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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A question about a relation between time-like world lines

Given any Lorentzian manifold containing three distinct time-like world lines $L$, $A$ and $B$ such that $L$ and $A$ have exactly one common coincidence event, $\mathcal{E}_{AL}$, $L$ and $B$ have ...
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3answers
236 views

Is the edge of our Hubble Sphere within our Cosmic Event Horizon and why?

I was recently shown a pretty cool video about common cosmological misconceptions. It got me reviewing the different between event horizon (current distance within which we will see/interact), ...
23
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5answers
3k views

Einstein's postulates $\leftrightarrow$ Minkowski space for a Layman

What's the cleanest/quickest way to go between Einstein's postulates [1] of Relativity: Physical laws are the same in all inertial reference frames. Constant speed of light: "... light is always ...
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0answers
76 views

Does quantum entanglement imply the existence of a non-causal structure connecting space-time together?

In contrast to a "time-like" or "causal" structure connecting space-time together, Does quantum entanglement imply the existence of a "space-like" or "non-causal" structure holding space-time together ...
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3answers
116 views

If we say the universe is expanding, shouldn't it be expanding relative to something?

I don't understand, if everything in this world is relative to something else, then cannot we essentially say that nothing exists independently? We say that the universe is considered to be the ...
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1answer
36 views

Spacetime and the conservation laws

I'm reading Peter Atkins' book, Galileo's Finger, and in the chapter on energy, he makes the points that the conservation of momentum stems from the shape of space (that it's smooth and not lumpy) and ...
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5answers
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Is proper time a vector?

Proper time is identical with the spacetime interval of a timelike movement. A spacetime interval is the dot product of two vectors and thus a scalar. Proper time however is always pointing exactly ...
4
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1answer
130 views

Non-stationary spacetime

What is an example for a spacetime that is non-stationary that is considered as a description of something in nature? So far all the spacetimes I encounted have always been stationary ...
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2answers
115 views

How is time distributed through space?

I was greatly confused when I saw video saying that if two wormholes were separated by time instead of distance, it would allow time travel. My question is what did they mean by this statement? What ...
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3answers
178 views

What exactly is charge? [duplicate]

If gravity is really the bending of space/time causing objects with mass to experience acceleration, is there a similar physical meaning to 'charge' besides 'a property of matter which causes it to ...
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3answers
2k views

Why is spacetime curved by mass but not charge?

It is written everywhere that gravity is curvature of spacetime caused by the mass of the objects or something to the same effect. This raises a question with me: why isn't spacetime curved due to ...
2
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1answer
92 views

Size of the Universe: Curved vs flat? Finite vs infinite?

I have recently heard the theory that the Universe may be smaller than observed but may be curved to the extent that light rays may have looped past us once already and hence appear to have originated ...
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1answer
63 views

Origin of Space-time fabric

I have seen on this site some questions regarding the nature of gravitational-force, and the ways in which it could be interpreted. Most of them mention that the space-time fabric came into existence ...
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3answers
74 views

Space distance between observers and Lorentz transformation

I'm not a specialist of Lorentz transformation. I would like to know if the Lorentz transformation from one observer to another preserves the space distance between these two observers. Example ...
2
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1answer
61 views

Does negative energy density (i.e. weak energy condition violation) create closed timelike curves?

I remember reading something about Stephen Hawking denying the fact you can't make CTC's (Closed Timelike Curves) without weak energy condition violation. If this is true, where do the light cones ...
3
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2answers
274 views

Is there an orthogonal time dimension in Minkowski spacetime?

Space dimensions are orthogonal one to each other. But what about time in the Minkowski diagram? At first sight, time seems to be orthogonal to space. But we have to consider that each Minkowski ...
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1answer
66 views

Is there no fourth dimension (time) in the universe? [closed]

In this article (and numerous others) http://phys.org/news/2011-04-scientists-spacetime-dimension.html and http://www.rebelscience.org/Crackpots/notorious.htm they suggest that there is no time ...
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1answer
63 views

Observation of light bending spacetime [duplicate]

Has radiation or energy bending spacetime ever been observed? If not, is it likely that it ever will, assuming current technology? Note: This is not a question of space bending light, but of light ...
3
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1answer
90 views

Is the concept of space-time curvature a recursive one? [duplicate]

A way some people explain (or try to explain) how gravity works is using space-time curvature: an object with high mass distorts the surrounding space-time plane like a bowling ball distorts a sheet ...
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6answers
521 views

More than one time dimension

We know that space-time dimensions are 3+1 macroscopically, but what if 2+2? Obviously it is tough to imagine two time dimensions, but mathematically we can always imagine as either having two ...
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3answers
92 views

Energy Required to Rip Spacetime

I have heard that the presence of an extremely strong gravitational field possesses the capacity to warp or tear spacetime and to potentially create a wormhole. Is any energy lost when spacetime is ...
2
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0answers
40 views

Hierachies of AdS/CFT holographies

One of the most disturbing aspects of General Relativity is the 'Marble versus Wood' duality of the theory: Matter creates curvature, and curvature doesn't create curvature (at least not directly) ...
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2answers
145 views

Does gravity actually contract space-time?

In the Big Crunch Theory it says that gravity (curvature in space-time) will stop the universe's expansion and gravity will cause the universe to contract on itself. My question is if gravity is a ...
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3answers
715 views

Has the concept of non-integer $(n+m)$-dimensional spacetime ever been investigated by theoretical physicists?

The following image serves to aid the reader in understanding the "privileged character" of $3+1$-spacetime. The wikipedia article on spacetime, and the sub-article "The priveleged character of ...
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0answers
72 views

Does quantum gravity entail that spacetime is emergent?

