Lecture 4 | String Theory and M-Theory

(October 11, 2010) Leonard Susskind gives a lecture on the string theory and particle physics. During this lecture he focuses on closed string theory as opposed to open string theory.
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

[@joabrosenberg2961]

Noether theorem; Closed string 5:45; Right moving wave 9:45; Reminder - the photon 30:00; Two photons or a graviton? Level matching 34:30; Axion and Deleton 49:00; Symmetry of a closed string 55:00; Philosophy (="bad question") of reductionism 1:15:00

[@PJ-he5zk]

Thanks!!!

[@petergreen5337]

❤thank you very much

[@voldym888]

Thank you Stanford University and thank you Dr. Susskind for such an outstanding generosity that allows ordinary people to get such a profound education.

[@pauloteixeira763]

Comments here have at least months... most of them are from 1 and 2 year ago. It makes me sad knowing that this kind of knowledge is not viewed, appreciated and certainly, not searched on the web. In the world of information... ignorance is not an excuse... it´s an option. Thank you again, Stanford University, for this amazing series of lectures.

[@juld55]

Well, a lot of people are interested - just look at the view count on the lecture 1 video. Unfortunately only ~15% of the people who watched the first video moved on to the second. And by lecture 7, only ~5% stuck around. I guess it's just too difficult for most people.

[@brainoutyakabrainout]

Not so much difficult, as there is a lot of info to absorb. I never had physics in school. I never went beyond geometry. So there is a ton of vocab to learn, tho the concepts themselves, are not hard. Just the jargon. So one must spend a lot of time looking up the jargon, then 'Aha, I get it now', then back to the next video.

[@andreya5788]

Says someone from 3 years ago : /

[@stephanmantler]

Four years later, it's still up here and people are still watching (I certainly am). However I rarely comment, even on videos that I thoroughly enjoy, because it is not often that I feel I have something to contribute that is of value. And I am sure that I am not alone in this sentiment, but that many others feel the same.

[@PJ-he5zk]

Been chewing through all these lecture series since 2013, first learned about Susskind back in 2011 from his book The black hole war. There's always someone out there.

[@pauloteixeira763]

Thank you Stanford University and thank you Dr. Sussking for such a generous gift and for feeding our curiosity.

[@radicalsquare]

Thank you Stanford for uploading these. What a unique and rare opportunity to learn from this brilliant man.

[@lucasspeer]

Great Stuff and amazing to get the chance to learn something from such a guy freely at no cost.

[@cliffhanger625]

Thank you so much for this series. I've been studying string theory on my own for about a year now, using Zwiebach's textbook and a series of lecture notes from t'Hooft, mostly. Thank you Prof. Susskind and Stanford! These lecture have been a lifesaver for me.

[@wtj11]

So where are you now? The comment was 11 years ago so wondering how is it going in a good way

[@Dilaton100]

LOL, Lenny Susskind explains where my nickname comes from ... I just love him for giving such nice lectures :-)

[@Pengochan]

Its a bit similar to applying the slater determinant but only allowing cyclic permutations and then making it continuous.

[@TomFynn]

@MAUERBAU1 As you can see from the commentary this is from "Stanford's Continuing Studies". Meant for the public, not for specialists. First read, then rant.

[@petergreen5337]

❤Thank you very much Professor and class.

[@ireallyamrumi]

Last 20 min are truly excellent

[@killshot7873]

1:05:00 he earned my respect

[@aryhegde7952]

At 21:20 shouldn't the lagrangian have a 1/2 term in it for both kinetic and potential energy? Please clarify

[@SalvatoreIndelicato]

Ottima lezione. Complimenti dall'Italia

[@tapwater128]

I think i can answer my own question, but just see if I'm right on this. At this point the Higgs mechanism hasn't been established - and by that i mean the interactions with its own field and so at this moment can be considered massless

[@FHB71]

Question and I am not a physicist: at the end the formula saying 0=int dx/dtau * dx/dsigma dsigma, the dx/dtau looks like a constant, can't it be eliminated leaving 0=int dx/dsigma dsigma? If so, doesn't that simply mean that the string is wiggling and the center of mass is not changed? ... just curious.

