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# General Relativity | Lecture 10

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Stecher University so the

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question is so as to make nite there more or less by accident there are exactly the ones that I want to address tonight weak gravitational fields Ranieri vs. nonlinearity and gravitational waves let's begin working out the equations of general relativity is all the ways on pleasant not going to do it on the blackboard Perry a word there are still a blackboard even for simple things than they probably would not be terribly illuminating to learn this subject really how the computer however solvent equations and so forth thing have to sit down and do it for the other hand the principles a straightforward enough and it's easy enough to say what you get when you do solve the equations so that we will talk about gravity waves our by writing down the equations and then writing down solution now we're interested in what could be called week gravitational waves weak gravitational waves means that amplitude of the gravitational waves are small enough that you could make approximation such actors the amplitude square of the gravitational field 0 or quantity is small and you're expanding equation about the smallness of our quantity the the usual rule is to ignore things which Ohio water In that small quantity but we start with an equilibrium what we're talking about fluctuations of perturbations about equilibrium situation and the simplest equilibrium solution aligned with the threat aren't equations 1st Einstein's equations but say the case without any matter Noah no energy momentum tensor and the equations of motion and just aren't you know minus 1 have Jim you know Our is equal to 0 Kobe Einstein cancer do you know of 0 they can be simplified if you take it traces of both sides are here is no trace of the year Ricci tensor I don't interest both sides you will discover that are the scalar curvature R is equal to 0 . you know that you could ignore the 2nd term in the equation equations said a doesn't really matter because we're not gonna write down the details anyway but this is the equation of Einstein in a context in which there is no energy momentum tensor on the right-hand side by Watson equilibrium situation and equilibrium situation 1st of all means a solution which has no time-dependent is really are and which also doesn't have any matter on the right right-hand side matters another word for the energy momentum tensor right-hand side not right hand sorry there's really only 1 equilibrium situation equilibrium situation is just empty space empty spaces tiny independent BTR and these entities thereby empty space I mean in B flat space no curvature no interesting gravitational field and that case the metric g and now I'm not going to say the metric is equal to such as such the method depends on the quarter to you if I wrote that the metric on a flat plane is just a crocodile assembled you would correct in saying no the metric color flat plane is not be Crocker Delta a possible metric expressing the flatness of space would be the crocodile the values the coordinates the that that would not be the crime use Kurt coordinates used polar coordinates used any other kind of course that's what's special about flat space is that you can find coordinates in which metric customizable form the same is true in general relativity flat space time with no gravitational field there is a choice of coordinates in which the metric has a simple forms of aid and you know just remind you what a them you Louis 8 immune Nova is a matrix that's a fright matrix laughter I think with notations we've been using in this class I forget some

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1 minus 1 minus 1 minus 1 0 0 0 no sorry correct 1 0 0 0 0 0 1 9 1 0 0 0 0 1 0 0 0 0 1 which throwing column corresponds time the first one T X Y

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Z T X Y Z so forth OK so that's metric of flat space-time 1 might 1 and 3 pluses Anderson equilibrium solution it is a solution of the Einstein field equations the Einstein field equations components of curvature are equal 0 this is a geometry of the symmetric has no curvature of all so it is a solution 4 billion you would find a solution factor will be trivial step now let's think about something which is close to empty space far away some complicated thing is happening far away so far only to be interesting to us at least to be close enough for us to directly observed there's something going on AT binary Paul Soros a rotating around doing some complicated emission of gravitational waves closer by the gravitational field may be very strong even the gravitational field waves might be rather strong but you go far enough away the gravitational radiation the gravitational waves produced by this are going to be very weak was weak means week means that the trough metric can't be chosen again emphasize can be be chosen people a them you know class something small small means that its components are much smaller than the ones which appear here and that small thing is usually called each as far as as I know it's core age because each is the letter after G don't know any of a reason for 4 h h mu Nova on like a them you know is in general a function of position it's also what I say position 19th position and time at a function of cordon that varies from place to place their might describe away will come back to raise moment but wary doing this take this metric calculate from it are new new and set equal to 0 and that's give us an equation for now the equator and that you actually get from H H it's not big enough to fill a hole blackboard big enough to move to quite the unpleasant and some scanning I'm going to be schematic show you what goes into 1st of all waters off our ears really is a combination of components Of the curvature and not right to curvature tensor underscore remind you would contain the curvature tensor itself with 4 indices contains 1 turned which is a Durant is with respect to position with respect to acts on on a right which X it contains lots of different derivatives depending on the index structure derivative of the off symbol solar contains a 1st derivative of the kerfuffle symbol and secondly it contains the Christoval Sonal quite radically squared opera star possible times Nova Christoval so this is really offices as much detail as we do I could put picked in the indices inherent remind you with the indices go in there are various turns of took like this there are various terms that but as a good enough for us what about gamma what about the staff or symbol of you remember that your 1st contains a factor 1 hailed it contains metric cancer with component upstairs the that with component upstairs is simply be in matrix of the metric tensor itself 8 can also be expanded powers of each of the 1st contribution to it it is just in Istanbul itself so the inverse aid is inverse matrix and so it begins with G the minus 1 on be more specific America and never has derivatives of GE it has various kinds of derivatives of GE Malcolm G in reverse that contains again things like leader probably minus H but whatever it is it contains he ate a symbol plus a small correction be metric tensor with upper indices if you only have

