Lecture 2 | Quantum Entanglements, Part 1 (Stanford)

[Baronstone]

Great school, horrible school band

[Eztoez]

I lost the plot when he started talking about vector spaces. WTF lol. Why is it so hard to explain quantum mechanics without assuming your audience holds a double doctorate in math from MIT?

[alecbrady]

@Eztoez I haven't got any maths beyond A-level, and it all seems quite simple to me.

[nitter93]

@Lodine66 All math and science courses are like that in college.

[Ymirheim]

@Lodine66 Actually I found most of the questions intelligent and insightful which is what you get when the students are not undergraduates.

[joeglimmix]

crazy sneeze 23:46

[cochisewolf]

Professor Susskind is excellent. I would have to agree with another comment, one sure way to improve the presentation would be to not allow questions to be asked until it was over. I had an instructor once who told everyone to hold off any questions till the end of the lecture at which time he would devote time to them.

[grunder20]

Excellent work if I may say.

[varunkryadav]

In the video it is mentioned that the new state of electron(turning on the magnetic field) depends upon the earlier state of electron. Can someone explain to me how and why?

[alecbrady]

@varunkryadav Doesn't everything depend on what its earlier state was?

[shenergy47]

Thank you Prof Susskind and Stanford for posting these amazing lectures on line.

[ericpaterson]

Quick question. Does the electron in the magnetic field release different amounts of radiation depending on the angle its situtated at? if so, then it seems there are many different states that it can be in.

It either releases no radiation, or there are many different amounts of radiation it can release. That does not sound like two states. Any help would be much appreciated, thanks

[pengfeiDu83]

@ericpaterson If I understand the lecture, what you are talking about it what you imagine in a classical way, but that's wrong, in reality or quantum world, you have only two states, photon released or not, which means two states only: either 0(no energy released) or 1(released).

[Samgurney88]

I tried giving this video a thumbs up... but I could only give it both thumbs up and thumbs down at the same time...

[av733]

Is this first year stuff?

[ukvaishnav]

@av733 not really but if you have done little idea of linear algebra and ket-bra notation (which is just the vector space in different suit) then go ahead you'll hardly have problem. If you don't then first finish the Quantum Mechanics lectures.

[alanthegreatestgreat]

love it

[Ogudaktig]

I think the "part"s in the title are nothing to worry about. It seems that at the end of each video is the end of the lecture, and everyone goes home for the day, and then comes back later.

[MEpianist]

Big THANKS to YouTube, Stanford & Prof. Suskind!

Although I disagree with your decision to edit out the questions...

[TheOvermaster]

I'm pretty sure the guy asking the most questions is the same guy from the C.M. course - I'm glad they turned up the volume on questions so I can get a sense of how stupid he actually is.

[TheOvermaster]

@HyperBorealOperator Correct me if I'm wrong - but I believe it was a statistical mechanics course - so there is no loss of continuity by skipping (in this case, we can't watch it) part 2.

[TTB711]

We live inside the body of our God. The earth is an atom in His body. Our solar system, a cell. The universe, His DNA make-up. We look at the sky for God, when He is the sky.

[abdullahnaeemamalik]

Why are the weights to the basis complex numbers?

[SZDarkhack]

Here's a question, when you prepare the electron for measurement, how do you know if it has been actually prepared to the "direction" you want? I mean, using the configurations on the board (vertical measurement and 45 degree prep), when you prepare the electron it should (if i understand correctly) again have only a *chance* to actually "point" where you want it, and a chance to point the other way. Do we just wait for it to emit a photon for however long it takes?

[ArcologyDesigns]

@SZDarkhack dude I got a better question. When you prepare the electron measurement, how the heck do you know what you're observing? There are billions of molecules composed of trillions of atoms which have that times 10 to the power of whatever huge number electrons (except hydrogen of course). So how do you just look at an electron, AN electron, not a gazzilion particles. Idk if that makes sense but it confuses me.

