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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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 ...
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727 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 ...
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What does it mean for objects to follow the curvature of space?

In science documentaries that touch on general relativity, it is often said that gravitational pull isn't an actual a pull (as described by classical physics), but rather one body travelling in a ...
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3answers
800 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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452 views

Some questions regarding $n+m$-dimensional spacetime [duplicate]

Possible Duplicate: 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 ...
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2answers
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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 ...
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3answers
447 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 ...
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4answers
665 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 ...
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2answers
262 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 ...
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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 ...
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256 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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0answers
312 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 ...
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Alternate layman's metaphors for illustrating curved space-time

The metaphor of a surface (typically a pool table or a trampoline) distorted by a massive object is commonly used as a metaphor for illustrating gravitationally induced space-time curvature. But as ...
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2answers
89 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.
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0answers
118 views

Does Spacetime have a “This Side Up” arrow? [duplicate]

Possible Duplicate: Does the curvature of spacetime theory assume gravity? Forgive my naivete as I am not schooled in Physics or Mathematics. I was watching NOVA's "The Fabric of the ...
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4answers
811 views

Does (it make sense to say that ) the universe has a center?

I was reading this page: http://www.guardian.co.uk/science/2011/oct/23/brian-cox-jeff-forshaw-answers and I found this sentence by Brian Cox: That seems to imply that everything is flying away ...
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3answers
676 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 ...
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3answers
597 views

Does the Opera result hint to a discrete spacetime?

Could the Opera result be interpreted as some kind of hint to a discrete spacetime that is only seen for high enough energy neutrinos? I think I've read (some time ago) something like this in a ...
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2answers
947 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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5answers
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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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1answer
210 views

Spacetime assumptions at OPERA

I would like to know what are the assumptions about the spacetime metric between the neutrinos emiter and detector, I mean, perhaps (just a stab in the dark) could be a mistake to asume the metric of ...
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2answers
1k views

Does space have to be filled with charged particles to carry electromagnetic waves?

I'm a newbie here so have mercy. I'm studying electromagnetic waves. This is the propagation of energy via the vibration of charged particles, as I understand it. A charged particle could be like ...
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3answers
251 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", ...
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130 views

Confusion with infinity and time [closed]

I have some confusion between with resolving the following situation. I know that no measurable quantity can have a value of infinity. For example, I just wrote something out, but clearly this ...
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1answer
258 views

Can a vortex be self-sustaining? [closed]

Can vortices be self-sustaining? I suppose vortices in water are not self sustaining, since they need constant supply of water at speed i.e. kinetic energy. But is the same case applicable to space ...
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1answer
495 views

Orientability of spacetime

In many theoretical setups it is implicitly assumed that the underlying manifold (i.e. spacetime) is orientable. Then our analysis depends on this implicit assumption. For example, Stokes' theorem ...
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531 views

Can a black hole be formed by radiation?

I'm trying to find out if black holes could be created by focusing enough light into a small enough volume. So far I have found (any or all may be incorrect): Maxwell's equations are linear, ...
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1answer
254 views

How to bend 3d space embededd in a 4d enviornment? [duplicate]

Possible Duplicate: Calculating position in space assuming general relativity I recently started to program a 4 dimensional visualization program. I have the 4 dimension space perspectively ...
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2answers
692 views

Why is time special?

In Special Relativity, the spacetime interval between two events is $s^2 = -(c{\Delta}t)^2+({\Delta}x)^2+({\Delta}y)^2+({\Delta}z)^2$ giving the Minkowski metric $\eta_{\mu\nu}=\text{diag}(-1, 1, 1, ...
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3answers
553 views

Getting back out of an Alcubierre warp bubble

Does the theory on paper provide a way for hypothetical travelers to get back out of the bubble that has gotten them close to their distant destination by compressing all the space in front of them ...
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1answer
303 views

What does scalar phi represent in spacetime?

Trying to understand one-forms and vectors via Schutz's A First Course In General Relativity. His example uses a spacetime diagram, a scalar field phi, a curve (worldline) parametrized using proper ...
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6answers
1k views

Proving that interval preserving transformations are linear

In almost all proofs I've seen of the Lorentz transformations one starts on the assumption that the required transformations are linear. I'm wondering if there is a way to prove the linearity: Prove ...
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0answers
127 views

Einstein's postulates <==> Minkowski space. In layman's terms [duplicate]

Possible Duplicate: Einstein's postulates <==> Minkowski space. (In layman's terms) In the spirit of Einstein's arguments using flashes of light, moving trains and mirrors; ...
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5answers
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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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3answers
610 views

Rotations in space-time

In Landau's Classical Theory of Fields, one finds the statement: Every rotation in the four-dimensional space can be resolved into six rotations, in the planes $xy,zy,xz,tx,ty,tz$ (just as every ...
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4answers
809 views

Why should one expect closed timelike curves to be impossible in quantum gravity?

From the Wikipedia article, it seems that physicists tend to view closed timelike curves as an undesirable attribute of a solution to the Einstein Field Equations. Hawking formulated the Chronology ...
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3answers
392 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 ...
3
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2answers
328 views

How is the complexification of spacetime justified?

As always the caveat is that I am a mathematician with very little knowledge of physics. I've started my quest for knowledge in this field, but am very very far from having a good grasp. General ...
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2answers
717 views

Is spacetime simply connected?

As I've stated in a prior question of mine, I am a mathematician with very little knowledge of Physics, and I ask here things I'm curious about/things that will help me learn. This falls into the ...
106
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6answers
44k 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 ...
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2answers
245 views

Is the Assumption That Space-time Has to Be a Continuum Just a Matter of Mathematical Taste?

Is the assumption that space-time has to be a continuum just a matter of mathematical taste? Isn't there any physical significance associated with it?
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Why is gravitation force always attractive?

Why is the gravitational force always attractive? Is there another way to explain this without the curvature of space time? PS: If the simple answer to this question is that mass makes space-time ...
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2answers
541 views

Does spacetime really exist in quantum gravity?

If there are no localized observables in quantum gravity, does spacetime really exist, or might spacetime really be an illusion?
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122 views

the search for calabicles

Pop-sci ST books say, "At every point of space, there is a Calabi-Yau space..." or similar. But uncertainty p. and noncommgeom say, no points in space, only planckons (not my coinage). Is 'point of ...
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2answers
661 views

Nature of spacetime 4-vector and tangent space?

An entry level confusion about spacetime. I understand that a 4-vector describes a point or event in spacetime. But I've also read (Bertschinger, 1999) that re spacetime "we are discussing tangent ...
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3answers
554 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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7answers
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Is 3+1 spacetime as privileged as is claimed?

I've often heard the argument that having 3 spatial dimensions is very special. Such arguments are invariably based on certain assumptions that do not appear to be justifiable at all, at least to me. ...
3
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1answer
702 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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289 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 ...
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3answers
3k views

Gravitational time dilation at the earth's center

I would like to know what happens with time dilation (relative to surface) at earth's center . There is a way to calculate it? Is time going faster at center of earth? I've made other questions ...