1. ## Heisenberg Uncertainty Principle

Recently, I was in a discussion (about Evolution vs Intelligent Design) and I made a point with an illustration, namely, how difficult it is to measure very small things without changing them.

The illustration that I used is one I read somewhere, but I have no idea where. If we want to measure the temperature of a body of water, we can stick a thermometer in it. That's fine for large bodies of water but what about small ones? Stick a thermometer into a drop of water, and the temperature of the thermometer will change the temperature of the drop of water, thus rendering your measurement worthless. I called this an example of the Heisenberg Uncertainty Principle because that's what the person I had read years ago called it.

My opponent simply replied, "No it's not." To double-check myself, I checked Wikipedia, and I see the true definition only regards particles, namely, we can't know their both their position and their momentum with precision, and the more precise we measure one, the less precise the other becomes. I think my illustration was what is commonly called the Observer Effect, but now I'm not sure.

So with my puddle illustration, am I all wet?

2. As I understand it, the Heisenberg Uncertainty Principle is a specific example of the Observer effect, so you were both right and wrong.

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I don't know. It seems to me that what jamesabrown described with the thermometer is a thermodynamic effect (i.e. explainable by classical thermodynamics), while the Heisenberg Uncertainty Principle is a quantum effect (a cornerstone of quantum mechanics, even)...

4. Originally Posted by Disinfo Agent
I don't know. It seems to me that what jamesabrown described with the thermometer is a thermodynamic effect (i.e. explainable by classical thermodynamics), while the Heisenberg Uncertainty Principle is a quantum effect (a cornerstone of quantum mechanics, even)...
I think the author of what I was reading was using the thermometer as a large, real-world example of what's going on at the subatomic level. The article wasn't about how to measure temperature but to illustrate the principle that measuring stuff changes stuff. My question really is, is that principle called the HUP? Or something else.

5. Originally Posted by PraedSt
As I understand it, the Heisenberg Uncertainty Principle is a specific example of the Observer effect, so you were both right and wrong.
Yikes, can you explain that further, please?

6. Heh. I can do the Observer effect. The act of observing X, changes X. It's quite a general statement, in the sense that you can apply it in many fields, not just physics. Psychology and economics for example.

So thermometer/water. As you said- if there is enough water, this effect (heat transfer from water to thermometer fluid, or vice versa) is negligible. With a drop of water, it's not. Observing the temperature of the drop, will significantly changed it's temperature.

As for HUP- all I can say is that, from what I remember, it's the Observer Effect when you're talking about small particles and small scales. But there are specific/unique aspects to HUP, and I'm afraid I'm not very good at quantum stuff. I could confuse you more! Sorry.

7. Originally Posted by jamesabrown
I think the author of what I was reading was using the thermometer as a large, real-world example of what's going on at the subatomic level. The article wasn't about how to measure temperature but to illustrate the principle that measuring stuff changes stuff. My question really is, is that principle called the HUP? Or something else.
No, what you describe is not the Heisenberg Uncertainty Principle. It would be an example of the observer effect.

Using the thermometer example as an analogy to explain Heisenberg only goes so far - it breaks down when you consider quantum values.

8. The two are discrete and different.

Quantum physics treats particles and their properties as the probability that they hold those quantities and positions. Define the actual value of one, and the probability that it holds the others spreads out and smears. THAT's the Heisenberg principle. Nothing to do with observation, it's in the theory.

On the other hand, observer effect is a crude, macrocosmic effect as described above, and is very real. It may also be present in softer sciences such as psychology and sociology, and in the design of clinical trials, but it has nothing to do with quantum physics.

The drop of water and the thermometer is only an anology, and as scuh cannot be compared too closely.
JOhn

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Originally Posted by JohnD
The two are discrete and different.

Quantum physics treats particles and their properties as the probability that they hold those quantities and positions. Define the actual value of one, and the probability that it holds the others spreads out and smears. THAT's the Heisenberg principle. Nothing to do with observation, it's in the theory.

On the other hand, observer effect is a crude, macrocosmic effect as described above, and is very real. It may also be present in softer sciences such as psychology and sociology, and in the design of clinical trials, but it has nothing to do with quantum physics.

The drop of water and the thermometer is only an anology, and as scuh cannot be compared too closely.
JOhn
I respectfully disagree. Trivially, of course measuring water temperature and particle positition are different acts of observation. Observing the behavior of chimpanzees by walking around with them is also a different act of observation. Still, it's useful to have a general category, call it the Observer Effect, to describe this general problem that plagues all sciences--even the so-called "hard" ones.

