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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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Can a system be engineered to cause a standing wave resonance with gravity waves

Can a system be engineered to cause a standing wave resonance with gravity waves? It appears that gravity waves can be reflected. Do Mirrors for Gravitational Waves Exist? Stephen J. Minter, Kirk ...
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Why is the past also represented as a light cone in the opposite direction? (layman)

I recently heard about light cones as mentioned in Stephen Hawking's A Brief History of Time. I understand light can be seen as a cone when a flash of light is spread from an ...
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2answers
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In the Schwarzschild Metric does $\sin^2\theta{\Delta}\phi^2={\Delta}\phi^2\sin^2\theta$?

In the Schwarzschild Metric as the spacetime interval between two points in spacetime approaches $0$ for any ratio between the length of time and space the spacetime interval between the points in ...
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Negative Quantum Space-Time? [on hold]

What is the Uncertainty Principle telling us? Is it saying the power of observation/measurement of a quantum object is not enough to make it a genuine 3D + 1 space-time object? If something on our ...
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7answers
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Do photons bend spacetime or not?

I have read this question: Electromagnetic gravity where Safesphere says in a comment: Actually, photons themselves don't bend spacetime. Intuitively, this is because photons can't emit ...
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2answers
67 views

What is meant by globally hyperbolic spacetime? [duplicate]

Can somebody explain what is meant by globally hyperbolic spacetime? What other kinds can there be?
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19 views

Can we see our past from a distant heavenly body? [duplicate]

Lets assume that a fictitious planet called Planet L is 10 light years away. Also,lets assume that it is inhabited by fictitious aliens.These fictitious aliens have high resolution telescope which can ...
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2answers
942 views

There's only time, no space in Quantum Mechanics

In this lecture (44:23) Nathan Seiberg: Topics in 2+1 Dimensional Quantum Field Theories 2. Nathan Seiberg says there's no space in QM and therefore fermions have spin 0. This sounds pretty revolting ...
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Distance to Proxima Centauri according to SIMBAD [closed]

What is distance to Proxima Centauri measured up to 4th decimal?
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31 views

Is the location of everything relative? [duplicate]

I have heard that everything in our physical reality is relative. But I wonder: Does the universe have an edge? In an expanding sphere, there would be an edge is it like that? If the above is true ...
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3answers
767 views

Why do we perceive time?

Since everything (at least what we observe) is travelling through the time in the same direction and speed why do we even perceive the time? Of course, there is the time dilatation, but it is ...
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1answer
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If all spacetime paths inside a black hole point towards the singularity, then does that mean entropy is decreasing?

Entropy is a concept of disorder. A drop of blue ink inside a glass of water only has 10^10 factorial possible ways of being an ink drop, but it has 10^11 factorial possible ways of being an ink soup. ...
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Volume of dark energy in space [duplicate]

As I understand it, Dark Energy is believed to reside in empty space. As the Universe exapnds, more empty space exists inside the Universe and so the small energy per unit of empty space adds up to a ...
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The Expansion of universe [duplicate]

if the space itself is expanding at considerable rate, that means we are also expanding along with it(as we are the part of the universe). so why can't we feel ourselves expanding or detect it?
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30 views

Casimir-Waves-Production [on hold]

https://youtu.be/Rh898Yr5YZ8 In this video, starting at 4:02-4:40 Matt speaks of the production of "waves" in the space-time by the casimir effect metal plates even dough the whole process is ...
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32 views

Why do we visualize space time as trampoline surface? [duplicate]

It's a silly question probably. But the thing is that what is the fascinating about fabric surface. I am so sorry for this silly question in advance. I read a lot articles regarding my question. But ...
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1answer
66 views

The relationship between time, relativity and entropy

I came across a discussion about the nature of time and whether or not time is an illusion on a physics forum. I'm not so much interested in the philosophical issues regarding time, but the following, ...
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2answers
88 views

Metric tensor: Why relate it to Cartesian/Minkowski coordinates?

