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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158 views

Can project LISA will bring us closer to time travel? [closed]

After looking at the current discovery of Higgs Boson and the next long term plans of NASA and ESA(European Space Agency), I can't stop myself asking about the possibilities in near future. According ...
4
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2answers
628 views

Can an “absolute” frame of reference be determined by measuring the compression of light?

General relativity tells us that there is no absolute frame of reference (actually, it tells us that all frames are relative, which is close but not the same as there is no absolute frame). Special ...
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1answer
257 views

If the universe is infinite, shouldn't I already have been contacted by a time and space travelling doppelgänger?

If the universe is infinite, by virtue of chance it means that every possible configuration of matter must exist somewhere (according to this documentary). Therefore, if we accept that the universe ...
0
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1answer
79 views

Is the spacetime interval between the Big Bang and a given event dependent only on the time component?

My thinking is this - at the time closest to the Big Bang when physics doesn't break down, the universe would have had a volume that was still very close to zero. If that were so, wouldn't the space ...
6
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2answers
581 views

Does String theory say that spacetime is not fundamental but should be considered an emergent phenomenon?

Does String theory say that spacetime is not fundamental but should be considered an emergent phenomenon? If so, can quantum mechanics describe the universe at high energies where there is no ...
7
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4answers
505 views

Hamiltonian and the space-time structure

I'm reading Arnold's "Mathematical Methods of Classical Mechanics" but I failed to find rigorous development for the allowed forms of Hamiltonian. Space-time structure dictates the form of ...
91
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6answers
35k views

If you view the Earth from far enough away can you observe its past?

From my understanding of light, you are always looking into the past based on how much time it takes the light to reach you from what you are observing. For example when you see a star burn out, if ...
1
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3answers
288 views

Could we enable ourselves to send messages to and receive messages from the future?

Based on John Isaacks' question, "If you view the Earth from far enough away can you observe its past?" and the responses, it appears that we could use mirrors to see into the past. Using Vintage's ...
2
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2answers
296 views

How does gravitation propagate along curved spacetime?

In this wikipedia article it is described how a beam of light, with its locally constant speed, can travel "faster than light". That is to say it travels a distance, which, from a special relativistic ...
3
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7answers
3k views

According to General Relativity, Does The Past “Exist”?

I'm curious about just what is meant by time being another dimension, like the three (observable) spatial dimensions. Does this imply, according to General Relativity, that the past and the future ...
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1answer
361 views

Speed of light is not fixed? [closed]

In my research, I found that the speed of light is not fixed. IS it true? Namely, We know that light refracts when the medium it travels through changes. Actually, light travels in the same medium ...
3
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1answer
110 views

Is GPS time measuring the proper time on the mean sea level or the GPS station itself?

LeapSecond.com states: Global Positioning System time is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves. Does GPS ...
3
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4answers
234 views

Can a huge gravitational force cause visible distortions on an object

In space, would it be possible to have an object generating such a huge gravitational force so it would be possible for an observer (not affected directly by gravitational force and the space time ...
2
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0answers
495 views

de Sitter and anti de Sitter metric

Is the following correct for the distance $d$ from the origin $(0,0)$ to point $(t,x)$ in the 2-dimensional de-Sitter and anti de-Sitter spaces? Here, $t$ is time and the distance may be called the ...
4
votes
1answer
154 views

How should one interpret the de Sitter slicings?

When 'constructing' the usual de Sitter space in $\mathcal{M^5}$ by invoking the contraint $-X^{2}_{0} +X^{2}_{1} +X^{2}_{2} +X^{2}_{3} + X^{2}_{4} = \alpha^2$ we quickly see that we end up with a ...
1
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1answer
443 views

How is traveling back in time possible in theory according to some scientists? [duplicate]

Possible Duplicate: Is it possible to go back in time? Is time travel possible? I get the idea of traveling to the future and it makes perfect sense as we'd be somehow trapped in a ...
0
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1answer
136 views

weak bosons and feynman-stueckelberg interpretation

from Wiki "The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle." Q:If each is the other's antiparticle then which is ...
6
votes
1answer
166 views

If a fundamental theory exibits e.g. a mirror symmetry, in what sense it the underlying geometry real?

Are the more recently discovered symmetries in string theory such that the theories based on mirroring geometries are absolutely the same from an observable point of view? I have mirror symmetry ...
3
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1answer
214 views

Hamiltonians and Lagrangians, Euclidean and Hyperbolic: Are they related?

The Lagrangian of a system is the difference between its kinetic energy $T$ and potential energy $V$, and is relativistically invariant: $L = T - V$ The Hamiltonian of the same system is the sum ...
0
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2answers
151 views

How is spacetime depicted in quantum field theory?

How is spacetime depicted in quantum field theory? Is space and time completely separate, and time is just nature of law as in Newtonian mechanics?
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3answers
254 views

The metric expansion of space and Hawking

In a Wikipedia article I read that the "Metric Expansion of Space" exceeds the speed of light. If this is true then we must be being disconnected from very remote parts of the universe since gravity ...
1
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0answers
34 views

How can we know the size of a dimension? [duplicate]

Possible Duplicate: Measuring extra-dimensions I was watching a layman's documentary and it said: "The 11th dimension is incredibly small, like a long tube about $10^-30m$ in diameter". ...
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1answer
258 views

future light cones and light paths

I understand that an event, in a four dimensional space-time, produces a light cone. As time increases the cones gets larger on either side of the event (past and future). For example the if the sun ...
0
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1answer
328 views

Time dilation at the speed of light

Does a massless particle travelling at the speed on light in a vacuum (c) experience an infinite time dilation effect? i.e. Would the time dilation extend from the perspective of the massless ...
0
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1answer
123 views

Is it possible to mould _ruptures_ in Space-time?