To say that it's the case that a particle never really has both a definite position and a definite momentum is pure metaphysics. There is not even in principle a way to decide whether positions and momentums are really discrete or not. I think that was Heisenberg's main point: the HUP was not intended as a description of what particles are really like independent of observers. Thus, particle physics is in worse shape than your so-called "soft" sciences--maybe we should reverse the appelations! At least a clever thermodynamisist has a hope of correcting for the observer effect and arrive at an accurate estimate of orignal temperature of the water drop. An ecologist can try to correct for observer effect by comparing results obtained with less intrusive methods. A clinical trial can at least try to correct for the observer effect by using double-blind techniques.

The poor particle physicist, on the other hand, must of necessity be forever ignorant of the true nature of his or her chosen subject matter.

ETA: actually there are some sciences, like astronomy, that rely entirely on passive observation, and are therefore not subject to the observer effect.
Last edited by Warren Platts; 2008-Nov-25 at 02:43 AM.

10. Originally Posted by jamesabrown
Yikes, can you explain that further, please?
The Heisenberg uncertainty principle is a bit more fundamental than what is being called the observer effect. It is not just a statement that a physical measurement requires a physical interaction between the system being measured and the measuring device, but rather a fundamental statement that the operators that represent measurements in quantum mechanics do not commute. So if you measure position and then momentum you will get a different result than if you measure momentum and then position. This boils down to the simple fact that if x represents the operation of multiplication by x (the position operator in QM) and d/dx is the usual operation of differentiation (the momentum operator) then the operators x*d/dx and d/dx*x are different operators (this is just a result of the product rule for differentiation as you can see by applying the operators to an arbitrary function f).

The Heisenberg uncertainty principle results in the determination of a minimum error that results from attempts to simultaneously measure both position and momentum, and this minimum error cannot be reduced by refinement of measurement technique.

The uncertainty principle itself and the essential uniqueness of operators obeying the commutation relations have been studies extensively.

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

http://en.wikipedia.org/wiki/Stone%E...eumann_theorem

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Originally Posted by PraedSt
Heh. I can do the Observer effect. The act of observing X, changes X. It's quite a general statement, in the sense that you can apply it in many fields, not just physics. Psychology and economics for example.

So thermometer/water. As you said- if there is enough water, this effect (heat transfer from water to thermometer fluid, or vice versa) is negligible. With a drop of water, it's not. Observing the temperature of the drop, will significantly changed it's temperature.
You can also get the water temperature by measuring the thermal radiation emission. Here you don't change the observed object...

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Originally Posted by Vallkynn
You can also get the water temperature by measuring the thermal radiation emission. Here you don't change the observed object...
What is observed here is the thermal radiation, which is modified in its interaction with the dectector; the water is merely implied. So the observer effect is respected - the thermal radiation itself was modified in the act of observation.

But there is a key difference between the observer effect and HUP. In particular, we can take account of the observer effect to calculate what we wanted to know. For example, in this case we effectively "destroy" the thermal radiation by absorbing it, and conclude what the thermal radiation was before we destroyed it. Whereas HUP says that there are things we inherently cannot know.

If the thermal radiation detector is anywhere near the water, you will have to be very quick before the detector sends some thermal radiation to the water and modifies its temperature.

13. Warren,
You accuse Quantum Physics of being mere metaphysics and then talk of there being no way "to decide whether positions and momentums are really discrete or not." They aren't discrete and must be considered as a cloud of probability - that's the point - .
There is no pretense that the quantum world as decribed is a real world description and it doesn't matter, because it the theory allows results that correpsodn to to real world behavious to be derived. This is not exclusive to quantum physics - pharmakokinetics describes volumes in the body that are measure in hundreds of mls/kg - impossible of course but no matter as the tecnique allows the prediction of drug concentrations over time.

The point is also that in QP, defining one parameter makes the others more uncertain.
In 'real world' sciences, hard or soft, clever observers have developed ways to avoid the observer effect, because there is no theoretical abberation introduced, just clumsy observing.

John

PS a thermal radiation sensor WILL affect the water drop, as it will absorb more radiation than than in the unobserved state!
PPS Oops, sorry, Ivor said that!

14. Originally Posted by Warren Platts
To say that it's the case that a particle never really has both a definite position and a definite momentum is pure metaphysics. There is not even in principle a way to decide whether positions and momentums are really discrete or not. ... The poor particle physicist, on the other hand, must of necessity be forever ignorant of the true nature of his or her chosen subject matter.
This is a mis-characterization of Heisenberg. The HUP allows quantum observables to be measured to any degree of accuracy. And there are several observables that can be measured without any discernible "observer effect". For example, you can make repeated identical photon polarization measurements and you will get the same answer each time. The photon is not disturbed by such measurements.

