Understanding Quantum Mechanics

This video is an simple introduction to quantum mechanics that explains why there is still so much controversy over the meaning of this scientific theory.

Thanks to Vlogbrothers for their sponsorship of this video! I really really appreciate it!

Homework questions:

1. We talked about doing the double slit experiment with small things like electrons. But is there any reason in theory we can't do it with bigger things? If not, what would you need to do to make it work?

2.We didn't measure where the electron went in the middle of the experiment. But it is possible. If we measured each door to see if the electron went through it, what would we find? And what would happen to the pattern on the back wall?

3. There is another way to interpret the double slit experiment, which I think is really cool- it's called Pilot wave theory. I made a video about it. In that theory, the particle does go through just one door, even though our argument said that's not possible. Find the flaw in our argument. Also comment on why particles still act strange in Pilot wave theory.

[matt]

I wanted to find a video about something over my head from someone with a nice voice so I could bore myself to sleep. I settled on this but now I'm about to start a series on "understanding" quantum mechanics. Super well made. Bravo.

[ScienceAsylum]

Loving the improvements and the background music. Well done!

[polgabaldon]

This channel is INSANELY GOOD! I just love these videos. Keep up the great work!

[shubhamshinde]

I love this channel!!!! So happy to see you active looking glass universe!

[rubyjohn]

omg I'm super hyped. what a great day to see a great channel updating again

[JohnKNMurphy-nz]

The part about one 'bin' per door when each is closed does not match what we see in real experiments with either matter or light.
When one door is closed then the pattern that appears is still an 'interference' pattern and it still exactly matches a pattern predicted by QM. The beam widths of the single slit pattern, together with the form of its surrounding bars, is inversely proportional to the aperture dimensions in a way that cannot be explained by random scattering. These are also disturbed if you 'look' at the particle transiting the aperture(s).

For this reason, I think the depiction of the difference between single and double slit patterns shown in the video is very misleading. Especially the idea that the single slit pattern reverts to a single 'pile' behind each opening (like a simple shadow) - I really question the ethics of propagating this trope and then claiming to represent some form of authority on QM.
In particular:
a) The single slit pattern is a quantum phenomenon with exactly the same explanation as to the double. and
b) In order to be mathematically consistent with QM, the single slit pattern needs to fully overlap the region of the double-slit interference.

For all aperture patterns, (single, double, grating) the observed particle scattering patterns correspond exactly to particles/photons acquiring a transverse momentum from a probability distribution that is given by the square of the Fourier transform of the aperture pattern divided by Planck's constant. This is independent of the type of incident particle or wavelength.

For a bright line in the two-slit. sin(theta)=n*lambda/d, substitute de Broglie wavelength and p=n*h/d. In effect, the de Broglie/photon wavelength is eliminated from the calculation of the scattering pattern.

For a single slit, the beam width is also an exact function that depends on QM = first dark line p=h/w (w=slit width). Same deal, the pattern depends on QM and from a momentum perspective, is totally independent of the incident 'wavelength'.

In effect, the experiment is explained if, at the instant that the particle transits the slits, it experiences a discrete momentum transfer from a probability distribution that is given by a simple formula predicted by QM. (provided nothing interacts during the transit).

When the particle transits the slits it exchanges momentum if it is deflected. The spectrum of possible momentum exchanges can be calculated using QM by calculating the energy distribution of virtual photons that mediate the momentum transfer, this is a fixed pattern that depends on the aperture pattern and has nothing to do with some painful confabulation and waves that is adapted from classical wave propagation.

The semiclassical arguments about waves and interference, not-looking, and the tortured nonsense about particles becoming waves and interfering harks back to Bohr's attempts to create a semiclassical model. it is actually unnecessary provided one uses QM to model the momentum transfer between the slit and the screen as a discrete interaction.

[kevc]

Love the way how you explain this difficult subject matter to us novice in this field.

[abhiz]

YES, MY FAVOURITE YOUTUBER IS BACK!!!

[hargisss]

This video is awesome! One of the best concise overviews of quantum mechanics I've seen. I'm totally completely psyched for the whole series. Keep it up! :-D
I'm going to give some probably incorrect answers to your questions:
1) I'm assuming to get the apple to behave in a quantum mechanically weird way you'd need to totally isolate it from all interaction with anything else. Shay Lempert below says that the wave-function of the apple is probably going to be smaller than the apple, but it seems intuitive to me that the longer the apple stays isolated from the outside world, the larger its wave-function will get. Eventually, given a crazy large setup and a crazy lengthy experiment, wouldn't it become larger than the apple itself? More importantly, if not, why not?
2) In the second example I happen to already know that if you measure which slit each electron goes through, the electrons form two clumps behind each door, much as you'd naively expect a particle to behave from the beginning. Measure the particle, and it assumes a definite position while passing through the slit. This still confuses me a bit... one might guess it's the interaction with the particle used to perform the measurement that causes this effect, but what if you create a system to measure only the particles going through the left slit, then observe the distribution of only the particles which went through the right slit? Do they fail to form an interference pattern because they would have been measured if they'd gone through the other slit? My science-enthusiast-level understanding of the matter says yes, they would fail to perform an interference pattern. In that case was it the potential but seemingly non actualized interaction that caused the difference? Very strange indeed.
3) Thirdly, in the case of pilot wave theory, the particle does have a definite position at all times... but that position is guided by the wave function, which is itself sort of a mathematical representation of all the different ways the particles could have gone, and how those ways interact with each other... including the particle going through the other slit. That last sentence was probably pretty philosophically extravagant, but I'm not a physicist or a philosopher of physics, so I don't have a reputation to lose :-)
Anyway, the fact that the particles are guided by a wave-function which itself, if my novice understanding is correct, is comprised of all the different ways the particle could have gone nicely explains why the trajectory of the particle is so insane.

