Heisenberg uncertainty principle practical

[L Drago]

TL;DR Summary

The Heisenberg uncertainty principle states that position and momentum cannot be determined at the same time.

There is an experiment to demonstrate it :

A laser light is fixed and there are two razor blades we are bringing slowly together. We are predicting positions of photons on screen but after moving it more it changes shape and momentum changes. Hence, we cannot predict position and momentum at the same time.

Please review this experiment I watched from a video in YouTube and I think is correct. If there are flaws or there is a better experiment of proving this principle, kindly tell me.

 

[Nugatory]

The Heisenberg uncertainty principle states that position and momentum cannot be determined at the same time.

That's an OK English language description, but there's a lot more to it and the principle can be generalized to observables other than just position and momentum. But with that said, yes, the experiment that you describe is a demonstration (not a proof, that's in the math) of the how the uncertainty principle works with position and momentum.

We start with a particle's wave function, from which we can calculate the probability of getting a particular result out of a position measurement or a out of a momentum measurement. Suppose t wave function says that the position probabilities are tightly clustered around a particular position; this will be the case for those photons that pass between the razor blades because they're so close together. Then we go to calculate the probability that the momentum values will be clustered around some particular value; we find that the narrower the range of probable positions, the wider the range of possible momenta.

Please view this experiment I watched from a video in YouTube and I think is correct.

Be very cautious about online videos (unless they happen to be the Feynman lectures). Some are good, many are garbage, and there's no easy way of picking the occasional good ones out of the junk.

Although there are no really good texts accessible to a seventh-grader (understanding QM requires a lot of math) you might try working through Giancarlo Ghirardi's book "Sneaking a look at God's cards". You won't find it an easy read, but I don't think QM can be made any easier.

 

[L Drago]

That's an OK English language description, but there's a lot more to it and the principle can be generalized to observables other than just position and momentum. But with that said, yes, the experiment that you describe is a demonstration (not a proof, that's in the math) of the how the uncertainty principle works with position and momentum.

We start with a particle's wave function, from which we can calculate the probability of getting a particular result out of a position measurement or a out of a momentum measurement. Suppose t wave function says that the position probabilities are tightly clustered around a particular position; this will be the case for those photons that pass between the razor blades because they're so close together. Then we go to calculate the probability that the momentum values will be clustered around some particular value; we find that the narrower the range of probable positions, the wider the range of possible momenta.

Be very cautious about online videos (unless they happen to be the Feynman lectures). Some are good, many are garbage, and there's no easy way of picking the occasional good ones out of the junk.

Although there are no really good texts accessible to a seventh-grader (understanding QM requires a lot of math) you might try working through

Are there any more experiments which are better and more precise to demonstrate and Thanks for the information @Nugatory.

 

[phinds]

In addition to @Nugatory 's excellent explanation let me add this, in more colloquial terms. The (HUGE) difference between classical mechanics and Quantum Mechanics, is that in classical mechanics, if you can exactly, and I mean EXACTLY, replicate the starting conditions of an experiment,, the results will always be the same because that is the deterministic characteristic of classical mechanics. In Quantum Mechanics, on the other had, if you can EXACTLY replicate the starting conditions of an experiment, you will NOT get the same results because that is the non-deterministic characteristic of Quantum Mechanics (exemplified by the HUP)

 

[Nugatory]

In Quantum Mechanics, on the other had, if you can EXACTLY replicate the starting conditions of an experiment, you will NOT get the same results because that is the non-deterministic characteristic of Quantum Mechanics (exemplified by the HUP)

On the other hand, EXACT replication of the initial conditions is only possible in a thought experiment. This is true even in classical physics - we can write down hypothetical initial conditions as precisely as we like but there's no way of setting up a physical system that exactly matches that specification.

