Lecture 3 | String Theory and M-Theory

(October 4, 2010) Professor Leonard Susskind reviews harmonic oscillators, the spin of massless particles (photons and gravitons), the low lying spectrum of strings, the tachyon problem, and the basics of string interactions.
String theory (with its close relative, M-theory) is the basis for the most ambitious theories of the physical world. It has profoundly influenced our understanding of gravity, cosmology, and particle physics. In this course we will develop the basic theoretical and mathematical ideas, including the string-theoretic origin of gravity, the theory of extra dimensions of space, the connection between strings and black holes, the "landscape" of string theory, and the holographic principle.
This course was originally presented in Stanford's Continuing Studies program.
Stanford University:
www.stanford.edu/
Stanford Continuing Studies Program:
csp.stanford.edu/
Stanford University Channel on KZbin:
/ stanford

[@nmarbletoe8210]

For me, one of Dr. Susskind's best and most accessible lectures. Stanford thanks so much for sharing these!

[@iytbazzie7723]

Oll

[@joabrosenberg2961]

Spin of massless particles; Creation and Annihilation operators 7:30; String theory needs more dimensions 17:45; Back to Massless spin 27:00; Low line spectrum of strings 43:30; Imaginary mass? Tachyon 57:00; Closed strings 1:16:00; Interactions, Coupling coefficient 1:22:20; Closed strings and gravitons 1:30:00; What makes things move? (Frame of reference?) 1:44:20

[@stillcantthinkaboutaname5396]

Thank you.

[@petergreen5337]

❤thank you

[@jalalabdallah7879]

Prof. Leonard Susskind [ My Father ], you are the Best Physicist for ever .

[@ThalesPo]

Congratulations Susskind! And thanks very much, Stanford, for sharing this and other pieces of free education.

[@petergreen5337]

❤Thank you very much Professor and class.

[@thelichking1242]

He is a genius! I met him a few years ago and he was a wonderful man to talk to on String Theory!

[@eriksundell1400]

The probability of self coupling for an open string (snake eating tail), might decrease with increasing angular momentum, due to centrifugal force, right? I wonder if this idea matches observed reality, or can/has guided the interpretation of what an closed string formation and a self coupling could represent in order to match reality. I love these lectures, I'm so glad to finally grasp some details of string theory, which has been so mysterious until now. Thank you all whom are involved!

[@jalalabdallah7879]

Prof. L.Susskind really should have a Nobel Prize.

[@MiguelGarcia-gq1yq]

Thank you for your answer Jonathan!

[@eligraham55]

Great professor, he's great.

[@SocioecologicalInterdependance]

Pitchpole 1:21:xx Pitchpole means for a displacement hull boat to come too fast down the face of a wave and the nose plows into the trough, causing the boat to invert bow to stern, as opposed to getting rolled port or starboard. ;)

[@HughesMath1]

Stubb uses a pitch pole In chapter 61 Stub Kills a Whale Moby Dick Melville

[@hasanshirazi9535]

@ 54:10 the "i" in the circular polarization corresponds to the phase shift between a and b.

[@epirvsflavivs]

i like how he's eating cookies all the time

[@dillirajbashyal2303]

i'm almost dumb at mathematics but i see this, not because of knowledge or for knowledge but for joy.

[@matharoofmaths]

Susskers is an absolute LEGEND!!!!

[@ndahtuuirne]

@gChinkin because the first excited mode is massless, which means E (the rest mass of the string) is equal to zero. He explains why it is massless by noticing that there are only two degrees of freedom which is impossible for massive bosons since they always have odd number of degrees of freedom (scalar bosons have 1, vector bosons 3, etc.)

[@rodovre]

Great lecture! He sometimes makes my head boil, and that feels good.

[@shaneilic6969]

Wow! I wish I knew WTF he was saying . Brilliant man! Seriously!

[@lucasthompson1650]

37:19 That look when someone's phone starts ringing … 😆

[@MiguelGarcia-gq1yq]

Hello! I am reading the book by Zwiebach (I am around chapter 20 now) and I also started to watch these lectures because I tought it would be a good complement. I have watch up to this third lecture but the Nambu-Goto action has not yet been introduced so I am starting to doubt whether this lectures will actually help me along the reading of Zwiebach's book...does he do this at some point later on? I am specially interested in D-branes, does he talks about those at some point? Thank you people!!

