# Tag Info

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Conserved quantities in GR In GR, energy (or mass) is typically an ill-defined concept. In flat spacetime, we define energy as the conserved quantity corresponding to time translational symmetry. Extending this to GR is quite tricky mainly because, what one is calling time is already observer dependent (this is of course also true in flat spacetime, but at ...

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Let us for simplicity work in units where the speed of light $c=1$ is equal to one, and assume that there is no cosmological constant $\Lambda=0$. A spherically symmetric vacuum solution to the EFE of the form $$\tag{1} ds^2~=~g_{tt}(r)dt^2 + g_{rr}(r)dr^2 +r^2 d\Omega^2,$$ such that it asymtotically becomes Minkowski space $$\tag{2} ... 3 Take a future-directed timelike curve \gamma= \gamma(\tau), \tau being the proper time along \gamma in the spacetime M. Assume that p = \gamma(0) is the initial point of \gamma. Fermi coordinates adapted to \gamma are constructed this way. Consider an orthonormal basis of T_pM with e_0 parallel to \dot{\gamma}. Transport the basis ... 3 The conservation of \vec{k}\cdot\vec{u} only holds in the test particle limit. That is, it considers the metric to be unaffected by the motion of the particle. In this limit, there are no gravitational waves, since the metric has no time-varying quadrupole. If you want to see gravitational waves, you need to allow the metric to evolve dynamically, ... 3 Any observer outside the Schwarzschild radius sees the same thing: matter approaching the Schwarzschild radius at slower and slower (asymptotically zero) speed, forming a thin shell around the event horizon. The matter takes an apparently infinite time to collapse, and infinity is infinitely larger than a large finite the same way it's infinitely larger than ... 2 You don't. Two given spacetimes can have their metrics written in the same way but may have different coordinate ranges. A simple example is just a spacetime with spatial coordinate identified , such as the cylinder spacetime :$$ds^2 = -dt^2 + d\theta Identical to Minkowski space, which is its universal cover. Of course, two things to watch out for : ...

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EDIT: My first answer seemed to imply that radiation is at rest in the Cosmic Rest Frame. Radiation is not in rest in any frame. See below. The sentence shouldn't be read as "[velocity of energy] forms", but "velocity of [energy forms]"$^\dagger$. The sentence refers to "energy forms", i.e. the different forms in which energy can manifest itself. These ...

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The concept of 'straight' is a bit ill defined in GR and has no real definition. In fact in a sense the geodesics themselves be seen as 'straight' lines; they are the shortest paths connecting 2 points (this is what in normal Euclidean space would be a 'straight line') In the LC connection they are the integral curves of some vector field $V$ with $... 1 Prahar is correct, but here are two more things to note. If spacetime is an$n$-dimensional Lorentzian manifold$(M,g)$, let$\{E_1,\dotsc, E_n\}$be an orthonormal frame, i.e.$E_i\in\Gamma(TM), T_pM=\mathrm{span}\,\{E_i\lvert_p\}$, and$g(E_i,E_j)=\eta_{ij}$, where$\eta=\mathrm{diag}\,(-1,1,\dotsc, 1)$is the Minkowski matrix. Then, if$M\$ is orientable ...

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Well when there is no answer available, I do not think it hurts to try to come up with one. As far as existence is concerned, Geometry and Topology can exist without physical matter, but not vice versa. In fact, there is so much empty space without matter, but no matter without being in space. Empty space has some Geometry and Topology. However, to ...

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Of course cellular phone systems could work without GPS. In fact, cell phones DID work before they carried GPS chips! Bottom Line (moved to the top): if there was never GPS or an understanding of General Relativity, the system would have been developed just fine without it. Spend time in a city now with narrow roads and tall buildings (or inside those ...

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Correct; in general the speed of light is constant only as measured by local inertial observers. As an extreme example, consider a photon emitted from a galaxy far, far away, in our direction. Although it moves away from the galaxy in the direction of the Milky Way, the expansion of space makes it increase its distance from us. Eventually, however, it will ...

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Actually, it's NOT true that in SR the speed of light in vacuum is the same for all observers, regardless of the motion of the light source. This is true only for inertial observers. The same applies for GR, in which the generalization is a "freely falling frame" (a local inertial frame without effects of gravity). A good reference: Speed of Light

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