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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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7answers
83 views

Are time and motion the same thing?

My question might be completely irrelevant or stupid - sorry in advance if it's the case. I'm kinda lost with the time and space notions, and even formulating adequate questions is hard at that point. ...
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Space time and its existence [on hold]

I was just thinking if we could analyze the ellipses in which all of the planets move around the sun and then see if they resemble a cone with the sun at the apex(because that what it looks in the ...
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1answer
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If space-time can be warped or curved, Doesn't that mean that It need space to be curved? [on hold]

There is a doubt that i've always have and no one has given to me a satisfing or good response, i hope someone give me the response. Lets Talk about space only. If space can be curved or warped, that ...
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4answers
63 views

Can motion be considered a dimension?

Okay, go easy on me, I’m in no way any kind of physicist, or even a scientist, I graduated High School and that’s it, but I do have a very mechanically based mind, (probably why I wound up a Diesel ...
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1answer
31 views

Does Electrostatic potential energy bend Space-time? [duplicate]

Okay, there are various questions. First, "matter and energy bends space-time" does this mean any form of energy can bend space-time? Does theory of relativity assume that there is no other form of ...
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2answers
168 views

Equivalence of two definitions of proper time in special relativity

According to Wikipedia, proper time along a timelike world line is defined as the time as measured by a clock following that line. This makes sense to me, but my book defines proper time in the ...
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2answers
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If gravity can be thought of as masses leaving dents on a spacetime 'sheet', what is holding up that sheet? [duplicate]

If the force of gravity can be thought of as masses leaving dents on a sheet of spacetime, what is holding up that sheet?
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Does the homogeneity and isotropy of space imply that the expansion of the universe is uniform?

I have asked this question. Now I wonder what could happen if I take a step further. If space is assumed to be BOTH homogeneous AND isotropic, can I prove that the expansion of the universe is uniform?...
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0answers
66 views

Does gravity need to be explained by quantum mechanics if spacetime is discrete? [on hold]

I have read that Einstein‘s theory of general relativity, explaining gravity as a warping of space time, is inconsistent with quantum mechanical theories attempting to explain gravity. My question is ...
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1answer
84 views

Will the track length increase to keep the light velocity constant?

Assuming light travels at 3 km/sec(for calculation purposes), what happens when a 1 km long train moving at 2 km/sec along a straight track switches on its headlight at 30 km distance from a wall.So ...
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0answers
37 views

Is the relativistic time dilation compatible with the gravitational field of a moving massive planet?

Assume that a satellite orbits a massive planet/star at a distance $r^\prime$ away from its center in a circular path in plane $x^\prime y^\prime$. Suppose that the gravitational field is $g^\prime$ ...
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4answers
97 views

Is the slowing of time with speed exactly proportional to the increase of mass with speed?

This article on time dilation explains how time slows and mass increases with speed. Are they proportional? And is time dilation with a specific speed-mass equal to the time dilation with a ...
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2answers
51 views

Does light comeback again?

I know light follow the space time curvature.I listen about gravity lensing.Is there any proof of total reflection of light due to gravity like total internal reflection of light? If this possible ...
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0answers
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Magnetic field “lines” in $D \ne 4$ spacetimes

The electromagnetic field is represented by an antisymetric tensor $F_{ab} = -\, F_{ba}$ (the faraday). In $D = 4$ spacetimes, it has 6 independent componenents: 3 describing the electric field: $F_{...
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0answers
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What is the best comparison to understand how space is formed without resorting to the design of the temporary space blanket? [closed]

For Einstein, space is like a blanket that curves according to the mass of the bodies, so objects should move up, sideways, and down very little, according to the mass of the bodies. This ...
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1answer
55 views

Positive local spatial curvature of the universe implies that the universe is compact (i.e. finite)?

I quote from the Wikipedia page about the shape of the universe: If the spatial geometry [of the universe] is spherical, i.e., possess positive curvature, the topology is compact. I'm trying to ...
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1answer
52 views

Does the homogeneity of space imply that the expansion of the universe is uniform?

