Lecture 4 | Modern Physics: Quantum Mechanics (Stanford)

[prchecker]

This is a great video

[grunder20]

the class was boring!

[MrPEDOCTOR]

Another good way to think of delta

[manupasricha]

Can somebody please tell me which book Professor Susskind recommends for an undergraduate Quantum Physics Course ?

[RavellaMusic]

@manupasricha I can tell you what I would suggest, - i.e., what's been helpful for me: Quantum Mechanics by F. Mandl and Quantum Mechanics - Modern Development by Rae. They both move at very good pieces, and kind of complement one another: what isn't explained too well in one is explained very well in another. There's a book by Greiner which I am told is very, very good, but it moves way too fast for me. Hope that's helped in some small way.

[RobertErac]

@RavellaMusic "move at very good pieces" - what the fuck could that possibly mean? I guess I must have meant speeds.

[1o618033988749894848]

@manupasricha I use David Griffiths... and have found it excellent. His book on electromagnetism is hella good too

[ghghghkkkkkkkkkkkkkk]

B-hulll Shite! Leonard is a legend! His explanations aren't supposed to be a text book lecture. He takes you through the thought process and is always concise in what he says. Only people who really understand stuff can teach like this. Wish Leonard had taut me at Uni.. I love you man! Peace

[molozful]

Here's another good way to think of delta: take delta_eps to be the eps-wide, 1/-eps tall function, and think of taking the limit as eps goes to 0 of whatever integral you are calculating: limit as eps->0 from integral (delta_eps(x-y)*F(x) dx) = F(y)

[skepticalinquier]

The problem, man has never thought he is part of a system. So, mankind again sees the Universe sees him as a part of its whole. Wink your eye and trouble a star.

[abhishek150390]

this guy is genius i have just started studying quantum mechanics newly, with the help of his lectures on youtube i will master it definitely.hats off to him.

[arsal09]

my QM teacher sucks!!He says just do the math, forget the concepts!!!

[TimeTraveller121]

wish I could hear the questions from the audience! Otherwise, I believe Susskind's an awesome teacher. Definitely better than my own QM's professor.

[jamma246]

I don't quite understand where the new equation for the wave function caused by the 2nd slit is coming from. Shouldn't it just be a displacement of the original function? I don't see why the momentum should be altered. Intuitively, I'd have thought that it would have just been the original function, but with y replaced by y+delta(x) where delta(x) is the displacement between the slits. So why has the momentum changed?

[OsrTenorio]

@jamma246 He explains that the momentum changes along the y axis. At a given point, the momentum due to a slit is different of the momentum of the other ( that's why one is p and the other is q).

The difference comes from the vertical momentum that a particle gains to get to a given point. If this point is in front of the slit, this vertical momentum is zero. If it's far away on the y axis, then it's a lot more.

At least this was what i got from it :)

[2regors1]

i listen to this while i do hw in hopes of absorbing that knowledge! lol

[dogbishop]

It's amazing that when someone clearly competent in some field tries to teach something for enrichment of others, there come out of the wood-work those anonymous "experts" who fault them and claim they aren't doing it correctly, yet no evidence of the critic's expertise is anywhere to be found.

[xIndca]

One of the best videos I've seen in a while! You definitely need to make more videos like this! You should configure your social account at xInd.ca with your youtube channel so we can promote your videos for you and get you more subscribers on youtube. It will automatically embed your videos into posts which automatically gets posted through our network and watched by our members and visitors.

[theerterek]

sir, pro teachers don't look at their paper!!

[cesarjom]

@theerterek ... that's really funny... btw Susskind is not a "pro teacher"... he's one of the most prolific theoretical physicists in the field of string theory among other related branches.

[jimmyordaya]

@theerterek uhhh you ever think he does that maybe so he dont forget to bring up a point and keep things smooth and organized. dont speak unless you know what your talking about

[mdinka]

how comes that we can speak of simultaneousness w/o having introduced the time yet. i.e. how can one be sure that AB uneq. BA actually means is not simultaneous measurable? and don't we need to introduce the time first before we can define what simultaneous measurable means?

[cesarjom]

@mdinka ... listen to the definition of "compatible operators A and B"... having common eigenvectors (basis) of both operators simultaneously ONLY means that for a given state of a system, there will be a common basis (set of eigenvectors) that both A and B operators can act on at the moment of measurement and yield an observable eigenvalue a and b for each operator. There is no need to mention time as a dimension here!

[tophernates]

thankyou

[PanZajko]

A question. Maybe I know the answer already. He says, that the electrons forget what they were doing before they hit the wall with the slits and start to behave as if they were shot from the slit itself. Now, if we took 2 separate sources of electrons, forgetting the wall with the slits just shooting them straight at a screen.

Would there be an interference pattern ?? The maths should be the same.

[FlamingPope]

I like the draic function interpretation, but he could have done better applying and showing the postulate of super position/probability. A simple die example, of 1/root6 with the 6 states would have demonstrated the postulate in a better manner. Also his use of "axioms" - axioms are taken as truths, postulates are assumptions which are tested.

[floopsie666]

@FlamingPope Postulate and axiom are synonymous.

[sweetsongman1]

i mostly click this because its the longest video ive ever seen on youtube xDDDDD

[bhigr]

Well, one thing is reassuring. He doesn't know an elegant solution either. I think it would be better to just drop the requirement of quadrature integrability for eigenfunctions of continuous observables and give the whole thing a new mathematical name. So, in case of continuous variables we are not operating in hilbert space.... But then, these topics have plagued my mind for a long time.