[AngelDemon]

This is a really well done video on the intro to QM, the one comment I would make is that the background music is a bit too loud and so it’s a bit distracting. Other than that though, it’s wonderful to see you remaking videos from the original series.

[Holobrine]

Quantum Mechanics in one sentence: Things get wavy when you aren't looking.

[williambunting]

The explanation of “observation” of larger objects was very good, and to make the distinction with observation of subatomic phenomena where to detect such requires absorption of all of their energy, hence collapsing the “wave” function, was ideal.

[ericpatterson]

1. You can do it with bigger things, but you need to ensure that the constituent components share the same quantum state and don't interact with the environment (i.e. "be observed") so you'd have to use something like a BEC.
2. You'd see which door the electron goes through, but the interference pattern would disappear and you'd see two bands behind each door on the screen.
3. The point is that some part of the electron must be non-local (goes through both doors). In pilot wave theory the electron only goes through one door, but its pilot wave goes through both doors. Essentially the pilot wave takes care of the non-local bit of the problem allowing us to feel good about our intuitions that particles can only be in one place at a time.

[whatishappening]

Lovely animation + impressive EDU skills + mind blowing topic = A Looking Glass Universe Video. Loved it!!

[roxastherogue]

MORE. I love this channel and i just found it. Ive just started studing the quantum world and out of all the people i know that are very certified to talk about the matter this video made it so much simpler. I wish you posted this months ago.

[SmajdalfFrogi]

1) Since bigger object consist of more particles it is hard not to make those interact and constantly "measure" one another; their wave fuctions are also too small to have a significant impact.
2) If we would measure through which door the particle went we would get a 50/50 split and two blobs behind the doors as the pattern
3) Although particle might only go through one door it can end up in the diffraction pattern due to the fact it's motion is defined by how many doors are open.

[danielfarbowitz]

Hi Mithuna,

Truly excellent work putting this video together. As a soon-to-be PhD physics student who has done quantum coursework extensively, I’m anxious to answer the questions for fear that they’ll expose how little I know or how misguided my thinking might be. So here it goes…

1) In principle, everything is a quantum object. Why wouldn’t an apple thrown in the middle of a double slit go through both?

In a double-slit experiment, the distance between visible peaks in the interference pattern is proportional to the wavelength of the object and the distance to the back screen and inversely proportional to the distance between the slits.

If you think of a thrown 70g apple as being one big quantum object, its wavelength (h/p) is on the order of 10^-33 m, or about a billionth of a yoctometer. The smallest width between slits we could currently create (a bit generously) is 10^-10m. So in one way of thinking about it, you couldn’t produce slits simultaneously big enough for the apple to pass through and small enough a distance between their centers.

And at a macroscopic distance to the back screen, the distance between the intensity peaks would be too small to resolve anyway – on the order of 10^-23m.

If you instead thought of the apple in terms of its constituent subatomic particles, and tried to run the experiment on an “electron-sized” double slit, the barrier with the double-slit isn’t the only potential to consider anymore. It is already in the presence of other subatomic particles pulling on it with various forces. The result on the back screen after you take into account the forces from the barrier and other particles would just be a splattered apple.



A couple questions came up in thinking about this:


What happens if you move the location of the electron emitter? I know it so often gets portrayed as being exactly in between the two slits, but how does the resulting pattern change as the emitter gets moved closer to (or past) a slit?

Also, how does changing the slit size affect the interference pattern?



2) So I’m not sure what the measuring device is doing, but my guess would be that if it stops the particle entirely then you would get the same result as the observed slit being closed with 50% particle detection from the detector. If the device detected and allowed the electron to pass through, you’d get the same 50% detection and I think the result becomes two piles behind the slits, as if you added the results of each of the “one door closed” scenarios together.

My suspicion has been that there’s something fundamental about measuring that causes this. My teachers hand-wavingly talked about “increasingly subtle” detection devices and that may be the reason my view is less nuanced.



3) I had an interest in Bohmian mechanics but never followed through, so this is a best guess – the particle is, as your illustration suggests, riding a wave. (What is that wave exactly? The fabric of the universe?) So when the double slit is encountered, the particle only goes through one slit but the wave goes through both and interferes with itself, affecting the travel pattern of electrons such that collections of them detected on a screen make an interference pattern.

[noahsmith]

I really love your explanations to quantum mechanics. I saw all your videos and it's great how you are able to explain all these phenomenon of qunatum mechanics in context to the basic principles/laws, because I used to struggle to find the connection between these concepts.
I was wondering if you could make a video Quantization in quantum mechanics. How can it be explained with the basic principles or do we have to simply accept it aswell as a basic law?

[Portablesounds]

1) For larger objects, this experiment is impossible because all of the atoms that make up the object "observe" each other through the interactions of their bonds.
2) If we measure the electrons mid-experiment, they would appear to only go through one door and the resulting pattern would appear like we originally predicted with 2 distinct patches of electrons.
3) Even if the particle goes through only one door, it's wavefunction interacts with the slits and guides the particles in the interfering waves pattern we observe.
Awesome video! This definitely seems like a great starting point for diving into quantum mechanics. I really liked the art style as well (especially that moebius strip title card).

[joshuabaker]

So excited! Just subscribed a few weeks ago and so happy to see new stuff already!