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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546 views

If the Big Bang theory suggests that the Universe is “expanding” then what is it expanding INTO? [duplicate]

I am a software engineer and not an astrophysicist but I want to know if anyone is working on sorting out what exists outside the universe? So what about outside spacetime? what is there? or shall I ...
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1answer
104 views

Why not $\rho^{2}=T^{\mu\nu}T_{\mu\nu}$ as an effective mass density (squared ) in general relativity?

Why not $\rho^{2}=T^{\mu\nu}T_{\mu\nu}$ as an effective mass density (squared) in general relativity? It's covariant, and as far as I can tell is zero for any electromagnetic field tensor. \begin{...
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1answer
38 views

Does the Komar mass density act like a four density?

The Komar mass is a means of measuring gravitational mass in spacetime. Via Wikipedia (https://en.wikipedia.org/wiki/Komar_mass) it is stated as (for a stationary metric): $$m=\int\rho d\mathrm{vol}=\...
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1answer
48 views

Can molecules/atoms/any subatomic particle cause space time curvature?

Space-time curvature is caused by huge objects in space like black holes, merging black holes, or planets. This curvature is what causes gravity. Can molecules cause at least a really tiny curve ?
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74 views

In the classical inflation and big bang theory how can the universe grow to infinity?

Some astronomers make the conjecture that the universe is actually infinite in extent (which is a bad manner of expressing, I know). If so, how could it grow to this size in the classical theory of ...
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2answers
68 views

How does rotation of a mass affect the geometry of space? [closed]

I understand (or rather, I have been told, but do not understand) that a large mass can distort space by its rotation. While I am somewhat familiar with the concept of a mass distorting space and ...
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4answers
114 views

Why is acceleration of expansion of space unexplained?

I didn't know how to better phrase the question so here we go. It says that the farther we see from our galaxy the faster the other object seems to be going away (accelerating). My common sense ...
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1answer
35 views

Conserved quantity in a spacetime with Killing vector

I am trying to prove that that the expression $Q=-\frac{1}{\kappa}\int_{S_\infty} \nabla^i \xi^k \mathrm{d}\sigma_{ik}$ is a conserved quantity for a spacetime with Killing vector $\xi^i$ where $S_\...
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1answer
41 views

Function to model deformed spacetime in 2D visualization of an Alcubierre drive

In the Wikipedia article on Alcubierre drive there is a top image. It is 2D visualization of an Alcubierre drive, showing the opposing regions of expanding and contracting space-time that displace the ...
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1answer
74 views

Questions about how the curvature of universe is described with a single number

I often read of physics talking about "the" curvature of the universe, giving it some letter $K$, and claiming that we have seen that $K = 0$ within such and such certainty. I have three questions: ...
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1answer
79 views

Is it possible to travel faster than the speed of light under extreme warping of spacetime?

The event horizon of a black-hole is defined as the point at which light itself cannot escape, supposedly due to a black-hole's immense gravity and effect on spacetime. However, would this not require ...
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1answer
242 views

What is the meaning of the particle horizon in conformal diagrams?

I'm reading "Physical Foundations of Cosmology" (Mukhanov) and in Chapter 2.3 conformal diagrams get introduced. They seem to be a (graphical) tool to understand the causal structure of the universe. ...
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1answer
63 views

How is Zeno's Dichotomy Paradox resolved according to physics? [duplicate]

I always thought of quantum space and quantum time as a solution, but then i figured out that "quantum space-time" is not actually true(yet). I've also heard of the calculus approach and limits, but i ...
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3answers
308 views

How can the past, the present and the future coexist at the same time? [closed]

We all heard it many times, the theory of Time suggest it is parallel rather than being linear. This leaves the door opened for alternate realities and well, immortality. However, even after reading ...
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1answer
101 views

Geodesic equation (free particle)

How to find a coordinate system whose geodesic equation does not have the "Christoffel symbol" term? (i.e. free particle - generalized Newton's second law.)
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1answer
222 views

Is the Universe infinite or it's just expanding to infinity?

