1. Emergence of Gravity

MIT 8.821 String Theory and Holographic Duality, Fall 2014
View the complete course: ocw.mit.edu/8-8...
Instructor: Hong Liu
In this lecture, Prof. Liu discusses the concept of emergent gravity, and proves the Weinberg-Witten no go theorem which forbids existence of massless spin-2 particles. The beginning of the lecture is devoted to course information.
License: Creative Commons BY-NC-SA
More information at ocw.mit.edu/terms
More courses at ocw.mit.edu

[@ThePritt12]

8:00 the sentence every professors say in the first lecture. no exceptions :D

[@docu73]

Lecture start 9:22

[@fourpanelszerowords476]

that is quite an introduction

[@stanhootzz1904]

Thank u fer the heads up.

[@onkar-mahamuni]

Thanks.... I was in need of it so badly

[@loouuiisssss296]

Thx

[@still2weirdfoU]

Ok but I really wanna know how much of this is gonna be in the test.

[@franciscoflores3231]

Hope you guys of OCW read my comment. Thanks for uploading such a great content. Since I started my undergraduate studies I've seen OCW physics lectures on the web. Curiously, They have been accidentally coincided with my carrer path. Thanks so much for this. Do you have plans to upload any Quantum Field Theory lectures? That would be amaizing!

[@christianjimenez1877]

This lecture is philosophical and very important.

[@ralphclark]

What a pro. Just marvel at those blackboard skills.

[@ChaojianZhang]

Best free KZbin video ever.

[@Olegitskillz]

Higher resolution = the more his shirt hurts your eyes

[@MrBlackMrBlue]

Thank you for this series of lectures.

[@fadyfarouk8635]

Thanks a lot for uploading this, keep up the good work!

[@mariapaz6103]

I am a med student. I have a psychology and anatomy test this week, yet here I am

[@krishnakumarsah632]

It is one of the topics at the peak of a mountain called physics

[@RJBenish]

What a math-geeky fellow. Oblivious of the flattening of his undersides (the PHENOMENON of gravity), no doubt.

[@martinsoos]

We have a true mathematician. His eyes are almost always on the chalkboard or his feet.

[@lt2557]

“the professor's accent is really cute，accent is really cute” LOL

[@naimulhaq9626]

Spacetime and the quantum field together forms the fundamental background giving rise to particles, strings, dark matter, dark energy, inflation, gravity, anti-matter puzzle, complexity etc. Quantum computing function is also simulated and enacted by the QF, when intelligent conscious 'observer', collapse the fields into particles creating the universe governed by a single probability wave function.

[@roberttrahan709]

Instead of being considered a force, why not a dynamic of space fabric displacement ?

[@physicsperadox7849]

Did anyone ask u to speak?? Shut up

[@heroncortizo1993]

Fascinante este assunto! :-)

[@tiamatbenoit7267]

Hello, or could we find the books that Mr. Hong Liu gives as references to these students?

[@mitocw]

See the Readings section of the course on MIT OpenCourseWare at: ocw.mit.edu/8-821F14.

[@tiamatbenoit7267]

thank you

[@sdcair]

Link in the description is not working. Course not online yet? Otherwise, BIG thanks for uploading this!!

[@mitocw]

+sdcr Link fixed! Thanks for the note.

[@kennethchow213]

If Einstein did not misconceive when he tried to unify gravity with electromagnetism, then gravity and electromagnetism must be one and the same thing, though they may operate in different phase spaces and thereby seem to be different. Einstein's this conviction was also reflected in Newton's intuition( On the Shoulders of Giants, 2002 edition, p. 1159-60).

[@kasel1979krettnach]

So does a free falling electric charge radiate , as it is being accelerated gravitationally ?

[@user-bk2xv1il1h]

Your question is not correct. Free falling is per definition NOT accelerating

[@mpicos100]

is the Weinberg-Witten result on the Weinberg QFT's books?

[@oblagiancarlos9573]

When the fan blades spin so fast, they seem to go backwards. In a way, when particles push in on each other, just to push back out again, and push back in and out over and over again, generally crushing a little bit more at each time, can't that be like the gravity waves we see? Like it's easier for me to walk towards a crowd (and can be interpreted as me being attracted to it) but most individuals in that crowd may have a harder time walking towards me, individually.

[@metalkokorea]

Professor! why should a graviton be a spin-2 particle?

[@TheGlintbeetle]

I'm not sure about a string theory derivation, but in general relativity one show (relatively simply) that a rotation of spacetime by some angle rotates the phase gravitational waves by twice that angle. Hence the 'graviton' (if one would be as bold as to quantise a classical field) is a spin 2 particle.

[@ShailendraKumar-ug4tn]

Because the field has 10 degrees of freedom.