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Ada would basically be the same data and then age would appear as a correction 5 Sergey Mercier this would contain things like this is a legally equal sign which means not equal it would contain things like Ada minus a small Correctional plus a small correction report for us but then the of GE that only contains a because the derivative of this metric 0 contains a zeros in 1 derivative

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zeroes in 1 0 themselves and the approximation working in the derivative of G just contains derivatives of age which call derivative of H OK let's look at what we have here this the Chris symbol it contains 1 turned which has won power of H incidentally H is small and will also assume the derivative is also small if you have a small number of small function in a differentiated that's equally small general has very shocked point or something which will show the doesn't so the theory of H it is also considered to be a small thing they had times H is wants a small beach the derivative 8 is twice a small but quadratic In the fluctuation are in the yard in a small gravitational field so ignore it too small to be important if beaches . 0 1 and derivative of 8 . 0 0 1 then aged times H is . 0 0 0 0 1 whoever is very small but we can ignore this a net h will just call derivative of age is called so the crisp Opel symbol itself it is just in the approximation of small grabbed a weak gravitational radiation the Crist awful symbol itself is proportional to some derivative box and collection of derivatives of H what about the Ricci tensor then the Ricci tensor well contained curriculum gamma

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and that's going to be 2nd derivative on each it's gonna take contain various kinds of 2nd derivatives of the gravitational field insolently H is called the gravitational field of a field of the gravitational waves and that over here that's correct contained plus a derivative of aged times derivative of H gamma times gamma is quadratic in derivatives of age salt immediately was say this is much smaller than is similarly nut but so what are the devil this Ricci tensor it is it's composed of simple 2nd derivatives of the metric tensor from that we can conclude that Einstein's equations have a relatively simple form the sub plenty of indices around and the number of different terms involving 2nd rivers of significant it's not its it's complicated enough I don't write it down on the blackboard but it is basically built out of 2nd derivative 2nd derivatives with respect to position 2nd derivatives with respect to and maybe even some terms which have a derivative with respect to position and derivative respect at time and furthermore there are there are several components of it is not just many components of it well the Ricci tensor has a mule and no so as to components were comes out has 2 components but the whole works with composed of 2nd rivers rich a sold are equation emotion is some kind of the equation that looks like that equations of that form especially In relativity I usually wave equation which just to remind you a wave equation for any ordinary wave looks like but wave moving down z-axis wave moving down the z-axis also satisfies an equation that looks like this but say the wave field with qualified the way I feel fine was satisfying equation like these 2nd 5 IDT squared is equal to be 2nd fiery by Desi square is the simplest wave equation cannot Nagin and the solution to it all waves which movie even tho right portable left Ucon airwaves which go to the right and the left is still a solution their way which reflect this a little way of moving the left the sum was still a solution because the equations linear OK so this is where we can persuade his mind this is equal 0 if we had more directions space the structure of the equation would do more more complicated instead of just derivative of fire with his ceiling would also have Have a sum of terms plus seconder afire with respect x squared yet plus secondary terrified With respect Kauai square equal 0 but evident that there's a young family similarity between the kind of equation which occurs here and the Cairo court which occurs in fact but a clever manipulation you could make these equations this is not just 1 equation incidentally this equations the each mule in no but Brack around here just remind ourselves the are 4 times for 16 component of our new know that not all independent and it was really quite a bit simpler than that but still principal arrest 16 components 16 equations and that respect somewhat similar to Maxwell's equations Maxwell's equations have the form of wave equations for example of electric and magnetic field but there were several components there are 3 components of electric field 3 component of magnetic field sounds like is only 6 equations balance factory 80 equations of Maxwell's equations and this is the same sort of thank several equations per or similar similar form any questions is I would lose doesn't what orders to go yeah but their contracted in many ways remember that that's that's correct remember where army looking from it came from a thing with nose in the town or higher upstairs of downstairs series and then we contracted set up and go upstairs he so that New disappeared as as an index in we just gotta be the Ricci tensor so yes with 4 derivatives sorry with 2 derivatives and tow components 8 we certainly can make go Our many whatever whatever number makes a number of different combinations much more than just 2 indices but the combination were interested in is contracted in such a way that is only 2 wins this season only 2 equations level not election flesh as part of all is that he does good right of part of part of contracting introduce a involves introducing the metric again but again the same thing is true symmetric News Ada plus hour is composed derivatives so it already is small it already is small and if you multiply by most small number the stubbly small I think that your worker it for each time said yes but that is infinitely small their strength there's anything that involves I'm 8 multiplied by another nature of the river the Straits Times another derivative turn each time derivative rage it is dropped in this approximation approximation but a well-defined approximation in that it is the little of technically it's the linearization of Einstein's equations which means which simply means you throw away everything of higher power yet but it is a good approximation when the gravitational radiation gravitational waves is weak OK so we have equations of the form which were not specifically right down the kind of messy but but before we have discussed a before we discuss solutions you might ask itself well by originally told you that that it's not that the metric of flap space is equal to a new site can be chosen to be equal Tatum you know that means that there are other ways to represent flat face make us more make coordinate transformation make coordinate transformation and made you know when the metric changes but still exactly the same exactly the same solution and saw what that means is that they must be solutions of these equations which look like they have what looked like a nontrivial but really are just that Wally really represent is flat space but In coordinates when coordinates have little ripples another words supposing take the flat by black I maintained the flat blackboard there's a solution of Einstein's equations well about real quality is yet but the since ending here on it is a flat space it can be represented by metric

