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.
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2answers
1k views
What did general relativity clarify about Mercury?
I frequently hear that Kepler, using his equations of orbital motion, could predict the orbits of all the planets to a high degree of accuracy -- except Mercury. I've heard that mercury's motion ...
8
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2answers
641 views
Is there an energy density limit in GR?
I am speaking about GR with classical fields and energy. One question, spread over three increasingly strict situations:
Is there an energy density limit in GR? (literally, can the energy density ...
4
votes
1answer
304 views
How does an object falling into a plain Schwarschild black hole appear from near the black hole?
I know that when viewed from infinity (or from a very large distance from the black hole event horizon), an object that falls into the black hole will appear to slow down and will become more and more ...
4
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3answers
1k views
Why is the equivalence principle so important to general relativity?
In its simplest form, equivalence principle states that the inertial mass and the gravitational mass should be the same. This is easy to understand.
But why is it so important to the formulation of ...
3
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5answers
484 views
How are the comoving coordinates NOT a prefered reference frame?
Physics me this:
The equivalence principle has rigorous physical definitions that say, for one, that the laws of physics are the same in all inertial reference frames. This is to say that the ...
1
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6answers
1k views
Does light have acceleration?
Light travels with the largest speed in our universe. Does light have acceleration?
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1answer
378 views
If photons are deflected by a strong gravitational field, then how come photons do not have mass? [duplicate]
Possible Duplicate:
Explain how (or if) a box full of photons would weigh more due to massless photons
It has been proved and showed through experiments that light can be bent by the Sun or ...
9
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2answers
587 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 ...
7
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6answers
983 views
What methods can astronomers use to find a black hole?
How can astronomers say, we know there are black holes at the centre of each galaxy?
What methods of indirect detection are there to know where and how big a black hole is?
4
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2answers
719 views
What is the escape velocity of a Black Hole?
The escape velocity of Earth is $v=\sqrt{\frac {GM}{R}}$, where $M$ is the mass of the Earth and $R$ it's radius (approximating it as a sphere), and is much less than light speed $c$.
What is the ...
3
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1answer
153 views
If a magnetic monopole falls into a schwarzchild black hole, what happens to the magnetic field?
By the no-hair theorem, black holes can only have mass, charge and angular momentum. Does "charge" include "magnetic charge" (such as from a magnetic monopole)? Can black holes have magnetic charge ...
2
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3answers
184 views
Why can't we think of free fall as upside down rocket?
/\
/ \
| |
| m |
| |
------ <--- floor (Rocket A)
This rocket is accelerated (g) upwards then mass(m) falls on the floor.
...
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3answers
181 views
Theory that gets rid of dark matter/energy
Is there any physics theory that either groups together gravity and dark energy/dark matter or eliminates dark energy/dark matter by modifying standard understanding of gravity or any force? If so, ...
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1answer
226 views
Why is matter drawn into a black hole not condensed into a single point within the singularity?
When we speak of black holes and their associated singularity, why is matter drawn into a black hole not condensed into a single point within the singularity?
1
vote
1answer
613 views
Neutrino unaffected by gravity
Are neutrinos affected by gravity?
If not, could that be a plausible reason for a neutrino taking a shorter path than light, since light is affected by gravity?
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5answers
1k views
Could gravity be an emergent property of nature?
Sorry if this question is naive. It is just a curiosity that I have.
Are there theoretical or experimental reasons why gravity should not be an emergent property of nature?
Assume a standard model ...
12
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7answers
992 views
How precisely does a star collapse into a black hole?
I think we all heard general statements like "once big enough star burns out there is nothing to prevent the gravitational collapse ending in a black hole". But I can't remember even seeing the ...
8
votes
1answer
504 views
Is String Theory formulated in flat or curved spacetime?
String Theory is formulated in 10 or 11 (or 26?) dimensions where it is assumed that all of the space dimensions except for 3 (large) space dimensions and 1 time dimension are a compact manifold with ...
14
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4answers
546 views
Redshifting of Light and the expansion of the universe
So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...
12
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5answers
1k 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 ...
11
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6answers
1k 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 ...
15
votes
2answers
407 views
is there a way to split a black hole?
Classically, black holes can merge, becoming a single black hole with an horizon area greater than the sum of both merged components.
Is it thermodynamically / statistically possible to split a black ...
7
votes
2answers
795 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) ...
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3answers
522 views
How could spacetime become discretised at the Planck scale?
I didn't have much luck getting a response to this question before so I have tried to reword and expand it a little:
In early 2010 I attended this inaugural lecture by string theorist- Prof. ...
11
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2answers
400 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 ...
8
votes
2answers
617 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 ...
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4answers
325 views
What makes the stars that are farther from the nucleus of the galaxy go faster than those in the middle?
It has no sense that stars that have a bigger radius and apparently less angular speed($\omega$) goes faster than the ones near the center.
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1answer
1k views
Explicit Variation of Gibbons-Hawking-York Boundary Term
Are there any references that present the explicit variation of the Hilbert-Einstein action plus the Hawking-Gibbons-York boundary term, and demonstrate the cancellation of the normal derivatives of ...
