A theory that describes how matter interacts dynamically with the geometry of space and time. It was first published by Einstein in 1915 and is currently used to study the structure and evolution of the universe, as well as having practical applications like GPS.

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Why a timelike geodesic maximizes path length?

I'm studying some GR and my book says that in Pseudo-Riemannian manifolds geodesics may even maximize the path locally. That's what happen to the timelike geodesics, for example. My first question: Is ...
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23 views

Gravitoelectromagnetism or graviton thory to derive equivelent formulas to relativity theory [on hold]

Is it possible to change the definitions of an event happening (which currently in relativistic theory is defined by the moment you see* something - the time light took to travel to you) such that ...
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83 views

If you shine a light away from the center of a Schwarzchild black hole, will it stay still?

$$ A $$ Light is travelling away from the absolute center of the Schwarzchild black hole (i.e. none of that spinning/stretching field) at $\ C $, and so shouldn't $\ \Delta C$ need to be greater than ...
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23 views

Maximum Power transmitted using General Relativity waves - cf Schwinger limit

In Electromagnetism, QED says that the linearity of Maxwell's equations comes to an end when field strengths approach the Schwinger limit. Its about 10^18 V/m. What is the corresponding formula for ...
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32 views

GR. Gauge fixing in 2-sphere [on hold]

I am working in GR with a tensor field ($A^\mu$) and a complex scalar ($\Psi=M e^{i\Theta}$). My geometry is $S^2$. I have my Lagrangian and my 2nd order equations. I want to fix the gauge, but I ...
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0answers
62 views

Lagrangian, geodesics and relativity [on hold]

My background is in maths, but I have been studying some basic physics with occasional input from a friend who is studying for a physics PhD. Due to my background, I am keen to visualize things ...
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2answers
114 views

When will physics understand it is the 'dark mass' not the 'dark matter'? [on hold]

There is new thinking occurring in physics where 'dark matter' is considered to be incorrect. The new thinking is that mass fills 'empty' space. The mass which fill's 'empty' space is being referred ...
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1answer
43 views

General Relativity - Four Velocity Derivative Question

I am trying to get my head around a small point used in a book I am reading about General Relativity. The book states that because $u_au^a = c^2$ it follows that $u_a \nabla_b u^a = 0 $ The first ...
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28 views

Is Twin Paradox solved in Special Relativity or General Relativity? [duplicate]

I'm getting pretty frustrated about finding the true solution to the Twin Paradox. If I open 10 relativity books, they all tell that they have the correct solution and the other books are wrong. So I ...
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1answer
45 views

How can we define a frame of reference in general relativity?

I have started reading general relativity. (A First Course in General Relativity, Bernard Schutz). I am finding very hard to understand a frame of reference. When I was reading special relativity ...
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1answer
39 views

Do two bodies spinning about the same axis in the opposite direction repel each other?

I recently read about the GEM equations, which look very much like the Maxwell's equations. Does this mean the behavior of mass is like the the behavior of the electric charge? So for example if you ...
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40 views

value of $n$ in modified gravity model

In the modified gravitational model $ f(R)=R+\lambda{R_{0}}\left(\left(1+\frac{R^{2}}{R_{0}^{2}}\right)^{-n}-1\right) $ Are there any restrictions on the value on $n$. Also is this model valid for ...
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1answer
56 views

Spinor notation in general relativity

I have a somewhat broad/big question, and I know that there are many references for it available out there. However, so far I couldn't find anything that I can really understand, that's why here is my ...
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0answers
39 views

What are the zero point energy densities of the individual quantum fields?

