A theory that describes how matter produces and responds to 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.

learn more… | top users | synonyms (1)

14
votes
5answers
855 views

Does GR provide a maximum electric field limit?

Does GR provide a limit to the maximum electric field? I've gotten conflicting information regarding this, and am quite confused. I will try to quote exactly when possible so as not to confuse ...
14
votes
3answers
1k views

Why can't General Relativity be written in terms of physical variables?

I am aware that the field in General Relativity (the metric, $g_{\mu\nu}$) is not completely physical, as two metrics which are related by a diffeomorphism (~ a change in coordinates) are physically ...
14
votes
6answers
2k views

Is the Planck length Lorentz invariant?

The planck length is defined as $l_P = \sqrt{\frac{\hbar G}{c^3}}$. So it is a combination of the constants $c, h, G$ which I believe are all Lorentz invariants. So I think the Planck length should ...
14
votes
3answers
352 views

Is the flatness of space a measure of entropy?

This is a bit quirky: For a very long time I've found Stephen Hawking's evaporating small black holes a lot more reasonable and intuitive than large black holes. The main reason is that gravity is ...
14
votes
2answers
366 views

Newtonian gravity from the holographic principle?

Can one understand Newton's law of gravitation using the holographic principle (or does such reasoning just amount to dimensional analysis)? Following an argument similar to one given by Erik ...
14
votes
1answer
218 views

Is period of rotation relative?

My question is inspired by the following answer by voix to another problem: "There is a real object with relativistic speed of surface - millisecond pulsar. The swiftest spinning pulsar currently ...
13
votes
3answers
1k views

Why is a black hole black?

In general relativity (ignoring Hawking radiation), why is a black hole black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a ...
13
votes
5answers
2k views

Newton's Bucket

Newton's Bucket This thought experiment is originally due to Sir Isaac Newton. We have a sphere of water floating freely in an opaque box in intergalactic space, held together by surface tension and ...
13
votes
4answers
3k views

Is there a maximum possible acceleration?

I'm thinking equivalence principle, possibilities of unbounded space-time curvature, quantum gravity ...
13
votes
4answers
636 views

Coulomb's Law in the presence of a strong gravitational field

I was under the impression that the $1/r^2$ falloff of various forces were because of the way the area of a expanding sphere scales. But that strict $1/r^2$ falloff would only be globally true in a ...
13
votes
5answers
1k views

Are elementary particles ultimate fate of black holes?

From the "no hair theorem" we know that black holes have only 3 characteristic external observables, mass, electric charge and angular momentum (except the possible exceptions in the higher ...
13
votes
4answers
1k views

To which extent is general relativity a gauge theory?

In quantum mechanics, we know that a change of frame -- a gauge transform -- leaves the probability of an outcome measurement invariant (well, the square modulus of the wave-function, i.e. the ...
13
votes
2answers
554 views

Brachistochrone problem in general relativity

This question Brachistochrone Problem for Inhomogeneous Potential has the obvious extension. Namely the same question, when gravity is treated according to general relativity. To make it specific ...
13
votes
4answers
463 views

Interpretation of a singular metric

I'm interested to find out if we can say anything useful about spacetime at the singularity in the FLRW metric that occurs at $t = 0$. If I understand correctly, the FLRW spacetime is a combination ...
13
votes
1answer
473 views

Suggested reading for quantum field theory in curved spacetime

I want to learn some QFT in curved spacetime. What papers/books/reviews can you suggest to learn this area? Are there any good books or other reference material which can help in learning about QFT ...
13
votes
1answer
646 views

How to prove that a spacetime is maximally symmetric?

