Lecture 6 | Modern Physics: Quantum Mechanics (Stanford)

[jamma246]

If a vertically polarised particle goes through a 45 degree polariser (with probability 50% I think) does it come out with 45 degree polarisation, or does it keep its vertical polarisation? He doesn't make this clear.

[jamma246]

@jamma246 Oh, he answers this at 1:00:00

[loopantenna]

@jamma246 He explains it in the three polarizers experiment. The photon comes out with a 45 degree polarization.

[marcf999]

It is one of the most confusing and weak presentation of QM I have ever seen. The point is WHY DOES VECTOR ALGEBRA APPLY IN THE FIRST PLACE? He just goes (poorly too, see 1h25) on about the trigonometry... if this is what a first rate education is about, some folks are being ripped off.

[QuaternionEM]

@marcf999

OK...........well......vector algebra applies because we have DIRECTIONS as well as magnitudes in the probability-space in which QM works. The first freakin line of the lecture he makes reference to THE DIRECTION in which a photon is polarized.

THAT is why vector algebra applies, but then again I think Professor Susskind gives the crowd the benefit of the doubt and assumes that the vast majority of us can make that elementary connection on our own without him spoon-feeding it to us.

[marcf999]

@QuaternionEM I know what a vector is, my point was does vector ALGEBRA apply. For example, Velocity is a vector but decomposing the vector doesn't have any meaning. you are NOT going in the x and y direction at the same time, you are going in one direction. With forces however you can decompose them and get meaningful results. Susskind just goes into the algebra to calculate probabilities without really explaining the principle of superposition. For a good expose I much prefer Feynman.

[MrRobotoToo]

It's very illuminating to see the formalism of QM applied to a concrete system.

[bhigr]

There is one thing I want to mildly criticize of the lecture. Susskind says a lot of extremely important things, but he doesn't right them down. So taking notes while following his lecture is extremely difficult. This is amplified by the fact that he continuously erases stuff on the black board. It would be ideal if the students had a small booklet covering the fundamental definitions. So you don't miss anything while taking notes - you just add explanatory remarks to the booklet.

[bhigr]

Great question by the students. It's a nice example of the weirdness of quantum mechanics.

[RobotTed]

HOLY CRAP! MY MIND IS BLOWN... HE JUST CHANGED SHIRTS! O_O

I miss the shirt and I hope he didn't lose it. D:

Also, Quantum Mechanics is pretty sweet.

[QuaternionEM]

May be the photon is literally a physical wave-packet: An undulating blob of primeval material traveling along a pre-defined path in spacetime which is nonlinear and counter-intuitive from our size-relative reference plane? In other words, given enough particles they will attempt to take all possible paths of transit; The most direct paths have the highest probability. At atomic size scales the particles travel pre-existing wavelike paths. across

[bhigr]

@QuaternionEM The photon is indivisible. Therefore, it cannot be a wave-packet per se. The wave-like propagation is the propagation of the probability wave. "In other words, given enough particles they will attempt to take all possible paths of transit; The most direct paths have the highest probability. " This corresponds to the idea that lead to the Feynman path integral.

[jamma246]

@QuaternionEM What, and then when it's observed it all collapses again?

I suppose it's difficult at this level, what does "physical" actually mean? Does the wave function actually exist in this world in any real sense? Or is this just a tool to aid us in visualise something which could be actually even more complicated?

[QuaternionEM]

@jamma246

Yes, the wave function is just a probabilistic tool. WHen you deal with staggeringly large distributions you can't keep track of each element but you can get an AVERAGE for ALL of the elements. This is the basis of QM. You can never predict the behavior of a single particle because it's too small and too fast...to ethereal...but you can use differential equations to predict the average behavior for a particle.

[QuaternionEM]

@jamma246

Now imagine this tiny photon traveling across huge expanses of this sponge. From our frame of reference (size wise) it looks like a straight path, but from the photons frame of reference it's a chaotic roller-coaster type path as it navigates the sponge while avoiding the holes. Throw in extra dimensions and you see the world the photon lives in.

It's amazing that it can move anywhere, let alone at the speed of light.

[QuaternionEM]

@jamma246

I believe that at the photon's size-scale spacetime is porous...like a sponge...and the holes in the sponge are spaces "outside" of our universe. Maybe its REALLY porous like a sponge with LOTS of BIG holes.

4D spacetime in our universe is a huge huge huge giant thing...even from our human

perspective...now imagine a photon which is billions of times smaller than we are.

[einstein5311977]

Thank you Prof. Susskind for the lecture!!!

MABUHAY! from the Philippines

[ErichoEricho]

@einstein5311977 u from UP? :)

[MrJohnchios]

Hello from Greece!!!!!

VERY VERY VERY GOOD LECTURES

[olcclw]

There is a undergrad level introductory physics video that I have seen several times on cableTV. In it the professer demonstrats the light going through a vertical and a horizontal polarizers with a 45 deg polarizer in between. This material explains it.

[Taer11sho]

I admire Dr. Susskind-- His lectures are interesting and very insightful!

[simfu314]

awesome

[quantumxd]

Absolutely grand ! I wish to thank Dr. Susskind and Stanford for suppling everyone with this series free of charge I am humbled...peace

[chainsahw]

To me he looks like John Malkovich.

[icedearthfan21]

holy crap it's DR. George Carlin

[Darkfoster22]

6:00 sounds like a cloaking device lol

[JeeeNahi]

that was a very good lecture, nice and clear

[ArjenDijksman]

At 12:13 professor Susskind says: "it is quite hard to think of a classical setup that would produce the same kind of probability distributions and how they depend on the orientation of the polarizers". I've tried to explain how we could design such a classical setup. You may find it on my channel: quantum probabilities with ordinary objects.

[molinoj]

Thank you Dr. Susskind. It cleared a lot of doubts.

[ArjenDijksman]

Thanks for this sixth lecture! This one clarifies very well the basics of quantum mechanics. It's my favourite so far.

[davidwizard2006]

why the listener are all old nerds

[davidwizard2006]

great, it solve my biggest doubt

[mrbondisback]

these lectures are great. I love hus approach

[mrbondisback]

great