# Why does measuring the position of a particle change its momentum?

1. Nov 6, 2014

### k9b4

Why does measuring the position of a particle change its momentum?

Is it because 'measuring' involves shooting EM waves or other particles at the particle, which changes its momentum?

2. Nov 6, 2014

### phinds

The HUP (Heisenberg Uncertainty Principle), which is what I assume you are asking about, has nothing to do with how things are measured, it is an expression of a fundamental limitation of nature.

You'll find lots of threads here on this forum about the HUP so I suggest a forum search.

3. Nov 6, 2014

### k9b4

In this wikipedia article http://en.wikipedia.org/wiki/Uncertainty_principle, it states:

"This ascribes the uncertainty in the measurable quantities to the jolt-like disturbance triggered by the act of observation."

Does that mean that by measuring the particle (ie shooting EM waves at it) changes its momentum?

4. Nov 6, 2014

### phinds

It may, and it seems likely but I say again, this has NOTHING to do with the HUP. Did you bother to read the sentence directly after that one "Though widely repeated in textbooks, this physical argument is now known to be fundamentally misleading.[4][5]"

5. Nov 6, 2014

6. Nov 7, 2014

### Staff: Mentor

It is misleading because it suggests that the particle has both a definite position and a definite momentum before we measure either, and it's just the unavoidable clumsiness of the measurement process that prevents us from discovering both.

In fact, the uncertainty principle is deeper than just unavoidable clumsiness of measurement. Any state in which the position is definite is necessarily a state in which the momentum is a superposition, and vice versa.

7. Nov 7, 2014

### sophiecentaur

Whilst this is undoubtably true, it is nice that a very understandable argument takes you to the same place. I would say that it's not a bad way into the topic of HUP, bearing in mind that it follows the historical pathway to it. Once the practical measurement idea has been accepted - and assuming that the newcomer can actually handle the more formal reasoning - then one can move forward. When you think of the number of times that people ask (nay, plead!!) for a "physical interpretation" on PF, it seems reasonable to indulge those people a bit, along with the caveat that there's something more to it.

The result from the HUP is not unlike the result from the Pauli Exclusion Principle. You can work with both of them without losing sleep about the deeper meaning. (That's an Engineer speaking, of course!)

8. Nov 7, 2014

### Khashishi

I don't know why it has become unfashionable to talk about measurement effects when talking about Heisenberg Uncertainty Principle. The measurement effects are a legitimate way of picturing HUP, and it's not wrong. There are different "pictures" of quantum mechanical phenomena, and it's hard to say one is more correct than the other. The HUP says that fundamentally the particle does not possess a precise value for both noncommuting observables (eg momentum and position). But that doesn't explain why we can't measure the precise values.That's where the measurement effect comes in. This way we can answer both the why and the how.