Lecture 6 | Quantum Entanglements, Part 1 (Stanford)

[MuggsMcGinnis]

Advice to the camera-person: It's less important to keep the speaker centered than it is to show what they are talking about. When he points to an equation, focus on the equation, even if only his hand remains in the frame. It's not like videoing a dance, you're helping people take notes.

[MuggsMcGinnis]

@MuggsMcGinnis At minute 36, the camera-person does it exactly right. Letting Susskind step into and out of the field while displaying clearly the entire contents of the white board.

[grunder20]

I am gonna work on this. GOOD VIDEO!!

[agapitoflores001]

I cannot relate much to this one. I can't say anything. But anyway thanks for the video.

[setnorm]

How wonderful it is to be able to see what he's writing again! (Unlike lecture 5).

[zeperf88]

Does anyone understand the Bell Inequality thing? How can the first electron be up at zero degrees and also down at 45 degrees? He said the second electron is positive 45 because the first is negative 45.

[MuggsMcGinnis]

@zeperf88 A given electron might be observed in different ways. You measure the spin of an electron with a magnetic field. The field orientation will always be observed to be either in alignment or opposition (anti-alignment) to the imposed magnetic field. If a beam of electrons are polarized to be perpendicular to the field, 1/2 will be observed to be aligned with the field (no photon) & 1/2 pointed the other way (photon emission).

[rmgorichanaz]

I'm getting tired of that one guy on the right who seems to delight in "showing off to teacher" how much he really doesn't know about QM

[TheGuyWhoToldMeToTel]

@rmgorichanaz It's the same dude in every lecture. I loved the one where he argued for 15 minutes in general relativity on whether or not you could buy a 1 farad capacitor (which had nothing to do with what Susskind was talking about). Asking questions is one thing, but if I were his classmate, I'd lock him out of the classroom.

[guillefix]

I am still not sure why measurements correspond to matrices. Is the number of possible eigen states, 1 or -1, the same as the number of dimensions of the space of states?

[PLecN]

@guillefix The eigenvalues are the possible values that could be measured. If the spin of an electron is measured, the result of the measurement will be +1, or -1. Also, you could have more than one eigenvector associated with a particular eigenvalue, so number of eigenvalues does not equal dim of space of states.

[twheeler76]

@san34451: These Youtube lecture series are somewhat mislabeled. The followup to this series (i.e. part 2) is simply titled "Quantum Mechanics," and what is labeled as "Quantum Entanglements, Part 3" is actually the first of a 2 part lecture series on Special Relativity. Wikipedia Leonard Susskind and scroll to the bottom and there you will find that somebody has made sense of how all of these lectures are labeled.

[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.

[bhigr]

There is one thing that is boggling my mind. The classical bell inequalities deal with a certain number of "classical states", which have the same property. The quantum mechanical "equivalent" deals with expectation values for measuring this property . The qm expectation values are somehow equated with the number of classical states.

[bhigr]

I wonder why he never mentions the term degenerate Eigenvalue.. I learned an Eigenvalue is degenerate if it has plural linear independent Eigenstates (Eigenvectors)....

[bhigr]

Although I already know most of this stuff, he is a great teacher! But, I guess if you don't know linear algebra, you won't be able to follow or it is going to be difficult.

[odysseus9672]

You can turn the inequality into a differential statement and find that it is violated most badly for small angles. IIRC, the inequality requires that the slope is greater than 1 and the initial slope of the quantum system is 0, or some such.

[fermista]

Water atoms, lol! Reminds me of the time when Young Einstein successfully split the beer atom :)

[myclicks]

Most high school science teachers misuse the words atom and molecule. They call most particles molecules, like, Sodium molecule, iron molecule. Although "a sodium chloride molecule" is excusable, technical it's not one.

[luzzie9]

Preparing elementary particles in *exactly* the same way may not be all that difficult. For example, if you line up a billion electrons in a magnetic field and measure their spin you would get the same value a billion times.

[luzzie9]

The comments about the water *atoms* were a joke but I understand true ionized atoms can be accelerated and thus would work as well as electrons, but probably the interference bands would be smaller.

[timcabc]

If we use water elementary particle (with only up and down spin as just like electron), I wonder the two slit experiment will give the same outcome.

[dmtr987]

The example of the physical experiment (with 3 populations of electrons), is it correct? I understand that in order to be correct all the 10000000 pairs of electrons have to be prepared in _exactly_ the same way, which impossible.

[keggerous]

are there water atoms?? hahaha

[openuniverse2003]

Seriously, you people need to hire professionals to videotape these lectures, ESPECIALLY to handle the SOUND. If you're going to post these lectures (and the assumption is that Stanford is putting its best foot forward), then you better make sure you're posting QUALITY.

[timcabc]

If you're referring to the horrible noise when student(s) speak, I think it has more to do with the microphone than the the person who record it.

[machobunny1]

The room mike is on a long wire. It picks up 120 hz from the fluorescent lights. The amplifier gain is high enough to amplify the hum. Can it be avoided, of course. Who cares, really? The real garble is provided by the professor's habit of starting definitions and branching a dozen times before completeing the basic statement.

[0101483]

Answer: the coefficients are not normalized.

[0101483]

In the 2 slit electron experiment, with unobserved paths, starting from point 0 on the left then to points +/-1, then to the symmetric midpoint 0 on the screen, say the normalized psi coefficients of the two paths are both equal to 1/sqrt(2). The probability of any given electron being found at the midpoint when either slit is open (but not both) is then 1/2. With both slits open, the probability of finding the electron at the midpoint would then be 2. What is wrong here?