Questions tagged [spacetime]
Within relativity (both special and general), changes of reference frames can change both the notions of space and of time, with one depending on the other as well. As a consequence, it is necessary to treat both concepts in a unified manner. Hence the term spacetime.
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Age of universe vs Hubble time in Milne universe
Consider an empty universe where energy density $\varepsilon = 0$, thus the Friedmann Equation can be reduced into:
$\dot a^2= -\frac{kc^2}{R_O^2}$
$k$ is the curvature of space, $R_0$ is the radius ...
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Does the "Andromeda Paradox" (Rietdijk–Putnam-Penrose) imply a completely deterministic universe?
Wikipedia article: http://en.wikipedia.org/wiki/Rietdijk–Putnam_argument
Abstract of 1966 Rietdijk paper:
A proof is given that there does not exist an event, that is not already in the past for ...
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How do we account for the 'one way' drag of moving space?
As I understand it, the rotating space outside a Kerr black hole drags radially falling particles into circular motion. Similarly the river model posits that the inward flow of space ensures particles ...
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Curves on LCDM spacetime diagram
I have a question concerning the LCDM spacetime diagram on this Physics Stack Exchange post.
How are the set of curves for $z = \text{constant}$ (1, 3, 10, 50, etc) constructed or calculated, i.e. ...
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Finding the correct Christoffel-symbols in a 2+1D space-time
I'm trying to calculate the Christoffel Symbols in a 2+1D space-time with the following metric:
$$ds^2 = N^2(\vec r)c^2dt^2-\phi(\vec r)(dx^1)^2-\phi(\vec r)(dx^2)^2$$
To find the Christoffel ymbols ...
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Angular Deficit of a Conical Singularity
I'm currently studying the Bonnor solution starting with this paper on Black Diholes. The metric is given by :
$$ ds^2 = \left(1-\frac{2Mr}\Sigma\right)^2 \left[-dt^2 + \frac{\Sigma^4}{(\Delta + (M^2 +...
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Are $i^\pm$ and $i^0$ codimension 1 surfaces?
Standard textbooks like Carroll's say that spatial and temporal infinities in Minkowski space Penrose diagram are points. But on the footnote in pg. 3 of some draft notes on Celestial holography by ...
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Is spacetime flat inside a spherical shell?
In a perfectly symmetrical spherical hollow shell, there is a null net gravitational force according to Newton, since in his theory the force is exactly inversely proportional to the square of the ...
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Can we glue the Schwarzschild and the de Sitter metrics at their event horizon?
Is there a way to glue the de Sitter metric inside the event horizon of the Schwarzschild metric, without an explicit reference to a particular coordinates system?
Using the standard radial ...
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How certain is it that quantum fields exist everywhere?
When people explain Quantum Field Theory, one of the first things they'll say is that quantum fields exist everywhere. This seems like a fairly reasonable framework to base QFT on, but I'm wondering ...
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Why does space "spring back" after mass has passed through it?
A comet travelling through space bends the space around it due to its mass. But after it has passed, the space is no longer curved. Why does it do this? Compare for example, the rut left in soft ...
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If gravity is not a force, what makes massive objects spheroid?
For most of my life, the explanation given for why celestial bodies like stars, planets, etc. are round is due to gravitational force. Simply put, if an object has enough mass, it will, in turn, have ...
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Thought experiment circumventing finite speed of light via relativity of simultaneity - what's wrong?
I just watched this video regarding block universe: https://www.youtube.com/watch?v=wwSzpaTHyS8&t=676s and it provoked the following thought experiment:
Let's assume two observers, O1 and O2, at ...
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Does the expansion of the Universe into a higher dimensional space imply that 4D objects are real?
It is my understanding that objects in the Universe are not just getting farther apart but space itself is expanding and so in some real sense, higher-dimensional geometry is "real" -- if so, on a ...
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Locally flatness in general relativity
My professor made following statement:
The spacetime of GR is curved in the presence of strong gravitational fields. The effects of curvature
manifest themselves at large ...
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