Lecture 5 | Modern Physics: Quantum Mechanics (Stanford)

แชร์
ฝัง
  • เผยแพร่เมื่อ 20 ธ.ค. 2024

ความคิดเห็น •

  • @afarro
    @afarro 2 ปีที่แล้ว +18

    @0:15 Concept of a periodic line
    @2:00 (pure) Momentum wave function on a periodic line
    @5:40 Possible values of momentum on a periodic line by applying periodicity constraint
    @14:15 Stretching eigenvectors to substitute Kroniker delta with delta function
    @19:40 Outer product of vectors as an operator
    @24:45 Identity operator
    @27:50 Connection between momentum pure states and position pure states
    @28:20 Wave function definition (the position representation)
    @30:45 Probability of a general state to have a given momentum
    @37:30 Duality of position and momentum representations of wave function (Fourier transform)
    @49:17 Describing a wave packet
    @53:30 Bridge between classical and quantum physics based on wave packets
    @55:45 Incompatible observables
    @58:30 Polarization and the wave theory of light
    @1:05:20 Photons passing through a (plain) polarizer
    @1:09:00 Polarizer as both a measuring apparatus and a preparer of states
    @1:11:00 QM formulation of plain polarization of a photon
    @1:21:40 QM formulation of polarization with a 45 degree orientation
    @1:30:04 Calculating the probabilities for a 45-deg-polarized photon passing through a vertical polarizer
    @1:35:30 Observable associated with polarization in the 45 degree direction
    @1:38:55 Eigenvectors and Incompatibility of XY-polarization and 45-deg-polarization observables
    @1:42:26 Q&A

  • @cmfluteguy
    @cmfluteguy 14 ปีที่แล้ว +9

    Susskind is like most physics professors I have had. He loves his topic, and he thinks about it an awful lot.
    Physicists are so devoted to understanding things, that they understand how to make things understood!
    Ask an "economist" do describe a financial "derivative" if you want to see the difference!

  • @loopantenna
    @loopantenna 13 ปีที่แล้ว +3

    @jamma246 In fact the photon has equal probability of being detected along x and along y, which is 1/2. The probability to detect it along 45 is certain. Any possible state can be represented in any possible base. A base is formed with orthogonal eigen vectors.

  • @ManojKumar-cj7oj
    @ManojKumar-cj7oj 3 ปีที่แล้ว +14

    If you have taken linear algebra course this stuff will be more and more clear .
    I'm about to complete Gilbert Strange's L.A. course that's great as it can be

  • @rajaradi802
    @rajaradi802 4 ปีที่แล้ว +14

    Its an amazing oppurtunity to be at his lecture but these videos make it possible

  • @QuaternionEM
    @QuaternionEM 14 ปีที่แล้ว +2

    I've had some very, very good professors and Susskind is as good as the best of them...............at lecturing...................
    When he explains the nature of commonly seen factors (like 1/(2*pi) factor
    that occurs as we move from a Kronecker delta to a Dirac delta) it really is an epiphany for me.
    Good, good stuff..............I watch these lectures every night now.

    • @ВладиславФотин-г4т
      @ВладиславФотин-г4т 2 ปีที่แล้ว

      Нжогждннжржжржнжжнжнжнжжжнолгн жду но нжожжжжннжнннжнжндннжнжнжлжжнржОл д ржнжннжнжзлннжнжннжо до джнннн джнннн лннжжрнннжнжннжрлрднжжжрж по лн не Джоли жлжжннннлжннжнжжнннжлнжжнннлннжжнжжжнннжржнржожрржжрржнннржж да ллжоржжнжннннжжнжнннжж РД жннжжнжннжжжжнрж жлжжннннлжннжнжжнннжлнжжнннлннжжнжжжнннжржнржожрржжрржнннржж ржнжннжнжзлннжнжннжо Ол доз РД нжлнжнжджжжннжжнлжнжнннрджжеждржрр в ООО жннжжжннжнгжэе рннннжжнднжннжнжнжннжрзржннжлнжлрлнож ООО знннннрнж по не жлжоннжннжнжнжнннжннннжн до одного год и города и раз н нжнлжжннжжж ЖЖ жнжж Ол эжнжжжжнннжжжжрнж РД нжнжнжннжжжжжннжжжгжнжннжж ООО жржржнжнжжржнрнлрлжжнжнллжннжнднжнжжнжжжнжжн РД нжнлннжжнжнжжнжрлнржрллрлжрнржрнжнжжжл РД ржнжннжнжзлннжнжннжо оно хз Дж же на нжлнжнжджжжннжжнлжнжнннрджжеждржрр для родного нжнлннжжнжнжжнжрлнржрллрлжрнржрнжнжжжл не любит днллржнжрожржнжлллжжнжжллжлжлжрж Орджоникидзевский дорогие нжнлннжжнжнжжнжрлнржрллрлжрнржрнжнжжжл лет на лннл в олжожлзле дорого по на р РД Лондоне лучше лннжржж РД не он нжжжннлнжжрнрлнлжннлрррлжлллллржж ох для лого дннлжл Ол для людей ли у Лены лнжллж лжнлжж в день обращения до ллл ООО ТД не долго думая об лрлжлллллллллллрл ООО лучше любого лог либо они лучше не рисковать оно лежит ООО лидер долларов за любую со следующим ООО для ООО лучше чем на оплату долга ООО лидер ли ООН ООО Лололошки о людях л долг Ло в этом л