In Carroll's book on general relativity, I found the following remark: In two dimensions, finding that $R$ is a constant suffices to prove that the space is maximally symmetric [...] In higher ...
13
votes
3answers
824 views

Swimming in Spacetime - apparent conserved quantity violation

My question is about the article Swimming in Spacetime. My gut reaction on first reading it was "this violates conservation of momentum, doesn't it?". I now realize, however, that this doesn't allow ...
13
votes
1answer
521 views

Lagrangian for Euler Equations in general relativity

The stress energy tensor for relativistic dust $$ T_{\mu\nu} = \rho v_\mu v_\nu $$ follows from the action $$ S_M = -\int \rho c \sqrt{v_\mu v^\mu} \sqrt{ -g } d^4 x = -\int c \sqrt{p_\mu ...
13
votes
5answers
1k views

The definition of an inertial reference frame in Einstein's relativity

I'm reading Sean Carroll's book on general relativity, and I have a question about the definition of an inertial reference frame. In the first chapter that's dedicated to special relativity, the ...
13
votes
1answer
489 views

What are the local covariant tensors one can form from the metric?

Normally in differential geometry, we assume that the only way to produce a tensorial quantity by differentiation is to (1) start with a tensor, and then (2) apply a covariant derivative (not a plain ...
13
votes
1answer
492 views

How does one measure space-like geodesics? Or: What is the physical interpretation of space-like geodesics?

In general relativity, time-like geodesics are the trajectories of free-falling test particles, parametrized by proper time. Thus, they are easy to interpret in physical terms and are easy to measure ...
13
votes
3answers
580 views

Extremal black hole with no angular momentum and no electric charge

A black hole will have a temperature that is a function of the mass, the angular momentum and the electric charge. For a fixed mass, Angular momentum and electric charge are bounded by the extremality ...
13
votes
1answer
1k views

Euclidean derivation of the black hole temperature; conical singularities

I am studying the derivation of the black hole temperature by means of the Euclidean approach, i.e. by Wick rotating, compactifying the Euclidean time and identifying the period with the inverse ...
13
votes
1answer
451 views

Current status of Penrose-Hawking “thunderbolts?”

I recently came across the interesting statement that most definitions of cosmic censorship (CC), and most work on proving it, don't exclude something that Hawking calls a "thunderbolt" (Penrose 1978, ...
13
votes
3answers
899 views

Does a Weak Energy Condition Violation Typically Lead to Causality Violation?

In the answer to this question: ergosphere treadmills Lubos Motl suggested a straightforward argument, based on the special theory of relativity, to argue that light passing through a strong ...
13
votes
0answers
433 views

Do intergalactic magnetic fields imply an Open Universe?

According to a recent paper on the arXiv, they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty intergalactic space, support ...
12
votes
8answers
4k views

Is gravity just electromagnetic attraction?

Recently, I was pondering over the thought that is most of the elementary particles have intrinsic magnetism, then can gravity be just a weaker form of electromagnetic attraction? But decided the ...
12
votes
8answers
5k views

What is a rocket engine thrusting against in space?

I know Newton's third law of motion might be the answer for this but still I am wondering how the rockets could thrust in the empty space and move in the opposite direction. I guess an astronaut ...
12
votes
5answers
2k views

What is the conserved quantity of a scale-invariant universe?

Consider that we have a system described by a wavefunction psi(x). We then make an exact copy of the system, and anything associated with it, (including the inner cogs and gears of the elementary ...
12
votes
5answers
4k views

Is the total energy of the universe constant?

If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
12
votes
3answers
836 views

Is “now” or “the present moment” properly defined in GR?

My question is about the extent to which "now" is defined in GR. In Minkowski spacetime, it's possible to define a "now" for an inertial observer by finding a spacelike 3-plane such that, in the ...
12
votes
4answers
487 views

Is the concept of tensor rank useful in physics?

The term 'tensor rank' is sporadically used in the mathematical literature to denote the minimum number of simple terms (i.e. tensor products of vectors) needed to express the tensor. This is ...
12
votes
2answers
985 views

What is the weight equation through general relativity?

The gravitational force on your body, called your weight, pushes you down onto the floor. $$W=mg$$ So, what is the weight equation through general relativity?
12
votes
1answer
1k views

Would the inside of a black hole be like a giant mirror?