Obviously, homogeneity implies that the density is the same everywhere at any time. However, does this imply that the expansion is uniform? By uniformity, I mean that if I pick three galaxies to form ...
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0answers
62 views

Confusion about gamma matrices in Euclidean spacetime

I have encountered a number of sources with differing definitions of the transition from Minkowski spacetime to Euclidean spacetime. I'd like some clarification as to how to go from Minkowski to ...
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1answer
41 views

What is the Komar mass of the de-Sitter spacetime?

The Komar mass of some spacetime is defined as an integral (volume or surface, depending on its formulation): https://en.wikipedia.org/wiki/Komar_mass The de-Sitter metric in static coordinates is ($\...
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1answer
63 views

What happened to a photon in the early universe?

So in the early universe where the universe was really small what would happen to a photon is it didn't collide with any material and made it to the edge of the universe? Because yes the big bang ...
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0answers
25 views

How should be 'time' understood? [duplicate]

Is time an invention or discovery? How to relate time with the gravity via general relativity theory?
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2answers
52 views

How to interpret negative time in Lorentz transformation?

I am somewhat confused about how to interpret negative time in Lorentz transformation. In the usual case of two reference systems S and S' where the distance X (the one that measures S) to an event, ...
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0answers
52 views

Is really there exists any kind constant that can explain expanding universe and increasing time?

I know far galaxies moving out from us I.e universe expanding I.e increasing space.There also time always growing up.From four dimensional analysis space time has covariance. I want to know is there ...
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1answer
35 views

Did the big bang start at ONE infinitely curved point (or a 4D curved ball shrunk to a point) with an infinite density of energy?

I'm confused. In this question it is said: The simple answer is that no, the Big Bang did not happen at a point. Instead, it happened everywhere in the universe at the same time. How could it have ...
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1answer
57 views

How to find a normal to an hypersurface

I have to apply the Israel junction conditions in a region in which a hypersurface with O(3) symmetry separates two spacetime with Schwarzschild metric (with masses $M_+$, the exterior one, and $M_-$, ...
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0answers
22 views

Problem of now. How physics deals with it? [duplicate]

i know this is kind of off topic. from my understanding in relativity spacetime is frozen, nothing changes, there's no "time" as we understand it in everyday life. is there anything in physics that ...
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0answers
41 views

Coordinate-free representation of invariant interval [closed]

Suppose event B is at the origin of space-time diagram. Let A and C be arbitrary events (A is in the backward light cone and C is on the forward light cone with B at the origin). The world line ...
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0answers
51 views

What would the eternal black hole look like?

The white hole and black hole regions in a Kruskal diagram are said to be actually two different locations. Given the problems with white holes it might be a silly question but, hypothetically, what ...
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4answers
108 views

Is mass persistent in spacetime?

As an object moves through spacetime is its mass persistent? What I mean is, if we find a way to look backwards into time, will a pen sitting on a desk when I was in college still be sitting on that ...
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1answer
47 views

Are all lengths multiples of the Planck length? [duplicate]

If the Planck length is the smallest unit of length that has any physical meaning then is it true to think that all the lengths are multiple of the Planck length?
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1answer
51 views

About space/timelike intervals and event horizon of observed universe

How does interval between us (e.g earth) and the spacetime outside of observable universe (above the event horizon) can be described in terms of timelike and spacelike intervals? Is it just spacelike ...
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1answer
60 views

Is space-time fabric around Uranus tilted?

I am completely new in this field, so please pardon me if I am asking something which makes no sense at all :) The Situation: According to Einstein's theory of relativity, there is a 4th dimension ...
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1answer
56 views

How does the fabric of space-time bends? [duplicate]

So I get the analogy of the fabric of space-time bending as a sheet would bend with the weight of a ball on it, but that explains the "outside" how would the fabric warp inside the earth? Like the "...
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2answers
93 views

Why is the 1-dimensional wave equation called like that when it seems to be 2-dimensional?