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Delta mn website Annan instead effects wife on Elian there's nothing to prevent me from introducing court coordinates with little wiggle in well we could start with coordinates what's we start with coordinates which other good Cartesian coordinates let's call them X XMY will occur with contracts that's a lot more than they would introduce no coordinates next time and why Prime where X prime is just X a small change quickly call that plus f sub x of excellent y as according to change according change X prime is some function of X and Y and Y Prime some other function of X and Y at . change here at the change is taken to be almost no coordinate change at all plus a small Will correction the same thing for y prime musical Hawaii plus let's call best why aleksandr White I have not seen the space it anyway all I've done is change coordinates and perhaps put some little wiggle sofa 1st test of wills and then the no coordinate axes might have revenue Courtney lines might have some wheels and but the 4 if I rewrite the metric for the current components are of let's say you are let's OK let's take the the case looks assumed the metric in the Klein Court trips is just be X Times squared plus BY scores I'm working backward and supposing that prime coordinates organized coordinates the mice Cartesian coordinates and the X and Y coordinates Of the year of the 1st slightly curved form but was the metric the metric this is just a blackboard now outgoing space time just a blackboard the magic of the black His DX prime squared forced to wipe right square let's look out for fun in exon wife OK so what is the former work at work DX primaries DX front commerce DX is equal to DX plus the derivative of f sub X with respect to its call x sub x Kobe the X or Y funds DX and and likewise for White we figure out work DY Prime Minister DYE plus a small correction is year I want find out you find out that the X Prize square plus 2 why Prime squared is not D X squared plus Y squared it contains DX square post y squared but also contains order of VAX times this Andy times that if you work it out but you find out that this is equal to D XTY sorry the BX queer Post e Weiss word assuming that a small we're going to assume that the small and that means when I Square D X prime squared I Icahn drop the F X squares likewise for why don't wanna get is a correction and the correction could be called a each and VXM excellent another words just by squaring up PX foreign square BY prime square substituting in it you'll discover that is a small correction to the metric tensor now there's a small correction the metric cancer mean that the geometry of a blackboard change now it just means that beyond that the accord the views of totals in Ukiah worked out it's it's it's nice to work out little exercise worked out with the correction in that is dropping it anything which is quadratic in an dropping anything which is higher or not want to get you get that H & & And now just run over X and Y is equal could be be F M D X In Class D In my DX and that will mean nothing to you want to you try the work as an example onto you tried approve it this is the a correction for the rhetoric it's correct from Electric which is small white because of because I chose F to be small but it has the form wars a small perturbation on that but doesn't represent anything it represents just a trivial love change so there are some let's call them perturbations perturbations small change perturbations the original form of the metric which are trivial they don't represent likewise here there are small fluctuations that you could write down which just represent curvy linear cordoned space how do you get rid of that I get rid it and eliminate these the phony solutions which automatically solve the equation because they just flat space but which don't represent any real physics Eddie Dillard by imposing more equation more equations on the metric that so cut away and divide I would eliminate b b unwanted spurious solution I I could write down the equations it's not important just equations which tell you get rid of the spurious accord that there were coordinate ambiguities and you could do that a wide variety of ways wide variety of conditions that will allow you to eliminate on physical mean lists solutions of Einstein's equation once you've done that and you've imposed all of the Einstein equations once you've done that incidentally the equations become pretty simple and in fact Ole become as wave equations they become wave equations Camara several different equipped to become 1st of all with ordinary way just perfectly ordinary wave equations for the components of H. Singh continuity equations I wrote Barrow USAir race there be 2nd foray by DP squared minus D 2nd 5 90 x squared blah blah blah why Square D squared is equal to 0 facts of this summer let me right now In the correct form instead of fighting we have each year each meal each component of the metric tensor a rich component of the fluctuation satisfies a wave equation that means that all the components of the wave equation sir all components of the metric just moved down the actors Roark warrior always there are also some constraints constraints calmed the lot more equations from the

