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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Are tachyons a mathematical impossibility (not just physical)?

I recently learned from a helpful SE user that, in general relativity, the "law of geodesic motion" is actually a mathematical law, not a physical one. That is, a "test particle" (...
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What is the cardinality of all points in space time? [closed]

Since the universe isn't a cartesian grid, and can bend and warp, what is the cardinality of all points in space time?
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Does Absolute Rest Imply Absolute Simultaneity in General Relativity?

In Special Relativity, absolute rest implies absolute simultaneity. In other words, although simultaneity is standardly taken to be relative to an inertial reference frame, if you think there are ...
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How can we perceive light if light doesn't move through time?

From my understanding in space-time everything moves at the speed of light $c$ in some direction of this four dimensional space. Light itself moves only through space, so it doesn't move at all in ...
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Is spacetime equivalent to changes in index of refraction of aether? [closed]

If space is made of aether, and it is denser in regions of mass concentrations and have higher index of refraction in those regions, if light goes where index of refraction is high number, it will ...
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Should time be a loop or a line?

It's interesting to see that torus was so popular in physics, that it worked so well. There was famous Gauss–Bonnet theorem which stated basically $$\oint_S K dS= 2\pi \chi(S)$$ where K was the ...
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What's incorrect with using the two formulas for Lorentz contraction and time dilation at the same time?

Lorentz contraction: $L = L_0 / \gamma$ Time dilation: $t = \tau \gamma$ Using the above two formulas at the same time leads to contradiction to the principle of the constancy of light velocity. What’...
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How equivalence principle explains curvature of space?

We know that the equivalence principle explains the slowing down of time, since, with respect to an inertial observer the time difference between sending two successive light pulses at the head of the ...
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Objective direction of time in general relativity

In general relativity, the coordinates $x^\mu$ we put on the manifold are arbitrary and need not have any physical interpretation. For this reason, it is said there is no objective notion of time in ...
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On the derivation of Rindler metric

I'm trying to reproduce the derivation of the Rindler metric in this wikipedia article but I came at a point which I cannot undertand. In the Rindler coordinates section, $\alpha$ is the proper ...
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Can spacetime be curved without matter inside it? [duplicate]

I can imagine space to be curved without matter in it. But can this happen in reality and what does it mean? A black hole is said to be a vacuum solution but doesn't it require matter to form a black ...
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Curvature of space without mass or energy [duplicate]

Are there solutions of Einstein's equation $$ R_{i,k}=0 $$ which imply there are 'curvature' without matter or energy? For example, space is curved but there are no matter or any energy there so an ...
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Warped space shouldn't cause movement in Gravity so what does? [duplicate]

I'm trying to get an intuitive understanding for how gravity works and have been reading various explanations from answers here to Sean Carroll and others' youtube videos. I have a set of university ...
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Another slices of the universe than space? [closed]

We observe a particular kind of "slices" of our universe: the slice that is called "space" (in the special sense of our 3 dimensional physical space). Are other kinds of slices in ...
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Anti de Sitter & de Sitter Spacetime [duplicate]

Can anyone please explain what are the Anti de Sitter and de Sitter spacetime and what is special about them? I am learning general relativity and I stumbled upon them a few times, even on the subject ...
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Is inertial mass actually reverse-time gravity reaching back from the distant future and pushing?

James Woodward of Cal State Fullerton says so. He say it's just like an advanced wave in QED. He says that gravity accounts for inertial reaction forces as long as the interaction of local stuff with ...
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On the dependence of time in transformation between reference frame in Special relativity

Suppose we have a reference frame S with (position,time) variable as $(x,t)$ and in another frame$ S'$ we have another set of coordinates $(x',t')$ moving at a speed of $v$ with respect to $S$ then ...
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A coordinate-free understanding of the space-time manifold

I study dynamics and continuum mechanics. Over the years I've gotten used to the coordinate-free, or geometric, way of thinking. A velocity vector, for example, is a tensor. It is the same object when ...
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What was the volume of the universe a short time after the big bang? [duplicate]

I assume this question is somehow ill posed, but I do not know in which way. I think it is not a difference here whether the universe expanded into space, or space expanded itself. Seen from the ...
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Is there a Maximum Data Density of a Physical Volume?

Is there an established or proven relationship concerning how much data can fit in a given volume of space / spacetime?
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How is spacetime curvature generated in string field theory?

In String Theory, the graviton originates from a mode of vibration of the fundamental strings and spacetime curvature itself is somehow generated by the strings themselves. In String Field Theory, ...
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An apparent paradox in General relativity using a binary black hole

Note: Quantum gravity effects are ignored in this question. Imagine 2 black holes each with mass $m$ approaching each other and after some time the Event Horizons (EHs) of both black holes touch each ...
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A perhaps naïve question, but will help with my understanding [duplicate]

Now, I get that gravity according to GR is merely an illusion caused by acceleration, and therefore the force I feel on Earth is not gravity at all, but the Earth accelerating ahead at $9.8 \;\text{m/...
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If position is fundamentally undefined at quantum scales, how can strings be Planck length?

From how I understand Quantum Mechanics and the Uncertainty Principle, quantum objects position is fundamentally undefined. So how could an object like a string with a clearly defined geometry exist ...
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Is there an exact constant value for the speed of light in a vacuum distorted by a gravitational wave?