[@TheRemixDenuo]

1:03:40 - 1:05:10 why did he get rid of the wavefunction? doesn't Psi depent on sigma?

[@Achrononmaster]

His blackboard style is very loose. You should imagine Psi is still there, and also should be in the integrals above for the R and L moving energies. In QM all observables get sandwiched between a bra and a ket, or integrated over if you are doing wavefunctions.

[@Binyamin.Tsadik]

Is it possible to get in contact with Dr. Susskind? I tried to send him an email at a few different addresses but he never replied.

[@Jipzorowns]

what song is that? 18:54

[@tapwater128]

When considering the operation where you generate the spin 2 states (from the ground state) and then also spin 0 states you also say the that the spin 0 is also massless, however I was under the impression this was a candidate for the higgs, which has mass. So i guess my question is, is this where the higgs idea is generated, if this is so where does the mass come from, or this just a coincidence and I've just been fooled?

[@perjespersen4746]

1:19:00 Gerad T Hooftt on what is fundamental

[@peterpetroff851]

To understand wave moving left, wave moving right - read Negative frequency in Wikipedia.

[@longalexislong]

wait wasn't he trying to exclude the angular momentum zero particles? They were allowed under the restriction that left moving energy = right moving energy, but all he proved with this jaunt in to proving their immutability to changes in the co-ordinate system was that left moving energy and right moving energy is the same thing as saying that they are immutable. Sure that's interesting and all but it doesn't change anything so far

[@grunder20]

stanford is the best!

[@rodovre]

Several hours into the lectures, I'm still confused: Are strings mathematical objects that explain something physical, or are strings real physical objects explained by the mathematical theory?

[@brainoutyakabrainout]

Combo of both. The math is used to detect what they now think are real physical objects. They can't run experiments to see them, so keep on using math to discern the properties, waiting to discover a way to test their discernment. It is like logic, but so much jargon that fact often goes missed.

[@universum-theuniverseexpla6565]

4:38 how does nobody get this, it is simply the coordinates systematic symmetries between particle dualities in the form of a certain variable summed up system, in the form of a interchanging divergence law

[@bohanxu6125]

1:00:16 Doesn't this just means the wavefunction is constant around the closed string? If so, then of course the left moving energy would equal the right moving energy... but it would be trivial. The left moving energy equal right moving energy sounds like a sigma reversal symmetry, instead of translational, no? Also, in simple particle quantum mechanics, translational symmetry only means energy eigenstate have the same amplitude everywhere (with potentially different phase indicating momentum). A specific particle configuration can have a wavefunction violating the symmetry. Only its time evolution would respect the symmetry. A closed string can also in principle have different energy for different direction. The two energy would only remain the same if they start out the same. Strictly equal energy at all time and all state is an additional postulate that is not provable, I think.

[@larspeters2587]

What a golden guy💪

[@Binyamin.Tsadik]

I agree, he does his math very loosely, however, I like it that way. We are not learning math here, we are using it, and writing everything down is tedious and takes away from the purpose of the lecture.

[@lsbrother]

agree but you are feeding him with exactly what he wants - a reply . the best thing for trollers is to ignore them - totally.

[@Achrononmaster]

@1:13:36 "Feynman diagrams are not very effective when the electric charge is large..." This is not so true. Divergent series often do sum, and can be summed more rapidly than converging series, if you understand asymptotics. Carl Bender has a terrific PIRSA lecture series on this.

[@GBart]

I need practice problems to do for homework. Also when's the exam?

[@Achrononmaster]

Barton Zweibach's book is a good resource at the same level with loads of exercises.

[@davsaltego]

So, this is Mike Ehrmantraut’s new gig?

[@wtj11]

Looking for this comment only 😂

[@ryanlafferty5948]

I was about to flag your comment, but then i read the one that it's a reply to. Couldn't have said it better myself

[@saikiranalvala502]

51:20

[@NWRsk]

1:03:30 Is it legit that he just replaced the P operator which acted on the wave function with just P(sigma)?? Don't we have to be very careful for that in qm?