Why does the metric tensor always relate to cartesian coordinates? Let's take the simple case for the metric tensor in 3D-space without a time dimension, $g_{ij}= \begin{bmatrix} 1 & 0 &...
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1answer
37 views

How can I calculate time difference of things moving at different speeds?

Satellites orbiting earth are faster than it, and hence from our perspective they age slower, or, for us time moves faster. Is there a way to calculate this difference? Even though earth is fast, ...
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3answers
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Rigorous procedure of gluing together two spacetimes

There seems to exist a procedure of "gluing two spacetimes together". In particular I've seem this mentioned in the context of gravitational collapse. The examples I've seem where that of gluing ...
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1answer
79 views

Why is light affected by time dilations in space-time curvatures

My previous question on this site gave me an answer to what affects light - space warping or time warping. The answer is- both. But what now doesn't make sense to me is why light is affected by time ...
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0answers
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What is the shape of the universe? [duplicate]

If it's flat then how a volume can be flat? And I've read that it's actually not flat .....it's a "Poincaré dodecahedral space". Any suggestions for books posts or articles are highly appreciated.
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2answers
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Is light affected by space warping or time warping?

Gravity, according to the General Theory of Relativity, is simply the curvature of space-time. Objects in the universe move through space-time in geodesic paths. Also, the most interesting part is ...
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Can the shape of our Universe be a Mobius strip? [duplicate]

The Friedmann Equations describe three possibilities for the shape of our using General Relativity, I read in a book that the shape of our Universe is a 3-sphere such that in any direction if you ...
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2answers
893 views

Does kinetic energy warp spacetime?

My interpretation of GR leads me to think that energy (namely kinetic) also adds to the curvature of space-time. Which, has raised a thought experiment. If a $10000$ kg ship closely passed a $1$ kg ...
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Where‘s the coordinate origin of the universe? [duplicate]

Given all mass originated from 0/0/0, what point do we have approximately? In km
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2answers
145 views

Newton's First Law and things that are very old [closed]

Recently I've been revisiting physics text books, and books by Feynman, and others. A curious thought has arisen, while I was reading about Mach's principle, and it seems to gnaw on my mind. I hope ...
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2answers
76 views

Is there a temporal difference between planets due to the sun's gravitational field?

since the Sun generates a gravitational field it also generates gravitational time dilatation. Hence, time further from the Sun should pass quicker than in its proximity. Can we, therefore, say that ...
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1answer
122 views

Is universe symmetric about a point?

We have a good amount of discussion and theories on the formation of universe. I want to ask is universe symmetric about a point? I think that the answer should depend upon the uniformity of ...
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1answer
133 views

Why does time slow down ONLY for objects travelling at high speeds?

Today in physics class, we learned about the theory of relativity and how it relates to time dilation. We were given the example of two photon clocks- one stationary relative to the Earth, and the ...
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1answer
82 views

How does acceleration affect time dilation and length contraction?

I just studied SR and I understand how velocity affects time and length; how do they evolve during the process of reaching a certain velocity?
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1answer
243 views

Is space itself expanding or is it just momentum from the Big Bang carrying things apart? [duplicate]

So, we observe that galaxies are moving apart (on average) and the further away from us they are the faster they are moving away from us. If space was expanding that would explain this. However, I ...
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5answers
168 views

Is curved spacetime a real thing or just math

I was curious if the curving of spacetime by mass/energy was actually a real thing or is it just a mathematical construct, a way of visualizing the force of gravity and explaining it and that there is ...
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1answer
58 views

Could electric charge be a property of spacetime? [closed]

I am always looking for geometric interpretations, and as I understand it, the Einstein field equation basically says 3 things: (1) spacetime is a manifold, (2) free particles follow geodesics in the ...
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1answer
101 views

Intuitive explanation for the Lorentz transformation for time

I've recently started learning SR, and while the Lorentz transformation for space is pretty obvious, just the Galilean transformation combined with space contraction, I can't figure out the ...
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2answers
89 views

Is the concept “space” actually needed?