In one of the documentaries hosted by Morgan Freeman, a reference was made that just like an ordinary three-dimensional object like a ruler has scratches and cracks, in the same way there might be ...
3
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2answers
318 views

About space-time and its four dimensions

I explained to someone I know about General Relativity (as much as I know). He said that he didn't see how it could be correct. He argued: How is 4-dimensional space-time space different to ...
2
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1answer
865 views

$\pi$ and the Curvature of Space

If one draws a circle on a sphere and measures the ratio of the diameter to the circumference, that value varies depending on the diameter of the circle compared to the diameter of the sphere it is ...
2
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3answers
309 views

How to distinguish 4D and 3D vectors in handwriting?

Usually vectors are denoted with bold font in printbooks and with arrows above in handwriting. In Thorn's e al. Gravitation, 4D vectors are denoted with bold and 3D vectors with bold italic. How to ...
3
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1answer
554 views

Is the Portal feasible in real life?

Firstly, I assume prior knowledge/experience with the game Portal. Should it be possible that two openings (circles/unidentified-constructions) would work as interchangeable gates? With one opening ...
1
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2answers
275 views

Confusion regarding geodesic of thrown ball - curved or Cartesian coordinates?

I'm confused trying to understand what's happening in terms of spacetime geodesics when a ball is thrown and its trajectory plotted, height against time to give a parabola. I read (from more than ...
1
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1answer
131 views

Classical black holes?

How big should the black hole be so we can consider it to be classical? When they claim that we can not probe shorter distances than the Planck length, can it be true? The argument says that, ...
4
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1answer
293 views

What are the current (popular(ish)) approaches to modelling the quantum nature of spacetime at the Planck scale?

My guess at a list of them would be: spin foams, casual sets, non-commutative geometry, Machian theories, twistor theory or strings and membranes existing in some higher-dimensional geometry... ...
5
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1answer
1k views

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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1answer
572 views

The nature of time, according to quantum field theory

I will try my best to ask the question that best fits something I have been pondering on for a few days. Are virtual particles really constantly popping in and out of existence? Or are they ...
1
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3answers
317 views

Evidence that “space exists rather than just particles”

What is (theoretical) evidence that you need to define all of space with properties rather than just stating where all particles are? I mean, does every single coordinate x, y, z have a meaning or ...
5
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3answers
839 views

Why are there 4 Dimensions and 4 Fundamental Forces?

Is it a coincidence that there are four fundamental forces and four spacetime dimensions ? Does a universe with three spacetime dimension contain four fundamental forces? Can magnetism be realized in ...
3
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2answers
1k views

Proof that Energy Momentum Tensor of Scalar Field Theory satisfies Weak Energy Condition

It's a question on Sean Carroll's Spacetime and Geometry, where we are supposed to prove that the energy momentum tensor of scalar field theory satisfies Weak Energy Condition (WEC). The energy ...
0
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1answer
199 views

Wormhole related experiments? [closed]

Are there any simple "experiments" that can be done in a high school science lab that could demonstrate some sort of basic principals of wormholes or spacetime? Or sort of proving how long something ...
2
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2answers
338 views

What does a closed time-like curve look like?

I want to see a plot of closed time-like curve in $(t,x)$. $t$ - vertical axis $x$ horizontal axis (the usual setting just neglect $y$ and $z$ components of $(t,x,y,z)$). What does it look like?
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2answers
262 views

How to concile flat spacetime and big bang?

After reading How do we resolve a flat spacetime and the cosmological principle? I still remain perplex. Please excuse my ignorance and try explaining to me : I thought that basically, when we ...
0
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2answers
572 views

Can the expansion space time reverse itself and contract the same way?

If there's a mechanism for space-time expanding faster than the speed of light, is there an example of It contracting in the same manner? If whatever mechanism is causing it to expand, can the ...
5
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5answers
1k views

ALL “forces” as manifestations of properties of space-time

I apologize if this seems like a quack question, but I need some insights by those who know much more than me in Physics. Anyway, the gravitational "force" (not really a force) is a manifestation of ...
3
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3answers
550 views

Defining a Riemannian manifold - made easy?

In the context of GTR spacetime, I'm trying to get the basic idea of a Riemannian manifold clear in my mind. Apologies for the longwindedness. Question 1. Is this a reasonable, simplified summary of ...
2
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3answers
244 views

Is there a single metric for a given system?

Let imagine a tunnel that connect two distant places at the globe (eastern-western or north-south) There are a lot of posible "distances" or metrics, defined by maps, routes, "as the crow flies", ...
2
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4answers
499 views

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 ...
0
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2answers
246 views

Is this alternate theory of gravity as cause instead of effect plausible?

I came across this video today on YouTube that presents an interesting alternate theory of Gravity and the "missing" matter in the Universe that Dark Matter/Energy theories try to account for. If I ...
12
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3answers
716 views

Swimming in Spacetime - apparent conserved quantity violation

My question is about the article Swimming in Spacetime. My gut reaction on first reading it was "this violates conservation of momentum, doesn't it?". I now realize, however, that this doesn't allow ...
1
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0answers
247 views

The structure of space-time

I came across this paper recently called The Small Scale Structure of Spacetime and the following idea occured to me: To uninformed humans the universe appears Euclidean but we know from GR that on a ...
3
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1answer
588 views

Universe Expansion as an absolute time reference

Why we call "constant" to the Hubble constant?, if the universe were really expanding then the Hubble "constant" should change, being variable, smaller and smaller..with "time". Other example/view ...
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2answers
86 views

Resources for current thought on time/spacetime?

Are any of the big-name physicists associated with the time in the same way that Hawking and Penrose are associated with black holes? I'm interested in some good books that focus on the topic.