I'm reading through "General Relativity - An Introduction for Physicists", by Hobson, Efstathiou, and Lasenby, and I have a question regarding one of the statements related to the cosmological ...
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2answers
134 views

Noether's theorem in general relativity

Noether's theorem yields a conservation law for every symmetry. Is that independent of the Lagrangian i.e. when $\mathcal{L}\neq T-V$? In general relativity the integral that is minimised will be the ...
3
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50 views

Escape velocity for Schwarzschild metric

I can't fill in the gaps in my solution to this and assistance or a reference would be appreciated. The question begins with the straightforward derivation of the EoM for a massive particle orbiting ...
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0answers
43 views

Geodetic effect and Frame dragging

Two gyroscopes pointing perpendicular to each other were housed inside Gravity Probe B which performed polar orbit around Earth to test Einstein's theory of relativity. As the probe is orbiting ...
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46 views

metric determinant and its partial and covariant derivative

question : $\nabla_a \nabla_b \sqrt{g} \phi =\partial_a \sqrt{g} \partial_b \phi$ is true ? because $\nabla_a \sqrt{g}=0$ so we can write $\sqrt{g} \nabla_a \nabla_b \phi$ , but because metric ...
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3answers
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Is a black hole a 3D hole? And doesn't it pull into the 4th dimension?

Hear me out here please: A body on a line (a 1-D world) causes a warp in the line, i.e. a curve (2-D) A body on a plane (a 2-D world) "sinks", causing a warp in the plane, i.e. a pit (3-D) Then does ...
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2answers
130 views

Does the universe have a different age to different observers?

Although I understand that the universe is approximately 12 billion light years wide, and that may mean that it took the light photons 12 billion years at 186,000 miles per second to reach the ...
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2answers
27 views

Does Energy have a similar effect on time as Mass would when considering the gravitational field both exert?

I was thinking about light and the different frequencies. The higher the frequency the more energetic the photon. The higher frequency photon being more energetic seems counterintuitive when ...
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4answers
122 views

Exceeding the speed of light using gravity? [duplicate]

From what I have learned, objects must accelerate as they approach a heavy mass. But I also know that they cannot travel faster than the speed of light. What if you had a proton travelling at ...
2
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1answer
84 views

Clarification with Einstein's explanation of inertial mass and gravitational mass from his book

In the book "Evolution of Physics" - Albert Einstein and Leopold Infeld, the following explanation is given for two types of masses: A body at rest gives way before the action of an external ...
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1answer
99 views

Neutron star: escape velocity

The textbook from which I teach physics at the end of secondary school calculates the escape velocity at the surface of a non-rotating neutron star Newton-style using $\frac{1}{2}\cdot m\cdot ...
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2answers
51 views

Schwarzschild radius in black hole density

The textbook from which I teach physics at the end of secondary school has a question about the density of a non-rotating black hole. Because the density at the singularity is perhaps infinite or ...
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2answers
75 views

Neutron star: free fall acceleration

The textbook from which I teach physics at the end of secondary school, has a question about a neutron star: $M_{star}=1.4\cdot M_{sun}$, radius 15km. "Calculate the free fall acceleration at the ...
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0answers
72 views

Phenomena in the intersection of general relativity and quantum mechanics

I am looking for physical phenomena that have aspects involving both general relativity and quantum mechanics. The only example I know is Hawking radiation. While black holes are objects that cannot ...
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28 views

Another universe in the other side of a black hole? [duplicate]

A common notion is that there is another universe in the other side of a black hole, past the singularity point....is this true? does it have any scientific support?
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2answers
89 views

Can a hypothetical universe have more than 2 types of dimensions: spatial and temporal?

Our universe is often described as having 3 space-like dimensions and 1 time-like dimension. Can hypothetical universe exist with more than space- and time-like dimensions? If so how would these ...
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0answers
33 views

Birkhoff Theorem proof reference [closed]

I am looking for a reference with a clear (and easy to understand) proof of Birkhoff's theorem in general relativity.
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4answers
78 views

How is gravity proportional to space-time curvature in the rubber-sheet analogy?

In General Relativity, Einstein established that gravity is due to the curvature produced by objects in space. We all know that gravity is proportional to mass. The picture Einstein painted looks ...
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3answers
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Black hole with two singularities?