I started making my mind around space and time and recently came to a point where I wondered if the concept of "space" is actually needed to describe physical processes at all and not just some ...
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1answer
36 views

Could the redshift interpreted as space expansion also be a time dilatation (change in tick speeds)?

This might be a very naive first thought but I was not able to find any further information if it can be ruled out by some observations or some theories immediately. Can the increasing mean redshift ...
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5answers
113 views

In general relativity, is there a difference between matter-energy and spacetime?

This is the third time I try to formulate a question in a proper format so as to understand what is going on with gravitational waves at a physical level. I am told that a spacetime oscillation and/...
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0answers
52 views

Why does Alfred Schild's time dilation experiment make sense?

From my understanding, the experiment is carried out between 2 observers under different gravitational acceleration. The first one fires 2 light pulses towards the second observer with a certain time ...
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0answers
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Why does entropy increase along the relativistic time dimension?

I've repeatedly read that time flows / is asymmetric because entropy increases as you move away from the big bang and towards the future. This makes sense. But why is this direction in which entropy ...
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0answers
38 views

From a gravitational wave detector perspective, what is the difference between a gravitational wave and a phonon?

I already asked a question but it seemed that it was not precise enough. My problem is that I cannot see how gravitational waves have anything to do with spacetime. An EM wave or a phonon can put ...
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1answer
80 views

How spacetime distortion can ever be noticed from inside spacetime itself?

this is a naive question from a non-physicist. It is my understanding that gravitational waves are a deformation of spacetime. However, those are noticeable through, for example, laser interferometry. ...
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1answer
62 views

Interpretation of the wave function in newtonian spacetime

A Newtonian spacetime is a quintuple $(M, \mathcal{O}, \mathcal{A}, \nabla, t)$ where $(M, \mathcal{O}, \mathcal{A}, \nabla)$ is a 4 dimensional differentiable manifold with a torsion free connection, ...
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4answers
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Does the universe have a fixed centre of mass?

Does the universe have a fixed centre of mass? If it does, doesn't it necessarily mean that every action of ours has to be balanced by a counteraction somewhere in the universe so as to neutralize the ...
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3answers
76 views

Non-existence of double time-derivative of fields in the Lagrangian and violation of equal footing of space and time

In classical field theory, we consider the Lagrangians with single time-derivative of fields whereas double derivative of the field w.r.t. space is allowed sometimes. I understand that the reason of ...
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1answer
58 views

Light moving toward a moving observer's reference frame

The speed of light is supposed to be the same for all observers in an inertial frame, no matter the observer's speed. As a result, time slows down for observers moving quickly, and this explains why ...
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2answers
217 views

How does the Penrose diagram for a spinning black hole differ in realistic scenarios (formed by stellar collapse)?

The Penrose diagram for a non-spinning Schwarzschild black hole is Notably, there is a second universe "on the other side" of the black hole. However, actual black holes form by stellar collapse, and ...
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1answer
59 views

Can a black hole have a finite perimeter but an infinite radius/diameter?

Recently, I attended a talk at UNC Chapel Hill by Kip Thorne and I recall that he mentioned that black holes can have a finite perimeter but an infinite radius/diameter (since spacetime is curved with ...
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1answer
84 views

What is the physical interpretation of the Wick rotation?

What is the physical interpretation of the Wick rotation? How is it that we can just propose there's a new time coordinate tau? Are physicists saying time is modeled by an imaginary number? Isn't ...
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0answers
47 views

Hausdorff property in Minkowski spacetime

In the 4-dimensional Minkowski spacetime, for a given point $x = (x^0,x^1,x^2,x^3)$, its timelike future/past set is defined as, $$ I^{\pm}(x) = \{y =(y^0,...,y^3) \in \mathbb{R}^4 : \eta_{\mu \nu}(y-...