Note also that QM does offer a thought experiment in which we will get the same result every time: If the initial conditions are that the wave function is an eigenstate of an observable that commutes with the Hamiltonian then it won't change. This is not possible, even in principle, with position and momentum - they don't come in well behaved eigenstates.

 

[PeroK]

TL;DR Summary: The Heisenberg uncertainty principle states that position and momentum cannot be determined at the same time.

There is an experiment to demonstrate it :

A laser light is fixed and there are two razor blades we are bringing slowly together. We are predicting positions of photons on screen but after moving it more it changes shape and momentum changes. Hence, we cannot predict position and momentum at the same time.

Please review this experiment I watched from a video in YouTube and I think is correct. If there are flaws or there is a better experiment of proving this principle, kindly tell me.

We don't usually recommend popular science videos here, but this one on the HUP (Heisenberg Uncertainty Principle) is better than most.



If you really want to learn QM (which is significantly harder than SR), then these notes are perhaps the most accessible serious introduction to the subject. Even if you don't follow all the mathematics, the insights are invaluable:

https://physics.mq.edu.au/~jcresser/Phys304/Handouts/QuantumPhysicsNotes.pdf

 

[L Drago]

We don't usually recommend popular science videos here, but this one on the HUP (Heisenberg Uncertainty Principle) is better than most.



If you really want to learn QM (which is significantly harder than SR), then these notes are perhaps the most accessible serious introduction to the subject. Even if you don't follow all the mathematics, the insights are invaluable:

https://physics.mq.edu.au/~jcresser/Phys304/Handouts/QuantumPhysicsNotes.pdf

Thanks a lot @PeroK for giving me the authentic sources to read from.

 

[L Drago]

In addition to @Nugatory 's excellent explanation let me add this, in more colloquial terms. The (HUGE) difference between classical mechanics and Quantum Mechanics, is that in classical mechanics, if you can exactly, and I mean EXACTLY, replicate the starting conditions of an experiment,, the results will always be the same because that is the deterministic characteristic of classical mechanics. In Quantum Mechanics, on the other had, if you can EXACTLY replicate the starting conditions of an experiment, you will NOT get the same results because that is the non-deterministic characteristic of Quantum Mechanics (exemplified by the HUP)

QM is actually weird though true. For example, in Schrodinger cat experiment, the cat is in a state of quantum superposition. It means it is both alive and dead at the same state as there is 50 percent chance of emitting radiation and the Geiger counter doing its job and spilling poison. Hence, the cat is both alive and dead at the same time before we open the box.



That means QM is very weird but interesting at the same time

 

[PeroK]

QM is actually weird though true. For example, in Schrodinger cat experiment, the cat is in a state of quantum superposition. It means it is both alive and dead at the same state as there is 50 percent chance of emitting radiation and spilling poison. Hence, the cat is both alive and dead at the same time before we open the box.

The point of the Schrodinger's cat thought experiment is that we know that the cat is not alive and dead at the same time. That makes no sense from our knowledge of macroscopic objects. But, QM appears to suggest that it should be. The experiment asks where do we go wrong when we apply QM to this scenario?

 

[phinds]

in Schrodinger cat experiment, the cat is in a state of quantum superposition. It means it is both alive and dead at the same

No, that is not only not correct, it is EXACTLY the misunderstanding that Schrodinger was pointing out --- the absurdity of taking the Copenhagen Interpretation of QM too literally. The moon, after all, IS still there whether anyone is looking at it or not (this is another misconception), and the cat is NOT simultaneously alive and dead, it is always just one or the other. It is not the cat that is in superposition, it is a quantum object (subatomic particle).

EDIT: I see @PeroK beat me to it.

 

[L Drago]

The point of the Schrodinger's cat thought experiment is that we know that the cat is not alive and dead at the same time. That makes no sense from our knowledge of macroscopic objects. But, QM appears to suggest that it should be. The experiment asks where do we go wrong when we apply QM to this scenario?