[@physicsjagat]

New insights from this lect are about massive & massless states of spin 1 particles & polarizations.

[@perjespersen4746]

1) harmonic asscilator quantized. Creation and annihilation and commutators 2) 30:00 particle spin

[@kwijung]

tachy- ... prefix meaning "swift or rapid": tachycardia, tachyphrenia, tachysystole

[@askarhmath3260]

can someone tell me that which textbook of string theory or notes Sir has following here?

[@tomaszdzieduszynski]

Does anyone know where are the solutions to Theoretical Minimum book exercises after Lec6Ex5? It looks like the official site doesn't have them :o

[@tomaszdzieduszynski]

+Tomasz Dzieduszynski Seems that I've found the unofficial version. The same guy also uploaded answers for the 2nd book: onedrive.live.com/?authkey=%21ACvGALCAMU4xPL4&id=21D08FA0C16B93A5%215776&cid=21D08FA0C16B93A5

[@bruinflight1]

"It's the unexcited oscillators." Speak dirty to me Lenny!

[@killshot7873]

Good for introductory studies ,sometime seems to be over simplified

[@zoltankurti]

Kind of wired how simple harmonic oscillators and the spin of massless particles is being explained in a string theory lecture, but I still like the style. It's relaxing, I like listening to him.

[@epirvsflavivs]

the end is just great... "Woody allen".

[@_bxrryYT]

At 1:08:23 he showed the positive p.e of the field, if we made it negative wouldn't that just be the lagrangian

[@johnstfleur3987]

WALKING ON

[@massimoandretta3529]

Great lesson, Prof. Susskind. Anyway, Tachions come from Acient Greel: "Takìs", that is in Greek for Speed

[@io.hadjidakis4620]

"Ταχύτης"(tachitis) is the Greek word that means speed and the term tachions are coming from. (Takis: a comon nickname)

[@zsuzsannanorin2702]

I don't get it. Why would you spend 45 minutes+ on explaining the polarization of light, related mathematics, easy things to understand and brush over in 2 seconds how the holograpic principle in relation having some kind of connection to boosting the system relativistically in one (z) dimensions. Deggrees of freedom, etc... ???

[@Hythloday71]

were is lectuer 1 and 2 of this series ?

[@aguante0]

It's not "BOLO", it's "boleadora". I'm not prepeared to comment on any other topic of this lecture, but this. I'll keep on listening :)

[@otonanoC]

At 17:38 Susskind tells a joke about 26 dimensions, which I did not catch on the first listen.

[@musicalfringe]

It took me a second. Hang on, that's not the right numb-- ohhhhhh.

[@nmarbletoe8210]

lol because they ran out of letters in the alphabet

[@Dilaton100]

Great :-)))

[@Jo3harker]

I just woke up and youtube had this playing. KZbins auto play went from music to string theory xD

[@mirijason]

I'm sorry but I have the exact same question and I still don't understand (although I watch the video 3 times), why would the energy be 0? Cause if the first excited state is a massless particule, then the energy formula he used to deduce E1=0 should be wrong shouldn't it? Indeed, it's a non-sense to speak about the proper energy of a massless particule as it's proper referential is undefined. Perhaps there's something I'm missing but I fail to understand, if someone could enlight me please.

[@siquod]

18:00 So, German string theory presumably has 30 dimensions, but three of the four additional ones are actually second time derivatives of the dimensions indexed by a, o and u?

[@musicalfringe]

Very good.

[@Gonnakillthehaters]

i wish i could learn physics and string theory in sophomore year ;~;

[@RepublikSivizien]

The missing c (speed of light) in the equations is somewhat irritating but I understand why he omit it.

[@roberthayter157]

1:21:43 When the Scots "pitchpole" they call it "tossing the caber". It is one of the events in the Highland Games.

[@CaptianKeyz]

Microphone level is a little hot. Thanks for another lecture.

