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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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On definition of time in non-inertial frames [closed]

Strictly speaking we can only compare times and lengths in inertial frames of reference because the measuring tools are in the same conditions and aren't subject to G-force. So they are identical ...
Марат Рамазанов's user avatar
2 votes
3 answers
421 views

Question on special relativity

I am trying to learn special relativity. If we consider two inertial reference frames with spacetime co-ordinates $(t,x,y,z)$ and $(t',x',y',z')$ and let there be 2 observers who measure the speed of ...
morpheus's user avatar
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Looking for papers that claim that spacetime is emergent

This article states: It’s really striking that for most of the plausible theories of quantum gravity that we have, in some sense their message is, yeah, general relativistic spacetime isn’t in there ...
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1 answer
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Question on spatiotemporal dimensionality about the contradictions of time being a dimension

We can axiomatically see that all spatial dimensions have a fundamental rule where they can either move back or forwards infinitely. However, the temporal dimension started when the universe began and ...
Mason Kang's user avatar
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Under what circumstances can a 4D singularity occur in General Relativity?

I've tried to find on the literature about 4D (single point) singularities, but most of the theorems about singularities pertain to either space-like or time-like singularities, which always have some ...
UnkemptPanda's user avatar
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1 answer
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What happens if we differentiate spacetime with respect to time? [closed]

Essentially, what would differentiating space-time with respect to time provide us with? What are the constraints associated with such operations? Is it possible to obtain a useful physical quantity ...
Kimaya Deshpande's user avatar
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Does gravity accelerate you towards the geodesic of light between you and the mass?

If there's a planet far away, you will accelerate straight towards it due to gravity. If you place a Schwarzschild black hole right in the middle between you and the planet (the distance between the ...
Zach's user avatar
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Do satellites in orbit create Relativity paradoxes? [closed]

Can someone point out the flaw in this very realistic scenario below? I will start by stating established first principles of the applicable orbital and relativistic conditions. Then I will describe ...
Anakin Skywalker's user avatar
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Can part of space be causally disconnected from the rest of the universe by being surrounded by black holes? [duplicate]

Is it possible for black hole event horizons to overlap and form a spherical wall around an island of space (that's not inside a black hole) while still being causally disconnected from the rest of ...
user3624007's user avatar
1 vote
2 answers
131 views

Is the FRW metric, based on spatial homogeneity and isotropy, rotationally and translationally invariant? If so, how?

The spatial part of the Minkowski metric, written in the Cartesian coordinates, $$d\vec{ x}^2=dx^2+dy^2+dz^2,$$ is invariant under spatial translations: $\vec{x}\to \vec{x}+\vec{a}$, where $\vec{a}$ ...
Solidification's user avatar
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Extrinsic Curvature in a conformally-flat spacetime that is also asymptotically-flat spacetime

I would appreciate if someone can confirm or correct my understanding of extrinsic-curvature (as in the ADM 3+1 decomposition of spacetime) when dealing with a conformally-flat spacetime. (I updated ...
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Do you always experience the gravitational influence of other mass as you see them in your frame?

You see a galaxy far away. That galaxy is attracting you with a certain amount of gravity. I'm wondering if the gravity influence of the galaxy on you, as measured by you, always ends up being what ...
Zach's user avatar
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How does loop quantum gravity handle spacetimes which aren't globally hyperbolic, like the Kerr metric?

Loop quantum gravity assumes spacetime is globally hyperbolic. However, the interior of a Kerr black hole isn't globally hyperbolic, containing closed timelike curves. So, how are Kerr black holes ...
Zee's user avatar
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1 answer
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Length Contraction: is $t'$ or $t = 0$?

To demonstrate my confusion - let's say there is a rod traveling with velocity +v relative to S, and in S, the length of the rod is measured to be $L$. If I want to go from S to S', the frame where a ...
Emil Sriram's user avatar
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Action principle dependent on spacetime-topology?