[@abhinav.thepianoman]

Yes, but in this case the math checks out. Replacing the p operator with p(sigma) is consistent because we then replace p(sigma) with the time derivative of x and it was only used as a step in understanding the conceptual derivation. We have to be careful of replacing the operator with p(sigma) in qm when working with harmonic oscillators or when we have values to plug in.

[@NWRsk]

@@abhinav.thepianoman thanks!

[@abhinav.thepianoman]

@@NWRsk no problem mate, hope that helped!

[@mrfrankincense]

Isn't Susskind using classical physics, while string theory is quantum mechanical? His Hamiltonian is not an operator, and as I understand it, the Lagrangian in QM becomes the path integral formulation, which the Lagrangian here is not. I suppose from this that the way he is teaching string theory here is to use classical these notions to make an analogy in order to teach the principles most easily, while string theory is actually expressed with a Hilbert space and the Dirac-VonNeuman postulates/axioms as applied to strings? In some sense, that is breaking his code of taking the listener beyond a 'Scoentific American' type cop-out. However, I suppose in order to teach so much about string theory in ten lectures, this short-cutting is necessary? I was wondering if I've got all this right, or someone can explain where I'm wrong.

[@mrfrankincense]

Of course, he is at the same time thinking of QM when using discrete energy states and creation/ annihilation operators, but regardless, it is not consistent.

[@jfdowd]

+mrfrankincense It is not inconsistent. Starting with a classical system and then quantizing it later is standard practice.

[@mrfrankincense]

Thanks. I hadn't thought about it like that :)

[@mrfrankincense]

I think what i meant was to just check that at some level string theory should ultimately be quantised.

[@vikramadityaarya838]

in fact he is d father of string theory

[@ryanlafferty5948]

I don't think philosophy is the right word. But you're basically right. It's a mathematical beauty. It's unclear whether it should be called a physical theory. It does make testable predictions. They're just really hard to test. But look, any string theorist would abandon the theory in a heartbeat if it was proven wrong by experiment. A philosopher would just say something cute and pretend like it didn't happen.

[@cmfluteguy]

Yes, but Horst has all the secrets of the universe in his head, but he is too important to tell us what they are.

[@QRebound]

This is a lecture, not a rigorous treatment. He lets constants and the like get absorbed into the units and constants, and instead focuses on the interesting pieces (time or sigma dependent values, or things that vary greatly). This is common practice, and complaining about it only shows your ignorance of mathematics.

[@musicalfringe]

I spent about 5 minutes shouting at the screen after he forgot to square the PE term of the Lagrangian. Awesome lectures though.

[@adamwatson7669]

You realize that relativity has been tested to better than one part in one billion, and quantum theory is our most successful scientific theory ever devised? I mean, I'm not quite sure what you're saying here. Are you suggesting that the thousands of experiments confirming either theory didn't happen?

[@tanchienhao]

he showed in the first few lectures that a relativistic theory in 3 dimensions is approximated with a non-relativistic theory in 2 dimensions, he taught this in non relativistic way to make it easier to drive across the ideas because relativistic theories require much more technicalities

[@PetraKann]

NOT a shred of evidence to support string theory The Best way to describe ST at the moment is a mathematical philosophy - is it not a scientific discipline (not yet anyway)

[@Sam_on_YouTube]

In spite of the fact that he doesn't like philosophical questions, in college I wrote a philosophy paper on the mereology (philosophical study of composite objects) of liquid helium. I came to the very same conclusion he did, whether or not something is composite or elemental, depends on the context. I called it mereological perspectivism. So this room full of brilliant physicists was silenced by the same conclusion reached by an undergraduate applying applications of philosophy. That doesn't seem like a bad question to me.

