Einstein's General Theory of Relativity | Lecture 4

[ilovephysics1122]

great lectures

[thegreeensky]

the best theory.. he's really fantastic... nicely discussed..

[adelle0001]

nice one!

[agapitoflores001]

Einstein was really great. He is really a foundation of Science.

[zhenwenlin87]

I like how from 577,250 views it drops to 123,148 views to stagnate around 57-60k views. It exhibits a exponential decay graph. It shows, to a certain extent, well with this sample size, a normal distribution is well established thus, to a high extent of scientific interest and knowledge amongst the community of youtubers curious about science and interested about science or general relativity.

[zhenwenlin87]

@zhenwenlin87 So I decided to do some math, see assuming(assume means i assume the statistics i got is correct) 800M pple on youtube/mnth , 3 B views in youtube/day. Work the math and u find that only 0.005% people are 0.888% or 1/112.5 percent curious about science and only 20% of this number is interested in science and 10% of the number is willing to through the theories. Of course this is only 1aspect of science. Base on my uni's no of basic sci courses, est 300* 0.005% =0.15%.

[zhenwenlin87]

@zhenwenlin87 Sorry I made a mistake. To generate a 1/112.5 interest will require 0.000025/40000 which is 0.0000000625%. seeing 300 basic mods are in my uni concerning science, we get 0.00001875%. Well even though this is highly inaccurate as probably other areas of science will generate more interest, it does indicate how ridiculously small the amount of people who are genuinelyy interested in science there actually are.

[FAZIL9999]

amazing prof.susskind

[StudyAcademic]

skip to 0:01:45 if you want to go straight to material and save one minute and 45 seconds of your life!

[sumitfreaks]

Sir... can u please explain Lorentz transformation.. or relationship between lorentz factor and spacetime...

[sumitfreaks]

Sir... can u please explain Lorentz transformation.. or relationship between lorentz factor and spacetime...

[sumitfreaks]

Sir... can u please Lorentz transformation.. or relationship between lorentz factor and spacetime...

[ScrewAttackChina]

well explained for the laymen.. good job

[linuxlvr]

Brilliant.

[calebp9503]

why is the first hour the same thing over and over!

[bangobangbang]

I'm sorry but I think this is a bit wrong... It's true that the proper time d(tao)^2 = dt^2 - dx^2 but not for the reason that is given here. In Minkowski space the metric is

g = diag(-1,1,1,1) and so ds^2 = dx^2 - dt^2. The proper time d(tao)^2 = -ds^2 => d(tao)^2 = dt^2 - dx^2. Usually though when measuring tao one assumes that dx = 0 which gives d(tao)^2 = -ds^2.

 I think...

[Diemedes]

@bangobangbang

It's actualy 'tau', not 'tao'.

[bendelafrance]

@bangobangbang no, you can use the space-time metric with signature either (-,+,+,+) or (+,-,-,-), physicists tend to use the latter and mathematicians the former, however its down to personal choice, this is a long debated topic

[Brawn]

holy shit this is confusing

[Bremsstrahlung1]

is felix bloch the same guy for which bloch waves are named after?

[faian0re]

yes

[Nikolay116]

yes

[nataher123]

This guy sounds like Al Pacino

[itaiinc]

I think he got it wrong with his "temperature, pressure and humidity does not form a vector argument because there is no transformation". Fore and foremost they don't form a vector, because they don't conform to abstract vector space definitions - temp. and humidity don't really comply to additivity, for example. Also it cannot really be a hilbert space (meaning having a scalar multiplication) since the components don't have the same units so A*B don't have any real meaning.

Am I wrong here?

[naughtymachinegun]

yes

[Salmontres]

I'm no expert on Hilbert Spaces, but I know you're first statement is right. The vectors he described didn't conform to vector addition. Honest mistake, I do it all the time in my oral descriptions/ talks. I don't mean to get philosophical, but this does raise a point: one shouldn't, in general, take a lecturer's words to heart. We do make mistakes; books (or generally, any written work), albeit also not perfect, usually contain much less mistakes.

[itaiinc]

Yes I'm sure its just an honest mistake. Just wasn't sure I'm right...

You are right about your point, but I found it easier, for myself at least (I don't think its the same for everyone), to learn material from lectures instead of a text book.

Even seeing lectures on youtube is very nice and "flows better" than reading the same material in a book but has its disadvantages compared to attending - for example having a question :)

[BagarozziBoi]

@Salmontres (+ itiianic) So prof. Sussekind is not in error, he is quite correct here, and i'm sure he's well aware of the formal linear algebra and differential geometry involved. Sure, always think for your self, challenge every statement and try to understand everything rather than taking things on "authority". But be careful and double check before you correct one such as Sussekind.... XD

[bbqltw]

Susskind is right. He referred to the tensor concept in physics. Hilbert space is not related to GR. (by the way any vector space has scalar multiplication. Hilbert space is a pure mathematical construct and has nothing to do with units. It means a complete inner product space.)

[itaiinc]

Sorry I meant inner product not scalar multiplication.