[@thomasbastos3869]

Besides all of the above you can also show that a consistent theory of massless spin 2 fields ( free of ghosts ) have the same lagrangian of the linearized Einstein-Hilbert action. More of that on Schwartz's quantum field theory book.

[@anurag6674]

what is prerequisite for the this course?

[@mitocw]

The prerequisites listed in the syllabus are 8.323 Relativistic Quantum Field Theory I, 8.324 Relativistic Quantum Field Theory II. For more information see the course on MIT OpenCourseWare at ocw.mit.edu/8-821F14.

[@davidazizov8864]

Sakharov is surname

[@gruminatorII]

I am trying to understand, but i alreaddy fail to understand why people want to unify Gravity with the other forces, if gravity is just spacetime itself that bends. And Trajectories are geodesics, so "straight lines". For me that means that Gravity just is not a force its a feature of spacetime.

[@apburner1]

You don't get tax dollars in the form of government grants with that kind of logic, now go away sheep.

[@stevenytcx]

Well for most situations indeed what you said suffices But there are questions that can only be answered by a unified theory. Prof Liu mentioned physics in black hole and big bang as two examples. If you are interested you might look into e.g. black hole information paradox.

[@ronaldderooij1774]

Correct. But it all is in one universe that we try to understand. Where is the bending of spacetime coming from?

[@JohnBedson]

We will never get a quantum theory of gravity until we realise that Space/Time cannot exist below the plank scale because of the uncertainty principle. Therefore gravity cannot be the bending of Space/Time because there is no Space/Time. There must be something else that our false idea of Space/Time is an approximate representation of. Once we figure out what we replace Space/Time with it is likely that a quantum gravity theory will emerge as an obvious result.

[@JohnBedson]

My hypothesis is that to an observer outside our universe, the information inside our universe is probably proportional to its area and not to its volume. Hawking has already shown that this is true of black holes. If that is true, what we experience inside our universe is merely a complex hologram generated from a membrane on the edge (area) of our universe and projected back inside our universe. That may account for the many paradoxes and "fine tuning" problems that we face. There may be no realistic answer to these questions because there does not need to be an answer. We are perhaps existing inside a computer (I use the word metaphorically) generated and projected fantasy hologram and 'reality' is not what we experience, which is an illusion, but rather, true reality only lies on the edge of the universe in the manipulation and projection of data. Our universe may be massively smaller than we think. To an outside observer it might be subatomic or even undetectable. If the expansion of our universe is generated by new space coming from the quantum vacuum, it is possible that it is only expanding on the inside and not on the outside. If it were expanding on the outside at the same rate as on the inside it would mean that the outside was also part of our universe and therefore our universe has no edge. But we know that our universe does have an edge. It is even possible that rather than there being an infinite number of universes side by side, there are an infinite number of microscopic, undetectable universes within our universe and an infinite number of macroscopic universes outside our universe like nesting Russian dolls inside each other. All of these universes would therefore likely have the same physical laws. If they did not have the same physical laws, the entire structure could collapse. - Just something to think about.

[@doubledogfoxyranger60]

When are they going to get whiteboards.

[@alexv3357]

When pigs soar across the skies of Venus

[@vinitchauhan973]

DoubleDogFoxyRanger white board are expensive and so is ink.

[@oussematrabelsi9429]

lol mit can easily afford white board they just prefer old school teaching. most elite american universities do that

[@subita7835]

Is this undergrad or graduate?

[@mitocw]

This is a graduate level course, see MIT OpenCourseWare for more information at: ocw.mit.edu/8-821F14.

[@stanhootzz1904]

Please go to a power[point format, the second hand chalk is killing me! LOL's

[@MrTeney]

i hope they did not pay much for that class

[@vinitchauhan973]

Why?

[@alfredomatias313]

GRAVITY

[@apburner1]

So much time wasted writing on blackboards. If only someone would invent computers, screens, and software that would make this more efficient...

[@earthstar7929]

You get time to think about meaning

[@ralphclark]

It’s to allow the students time to write down the notes and give them time to digest what it means. In the course of doing so, questions will arise in their minds, and JOB DONE.

[@48acar19]

The subject is interesting but the presentation is awful.

[@salimjabr6058]

This guy is just copying Polchinski's chapter in : "String Theory and Its Applications: TASI 2010, from MeV to the Planck Scale ... By Michael Dine, Tom Banks, Subir Sachdev Makes it look like making a spin 2 particle fro 2 spin1s is his idea ! Bad !

[@colaforest1]

Salim. It's called attending a lecture. Not watching discovery channel

[@anandbalivada7461]

Salim, he's teaching something in a class, not giving a presentation on his research or professing the idea to be his. Thanks for citing this resource though.