[@bindon8581]

EXCEEDINGLY FASTER THAN LIGHT, MR EINSTEIN Well, if we must postulate symmetry in all things [despite being broken at a BB], then Dirac's Sea Of Positrons/Particles must be balanced by a Faster Than Light theory to balance slower than light. That's because we mistake the part for the whole, the wave field. FTL is merely the past and the future. [ see Newton's inverse-cubed law, sadly neglected, to see how particles can bob up or down or/and in and out without losing position. ] We can't grasp the actualised past or future in the moment. But we can see the effects. See the future? Of course. That's teleology . Einstein's equations, as is, can be used to influence the future; polarised light can tell us what the partner of the paired photon will be. If it's up now it's partner will be down. We can also prime the future. Einstein limited the speed of light to what we can measure. We have to contemplate negativity as we now do value-systems [cf. Benatar]. That is, minus mass beyond massless photons. It's the implication of 'balancing the books', as physicists love to do. You can't have your cake and eat it, unless you're Mr Kipling! Instead physicists have ducked out and opted for multiverses, big bounces, strings and yes, even holographic principles. [ as I noted before, information is lost or/and fuzzy even as you use it. since memory is involved, i.e. brain or/and mind. who knows what reverberations, say, "Season of mists and mellow fruitfulness" sets up? ]

[@brainoutyakabrainout]

Agreed, in part. Light cannot be a constant. But for testing they have to treat it like one. The negative-mass 'tachyon' idea in Lecture 3 beginning about 57 minutes, is the closest they can come to playing with speeds faster than light, and of course they do it backwards.

[@drdca8263]

Have you attempted to formalize the ideas you are attempting to describe? Perhaps actually doing so could be too time consuming to be feasible, but it seems to me that, if your ideas make sense, then it should be, at least in principle, possible to formalize them. You say that “FTL” “is” the past/future, but I’m not sure what you mean by that. By “FTL” do you mean “some object moving faster than light”? Or do you just mean “paths through spacetime that are faster than light at a given point on the path point in the past and/or future direction(s)”(wait, no, that’s backwards, hmm), or, uh, “paths through spacetime that are faster than light are spacelike”?

[@Taqu3]

LMAO!

[@tinkeringtim7999]

Perhaps the only mistake he's said so far is that what strings are made of and whether or not they're made of smaller things is a bad question. It is a philosophical question, and a good one, and it has a clear answer - they are made of ideas.

[@MAUERBAU1]

is this science or kindergarden? i cant imagine, that this guy wants to be taken serious.his treating of mathematics is a slash in the face for every serious mathematician. this of course is speculative so called science at its best.

[@YoutubSUCKZ]

creationists

[@PAtch3s1024]

He came up with the idea of string theory.... I think he is a little smarter than you. You'll find that most physicists dumb down the math when they teach a topic for the first time to make it easier to understand.....

[@fartreview1739]

Lol string theory, there is zero evidence for this if u ask me. Imagine this dude doing math magic just for it to be a big nothingburger if u ask me.

[@Achrononmaster]

Yeah, probably no one is asking you. Susskind is clearly on the record stating string theory is probably not good physics (kzbin.info/www/bejne/rpyXaa1ug5KmopI - watch for a few minutes from there). But he also notes that it has been enormously influential and was the reason holographic duality was discovered, and that is a lasting contribution even if string/M-theory is fundamentally just cute math.

[@MAUERBAU1]

how nice to see that you react on my comment like religious radicals on critics about their religion..... this shows in another way the low level horizon of you so called scientists string theory is a funny fantasy.....like relativity and quantum theory so lets discuss the subject in a few years, when einsteins fantasies will be part of museums and quantum theorie will be a relict from the past....

[@_John_P]

11 years on and Einstein still strong than ever. After the success of NASA's Gravity Probe B (2004), which confirmed (2011) the geodetic effect and frame dragging, both predicted in GR. LIGO (2016) confirmed the existence of gravitational waves, also predicted by Einstein's theory, and more observatories were built (Virgo 2017, KAGRA 2020), independently confirming LIGO's results. They were then connected together and carried out 9 tests of GR (2022), finding no deviations from GR. In the meantime, the first and second pictures of a black hole were obtained (2019, 2022) by the EHT (2009), again showing no deviations from GR.