More on superposition. General state of a photon and spin states

MIT 8.04 Quantum Physics I, Spring 2016
View the complete course: ocw.mit.edu/8-04S16
Instructor: Barton Zwiebach
License: Creative Commons BY-NC-SA
More information at ocw.mit.edu/terms
More courses at ocw.mit.edu

[@justindrew7401]

This guy is a genius teacher. As a teacher I can tell you this is the result of great intelligence, experience, and mastery of the subject.

[@adrienrandall9483]

i realize I'm kinda randomly asking but does anybody know a good website to stream new series online?

[@jimhalpert9898]

yes I do

[@joesmith8288]

I like how much time he spends on clarifying the superposition principle and measurement.

[@thisisFaye1]

he got a hair cut

[@MrDroenix]

Haha that was my first thought too

[@endogenic6913]

@@MrDroenix same

[@ChrisShawUK]

:) my first observation too

[@ldanielfch]

Yes

[@eightysevenf]

Lolol thanks i was like.. something’s different i’m confused 😅

[@drdd333]

I thoroughly enjoy this gentle teacher. Brilliant.

[@nst1058]

Awesome mathod of explaining the theory, respect from India🙏

[@brainstormingsharing1309]

Absolutely well done and definitely keep it up!!! 👍👍👍👍👍

[@andressalvadorsartarelli6393]

Muy buen docente. Explica con mucha claridad

[@tangye8243]

Mind-blowing!

[@sayedahmed7465]

Excellent way of teaching , absolutely fascinating !

[@mona82]

Thanks MIT

[@zack_120]

These Superposition lectures did not explain the mechanism how a particle can be in both positions at the same time but only talked about what it is which is already known. It looks really weird, even hard to teach by experts. By comparison, the entanglement topic is taught clearer in a simple but convincing eq.

[@ryanrizzo3866]

Excellent Lectures! Very helpful.

[@nimeshdwivedidwivedi1530]

Wonderful session sir proper understand thats superposition sir

[@georgesbegon4252]

Données claires excellent professeur l écriture est très lisible ce qui est déjà révélateur le tableau est bien filmé quant à la validité des théories je reste très modeste pour ma part vu que l on a constaté tant de remise en question depuis Newton ..

[@p.s.design4338]

Really great lectures.

[@Mizraab2912]

But you have to look in another direction-Such a nice line to convey that perspective matters

[@brandomiranda6703]

how did he resolve Einstein's paradox (e.g. how do we know if half were down already before the measurement rather than this superposition)?

[@akindeleoduwole1094]

Quite enlightening and helpful. Thanks. From Nigeria

[@joseluisrodriguez2605]

Exelente mi profe saludos desde la fiee lima peru especialidad ing electronica de potencia

[@mareksozolins6347]

Good old days at school i have getting sleep to listen to teacher -)

[@kaushaljain5999]

2:04 to 2:09 _ how will we get state minus A or iota A by superposition of A by itself. Since to get state 2 A is obvious by superimpose one A by another A.

[@kaushaljain5999]

4:41 to 4:57 in previous lecture L1.4 you have said that after polarization state becomes either or . There is no significance of cosAlpha or sinAlpha coefficient which is in front of them. So after polarization there should be no or One real parameter which is no. "1". Another Query_If there is one parameter then before polarization there should be three real parameter (one from real no 1 and two from complex no beta by alpha)

[@theconstellation__]

If a photon is polarized in x direction by a polariser ,then what will happen when it passes through another identical polariser, as on Photon level it's not predictable, so do all the photons come out or ,they again come out in some probability

[@surendrakverma555]

Very good 🙏🙏🙏🙏🙏🙏

[@furkanunsal5814]

11:07 Up allons y and down allons y

[@saskiavanhoutert6081]

Very clear explanation, thank you very much Einstein was right, your explanation also, kind regards, Saskia van Houtert engineer/office-manager graduaded at the Graphic Lyceum, the Netherlands 29 june 1995 as a Graphic Technical Engineer. And graduaded at 19 july 1997 as a office-manager. at the Economical Administration Education ECABO-BOVA at Breda. So I have 2 profession possibilities; engineer/office-manager.

[@elsayed111]

Dear Sir, in case of super position u tl about polaristion in two diretion where in the previous lec. u told that only one direction is the valid and other vanished, THEN HOW COULD YOY TAL ABOUTH SUPERPOSITION OF BOTH OF THEM THANS

[@kaushaljain5999]

1:47 to 1:52 Why is it physical assumption? Because i think it seems obvious.

[@kaushaljain5999]

8:04 why have you given thanks at this time?

[@jorfoas]

I would really appreciate if the teacher labeled axes properly. For example, in minute 7:23, I am assuming that both X and Y axis represent electric fields, but that should be clarified.

[@magnfiyerlmoro3301]

i dont understand when he says, for 2 complex parameters there is 4 real parameters, thanks for explaining!!