In a vacuum, construct a cylinder of photons arranged as follows: A 'measuring apparatus' computes the speed of light as 299,792,458 m/s. However, suppose the vacuum is distorted by a gravitational ...
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If you could zero out your momentum, could you travel faster than the speed of light [closed]

I figure I'm going to get ridiculed for this question, and maybe deservedly so. The sensor images and eyewitness accounts from the Naval Aviators released last year of unexplained aerial phenomena ...
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Confusion over Special Relativity - Time and Length

I've just started learning about special relativity. Apart from realising that everything I thought I knew about reality was a lie, I'm trying to wrap my head around a few concepts relating to the ...
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Definition of Isotropy and Homogenous in Cosmology [duplicate]

Please explain the meaning of the isotropy and homogeneous in cosmology. I will be glad if someone can provide the mathematical definition also.
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Lorentz Transformations involving Two Dimensions of Space

I'm a high school student trying to understand the basics of special relativity, and I've been learning about the Lorentz Transformations. I understand that one transforms between the reference frames ...
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Can there be simultaneity without a reference frame?

I have been reading about the concept of the relativity of simultaneity which states that whether two events, separated by space, occur at the same time is relative to the observer's reference frame. ...
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6answers
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Why does FTL imply that BACKWARDS time travel is possible? - Very Detailed! [closed]

I know this question has been asked before and I have read and understand the answers however imo none of them still imply that backward time travel is possible. I understand moving forward in time ...
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Why is absolute time considered an axiom of Newtonian mechanics? What statements are based on this axiom?

I guess absolute time is associated to classical mechanics because people like Newton believed in that concept, but are there actually any statements whose derivation is based on this assumption? I've ...
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Geodesic deviation equation in flat spacetime $\sim$ divergence of geodesics

Consider the above two neighbouring geodesics $\mathcal{Y}$ given by $x^{\alpha}(\sigma)$ and $\mathcal{\tilde{Y}}$ by $\tilde{x}^{\alpha}(\sigma)$ for top and bottom curves respectively. Vector ...
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If MOND were true, what would that imply for the geometrical description of gravity as curved spacetime?

As I understand it, Modified Newtonian Dynamics, or MOND (Milgrom M., 1983, ApJ, 270, 365), slightly alters Newton's Law of Gravity by introducing a low acceleration limit below which (for an object ...
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Why do physicists say that spacetime is not bending “into” or “out” of a fourth dimension?

I understand that there is no need for a fourth-dimensional space to bend into, but why do physicists seem to be against the idea? Is this simply because there is no proof of a fourth dimension, or is ...
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Difference between spacetime interval and metric

Theory of relativity novice here. I understand that the spacetime interval $(Δs)^2$ is the analog of the Euclidean distance in spacetime. But what exactly is the relation between the spacetime ...
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Can a frame of reference travel with the speed of light? [duplicate]

I read that nothing with mass can travel at the speed of light. Therefor one can not ask what happens when an observer in a car moves with the speed of light. But, is it possible for a frame of ...
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How gravitational time dilation affects free-fall time

Consider a point-like planet having a large mass $M$ and two small point-like objects with masses $m_1<m_2$. Drop down each of them separately from distance $d$ on the planet, let $T_1$ and $T_2$ ...
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What are some of the uses of the invariant spacetime interval? [closed]

I am currently researching special relativity and I have come across the invariant interval. So far, I know that all observers will measure the same interval regardless of position or velocity, and ...
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Can the curvature of space-time be also described as a 'frame' (space) whirlpool?

Can the curvature of space-time be also described as a 'frame' (space) wirlpool? When we talk about the Schwarzschild metric it can be immagined as a curved manifold but in the Kerr metric it seams ...
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Is the concept of Planck Length in this video corrrect? [closed]

I'm asking this question because I'm a 10th grader and interested about learning Planck Length. Here's a video I made https://youtu.be/VECJqYCTwbo. Is all the content in this correct. Can I now show ...
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Is time dilated for objects that make a close orbit around (massive) bodies?

I understand that, according to general relativity, matter warps space-time in such a way that the 3-dimensional projection [in space] of the geodesic of an object subject to no forces looks like ...
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Galactic rotation curves question

I think I need to ask this question now, because it's in my mind for several years and I just can't find an answer to it. After observations of those galaxtic rotation curves, it's pretty clear, that ...
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MTW Box 9.1 Tangent Vectors and Tangent Space of a metric-free, geodesic-free spacetime. What necessary properties remain?

With no intent to violate se rules, I wish to as specific questions related to this general question. For that reason, I will attempt to specify the kind of general answer I seek. It is most likely ...
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Is there any experiment to check discreteness of space?

This article from 2015 seems to suggest that there will be experiments to check discreteness of space: If space-time is discrete, there should be imperfections. And even if rare, these imperfections ...
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Why does time seem to flow in only one direction? [duplicate]

According to my understanding. Since Einstein, physicists have thought of space and time as forming a four-dimensional structure known as “spacetime.” But space differs from time in some very ...
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A space ship has maximum proper acceleration of $a_0$. How close can it fall freely towards a black hole before it can no longer accelerate away?

Edit: To clarify, all motion is radial only. Classically, the answer to this is pretty obvious. You just find the distance from the black hole at which the gravitational acceleration matches $a_0$ and ...
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Can someone please tell me the problem with this method to measure one-way speed of light? It's driving me crazy!

I am having a hard time grasping the idea that one-way speed of light is immeasurable. I have watched several videos (including the Veritasium one), read the Wikipedia article, and read some questions ...
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64 views

What is the meaning of “the moment in time of closest distance” between two moving objects?

Two objects $A$ and $B$ are moving along skew lines. Is there a reasonable definition of the moment in time where the objects have minimal distance from each other? In a Newtonian world with global ...
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Questions on the notion of an inertial frame of reference

Imagine an astronaut flying in an spaceship at a constant speed in completely empty space, with no acceleration whatsoever, and no outside references, just dark empty space. Q: Can the astronaut tell ...

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