As any light reflected or emitted from objects inside a black hole (if it is possible to be there) does not leave the event horizon and comes back inside, would it be like seeing yourself? What I ...
12
votes
1answer
773 views

Why is there no dynamics in 3D general relativity?

I heard a couple of times that there is no dynamics in 3D (2+1) GR, that it's something like a topological theory. I got the argument in the 2D case (the metric is conformally flat, Einstein equations ...
12
votes
4answers
932 views

Why is there a search for an exchange particle for gravity?

If I understand correctly, according to Einstein's General Theory of Relativity, mass results in a distortion in space-time. In turn, the motion of the mass is affected by the distortion. A result of ...
12
votes
2answers
660 views

Is spacetime simply connected?

As I've stated in a prior question of mine, I am a mathematician with very little knowledge of Physics, and I ask here things I'm curious about/things that will help me learn. This falls into the ...
12
votes
6answers
9k views

Why can't light escape from a classical black hole?

Photons do not have (rest) mass (that's why they can move at speed of "light"). So, my question is how the gravity of classical$^1$ black hole can stop light from escaping? -- $^1$ We ignore ...
12
votes
2answers
3k views

Does the speed of light vary in noninertial frames?

The speed of light is the same in all inertial frames. Does it change from a non-inertial frame to another? Can it be zero? If it is not constant in non-inertial frames, is it still bounded from ...
12
votes
5answers
1k views

Does the amount of gravitational potential energy in the universe increase as it expands?

It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
12
votes
2answers
849 views

If two ultra-relativistic billiard balls just miss, will they still form a black hole?

This forum seems to agree that a billiard ball accellerated to ultra-relativistic speeds does not turn into a black hole. (See recent question "If a 1kg mass was accelerated close to the speed of ...
12
votes
1answer
615 views

Is this a quaternion representation of the equations of motion of General Relativity?

In The Quaternion Group and Modern Physics by P.R. Girard, the quaternion form of the general relativistic equation of motion is derived from $du'/ds = (d a / d s ) u {a_c}^* + a u ( d {a_c}^* / ...
12
votes
3answers
1k views

What happens when a black hole and an “anti-black-hole” collide?

Let's say we have one black hole that formed through the collapse of hydrogen gas and another that formed through the collapse of anti-hydrogen gas. What happens when they collide? Do they (1) ...
12
votes
1answer
219 views

Gravitational collapse of a photon cloud

The Schwarzschild solution shows that a spherically symmetric, static fluid will undergo gravitational collapse if too much mass-energy is concentrated together (i.e. if the fluid's radius is less ...
12
votes
3answers
520 views

Is Einstein-Hilbert action the unique action whose variation gives Einstein's field equations?

I know that scaling the action with a non-zero multiplicative constant, or adding a total divergence term to the Lagrangian density do not change the Euler-Lagrange equations, cf. e.g. this ...
12
votes
3answers
673 views

Why is there a flux of radiation in the Hawking effect but not in the Unruh effect? (and other questions)

This question is slightly related to this one Do all massive bodies emit Hawking radiation?, which I think was poorly posed and so didn't get very useful answers. There are several questions in this ...
12
votes
1answer
498 views

Asymptotic symmetry algebra

So after a lot of research, and tons and tons of papers that I've went through, I finally have some idea how to solve the equations that will give me candidates for the asymptotic symmetry group for ...
12
votes
4answers
1k views

Is there a fundamental reason why gravitational mass is the same as inertial mass?

The principle of equivalence - that, locally, you can't distinguish between a uniform gravitational field and a non-inertial frame accelerating in the sense opposite to the gravitational field - is ...
11
votes
3answers
3k views

Did Hilbert publish general relativity field equation before Einstein?

Did Hilbert publish general relativity field equation before Einstein?
11
votes
2answers
1k views

Is dark matter repulsive to dark matter? Why?

I think I saw in a video that if dark matter wasn't repulsive to dark matter, it would have formed dense massive objects or even black holes which we should have detected. So, could dark matter be ...