[SZDarkhack]

@ArcologyDesigns That you can achieve with multiple ways. The "easiest" would be a vacuum (obviously) into which you shoot gamma rays until a pair of an electron and a positron materialize, then you guide them away from each other (so that they don't collide and disappear) using a magnetic field. You can "see" that an electron has been created by the increased load on the electromagnet (it gives energy to the electron). Then you guide the electron to a chamber using electric and magnetic fields.

[electrophiliac1]

A bit off-topic but his voice is really similar to George Carlin

[GROGANIOUS]

Quantum states are always superposition of states and therefore a Hilbert space must be used.

[whactya]

Is there a fundamental reason why the observable states should always correspond to a (vector space) basis for the quantum states of a system?

[whactya]

At 28:35 the question was misunderstood by Prof. Susskind it seems. I believe the student wants to know the detailed process of emission of a photon by the transition of spin direction. Perhaps the professor did get the question, but a clearer answer would have been `I don't know.' I assume QM does not specify a process for the emission, just probabilities of an emission based on strength of external magnetic field.

[GROGANIOUS]

@whactya There is only one photon produced for whatever transition or none at all. I think he has explained this previous.

[1HumanKind]

why are such stupid people allowed into Professor Susskinds class?

[TehAl3X]

@1HumanKind because everyone is entitled to a chance to learn

[1HumanKind]

@TehAl3X

Indeed. In that case the class should be filled with retards who continually re-ask the same question. They are entitled to the (wasted) chance to learn.

Now then. Do you have an answer to my question, or are you on your way to that class?

[TehAl3X]

@1HumanKind why are you such an asshole?

[TehAl3X]

@1HumanKind To answer your question: everyone is entitled to a chance to learn.

[JoeyMars1]

Some of this is cool but all I see out of all this is just numbers! Where is the fun stuff like the realm of mystery and demensions and stuff?

[jimmyshitbags]

@JoeyMars1 im sure you have to build up from the simple foundations before you get into anything weird

[checklic]

shmector...lol

[captainhampton]

How did the people in this class....get in this class? I love Susskind, and I know he's fully capable of teaching at a faster pace. Wish his class would allow that.

[mnfchen]

This lecture series is a bit slow, but gotta be patient. I went through Linear Algebra in college, and these people obviously haven't =/ Great lecture btw!!

[BeautifulBluez1]

Ok, I read further down on the comments, answering my question about his illustration for the electrons orientation in space. An electron is usually being pulled one way or another and if it needs to 'move' a photon (radiation) is emitted. After the photon has been emitted, where does the electron gain another one? I am at the basic level, so sorry if silly questions :)

[BeautifulBluez1]

I am interested in this topic, but a little confused on his illustration of setting an electron to a certain position. Now, if an "electron" is a sphere, how do you go about 'setting its position/orientation' in space, if an electron has a negative charge, which 'side' is which?

[luisescobariii]

@BeautifulBluez1 If I understood correctly, the electron is attached to a directional vector or "spin", this vector is what you position not the electron itself.

[DoubleHeIix]

@luisescobariii Well sorta yes. The electron (or any other particle in fact) is not a spinning ball, or whatsoever. since we know that particles are like a propagating wave, its better to imagine the spin as the circular plarization of a wave. But dont take that by word. The "rotational axis" of the spin is not bond to the waves direction of propagation.

[san34451]

Does anyone know where to find "Quantum Entanglements, Part 2"? Was it video-recorded? Only Part 1 and Part 3 are available on YouTube. StanfordUniversity did clarify that Part 2 "is unavailable at this time", but that was an year ago.

[gavfitzy2000]

this guy knows his stuff but gets a bit frustrated with teaching

[Kisa8092]

Goods

[oringent]

wow, how did these ppl make it into stanford??

[Zee96969696]

jesus crist i have such a difficult choice to make, do I study psychology, neuropsychology or physics or maths? and how and when am I gonna study chemistry? when am I gonna get a job? this is very difficult choice I donno what to do

[oringent]

@Zee96969696 do math. if you are not smart enough for math do physics. if you are not smart enough for physics do chemistry... neuropsychology...psychology

[Zee96969696]

@oringent fuck you

[reticulum78]

@oringent I guess you are not smart enough to do any of these otherwise you would respect science.