[@fcwong1]

alpha = a + bi beta = c + di. a, b, c, and d are the 4 real parameters.

[@eshankvaishnavi3821]

fcwong1 thanks

[@haiekmohammed6548]

Thankx professor from morroco

[@jessstuart7495]

My gut tells me using the two measured outcome states (spin-up and spin-down) to describe the particle's state before measurement (superposition) might not be the best approach.

[@phononify]

Would the argument hold also for the Hydrogen atom (assuming a nuclear spin of zero): Would the state of the bound electron would be also N*(|up>+|down>), or is the ensemble of that H atoms a mixture of up and down electron spin variants ? :) How could somebody distinguish these two cases experimentally ? Could you ? Thanks for your opinions.

[@cakeathon9983]

I'm not sure what you mean by "N*(|up>+|down>)", the electron of each atom will have spin up or down with 50% probability. The experiments for spin are done on the 47th electron of silver atoms, the same thing should apply for all atoms. That being said, I don't think it can be done experimentally, it is just assumed that what is true for silver atoms is true for all atoms. The problem is that orbitals cause an angular momentum which dwarfs the angular momentum of electron spin, so you need large atoms for which the angular momentum due to orbitals cancels itself out enough to detect electron spin.

[@mishalkiranreddy1214]

Sir what exactly a state is?

[@brandomiranda6703]

Does he ever explain what "physical equivalence" really means? Is the rescaling valid due to quantum probability measures adding to one?

[@vwcanter]

Yeah, it’s just a consequence of the same linearity he spoke of. If A and B are solutions, then A+ B is a solution. Therefore, A + A is also a solution. So 2A or 3A or iA or -A are also solutions. I think you could think of it as a result of probabilities adding to one, because he describes it as “normalization”. But someone else can perhaps explain that better.

[@guitarttimman]

Look up schrodinger's equation.

[@brandomiranda6703]

Also I wasnt super convinced about his argument of how we really know they are in a superposition rather than apriori being 50-50 split, does he explain it better latter or did someone catch that better than me?

[@mohamedakrout9742]

This lecture kzbin.info/www/bejne/oouWk4OLgNRosMU of the same course (taught during another semester) explains exactly how and more importantly why there should be a superposition.

[@PrivateSi]

I don't get it, he starts by saying two photons in superposition don't add or substract from each other, yet we know they constructively or destructively interfere.

[@ryanjbuchanan]

How beautiful the internet is. If only Einstein and Newton had such tech.

[@abdurrauf7525]

Respected Sir, which books you used for these letters?

[@jessstuart7495]

Zwiebach recently published a textbook _Mastering Quantum Mechanics: Essentials, Theory and Applications_ , MIT Press, 2022

[@abdurrauf7525]

@@jessstuart7495 thanks

[@sauravmittal538]

how cn i build a new quantum state by superposition of spin of two electrons

[@GrandGobboBarb]

11:19

[@joedarkmatter5051]

1:05 After he said "strange" , i see Dr Strange in him. Or is it just me =)

[@silberlinie]

0:49 best. How he stands there, how the many contradictions expressed by others to what has been said here in his thoughts rush past. And he then stomping his foot inwardly and he defiantly arrives at an SO.

[@stormtrooper9404]

May someone explain what is IA> ? And how possibly IA> ~ 2IA> ~ -IA> ~ iIA> How can one "state" be equal to that same state x2?

[@compilationsmania451]

1) i is a complex number equal to square root of -1. The teacher explains that in the previous lectures. You should start from there if you haven't seen them. 2) In a superposition equation, the states refer to the possible results and their coefficients squared are proportional to the probability of getting that state. So, the coefficient just refers to the probability, not an actual physical quantity. So, if you have a single possible state, then it doesn't matter what coefficient you add in front of it, physically it doesn't make a difference. To give you an example, if you take a bag with any number of red balls in it, if you choose any random ball from the bag, the probability of that ball being red is 1 irrespective of how many balls are in the bag, because there are only red balls in the bag.

[@afifakimih8823]

He is just not a good Professor,and an excellent teacher,he is one of the leading string theorist as well !!

[@samybouzida7956]

Your comment could be misunderstood. Let me help you out here: ´´He is not just a good Professor, and an excellent teacher, he is also one of the leading string theorist as well !!`` The location of the word ´´just´´ in your sentence could lead to understand that you actually didn´t like the guy. We all know what you meant. Hope you won´t find me snob or anything. I 100% agree with you, this guy is beyond brilliant.

[@bobbwc7011]

But string "theory" is scientific trash and should have been abandoned decades ago.

[@themasstermwahahahah]

he looks a lot more whimsical with the longer hair

[@rai2134]

How can we design a superposition state of spin with probabilitis of 40% and 60%?

[@schmetterling4477]

We change the strength of the magnetic field.