I hope this question isn't too naive, but would it theoretically be possible to have a black hole with 2 singularities (or 2 black holes at the same location). If this is possible, would there be any ...
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1answer
69 views

How many solutions to the Einstein Field equations are known? [closed]

Also, I would like to know if there's some list of all the known solutions.
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119 views

Is time an illusion? [closed]

IS time an illusion? I have tried thought experiment after TE and quite frankly I can't find any instance where time is defined wholly on its own. It is always a measurement of the interim between to ...
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1answer
46 views

Sign of matter Lagrangian term in curved space

In field theory the (matter) Lagrangian $\mathcal{L}_m$ is uncertain upto an overall constant multiplying factor (i.e. $\mathcal{L}_m$ and $a\mathcal{L}_m$ yield the same field equation(s) on ...
4
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1answer
85 views

How GR, QFT, or string theory address the one-directional feature of time?

It seems to me today's theoretical relativistic physics treat time and space on equal footing, with manifold diffeomorphism structure decoded in metric. However an obvious difference is that time is ...
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22 views

Accretion disks on neutron star binaries

Why does hydrogen gas from accretion disks not constantly get sucked onto/into a neutron star or into a black hole? I understand that some gets sucked into the black hole and some may come down and ...
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0answers
23 views

Who orbits who? Earth or Sun [duplicate]

We always say that the earth orbits the sun, but how can we prove this? Could we not say that the sun orbits the earth or that the earth orbits Mars?
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0answers
22 views

FRW Metric maximally symmetric, derivation, $R=3K$ or $R=6K$ confusion, two different texts

I'm looking at Tod and Hughston Introduction to GR and writing the metric in the two forms; [1]$$ ds^{2}=dt^{2}-R^{2}(t)(\frac{dr^{2}}{1-kr^{2}}+r^{2}(d\theta^{2}+sin^{2}\theta d\phi^{2})) $$ [2] $$ ...
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65 views

Schwarzschild metric circular orbits and kepler's 3rd law

I have been looking at the Schwarzschild metric presented to me as the following within lectures: ...
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2answers
49 views

Is there any physical meaning for the inverse metric?

I've been wondering if we can attribute any physical meaning to the inverse metric. I mean when we talk about the metric itself, there are lots of insights we can have towards its role in spacetime, ...
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2answers
124 views

General Relativity visualization software

As I am approaching the study GR, I was wondering if there are softwares that allow a quick visualization of custom metrics, curvature, and particle motion even in the limited context of 2D space. ...
4
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1answer
83 views

Light clocks measure conformal time - detailed argument

Let us assume that an observer is stationary at the origin in expanding space. We assume the FRW metric near the origin is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Let us assume that the observer ...
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31 views

What is the speed of gravity? [duplicate]

Hypothetically, let`s say that one day our Sun is suddenly removed from the galaxy. For us, on earth, would take up to 8.3 minutes to notice that the sun was removed, since the distance from the earth ...
0
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1answer
61 views

How can electromagnetic waves be affected by gravity if photons have no mass? [duplicate]

I have found answers that say that gravity interferes with the oscillating electric and magnetic fields of the waves and others that say that since gravity is a bend in space-time, in which case the ...
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0answers
58 views

using tetrads to glue local currents into global currents

According to John Baez it is possible to take a locally conserved tensor $\nabla_\mu\: T^{\mu\nu}(x)=0\ \ \ \ \ \mbox{(locally)}$ and convert it to a globally conserved tensor by "patching" ...
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37 views

Local translations included in covariant general coordinate transformation

It is known that if we use the constraint $R_{\mu\nu}(P^{a})=0$ , i.e. the curvature of local translations vanishes, then we can modify general coordinate transformations (gct) to a covariant ...
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Is our locally measured time actually conformal time?

The FRW metric at the origin $r=0$, with $c=1$, is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Now one can change variables so that near the origin the FRW metric is approximated by the Minkowski metric ...
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1answer
45 views

Does one *feel* the electrostatic force while not resisting it?

Suppose that I put lots of big magnets around me, or say, that I charge myself up to a macroscopic charge. Now, suppose that there's a huge magnet in front of me (or a huge object with opposite ...
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51 views

Lorentz contraction in continuously accelerating rod

I was having a little thought experiment about Lorentz contraction, and I couldn't really figure out what would actually happen. Note that I'm not looking for a answer 'this effect is barely ...