According to the WMAP data in the past years we can say that our universe is considered flat or at least nearly flat and we also know that flat universe is allowed to be both infinite or finite in ...
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1answer
81 views

Gravity vs. Velocity time-dilation [duplicate]

Is there any difference in effects from time dilation of that due to gravity vs. that due to velocity?
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1answer
61 views

Difference between Distance and Space

So I take particle A and place it in space, then I place particle B 1,000,000 light years away from particle A. I strike a line between the two, and measure that line as having a length of 1,000,000 ...
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1answer
222 views

Is this statement correct or incorrect: Moving objects undergo length contraction because they appear in space-time cross-section

Why or why not? This is closely related to another question I posited here, Does it make sense to say that objects moving at relativistic velocities appear in space-time cross-section? What I mean ...
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1answer
383 views

Can gravitational waves carry momentum, and if so what mass/energy produces it, and is space-time capable of vortex formation?

This question is about gravitational waves postulated by Einstein. Is gravitational radiation a form of mass/energy that can form momentum? If it causes ripples in space-time, would space-time ...
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1answer
75 views

Understand the relation of time and the space dimensions of spacetime?

I assume the relation of the three spacial dimensions and the time dimension is handled purely in the mathematical domain, usually. But is there any intuitive description of this relationship, how ...
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2answers
72 views

What does “simultaneously” mean in relation to a detonation shockwave?

I understand that the difference between a deflagration and a detonation is that the reaction front of a detonation moves faster than the speed of sound in the material, which means that the pressure ...
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1answer
163 views

Spacetime in Loop quantum gravity

In LQG, does spacetime consist of interconnected loops? Are those loops real? If spacetime does not consist of the aforementioned loops, what it consists of? The definition of real would be: they ...
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188 views

Why does the thought experiment of a photon bouncing of a mirror imply anything for other objects?

Okay, so I am reading a book, "The Elegant Universe" by Brian Greene, which talks about motion and its effect on time. Greene makes the point that time changes with motion by saying that if you have ...
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1answer
118 views

Empty Space - Could we exist without it? [closed]

I have been giving empty space much thought. I imagined reaching the edge of the Universe and thought of 2 scenarios(focusing on 1): There is no more empty space 1.1 Would space extend with me if ...
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6answers
885 views

Mass without Matter?

My cursory studies of general relativity had introduced me to the concept of how gravity is associated with the warping of spacetime and that the presence of matter in space warps space and this ...
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1answer
406 views

Time coordinate inside black hole horizon [duplicate]

I am new to physics and was trying to learn more especially about general relativity. The Schwarzschild metric, changes the sign of the time and radial parts of the metric once we cross the event ...
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2answers
74 views

Gravitational force of several massive bodies, from the viewpoint of general relativity

According to Wald's GR, "absolute gravitation force has no meaning". The text goes on to describe two cases: one where a gravitational force can be defined, and one in which it cannot. I'd like to ...
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1answer
408 views

Is there an absolute center of mass exist in the universe? [duplicate]

By the "absolute center of mass" I mean the center of all the matter that exists in a given moment. My friend asserted that today and it intrigued us very much but since we only have a CS degree and ...
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1answer
156 views

The universe appears to have a lower bound in the time dimension, why not an upper bound?

The Big Bang looks like a lower bound to the "size" of the universe in the time dimension. Could it also have an upper bound, some furthest point in time from the Big Bang?
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1answer
96 views

Analogues of mass, space, or time? [closed]

I began wondering about this when working through this question: is there anything analogous to spacetime or matter? Both spacetime (note: most of what I know of about the concept of spacetime comes ...
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1answer
4k views

If gravity bends space time, could gravity be manipulated to freeze time? [closed]

You age at a different rate depending on the force of gravity. Astronauts age fractions of fractions of fractions of a second less than earthlings. If you took a sphere of equal masses, separated by ...
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1answer
110 views

Knots and singularities

Can space-time singularities be treated as mathematical knots occurring in dimensions greater than four? I just drew an analogy with knots in one-dimensional strings. When a rubber-band is looped over ...
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1answer
141 views

Is cosmic background radiation absolute [duplicate]

