fiber so that their distance was significant enough that, when measured, any communication would have to take place between them faster than the speed of light for spooky action to be observed- Spooky action was still observed. I don't have time atm to pull up the article on this, I'll do so later when I get home.

I'll need to research the info on bell's inequality a bit more before drawing a conclusion on that, I don't have time atm.

 

[Tenka]

heh..