  • @inox1ck
    @inox1ck 5 ปีที่แล้ว

    1:45:00 the probability refers to multiple particles anyway , observable is for a single quantum object

  • @ytLeou
    @ytLeou 12 ปีที่แล้ว

    Question at 01:16:21
    The apparatus corresponds to P_⊕ is the horizontal polarizer in the video.
    P_⊕ |x>=|x> corresponds to no change to the prepared state |x> which is horizontal.
    P_⊕ |y>= -|y> literally means the state is changed to -|y> after P_⊕ interferes the prepared input state |y> which is vertical state.
    But P_⊕ |y>=0 because the measure device is a horizontal polarizer.

  • @RexGalilae
    @RexGalilae 8 ปีที่แล้ว +2

    41:40
    Oh. This usage of k instead of p makes everything finally line up on why the momentum operator is related to that wacky equation.
    the d/dx part extracts the k and the i from the wave function and we place a "-i"so that it cancels out to give a simple k psi(x). If you multiply throughout by hbar, then you have momentum!
    Interesting indeed!!😅

  • @RCrosbyLyles
    @RCrosbyLyles ปีที่แล้ว

    A Dyad is a tensor of rank 2. Scalor is rank zero, vector is rank one ect.

  • @ashoknaganur8551
    @ashoknaganur8551 หลายเดือนก่อน

    As i am seeing the lecture for the second time I am understanding well

  • @Entropy3ko
    @Entropy3ko 12 ปีที่แล้ว +1

    Experiments tell us that 1/2 of the photons with a 45 degree polarization pass through a vertical (90 degree) or horizontal (0 degree) polarizer, so... no, the probability can't be zero.

  • @negarashariastani1863
    @negarashariastani1863 4 ปีที่แล้ว

    In 16:22, he says Kronecker delta which is mistakenly written chronica in the subtitles.

    • @wafikiri_
      @wafikiri_ 4 ปีที่แล้ว

      Subtitles are machine-generated from the audio track. It is common to see a lot of mistakes in such texts: the subtitles generator doesn't have a really good "ear." It has been trained but the training is not perfect.

  • @realCevra
    @realCevra 14 ปีที่แล้ว

    @PanZajko all transverse waves have a polarization (p.). it might be simple (linear, in on plane) or complex (2d, circular as mentioned in this lecture).
    em-waves are mostly 2d-transverse, longitudinal em waves (plasma waves, without any p.) are rather a specialty and cannot be seen in free space for long.

  • @Rene-uz3eb
    @Rene-uz3eb ปีที่แล้ว

    Seems like the can't have momentum and position at the same time is just an arbitrary interpretation, when one is defined in terms of the other. In particular, how do you make conclusions about a single particle from a probability distribution of particles, you don't, really. I'm starting to think the quantization is just a mathematical construct that holds when wavelength is greater than some constant, just like in fourier transform you can reconstruct a signal exactly below a max frequency with a discrete number of waves. Admittedly this lecture is all i know about the subject so far.
    1:39:54 I think people confuse math with physics.
    Just like in classical mechanics, energy is simply a mathematical invariant of the system given by acceleration being proportional to the ratio of the masses.

  • @kamrannasir3871
    @kamrannasir3871 11 ปีที่แล้ว +1

    when he discusses about the polarization of the photon, he says that when the polarizer is set at 45 degrees, the photon emerges out with a zig-zag polarization (45 with either x or y, in the x-y plane, but when I saw his lectures on quantum entanglement, he told that in such a situation, a large number of photons pass through with x-polarization, and half with y-polarization, the average is that zig zag configuration (he explained a similar situation of an electron spin inside a mag. field.)