6
votes
2answers
212 views
What happens to orbits at small radii in general relativity?
I know that (most) elliptic orbits precess due to the math of general relativity, like this:
source: http://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity
I also know that something ...
5
votes
1answer
188 views
Are objects in a gravitational well shortened?
Bob is in a gravitational potential well, he moves a long vertical stick up and down a distance of 1 meters. Alice observes the upper end of the stick, at upper location.
There is the phenomenon of ...
2
votes
2answers
289 views
Diff(M) and requirements on GR observables
This question is kind of inspired in this one:
Diff(M) as a gauge group and local observables in theories with gravity
The conundrum i'm trying to understand is how is derived the (quite) ...
12
votes
3answers
104 views
What is meant when it is said that the universe is homogeneous and isotropic?
It is sometimes said that the universe is homogeneous and isotropic. What is meant by each of these descriptions? Are they mutually exclusive, or does one require the other? And what implications rise ...
10
votes
2answers
425 views
Can “big rip” rip apart an atomic nucleus?
Some scenarios describing the fate of the matter vs dark energy tug of war on the universe involve the acceleration of the universe increasing to the point that it ends up ripping apart even atoms. ...
7
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4answers
379 views
How does the star that has collapsed to form a Schwarschild black hole appear to an observer falling into the black hole?
I understand that to an outside observer, the light from a star that is collapsing into a black hole will become more and more red-shifted as the surface of the star appears to approach the black hole ...
6
votes
2answers
246 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?
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votes
5answers
2k views
What is the evidence for Inflation of the early universe?
The theory of Inflation explains the apparent consistency of the universe by proposing that the early universe grew exponentially for a 1E-36 seconds. Isn't a simpler explanation that the universe is ...
4
votes
2answers
329 views
Can a black hole be formed by radiation?
I'm trying to find out if black holes could be created by focusing enough light into a small enough volume.
So far I have found (any or all may be incorrect):
Maxwell's equations are linear, ...
3
votes
3answers
157 views
What does Brian Greene mean when he claims we wont be able to observe light from distant stars due to the universe's expansion?
Brian Greene in this TED talk about possible multiverse, claims tomwards the end (At around 18:00 mark) this statement. 'Because the expansion is speeding up, in the very far future, those galaxies ...
3
votes
2answers
215 views
Does throwing a watch into the air cause it to gain or lose time?
Suppose I'm on a non rotating planet. I have two identical, perfect watches. I synchronize them. Then I throw one of them into the air and catch it. Does the one I throw into the air gain or lose time ...
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3answers
431 views
Is the curvature of space-time a smooth function everywhere ? (except at black holes)
Is the curvature of space-time a smooth function everywhere (except at black holes) in view of General relativity. By 'smooth' it is meant that it possesses derivatives of all order at a given point.
...
7
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1answer
110 views
Cancelling special & general relativistic effects
We know that for a GPS we need to make a correction for both general and special relativity: general relativity predicts that clocks go slower in a higher gravitational field (the clock aboard a GPS ...
6
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3answers
547 views
Maximum time difference between clocks in a gravity field
From Surely you must be joking, Mr Feynman.
You blast off in a rocket which has a
clock on board, and there's a clock on
the ground. The idea is that you have
to be back when the clock on ...
5
votes
2answers
1k views
Does the equivalence between inertial and gravitational mass imply anything about the Higgs mechanism?
For example: the role it might play in a theory of quantum gravity (ie causing space-time curvature)?
I realize that inertial mass can result from binding energy alone. Has the equivalence principle ...
4
votes
2answers
395 views
Positive Mass Theorem and Geodesic Deviation
This is a thought I had a while ago, and I was wondering if it was satisfactory as a physicist's proof of the positive mass theorem.
The positive mass theorem was proven by Schoen and Yau using ...
4
votes
1answer
219 views
Calculating position in space assuming general relativity
Suppose two pointed masses are given in space. Suppose further that one of the masses has a given velocity at (local) time 0. Is there a way to compute its position in a future time?
Neglecting ...
4
votes
2answers
405 views
What experimental proof has been found of Einstein's theory?
As a casual science reader, I've always found the implications of relativity (inconsistent clocks after near-light-speed travel and various space-time paradoxes) to be confusing and magical-sounding. ...
3
votes
2answers
273 views
ergosphere treadmills
suppose you place a number of rotating black holes in linear sequence (rotating around the same axis) between two stars at distance $d$ (assume as tightly packed as practical for purposes of ...
2
votes
4answers
342 views
Surely space-time Curvature does not explain gravity, it just describe its effects?
In special relativity co-moving objects see the other's 4-velocity as being only temporal.
When they move relative to each other they see the other's 4-velocity has rotated so that it points less in ...
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0answers
249 views
composition of space expansion and movement as a gauge invariance
suppose i have a space-time where we have one point-like object* which we will call movement space probe or $\mathbf{M}_{A}$ for short, and it will be moving with constant velocity $V^A_{\mu}$ in ...
2
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1answer
156 views
Testing General Relativity
Ever since Einstein published his GR theory in 1916, there have been numerous experimental tests to confirm its correctness--and has passed with flying colors.
NASA and Stanford have just announced ...