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equations are their arms world there are more equations then 1 for each new until the reason that there are more equations is because you have to divide How wrote this spurious they solutions there because of the court of ambiguity want to that you find out that be physical solutions the ones that really have meaning classify them out for you supposing we have a way of moving down z-axis but suppose there moving down z-axis was a way of moving down z-axis look like wave moving down the z-axis Italy just a simple way you would just be fired is eagle something like some number times sigh of Katie Current X is a stark terms in team but came out it is in Manistee tonight as it is now said is a wave that fixed incident of time is just a sine wave and most family taxes unit velocity unit velocity here means a speed of light now each 1 of the components will have a solution like that each component will be proportional to sign Kate times what was Katie insolently after is close wave number but sister frequency of away is the frequency of away it's also the wave number the number of oscillations per unit weight the because could really think beginning number at short wavelengths of large a long wavelengths of more of a small pockets of that that's a typical solution of a wave equation and the solution of this equation here are all the same type H not is equal to some function coefficient elliptical coefficient trying not fight the number I each component as a function of teens E which is proportional saying things sank 8 times minus times and number which we can call each lot of new and here new window this is not a function of position it's just a numerical coefficient and multiplies the sign of human misery and Arsenal right that this is the nature of a gravitational waves each component of the metric behaving like a wave moving down the axis however as I said there are more equations additional equations is a good number of equations here and when you colors all you find out all before we do this looks just remind us something about the electromagnet magnetism and electromagnetism we do the same thing we solve Maxwell's field equations with song Max was a field equations for the vector potential of electric and magnetic fields but we write down all the equation we find some constraints constraints are called transverse our view of the fileserver nor transit salary of an electromagnetic field things it means of electromagnetic field electric and magnetic fields always point in the direction perpendicular to emotional not only are the solutions waves but that transverse waves meaning to say that the electric field and the magnetic field magnetic field . 1 way the electric fuel policy of golfing goes down the axis oscillating is a goes down the axis very similar things happen here the way you have to be transferred to say the way a transverse it means that the kind of components and the span Z components of H 0 0 only components of a which are allowed to be non-zero other components in the plane perpendicular the direction of the way so his substitute this year but all the equations the full set equations here including the ones which removed the fake spurious flocculation you find that a gravitational wave has a very very simple form does have this form a set of numbers fictional but the only component several hours this Izzy Texan Y I'll call jail eye injury represent the directions in the plane perpendicular to the motion of the way of the only components which are large part each I J climbs sign KM Casey Kizzie miners Kate t but but so if you're looking down z-axis if you looking down the z-axis you see in front of you the X-Y plane received from the X-Y plane as the x plane and the x plane but each ZTE lures a slice of slice by slice of a given location at a given time at a given location and a given time to the left of the two-dimensional plane and I slice the wave in two-dimensional planes as a good axis here at fixed instant of kind and what I have here is a metric for each Zee which looks like Where is it right over here it With only comport with the only components the components only in the plane perpendicular to the motion of the year of the year of the way and the composer simply numbers and each ZCU the the metric simply a set of numbers there's 1 more equation 1 equation that comes from Einstein's field equations and it says that the traces of H I J is equal to 0 in other words H X X plus each y y is equal to 0 that's it that's a whole set of equations and what it tells you is that the metric the metric of a gravitational waves as age not not equal 0 age zeal Nora physical 0 H Z easy is equal to 0 in fact H anything with either at time component or a space oryzae component Oracle 2 0 the only components which are not 0 Laura age X X H Y Y H ex-wife HYX comedic cancerous symmetric cancerous research brick and so H X Y N HYX of the same and finally HXX force HYY is equal to 0 which means that each HYY is minus HXX HYY physical commodity sex but trisyllable force amenities that 1st of all of these very With

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Z & very holy was eager to if the wave is going down z-axis than by definition variations along the c-axis the sole HXX is some number to begin with punch signed KX signed hazy minus being sworn h Norwalk YYA sign K times in my and likewise Suarez H. North ex-wife sigh case that OK so think of a series of layers let's let's look at it at a fixed instant of kind of fix since time for example currently equals 0 as we moved down the exodus OK is a perturbation On the Net at the Met cancer is a little bit different from just a flat space metric it oscillate as you go down Zia third-down z-axis end the components that oscillate other components that have to do with the metric of the plane perpendicular to the wait what does it mean the have HXX well with just a case of accepts differs MHX acts that means the component of the metric is a little bit different than 1 the XX component of the metric was originally warned that Ada Ada XX it's 1 plus this little bit Obama bus so big that means that proper distance along the X axis are a little bit different that actual distances but say between x equals 0 0 and 1 Sea between X equals minus 1 and 1 the actual disk and the actual distance remember the metric tensor is thing which tells you the actual distance between points the coordinates the labels the actual distance between point minus 1 . plus 1 is a little bit different then it would be if this perturbation Syria a little bit larger separation or a little bit smaller separation depending on the sign of age except for example HXX is positive and the actual physical distance between here and here would be a little bit larger than 1 meter supposing working in meters that would mean the meter doesn't quite extend from minus one-to-one What about GYY well with GM will take HXX to be positive if HXX positive a yard to get that right the distance between minus 1 1 was a little bit bigger than a meter and therefore a real meterstick would look when without look it would be a little bit shorter the interval between minus 1 1 not the meter stickers necessarily shorter but because the coordinates of such that bet on that surface the distance between minus 1 plus 1 would be a little bit bigger than however GYY GYY is equal to 1 minus H X X minus HXX because Einstein's equations tell us that some of these 2 are equal to 0 that turned tells us that meterstick oriented along the Y axis what's safe from white equals 1 2 minus 1 here's again warned minus 1 meter stick would look a little longer the distance between plus 1 of my 1 would be a little bit less if you only have the 1 he and so are meterstick over here would look a little bit longer if it now this would be this will be nothing but a squeezing of the coordinates so that you would be choosing coordinates along the x-axis with meterstick was a little bit less than 1 unit and the music along the Y axis was the big greater than 1 will be nothing at all will be Nolan nothing significant accept the the fact that it changes as you go down the axis as you go down the exodus from 1 point or another or if the wave passing you imagine that even yet the wave was passed in the wake of his passing these sticks right after a brief amount of time these beaches would sign that sign of assign wave oscillate so after a brief interval of time 1 half cycle of the ways 8 X X has changed sign HYY his change sorry and the result is after a brief interval of time meterstick will look a little bit short in these coordinates along the y-axis a will belong along the x-axis yacht readers it's not know what express that little test In fact a little test ministers would go back and forth but not because the meter stake is Elliott 1 meter long always 1 meter long but right but an oscillating leave is going back and forth perturbing real waiver real wave of curvature this is real curvature and if you like Mr. Guo Li told Maginot around well the time dependence of this way you've really does exert tidal forces that tidal forces a wave has curvature of the waves moving past here that kind of tired force in the nature of the tidal force is actually cause meterstick be compressed stretched compressed stretched the way that compressed and stretch is that when it's compressed horizontally stretch vertically when it's compressed vertically it stretch horizontally so if you like this wave going by means of all art for that matter through which is to test masses really were just 2 test matches the 2 test massive oscillate back-and-forth here from it instead of taking a meterstick Turk a square piece of plywood square piece of plywood would be deformed this white and red river this way and then reversed now it's interesting there is another solution was another solution where it In which but not H X YEAR nonzero but in which ex-wife but is nonzero that would be a metric which would look alike crocodile filter and then what they'll IJT a little matrix which haven't HXY over here HXY over here still has HXX 6 y y equal to 0 knowledge XXH for 1 if you think about a little bit just spend a little bit of time figuring out what this means it's also the exactly the same kind of thing except that we accept that the squeezing stretching our

