Question

Quantum Mechanics Chat :)

https://www.youtube.com/watch?v=NF4GiR1Dghs&index=50&list=UUNIEAv633WRg4ubBIhOcwMg

Answer 1

Anyone who wants to watch this video or others from this series, check it out and let's discuss it! Watch it on 2x speed or whatever is comfortable to you. =P

Answer 2

awsome

Answer 3

is it backwards 51-->1

Answer 4

He has another playlist, this is sort of a weird one I guess.

Answer 5

I think we should also, link the chemistry videos because you would need some sort of basics to dwell into Quantum Mechanics. (Knowing what Planck, Thomson, Einstein, etc) did.

Answer 6

oh okay, ive seen this guy hes a good teacher

Answer 7

https://www.youtube.com/watch?v=NF4GiR1Dghs&list=PL65jGfVh1ilueHVVsuCxNXoxrLI3OZAPI&index=2

Answer 8

starting with last video ?

Answer 9

Try this link I posted, it should look like the second video int he series.

Answer 10

Is this Leonard Susskind or something? (Haven't clicked the link, spitballing). Stanford?

Answer 11

This is sort of our quantum mechanics learning over winter break group. It will be an ongoing daily thing and anyone can start a new thread not just me or batman.

@Mendicant_Bias It's from Carthage University by Brant Carlson, it's a sort of ground up video series of quantum mechanics in a kind of Khan Academy format.

Answer 12

Aww yiss, considering I'm literally going to have to take Quantum, I might as well get ready now, lol.

Answer 13

leonard susskind is very good

Answer 14

ill join u guys after my exam tmr :)

Answer 15

Good luck, @dan815 !

Answer 16

thnx

Answer 17

We have several other good playlists that I'd like to get through, just sort of go through it at your own pace and if you have any doubts or ideas this is how we'll all sort of be able to study together and help motivate each other to learn and figure it out. =P

The main thing is, these will get kind of long so every other day or so if anyone in here wants to make a new one and link to the previous one or tag everyone who's participating that seems to be interested that would be great. Good luck dan d00d!

Answer 18

I don't have speakers :c

I looked up the first standalone video, but don't have a way to find a playlist necessarily.

Answer 19

(From my tablet, wow, that made no sense without that context. I looked up the standalone video on my tablet, with speakers.)

Answer 20

Just some things of classical mechanics that will help you with QM and hopefully we can try figuring out what they really mean through out this chat :D

`Mass` - Physicists still have a hard time explaining this, why do electrons have mass? And what's so unique about the mass "they" do have? (Einstein postulated mass and energy are interchangeable, remember the formula E = mc^2 :)

`Motion` - In mechanics it is known as kinematics. In a classical sense, we learn that motion and space are seen to be smooth and continuous, as in point A to point B.

`Force` - springs and electrical forces (will learn ore about this later, as we progress I'm sure :)

`Momentum conservation`- Ability for mass to do what it keeps doing (weird?) Classically it's mass x velocity

`Energy conservation` - Usually thought of as driving change. No energy means, no movement, and no change to be taken place. Energy is all around the Universe (many forms of it).

Knowing this will help us understand and dwell deeper, because if we know classical mechanics well I think it gives us something to compare with and understand the thinking of the time and see how to approach some problems.

Answer 21

I don't know, perhaps one of these links will work for you @Mendicant_Bias

https://www.youtube.com/channel/UCNIEAv633WRg4ubBIhOcwMg/playlists
https://www.youtube.com/channel/UCNIEAv633WRg4ubBIhOcwMg/videos?sort=dd&view=0&shelf_id=0

Answer 22

And as Kai mentioned, you can always open up a new thread, it doesn't have to be this one specifically, if you want to discuss it any times, we're more than happy to :D also...take your time :)

Answer 23

Yeah, no, I can see the links just fine, my tablet just doesn't deal with OS very well so I'm going to have to separately find the playlists on my tablet without OS. I'm on a PC right now with no speakers, but yeah, shutting up and watching now.

