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I think this can be answer but I am not sure an this is my idea.Copenhag Interpretation says that measurement affects reality so our measurement affects universe

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I think this can be answer but I am not sure an this is my idea.Copenhag Interpretation says that measurement affects reality so our measurement affects universe

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Nugatory

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There's no easy answer to this question. If you learn quantum mechanics, the answer will show up in the math with absolute clarity, so the question comes down to asking how we know that quantum mechanics is a correct theory. That's an easy question to answer - there's more experimental evidence in favor of QM, and less experimental evidence against it, than for any other physical theory ever.How can physics says that this is real and not a experimental mistake.

However, for this to be answer to your original question, you have to learn quantum mechanics... And that's going to take some time and work, and a fair amount of mathematical background. Figure a year or so of college-level work after you've nailed classical mechanics and elementary differential equations. That's why I say there's no easy answer.

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Please note that unless this thread has substantial physics content and not just philosophy, it will be closed, per our PF Rules and the restriction on purely philosophical discussion.

I think this can be answer but I am not sure an this is my idea.Copenhag Interpretation says that measurement affects reality so our measurement affects universe

Zz.

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BruceW

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Interpretation is ... messy for quantum mechanics. It is better to learn the mathematics, and how to apply quantum mechanics to certain problems. Then let interpretation come afterwards. At least, that is how I have been taught it, in undergraduate physics.... I think this can be answer but I am not sure an this is my idea.Copenhag Interpretation says that measurement affects reality so our measurement affects universe

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Because the experiments are not done just once! Remember, everybody can't be wrong.

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So math says this is real.Its strange

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Nugatory

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Yes, it is strange, but also utterly fascinating. Learning QM is a fair amount of work, as I said above, but it's worth the effort - that's why people do it.So math says this is real.Its strange

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Mathematics cannot say something is "real". This is a fallacy.So math says this is real.Its strange

What makes it "real" is that this concept is verified by experimental evidence. That is the only way for something to be valid. Once the theory matches the experiment, then we know that we have a good understanding of a phenomenon.

Quantum mechanics arose out of classical theory not matching experimental observation at the time. Remember, you can also use math with classical mechanics to derive at a number of things. Doesn't mean these were "real" even when the mathematics said so. And the fact that these classical mechanics predictions didn't match experimental observations was a big clue that classical mechanics was missing something.

Mathematics alone is insufficient to show if something is "real". You are missing out on an extremely important component of science - experiment!

Zz.

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cb

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bhobba

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That isn't really what it says.Copenhag Interpretation says that measurement affects reality so our measurement affects universe

What it says is quantum objects do not have properties independent of observational context. Also note in QM observation is anything that has some kind of outcome here in the common-sense macro world - nothing to do with a concious observer.

I suggest you start at the beginning with its conceptual core:

http://www.scottaaronson.com/democritus/lec9.html

Once you understand the detail its less bewildering and much more beautiful.

Thanks

Bill

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bhobba

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Nor do I - but popularisations and even some actual textbooks IMHO don't help by being a bit vague on some points eg they don't make clear an observation is something that occurs here in the common-sense world - concious observer not required.However, I don't really blame them because it is really hard and confusing. Even Einstein could get his head around the quantum theory, then why should "normal" people do?

IMHO it doesn't require much effort to dispel the misconceptions - but, possibly because it makes it more mundane and less sensationalist, it's not done.

Thanks

Bill

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To just add one thing to what zapper and nugatory have said, it is experimental fact that makes QM "real", not the math, and at least 40 years ago Feynman was fond of pointing out the the experimental results of measurements made in support of QM were accurate to the same degree as measuring the width of the United States and getting it right to within the width of a human hair.So math says this is real.Its strange

And keep in mind "within the width of human hair" does NOT mean that it is off from the theory by that much but rather that that is the range of measurement error. This is a terrific agreement between measurements and theory.

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Nugatory

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This is something of a glass-half-empty/glass-half-full thing. I'm a total and irredeemable Pollyanna, so I look at the level of traffic here at physicsforums.com as evidence that there are a lot of people who are interested in physics. Also, the guidelines and mentor activity screen out the intellectually lazy - so I see a lot of people who are not only interested but also willing to put a fair amount of their energy into understanding physics.@Nugatory However, it is a shame that most people aren't interested in physics.

I'm much more unhappy about the poisonous effect of the populizers who exploit these interested and motivated people. Aside from an unnecessary upper-case 'H', I agree with Bhobba:

IMHO it doesn't require much effort to dispel the misconceptions - but, possibly because it makes it more mundane and less sensationalist, it's not done.

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A lot of people interested in getting their boring homework done.This is something of a glass-half-empty/glass-half-full thing. I'm a total and irredeemable Pollyanna, so I look at the level of traffic here at physicsforums.com as evidence that there are a lot of people who are interested in physics.

cb

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Yeah, but they don't stay around. If you stay around you'll find that there are a lot of people with more than the dozen or two posts that is usually the max for someone who just comes here for a homework question or two. There are probably a LOT of "registered users" who only have a couple of posts at most, but last time I looked there were over 350,000 registered users and that doesn't count the lurkers who haven't posted yet.A lot of people interested in getting their boring homework done.

cb

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bhobba

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Yea - too true.A lot of people interested in getting their boring homework done

I didn't really get into QM until I got away from this business of passing exams, doing assignments - yada, yada, yada.

Thanks

Bill

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BruceW

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well, quite a few seem interested in their homework. Or at least they need to do it, but also don't find it a completely mundane task.A lot of people interested in getting their boring homework done.

cb

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We have uncertaninty principle because experiment show us that ,we cant measure both of them(momentum and location).But How can we sure that this is works in real universe.Nugatory answered my question and he says beacuse of math now you are talking about experiment. Experiment results are cant say us about real world Because experiments changes real world.Like Double slit experiment.I am confusedMathematics cannot say something is "real". This is a fallacy.