  • @dhruvb2689
    @dhruvb2689 7 ปีที่แล้ว

    1:51:19
    is P^ = |x>

  • @arthurmee
    @arthurmee 13 ปีที่แล้ว +1

    This series gets a great thumbs up from me. To be able to access such a valuable for free is something i shall forever be grateful for. I love this man's style and his accent. I'm European, so is this a New York accent?

    • @NeelTigers
      @NeelTigers 3 ปีที่แล้ว +1

      Yep..Brooklyn accent haha

  • @eustacenjeru7225
    @eustacenjeru7225 4 ปีที่แล้ว +2

    Learnt alot of quantum mechanics from these lectures from basic principles

  • @Rakeshkumar30
    @Rakeshkumar30 5 ปีที่แล้ว +3

    35:16 Aha!! Moment ,I always wondered where the Fourier transform came from

  • @science-philosophy
    @science-philosophy 6 ปีที่แล้ว +1

    why after 4 lectures we didn't reach to particle in a box, harmonic oscillator, i think he should divide the topic to titles and subtitles.

  • @marlonbrade9004
    @marlonbrade9004 4 ปีที่แล้ว +1

    Good day everyone. I just have a question, i think it is just simple but i just want some closure.
    Suppose i have a polarizer at angle 45 degrees and i send light to it. Now the state of the system can be written as 1/sqrt(2)* ( |x> + |y> ) . Now i put a second polarizer, with vertical configuration. Now according to the postulate, the probability that the light goes through is 1/2. My question is, if i repeat and repeat the experiment, with same configuration, there exist an experiment where there is no light that goes through? Please, correct me if i am wrong . Thanks.

  • @AonnrecordsBlogspot
    @AonnrecordsBlogspot 10 ปีที่แล้ว

    Interesting perspectives on said subject

  • @eustacenjeru7225
    @eustacenjeru7225 4 ปีที่แล้ว +2

    This gave me alot insight on the origin and use of fourier transform

  • @jianhuazhang8246
    @jianhuazhang8246 6 ปีที่แล้ว +2

    Are there lecture notes on this?

  • @ashoknaganur8551
    @ashoknaganur8551 หลายเดือนก่อน

    I could understand polarization very well

  • @enricolucarelli816
    @enricolucarelli816 2 ปีที่แล้ว

    I got confused in the last minute: If I use a x oriented polarizer I can only get one result, x oriented photon. Similar with a y oriented polarizer. So how can I set up a measuring device that can tell me if a photon is x or y oriented?

    • @rasmusturkka480
      @rasmusturkka480 ปีที่แล้ว

      Use a x-polarizer, if the photon goes through it's x-oriented. If it doesn't, it's y-oriented.

    • @enricolucarelli816
      @enricolucarelli816 ปีที่แล้ว

      @@rasmusturkka480 yes, but how do I know that there was an y-oriented photon, if it didn’t get detected? I mean, this apparatus counts the x-oriented photons, but it says nothing about any y-oriented photons. Between two x-oriented photons there could have been any amount of y-oriented photons, or none.

  • @algebra5766
    @algebra5766 11 ปีที่แล้ว

    This is wonderful. I am a mathematician, and it is interesting how fourier analysis comes into this....

    • @RexGalilae
      @RexGalilae 8 ปีที่แล้ว +1

      Al Gebra
      Engineer here and yes, me too! Just pops in out of nowhere!!
      Tbh, I've always felt that it might happen since it's all about waves since the beginning

  • @imegatrone
    @imegatrone 13 ปีที่แล้ว

    I Really Like The Video Modern Physics course concentrating on Quantum Mechanics From Your

  • @dhruvb2689
    @dhruvb2689 7 ปีที่แล้ว

    Why are states of quantum systems with countable states being represented by a vector space consisting of C^n (n-tuples of complex nos.), while the states of a system like a particle on a straight line (which can have an infinite no. of possible positions) being represented by a vector space consisting of complex valued functions ?

    • @joefagan9335
      @joefagan9335 7 ปีที่แล้ว

      Dhruv B the Hilbert space can accommodate both vectors (for discrete states) and continuous functions.

  • @selsebilx
    @selsebilx 9 ปีที่แล้ว

    Could anyone please prove the last quation at @47:30, he said it is easy but i am confused...

    • @RexGalilae
      @RexGalilae 8 ปีที่แล้ว +2

      Sapiens Sapiens
      Well, Psi (k) = e^[-ikx] so i times the derivative with respect to k will bring the -ix down to give x Psi(k).
      It's like setting up an eigenvalue equation of the form
      H (some hermitian operator) Psi = x Psi
      In this case, we know what the eigenvalue is so it's simply a matter of guessing the operator which turns out to be *i d/dk*

  • @TheVnom
    @TheVnom 11 ปีที่แล้ว +1

    Susskind, a light-wave electromagnetic graphist expert. 0:59:29

  • @IdentifiablePerson
    @IdentifiablePerson 13 ปีที่แล้ว

    How is this polarization working? A 45 degree light is passing through a vertical detector at probability half. Why so? Shouldn't the probability be zero?