It is being said by some people, that quantum gravity entails that spacetime is an emergent phenomenon, and thus that the immaterial gives rise to the material. Is this what quantum graivty entails, ...
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1answer
58 views

Does the earth gets closer to the sun?

We know that the sun loses an amount of it's mass equivalent to the amount of energy it produces, according to the E=mc² equation. so the sun is losing mass every second. Does this affect the ...
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1answer
139 views

Does the Alcubierre drive use negative energy to compress space infront of it?

The contraction in most images looks like a gravity well. Is it that you use negative energy to BOTH contract and expand space or positive and negative to contract (positive energy) and then expand ...
3
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1answer
116 views

Quantum Field Theory with (2,2) metric

Does someone know some reference which treats QFT in a space with the following non-Lorentzian signature: $g_{\mu\nu}=\text{diag}(-1,-1,1,1)$. I'm interested in basic stuff like the shape of the ...
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2answers
97 views

What is an $n$ dimensional space? [closed]

How an $n$ dimensional space looks like? Is it possible that we are really in a space of dimension greater than 3?
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3answers
844 views

Time travel outside of light cone without causality violation

If one is able to travel into the past but at a spatial distance that puts him outside of his own past light cone would this be considered a causality violating trip? Looking at a Minkoski diagram, it ...
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1answer
71 views

Why could you not travel back in the time before a tipler cylinder is created?

Is it because the lightcones are not tipped all the way over in which the time axis points downwards?
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3answers
246 views

Why don't we define absolute coordinates?

Why don't we chose a random point in the void present between galaxy clusters and define it as the absolute origin? I know it is not at absolute rest and space expands, but we can easily keep that ...
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2answers
65 views

Are black holes trapdoors to the center of the universe? [closed]

Correct me if i'm wrong here, but if you consider the analogy of inflating balloon when explaining the universe expansion, then the center of the universe lies within the center of the inflating ...
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1answer
86 views

Superluminal travel without time travel

Can superluminal travel or communication be possible without leading to the possibility of closed timelike curves (CTCs) and causality violations? I have seen conflicting opinions regarding this ...
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1answer
71 views

Knots and singularities

Can space-time singularities be treated as mathematical knots occurring in dimensions greater than four? I just drew an analogy with knots in one-dimensional strings. When a rubber-band is looped over ...
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1answer
129 views

If gravity bends space time, could gravity be manipulated to freeze time? [closed]

You age at a different rate depending on the force of gravity. Astronauts age fractions of fractions of fractions of a second less than earthlings. If you took a sphere of equal masses, separated by ...
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2answers
77 views

Special Relativity Problem [closed]

I am having trouble with the following problem: Fry travels in a rocket ship towards Leela, at constant relative speed $v$: Fry is delivering a pizza, which in its rest frame stays hot for ...
11
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4answers
4k views

Is spacetime discrete or continuous?

Is the spacetime continuous or discrete? Or better, is the 4-dimensional spacetime of general-relativity discrete or continuous? What if we consider additional dimensions like string theory ...
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1answer
30 views

Voyager Golden Record

Has there been any subsequent researches or any respond from deep outer space in any type of form with these images and sounds?
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0answers
71 views

Big Bang, Heat Death, and cause and effect

If the Universe has two 'end points', one being the Big Bang, and the other being heat death, is there anything in the laws of physics which forbid a random fluctuation in the heat death state from ...
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2answers
73 views

Understanding emergent phenomena in the block universe. (Reworded question)

Each person exists as an unchanging 4D worldtube in the block universe. At each slice of the worldtube there is a present, past and future. However, there is a black box* which appears to exist in ...
9
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5answers
2k views

Did spacetime start with the Big bang?

Did spacetime start with the Big Bang? I mean, was there any presence of this spacetime we are experiencing now before big bang? And could there be a presence/existence of any other space-time before ...
0
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1answer
57 views

does space-time expansion affect on fundamental particle? [duplicate]

does space-time expansion affect on fundamental particle or point particle?
5
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1answer
211 views

Why is $S^1\times\mathbb{R}^{n-1}$ the topology of $AdS_n$?

Anti-de Sitter $AdS_n$ may be defined by the quadric $$-(x^0)^2-(x^1)^2+\vec{x}^2=-\alpha^2\tag{1}$$ embedded in ${\mathbb{R}^{2,n-1}}$, where I write ${\vec{x}^2}$ as the squared norm ${|\vec{x}|^2}$ ...
0
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1answer
191 views

Does the actual curvature of spacetime hold energy?

My understanding of GR is that curvature of spacetime reflects the density of energy-matter. Does the curvature itself have energy? Or if energy is assigned to curvature it simply reflects the energy ...
5
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2answers
382 views

Need explanation for a sketch of various spacetime dimensions

I came across this picture/sketch on the internet, however there is no any explanation about it: What is "UNPREDICTIBLE"? "UNSTABLE"? "TOO SIMPLE"? "elliptic", "ultrahyperbolic"??
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0answers
61 views

If tachyons exist, what would they do to spacetime?

Mass (Positive) bends space-time generating gravity. If Tachyons are supposed to have imaginary mass, what would they do to spacetime (if they do exist).
3
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0answers
61 views

What Would Negative Mass Do To Spacetime?

It's known that positive mass bends space-time to create a curvature. But if something had negative mass what would it do? Make it flat or like a crest?
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9answers
2k views

If an event happens 10 light years away and we observe it here, did it actually happen 10 years ago?

I'm sorry if this question has been answered a squillion times, I need someone with smarts to explain it to me. If an event happens 10 light years away, and we observe it here, did it actually happen ...
2
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2answers
119 views

Deriving formula for time dilation

Last week in class we derived the formula for time dilation using light clocks and got $$t=\gamma t_0\quad \gamma=\left(1+\left(\frac vc\right)^2\right)^{-1/2}$$ So far so good. However, after class I ...