[bhigr]

The mathematical definition of he delta function is simply. The operation that turns a function F to the value of the function at the location a F->F(a). But, Susskind is absolutely right. This definition is just circumventing the problem of integrating the delta function.

[bhigr]

Delta(n-m) is infinite for n=m and zero in all other cases This is the dirac delta. In case of continuous eigenfunctions the eigenfunctions are not normalized. That's the problem when dealing with the eigenfunctions of location and momentum. They are not normalized. He is dealing with a really difficult mathematical problem in quantum mechanics.

[TheKarenmuldoon]

Lenny, Lenny, Lenny . . .

Why do you always seem to stop short of discusssing the cinematic hologram? Isn't that really the bottom line after all of this is said & done? Am I just not digging deep enough?

I've read "The Black Hole War' at least 300 times, and this seems to be the single most important concept of all. Help me out. Love you to death, Miss Karen Muldoon

[RobotTed]

At 0:13:00 An operation you do on a function! But his y parameter could be t instead, to make it clearer.

The more symbols used the better : Leibniz to the MAX!

[marawanish]

Leonardo is the king Of Physics...But,no body cares..he deserves a statue..

[x1x2x3ct]

@marawanish actually not... he's quite good, but look, compared with heisenberg, schrödinger and others, he's meaningless

[nickharvey7]

Could Heisenberg Uncertainty Principle be the same uncertainty that the observer will have with any future event? Could time and the geometry of spacetime be continuously formed by the momentum of EMR or light form one atom to another? There is no understanding of time in modern physics or why we have a future and a past. Could this be why we have the paradoxes of QM?

[Angiegirl2112]

OMG how did u get this long video on here? I thought it could only be 10:00 long!

[debajyotisaha]

very realistic

[9906197439]

Leonard Susskind is really the best teacher for Physics.

I have never seen quantum mechanics so much closely and clearly as I can see it when I listen to these lectures.

Many many thanks to Stanford University and of course Dr. Susskind.

[ripperduck]

I'm taking QM now and his lectures hew closely to the ones being given at the undergraduate level. The reason I watch him is that he is a FAR better lecturer than the prof I have. Dr. Susskind has decades of classroom experience and my prof is teaching this for only the second time.

[earizon]

I found the lectures interesting if just to remember all that I studied nearly ten years ago. Still for anyone like me that have not the patience to stay for two hours watching the video here comes a trick: Download the video as MP4 (for example with the "Easy Youtube Video Downloader extension in Firefox). Then play it with the help of VLC at 1.5x or 2.0x times the original speed. In the mean time, thanks to the Standford University, Youtube and the VLC guys for their work!

[mathsboy]

This guy isn't a very compelling lecturer. He seems nervous, at least in the few minutes I watched

[IdoWORK2]

im a sophmore in highschool and my brain exploded in the first two minutes. Is that bad?

[mkifyousayso]

same here bro. Im in 10th and havnt taken physics yet but am in advanced math and I have no idea what hes talking about lol

[Pentazoid111]

Is this senior undergraduate level quantum mechanics?

[SasquatchKiller]

This is a continuing education course (i.e. for people in the community at large who are interested and don't mind paying money to listen to Dr. Susskind lecture)

[Pentazoid111]

but is it a senior undergraduate level quantum mechanics class?

[SasquatchKiller]

I'd say its a introduction to quantum mechanics... only real requirements are an understanding of basic linear algebra... again, hard to compare to undergraduate courses I've taken because the audience is so different... but its definitely just an intro. Wouldn't call it "senior."

[ripperduck]

I'm taking QM now at the undergraduate level and his lectures are the same, except far better, then what I get now. QM as is being taught to me now requires linear algebra, but seems to be far more conceptual than math based right now.....at least at my level.

[DNHshuffle]

but if the electron gets a kick from the hole... isn´t it a measure as well? if the electron gets the kick, you could measure the impuls and say where the electron will hit the screen!?!? so, what has it to do with a wavefunction?

[eb227]

If you "measure" the impulse, the wavefunction collapses.

[keysle]

i've just been kinda listening in... is sthis still "high school" stuff...

becuase it wasn't HS for me... I'm not sure if it was a terminology issue or whatever... but I lost him after video 1

[testoviron]

Is that what you Yanks pay for at Stanford? (Ya can get better lectures for free in Europe!!!!!, BTW the dude on the videos is fuckin' lame....he wouldn't do shit without his copybook...and still he doesn't know anything on the subject!!!!!). Ivy league can suck on my cock!!!!!!!!.

[cyberflo999]

The "dude" you're talking about - oh and by the way i'm european - is still Susskind: the guy who discovered a lot of things at the basis of string theory. And without his paper, he would be able to do a lesson, just maybe not in the amount of time required.

And I know great american professors and very boring european professor.

You think he's a bad teacher? Ok, fine. Find a better one. (At least his lectures are available online).

But don't go posting that kind of nonsense...

[Euquila]

Very rude and without class...

I'm Canadian (like that matters) and I find him (Susskind) thoughtful, kind, and a master of his work.

[kalleandren]

Well, compared to my lectures in this class, Susskind rules. I had a respected European professor, he did know how to do research, but was terrible as a teacher.

[molinoj]

Great question session. Thank you Sir.

[ArjenDijksman]

great lecture and great questions session at the end. thank you!

[Calabrese9876]

this dude must be WICKED smot

[zaxlj]

Where is lecture 3?