But in the internet, I found this Schrodinger cat experiment and his theory. Now I think I realize that Schrodinger was wrong. Even the great scientist Albert Einstein regarded this as absurd.

 

[L Drago]

No, that is not only not correct, it is EXACTLY the misunderstanding that Schrodinger was pointing out --- the absurdity of taking the Copenhagen Interpretation of QM too literally. The moon, after all, IS still there whether anyone is looking at it or not (this is another misconception), and the cat is NOT simultaneously alive and dead, it is always just one or the other. It is not the cat that is in superposition, it is a quantum object (subatomic particle).

EDIT: I see @PeroK beat me to it.

Yes, Einstein regarded it as absurd but Niels Bohr confronted that

'Automobiles aren' t subatomic particles '

But the laws of physics which work only to subatomic particles and not to real life objects seem absurd and weird.

 

[Nugatory]

For example, in Schrodinger cat experiment, the cat is in a state of quantum superposition. It means it is both alive and dead at the same state as there is 50 percent chance of emitting radiation and spilling poison. Hence, the cat is both alive and dead at the same time before we open the box.

That is a common misconception, something you think you’ve learned that you’ll have to unlearn, and an example of why you should be cautious about online videos and other pop-sci sources.

Schrodinger proposed his cat thought experiment to show that something was bad wrong in the then-current (close to a hundred years ago now) understanding of QM: it’s absurd to suggest that the cat is in that superposed state (and later paradoxes such as “Wigner’s Friend” put a sharper edge on this point) but that’s still what the math seemed to be saying.

Since then this problem has been resolved with the discovery of quantum decoherence: the cat inside the box is either dead or alive, the same way that a tossed coin is either heads-up or tails-up whether we look or not.
You might give David Lindley’s book “Where did the weirdness go?” a try. It’s pop- sci, but not at least not misleading pop-sci. For the real thing, Google for “quantum decoherence” but be warned that this will take you into some mathematical deep water pretty quickly.

 

[Nugatory]

Yes, Einstein regarded it as absurd but Niels Bohr confronted that

'Automobiles aren' t subatomic particles '

But the laws of physics which work only to subatomic particles and not to real life objects seem absurd and weird.

You are now very deep in “It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so” territory.

 

[phinds]

Now I think I realize that Schrodinger was wrong.

NO. He was not wrong, he was exactly right in pointing out the absurdity of the "alive/dead" statement.

 

[L Drago]

That is a common misconception, something you think you’ve learned that you’ll have to unlearn, and an example of why you should be cautious about online videos and other pop-sci sources.

Schrodinger proposed his cat thought experiment to show that something was bad wrong in the then-current (close to a hundred years ago now) understanding of QM: it’s absurd to suggest that the cat is in that superposed state (and later paradoxes such as “Wigner’s Friend” put a sharper edge on this point) but that’s still what the math seemed to be saying.

Since then this problem has been resolved with the discovery of quantum decoherence: the cat inside the box is either dead or alive, the same way that a tossed coin is either heads-up or tails-up whether we look or not.
You might give David Lindley’s book “Where did the weirdness go?” a try. It’s pop- sci, but not at least not misleading pop-sci. For the real thing, Google for “quantum decoherence” but be warned that this will take you into some mathematical deep water pretty quickly.

I now realize that the YouTube videos I thought were preety authentic and I regarded as reliable are not authentic except a few.

 

[phinds]

But the laws of physics which work only to subatomic particles and not to real life objects seem absurd and weird.

Again, you are overextending your knowledge and making statements that don't actually make sense. QM IS, technically, completely valid for "real life objects", even if in practice most of the "weirdness" evens out due to the presence of huge #s of particles. There are even special circumstances (e.g. superconductivity) where QM is directly applicable.

 

[L Drago]

Again, you are overextending your knowledge and making statements that don't actually make sense. QM IS, technically, completely valid for "real life objects", even if in practice most of the "weirdness" evens out due to the presence of huge #s of particles. There are even special circumstances (e.g. superconductivity) where QM is directly applicable.