The EPR Paradox, in fact, was created to demonstrate that quantum theory was "incomplete" as you describe. Unfortunately, EPR came to the wrong conclusion on this point; they did not know about Bell's Theorem and the Aspect experiment - both of which came decades later.

So it is not just semantic or metaphysical to say that particles do not have simultaneous values to some observables. It is a physical reality (per Bell's Theorem etc.) that either they don't have such simultaneous attributes, or there exist non-local (i.e. faster than the speed of light) forces of which we are otherwise unaware. It is not clear that either of these alternatives have preferential footing currently. However, the general belief is that reality is shaped by the "context" of an observation... i.e. that reality is contextual. If reality was fully observer independent by classical standards, then it would need to be non-contextual.

So the usual general conclusion is: classical observer independence of particle attributes is ruled out by physical experiment.

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Originally Posted by DrChinese
So it is not just semantic or metaphysical to say that particles do not have simultaneous values to some observables. It is a physical reality (per Bell's Theorem etc.) that either they don't have such simultaneous attributes, or there exist non-local (i.e. faster than the speed of light) forces of which we are otherwise unaware. It is not clear that either of these alternatives have preferential footing currently. However, the general belief is that reality is shaped by the "context" of an observation... i.e. that reality is contextual. If reality was fully observer independent by classical standards, then it would need to be non-contextual.
You hit the nail on the head: there are two, quite different underlying physical theories that can explain the data, yet it is in principle impossible to collect any more empirical evidence that could decide the issue. That's about as dismal as it gets. Physics has nothing on ecology or economics!
So the usual general conclusion is: classical observer independence of particle attributes is ruled out by physical experiment.
Ah, now the philosophy of physics--my favorite subject! I see one major problem with your thesis. Science says the universe existed for billions of years before any observers evolved. Therefore, the universe has been observer independent for the vast majority of its history. Or have I just inadvertantly discovered a new proof of the existence of God?

16. Originally Posted by Warren Platts
The poor particle physicist, on the other hand, must of necessity be forever ignorant of the true nature of his or her chosen subject matter.
Actually, a more sensible (and accurate) way to phrase that is that a particle physicist needs not pretend there is an accessible "true nature" out there in the first place, which might emerge if he/she could just get a little more bias out of the experiment, or slightly more convincing rhetoric. Instead, the particle physicist knows exactly what it is possible to establish as true, and what it is not. I'd say that's a perfectly wonderful state of affairs-- especially for philosophers.

17. Originally Posted by Warren Platts
I see one major problem with your thesis. Science says the universe existed for billions of years before any observers evolved. Therefore, the universe has been observer independent for the vast majority of its history.
As I have pointed out elsewhere, science is for intelligent observers-- it is precisely the effort of said observers to understand, predict, and gain mastery over the objectively repeatable elements of their existence. That's just what science is, any other claim would need to be completely blind to the entire process. So when there were no minds, there was no science. That's perfectly obvious, and presents no philosophical quandaries. Once there were minds, observer-dependent effects were established within the language of science, and it has always been workable (and perfectly reasonable) to imagine the same physics was in play prior to the appearance of physicists, to the same extent that the physics works after the appearance of physicists. What works now, worked then-- it's a basic assumption of science that suffers from zero contradictory evidence. Again, there is no philosophical challenge there at all, if one only understands what science actually is.

Now, in terms of this "observer dependent" quality, it is important to recognize that observers, making observations, have access to no magical processes-- they have only natural processes at their disposal. As such, analogous processes occur inside, and outside, of formal experiments. Nothing about Heisenberg's uncertainty principle requires invoking something that an observer can do, that nature cannot do without the observer, expressly because the observer is of nature. The only thing the observer can do, that nature cannot, is science on the outcome.

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Originally Posted by ken g
actually, a more sensible (and accurate) way to phrase that is that a particle physicist needs not pretend there is an accessible "true nature" out there in the first place, which might emerge if he/she could just get a little more bias out of the experiment, or slightly more convincing rhetoric. Instead, the particle physicist knows exactly what it is possible to establish as true, and what it is not. I'd say that's a perfectly wonderful state of affairs-- especially for philosophers.
GIVE ME NOUMENA
or
GIVE ME DEATH!