[@JonathanGleason]

He is probably not going to introduce the NG action. You'll recall he has already written down the Lagrangian (dX/dt)^2-(dX/ds)^2. This is essentially the Polyakov Lagrangian after a choice of gauge (see wiki/Polyakov_action#Equations_of_motion), and the Polyakov action, I'm sure you know, is classically equivalent to the NG action. As Polyakov is more useful, he's probably just going to stick to that.

[@karabomothupi9759]

Please shut up

[@SalvatoreIndelicato]

Ottima lezione Saluti dall'Italia

[@Seyeiin]

Oh yes, what happens if 3 strings attach and form an Audi or 5 attach and form an Olimpic sign?

[@CrimsonRGU]

lol shit, i could learn alot about string theory without paying thousands. i mean i dont want to major in it, but im still interested in learning about this. thanks for the video upload!!

[@sidewaysfcs0718]

Bola is another term for it i believe

[@digxx]

Anyone know the following: He said one can get rid of Tachyons but one cannot get rid of Photons. Now on the other hand he said Photons are represented by open strings and one can have closed strings without open strings. So one should also have stringtheories without a photon. This is somehow contradictory

[@mattryan2006]

around 1:16:10 he makes it clear that everything up to that point only included open strings. His earlier point was that in string theories with open strings you must have photons (which are open).

[@digxx]

So why does a closed string theory not necessarily have to have open strings? I mean he argued just like that open strings can fuse, they can also fiss... So why cant closed string of an interacting theory also fiss?!?!?

[@mattryan2006]

of course they can 'fiss' as he puts it i.e. break apart and become open strings but you have an open string theory. you can however have a complete theory of closed strings without letting them do that which would be a closed string theory. in that type of theory two loops meeting open at a point and create another closed string. if a closed string interacts with itself in the same way as it meets another closed string it can only form multiples of closed strings. you need to add something extra to that system to get an open string (a fission operator). if however you start with open strings and have an operator that joins open string ends then one possibility will include closed strings as he has demonstrated.

[@Unidentifying]

mirijason here susskind was describing the first excited states of the strings at rest, so it refers to rest mass being zero

[@muhsam]

boost it

[@awaissayyed5526]

Dr.sir i have one question, can we use it for stock trading... ?

[@ethannguyen2754]

“What do all string theories have in common?” Well… strings

[@williamotule]

Tachy= Fast!

[@Alexandru_Iacobescu]

Tachyon=> m sqrt =-1 maybe it is a hole or a tear in the universe.

[@nmarbletoe8210]

'imaginary mass is a bad thing' 57:25 'it's hard to have a string with one end. unless you're a buddhist... a zen buddhist.' 1:16:15 A string with one endpoint is a point?

[@alicewyan]

Mathematically, a string with one end is infinitely long (the other end is "at infinity")

[@stevengorlich4993]

kzbin.info/www/bejne/nXTceqCCqpV7ias - did anyone remember lecture 1? The (amazing!!!) prof. Susskind requested "as long as you bring me cookies" - just think this is hilarious anyway. i graduated in 2015 and felt the need of a refresh. Wish I had discovered this series of lectures earlier. thanks to everyone who has contributed to this playful and sometimes even more funny than I thought strings could get informative experience. i just love it

[@Urdatorn]

Woody Allen!

[@hasanshirazi9535]

@1:25:00 The Prof. says that all processes in QM are reversible. However, electron-positron annihilation produces two 0.511 MeV gamma ray photons, but these two photons can never recreate electron-positron pair.

[@t8m8r]

Time reverse

[@nmarbletoe8210]

why can't they produce a pair? they have enough energy...

[@hasanshirazi9535]

@@nmarbletoe8210 To produce a e+ & e- pair, photon must have minimum energy of 1.022 MeV. Photon with energy of 0.511 MeV will not be able to create a pair.

[@nmarbletoe8210]

@@hasanshirazi9535 oh yes, thank you! Perhaps he meant if two 0.511 photons collide they could produce a pair...

[@hasanshirazi9535]

@@nmarbletoe8210 Photons do not collide like particles. They simply pass through each other.

[@daujok1301]

All physics should have a rigorous proofs like that of mathematics that way all doubts are no more in physics.