  • @joshuablackmon3419
    @joshuablackmon3419 10 ปีที่แล้ว +1

    Before I continue I'm gonna go learn quantum entanglement. I got this far but its getting complex for me again

    • @RexGalilae
      @RexGalilae 8 ปีที่แล้ว +1

      Joshua Blackmon
      Since it's already been two years, i wanna know how it all went for you 😅
      If you found this too hard to follow, you must be really familiar with linear algebra and bra ket notation that he addresses in the quantum entanglement series lectures in the beginning.
      I thought that, in order to learn about quantum entanglement, I'd need knowledge of single systems and this is why i started here.

    • @paulnewton3556
      @paulnewton3556 6 ปีที่แล้ว

      Stick in....quantum entanglement is much harder -weirder than this. ....stick in you can do it!

  • @danielketterer9683
    @danielketterer9683 11 ปีที่แล้ว +1

    I can try
    See, it looks like the first post by luzzie was speaking on behalf of his teaching ability, and that he has made the process so simple that IF you cannot learn the watered down version, you have no hope of doing the real stuff.
    Whereas you responded to "Quantum mechanics is hard" or some variant thereof it seems, or do not think he is making it simple, presenting almost only the bare bones of the theory?
    Perhaps I can follow up, too, care to explain how?

  • @RexGalilae
    @RexGalilae 8 ปีที่แล้ว

    23:00
    Huh so basically |n> is a vector where the nth term is 1 and the rest, zero? If that's true, then all of this makes sense.

  • @AMANP2021
    @AMANP2021 12 ปีที่แล้ว

    nothing like that. Just sit and ponder over it. All one needs is patience.

  • @jamma246
    @jamma246 13 ปีที่แล้ว

    There is something that I don't understand:
    When he is looking at polarisation, he states that a particle polarised at 45 degrees is |x>+|y> / (root2). But following the logic before, wouldn't this be the mixed state that would be equal probability of being one state OR the other? Shouldn't you technically create a new state which is 100% certain of being at 45 degrees to remain consistent?

  • @AMANP2021
    @AMANP2021 11 ปีที่แล้ว

    I surely will. I actually commented on the part "... you never will". While lecture series like Susskind's are valuable resources, I personally feel he could make the presentation more intuitive. I have seen his lectures on relativity too. There also he focuses more on math and less on the physical picture. That is not to say that his lectures are bad. I said that even if you can't understand what he is saying, if one spends some time pondering over it then he will definitely get it.

  • @IdentifiablePerson
    @IdentifiablePerson 12 ปีที่แล้ว

    Yes, anything that is out of main line will always appear "crackpot zone" to most people. But if you are a scientists, and have given time to understand the logic, you should point out the zones in the blog site comments blocks. Thank you for visiting the site.

  • @TheVnom
    @TheVnom 11 ปีที่แล้ว

    the link for the playlist sends to the classical mechanics course :P

  • @joshuapasa4229
    @joshuapasa4229 6 ปีที่แล้ว

    i'm confused on the Fourier transform, I thought it was 1/sqrt(2 [pi] [hbar]) X integral(e^(i p/[hbar] x) X [psi](x))

    • @riccardolui3400
      @riccardolui3400 3 ปีที่แล้ว

      it's just a normalization factor, if you have hbar in the exponent then you plug it with the square root of 2pi so that the integral of (psi)(psi*) is 1

  • @IdentifiablePerson
    @IdentifiablePerson 12 ปีที่แล้ว

    If a 90 degree polarizer allows 45 degree light that means polarizer is not good, right? A high quality 90dg polarizer should not allow any light other than 90dg. If this is not true then maybe we should define what 45dg light is and what 90dg polarizer is.