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along 45 degree acts stretch this way squeezes way ocellated it's none the thinking of Stalin news solution except that the original solution by an angle in fact any solution that you right there which is a linear super butchers superposition of these 2 kinds of solutions any solution linear superposition of 2 solutions and they correspond somewhat orientation of taxis were along those taxis we Yarborough compression and expansion Our along those that use all there is all the kind of gravitational waves are Picasso's Maxeys perpendicular to 1st of all you pick a direction that the gravitational waves moving down to you has a strained age estranging this will create honest real stresses In the year in the piece of plywood on a strain gage will register our when this waves going that's the wave was that if the way was static and not moving vans this piece of plywood would really be unaffected everything would be simultaneously squeezed illustrates the same way the rulers which measure the apply would strain gages which measure but the oscillating character of the way the year of the solutions which really does create real honest stresses and strains of it does have a real curvature and this award look it it looks like a magnetic waves triggered by her or by a moving charged by or a charge for example 1 charge charger electronic going around a Proton will create electromagnetic waves the same things over a year start black hole or pulse on every in orbit around the other 1 binary pulses are for example the binary pulsar 2 very very concentrated masses which happened to be rotating around each other at rather small distanced the 1 was a black hole the whole thing is not a black hole but it is a very very strong gravitational field in or around each other and going around each other be accelerating masses create don't create the gravitational waves so it now near the binary pulses are this as a bad approximation of gravitational field is too strong the linearized equations way but did Back in all of fewer a few radio distances away and the waves spreads out there loops itself and gets weak enough that this becomes a good approximation so workers with a few had the mind if you have the binary pulsar Soros over there and you're standing back this coming at you but say the axis between you and the binary pulsars z-axis there you'll see coming past you will look like and what will go usable cause stresses and strains a longer perpendicular the line site for the posts now not less Year's good luck dark red the strike did not former mural leadership change has prices stressed by the way measures by the stresses and strains of induced and now they are they're tied to say that's not meter because by definition of meter stick but found the guys who had that if you had to it did to have a steel meter stake in a wooden leaders that they would probably react differently they would probably react differently so once these stresses are applied stress due to the curvature tensor he and real thing curvature tensor it is a kind of a tidal force once sees tidal forces are applied the meter stick 2 meters the steel won award 1 was stressed by different amounts in general so if you measured you wouldn't stick would still need a stack of Russia which 1 would be bigger which will be smaller but they would match up precise for young would build a rather than resort as can make for of seal leaders last but it's not way but if best right and that they had no less space dust while not known from what was quite was based on the B accelerated are 2 start ringing that it wanted detected gravitational waves what you want to do is create some system which has a resonant at the frequency of the gravitational wave in or residents a resident means that the system has its own natural frequency frequency of oscillation which is the same as the oscillation frequency of the wave than you would have a driven system being driven and so natural frequency and under those circumstances the response will be particularly big so if you want to detect the gravitational wave fault but say that the gravitational waves that we expect astronomy from different sources are not grumbling among expiry press but there are no enough about it to know that each new New which are produced by adjusting gravitational source of very very small so small that the dimensionless drain from a steel rod or something like that would be something like 10 to the minus 21 really really small effects are we fact that people even contemplate measuring then quite astonishing but Ginger can be measured and dumb not not that says that those Lagos and resist them a what Louisiana and some of the that but it does Barisal of lizard they not the rest of my life and I've got a lot of the gravitational because not based Sarsfield Ron marriage which are monitored by laser beams interference effect due to a relative motion and so forth but basically the gravitational detector is a system which is allowed to be 7 consolation you go this way or that way and there is a lot easier that the father was still alive but died it may not be the last 2 or fewer reduced or gravitational collapse gravitational collapse for the collision of 2 black holes would make that happens with prime from time where it happens I mean to say that you could estimated roughly from observed fact Germany black holes are out there and you can guess how many black hole collisions take place every year every year the final resting within within a range of such of the best imagined detectors value is not 1 black hole collusion per year a lot on when I throw in principle if it was possible to detect the black or collision through other means other gravitational radiation you would you ought to see the signal at the same time as a revocation a radiation bomb gravitational radiation is a pretty weak effect renewed far back but from the collision of 2 black holes it's an enormous effect

