Okay this is probably a numby question but I just thought of a question about the Uncertainty Principle:

In order to measure the velocity don't you already have to know the location?

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The first principle is that you must not fool yourself - and you are the easiest person to fool.
~~~ Richard Feynman ~~~
It is imperative in science to doubt.
~~~ Richard Feynman ~~~
Common sense is not so common
~~~ Voltaire ~~~

I would say yes, which is why an uncertainty in position leads to an uncertainty in velocity and vice versa.

No. The particle is moving, remember, and it's location is only a statistical probability. If you perform an experiment to measure its exact location over a narrow interval of time, you lose all possibility of determining its instantaneous momentum. If you perform an experiment to determine its momentum, all information about its location is lost. It isn't simply a matter of inadequate measuring tools. It is mathematically impossible to acquire both pieces of information on the same particle within the same interval of time.

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"To complete the picture all the photons can be seen to be synchronising friction on and off throughout the overall cone which itself is synchronised to the equal and opposite reaction of equilateral triangulation"... by a scientificator in ATM, too priceless to be lost to posterity.

I think you are basically asking, how do you measure velocity? First of all, the uncertainty principle involves momentum, not velocity, but since you generally know the mass of the particle, that's a minor issue. The point of the uncertainty principle is that, amazingly, even though there are many different ways to measure or infer momentum, they are all constrained by the fact that the better you know the momentum after the measurement, the more limited is your knowledge of position after the measurement.

It sounds like your question is, if I make two position measurements, can I not infer the velocity simply by noting the temporal separation between those measurements? The answer is, you could, but the result you get would not be the current velocity of the particle, after the second position measurement. Indeed, if you did that second measurement extremely accurately, the particle could next show up almost anywhere (within reason-- we don't want to consider relativistic speeds or the quantum mechanics gets a whole lot tougher). The uncertainty principle concerns what will happen next, not what has already happened.

IMHO, you can measure the "velocity" AND the location, but just that it's not precise.

See here and here.

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I think the question may be simpler than that. For example, the measure the speed of a car, you have to know where it is. Otherwise, you don't know if you're measuing car A or car B. So the question is probably like this: if you want to measure the speed of a baseball, and point the camera in the wrong direction, you won't get a good reading. So how do you know how to point the "camera" if you don't know the location of the object you're measuring?

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As above, so below

It's true that many types of velocity measurements are going to include some position information. The uncertainty principle says that the position information will limit the precision of the velocity determination. A nice example of this is the "diffraction limit" of the human eye. To know which direction the light is coming from, you must have that light pass through your pupil. So you might ask, how can I know what direction it is coming from (I know its speed is c), if I also have to know that it passed through the location of my pupil? The answer is, if it passes through your pupil, then that position knowledge does limit how well you can know the direction-- that's diffraction in a nutshell.

What I'm trying to ask is: Your supposed to be able to theoretically measure the velocity of a particle with infinite precision, however you would have no clue about location... so my question is how could you know the velocity so precise if you don't know the location?

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The first principle is that you must not fool yourself - and you are the easiest person to fool.
~~~ Richard Feynman ~~~
It is imperative in science to doubt.
~~~ Richard Feynman ~~~
Common sense is not so common
~~~ Voltaire ~~~

You are not supposed to ask that kind of question.

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The problem is that your assumption that one can know the velocity with infinite precision is inconsistent with the practical requirement that you always get some position information. So the resolution is, the assumption is untrue. So why do we talk about "eigenstates" of momentum measurements, as if you could really get an infinitely precisely determined momentum? We do because physics is built around idealizations, that are not exactly realizable in practice but serve the purpose nevertheless. One should never take a "plane wave" solution too literally, all real wave functions are "wave packets", and do not have a perfectly precisely determined momentum-- and for just the reason you are asking about.

But I think the answer he is asking is a very simple one, "how can you measure something if you don't know where it is?" On an intuitive level, this makes a lot of sense. For instance, how can I measure how fast an Olympic runner is running if I don't know where the runner is? For example, if there are 10 runners in a race, how can I know how fast runner A is going if I don't know which one is runner A?

I'm not sure if this is really a good answer, but my very laypersonish assumption would be that for example, electrons exist in shells and only one can occupy that space. So you can measure that speed of the object that exists somewhere within that shell, though you don't know where it is.

I don't know if this is a very good analogy, but for example, you may be able to determine the velocity of an ambulance just by hearing the sound of the siren, without knowing its location. So I don't think it's always true that you have to know the location of an object in order to measure its velocity. Or for example, you can tell if another boat on the water is going to hit your or not by looking at whether it moves compared to a distant object. In a sense, you are understanding something about its movement without understanding its position.

