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Do I understand Heisenberg?

Brian

Graduate Poster
B
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Jul 27, 2001
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My knowledge of science can be best (only) described as Trivia. Do I have a handle on this?

If you say that a moving object is in a certain place between 12:00:01 and 12:00:02, what I'm saying is that it's at some point between

Here........and here

If I say it's in a certain place between 12:00:01 and 12:00:1.5 it's between

here....and here.

Downward into infinty.

In case I'm not being clear, the numbers are HH:MM:SS

Is that more or less why you can't say a thing is moving and that it has a certain position at the same time?
 
No. I'll leave it to those more qualified than I to explain, so as to minimize confusion.
 
The easiest way to think of it is to ask yourself how you would ascertain the location of an electron (for example). In order to find out anything about it, you need to bounce a photon or whatever off of the electron. However, the act of bouncing a photon off of it will cause the electron's postion and velocity to change. You can change how you bounce the photon, to either alter the position more or the velocity more, but you can never set up the photon such that you know both to absolute certainty.
 
Another trival knower here so correct me where I'm wrong.

Heisenberg literally means that because of how we measure the location/velocity of the particle we can't know both and it's simply a side effect of how the quantum world relates to our world and vice versa.

So it isn't really this big, esoteric QM thing that the woos try to use it as then?
 
It's probably fair to say that if you see someone refer to quantum physics or the Heisenberg uncertainty principle in the process of discussing their new invention, they're using it in a woo-woo manner. (Exception: If someone mentions "quantum teleportation," they're probably legitimate, though the teleportation probably isn't what you think.) There's a good quote, probably originally said by Stimpson J. Cat (the guy who posts here), which I always keep in mind regarding quantum physics: "If a person tells you that quantum physics supports their theory, ask to see the equations they used to prove that. If they don't have equations, they're almost certainly woo-woo."
 
I don't have the physics textbook here, and I would appreciate a higher answer as well. I believe based on the one development of the principle I've read, that the Heisenberg Uncertainty Principle doesn't actually state that we can't know position and velocity beyond a certain certainty, but that it may not even be definable to such a level. But that I mean that physical an electron is never in position P with velocity V, its wave function just never gets collapsed down to that level.

As an example, in radioactive decay how do we know that the particles (alpha, beta or gamma) are generated and not already within the nucleus of the source? Because with the momentum they have, Heisenberg states that they are not contained a volume that small.

So Heisenberg just isn't a statement about what we can measure about the particle, it is a statement about the physical reality of the particle. Somebody correct me if I'm wrong.

A final note, in the traditional form of the equation it is not position and velocity which can never be defined beyond a certain accuracy, but position and momentum. Its just usually phrased as velocity often as more people are familiar with velocity than momentum (mass * velocity).
 
So aside from the equations, this whole thing isn't really that complicated and (to abuse a word) esoteric is it? The concept is relatively (pun intended, I suppose) mundane almost.

Or are you folks just really good-like at them splainy stuff?
 
The simplest way of stating it I know is

(uncertainty of momentum) * (uncertainty of position) > a constant value.

The constant value is very, very small (Planck's constant/4 pi I think) which means that you can know both momentum and position fairly accurately from a macroscopic point of view.

Walter Wayne is correct, the wave equation cannot be compatible with both measurements simultaneously. It's independent of how we measure them.

A consequence of HUP is that the vacuum cannot contain nothing. Wow.
 
Also, wasn't there also another formulation which said:

(uncertainty in total energy (including mass)) * (uncertainty in time) > constant

(with the constant also equaling h bar, maybe; I'm not certain of that)

The importance of this one being that with a small enough span of time, the amount of energy in a region can fluctuate wildly. Hence vacuum energy.
 
Schrodinger's Cat is the classic example of quantum probabilities (it's probably the best way to go about understanding a quantum bit, for example), but it's only tangentally related to the HUP. One thing the HUP says (subjectively, not mathematically) is that the act of observing an event affects the thing being observed. Thus, observing the cat causes its quantum waveform to collapse and forces the cat to be either alive or dead. For a discussion of Schrodinger's Cat which is far more eloquent than I could ever be, check out the Straight Dope's take.
 
Vacuum energy

Maybe I'm not remembering this correctly (it's been a while), but I thought the non-zero vacuum energy was implied by the fact that the measurements of energy and rate of change of energy are incompatible. If the vacuum energy was just zero all the time you would know both measurements simultaneously with perfect accuracy.

As rwald said

(uncertainty in total energy (including mass)) * (uncertainty in time) > constant

This allows virtual particles to come into existence for very short periods of time. The more massive the particle the shorter the period.

There are many variations on HUP. It applies to any pair of incompatible measurements. Unfortunately, I don't know of a simple way to explain what is meant by "incompatible". Anyone else?
 
The mathematics of quantum theory forbids assigning both a precise position and momentum to a particle at the same time because no such states exist in the mathematics which allow the widths of the position and momentum wave packets to violate the inequality which is Heisenberg's uncertainty principle.

Mathematically, as the position wave packet gets smaller, the momentum wave packet gets bigger and vice versa. It's not just that we can't just measure it better in a laboratory, if the mathematics we use is the correct way to describe nature then more precise values of position and momentum than those allowed by the mathematics don't even exist to be measured! :D

However, there is always the possibility that the mathematics we're using isn't the correct way to describe nature, but good luck to anyone in finding a better way as what we have right now works very well. :)
 
Schrodinger's Cat is a macroscopic quantum superposition state, and these MQS states only really exist only very, very briefly and in very, very small areas before decoherence eliminates them.

It's not that the cat is dead and alive until we open the box and thus measure it, all the particles in the cat get affected by the other particles of their environment (air molecules and radiation in the box) and in the tiniest fraction of a second the cat's state of quantum weirdness vanished, long before the box was opened.

Basically, these other particles "measure" the cat's particles. :)
 
wipeout wrote:
It's not just that we can't just measure it better in a laboratory, if the mathematics we use is the correct way to describe nature then more precise values of position and momentum than those allowed by the mathematics don't even exist to be measured!
Click to expand...
This bears repeating. The HUP is not merely a measurement problem, but nature herself doesn't know the position and momentum with certainty. Although philosophically these are the same, I think many armchair physicists like me come to think of the HUP as just that we aren't quite good enough at measuring.
 
CurtC said:
This bears repeating. The HUP is not merely a measurement problem, but nature herself doesn't know the position and momentum with certainty. Although philosophically these are the same, I think many armchair physicists like me come to think of the HUP as just that we aren't quite good enough at measuring.
Click to expand...


In other words:


If you do not have a tool ( instrument ) to measure something, you cannot measure it ..( ? )
 
Diogenes said:



In other words:


If you do not have a tool ( instrument ) to measure something, you cannot measure it ..( ? )
Click to expand...

More like: If it's impossible to have a tool to measure something, then that something doesn't exist.
 
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