[TurKdiRty]

@Zee96969696 choose the one you know least about but the most interested in. you want ot be challenged, whilst being interested.

[jcbmack]

These lectures accomplish in a gentle way teaching the fundamentals.

[ssss133hotmailcom]

Why are his lectures so slow? Why is the audience asking so easy questions? Does the audience have no mathematical background? When i heared quantum mechanics lessons in germany the professors were all explaining at least 5-7 times as many things in the same amount of time. Are there any faster lessons? its really hard to listen to such slow lessons. its nearly as bad as the boring and bland stuff engineers hear.Can you suggest some faster vids about maybe more advanced physics? any QED maybe?

[ldl1949]

also I have questions for him:

1. if the electron is pointing up (to the south)? and then you reverse the magnets the electron will release a photon... and also by what I understood from him if you keep reversing the magnets you will always get a photon because it definitely moves. and in the states between there is a variating probability then maybe we can not measure the amount of energy in the so called ""single photon released""?

[ldl1949]

also I have questions for him:

1. if the electron is pointing up (to the south)? and then you reverse the magnets the electron will release a photon... and also by what I understood from him if you keep reversing the magnets you will always get a photon because it definitely moves. and in the states between there is a variating probability then maybe we can not measure the amount of energy in the so called ""single photon released""?

[ldl1949]

why does he not explain anything about how any of this was found and measured?!

[QuaternionEM]

Jeez.... I don't want to be mean but some of these questions are killing my Feng Shui so to speak.... I'm having these beautiful epiphanies listening to this, and then around 0:21 the questions start and totally ruin the vibe... But that's just me. I've had Complex Analysis, Vector Analysis and Random Processes ......hell I even had Finite-Element-Analysis.... .....so I'm trying to be nice about the questions... I think Susskind likes the questions anyway..........but I don't

[fragmentsoferik]

I don't understand what he says around 1:21:00 that the probability for the observable (which is either + or -) is [P+] - [P-], which he calls the averageness of headsness or tailsness..

That should be 0,5 - 0,5 and I don't understand what he wants with that..

Sombody please explain..

[kadigan1979]

@fragmentsoferik It was just a very simple example of assigning a value to each result of a test. Assigning +1 and -1 to each state is arbitrary. It's not of much interest in the case of a coin, other than to say that the average expected value is 0 - you see the same number of occurrences of each result, which sums to 0. A more interesting example might be if you bet on the coin and receive $5 for a head (P+) = 5 and lose $10 for a tail (P-) = -10, in which case your average value is -$5.

[fragmentsoferik]

@kadigan1979 Thank you for explaining! I think I got hung up in the wording: "probability for the observable", thinking this translated to "probability for a given observable", in this case: "probability for observable A (P.heads=0.5) and probability for observable B (P.tails=0.5)". But as you say what he means is "the average value of the observable". (I still don't quite get how "probability for the observable" is a good way of describing this..) Thanks again.

[Fussinated]

Reverse the polarity? ... again?  Captain Jane way must be in trouble again.

[MaximumPopcorn]

Someone should compile a best of Leonard Susskind drinking coffee video.  : )

[dweeper1020]

QUESTION:

If we prepare the electron to be horizontal and put it in a vertical magentic field, there is a 50% probability that it will emit an electron.

 If it does, we know it is now aligned with the magentic field.

If it does NOT, what alignment is the electron in (or can we ever know).

Please only respond if you KNOW if the answer, not if you think you know it. I'd like to know.

[shakagenghis]

do you mean 50% probability it will emit a [photon?] In the same camp, would love to know. Sorry to respond without knowing, but it will cc me when you get an answer, and I'm pretty sure you mean photon.

[dweeper1020]

Yes, oops, meant "emit a photon" not an electron.