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I thought a big part of the problem with electrons was that to know where it is you have to see it, and that means an interaction with a photon, which changes it's velocity. So you can't measure without changing what you are trying to measure.

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Well, that is a problem, but that's not the basis of Heisenberg uncertainty. As Thorkil said, "It isn't simply a matter of inadequate measuring tools. It is mathematically impossible to acquire both pieces of information on the same particle within the same interval of time."

Nick Herbert put it, "The Heisenberg uncertainty principle follows solely from the waveform-attribute connection and has nothing to do with the 'unavoidable disturbance of the system by measurement.'"

And remember, it was Heisenberg who said, "Atoms are not things."

I'm glad I could clear all that up.

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http://en.wikipedia.org/wiki/Heisenberg_uncertainty

Somebody better go set those wikipedians right.

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Yes, I'd have to say that is a weak characterization of the Uncertainty Principle, but it's somewhat passable, and resembles a form that is often used because it resonates with our intuition that everything that happens was caused by something else (which is not necessarily the case). I see two problems with that statement of the UP:
1) It assumes there was always a momentum there, which got "changed" by the measurement, when in fact the way science uses momentum is as an answer to a question. When the question is not posed, it is also not answered, at least not by quantum mechanics.
2) In contradiction to what thorkil and Cougar have explained, it suggests that uncertainty is not fundamental to our description of a state, but rather that it is induced by the measurement. There is nothing in quantum mechanics that requires that view, though it can perhaps be made into a consistent interpretation if one wants to badly enough and applies great care.

Nevertheless, I do not bash the Wiki entry-- it really all depends on the target audience for the words. It is very hard to give a description that is at once technically accurate and widely understandable, and even what constitutes technical accuracy depends very much on the expertise of the judge.

Ha. Good point.

I'm not sure how authoritative this guy Nick Herbert is, but he seems to have said a few things right:

Electrons cannot really be said to have dynamic attributes [position, momentum, etc.] of their own. What attributes they seem to have depends on how we choose to analyze them... the kind of parts a wave seems to have depends on how we cut it up.

A couple other Herbertian bon mots...

"The pragmatist treats his theory like a cookbook full of recipes which are useful for ordering and manipulating the facts. The realist sees theory as a guidebook which lays out for the traveler the highlights of the invisible landscape that lies just beneath the facts."

A visitor to Niels Bohr's country cottage asked him about a horseshoe nailed above the front door. 'Surely, Professor Bohr, you do not really believe that a horseshoe over the entrance to a home brings good luck?' 'No,' answered Bohr, 'I certainly do not believe in this supersition. But you know,' he added, 'they say it brings luck even if you don't believe in it.'"

"Quantum theory is like Bohr's horseshoe: it works no matter what a person believes."

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Everyone is entitled to his own opinion, but not his own facts.

This is Hillman's Uncertainty Principle. We all know where Wiki is, but we're not sure if it's correct.

With best regards to Chris and Wikipedia, John M.

And although we may, with some effort, establish its correctness at the time of a particular observation, we cannot guarantee that it remains correct while we are not looking at it.

Grant Hutchison

I think Werner Heisenberg explained it best in his 1927 paper:

"The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa."

from Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik

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Like most things in Quantum Physics, if you don't find it baffling, you just haven't read enough about it.

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I was going to say "The more I pick up, the more confused I get." Maybe that means I'm indeed learning ... but most likely not.

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I'm like one of those idiot savants...well, except for the savant part.
"In order to increase awareness of the homeless, security have been given binoculars."

Yeah it was really two questions in one... the first part ken answered in post # 11... the other part is the one you tried to tackle...

So if you can't ever "precisely or infinitely" know the velocity, let's say you know it with a very good probability (as high as possible) but if this were to be the case you would have little idea of where the particle is... thus wouldn't it be very hard to measure the velocity of something if we're not sure where it is?

__________________
The first principle is that you must not fool yourself - and you are the easiest person to fool.
~~~ Richard Feynman ~~~
It is imperative in science to doubt.
~~~ Richard Feynman ~~~
Common sense is not so common
~~~ Voltaire ~~~

The point is, to whatever extent a measurement requires you to know the location, that is the limit on how well you can know the velocity, no matter how accurate you think you are making that determination.

The cat article is notoriously unstable, apparently due to random cosmic ray hits in the memory storage which go undetected until the next time it is read.