Questions tagged [spacetime]
As a consequence of the Lorentz transformations, time and space transform into each other when changing reference frame. This calls for a unified description: Minkowski spacetime.
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How to correct the Schwarzschild metric in the presence of Dyson sphere?
I was solving a problem in general relativity about Dyson sphere around a star. I don't have any problem solving the problem but in the question Dyson sphere was assumed to be massless. I checked ...
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Did the Big Bang cause an outward push of gravity?
There is a theory that the big bang’s blast caused an outward push, a kind of reverse gravity if you will, of our universe and everything within it.
My question is how could this have happened? If it ...
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How is spacetime locally Lorentzian at the center of a large mass?
Following up on the questions raise here and trying to get a little more clarity on what it means to be 'local' and 'flat'. In the first chapter of Gravitation, the author(s) state:
The geometry of ...
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A question about space-time diagram
Examine the image carefully, this is a space time diagram plotted by Brian Greene. The event co-ordinates are shown according to both reference frames.
My question is, did the event, $E$, happen ...
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Are topological changes to dynamic spacetime quantized? Can the Chern-Gauss-Bonnett theorem illuminate dynamics?
I was looking at the Chern-Gauss-Bonnett theorem in dimension 4. Here we can write the Euler characteristic of a compact 4-manifold as:
$$\chi(M)=\frac{1}{32\pi^{2}}\intop_{M}\left(|\mathrm{Riem}|^{2}-...
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A rapid question on $f(R)$ gravity
Is it possible to construct a $f(R)$ where,
$$f(R) = \alpha R \tag{1} ?$$
I'm asking this for two reasons: $1)$ I'm quite a freshman on modified theories of gravity and $2)$ Most of the times I see ...
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Why is Minkowski metric diagonal?
Why is the Minkowski metric a diagonal in a 4x4 matrix? What does the diagonal do?
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How to tell who is experiencing time slower and who faster when travelling at different speed?
I am trying to understand time dilation w.r.t velocity. Its said that when a satellite travels around earth (at speed more than a synchronized clock on earth which is stationary w.r.t earth), it ...
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Spin connection for a paralellization takes more general forms than $SO(3,1)$ in different spacetime topologies?
I'm interested in a frame bundle over spacetimes with different topologies. In the trivial case of Minkowskian space ($\mathbb{R}^{3,1}$), a frame (or tangent space) at one point is going to be ...
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Do objects in a 2D universe have an edge?
When discussing a 2D universe, many assume that an object would only be seen as "a line". This would imply that you are seeing the "edge" of the object. But, if there are only ...
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What is the role of the laws of physics in a block universe?
Definition of a block universe - The idea that the whole universe exists simultaneously and time doesn’t flow.
For those who favor this kind of theory (the few of you), what is the role of the laws of ...
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Mass as space curvature [duplicate]
General relativity states (in simplified vocabulary) that mass curves the space-time, being gravity one of the observable effects of this curvature.
Is there any theory or works starting on the ...
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Special relativity with horizontal mirror clock? [duplicate]
Every explanation video about special relativity comes with a constantly moving train and a vertical (according to the train's speed direction) mirror clock. So first of all, why in the classical ...
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Twin Paradox Explanation via Spacetime diagrams [duplicate]
Can someone explain to me the twin paradox with the help of spacetime diagrams? (No algebra and calculus)
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Schwarzschild geometry
In the text "Exploring Black Holes" by Taylor ,Wheeler I came across the following thought experiment
"We consider two imaginary concentric spherical shells and suppose we measure the &...
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How to interpret reference frames in special relativity?
I am studying special relativity but it isn't making any sense. I know that if I have space time co-ordinates in a stationary reference frame I need to use the Lorentz transformations to get the co-...
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Age of a person going to space [closed]
When a person goes into space and stays there for a specific time without moving the spacecraft, will he age faster than those on earth?
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Why is the proper distance written like this? - General Relativity
I came across the following equation for proper distance:
$$ds^{2} = -c^{2}dt^{2} + [dx - v(t)f(r(t))dt]^{2} + dy^{2} + dz^{2}$$
where the function $v(t)$ tells you the velocity of the warp spaceship, ...
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What are the possible candidates for entities which are more fundamental than spacetime?
Many physicists now believe that there are more fundamental entities than spacetime. I would like to know if this is at this point an educated guess or do we know possible candidates which replace ...
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How do you derive Newtonian Sum of Forces equation from GR?
This is what I have so far.
We start with the definition that force is the change in momentum with respect to time:$$\Sigma F_i =\frac{dP}{d\tau}$$Where $\Sigma F_i$ is the sum of forces, both applied ...
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Amount of matter in two infinite universes [closed]
I don't know exactly where to find any other information on this question so I thought I would ask here. If there were two infinitely large universes one where 20% of the space in the universe was ...
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While space itself expands, why does it seem to expand more between the stars than between me and my desk? [duplicate]
this is my first post in this community and I am not entirely sure if it is appropriate.
I am not an educated physicist, just an ethusiastic person trying to understand the universe.
So I apologise if ...
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Schwarzschild with cosmic strings: How can I visualize the string?
Geometrically, you can talk about a cosmic string in Schwarzschild black hole with this metric $[1]$:
$$ds^{2} = -\Big(1- \frac{r_{s}}{r}\Big) dt^{2} + \frac{1}{\Big(1- \frac{r_{s}}{r}\Big)}dr^{2} + ...
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How can I "match" energy-momentum tensors in Einstein Field Equations?
I) CONTEXT:
When we study exotic solutions of Einstein Field Equations (EFE), such as Traversable Wormholes, the elementary modus operandi to extract some analysis is to deal with EFE in the following ...
