# What is uncertainty principle

1. Apr 29, 2010

### Rico L

what is uncertainty principle?

thanks ppl..

2. Apr 29, 2010

### bapowell

This is an easily researched topic. Why not do some poking around on the web, and then come back here with more specific questions.

3. Apr 29, 2010

### ansgar

standard deviation in position times standard deviation in momentum is greater than h-bar

simple

4. Apr 29, 2010

### LostConjugate

greater than 1/2(h-bar)

HUP applies to other operators as well.

5. Apr 29, 2010

### ansgar

yes sorry forgot the factor of one half!

it applies to all operators which does not commute, eg. S_x, S_y

6. Apr 29, 2010

### bapowell

You really shouldn't discount the energy-time relation just because time is not a canonical operator. Since the energy of a state is proportional to its frequency, the longer it exists (the more cycles it undergoes) the more certain the energy i.e. the sample variance of the system energy goes down as more cycles pass.

In any event, the energy-time uncertainty relation governs quantum vacuum fluctuations, whose existence has been experimentally verified.

7. Apr 29, 2010

### stevmg

I have never understood the logic behind the Heisenberg Uncertainty Principle and neither did Einstein. One of those quirks between Relativity and Quantum Physics and I don't know if it has been resolved. I do know that if one follows the procedures in the calculations, it does work out...

"God does not play dice with the universe..." Einstein

"Don't tell God what to do..." Bohr

Last edited: Apr 29, 2010
8. Apr 29, 2010

### physixlover

As Hesinbergsaid -particle position and momentum both cannot be known at the same time
the best explanation you would get at this link-

Last edited by a moderator: Sep 25, 2014
9. Apr 29, 2010

### Rico L

just reminded me... lol. thanks

10. Apr 29, 2010

### stevmg

That video

still begs the issue... It states that the electron is a cloud because of its small mass (and consequently less internal attrcation to hold it together) and then states that as you leave the tight proton nucleus (the H atom) you increase your probability of finding the electron. If the electron is the cloud, then once you hit the cloud you $have$ found the electron. No probability about it.

But the old big argument about the inability to know the position and the speed and direction of the electron at the same time because if you look at the electron you alter its momentum is pure bunk. That just means we have no way currently of figuring this out, not that it is impossible.

However, the statistical formulas do work so I won't argue with them.

Last edited by a moderator: Sep 25, 2014
11. Apr 29, 2010

### Anthony A

The notion that one will never be able to understand the exact location of a particle.

12. Apr 29, 2010

### Fredrik

Staff Emeritus
It's not just that you can't know the particle's position and the momentum. It doesn't have a well-defined position or a well-defined momentum at any time. (In particular, the claim made in the video about electrons spending most of their time at a specific distance from the nucleus, is wrong). What I'm talking about isn't even a consequence of the uncertainty theorem*. It's just the standard interpretation of a wavefunction.

(I must have said this in at least 10 different threads. See any of them for more details).

*) The original idea could be described as a "principle", but the modern uncertainty relation is a theorem and nothing else.

Last edited: Apr 29, 2010
13. Apr 30, 2010

### ansgar

that relation is derived in a more "non rigour" way than the HUP though...

14. Apr 30, 2010

### ansgar

wrong, HUP is a statistical relation, not applicable to one measurment but to several

Last edited by a moderator: Sep 25, 2014
15. Apr 30, 2010

### ansgar

the logic is simple, the derivation is done in school, I can show it to you.

16. Apr 30, 2010

### Born2bwire

I don't recall Einstein having a problem with the Heisenberg Uncertainy Principle. More over, he had larger problems with the Copenhagen interpretation and he was seeking for a local realism interpretation. The Heisenberg Uncertainty Principle falls out simple from the mathematics of quantum mechanics. It is not taken via empirically like the Pauli Exclusion Principle.

The Heisenberg uncertainty principle makes NO ascertations about the precision of a measurement. It is a consequence that describes the relationship between the statistical measurement of what are called incompatible observables. It does not mean that the measurements are inaccurate or incorrect. It just means that between certain observables, we will not get the same measurements over and over again over a statistical set.

Think of a machine that measures some quantum state. The machine projects the measurement in the form of marbles. Each measurement makes a sack of marbles that varies in color, number, and size. We will consider color and size to be compatible (or commutable) observables. That is, every marble that is 0.5 in in diameter is always green and vice-versa. However, color (and by extension size) is incompatible (noncommutable) with number. That is, if we measure the state and have the machine make our sack of marbles, we might get 5 green marbles, or 3 green marbles, or 3 red marbles and so on.

So make 10,000 measurements and we get 10,000 sacks of marbles of varying colors and numbers. If we were to separate out the sacks by groups of numbers, we would find that for sacks of 5 marbles we have 10% red, 50% green, and 40% blue. For sacks of 6 marbles we have 20% red, 30% green, and 50% blue. And so on. Thus, we measure the number of marbles EXACTLY, but because a sack of five marbles can be red, green or blue then we get a spread of colors in our set of measurement. This spread will be described by the wavefunction in terms of color for the given eigenvalue of N marbles. This is the same as when we get an eigenfunction of position for a given eigenvalue of E energy. If the system has energy E_0, then the eigenfunction describes the positional distribution of the measurements of the system in this state (assuming time-independence). So if we measure the position of a particle in a system of state E_0, there are many many positions that it could be in and thus we get a statistical spread of position measurements.

Likewise, if we arrange the sacks by color we may find that green came in sacks of 5 10% of the time, 6 40% of the time and so on. This is another eigenfunction that gives the distribution of number for an eigenvalue of color. In this manner, we see that the eigenfunctions that describe the system in terms of color are different than the eigenfunctions that describe the system in terms of number.

Heisenberg's uncertainty principle then gives us the relationship between the variance of color and number in all our measurements. If we go back to our sacks and map out the number and color of the marbles in the sacks, we will find that we have a mean color and number (if we can allow for a mean color, we could allow the color to gradually transist over the visual spectrum as opposed to being three discrete colors). However, there will be a spread in the measurements in numbers and colors. The minimum spread is related by the uncertainty principle.

The problems of measurement and precision do not come into the argument yet, this is purely a consequence of the mathematics of quantum mechanics.

17. Apr 30, 2010

### bapowell

umm...so what? It's a valid relation. The OP asked about the uncertainty principle. He didn't ask that we critique which versions of it are most rigorously derived.

18. Apr 30, 2010

### ansgar

it is only valid for some specific systems, that is why it is not properly to call it a principle

19. Apr 30, 2010

### bapowell

really? who doesn't call it a principle?

EDIT: Anyway, there's little point debating what one calls it (I personally refer to it as a 'relation'). I don't think the OP is benefiting from this side discussion. However, given its importance in physics, and since it is typically discussed along with the position-momentum uncertainty relation, I think the energy-time relation is germane to the discussion and the OP would benefit from learning about it. It seems ridiculous to selectively withhold information because it's "not as rigorous" as something else, or is technically "not a principle". Is the Pauli exclusion principle not a 'principle' because it only applies to fermions?

Last edited: Apr 30, 2010
20. Apr 30, 2010

### ansgar

now you are making a huge mistake, the pauli principle says that for FERMIONS so of course it is only valid for fermions...