Answer 24

Since "Intro to QM", "Domain of QM", and "Key Concepts in QM" should be a good first goal, but feel free to go up past that through complex numbers and further. Just say wherever you stop or from what video you were in when you have your ideas/questions.

Since these videos are basically all intro and giving you the structure of the course, maybe talk about what your impressions of QM are, what sort of things you're expecting, or if there's any kind of interesting philosophical ideas you think are cool. We should try to have fun with this as much as possible. =P

Answer 25

For everyone who has completed the first video, reminding of the last two prompts at the end:

(1) "If the timescale of interaction of two helium atoms is 10 ns, what is the energy scale where quantum effects become important?"

(2)"If the energy scale at a temperature T is given by \[\triangle T \sim k_{B}T\]where \[k_{B}=1.38\cdot 10^{-23}\rm J/K,\]what temperature must helium be cooled to for quantum effects to become important?"

Answer 26

We should try to work through the given problems, in addition to thinking about the philosophy of it. Give a little bit of something to work with in the meantime.

Answer 27

I'm wondering how we're supposed to interpret/solve the first one; we somehow have to get the given information, the timescale of interaction of two H atoms being 10 ns, into the form \[\triangle x \triangle p.\]Any takers?

Answer 28

(And I'm guessing when it says timescale of interaction....well, no, actually, I don't know what it means, because this isn't Classical Mechanics, can we use impulse and other known info about a Helium atom or something like that to find momentum?

Answer 29

I want to try to solve it on my own first and I'll compare with you in a little bit. But I think you're right, everyone should solve these end of video problems before moving on.

Answer 30

I'm going to guess that when it refers to "timescale of interaction", we can assume that since the Helium atoms are interacting, they are colliding, or the distance between them is equal to their two radii added together, so that's our delta-x.
\[\triangle x \triangle p = (2)(31 \rm \ p m)(m \triangle v)=(2)(31\cdot 10^{-12})(m \triangle v)\]I have no idea how to solve for elta v in this sence, or how to phrase it with the equivalent form of \[F \triangle t\] since we have been given neither, but that's the approach I took. Seeing what else can be done from there.

Answer 31

\[\triangle x \triangle p \approx \hbar=6.62\cdot10^{-34} \rm \ m^2kg/s\]

Answer 32

Remember, they also said at around 3:25 that \[\large \Delta E \Delta t \approx \hbar\] That might help you quite a bit =P

Answer 33

Uncertainty principle:
\[\large \Delta E \Delta t \ge \frac{ \hbar}{ 2 }\]
\[\large \Delta p \Delta x \ge \frac{ \hbar }{ 2 }\]

Answer 34

What you need for your problem is what kai stated.
Also here is a nice video for uncertainty principle :) https://www.youtube.com/watch?v=a8FTr2qMutA

Answer 35

I think I'm going to get this out of the way and memorize these two values approximately.\[\Large h \approx 6.626 \times 10^{-34} J s \\ \Large \hbar = \frac{h}{2\pi} \approx 1.055 \times 10^{-34} J s\]

Answer 36

I totally missed that other relationship with delta E given in the video, thank you.

Answer 37

Man, I messed up like.....75% of what I wrote, from h bar to the value of it to how to do it, lmao. Will write out an attempt shortly.

Answer 38

No worries, the fact you attempted is good enough for me! Hehe, you don't have to always have to get the solution sometimes the struggle is more important ;).

Answer 39

\[\triangle E \triangle t \approx \hbar; \ \ \ \triangle E(1\cdot 10^{-9}) \approx 1.055 \cdot 10^{-34}\]
\[\triangle E = \frac{1.055 \cdot 10^{-34}}{1 \cdot 10^{-9}}=1.055 \cdot 10^{-25} \rm \ J\]I'll need to double check on those units, though, and the second one seems more complex.

Answer 40

Oh, yeah, units are fine, nvm.

Answer 41

I'm not sure how to do the second one honestly. He says near the end that temperatures <100K are when quantum effects come into play, but that seems to make it meaningless for him to say the kB value.