OK, now I understand is this the correct statement kindly check

Cat is either dead or alive until we open the box and is in a state of quantum superposition according to QM and Schrodinger's cat experiment which was correct.

 

[Nugatory]

But the laws of physics which work only to subatomic particles and not to real life objects seem absurd and weird.

The same quantum mechanical laws of physics that work for subatomic particles apply just as well to macroscopic objects. We use the methods of classical physics to analyze the behavior of automobiles because for car-sized objects classical physics is a really excellent approximation (an approximation that is accurate far far beyond the error of any imaginable measurement) and because it’s way less work (the difference between computationally possible and computationally hopeless).

There’s an analogy with the way that we use concepts of pressure, volume, temperature, density, flow and turbulence to analyze the behavior of macroscopic quantities of gases. All of these properties are results of Newton’s laws describing the behavior of each gas molecule. But we don’t solve Newton’s laws for the trajectory of each individual molecule to calculate the behavior of the gas, because we know that (for example) pressure is a really good approximation for the collective force of all these molecules bouncing off the side of the container. And we’d rather work with $PV=nRT$ then try to derive that force by calculating the constantly changing positions and speeds of $10^{25}$ individual molecules from Newton’s laws.

 

[phinds]

OK, now I understand is this the correct statement kindly check

Cat is either dead or alive until we open the box and is in a state of quantum superposition according to QM and Schrodinger's cat experiment which was correct.

NO. The cat is ALWAYS either alive or dead and is never in a state of superposition. That is the POINT of the (thought) experiment. Reread post #10. You don't seem to be paying close attention to what we keep telling you.

 

[L Drago]

NO. The cat is ALWAYS either alive or dead and is never in a state of superposition. That is the POINT of the (thought) experiment. Reread post #10. You don't seem to be paying close attention to what we keep telling you.

Okay thank you for clearing my misunderstanding.

 

[PeroK]

But cats aren't subatomic particles and QM is all about them.I don't want to say that he was completely wrong or something but I think there is a flaw in this experiment and his theory.

What Schrodinger did was link fundamental QM behaviour and macroscopic behaviour in one thought experiment. It was up to the proponents of QM. (I.e. Bohr and Heisenberg) to explain this. Which at the time they were unable to do satisfactorily.

 

[L Drago]

NO. The cat is ALWAYS either alive or dead and is never in a state of superposition. That is the POINT of the (thought) experiment. Reread post #10. You don't seem to be paying close attention to what we keep telling you.

That means -
the cat is either alive or dead but never in the state of quantum superposition.

Kindly verify is this correct

I think I need to start learning from authentic sources instead of following videos of YouTube.

 

[Nugatory]

But cats aren't subatomic particles and QM is all about them.I don't want to say that he was completely wrong or something but I think there is a flaw in this experiment and his theory.

Stop posting until you’ve had time to read the replies already posted to this thread.

Pretty much everything you think you know about this particular topic is wrong. That’s not your fault, you’ve been misled by badly oversimplified pop-sci stuff on the internet, but that’s why we tell you to be cautious about trusting the pop-sci.

 

[PeterDonis]

I think I need to start learning from authentic sources instead of following videos of YouTube.

Yes, definitely. @Nugatory gave you a reference to a good book to start with. It's not a textbook, but it's a good book for a lay person.

To really understand QM you will need to learn the required math at some point. Normally that means at the college undergraduate level.

 

[L Drago]

Stop posting until you’ve had time to read the replies already posted to this thread.

Pretty much everything you think you know about this particular topic is wrong. That’s not your fault, you’ve been misled by badly oversimplified pop-sci stuff on the internet, but that’s why we tell you to be cautious about trusting the pop-sci.

Thank you @Nugatory . I have deleted that post to avoid misunderstanding and now I will start following some authentic sources instead of those videos.