19. As a matter of fact, death, for the dead, is the quintessential noumenon.

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21. Sorry, I'm just laughing so much...

22. Warren wrote:
To say that it's the case that a particle never really has both a definite position and a definite momentum is pure metaphysics. There is not even in principle a way to decide whether positions and momentums are really discrete or not. I think that was Heisenberg's main point: the HUP was not intended as a description of what particles are really like independent of observers.
I think this is the core of the problem in the discussion. Warren appears to hold as axiomatic that a 'particle' is, well, a particle. By ignoring the wave properties of matter, he seems to want to change the embedded uncertainty to an extreme aspect of observer effect. That there are thresholds below which qualitative differences appear in the behavior of matter/energy is explained away with examples that don't really apply.

For that matter, I could posit that all particles fall in a gravitational field, therefore the molecules of gas in the atmoshpere should be settling to a thin scum on the surface of the earth sometime soon.

I would also mention tunnel diodes and the Lamb shift as consequences of inherent uncertainty at the level of electrons.

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I've got nothing against waves--but you look at a wave on the ocean moving, it's got both a definite position and momentum at any given moment.I'm just pointing out that the claim that there isn't both definite momentum and position "in reality" for subatomic particles is a philosophical surmise that's not very philosophically satisfying. I prefer the Bohmian, hidden variables approach. Just because we can't detect something, that doesn't mean it doesn't exist.

24. Originally Posted by Warren Platts
I've got nothing against waves--but you look at a wave on the ocean moving, it's got both a definite position and momentum at any given moment.
You think a wave has a definite position? No, the excellence of using the "wave" concept to describe the information we have about a particle's notion is precisely that we do not know "the position of the wave". Remember, for a wave to be associated with a definite momentum (in the quantum mechanical sense), it must be the mathematician's sine wave, not the surfer's "ultimate wave"-- the mathematician's wave repeats periodically over a huge distance. Ergo, it has no "position" to speak of. Our descriptions of particles' position is just like that.

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Originally Posted by Warren Platts
I've got nothing against waves--but you look at a wave on the ocean moving, it's got both a definite position and momentum at any given moment.I'm just pointing out that the claim that there isn't both definite momentum and position "in reality" for subatomic particles is a philosophical surmise that's not very philosophically satisfying. I prefer the Bohmian, hidden variables approach. Just because we can't detect something, that doesn't mean it doesn't exist.
Tell me the exact position of a moving object please...

26. Originally Posted by Warren Platts
GIVE ME NOUMENA

or

GIVE ME DEATH!
To whom does the choice belong ?

27. Originally Posted by Vallkynn
You can also get the water temperature by measuring the thermal radiation emission. Here you don't change the observed object...
But through the emission the temperature does change. And as part of the surrounding environment the measuring device does participate in the radiation balance of the system. The magnitude of the effect of the measurement on the system being measured can be affected by the technique chosen but it is never zero.

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Originally Posted by Neverfly
Tell me the exact position of a moving object please...
x = vt

29. First of all, that equation is actually an equation for the change in x, not x itself. So even if it were uniformly valid, rather than being a classical simplification we teach to high school kids, it would still not tell you x unless you knew the original x at t=0. In other words, that answer is no answer at all-- it merely passes the buck for specifying the position to the initial condition. Secondly, if the initial x is well known, then it is your v that is not well known. So the equation tells us nothing at all in the context of the question you were asked-- which is why there is such a thing as quantum mechanics.

Moreover, these fundamental issues are not limited to our ability to know various things, say due to technological limitations, they have physical ramifications, like diffraction. The Bohmian picture that you favor does nothing to change that situation-- it allows one to imagine, with no predictive benefit whatsoever, that certain things are knowable but never actually known. To even entertain that notion is to have left objective science, obviously. Bohmian mechanics is not only pure philosophy, it's not particularly useful philosophy either. All it does is allow people to use classical pictures behind the quantum mechanics, while others use quantum mechanical pictures behind classical mechanics. In my opinion, either way that represents a simple loss of contact with what science actually is.

30. Originally Posted by Warren Platts
I've got nothing against waves--but you look at a wave on the ocean moving, it's got both a definite position and momentum at any given moment.I'm just pointing out that the claim that there isn't both definite momentum and position "in reality" for subatomic particles is a philosophical surmise that's not very philosophically satisfying. I prefer the Bohmian, hidden variables approach. Just because we can't detect something, that doesn't mean it doesn't exist.
hmmm now that gives the rogue wave phenomenom happening here in the oceans a whole new twist.

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