[shakagenghis]

conceptually I am having a hard time understanding how an electron can be "pointing" up or down. Is anyone qualified capable of humoring me. Please don't reply if you don't know, armchairs scientists like myself.

[dweeper1020]

Don't think of the electron pointing up or down (this is just a convenience he uses), think of it as the spin state - spinning around it's axis (either spinning in one direction or the other).

All elementary particles have a spin quantum number. These spins create a magnetic dipole moment that "point" in a certain direction depending upon the direction of spin.

You can find more details on spin state in wikipedia.

[shakagenghis]

conceptually this is very helpful.  up/down could be yellow/purple- some states x and y that are connected observationally to a certain behavior. Very helpful. I really should have watched the whole lecture before I asked the question, but nevertheless I appreciate the help.

[LuckyStarsClan]

Rofl. Trolled by fisics.

[learnerL]

Free education online is a real gift. Thank you. Could you recommend a reading list with your physics courses (perhaps included in the course blurb).

[Mattprole]

I wonder if Mr. Susskind ever visits his own videos to see how his viewers respond? In case he does, you are loved sir! ; really excellent content you provide.

[MrHunter920]

@Mattprole im guessing its actually Dr. Susskind

[MunchVids]

At 4:13 his stick looked rainbowish.

[xmksq]

Hi, anybody knows where THE SECOND PART of the course could be found?

[quantomic1106]

the lecture is great but the video quality is poor, I'm having a hard time reading the board.

[fassterblade]

@quantomic1106 get a better monitor

[quantomic1106]

Its not my monitor, I got a samsung 42" LN46A650, 4ms response time, 50,000:1 contrast ratio, with 120hz refresh-rate. Its Connected via DVI-HDMI to an NVIDIA GTX 260 GPU in my system. But yea I can see the board okay, but poorly when the camera is moving or zoming due to bad video quality.

[fassterblade]

just fuckin' with you guy...it's cool

[quantomic1106]

aight..

[luzzie9]

I finally see the difference between spin space and vector , pointer, space. Why could not other writers explain this? Susskind makes it seem easy. Incredible!

[pzmtuthcvpvl]

Mr Susskind does an exemplary proof of explainations can be better, I note for the record that in the previous ( Lecture 1 ) it was stated that all attendees were there because they wanted to be. We see this often in lecture circuits. Also, I just found what appears to be an exceptional book on this: "FIVE-DIMENSIONAL PHYSICS" ( Classical and Quantum Consequences of Kaluza-Klein Cosmology ) by Paul S Wesson { imprint: World Scientific Books }

[wladwlad]

a huge THANK YOU to stanford for making these available, i am fascinated both by the bold step and by the content. finally spending hours on youtube will be actually worth it :)

btw. shmector really rules :)

[SolidState92]

lol, schmector!

[sponsoredwalk1]

LMAO @ Hermite's eating polynomials!!!

[pete2331]

I'm currently working my way through the lectures of part 1. Does anyone know if there is a part 2 around somewhere or does it just move to part 3? I just don't want to miss out on any of it if I can help it.

Thanks

[stanforduniversity]

Part 2 is unfortunately unavailable. However, the content presented in each of the parts should be self-contained.

[neobadandy]

@stanforduniversity any reason why it's unavailable? will it become available? lost? destroyed? never filmed? top secret? need to pay for it?

[MrBartusX]

@stanforduniversity Is there anyway we can get part 2?

[YoLninYo]

@pete2331: I think part 2 of quantum entanglements is now avaialble. Look in the playlists section of the StanfordUniversyt channel.

[ronljudd]

@pete2331 now available

[1yamaboy]

@pete2331

From Prof. Susskind's intro in this video, this appears to be the second part - I think it has been labelled incorrectly.

[qigong1001]

I question the intelligence of those against questioning.

[cochisewolf]

I've had a few classes where there were people, usually one or two, that asked so many questions that we couldn't get through the subject. So those of us who actually studied and met the prereqs suffered. I agree there should be a Q&A period after class for those not resourceful enough to find the answers on their own.