Now that we don't have permeating Ether anymore, why don't we consider the omnipresent Cosmic background radiation in place of it? and measure Speeds with respect to the CMBR? In that way an object ...
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1answer
333 views

Questions about MTW's “thousand” tests of the Einstein principle

In Misner, Thorne, Wheeler (henceforth written as "MTW"), "Gravitation", Box 16.4, there's an experimental setup construction (or method) presented by which "Each geodesic clock is constructed and ...
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1answer
245 views

How is Momenergy frame independent?

momenergy = mass * spacetime displacement/proper time for that displacement What I dont understand is how can the momenergy be frame independent? The unit-4 vector always points in the direction of ...
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1answer
143 views

Best way to check for anisotropy given a metric tensor

Carroll gives the definition of isotropy at a point as given vector $V$ and $W$ in $T_{p}M$, there is some isometry that can push $V$ forward such that it ends up parallel to $W$. I understand what ...
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2answers
705 views

In which direction do gravitons travel? Do they originate from mass or are they collected by masses?

This is a kind of silly question, that mostly goes unanswered: are gravitons detatched from the objects they act on, like photons in free space. Maybe gravity or gravitons are collected by mass (...
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1answer
146 views

Can the vanishing of the Riemann tensor be determined from causal relations?

Given a Lorentzian manifold and metric tensor, "$( M, g )$", the corresponding causal relations between its elements (events) may be derived; i.e. for every pair (in general) of distinct events in set ...
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2answers
398 views

If inherent randomness exist in quantum mechanics, what then of eternalism implied by relativity?

I am nothing but a curious layman so don't go too technical on me. First of all, I am well aware that a lot of people consider the question of determinism vs indeterminism to be unsolved and others ...
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2answers
3k views

If the universe is 3D, how is space-time like a “fabric”? [duplicate]

I have been taught that space-time should be viewed as a fabric and that objects with a large gravitational influence indent that fabric. My question is, if the singularity of a black-hole punctures ...
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2answers
213 views

How to find a curvature of the space-time by having $g^{\alpha \beta}$ in the following case without cumbersome calculations?

The metric tensor for Fock-Lorentz space-time, $$ \mathbf r_{||}{'} = \frac{\gamma (u)(\mathbf r_{||} - \mathbf u t)}{\lambda \gamma (u) (\mathbf u \cdot \mathbf r) + \lambda c^{2} (1 - \gamma (u))t + ...
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2answers
390 views

about the 1D singularity of black hole

I saw some responses here saying that the singularity into the black hole is one dimension object so my question is : is it possible that the singularity is simply a merger of the 4 dimensions of the ...
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1answer
82 views

Are branes in 4D-spacetime moving, or are they static?

Given that a worldline, worldsheet, worldvolume, are representation in a 4D-spacetime of a point particle, a string or a brane, respectively, I was wondering if those objects necessarily have to be ...
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1answer
437 views

Time contraction

This is my first time posting on this site. I am a computer programmer that stumbled across a physics text book and have a question on special relativity. So firstly, I understand that there is no ...
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1answer
156 views

Symmetries of spacetime and objects over it

I guess according to mathematical didactic, we first think of spacetime as a set and we reason about elements of its topology and then it's furthermore equipped with a metric. Appearently it is this ...
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1answer
137 views

Classical black holes?

How big should the black hole be so we can consider it to be classical? When they claim that we can not probe shorter distances than the Planck length, can it be true? The argument says that, ...
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2answers
366 views

How to concile flat spacetime and big bang?

After reading How do we resolve a flat spacetime and the cosmological principle? I still remain perplex. Please excuse my ignorance and try explaining to me : I thought that basically, when we ...
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2answers
56 views

How do we know singularities/black holes actually bend time?

Recently I read up on some articles about singularities / black holes, how they form and how they affect their surroundings. Just to clarify, I am no physicist and I found those articles on the ...
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0answers
47 views

The expansion of the universe and its edge

I have understood that the universe is expanding at an accelerated rate. I have also thought, perhaps mistakenly, that this expansion means not that the masses in the universe are moving away from ...