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Unformatted text preview: Scott Hughes 13 May 2004 Massachusetts Institute of Technology Department of Physics 8.022 Spring 2004 Lecture 24: A (very) brief introduction to general relativity. 24.1 Gravity? The Coulomb interaction between two point charges looks essentially identical to the grav- itational interaction between two masses. Does this mean that everything we have done so far can also be used to describe gravitational interactions (perhaps with some slight modifi- cations)? The answer to this turns out to be almost but not quite. To understand what this means in detail, we need to do a little bit of background work. The key point to be made here is that Maxwells equations and electricity and magnetism are relativistically correct: all of E&M knows about special relativity, in the sense that the equations make sense in any frame of reference. (When we transform reference frames, we modify the ~ E and ~ B fields according to those relativistic transformation laws we all learned to love ages ago. However, the new fields still satisfy Maxwell equations.) This is NOT the case for gravity! Significant modifications need to be made to make gravity relativistic. What we end up with is Einsteins theory of general relativity . In this lecture we will look at some basic properties of general relativity. To begin, we need to introduce a bit of notation. 24.2 4-vectors Since space and time are unified in relativity, it doesnt make much sense to treat them separately. Accordingly, many concepts are described using 4-vectors , quantities that are essentially just like the vectors weve known and loved since kindergarten, but with an extra timelike component. For example, the position 4-vector is written x = ( ct,x,y,z ) . The way to read this is that is an index , ranging from 0 to 3: x = x =0 = ct ; x 1 = x =1 = x ; x 2 = x =2 = y ; x 3 = x =3 = z . Whenever you see an x with a number superscript, it means the index it does NOT mean take it to a higher power! This system can be a little confusing when you first encounter it. That the timelike component of position is time makes perfect sense. What do we get when we try to make something like momentum into a 4-vector? It turns out that the timelike component of momentum is the energy : p = ( E/c,p x ,p y ,p z ) . Well introduce a few other 4-vectors before were done here. 24.2.1 Invariant interval Back on Pset 6, we showed that the quantity s 2 =- c 2 t + x 2 + y 2 + z 2 is invariant : all inertial observers agree on its value. This can be written this in terms of a quantity called the metric : s 2 = 3 X =0 3 X =0 g x x where the metric tensor g can be represented as a 4 4 matrix: g = - 1 1 1 1 ....
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