My point about Hilbert space was that if its the usual inner product (a,b,c)*(d,e,f) = ad+be+cf then all of the elements in the vector needs to have the same units.

Didnt get the "tensor concept in physics" thing.

[createsutoo]

Check Out Mr. Rei Weiss Now.. he following behind Einstein, check-out His Invention.. WOO WEEE......

[xxkiki2xx]

lol checked wikipedia, supposedly Einstein actually derived his own Lorentz Transforms... Well what a guy lol... Maybe Minkowski noticed the Lorentz one or some future physicist and decided to name them after him... But even more supposedly, some guy in 1897 derived them also so they add to the bucket of theorems named after the incorrect founder lol

[yooo0oitsharry]

einstein didn't derive the lorentz transforms... poincare did and named it after lorentz.. einstein just used them to derive the electrodynmanics portion of special relativity and he came up with his two postulates of special relativity

[JennyFarlopez]

He's going sooooo slow...

[liopaisa]

guys using LORENZ TRANSFORMATIONS you can prove Einstein's theory of relativity in 10 MINUTES !!!

[weweallthewayhome]

lorentz transformations are not means of proof, they themselves are results of the theory. you cannot prove a theory by means of the theory itself.

[LeMegasandwich]

Special relativity not general.

[ShootBigBird]

d(tau)^2 = c^2dt^2 dx^2 dy^2 dz^2

[hcohl001]

Shouldn't the metric be d\tau^2=dt^2-a(t)dx^2 instead of d\tau^2=dt^2-a(t)^2dx^2 ?

[yoshidabrcg]

Correct. He made a little typo there.

[ShootBigBird]

Although a(t) is inserted into the matrix of the metric, I feel it is represented (perhaps a given) as a square function. If it were inserted in the top and left column, it might look messy with all the 11, 12, 13...21, 22, 23 coordinates.

[hcohl001]

In a 10 dimensional space such that there is for string theory, would d\tau^2=dt^2-(dx^1)^2+...+(dx^­10)^2 ? Or have I missed something about the extra dimensions. Does relativity have to be redone?

[dramon231]

i love how the students are getting quieter from one lecture to the next..

[danielatha4]

I like how the views on these videos drop from one lecture to the next, and that most sudden drop is form 1 to 2, ha

[03Kabbotta11]

I know, in the early lectures you could tell a lot of the students were trying to "show" how much they knew, but now they can't even try anymore...

[peanorg]

@dramon231 Yes, thats because all questions are either stupid or trying to prove their own intellect..

[fialkool]

@dramon231 so that they do understand what he says.

[awoinar]

do you know, can i bookmark a video at a specific time ? i want to restart from where i stopped.. thx

[HMiste]

No, but you could mark down where you need to start again on a note or something similar. YouTube allows you to go forward to unloaded parts of the video, but it needs to refresh the video.

[adkinsjr]

I got thrown off around 21:20. I can see how the derivative is one when x-r is equal to x-s, but I don't get how it is zero otherwise. He said dx with respect to dy is zero?

[ufaxx]

x(sup)1, x(sup)2 and x(sup)3 are coordinates in 3 orthogonal axes. He uses superscripts 1,2,3 to denote those axes. Now, the infinitesimal position change dx1 on an x1 axis, divided by the same change dx1 is 1. Here, by dx1 I mean dx(sup)1 in his notation. The change dx1, however, divided by dx2 is 0 because, as You move along the x2 axis, the position on x1 axis does not change. It's just that the usual 3D coordinates (x,y,z) are referred to as (x1,x2,x3) [using superscript].

[adkinsjr]

AWESOME!!! thanks for helping me out with that. I got stuck on this lecture and decided not to continue (I hate leaving things behind). Now I can move on.

[listminusv]

yo peepz never give up be patient and stick it out to the end....lots o fun when it makes sense!!!

[cmfluteguy]

dx/dy is zero, because coordinate systems have "orthogonal" axes. Of course dx/dx =1.

[greg5566]

Ufaxx's answer is correct, but what Susskind is saying is simpler than that. It's simply the definition of a partial derivative. "With respect to" means you leave everything else constant.

If you take the derivative of xy wrt y, then x is thought of as a constant. so d/dy xy = x.

d/dx xy = y, because then y is the constant.

So, the derivative of any constant is 0. The derivative of 5 wrt x is 0, wrt y is 0, and so on.

Simple calculus:

d/dx x^2 = 2x

d2/dx x^2 = 2

d3/dx x^2 = 0

[potetgull0]

quote: "you sit with a beautiful woman for 1 hour and it seems like one minute , you sit on a hot stove for 1 second and it seems like an hour." Einstein made that up not YOU.

[listminusv]

gees language....its only physics chillllll : )

[forgotaboutbre]

I can see how one of those comments would flow from the others

[hamderjakob]

omg. Is there a 5th lecture in this series of general relativity? I'm finally understandig things, which I didn't understand before... please post the next lectures!

[robkim55]

Very good idea. Now I will understand General Relativity in my time