Thread: question about the heisenberg uncertainty principle

Hello,

This is my first post here.

I've been reading about the Heisenberg Uncertainty Principle and I've got a question about it. I understand why an increase in the precision of position measurements results in a degradation of the precision of momentum measurements, but I don't quite understand how the reverse works - that is, getting precise measurements of momentum even though your position measurements are more uncertain. I mean, the only way I know of for getting momentum measurements is to get two position measurements, and using the amount of time between them, deriving the velocity. Multiplying that by the mass gives you momentum. But how are you supposed to get the two positions if the measurements you take of them are degraded? Is it a matter of taking many such measurements and deriving the average?

According to modern scientific "theory" you can measure the exact locations, but in practice the story is much different.

The Uncertainty principal ties in with the "cat alive - cat dead" concept of Schroedlinger.

It's one of those things about modern scientific theory that has required proposition after proposition to account for the concept of trying to predict the exact location of space-time and failing.

The chances are is that if you don't understand, no one else does. It's like in this forum. You know the general momentum of the questions, but there is no way you can predict the answers.

If you want an answer to your question, be prepared for a theory that has yet to be proved, or a ball-park figure, and that's from contemporary physics. Still, understand that those who propose such theories, wel, they completely rubbish anyone else with other theories, no matter how many equations they have to back their own theories up.

If you are able to contact a moderator and send them a private message regarding your question, they may give you a sincere answer, I have found.

Basically, contemporary physics doesn't have the exact answer you seek, only propositions. Understand that the contemporary/official understanding of space-time is far from perfect.

One should never be a fan of ball-park figures, especially in physics.

What is the surface tension of mercury closest to, a solid or a liquid?

Ball park figure.

Yes.

The best way to handle contemporary physics is to first understand the difference between what is theory, and what is FACT. What theory perfectly describes a space-time phneomena, and what theory tries it's BEST to describe space-time phenomena. One must be certain that contemporary physics has the courage enough to write such a book for the public.

The other thing to perhaps note is that if an equation seems complicated and knotted, it is because that equation is trying to explain one of those "best-fit/ball-park" theories. Don't be mislead by complicated equations that only support ball-park theories. Look at them for what they are. Compared to what we are really in search of, the certain theories and associated equations, most contemporary equations are like lantana in front of a weedy hollow.

BUT, and moderators take note, HATS OFF to all those who at least make the effort, without whom we would have NO idea whatsoever.

Your question basically sought a perfect answer, when according to contemporary physics one doesn't exist, not as they know it.

Well, it's not so much a matter of how one "takes measurements", it's more a matter of principle; the energy and position of a quantum entity cannot in principle be known simultaneously to equal precision.

I explain it (poorly) here

It's something to do with the non-commutativity of operators acting on an Hilbert space, I believe. Ask a physicist!

Originally Posted by Guitarist

Well, it's not so much a matter of how one "takes measurements", it's more a matter of principle; the energy and position of a quantum entity cannot in principle be known simultaneously to equal precision.

I explain it (poorly) here

It's something to do with the non-commutativity of operators acting on an Hilbert space, I believe. Ask a physicist!

I see. So then what was Heisenberg trying to say? Was he saying that if the day ever came when we would be able to measure a particle's momentum technologically, we would find that it comes at the cost of precise position readings? Can that day ever come?

No, for the reason I tried to give in my link, that day will never come.

Let's see. One finds in populist literature an idea that (i believe) was first offered by Wolfgang Pauli, an extremely eminent physicist in his day, and still revered. It goes something like this:

In order to observe (measure) anything, we need at least one photon to hit that "thing". If our "thing" is less energetic than that photon, then all information about the state of our "thing" before hitting it with the photon is lost, because the energy of the photon is sufficient to alter its state.

Now, physicists, I am sure could explain it better, but this sort of explanation does lead to the question that you implied: could we ever build an apparatus so non-perturbative that both position and momentum could be simultaneously measured?

My link attempts to explain why not.

Originally Posted by Guitarist

No, for the reason I tried to give in my link, that day will never come.

Hmm... that's interesting. So tell me if I'm wrong then - doesn't this mean that another way of construing the HUP would be to say that position can be measured either with high precision or low precision, but in either case, momentum can never be measured at all?

Or vice versa, yup that's the HUP (ha ha).

Bur seriously, you need to talk to a physicist about this, I've told you all I know (which is not a lot)

Originally Posted by Guitarist

Or vice versa, yup that's the HUP (ha ha).

Bur seriously, you need to talk to a physicist about this, I've told you all I know (which is not a lot)

Well, I really appreciate your answers (however professional they are).

Something else HUP-related that I wanted to ask is this: I've been told by some sources that the imprecision with which one measures position when using long wavelength photons is due to the spread out character of that photon (i.e. it is not localized so it's hard to say where it hit the other particle). But another account I've been given says that long wavelength photons tend to diffract when they collide with another particle rather than reflect. Diffraction makes it difficult to read position. Maybe these are two ways of saying the same thing. Know anything about this?

Remember, the Heisenberg microscope was meant to be a thought experiment to show that classical mechanics is insufficient to explain the mechanics of electrons and atoms and not as a rigorous deduction of the uncertainty principle. I found some nice notes which explain the thought experiment in the most clearly thought out steps i have ever seen it

http://spiff.rit.edu/classes/phys314...heis/heis.html

Go give it a read