2 Quantum Mechanics questions

[SkepticalScience]

I asked about QM before - and have since read a bit more about the topic. I still have 2 questions though:

1. Can scientists "see" the particles that make up protons and things? Meaning - do scientists have some sort of super-duper microscope that can actually see these particles? If not - how do we know they exist really??

2. At the quantum level, are the motions of the sub-atomic particles TRULY random - or is it that we don't know what controls it yet??

Thanks,
SS

 

[Bodhi Dharma Zen]

I asked about QM before - and have since read a bit more about the topic. I still have 2 questions though:

1. Can scientists "see" the particles that make up protons and things? Meaning - do scientists have some sort of super-duper microscope that can actually see these particles? If not - how do we know they exist really??

2. At the quantum level, are the motions of the sub-atomic particles TRULY random - or is it that we don't know what controls it yet??

Thanks,
SS

1) You see their traces after a collision on a particle accelerator, certain trajectories are predicted to be definite particles. AFAIK, thats about it.

2) What is "truly" random?

 

[SkepticalScience]

By truly random, I mean that there is no rhyme or reason causing the particle behavior.

And that there is no way of observing and then predicting the motions of a sub-atomic particle.

I can roll a ball down an incline, and if i knew all of the variables, I can tell you it's exact motion. From what I have been gathering, you can't do that with sub-atomic particles.

What I am can't figure out, is if the reason we can't predict sub-atomic particle motion because of a limitation of our current technology, of if those particles motions are inhernetly random.

 

[Ziggurat]

I asked about QM before - and have since read a bit more about the topic. I still have 2 questions though:

1. Can scientists "see" the particles that make up protons and things? Meaning - do scientists have some sort of super-duper microscope that can actually see these particles? If not - how do we know they exist really??

I'll give a little more detail about what BDZ said. If you whack protons and neutrons hard enough in particle accelerators, you can break them. These collisions are extremely energetic, and the resulting particles are detected in large particle detectors as trajectories, from which you can infer things about them such as their charge and their energy. It gets quite tricky, because you're not only breaking apart the particles, you're also often forming new particle/antiparticle pairs. So analyzing all this gets quite complicated. But by looking at conserved quantities (such as charge and energy/mass), you can infer quite a bit. One of the things they've infered is the existence of quarks: three quarks make up protons and neutrons (the combination determining which you get), but you can also have exotic particles with only two quarks (though I think they decay fairly quickly). But you NEVER see solitary quarks. We infer their existence because of the way the properties they have (such as mass and charge) get conserved across all the different products of collisions.

2. At the quantum level, are the motions of the sub-atomic particles TRULY random - or is it that we don't know what controls it yet??

I would say that this isn't really a well-formed question in the quantum mechanical sense. Quantum mechanics is completely deterministic: the waev function for a system evolves according to an equation with no randomness involved. The problem comes from the measurement process: what exactly does the wave function mean? If we want to measure the position of a particle, we can treat the wave function as a probability distribution and say that the rest is just randomness, but that's not satisfactory. We can also postulate that there are some hidden variables (what you allude to), factors which, if we could know them, would completely determine things unambiguously. But that's actually not satisfactory either, since it's been proven that any possible hidden variables theory is necessarily non-local (shorthand: there's still weirdness going on). And besides, we have no idea where to start with such a theory, so it really doesn't get us any farther.

But there's even a third point that I think gets brushed under the rug far too often. Namely, quantum mechanics makes no distinction between microscopic and macroscopic systems. Our measurement apparatus, in quantum mechanics, is some EXTREMELY complicated system with LOTS of particles. Modelling it with quantum mechanics is, practically speaking, an impossibility, but that doesn't mean it couldn't be done in principle (if, for example, we had a truly infinite and infinitely fast computer, and we could somehow know the exact initial quantum mechanical state of our instrument). IF you could do so, then the complications that arrise from our single particle interacting with this massively many-bodied system (which, due to thermal fluctuations, is never going to be in the same initial state for any two measurements) might make what looks initially like randomness turn out to be not so random after all. But unfortunately, we really can't do that, so this too is kind of just speculation, although I find it much more satisfying than hidden variables (since there's only a practical, not an in-principle, obstacle).

So as a practical matter, we're kind of left with our first option. It does look like there's some inherent randomness in quantum mechanics, but we only need to address it if we do a measurement. The time evolution of a wave function in quantum mechanics is not, itself, random at all.

 

[Rob Lister]

By truly random, I mean that there is no rhyme or reason causing the particle behavior.

And that there is no way of observing and then predicting the motions of a sub-atomic particle.

I can roll a ball down an incline, and if i knew all of the variables, I can tell you it's exact motion. From what I have been gathering, you can't do that with sub-atomic particles.

What I am can't figure out, is if the reason we can't predict sub-atomic particle motion because of a limitation of our current technology, of if those particles motions are inhernetly random.

I'm Ultra-novice (read mostly ignorant) but the two things that struck me with your original two questions were:

1) how do you define "see", and
2) how do you define "random".

Bodhi touched on both but specifically on no.2. Random has no meaning other than "the rhyme or reason is unknown." Really smart quantum-guys (who are not mostly ignorant) postulate that not even god can know the rhyme or reason of quantum radomness. I expect to be corrected but that is my understanding. If true then that pretty much meets the test for random.

