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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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Is space continuous? [duplicate]

Take a particle, it has a position $(x, y, z)$ maybe it is $(0.231, 8.962, 10.567)$. Is there a maximum precision to this? Is the space discrete or continuous? If it is discrete, how thin the ...
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The chicken-egg nature of bending a photons pathway

I'm a chemist not a physicist, so I have a loose relationship with its laws. I read that light (specifically a photon) doesn't have mass, gravity doesn't act on a photon to alter its trajectory. ...
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Questions about tear in spacetime [duplicate]

Sorry if there's any mistake in the language. In Einstein's General Relativity Theory gravity is described as the curvature of the spacetime fabric. So can analogous to a real fabric can this ...
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Is spacetime wholly a mathematical construct and not a real thing? [on hold]

Speaking of what I understood, spacetime is three dimensions of space and one of time. Now, if we look at general relativity, spacetime is generally reckoned as a 'fabric'. So my question is, whether ...
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What do you think of this conceptualization of the universe and time dilation? [on hold]

So the universe is ever-expanding. It's been expanding in all directions since the Big Bang, like how a sponge expands when you give it water. The universe is made of a fabric known as space-time. The ...
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I have a question about the gravity probe B geodectic precession formula and why it seems to be twice the planetary precession formula

Specifically, I can work out the planetary precession formula to give radians per orbit as 6*pi*GM/c^2r (and I find that on the web at places like http://farside.ph.utexas.edu/teaching/336k/...
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Mass bending spacetime: what is the mechanism? [duplicate]

What is the mechanism that causes mass to bend spacetime? Merely inferring that it does is not an answer. How it does it is the answer.
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Non-static spherical symmetry spacetime

The Schwarzschild solution is a static spherically symmetric metric. But I wanted to know that how would the space-time interval look in a Non-Static case. I tried to work it out and got $$ds²= Bdt² -...
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Kaluza-Klein approach and Gauss-Cadazzi approach

Can you tell me the difference or physical application of Kaluza Klein approach and Gauss Codazzi approach? In Kaluza Klein theory, 5 dimensional theory can be dimensional reduced to 4 dimensional ...
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Spacetime has an infinite number of choices for differentiability. Coincidence?

Spacetime can be modelled using a four-dimensional topological manifold. Say we denote the manifold using $(M, \mathcal{O})$ where $\dim M =d$. The structure $(M,\mathcal{O})$ is not sufficient for ...
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Time for a photon to pass through a massive body

Suppose there is a neutron star with a hole in the middle. Through this hole, scientists shoot a laser beam in such a way that it will pass through the gravity well of the neutron star but will not ...
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2answers
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What if humans doubled size… and everything else… could we notice? [duplicate]

After the big bang, everything expanded from a small mass. That expansion is said to be still happening. Imagine if everything observable constantly grew in size. EG. Everything slowly doubled in ...
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How do we measure the age of the universe? [duplicate]

How do we measure the age of the universe if time is relative to the observer? What is the reference frame we use to measure the age of the universe?
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Ricci Scalar as Curvature

So I understand that the Ricci scalar represents the curvature of the space. Since any manifold can be considered locally flat, is Ricci scalar always zero locally for any manifold? On one hand it ...
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Could uncomputable processes and things exist in a spin-networks-based universe? Even observable uncomputable processes and things?

Roger Penrose proposed a series of networks from which, fundamentally, space-time would emerge, called spin networks (https://en.wikipedia.org/wiki/Spin_network) Also, in this paper https://...
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How can the universe be dodecahedron-shaped?

Physicsworld references "Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background" by J.-P. Luminet et al., published in Nature (...
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Another question about Four-velocity derivation

Consider,again (Question about derivation of four-velocity vector) the following: For a massive particle with position $x^{\mu}(t) = (x^{0},x^{1},x^{2},x^{3}) \equiv (x^{0},\vec{x})$ we define the ...
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Is spacetime-curvature relative?

Velocity is relative, which means kinetic energy is. Since, according to general relativity, energy bends spacetime around it, wouldn't this mean observers moving in different inertial frames measure ...
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Naive question about the need to construct the 4-velocity

Special relativity was not born as a 4-covariant theory. Instead, Einstein derivated the kinematical quantities without spacetime,therefore without 4-vectors. Consider then the following: First ...
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Gravitational Lensing: Up, Down or Sideways

I have been reading about Eddington and his measurement of 1.75 arc seconds in 1919. What I am wondering is why was the measurement lateral? I picture a beam of star light entering a curvature of ...
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2answers
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If the universe is flat, does that imply that the Big Bang produced an infinite amount of energy?

Too much density and the universe is closed, analogous to a sphere in four dimensions: you travel in a straight line and you end up where you started. Too little and you have a saddle: not sure about ...
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Direction of gravity

General Relativity explains the path a falling body makes (ex. An apple falling toward the center of the Earth) as a geodesic in curved spacetime. What explains the direction the apple falls? In other ...
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Noether's conserved charges: why are there not “spatially” conserved charges? [duplicate]

Noether theorem implies that there is a conserved current $j^\mu$ for every continuous symmetry of the action, i.e. $$\partial_\mu j^\mu=0 $$ to each conserved current we can associate a conserved ...
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3D positively curved space [migrated]

If we consider 2D euclidean surface consists of infinite concentric circles and 3D euclidean surface consists of infinite concentric spheres. If 2D surface is positively curved the radius of the ...
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Do mass and motion affect space-time differently?

Mass is said to create curvatures in space-time thereby creating gravity, yet technically the smallest movements, even on Earth, create gravitational waves. Are there different "types" of disturbances ...
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Why do we differentiate a 4 vector with respect to proper time to obtain 4-velocity?