The wave equation in one dimension traveling along a string is: $$ \frac{∂^2y}{∂x^2} = \frac1{v^2} \frac{∂^2y}{∂t^2} $$ but this equation has 3 variables $x, y,$ and $t$, shouldn't it be in 2 ...
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2answers
118 views

Does the relative speed of time mean there is less energy where time is slower?

Time runs relatively slower near a planet than in outer space. Does this mean that there is less energy near the planet? Is there a relationship between energy and the speed of time? If so, this ...
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1answer
61 views

What does bent in space-time means exactly? How does mass of an object affect space and time? [duplicate]

I don't understand how does of mass an object for example say earth causes distortion in space and time. I am just new to this field so it is difficult imagine this phenomenon.
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10answers
8k views

Is the “spacetime” the same thing as the mathematical 4th dimension?

Is the "spacetime" the same thing as the mathematical 4th dimension? We often say that time is the fourth dimension, but I am wondering if it's means that time is like the fourth geometrical axis, or ...
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1answer
115 views

Why are worldsheets of strings _holomorphic_?

Disclaimer. I am a mathematician (algebraic geometer) who knows nothing about physics. Even worse, I might have major misconceptions about the objects I'll ask about. The level of the question is pop-...
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1answer
60 views

If two places in the infinite universe have the same state and history, is there a way determine they are different locations? [closed]

I've heard it said that in an infinite universe if you could go far enough in one direction you would end up in a location identical in every aspect to the one you left. Same Earth, same culture, same ...
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1answer
127 views

Simultaneity in General Relativity

Take the following situation: An astronomer is on the surface of Sun (assume he's not rotating around the Sun). He measures two stars from two locations in the universe exploding. Both stars ...
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1answer
23 views

What is the definition of the “characteristic radius”?

Upon solving exercises regarding relativity, I have run into the problem below. The inverse square radius of curvature of spacetime is of orer the tidal field, $R^{-2} \approx \nabla^2 \phi$ where $\...
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1answer
49 views

The description of a universe with one or two objects

I understand that in general relativity, we describe the universe by the relationship between objects "in it". I quote "in it" because in this model, there is no background where the objects exist. ...
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1answer
69 views

Geodesic equation in Einstein-Cartan manifold

In GR we use the Riemannian manifold without any torsion to describe the theory. Hence, the geodesic equation can be interpreted as "a trajectory of a free falling particle" or "equation of motion". ...
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1answer
80 views

Terminology used in the Wikipedia article for wormholes

The lead section of the Wikipedia article for wormholes says the following: More precisely it [a wormhole] is a transcendental bijection of the spacetime continuum, an asymptotic projection of ...
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2answers
91 views

Is there a significant difference between time at sea level vs top of mount Everest?

I heard about time dilation is somewhat proportional to gravity, and that gravity force may vary by latitude and elevation. So I am wondering, is there a significant time "speed" variation between if ...
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2answers
60 views

Connected and disconnected dimensions

The usual way of determining the dimensionality of space is from the number of values needed to define a unique point. However, when choosing a ski, my body is defined by two numbers - my mass and ...
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3answers
1k views

When exactly is a dimension spatial?

I every so often hear claims like: M-Theory predicts that there are 10 spatial dimensions! Now I'm not really sure what these claims mean. There are three spatial dimensions that I normally ...
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1answer
160 views

How do we know the laws of physics remain the same in different dimensions?

Section on Wikipedia dealing with the possibility of different dimensions. When reading this section it feels like there's a giant elephant in the room that is not addressed. For example, here's a ...
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2answers
82 views

Relativity geometric analysis without spacetime

I'm an engineer and I'm used to classical mechanics "spatial-vectors" approach, which allows a geometric analysis of rigid bodies motion. In this context, as you know, it is very useful to imagine (...
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1answer
106 views

Equivalence principle doubt

There is something about Einstein Equivalence Principle that I don't quite get. This is my reasoning: Equivalence principle $\rightarrow$ locally, acceleration is equivalent to a gravitational field ...