[gricka31]

this is one of the best of this guys lectures. shame about the audience,

they should just stfu, listen and stop asking irrelevant questions

[ff7522]

I must agree, some of them come off as little pricks.

[tcb666]

I must respectfully disagree. Scientists are not oracles in ivory towers who must not be questioned. Science is all about asking questions, some of which turn out to be stupid. I do agree that some of the interruptions of this _particular_ lecture are just goofy.

[gricka31]

of course, only idiots don't ask questions. it's just the manner of the questions which annoy me. they ask a question and then use that opertunity to keep talking about how amazingly clever they are and don't give him a chance to explain anything

[timetraveler2006]

nothing wrong with questions.

do YOU know everything? the mere fact that you're ranting against questions makes YOU an eyesore!

[gricka31]

haha, eyesore. in my comment i said there was nothing wrong with asking comments. maybe you misunderstood me

[cochisewolf]

You shouldn't allow students to ask questions. That's what books are for. They slow down the momentum of the lecture and for the most part the questions demonstrate their laziness. The questions are ill conceived and could be worked out if they gave them some thought.

[gricka31]

well questions are always helpful, often for multiple people, but they should be asked at the end of the lecture, because they are often resolved later on in the lecture

[Perrickan]

Leonard Susskind drinks a lot of coffee.

[lildog908]

@Perrickan my science teacher looks just like him but drinks 3 times the coffee he does

[barrackisastupidnig]

He looks like a late Bobby Fischer.

[liawol]

So let me figure this out. If you prepare an electron in a 5 degree axis regarding to the north pole and then measure it you get either a photone or not. If you prepare it 100 times in the same axis you got a 50% chance to get a photon or not? Does the degree of preparation axis effects the statistic result of the experiement or is it always a fixed 50% ratio?

[vanchuck]

Not quite-- if you prepare the electron so it is aligned perpendicular to the magnetic field, when you apply that field it would have a 50% chance of releasing a photon.

If it is prepared so that it is 5% away from magnetic north in the applied field, it will very rarely emit a photon (I'm not certain, but my guess is that it would emit a photon far less than 5% of the time). But probabilistically, it COULD.

[Aryanpars]

Total Awesomeness!!! :P

[F00dTube]

At around 1:27 on guy in the audience makes the remark that P(+) - P(-) isn't the expectancy value and that when both Probabilities are equal the total probability is 0. The formula should be 1P(+) + -1P(-).In this case it works because the absolute value of each possibility is 1.

[F00dTube]

huh nice 1:27:00

[Mazmatickz]

P(+)-P(-) is the same as 1P(+)+-1P(-).

[F00dTube]

Correct in this case it works. But what if we had a die(1-6). Than the outcome would be very different: 1P(1) + 2P(2) + 3P(3) etc. So only in this case, with only two possibilities and with 1 and -1 having the same absolute values, subtracting the probability will work. So the confusion for the student is understandable.

[Mazmatickz]

Yes, in this case it works. For a die(1-6) the expectation value is 3.5 which is also the average value. You can assign arbitrary numbers to represent the possible outcomes, if the probability of each and every outcome is equal and adds up to 1, then the expectation value is the same as the average value of the representational numbers. I think maybe the confusion of the student was caused by that he misunderstood 0 for being a probability value.

[F00dTube]

Exactly my point.

[Glenn001au]

Thank you for the great vids Stanford and Leonard Susskind!

[istoleyournikes]

i've been sittin here gettin stoned for the past few months first watching physics for future presidents from some dude from berkley and now this dude. i love this shxt. but im in electronics class and it seems figuring out total resistance and voltage drops of resistive-inductive-capacitive circuits is more complicated than this stuff. and thats easy. im waiting for this to get hard.

[mdiem]

If it's like most science classes I've taken, it doesn't start to get hard until over half-way through the course.

[Kingoftorts1]

dido with both

[teknos1s]

what the guy below me said.

[bear630]

Thank you for this. I cannot express to you how much I am enjoying this series of lectures. That they are available here is wonderful for someone like me.