59:34

when you're at a much bigger than any other kind of radiation emitted so if these coalition taking place a large cosmological distances it can't be the case that the only way to detect it would be to gravitational radiation gravitational waves like this that's what's interesting because it's a new window her on for astronomy OK that that's that's a gravitational radiation is less a gravitational waves are they real an hour ice U.S. gravitational radiation is detected yes and no it's never been detected by direct detectors but just like electromagnetic radiation for example from an orbiting system radiation carries off energy carrying energy means that has 2 systems a rotating about each other they will lose energy and losing energy they may speed up a little bit they will speed up a little bit there are a total loss of energy and so the study of the binary pulsar that's out there which Is has all the properties necessary for gravitational radiation be emitted from red when the timing of the procession a procession a rotation of the 2 systems about each other they are frequency is changing a little bit time exactly as would be expected quantitatively exactly was would be expected from the system if a worry meeting gravitational radiation are gravitational radiation is mean the thing which would cause its energy to change end it works out exactly right so much for 4 milliseconds ms out I think you will go up a master of will slow millisecond ms as were pulse or does what rotator Anton access met will slow death old days forbade days AIDS is to start going around each other saying Ms now arms Ms is for the rotational pulse on a visit to stars Gore around each other that's not of the quest of yacht a basic life desirable over did of excess heat In other words the always a cease fire other this is the direction wave has directionality along with the more the waves transverse which means the only component that have physical significant other ones in the plane perpendicular to the direction emotional the other components are alarmed by the not there or they correspond to wiggle in the coordinates the other components are eager wheels in the quarter notes or them out there now but he also said that took geometric mean of a daily and remember how right how much is it changed over the period of the juror go for a very small number but that much of the at their now I was actually lost as much time as not oscillating the wave was oscillating with time reduced demand place the ocelot time yes but you would ask for a time of court we note O'Donnell when I set orthogonal I meant the space directions of Barbara Walters 3 bomb to restored the components of the metric whichever time component in them all Oracle if much like an electromagnetic waves electromagnetic waves also has space and car electromagnetic field also has space and time component on the vector potential has space and time component in a way of moving down the z-axis only the transverse space component of the vector potential are of their very much like that for the 0 0 0 all year yet so there's no time while another Misko current direction remember something arm B H is in those directions on 0 but when you differentiate different you with respect time yet climb into it so derivatives are H. Schwartz said uh XYA with respect to T is not equal to 0 and remember the curvature tensor has to derivatives on age so you could get a whole load of whole collection of different kind of component but still it's a very special kind of curvature of the of generic OK that's gravitational waves you can learn more about them by any good book Unger wrote committee which will solve work right down the details of these equations an will they will feel about 1 blackboard there will be terribly illuminating but there are the basic principles rep told you any other questions or are planned about this a lot has been made the approximation of a very small H set was getting rid but no no no laniary was because the this weak generally speaking I tried to explain before a small oscillations about an equilibrium in the approximation that be oscillation is very small are linear linear because of approximation that age is so small that the square of 0 base never question yes you do this for a that you see for us at the year's target if a 2 . 0 0 1 the corrections will be . 0 0 painters one-tenth the corrections will be 1 per cent whatever at whatever age is the corrections will be of aboard a square of it so writer but or something yes that that's something you always do when you make a linearized approximation you always Jacoby and that the corrections to it it would be small brokers or they will be if they too small enough but there's value made each watch warhorse is a component of a hot water that affect these fat that I had years to get with that now that but

1:08:52

what will affect is whether waves will pass through each other they won't it will scatter of whichever now the way will maintain that's a good question Rabin will maintain its transverse care can wait show that always the energy nearly is in this this step this area it said the young stiff most of the material does determines the velocity of propagation material you could substitute for stiffness sugar substitute velocity of propagation electromagnetic gravitational waves are extremely staff they have the stiffest things possible and that the year at the speed of light and that they are both gone by the speed of light serviced in in the old days when people thought about ether things and mechanical properties of the ether they attribute his stiffness was a core Young's modulus Everett called 2 were about to to eat there and it was a very very big young Marmara there'd big coefficient of whatever it's called because the speed of light is so much larger than the speed of propagation for example through a metal beams armed with no longer really think of it as a step this month he did think about this the stirred as you would say that the year of the space on which these waves moves step and there is that Norlien only the combination of them appears in New year and the propagation of the propagation of like just a which is what combination namely velocity which appears Her Truro OK whatsoever normal but finally we have just want to tell you about another way of thinking about the equations for general ability again huh the complexity of actually carrying out the equations is too complicated but nevertheless basic figures on what other things I've told you and emphasized you particularly in classical mechanics is the principle of least action is the most central principle on classical physics that we have all systems it we know of the chemical sisters electromagnetic radiation quantum field theory all of these things the Standard Model particle physics all governed by an action principle reaction has something which field theory in front of a particle mechanics the action is an integral along the trajectory of the particle so the rule over time the action of a field is an interval of space and kind of field configuration is a value of the field at every point of space and kind of withdrawing airspace time or that rolled down on the blackboard kind resist way a space because this way there'd been a field configuration of Field history you better field history is just label by Abbas is basically a value of the field below fields at every point on here so it's some field score genetically 5 at a function of X Anthony field configuration not every configuration the solution of the equations of the theory just like every trajectory our is a solution of the equations of motion every trajectory is a thinkable trajectory but not every trajectory satisfies inquiry motion in the same way every 5 sacks and is a thinkable possible value of a field in space and time but they're not all solutions and in general the best of our knowledge every such field is governed by a principle of we stature the principle of least action involves some action which is composed of the fields this is just a generic term field into go into go over space and time be X T and X now means x y and z Z sort of four-dimensional the rule of some things usually call the Lagrange density which is a function of the fields and their derivatives of corn of mule derivatives of a field of respect X new nobody don't you minimize it when you make it stationary you find the field configurations you shorthand which minimize the action subject constrained you go out to a large space of some larger dissidents and some large times Dixon region of space time and you will see what the values are on the boundary and it's the same deal as when you look up particle motion and is interested in the solution giving the particle goes through point given that the particle start some place and in some place then what's the trajectory of between the same way you say supposing you know that the field at some later time has the value of safe final initial ask what is the solution which interpolate between these 2 and so will be the middle of the field that solutions that make reaction stationary OK some of the point here is that the field equations the Einstein field equations can be derived from 80 action principle that's long computation but action simple undiscovered tell you what the action is and then you can go home try to take that action and apply the Lagrange equations to it and see if you could derive Einstein's field equations I'll I guarantee you it can be done also could tell you I never finished it started several times and never quite got to to be in love but it is it's just tedious it's straightforward but tedious sir you go back and look at what Lagrange equations are arguably action has some interesting pieces in 1st of all let's do a simple thing 1st let's just calculate the space current volume I'll tell you what I mean no well but that's actually even think about ordinary space for but suppose we have ordinary space and it has a metric g M and thinking about space savers use and GM and EX and VX end that's symmetric square the distance between 2 neighboring points was given by the formula that's a positive I'm interested in the volume of a small regional space let's say DXT y let begin by supposing the metric has 2 components GXX GYY but I have a smaller double D X in the small but will be able to worry I want to know area called the volume volume needs 3 dimensions area means to dimensions but at the same concept OK what sea area of this square the area will square I would know it