[@nmarbletoe8210]

I thought mathematics can't be derived from logic

[@nikn2621]

Tachy-(Ταχύ) a Greek word for "Very fast"

[@fredrickvanriler7986]

Absolute Genius Susskind is❕🥇 I agree with other commenters here, that he should have won a Nobel in Physics by now, IMO!!

[@life42theuniverse]

1:03:00 dE/dP = p /(p^2 - (i m)^2 )^0.5 or p/(p^2 + m^2)^0.5 ???

[@Ryan_Perrin]

What's the difference?

[@life42theuniverse]

@@Ryan_Perrin I think (i*m)^2 implies something about mass/momentum vector change in the imaginary plane while it's square is just a magnitude. While leaving out the you lose the vector in the imaginary plane and only allow motion in the real plane. Maybe.

[@Ryan_Perrin]

@@life42theuniverse (i m)^2 is just -m^2.. I don't understand what the issue is

[@life42theuniverse]

@@Ryan_Perrin Just that if mass is a complex number and not a real number then such things as P =i mv is complex

[@Ryan_Perrin]

@@life42theuniverse where did you see this at in the video? Or are you just asking what if mass was purely imaginary or complex (consisting of both real and imaginary parts). Also, some people teach minkowski space and special relativity as having an imaginary time part. Is this why you are confused? Because that isn't proper

[@MrGiuse72]

unit = 1 hbar

[@sidewaysfcs0718]

there you have it, Woody Allen is god.

[@Dyslexic-Artist-Theory-on-Time]

This is an invitation to see an alternative view! Where the different dimensions of String Theory are just future possibilities in our one 3D Universe Based on: 1 Is that the quantum wave particle function Ψ or probability function represents the forward passage of time itself photon by photon 2 Is that HUP ∆×∆p×≥h/4π that is formed by the w-function is the same uncertainty we have with any future event within our own ref-frame that we can interact with turning the possible into the actual!

[@LookToWindward]

Don’t get this man started on cones.

[@aguante0]

Nope... it´s just "boleadora"... anyway, we are here for string theory. thanks for your reply

[@NerdGlassGamingPA]

I didn't understand :D

[@sidewaysfcs0718]

i think what Leonard is getting at is that all strings are massless, but when strings interact with the higgs field , they get mass. so in a way, the initial standard model wich predicted all particles are really massless, is right, all particles should be massless, but have energy, it's only the particles that can interact with the higgs field, that gain their mass, and in a way "slow" down

[@abdulhameed2603]

only about 1-2% of the mass of protons and neutrons come from the masses of their quarks due to the higgs field. The rest of the 98% is due to the gluon field energy of the quarks strong interaction. Therefore they don't only gain their mass through interaction with the higgs field.

[@mirijason]

Oups "its" proper referential

[@CurtZilbersher]

Brilliant, yes. But can he make a radio out of a coconut?

[@mirijason]

But the problem I have is that I don't see the unacceptable conclusion as the way he concluded there's something wrong seems uncorrect to me. Indeed, the energy of a photon has nothing to do with its mass (as it has none) but it does have some (in an usual referential) except in its proper referential I admit but as such a frame of reference is undefined, the way he describes the string itself is uncorrect in the case of a massless particule. I just don't see how E=1+m^2 can still stand.

[@Seyeiin]

So can we have an oblect like a tennis racket having a string's end attaching to it's own middle and the other end wawing freely? ---O Let's call this the Super String pre Quantuum DNS theory.

[@32bikkeltje]

Nice to see something less complicated then women

[@factsheet4930]

Than*

[@matttonkthetank5619]

@@factsheet4930 Maybe he's a genius and this is very simple for him and he was planning on going to a club right after he posted this to check out some women. You never know.

[@Urdatorn]

@@matttonkthetank5619 This made my day, haha! String theory then picking up girls, fucking ace

[@konstantinosmei]

he keeps calling it blackboard, aaaaaaaa

[@NothingMaster]

I corrected him, but he didn’t hear me. LOL

[@MAUERBAU1]

what about super super string theory or maybe super-super-super string theory, or super-super-super-super-super......string theory? keep on dreaming mr. susskind.

[@JiveDadson]

This didn't age well.

[@timthomas3281]

Hesz no Feynman. Takes credit for everything, involved in nothing.

[@romanpierantozzi]

You are ignorant. Clearly.