The test for what "see" means, or can mean, remains the key to your first question.

 

[pgwenthold]

I can roll a ball down an incline, and if i knew all of the variables, I can tell you it's exact motion. From what I have been gathering, you can't do that with sub-atomic particles.

Can't do what? Predict the behavior? Or know all the variables?

For example, even with your ball rolling down an incline, you may be able to calculate it's exact motion but you can't know all the variables. You may know them to a precision at which it doesn't matter any more, but that doesn't mean you know them exactly.

 

[voidx]

As far as particles go in relation to quantum physics, the uncertainty principle is fairly important.

It basically says that if we know a particles location at rest, then we cannot know its velocity and vice versa, if we know its velocity, we cannot know its location. So any measurement of a particle suffers from this uncertainty to some degree. Its not random, its just that we are uncertain about one variable or the other and so we can only say so much about it.

But keep in mind that what we can say about it has drastically changed our understanding of the world around us.

Posted by SkepticalScience
I can roll a ball down an incline, and if i knew all of the variables, I can tell you it's exact motion. From what I have been gathering, you can't do that with sub-atomic particles.

You by definition cannot know all the variables. By knowing its exact motion or velocity, you then cannot say with certain its location at any given point. When you can say something about its location, you can then no longer say anything about its velocity. Annoying yes, but hey, its the way things appear to be.

 

[Ziggurat]

As far as particles go in relation to quantum physics, the uncertainty principle is fairly important.

It basically says that if we know a particles location at rest, then we cannot know its velocity and vice versa, if we know its velocity, we cannot know its location. So any measurement of a particle suffers from this uncertainty to some degree. Its not random, its just that we are uncertain about one variable or the other and so we can only say so much about it.

I would replace "we can't know" with "it doesn't have." The former suggests that we're just missing something. But quantum mechanics doesn't require that anything be missing, only that what are classically exact properties do not apply in the same sense to wave functions. Because you CAN, in principle, know the wave function exactly, with absolutely no uncertainty at all.

 

[John Bentley]

I can roll a ball down an incline, and if i knew all of the variables, I can tell you it's exact motion.

From what I have read about "Chaos Theory", you can't even do that. You have described the classical "Pinball" example for chaotic systems. No matter how carefully you position the ball at the top of the incline, it's endpoint position will be different for each roll. Aggravating, isn't it?

As an aside to the mathematicians out there, is the above chaos math example a result of unknowable, macroscopic variables, or quantum mechanical weirdness?

 

[uruk]

1. Can scientists "see" the particles that make up protons and things? Meaning - do scientists have some sort of super-duper microscope that can actually see these particles? If not - how do we know they exist really??

To date, they can only "see" electron shells using scannin tunneling electron-microscopes.

see: http://www.deutsches-museum.de/ausstell/meister/e_rtm.htm

Subatomic particles can only be detected by scintillators and cloud chambers.

2. At the quantum level, are the motions of the sub-atomic particles TRULY random - or is it that we don't know what controls it yet??

See brownian movement. But I think the jury is still out on randomness

 

[drkitten]

As an aside to the mathematicians out there, is the above chaos math example a result of unknowable, macroscopic variables, or quantum mechanical weirdness?

Unknowable macroscopic variables.

If I have a "normal" system and I calculate (iteratively), f(x), f(f(x)),.... I will get an answer. If I calculate f(x+epsilon), f(f(x +epsilon)),... , I will get something very close to the first answer.

In a chaotic system, the differences magnify, so the final answer(s) may be worlds apart. But there's no necessity for QM wierdness here. Straight up Newtonian deterministic math will do the same thing. Unless you can measure to infinite precision, the inevitable round-off errors will accumulate and turn your predictions into oatmeal.

 

[Ziggurat]

From what I have read about "Chaos Theory", you can't even do that. You have described the classical "Pinball" example for chaotic systems. No matter how carefully you position the ball at the top of the incline, it's endpoint position will be different for each roll. Aggravating, isn't it?

Chaotic systems are not inherently unpredictable. The key is the rate of divergence for different initial states. In most systems, the divergence between two slightly different initial states grows as some polynomial (and often only first order) of the time you let them evolve. In chaotic systems, the divergence is exponential, meaning that they diverge from each other very quickly. As a practical matter, this places limits on predictability, since you're always going to have some error in your measurement of the initial state. Even if this error is very small, since it grows exponentially, it won't take long for even miniscule errors in initial state to manifest themselves as huge discrepencies in the end state. Each time you shrinking your error by an order of magnitude, it only buys you a fixed amount of additional time before the divergence is big for a chaotic system (unlike for a non-chaotic system, where you might increase the effective time of accurate predictions by an order of magnitude each time as well). But in principle, chaotic systems are no less deterministic than non-chaotic systems.

 

[John Bentley]

I'm obviously no mathematician, so thanks for the explanations of chaotic systems.

I realize that these systems are theoretically predictable, but I was under the impression that it was a practical impossibility because they were "infinitely regressible" (I know that's probably not a real word, but it's late). By that, I mean that no matter how detailed you get in your measurements, there will always be a level of detail "smaller" than that which will cause the endpoints to diverge wildly. Is this what you meant newdrkitten and ziggurat?

Hence, I was wondering if the ability to predict these systems was ultimately constrained by quantum weirdness.

Apologies to SkepticalScience for hijacking your thread.