1:18:04

if I knew the distance the the actual proper distance associated with the X and the actual proper distance associated with be while I was a proper distance associated with them X VXC but square root GXX the square root of GXX DX period that but make it bigger Mr. the square root of GX X DX and this distances square root GYY wife cell what's the area of a rectangle square root GXX squared you DXT why it's not just CXT wife important thing is it's not just the exit to want no more so than the distance between 2 points is just XOD while the actual physical area is a square in this case is a square GXX GYY now I use the case here where it was only GXX Angie why there could be other components of the if you work it out this is a special case this is a special case with no off Bagwill GXY terms of strips of this way metric matrix GXX GYY now there may be some other components year GXY GXY thought I tell you it is that when there's no GXX and you want to share with was not off diagonal components the volume element involves a product of G X X GYY you guess with the general formula for replaces the proper thing determined determined GXX GYY miners GXY square the determinant is exactly right so if you actually work out the volume element of a little square like this or rectangle BXT White they actually answers not GXX URY GXX GYY miners G X Y Square which is as the determinant determined of the 10 it's usually just written with Tom bars to represent determinant Gee that's a formula for the volume element of volume of our small differential be ex ante why what would be a B and this can be a curved space the 1st any space for ever will rectangle BXT in coordinate space area or a higher dimensions it would be the volume would just be the square of G square of the determinant of G terms DXT worry if I have some large regions and I want a total volume of it I integrate into growth I love square root of the determinant of metric tons DXT why that is equal the area of the entire region and close with some members What about the volume of space if we're talking about the volume of words three-dimensional space what exactly the same except you have these here exactly the same square root of the determinant of the metric is called the volume element usually called volume already if you have a fuller dimensional space-time it's useful that finest simple a similar Carter concept which just involved the 4th component called the space volume it it's an interesting quantity it's invariant same in every coordinate system as an important thing about it if you re cordon the ties region change coordinates this quantity stays the same volume in some sense the physical volume of a region of space time of physical four-dimensional volume of regional space and this and the goal is invariant quantity on 0 invariants other quantities which are invariant them which will not change when you change coordinates are based on the squad if you take the volume element a new integrated with any scalar let's call scalar passive acts as aleksandr Cheney BXT Cheney Miss also is invariant quantity which doesn't depend on the court it's just the stuff you take each little volume element you take his volume and you multiply by the square root of starting the scalar field that the volume of each element multiply it by the scalar quantity in there and get them all out that's a quantity which is really not dependent in any way on the quarter to use even writing it down the uses of court action is a thing which is supposed to be in area action in relativity theory is always supposed to be the same in every quarter NetFrame otherwise the laws of physics which came out of it would depend on the coordinate system if the laws of physics was supposed to be the same in every quarter frame 1 way of ensuring that is to make action In invariant the quantity which doesn't change from cordoned the the court reporter 1st quarter so it's natural and to take the action could beef we call it before exiting some kind of squared and Janey the square of the determined from metric some scalar that was just take b Einstein's theory without any energy momentum and on the right hand side just pure gravitational field nothing else what kind of scalar cut I put their where the many dealers are there that I could make up only out of metric well indeed so that's called are the Safina exceed that's a scalar Brazil sir there was 1 more thing the other thing His any number 7 4 or 16 or whatever number you light so you could enter this numerical numbers numerical numbers are always regarded as invariants everybody 7 sees the same number and the number you can't hear depends on the laws of physics it is a law physics in itself was a law of physics has a name was called up they would know they were symbol is Simone slander now what's a cold the cosmological constant gets it termed the action which is just proportional to the space time volume itself turned the action when the Kansas the reserve is based on chair of the cosmological constant has not been and not included in these equations it is a solution is the equations without cosmological constant or about cosmological constant This is the action and Einstein's equations are completely equivalent saying minimize their make it stationary maggots

