Superposition of Quantum States

Quantum Superposition and the Stern-Gerlach Experiment.

Allan Adam's MIT lecture:

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Quantum Physics Playlist

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*Music*

[upandatom]

This is the beginning of a quantum physics series I'm starting on the channel. In future videos we will explore the physics and mathematics behind the results of this experiment, plus much more. Please let me know if you have any requests for the quantum series!

[FulvioCarrus]

I once found a nice way to wrap my mind around the "can't know both" thing:

Imagine you see a very fast car, you want to take a picture of it, to know both where it is and how fast it is going.
If the car comes out blurry, you have some idea of the average speed of the car (width of the blur divided by the lens aperture time), but have some uncertainty on where exactly the car was during the shot.
If the car comes out sharp, then you know exactly where it was, but with no blur, you can't know how fast it was going.

You can't know both because both quantities depend on one another. This helped me *a lot*

[ScienceAsylum]

I'm back revisiting this a year (and a couple months) later and it's still my favorite video of yours. Well done 😊

[Imilmano]

I feel like I'm being pranked by the universe.

[ScienceAsylum]

2:45 _"The actual mechanisms of the machine don't matter at all."_ This is a point that's really hard for newcomers to accept, but needs to be made. Thank you for saying it.

[nicholaslau]

General Physics I: Classical Mechanics
General Physics II: Electromagnetism
Modern Physics III: Forget what you have learnt and start from square one

[omkargaikwad]

I just subscribed about a week a ago and have already watched many videos some more than once. So glad I found this channel. Why didn't I find this channel sooner but better late than never.

[KhAnubis]

This is a really interesting new format. I feel like you could use it on occasion, in addition to your regular videos, especially to make this kind of stuff more accessible to those less knowledgable in this field (e.g. myself)

[Deuphus]

Even more confusing is a 8:30, the Neutrons became Electrons. Then at 8:46 they became Neutrons again.

[ininfinitygeometry]

Ok. I had a look at the lecture you mentioned. I notice there are a few errors. 1: Electron exhibit up and down spin, just two quantities. The idea of colour and shape is derived from the orientation of the magnetic field in the box, not the electron itself. 2: you need to understand spin to get superposition. 3: Neutrons have 1/3 and 2/3 spin, not binary. 4: colour and shape 'machines' are the same thing, orientated at 90 degrees to each other. 5: the machine organises the neutrons by aligning them into Up and Down, it rotates them and so does change their orientation. 6: the mechanism does matter, there is no tiny demon inside the machine, you need to have a magnetic field in a certain orientation for the effect to work. It is completely to do with the orientation of the magnetic field. 7: The reason it is always 50%/50% is due to the wave nature of the electron. Waves go up and down in equal proportion. Therefore, it in not completely random. 8: after the 1st pause, the solution as to why 50% are 'green' and 50% are blue is because the machine reorientates the electron wave, the X, Z, X axis will obviously produce a 50% mix. 9: the machines reorientate the neutrons on the x and z axis, which is why you get the 50%/50% mix. 10: no one has ever isolated a neutron or even a photon. 11: Experiment 2 produces all 'green' neutrons, as the recombination reflects the 'round' neutrons, but not the 'square' ones. The mirror 'inverts' the wave, in the same way that Circular polarization is inverted. As only the 'circular' waves are inverted at the last step, so the result produces all 'green' neutrons. 12: When the 'round' neutrons are blocked, there is no recombination in the final mirror, so all the 'square' neutrons are now polarised in the x-axis, so when passed through the z-axis (colour box') at the end, the result is 50%/50% split. 13: The reason you can't know colour and shape at the same time is because the electron only has 1/2 spin, (neutrons only 2/3 and 1/3). As there is only one property, not 2 as you have suggested. The only difference between 'shape' and 'colour' is the orientation of the magnetic components inside the box at 90 degrees. 14: the path is not a million miles long. 16: you forgot option number 6. You can change the polarisation of the electron/neutron with a mirror, which makes logical sense of reality, once you reintroduce the proven wavelike nature of matter, and stop proposing the neutron is just a particle, like a football, which it is not. Btw, you can't place a detector on both paths, as detection is a destructive process, which is the foundation of the uncertainty principle. 17: No the neutron is not square/circular and blue/green at the same time, which are orientations of spin at 0 degrees and 90 degrees to each other. The up and down spin exist at any orientation until passed through the magnetic field. 18: I am glad you mentioned the Stern-Gerlach experiment.


For a clearer explanation, check out these videos:

btw: this is not a dig at your understanding of atomic physics, but I have noticed that this concept seems to have created a lot of confusion about the nature of electron spin. Thanks to Allan Adams, who has taken it upon himself to redesign quantum mechanical principles.

[equesdeventusoccasus]

This is one of the best descriptions for the lay person of quantum superposition I have heard. If professors started their discussions about QS this way, I think a lot of people would understand it quicker and with greater easy.
We are hunters by nature. A hunter, regardless of quarry, starts by looking at a general area and then focuses on areas of increasing detail until they find what they are looking for, whether an animal, berry, or a quantum entangled pair. Your approach to the topic follows that path.

[ritchiemx]

Although I have heard this type of explanation before, this is the best presentation of it I have seen. Thank you for posting it.

[aryanduttaschannel]

I saw the MIT lecture like 3 times, to understand your animations just made my concepts more concrete

[thesentientneuron]

Crap. execute uncertainityprincle.exe failed. Error log location and information simultaneously inaccessible.

[danielspivak]

I think it still makes sense to say it took both paths. The reason is that if you put a detector in, then sure, you will find that it took only one path - but if you detect which one it took, it will come out 50/50 blue/green again.

[omkargaikwad]

Please could you make a video on quantum computer and why is it way better than regular computer. I like how easy you make the topics.

[yacc]

7:53 very important! Green color state "IS" ONE case of superposition of the 2 shape states (for example, +) And the blue color state is THE ANOTHER one (-). And viceversa, the shape states are superpositions/combinations of color states

[ichigokurosaki]

The real question is, I understand super position... it's both square and circle at the same time. However why is the split always 50% 50%, I always thought super position meant it could be in either state, and once you measure it condenses and chooses one or the other. However if were talking about 100 particles, 50 will be square and 50 circle. Almost like they can talk with each other.... and the even wierder thing is how the particle knows were measuring it, I've seen other videos on particle physics and superposition and they said this is the same with all particles, neutrons, protons, photons, electrons, and everything else. And we've also used many different techniques to measure their states like you said, lasers, magnets, etc... it's very unusual that the particles know we're measuring them and what we're measuring. Because if you measure a particle and it's square as long as you dont measure the color I'm pretty sure it stays square no matter how many times you measure the shape it'll always be square. But once you measure color and shape again it could be square or circle. This is why I believe there is a higher being like God because that seems a lot like intelligent design(almost like a programmers code).

[animistchannel]

Another great video, thanks! These quantum ideas have never seemed that strange to me... in my tradition, we often treat the whole world that way. It makes a lot of survival tasks easier. The harder part is keeping track of what you already know :)

[sethsagnik]

This video follows Prof Allan's quantum mechanics course (MIT OCW) very closely. It is really beneficial for me to see this video after the lecture. It cleared all my doubts.