Quantum uncertainty?

Question

I have just started reading about this, so I will try to make sense, hopefully someone will get the idea.

Since it is not possible to measure a particle's position and speed at the same time, physicists say that this dismisses the clockwork universe. There is something I don't get here, if we measure a particles speed, we know it has a position, we just don't know what it is. So wouldn't it still be governed by certain rules and behave in a predictable manner?

And what would it mean if in the future an experiment was developed that allowed both speed and position to be measured?


As I said, I don't know very much about QM, so there is probably something I am missing.

Answer 1

When I asked my physics teacher, "even if we can't know the particle's exact position and velocity, does it have one?" and his answer was, "THAT is the question". So he didn't answer it. But he did tell the class that we would not get anywhere thinking of these particles as little dots. Instead, think of them as little wavy lines, like this ~ . So I have come to believe that they really don't have an exact position or velocity, because they aren't really particles. They are "wavicles". They are probability density functions that have a certain probability of appearing to be particles at some position and velocity, but that is only an appearance. They are really spread out, but will appear to be points in response to photons bouncing off of them, or whatever method you use to detect them. Remember, when we see things, what we are really doing is using our eyes to gather photons that have bounced off of them.

Answer 2

That's Heisenberg's Uncertainty Principle. It set a lower limit to how accurate u can measure the position and velocity of a particle. The more accurate u measure one, the less accurate u can measure the other. There is logically no equipment that can do any better than the limit no matter how technology improves.



To observe, you would need to bounce light off the particle. Since the particles are so freaking small, by shining light on it could significantly bumb it out of its initial position and velocity. with this fact, Heisenberg derived the following limit, dxdp>=h.



The interpretation of QM is very counter intuitive. What u mentioned, "So wouldn't it still be governed by certain rules and behave in a predictable manner? " is the Bohm interpretation, where he favours underlying mechanisms, which only its probabilities are observable as opposed to definite values. Bohm interpretation still favours "determinism".



The philosophical argument here is, since u cannot observe position and velocity to an accuracy of higher than dxdp>=h, is it meaningful to talk about their existence? When a tree falls and nobody's there to hear it, would it make a noise?







There is no way to describe the quantum realm perfectly with English. You can remove the picture completely by applying Q-algebra and all u get are numbers. Or u can give incomplete but helpful physical pictures to help someone grasp the logical concepts. I've discovered in my works, even if u r real good, when u talk in pure mathematical terms, u can lost yourself.

Answer 3

You're not missing anything that you'd expect to get in an introduction to QM. In fact, you're asking a very good question. It's such a good question that I'm not even going to try to answer it here because I don't feel like writing a book. Instead I'll suggest a few things you might want to look into that will answer your question.



Essentially you're asking if the quantum scale is deterministic and we just don't know it because of measurement limitations, right? This view was a start of a school of thought in QM called hidden variable theory. It moved along quite successfully and had a number of noteworthy adherents, but there were problems. To make a long story short, there proves to be a measurable difference between a local hidden variable theory and an interpretation in which QM is taken to be stochastic at its core. So, the issue goes well beyond the lack of determinism being simply a measurement problem. Notice, however, I said a *local* hidden varaible theory. It is possible that there is a universal hidden variable theory, and for more on this you should read some of David Bohm's later works. Bohm had some very eccentric ideas, many of which aren't easily testable, but which are fascinating explorations of what is knowable in the context of QM nonetheless.



For why a local hidden variable theory won't work, read up on Bell's inequality or Bell's theorem.

Answer 4

People often answer this in terms of "by bouncing a photon off something to measure its position, you scatter it and now don't know its velocity", which I think kind of obscures the fundamenal point, because the next question is always, "well, what if I invented some new technology that could ...?"



Quantum mechanics states that one cannot measure the position and momentum of an object to arbitrary precision, and this is not because of any handwaving arguments about bouncing photons off things, but because of the way position and momentum are defined in quantum mechanics. They are mathematical measurements that have different eigenspaces. We say they are "complementary" or "non-commuting". In quantum mechanics, if you want to define the position of something, you write down a function which is related to the probability of locating the object at a given position. We refer to this as the wave function in spatial coordinates. Given this function, you can calculate many things. Note that this function is related to the probability of observing the particle in a given place, so you can only say "if I measured this particle 1000 times, what would be the average position I measured?" You can also calculate the average of the square of the position, the average of the momentum, and the average of the square of the momentum. The mathematics of quantum mechanics forces the uncertainties (think standard deviations in statistics) to be related, and their product to be bounded below. It's not that we cannot measure both quantities together, but that it is actually mathematically inconsistent for an object to have a precise position and momentum.



To put this into a context that is mathematically similar, and possibly more familiar, think of wave pulses. Exactly the same math requires that bandwidth and pulse width are reciprocally related. If you try to make a very short pulse, you need a large spectrum of wavelengths. If you want to make a very pure tone, you need to make the pulse extremely long. The fourier transform of a narrow object is wide, and vice versa.

Answer 5

Well, first, if a future experiment can measure both than the uncertainty principle would not be a principle anymore.



Let me try this out:



Lets say you are looking at one of those lotto machines with the balls that race around. You want to grab #13. The balls are racing around quite quickly. You finally think you see #13. You grab abd get #23 instead.



Now someone tells you that he can stop the balls momentarily for you to look at them. He does this and you see #13. You get ready to grab. He starts the balls again and you grab. But #13 has moved out of its position so you miss. Why? Because in order to find out EXACTLY where the ball was you had to stop it. If you stop it, you cant very well see where it is going or how fast so you can grab it.



Again, the man tells you, you know what, I can tell you what its speed is right now. It is going up at 1mps. As you look into the orb of flying balls, you realize that that is never going to tell you where to grag to get it.



The uncertainty principle has to do with the act of measuring, not the physics of the particles themselves. Every ball in the machine has a position and a speed at the same time. The idea is that we can not MEASURE them at the same time with any certainty. Hope this didn't confuse you further.

Answer 6

heisenbergs uncertainty principle is what you are describing, the point being that the more you know about a particles state the less you know about its position and vice versa. Taken to its logical conclusion if you attempt to measure a particles speed and position at the same time it would become unobservable, basically it would vanish.

This is starting to be observered in quantum computing where the state and speed or findividual particles are being used to create quantum effects like pairing where two particles will still interact with each other even though they are far apart.

Answer 7

The world of QM is much different than it is in what is termed the 'macro' world. Meaning, our everyday world of objects of size we can see, and touch.



Yes those particles have mass and all, but there are many other factors that come into play. They can give off photons and various other mechanisms that affect their flight that regular translational speed mechanics just can't deal with. The Uncertainty Principle puts this into a qualitative relationship.



There is much that is not known about this part of our world. Remember....you're taking about an object that is so small that one cannot use light to illuminate it, because the photons of light would batter the particle influencing it just by virtue of trying to look at it. It would be like using a cannon that shot out bowling balls onto your car while you tried to take its picture.

Answer 8

"I think it is safe to say that no one understands Quantum Mechanics." -- Richard Feynman



"Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it"- Niels Bohr