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Special Relativity can my past be in someone's future
My question is the following:
Is it possible that an event which is in my past light cone is currently in another observer's future light cone?
(i.e. for an observer sitting at a point in the set of ...
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Length contraction, time dilation, and spacetime intervals contradiction
I'm learning about spacetime and relativity in class. Default units are seconds and $c$. I'm using the length contraction calculator and time dilation calculator here: https://www.omnicalculator.com/...
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Does the Schwarzschild metric prove that around the earth, 99.9999% of the warping of spacetime is temporal?
In several posts on Quora, here Howard Landman writes that the Schwarzschild metric shows/proves that around a smaller body, like the Earth or the Sun, 99.9999% of the warping of spacetime is time, ...
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Motion vs space expansion
Let us consider two particles A and B in empty space, and let us assume that I am an observer attached to particle A. Also let us assume that, according to my measurements, I see that the distance ...
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Does time expand?
If the time component of spacetime does not "expand" in the sense that space expands, doesn't that imply that time must already exist in its entirety, past and future? I believe I have seen ...
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What does spacetime interval really mean? [duplicate]
Is there any simple way to intuitively understand spacetime interval, proper time and proper length?
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How much of gravity is caused by time dilation?
I just watched this video where it is explained how time dilation causes gravity. It is said in this video that time dilation (caused by masses such as the earth) is the only cause for gravity.
But ...
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Motivation for curved spacetime on General Relativity
I'm trying to motivate the theory of general relativity. Concretely, using which arguments is it deduced that spacetime must have the structure of a curved Riemannian manifold?
From the strong ...
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Ambiguity in measuring the speeds of galaxies
People measure the speeds of galaxies via the redshift effect. However, considering our spacetime as a non-flat manifold implies that measuring the speeds of any two objects not in the same tangent ...
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Example of curved spacetime with flat space layers
curved spacetime is a famous geometrical and physical object. on the other hand there are debates on if space itself is curved or not. but is it really possible to a curved spacetime that all of its ...
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The effects of Lorentz transformation on shape
Imagine a solid 3D cube. Now imagine that this cube is traveling close to the speed of light. To what degree will the spatial geometric properties of this object (or in general of any 3D object) ...
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What is the sum of forces equation in curved spacetime?
I've been thinking about the Sum of Forces equation and the fact that gravity isn't really a force. So what is the proper way to think about this equation when dealing with objects in a gravitational ...
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Why does Misner, Thorne, and Wheeler’s Gravitation visualization technique breakdown in dimension $> 4$?
In this chapter we will introduce and discuss at length one of the ways that physicists sometimes visualize “nice” differential forms. In essence, we will be considering ways of visualizing one-forms, ...
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Is there physical reason for a stably causal spacetime, or the existence of a Cauchy surface?
In their 1979 essay Global structure of spacetimes, Geroch and Horowitz describe methods of determining the topology, causal structure and singularity of spacetimes. Their (mathematical) arguments are ...
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As space expands, should the Planck length too?
The Planck length is usually defined as the scale at which QM effects become dominant. This is what I am referring to.
I found one question, but that one has no good answers.
How does universal ...
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How does Einstein Field Equations (EFE) work with more than 4 infinitely large spacetime dimensions?
How does Einstein Field Equations (EFE) work with more than 4 infinitely large spacetime dimensions?
But is gravity in Newtonian physics valid with more than 3 infinitely large spatial dimensions?
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Is it enough to give a time-orientation to define a spin structure?
Maybe I got it wrong and my question doesn't make sense, excuse me if that's the case. For a smooth Lorentz 4-manifold $(M, g)$ with signature $(- + + +)$ is it enough to give a time-orientation to ...
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Is there any model or theory which considers that spacetime and symmetries are all emergent? [duplicate]
There are a lot of models which propose that spacetime is actually emergent. I am not sure if this necessarily means that all symmetries associated with spacetime would also be emergent.
Do you know ...
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What does it mean for a physical object to have size?
When I think about describing what it means for something to have a certain size, I keep running into tautological explanations:
An object has size if you can compare its size to something else (that ...
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Proper force and relativistic force
Relativistic force of an object is $F$ = $dp/dt$ where the object is moving with velocity $v$ with respect to our frame of reference. $p$ is the relativistic momentum given by:
$$p = \frac{m_0v}{\sqrt{...
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Proper time and 4-velocity
Now, 4-velocity is defined as $\mathbf u^\mu$ = $dx^\mu/d\tau$, $\tau$ being the proper time.
And then the four momenta is $\mathbf P^\mu$= $m_0$$\mathbf u^\mu$.
My doubt is, suppose we have two ...
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Are there 4D spacetime not described by GR? [closed]
I heard the EFE is specifically for 4 spacetime dimensions. Does it mean whenever there are 4 spacetime dimensions, there is automatically GR? In other words, are there 4D worlds or spacetime that ...
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Tidal forces of radial geodesics in Schwarzschild spacetime [closed]
Let me start with the questions I have:
Since infinite tidal forces at the event horizon are meaningless, is this yet again a result of the choice of coordinates?
What exactly do we call the tidal ...
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Invariant line element in GR
In my General Relativity course, we had argued that even in a curved metric space, light follows a trajectory that obeys $ds^2 = 0$. The reasoning we were given is as follows:
The line element in any ...
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Is there a limit to how much spacetime can be curved?
My question is about the fact that general relativity predicts bending of space and time. However, I have not seen anything regarding the limits to how much space or time can be bent.
So my question ...
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What does it mean when we say, "Gravity is a source of gravity"? [closed]
This is something one encounters quite often when reading up on general relativity. How am I to understand this statement in comparison to, say, electromagnetism where light is understood as ripples ...
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