Consider the Lagrangian density $$L(\phi, \nabla \phi, g) = g^{\mu \nu} \nabla_{\mu} \phi \nabla_{\nu} \phi$$ If one varies the action as usual, then one finds the equation $$\delta S = \int_{\mathcal{...
Octavius's user avatar
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Does Matter Cause Curvature or Vice-Versa [closed]

From the way explanations about gravity-acceleration-curvature equivalence are usually phrased here or elsewhere, it would appear many or most think that matter causes space-time curvature. I cannot ...
Prototypist's user avatar
9 votes
4 answers
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Understanding expansion of the Universe as things flying apart

Say that we have a Universe uniformly filled just with matter (let's not bring dark energy into this). And say that we fill it with very light particles (so that the gravitational interaction between ...
Negredol Nekaj's user avatar
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1 answer
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A few doubts regarding the geometry and representations of spacetime diagrams [closed]

I had a couple questions regarding the geometry of space-time diagrams, and I believe that this specific example in Hartle's book will help me understand. However, I am unable to wrap my head around ...
amansas's user avatar
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1 answer
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Confusion about local Minkowski frames

This is sort of a follow-up to the question I asked here:  Confusion about timelike spatial coordinates The important context is that we imagine a metric that, as $t\rightarrow\infty$, approaches the ...
Aidan Beecher's user avatar
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1 answer
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Confusion about timelike spatial coordinates

I'm pretty new to general relativity, and I'm self-studying it using Sean M. Carroll's text on the subject. In Section 2.7, he introduces the notion of closed timelike curves. He gives the example of ...
Aidan Beecher's user avatar
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3 answers
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How did Einstein figure out mass (and hence energy) bends spacetime?

I can understand that once I fix the velocity of light at $c$, there is a relative variation in space-time based on special relativity (inertial frame of reference). It's not clear to me how Einstein ...
iVenky's user avatar
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Inflation in background free models of the universe

There are many authors who are attempting to construct a model of physics that doesn't rely on the objective existence of spacetime. This is part of the work in quantum gravity. This leads to things ...
Ben Sprott's user avatar
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What is the dual asymptotic spacetime of a CFT on a particular flat manifold?

According to AdS/CFT correspondence, the dual theory of a boundary CFT on flat spacetime is defined on an asymptotically AdS spacetime. The nature of the bulk spacetime depends on the topology of the ...
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When you are in a gravitational field, do object far away get physically closer to you as you get closer to the mass?

An observer A is close to a black hole and an observer B one light year away. They are both remaining at constant radial distance from the black hole. A is at 2 Rs away from the center of the black ...
Zach's user avatar
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How to derive Feffermann-Graham expansion for AdS Vaidya geometries?

Introduction The Feffermann-Graham expansion for an asymptotically AdS spacetime [0] looks like Poincare AdS but with the flat space replaced by a more general metric i.e. $$ds^2=\frac{1}{z^2}(g_{\mu \...
Sanjana's user avatar
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Time component of four-velocity

While reading through Spacetime and Geometry by Sean Carroll, I came across the following passage: "Don't get tricked into thinking that the timelike component of the four velocity of a particle ...
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How to Understand Negative Energy in the Ergoregion?

I am trying to understand the Penrose process and having trouble explaining negative energy in the ergoregion. How I interpret it is: Energy is the dot product between the four momentum of the object ...
Gene's user avatar
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2 votes
2 answers
915 views

Theoretically, can perfectly flat space exist in the universe?

According to general relativity, mass and energy cause the curvature of space. To have perfectly flat space, there must be a completely empty vacuum state with no mass or energy. Theoretically, is it ...
NOH WHIREA's user avatar
1 vote
1 answer
71 views

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 ...
Polaris5744's user avatar
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2 answers
108 views

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 ...
KDP's user avatar
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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 ...
Sanjana's user avatar
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1 vote
1 answer
149 views

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 ...
Giorgos G's user avatar
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4 votes
3 answers
198 views

Change of variables from FRW metric to Newtonian gauge

My question arises from a physics paper, where they state that if we take the FRW metric as follows, where $t_c$ and $\vec{x}$ are the FRW comoving coordinates: $$ds^2=-dt_c^2+a^2(t_c)d\vec{x}_c^2$$ ...
Wild Feather's user avatar
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2 answers
98 views

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 ...
Uffe Poul Hansen's user avatar
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3 answers
129 views

Can you tell who is moving through time? [closed]

In relativity, there is no way to tell if you are moving through space. So, if you were inside of a box, there would be no way for you to tell if you were moving or not. However, can you know who is ...
John W's user avatar
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1 answer
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Time function as a function of energy (from velocity and gravity)?