[@nickknight5373]

He seems to be talking at a similar tempo whatever the playback speed.

[@redberries8039]

6:50 beta/alpha .... '2 real parameters' or gamma 'one complex parameter' ...How are we getting the complex from a division of the reals? ...and I thought the alpha beta has to be complex anyhow?

[@kaushaljain5999]

Explain 3:19 to 3:29

[@brandomiranda6703]

why does scale affect the physics?

[@anonymous.youtuber]

When you do an experiment in classical physics you act on trillions and trillions of atoms at the same time and you only measure some average result that is the sum of many individual results. The atoms don’t scale, there are just many of trillions of them. You can’t grow an atom the size of a football and experiment on it.

[@jamesfullwood7788]

It's amazing how long a physicist can go on talking about something without even defining it (i.e., the wave function). If I was one of the students in the class I'd be like "Ok great, but what IS |A> ?"

[@flygonfiasco9751]

This is a continuation of other lectures, not the first day of class.

[@jamesfullwood7788]

@@flygonfiasco9751 I watched all the lectures leading up to this one....

[@flygonfiasco9751]

James Fullwood Rewatch L1.4. In the latter part of the video, he goes into depth about Dirac notation describing quantum states using a photon as an example.

[@jamesfullwood7788]

@@flygonfiasco9751 He still doesn't give a precise definition of the wave function. In mathematics, a function is a relation between two sets, one called the domain (the set of inputs)and the second called the codomain (the set of outputs). It's obvious that the codomain of the wave function is the complex numbers, but he never defines the domain.

[@m.s.6449]

Defining something before you know anything about it is impossible. First you learn of the existence of some new object, investigate its properties and only then, based on acquired knowledge, you can invent a definition for it.

[@kaushaljain5999]

4:04 meaning of QM state

[@timmy18135]

11:10

[@andreeapopa9635]

can he teach our online classes? he's a much better teacher than ours LOL. you should always focus on the explanation instead of reading your PDF @my prof. *cough cough*

[@mp3lwgm]

The title is misleading. You do not discuss the spin states of the photon. Instead you Segway from the polarization of the photon to the spin states of the electron; awkward !

[@elhaditalbi346]

🌎

[@kaushaljain5999]

9:27 Explain

[@brandomiranda6703]

does he depend what physically equivalent means?

[@davidwilkie9551]

Spin, the concept, is a particular identification of probability positioning states in the context interpreted Superspin Modulation, the Conception of the infinitesimal sync-timing, of zero difference synchronicity universally distributed by the mechanism suggested by Wheeler and Feynman applying to the "One Electron Theory", an infinite-closed loop equivalent to the meaning of a Black Hole Singularity Conception of inflation-collapse singularity = Eternity-now +/-.., the actual circumstances of time duration timing, QM-TIMESPACE. (Requires a pictorial observation technique/imagination) If QM-TIMESPACE e-Pi-i in-form-ation of Time Timing is a Holographic brane, an objective of resonance imaging that "floats on nothing" (Avagadro's? No, Archimedes principle is the same positioning here-now forever Principle as Superposition-Exclusion holographic image projection drawing. Multi-phase reciprocal-resonance-> "loop" between micro-macro vanishing point

[@bobbwc7011]

You need to stop taking drugs. Your comment is insane rubbish.

[@SfhjjFghjki]

For sy contest sup calculate Never RE MINDED ,,QUESTION QUALITY WE HAVE ,WHERE A SUBJECT, 2()+ ing¿ Like similar EYES ARE GOING STAGE // «««««| NEW SWORDS ,IM XOING AND WHAT // Energy free ,quanta Est MYTH DES PROBS ,

[@scimad]

1.25x

[@elosant2061]

when he says "superimpose" doesn't he mean "superpose"?

[@bustercam199]

No, this is incorrect.

[@jpzhang8290]

Laudau-style teaching. I like it.

[@ken91656]

It looks like a circular argument to me.

[@ericreiter1]

I retract my comment. I just have experiments showing qm fails.

[@ericreiter1]

What a terrible professor.

[@mubeendewan2287]

how so?

[@ajaymishra7212]

Utter nonsense!

[@mubeendewan2287]

how so?

[@glenmenas9424]

Describing spin up or down along the z axis is so far from what is actually happening. You cannot describe the wave two dimensionally and then describe spin on a third dimensional axis, especially if we're talking about photons because of how the electric field interacts with the magnetic field in a light wave. If you're going to describe the wave function of the nature of light and photons specifically, you must start by describing the perpendicular electric and magnetic fields in three dimensions instead of two, which I have seen no one do ever. Stop thinking in terms of sign waves!

[@ericreiter1]

I retract my comment. I just have experiments showing qm fails.

[@ericreiter1]

I retract my comment. I just have experiments showing qm fails.

[@ericreiter1]

I retract my comment. I just have experiments showing qm fails.