What makes it "real" is that this concept is verified by experimental evidence. That is the only way for something to be valid. Once the theory matches the experiment, then we know that we have a good understanding of a phenomenon.

Quantum mechanics arose out of classical theory not matching experimental observation at the time. Remember, you can also use math with classical mechanics to derive at a number of things. Doesn't mean these were "real" even when the mathematics said so. And the fact that these classical mechanics predictions didn't match experimental observations was a big clue that classical mechanics was missing something.

Mathematics alone is insufficient to show if something is "real". You are missing out on an extremely important component of science - experiment!

Zz.

- #20

BruceW

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Nugatory

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The uncertainty principle should not be understood as the experiment disturbing the system. Heisenberg himself first thought about it that way, but he and the rest of the physics community soon realized that it wasn't quite right. Unfortunately by then the bogus explanation had leaked out into the popular understanding and a half-century later it's still being repeated. Yes, it is true that the measurement disturbs the measured system, but that doesn't stop us from measuring any physical quantity to arbitrary precision.We have uncertaninty principle because experiment show us that ,we cant measure both of them(momentum and location).But How can we sure that this is works in real universe.Nugatory answered my question and he says beacuse of math now you are talking about experiment. Experiment results are cant say us about real world Because experiments changes real world.Like Double slit experiment.I am confused

What's really going here: We can measure property A with as much precision as we want, and we can measure property B with as much precision as we want. However, if we set up ("prepare" is the term you'll see used a lot) the system so that we know what value of A we'll get if we measure A, we'll find that we can't predict the value of a B measurement as accurately. We can do this experiment many times, preparing the system in the exact same way every time, and then look at the spread of B values that we get. We find that the more tightly we control the A values, the more spread out the B values are, and this spread is exactly what the uncertainty principle predicted.

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Just to expound slightly on what Nugatory said (and I'm not really adding anything, just emphasizing) we CAN in fact measure both at the same time, BUT what we can NOT do is set up ("prepare" as he mentioned) an experiment where we will know in advance what BOTH will be. We can only prepare an experiment where we know what ONE of them will be (to some degree of accuracy) and then the other one becomes unpredictable to a degree corresponding to the precision with which we know the first.

I had a lot of trouble with this myself when I first read about it because popular literature had convinced me that it was, as you believed, impossible to measure both at the same time, which as it turns out is not really what the HUP says.

So, again, for any given electron, we can measure both at the same time to any degree of precision that our instruments allow, because there is no theoretical lower limit but if we were to try to repeat the experiment we would get different results.

This is SO unlike classical physics where you can predict, repeatedly, exactly what the characteristics of a pool ball will be for a given setup.

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bhobba

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The uncertainty principle is a theorem from the math of QM - if QM is true then the uncertainty principle is true just like if the parallel axiom of geometry is true the angles of a triangle add up to 180%.We have uncertaninty principle because experiment show us that ,we cant measure both of them(momentum and location).But How can we sure that this is works in real universe.Nugatory answered my question and he says beacuse of math now you are talking about experiment. Experiment results are cant say us about real world Because experiments changes real world.Like Double slit experiment.I am confused

Another way of looking at what QM is, and indeed physical theories in general, as a mathematical model. Euclidean geometry you learnt at high school is the mathematical model par excellence.

Thanks

Bill

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BruceW

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Instead, if we say measurement is when the electron state becomes entangled with classical measuring apparatus, then... uh I suppose it's reasonable that the whole lot can collapse to some state where we get a precise value for both momentum and position. But how can we calculate probabilities for one of these simultaneous position & momentum measurements? Do we just multiply together the probabilities for each measurement? I am not familiar with this kind of thing. We could define a new physical observable 'vector' operator (x,p) But there are no possible eigenvectors for this operator, because we cannot have a state with both definite x and definite p.

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bhobba

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What a measurement is in QM is a tricky beast when you try and pin it down exactly. In practice its usually utterly trivial to tell what one is - but when speaking of matters of principle - things are a lot trickier.To phinds: It depends what is meant by 'measure'... If we define 'measurement' to mean that we make the electron go into one of it's eigenstates for the corresponding observable, then we can't measure both momentum and position at the same time. I'm guessing you're not using this definition.

In modern times to avoid such things the tendency is to consider a measurement to have occurred once decoherence has occurred ie we have an improper mixed state that admits an easy interpretation of the system actually being in one of the components of that mixed state. This is unambiguous and doesn't rely on any arbitrary quantum classical divide.

But it doesn't matter precisely how we define a measurement, general theorems about non commuting observables imply the uncertainty principle - its unavoidable.

That's more along the lines of the modern view.Instead, if we say measurement is when the electron state becomes entangled with classical measuring apparatus, then... uh I suppose it's reasonable that the whole lot can collapse to some state where we get a precise value for both momentum and position. But how can we calculate probabilities for one of these simultaneous position & momentum measurements? Do we just multiply together the probabilities for each measurement? I am not familiar with this kind of thing. We could define a new physical observable 'vector' operator (x,p) But there are no possible eigenvectors for this operator, because we cannot have a state with both definite x and definite p.

It's interpretation is easy - entanglement with the measurement apparatus and environment leads to decoherence which changes the superposition to an improper mixed state that can be assumed to actually be in one of the 'basis' elements of the mixed state.

See:

https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20&tag=pfamazon01-20

Fantastic book - cant recommend it too highly.

Thanks

Bill

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