The coordinates of an event in spacetime are given by the 4-vector $(ct, \mathbf{r})$, where $\mathbf{r}$ is the spacial coordinates of the event. This 4-vector can be seen as 4-displacement of a ...
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Fiber manifold Ricci flat, physical meaning

In a warped-product spacetime, what a physical meaning we have for Ricci-flat Fiber? I'll explain.. it is well known that a Ricci-flat spacetime means that the cosmological constant need not vanish, ...
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Energy spacetime warping

If energy warps spacetime, then does light warp spacetime? And if special relativity says that things near the speed of light increase in relativistic mass, then does light have a relativistic mass? ...
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Proper time vs time

I'm reading about relativity and I thought I understood things clearly about proper time / coordinates until I read the analysis of the Doppler effect. A and B are 2 fixed points, there is a clock in ...
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Are particles always represented by 4-volume preserving transformations?

I've been reading about affine gauge gravity, which uses the affine group A(4,R) (for example here ). If I'm getting it right there seems to be an “affine higgs” mechanism that breaks symmetry down ...
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$ct' = γ(ct − βx)$

I was reading a paper on S.R. saw this eq: $ct' = \gamma(ct − \beta x)$. However I am confused by the Minkowski's diagram: We know $\tan \theta = \frac{x}{ct}= \beta$. However, wouldn't it also mean $...
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Warped space and gravity

I'm trying to combine in my head a warped space (e.g. an Alcubierre "warp bubble") with the curved space of gravity. How do the two interact? More specifically, if a warp bubble moves near a source of ...
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Invariance of coincidence

I am reading the book "Space and Time in Special Relativity" by David Mermin. In chapter 13, at page 128 in my print, he says the following (screenshot): I'm referring specifically to the sentence "...
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Is ER= EPR a Hidden Variable Theory & If NOT Why?

John Bell showed that entanglement cannot be a local hidden variable theory in 1964. Leonard Susskind and Juan Malcadena have proposed that non-traversable wormholes, Einstein-Rosen bridges (ER) are ...
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Is it spacetime or space that is expanding?

I have read answers to several similar questions but I still don't get it. Earlier explanations seem to say that laws of quantum physics and general relative are different. Let me get this straight. ...
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4answers
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When photons are moving at the speed of light, how come they change in position?

From what i know, photons are moving at the speed of light and that means the time that photons are experiencing is zero, but here is the thing... If the time is zero how can they travel astronomical ...
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“It is the gist of general relativity that it admits, on an equal footing as it were, every possible coordinatization.”

The title is a quote from Hermann Weyl in a 1955 article: Weyl, Hermann. "Why is the world four-dimensional?" In Levels of infinity: Selected writings on mathematics and philosophy. Courier ...
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Could gravitational wave permanently distort a region of space time?

Imagine a flat spacetime being disturbed by a gravitational wave from a very distant source, say given enough time would that flat spacetime recovers meaning it must be at the same state as before, ...
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Why does spacetime propagate gravitational waves?

Setting the scene: If I drop a stone into water, the stone will create a depression in the water that the surface tension of the water and gravity (so says the Wiki article: Dispersion (water waves)) ...
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Deriving the equation relating the metric and the coordinates to the proper time in general relativity

If $x^a = x^a(\tau)$ is the worldline of a particle in general motion, then $$V^a = \frac{dx^a}{d\tau}$$ is a four-vector field along the worldline. If $$g_{ab}V^aV^b = g_{ab}\dot{x}^a\dot{x}^b = ...
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Why was pseudo-Euclidean geometry not enough for general relativity?

How would you explain to someone the change that Einstein needed in geometry for his new ideas about gravity and spacetime, what did he seek but could not be described by pseudo-Euclidean geometry? ...
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Areal radius, cosmology

In cosmology, what areal radius means exactly? Is defined as \begin{equation} R(t,r)=a(t) \, r \end{equation} Where $a(t)$ is the scale factor.
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Twin paradox with one twin in orbit, one in radial free-fall

Many questions on Physics SE relate to the twin paradox, but I did not find any that ask this specific question. Suppose that object A is in a circular orbit around a spherically symmetric, non-...
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1answer
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(3+1)D solution to (2+1)D einstein equations?

Imagine a grid in 3D made of pipes smoothed so that it forms one continuous infinite surface. The surface is 2D but it fills 3D space. Like this (at one instant): Could any surface like this be a ...
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1answer
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Is the Big Bang a naked spacelike singularity?

While reading some answers on similar topics, I was wondering about the nature of the Big Bang singularity in the standard cosmological model. I know it's a spacelike singularity that have a ...
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1answer
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Minimal Requirements for Space and Time

I read recently that to consider the Planck length of less than 1.6 x 10 exp -34 meters the smallest unit that can manifest 4 coordinate space time to be incorrect in its interpretation. The reason ...
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By how much does space-time curvature modify the density of an object?

Let's imagine a transparent object with a density $\rho_0$ [$kg/m^3$] (A sphere of glass as example). Now I send through this object a long flat impulse of electromagnetic plane wave with frequency $\...
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If I could halt all motion would I stop time? [duplicate]

I've never really understood time. Time is not a force. It isn't energy or matter. Time doesn't MAKE anything do anything. Time doesn't make a clock tick (motion or batteries do). Time doesn't make ...
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What does it actually mean to say that time “speeds up” or “slows down”?

I was thinking lately about relativistic time dilation and time passing differently on other planets (which both, eventually turned out to be caused by the same principle). Now, quite naturally (as ...
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Gödel Universe and second law of thermodynamics

In the Gödel universe, which contains closed timelike curves and is a solution to Einstein’s G=T field equation, time travel is thought to be physically possible. But as you need a spaceship (which ...