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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Measuring the speed of light and defining the metre - absolute or relative?

If the metre is now defined as the distance light travels in vacuum in 1⁄299,792,458th of a second and the speed of light is accepted to be ...
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Why would spacetime curvature cause gravity?

It is fine to say that for an object flying past a massive object, the spacetime is curved by the massive object, and so the object flying past follows the curved path of the geodesic, so it "appears" ...
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Is String Theory formulated in flat or curved spacetime?

String Theory is formulated in 10 or 11 (or 26?) dimensions where it is assumed that all of the space dimensions except for 3 (large) space dimensions and 1 time dimension are a compact manifold with ...
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4answers
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Is space “real”, or is it some sort of accepted postulate?

What is space? It seems to be everywhere in the equations of physics, as some sort of postulate or hidden hypothesis. We also have a direct experience of it, but is it "real"? The fact that we ...
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Minkowski spacetime: Is there a signature (+,+,+,+)?

In history there was an attempt to reach (+, +, +, +) by replacing "ct" with "ict", still employed today in form of the "Wick rotation". Wick rotation supposes that time is imaginary. I wonder if ...
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What is the extent of the universe?

Is there any realistic, understandable, provable (even in some extent) explanation/model for the extent of the universe? What is its shape? and Why? I mean physical explanations not philosophical ...
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230 views

Visualizing gravity in 3D

We've all seen the depiction of gravity bending space downwards, and so attracting objects into the dent it creates, cf. e.g. this and this Phys.SE posts. That's intuitive and makes a lot of sense, ...
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6answers
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Does vacuum (empty space) exist? [closed]

Added: 5 times down vote for now! Down voter is this religion or physics, please try to explain your decision. I'm confused about this. In physics we know for a vacuum, but I think that there is a ...
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205 views

Are the implications of an infinite universe necessarily so unsettling

I have often heard it said (by professional cosmologists) that if the universe is infinite, then there necessarily exist infinitely many copies of me repeated throughout. The reasoning seems to be ...
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852 views

What are some approaches to discrete space-time used in modern physics?

This thought gave rise to some new questions in my mind. What are the consequences for: How would it affect duality i.e. particle, wave property of photons? How does this statement affect the ...
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320 views

What is meant when it is said that the universe is homogeneous and isotropic?

It is sometimes said that the universe is homogeneous and isotropic. What is meant by each of these descriptions? Are they mutually exclusive, or does one require the other? And what implications rise ...
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5answers
670 views

Is it possible for one black hole to pull an object out of another black hole?

Suppose we have a spacecraft just inside the event horizon of a black hole, struggling to escape, but slowly receding into it. Another (bigger) black hole expands until its event horizon includes the ...
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Is anyone studying how the general topology of spacetime arises from more fundamental notions?

Stephen Wolfram in his book A New Kind of Science touches on a model of space itself based on automata theory. That it, he makes some suggestions about modelling not only the behaviour of matter ...
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1answer
311 views

Can an object be infinitely small?

I read somewhere that the earth has to be smaller than 1 cm to become a black hole, according to Schwarzschild. Since big bang came from a singularity, I am wondering, is there any minimum volume for ...
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4answers
89 views

Is light amplitude spacial?

In diagrams I often see light waves depicted as little sine waves that travel through space. And often when describing polarizers, the explainer will angle their hand to show the angle of ...
2
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2answers
189 views

How can space expand?

How can space expand when it is only a perception of the separation between at least 2 objects. Isn't saying "space expands" implying it has properties?
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3answers
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Why does string theory require 9 dimensions of space and one dimension of time?

String theorists say that there are many more dimensions out there, but they are too small to be detected. However, I do not understand why there are ten dimensions and not just any other number? ...
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Which causal structures are absent from any “nice” patch of Minkowski space?

Which "causal separation structures" (or "interval structures") can not be found among the events in "any nice patch ($P$) of Minkowski space"?, where "causal separation structure" ($s$) should be ...
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2answers
232 views

Why time is considered a dimension?

Why time is considered to be a dimension? And the other 7 (except the 3 dimensions of space, and the dimension of time) dimensions that string theory suggests, why can't they be realized?
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1answer
130 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
493 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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7answers
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What grounds the difference between space and time?

We experience space and time very differently. From the point of view of physics, what fundamentally grounds this difference? Dimensionality (the fact that there are three spatial dimensions but only ...
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3answers
895 views

A Sphere of Black Holes

Imagine a sphere of black holes surrounding a piece of space. Will this piece be separated from the rest of normal spacetime (at least for some time, till these black holes finally attracted ...
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1answer
251 views

Plausible explanations for 3 local space dimensions

Every now and then I see accounts of models that claim to explain why we experience only 3 space dimensions (locally, i.e. within cosmic horizons and outside black hole horizons). One such of course ...
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2answers
157 views

Visualizing GR spacetime distortion in 1+1D spacetime instead of 2D space

Usually spacetime distortion due to mass in GR is visualized as a rubber sheet distorted under some massive object, like this: source or as shown in this link. But what has always bothered me is ...
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Can spacetime exist in the absence of matter and energy?

