Physics Asked on August 23, 2021
My Level/Background:
I have just completed my first year of undergrad. In high school, I completed AP Physics C Mechanics and Electricity and Magnetism. In my first year of undergrad, I completed a course on Newtonian Mechanics and a course on Special Relativity and Electromagnetism which both approximately followed the sections on those topics in the Feynman Lectures on Physics.
The Question
I am starting to dive into tensor analysis and general relativity in my free time and I’m having some confusion about the Einstein Field Equation.
The Einstein Field Equation (without cosmological constant) states that $G_{munu} = frac{8 pi G}{c^4} T_{mu
nu}$ where $G_{munu} = R_{munu} – frac{1}{2}Rg_{munu}$ is the Einstein curvature tensor.
In most pop-science explanations of GR they say that matter and energy (or their density and flow I guess), which are represented by $T_{munu}$, cause spacetime to curve, which I assume is represented by the curvature tensor $G_{munu}$. Objects then move along the shortest path proper time path (geodesic) in this distorted spacetime.
They often do this by giving the pretty misleading picture of placing a large mass on a trampoline, where the fabric of the trampoline is spacetime, and showing how the large mass causes the fabric to bend and how this affects the motion of smaller objects thrown on the trampoline.
In the case of a spherical non-rotating planet, I’m assuming $T_{munu}$ is $0$ everywhere except for where the planet is. So that means $G_{munu} = 0$ everywhere not inside the planet.
My question is does that mean there is no curvature outside the planet (or is Einstein curvature a different thing than regular curvature)? Since this seems to imply that there would be no curvature in spacetime outside the planet which is clearly wrong since objects do orbit the Sun.
Or does the value of $T_{munu}$ inside the planet (where it is nonzero) affect the curvature of spacetime outside the planet (where it is zero) in a large radius around it?
In summary, what is the best way to think about how mass and energy affect the curvature of spacetime around them?
There are four different curvature tensors at play here. The complete information about the curvature is encoded into the Riemann tensor $R^{sigma}_{;mu tau nu}$, and the other three tensors are all derived from it.
The Ricci tensor is a contraction $$ R_{mu nu} = R^{sigma}_{;mu sigma nu} = g^{sigma tau} R_{sigma mu tau nu}. $$
The Ricci scalar is a contraction $$ R = g^{mu nu} R_{mu nu}. $$
The Einstein tensor is $$ G_{mu nu} = R_{mu nu} - frac{1}{2} R g_{mu nu}. $$
The vanishing of $G_{mu nu}$ implies the vanishing of $R_{mu nu}$. It is easy to show: contract the definition of $G_{mu nu}$ with the inverse metric $g^{mu nu}$, you will obtain $$ 0 = G_{mu nu} g^{mu nu} = left( 1 - frac{d}{2} right) R. $$
Here $d = g^{mu nu} g_{mu nu}$ is the dimensionality of space-time. Unless $d = 2$, we must have $R = 0$. Now plug this result into the definition of $G_{mu nu}$ to obtain $$ 0 = G_{mu nu} = R_{mu nu} - frac{1}{2} cdot 0 cdot g_{mu nu} = R_{mu nu}. $$
Hence, in the vacuum, the Ricci tensor vanishes. In fact, Einstein came to this conclusion even before the final form of his equations for gravity was finalized. He tried generalizing it as $R_{mu nu} = kappa T_{mu nu}$ first, and that didn't work out, which lead him to the definition of $G_{mu nu}$.
However, $R_{mu nu} = 0$ does not imply $R^{mu}_{;nu sigma tau} = 0$. The space-time outside the region where the planet is located is still curved, even though the Ricci tensor vanishes. Your intuition is also correct: if the full Riemann tensor was to vanish outside the inner region occupied by the planet, test bodies in its vicinity wouldn't feel its gravity, which is not at all what we observe in nature.
Correct answer by Prof. Legolasov on August 23, 2021
It is true:
$$G_{munu} = 0$$
at, say, the space station...yet it doesn't just sit there, does it?
Look at Maxwell's equation:
$$ {bf nabla cdot E} = rho/epsilon_0 $$
we could just as well say "charge tell the electric field how to diverge, and the electric field tells charge how to move" (to paraphrase J. A. Wheeler), but a zero divergence near a charge doesn't mean zero electric field.
Likewise, $G_{munu}=0$ doesn't mean $g_{munu} = eta_{munu}$.
Answered by JEB on August 23, 2021
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