Answer 42

The 2 is only important if you're wanting to calculate the uncertainty exactly, otherwise you're really just looking at a good estimate. We haven't really derived the uncertainty principle yet, so don't worry too much about that, this is just an introduction to get you to see that quantum mechanics is really tiny lol.

Answer 43

The units work out to just be joules when you multiply through, so I think it's just a typo, and it's really
\[\triangle E = k_{B}T, \ \ \ k_{B}=1.38 \cdot10^{-23} \rm \ J/K\]

Your T is definitely going to be in Kelvin, so the only thing it makes sense, given the units of that constant k_{B}, is that's delta E itself.

Answer 44

Yeah haha, you don't need to 2 as I mentioned above I was just showing the formula of the uncertainty principle, you did it fine, so what can we conclude? Is it QM or more so Classical?

Answer 45

Yeah, I think I'm going to leave it and move on, I'll come back later if I feel compelled but I'd rather think hard in concepts than get swamped in basic algebra at this time.

Answer 46

I'm moving on to video 2, where is everybody else at?

Answer 47

Hey, that's great, take your time, and I'm watching the key concepts in QM :)

Answer 48

I watched ahead already to make sure the videos were good quality for this, I'm just waiting to discuss this stuff and around the 10th video or so I will be sort of back with the group again watching them as we go.

Answer 49

So Kai, Schroinger's equation can be treated as fundamental to QM as Newton's laws are to mechanics and Maxwell's equations are to electromagnetism? :p

Answer 50

Yeah, for the most part. There are a couple other versions of QM, but this is the standard version of it. There is also Feynman's Path integral formulation as well as Dirac's and a couple other relativistic wave equations. =P

Answer 51

Though I'd take a pic of this, quantum mechanics concept map (from the video) http://puu.sh/dvmhi/aed1ec0bb0.png

Answer 52

I still don't understand Dirac's work, so he discovered anti-matter, and that electrons have a spin? But I remember once you mentioned the spin actually doesn't exist, so it's a hypothetical sort of thing?

Answer 53

He didn't discover spin, but his relativistic theory allowed for it. You know how there are 4 quantum numbers? Schrodinger's equation only gives us 3 of them. The electron spin is something you just have to accept at this level as something that is observed from the Stern-Gerlach experiment. But we don't nee to get ahead of ourselves haha.

Answer 54

Ah, that makes sense though haha, thanks!

Answer 55

Here's a fun challenge question I just came up with. Are sound waves quantum phenomena? Feel free to make any assumptions you need to, such as saying that air is purely nitrogen gas. Your approximations can be fairly liberal considering the incredibly small size of planck's constant.

Answer 56

I'll catch up to video 3 tomorrow morning, and then anybody else interested (gonna throw some tags some people's way soon) can start from there, but Kainui with what it seems like your already knowing QM and being 10 videos ahead, lol...

Answer 57

Well there's always a way to take it deeper when you review. Going through these videos is giving me a lot of things that I didn't notice or realize the first time through, so it's nice. Plus I get to help you guys out. =P

Answer 58

What is everybody else at video-wise? Was there another thread made today, or are we keeping on this one?

Answer 59

I was feeling sick, so I am going to start back up in a few minutes since I just woke up! =P

Answer 60

I haven't had much time today either :P, but I will try going over some of the videos hehe.

Answer 61

Looks like a good presentation.

May I suggest using electron volts as a unit of energy rather than joules.

1 eV = 1.602 E-19 joules which is a good unit for energy of visible light

Kilo eV - KeV is useful for X-rays

Million eV - MeV is useful for gamma rays and mass

Planck's constant h = 4.136 E-15 eV-sec

Answer 62

I like putting them in Joules, but eV is sometimes more useful, but it's an easy conversion from joules to eV, \[1 eV = 1.609 \times 10^{-19} J\]
(eV = electron Volts)

Answer 63

For a particle In a one dimension box, if uncertainty in position of the particle is 5% then find out the percentage uncertainty in the energy of the particle.
@iambatman
@kainui

Answer 64

You seem to be lacking information, what is the length of the box?