I'm pretty sure Ernst Mach would have said that spacetime cannot exist without matter in it. But I'm also pretty sure that a black hole can be described as a self-sustaining gravitational field, ...
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Reducing General Relativity to Special Relativity in limiting case

I understand that general relativity is applicable to gravitational fields and special relativity is applicable to case when there is no gravity. But is there a derivation on how to reduce General ...
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7answers
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Does Quantum Mechanics assume space and time are continuous?

I was confused when I was listening to a Quantum Mechanics lecture online. Are space and time assumed to be continuous or discrete in Quantum Mechanics? I can see the question is vague, but this is ...
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5answers
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How to measure the curvature of the space-time?

I know G.R. change our vision of space and time as a unique surface than can bend. We can associate the curvature of the space-time as the gravity created by the mass of planets, stars... But how can ...
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2answers
4k views

Do we live in a world with 4 or more dimension?

A NOVA show have told the audience that we are live in 3 dimensional world, the world we lived in is compose by 3 element: the energy, matter, space. By the time Einstein have invented the ...
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1answer
971 views

Why does the universe exhibit three large-scale spatial dimensions? [duplicate]

Possible Duplicate: Is 3+1 spacetime as privileged as is claimed? Regardless of your favorite theory of how many dimensions the universe has in total, the universe seems to have a deep ...
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3answers
259 views

In a very small static universe with only a particle, does it make sense to talk about time?

I am sorry if this question is silly; it′s just one of those things I wished I asked before leaving university. If there were a static universe only as big as the size of two particles, say ...
6
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2answers
317 views

What really is Spacetime?

What is a good definition on Space, Time and the most specific topic "Spacetime"? Because someone told me that spacetime is the foundation of the entire universe? And also, Is it possible to create ...
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5answers
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How does the curvature of spacetime induce gravitational attraction?

I don't know how to ask this more clearly than in the title.
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3answers
125 views

Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?

IIs it valid to apply Einstein's Relativity to scenarios involving expansion of space? For a practical example of this: Is it legitimate to speak of distant red-shift galaxies as experiencing time ...
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2answers
86 views

Curvature gravity and a falling apple? [duplicate]

I know very little of physics after Einstein. I am aware of that Einstein's gravity theory says that the existence of matters creates curvature of a space-time, so that our Earth orbits our Sun. I ...
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2answers
193 views

How much faster would a Clock without gravity run?

Pardon the misleading title. It is to my understanding that moving/heavy clocks run slow. The Earth itself is under gravitational influence from many sources, and is moving. Is there a way to know ...
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6answers
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Why are objects at rest in motion through spacetime at the speed of light?

I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ...
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4answers
924 views

Time dilation at zero velocity (and zero gravity)

From what I've learned, the more an object travels closer and closer to the speed of light, the more time will slow down for that object.. at least from an outside perspective.. It was shown that ...
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0answers
255 views

Twin Paradox Without Acceleration

So I've been doing a lot of reading about the twin paradox and have encountered several different explanations that strive to resolve it. First off let me start by saying general relativity is not an ...
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1answer
344 views

Multiverse theory and infinite individuals

I read an article about possibility of existence of multiverse and came up with a conflicting view with one of the sentences written in the article which goes as follows: "If space-time goes on ...
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3answers
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Where in, Einstein’s “On the Electrodynamics of Moving Bodies” (Relativity), is “Time” reasonably shown to exist, or why is it a valid assumption? [closed]

Given the widespread acceptance that “Einstein’s Relativity” reasonably proves the existence of Space-‘Time’, and thus “Time”... Can any member of the Physics StackExchange please show precisely ...
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381 views

How can I vizualize and understand curved spaces in general relativity?

I'm taking a basic physics class and the teacher described space with a special table that has curves and black holes etc. He would throw a metal ball down onto it and the class would watch it circle ...
5
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1answer
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Is the universe finite and discrete?

Is the universe finite, both in the sense of being a closed spacetime manifold, as viewed from the macro level, but also in the sense of being fully discrete and finite in all of its intricate quantum ...
4
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3answers
260 views

How does relativity explain gravity, without assuming gravity [duplicate]

I have seen the "objects pull down on space-time" explanations, but they assume a "pull down" force themselves. Could anyone explain the space-time explanation without assuming gravity in the first ...
4
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1answer
612 views

How much time has passed for Voyager I since it left the Earth, 34 years ago?

34 years have passed since Voyager I took off and it's just crossing the solar system, being approximately at 16.4 light-hours away. How much time have passed for itself, though?
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3answers
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Example of space-like intervals in spacetime

From wikipedia: When a space-like interval separates two events, not enough time passes between their occurrences for there to exist a causal relationship crossing the spatial distance ...
3
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2answers
178 views

Can a curvature in time (and not space) cause acceleration?

I realize that the curvature of space-time causes acceleration (gravity). Is it possible to have a curvature only of space, or a curvature only of time? If so, would a curvature only of space, or a ...
3
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1answer
88 views

Is force the unique possible relation between particles? [closed]

Is force the unique possible relation between particles? It was requested to further specify my question. The followings paragraphs contain some of my thoughts and confusions. Historical ...
3
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1answer
426 views

Does time dilation correct for the Doppler effect?

Knowing that a body in motion experiences time dilation, "also" knowing when two objects travel at a great speed away from one an other, both observers experience the others clock as moving slower ...