1:27:17

variation equal to 0 away from a solution of minimizing something making your stationary yacht 1st distress image attempts a would comment as additional terms reaction which would pay and on material and other fields for example it could be electromagnetic energy there could be other kinds of energy they would come and as additional terms here yup This is a vacuum is back in theory is vacuum theory for example the theory which would govern gravitational waves of the other things could be in here but those are the things would be made up of different fields they always have a square GE they always have a scalar if you remember your electromagnetism electromagnetic field was described by man dosimetric pants that you know 1 of the things that you would put here would be f new half at work cover electromagnetic field but let's leave that out what's Weaver here's here's the Einstein Gilbert Einstein action discovered I think I just about simultaneously by Einstein Hilbert got Einstein already had bad you really have the whole works hurl work but they In the independently he sat down and said can these equations be derived from action principle and both of them Gilbert on Einstein both came up with the same answer this is a the it was shorthand but not just assured him the physical a physical principle that contains all the Einstein fuel would incidentally what replaces this field said a field on space would you put in metric metric this is an action is made up of the metric then you very dear symmetrical little bit to Mayor varied electrical little bit could you look for places where you formed a minimum reaction Wearing magical will bet if you very all waif from a solution the action should grow bigger in any direction that you've area so this is the Einstein Hilbert form of general relativity as I said you could sit down and see what you have to do you have to calculate the curvature tensor in terms of derivatives metric you do that prefers calculating Microsoft will soon and differentiating the Chrysophyllum symbols multiplying them together sticking them altogether multiplied by the school do whatever I contractions unnecessary to produce the year of the curvature scalar multiplied by the square root determinant of the metric for by Fulham attract for rows or columns the determinant is a great big thing made up of auto components calculated the tenant and then applied or a Lagrange variational equations that action ended takes about 5 minutes the whole operation and out of it you will get signs field equations of can't do in 5 minutes you the of ever trying to do it for 50 years and never quits its he did not love my friends have that it I had a chance to get and care for them Utah and Fox's out here he's a very nice now known as a toddy repeatedly the principles themselves are not terribly hard for whenever a reason somehow anemia I know there are people who have such a good visual field for a threat curvature and arm the differential geometry that they can more or less see through it you have a choice either a person with a great deal of visual ability to be able to see through the complexity of our curvature tensor is and so forth and figure out how to do these apostasy bodies almost visualization while you work amount by the algebra a few good algebra you go 1 way if good that the visualizing geometry and you have a great command of differential geometry you probably could short-circuit a lot of that I frankly have neither I am not very good at algebra and I certainly don't have the visual ability to be able to visualize four-dimensional curvature answers so repeatedly have sat down and try to do this calculation usually in the process of teaching the subject and give up after about 2 hours if I had to do it I could but since I don't have to adopt down good don't ever tell you my friends and I can't do it OK meet yes they know I can't do it this but I doubt if that it is not only used those pages that concerns written where a dozen help me when you have a thing that complexity it really doesn't help follow the steps from 1 step to indeed do them yourself and you figure out what the uh what the relation between the steps or you don't for very long calculation really is not very illuminating unless you try to do it yourself and there when you rent a troubled you peek all that's the the next step on the new understand why its next step by Vera I'd never learned anything by looking at his longer calculation these days people don't do that people love from a computer b the numerically are analytically than what the computer grind out because is just too nasty complicated yet I'd say this more motivated than originally thing away from what I what we think is a parent firms papers what is taking so long on the on peppers I read the early ones around 1907 and in 1916 paper and your 50 years ago right now I really don't know what they were

1:35:02

barred part of it is that very very few people know very much about Riemannian geometry I still know he had the he basically had to invent the whole of Riemannian geometry I think 1 of his friends helped them with food Newsom mathematics was Russell Grossman a murderer but but take years to put all this together doesn't sound like a lot of them are you he literally had to build all the ideas of curvature are so it took a long time here think I think he knew it was doing pretty well but there you flow back and look yourself papers are in English there were translate was you could find Boca Our Antigua finish work or you're doing rushed for more

1:36:16

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### Metadaten

#### Formale Metadaten

Titel | General Relativity | Lecture 10 |

Serientitel | Lecture Collection | General Relativity |

Teil | 10 |

Anzahl der Teile | 10 |

Autor | Susskind, Leonard |

Lizenz |
CC-Namensnennung 3.0 Deutschland: Sie dürfen das Werk bzw. den Inhalt zu jedem legalen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen. |

DOI | 10.5446/15033 |

Herausgeber | Stanford University |

Erscheinungsjahr | 2012 |

Sprache | Englisch |

#### Inhaltliche Metadaten

Fachgebiet | Physik |

Abstract | (December 3, 2012) Leonard Susskind demonstrates that Einstein's field equations become wave equations in the approximation of weak gravitational fields. The solutions for these equations create the theory of gravity waves. This series is the fourth installment of a six-quarter series that explore the foundations of modern physics. In this quarter Susskind focuses on Einstein's General Theory of Relativity. |