Is there any formula, preferably in terms of energy, for the time dilation an object experiences taking both relativistic velocity and mass into account? I see both formulas frequently, but haven't ...
jamschreib's user avatar
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1 answer
82 views

What objects are solutions to the Einstein Field Equations?

The usual way the solutions of the Einstien Field Equations are introduced is by saying they are (pseudo-) riemannian metrics that satiafy the diff equations for a given EM Tensor. My question is: ...
emilio grandinetti's user avatar
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0 answers
39 views

How many dimensions are in string theory? [duplicate]

How many dimensions are in string theroy? I heard that there are 11 but to my understanding, there is an infinite, also can strings be on a 2D plane?
Lucas Dewan's user avatar
2 votes
1 answer
68 views

Understanding Wormholes Geometrically

Is the folding sheet analogy really that good for understanding what a wormhole is? After all, space-time curvature doesn't require any ambient space (it's intrinsic), as such a picture would suggest. ...
user345249's user avatar
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2 answers
65 views

What is $r$ in a metric signature in general relativity? If $v$ and $p$ are the time and spatial coordinates?

The Wikipedia article on metric signatures says that the signature of a metric can be written $(v,p,r)$, where $v$ is the number of positive eigenvalues, $p$ is the number of negative eigenvalues, and ...
Kurt Hikes's user avatar
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26 votes
10 answers
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How do black holes move if they are just regions in spacetime?

If black holes are just regions of spacetime, how can black holes even move? When matter moves through spacetime, it bends the spacetime around it, but if black holes are just regions of spacetime, ...
Rick Gennings's user avatar
1 vote
0 answers
48 views

How can you use gravity while trying to model gravity? [duplicate]

So consider the usual pop-science spacetime model, a bowling ball on a trampoline. Apparently, the ball should sink into the trampoline, causing a dip in the fabric which causes nearby objects to fall ...
stickynotememo's user avatar
0 votes
5 answers
210 views

Vacuum solutions in presence of mass?

Here is the page I will be referencing: Vacuum solution (general relativity) - Wikipedia My point is: if $T_{\mu\nu}=0$ implies that there is no mass, how can Schwarzschild vacuum be a solution, if ...
Elvis's user avatar
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0 answers
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Does it make sense to talk about time in absence of matter? [duplicate]

The equations of general relativity should predict (although I might be wrong) that in absence of mass the spacetime is everywhere flat. That is, time is the same everywhere. However, I'm not sure ...
Elvis's user avatar
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1 vote
3 answers
175 views

Seemingly equivalent linear form of the Sagnac effect

This is a derivative of the question regarding the Sagnac effect. Judging from the metric $ds^2=-dt^2+(rd\phi)^2$ for a constant $r$ for this question, it should be no different from that on a line ...
Hans's user avatar
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1 vote
1 answer
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Does dark matter have mass?

When trying to understand what dark matter is, it is helpful to know that some properties of it can already be derived from various observations, such as, it only interacting via gravity and no other ...
Quantum Wonder's user avatar
7 votes
3 answers
5k views

If we consider the spacetime of the universe to be four-dimensional, does the Big Bang lie in its center?

Apologies for the (hopefully now somewhat less) clickbait-y title. Now, of course, I know that the Big Bang did not happen at any point connected to a single point in our current $3$-dimensional ...
paulina's user avatar
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-3 votes
1 answer
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Does quantum entanglement arise from perpendicular time vectors? [closed]

From what I understand, "quantum entanglement" is a phenomenon where certain information travels instantly between entangled particles, regardless of distance in space. When thinking of ...
Quantum Wonder's user avatar
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1 answer
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Does time arising from entropy agree with GR?

There's a theory that the direction of time arises from entropy and the correlations (interactions) between bodies. However, I don't see how this would incorporate the effects of General Relativity, ...
Flamethrower's user avatar
2 votes
1 answer
44 views

Are non-point spacetime events partially ordered?

When describing events in spacetime, we usually use points. We then phrase the relation between points as a trichotomy: either they are timelike, spacelike, or lightlike separated, based on the ...
Corbin's user avatar
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