  • @dumlink586
    @dumlink586 3 ปีที่แล้ว

    Ya i went to Stanford

  • @aslichg
    @aslichg 7 หลายเดือนก่อน

    Exact statement is mentioned at 19:40 >>>> "what the hell this mean" :)

  • @ashoknaganur8551
    @ashoknaganur8551 4 หลายเดือนก่อน

    I am getting interested to study quantum mechanics

  • @AlecBrady
    @AlecBrady 13 ปีที่แล้ว

    @PanZajko
    The polarization along x and y are mutually exclusive - if it's x, then it isn't y. But if it's x, then it might or might not be 45deg - prob (x'|x)=cos(x-x'), where x-x' is the angle between the two directions.
    The NOT-polarized state doesn't exist for individual photons - it's just what happens when you have a beam of light whose photons are polarized in random directions.
    HTH

  • @ytLeou
    @ytLeou 12 ปีที่แล้ว

    My guess is that the lecturer meant to design the polarizer as follows to allow both horizontal and vertical polarized states through.
    |
    --- ---
    |

  • @Emiffe
    @Emiffe 13 ปีที่แล้ว +1

    I wish I attended Stanford. :(

  • @badazzanon7060
    @badazzanon7060 3 ปีที่แล้ว +1

    And I brag to my friends how smart I am ! I just woke up to this playing somehow and I have no clue nor could I learn a clue as to what this guy is remotely talking about. I can’t even believe someone could understand this stuff. Wtf is a side widdler ?

  • @AMANP2021
    @AMANP2021 11 ปีที่แล้ว

    A teacher can never make you successful. Teaching method matters. Some are easy with abstract concepts and some are not. And just because someone doesn't share your opinion doesn't make one an idiot. Give respect and you will recieve too.

  • @IdentifiablePerson
    @IdentifiablePerson 11 ปีที่แล้ว

    TH-cam does not allow web address. So you can Google for uncertainty principle is wrong subhendu. This will find the URL.
    Thank you for trying to visit the site. If you cannot find, I will write to you directly using TH-cam email system. You can also email to me at subhendu.das@excite.com

  • @AMANP2021
    @AMANP2021 11 ปีที่แล้ว

    I am here because there are many subtle things I hope to learn. It's not like I memorize lectures and I never check it again. It is about concepts. Anyways why are you concerned about what I am watching? Find some useful employment.

  • @asontehunsthanshhl
    @asontehunsthanshhl 14 ปีที่แล้ว

    It'd be nice if these were organized by content

  • @danielketterer9683
    @danielketterer9683 11 ปีที่แล้ว

    site didn't work

  • @DasKrabbe
    @DasKrabbe 14 ปีที่แล้ว

    @bhigr
    ... Not really, you could explain that classically.

  • @joppadoni
    @joppadoni 14 ปีที่แล้ว

    lenny is a shit teacher. however its all we have outside of any tutionioned state. so i love him. i also love him because like most quality minds when trying to explain anything, be it obvious and easy or otherwise, he struggles sometimes to explain. repeated viewing usually gets it i think. i hope we get more and immmm glad for them. dont hate me coz i can say shit and love in one too long a paragraph.

  • @jamma246
    @jamma246 13 ปีที่แล้ว +6

    I want him to be my dad.

  • @AMANP2021
    @AMANP2021 11 ปีที่แล้ว

    care to explain why? I am a physicist myself and I believe he could explain in a better way.

  • @RolandElvira-l4y
    @RolandElvira-l4y 3 หลายเดือนก่อน

    Robinson Christopher Moore Donna Martinez Donald

  • @Postermaestro
    @Postermaestro 8 ปีที่แล้ว +1

    I'm triggered by some of the questions..

    • @RexGalilae
      @RexGalilae 8 ปีที่แล้ว +5

      Postermaestro
      They're pretty good questions especially after you know they're coming from old men attending a continuous course with little to no background in higher physics required

  • @vjmadhudath9482
    @vjmadhudath9482 4 ปีที่แล้ว

    John Malkovich

  • @JonnyBursa
    @JonnyBursa 13 ปีที่แล้ว

    @luzzie9 lol complex topic, get it?

  • @danielketterer9683
    @danielketterer9683 11 ปีที่แล้ว

    If you are a physicist, I would like to ask why you are here relearning rather basic material.. and not say doing something more physics y.. like watching "Charles Ramsey's Reward: Free Burgers for Life"

  • @IdentifiablePerson
    @IdentifiablePerson 12 ปีที่แล้ว

    see my site - uncertainty principle is wrong - one word.

  • @mechapple
    @mechapple 14 ปีที่แล้ว

    all hermitian operators are observables? 1:48:14. really?

    • @ruchi9917
      @ruchi9917 4 ปีที่แล้ว

      Yes because hermitian operators are the one operators which have real Eigen value

  • @Highley1958
    @Highley1958 3 ปีที่แล้ว

    This guy might KNOW a lot, but he is a TERRIBLE teacher.

    • @enricolucarelli816
      @enricolucarelli816 2 ปีที่แล้ว

      It’s a matter of taste. I find him to be the best teacher I ever met. And I met a huge lot.