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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140 views
Does the recent Gravity Probe - B mission mean both the mass of an object and the spin of an object affect time?
I'm a non-engineer interested in the recent GP-B mission results: http://www.engadget.com/2011/05/06/nasa-concludes-gravity-probe-b-space-time-experiment-proves-e/#disqus_thread
Is it correct that ...
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3answers
276 views
If the Big Bang theory suggests that the Universe is “expanding” then what is it expanding INTO? [duplicate]
Possible Duplicate:
origins of the universe questions.
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 ...
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2answers
300 views
Light speed in space-time. is it really constant?
I am by no means an expert in this field, however something puzzles me about the speed of light and the relativity of time and space (space-time).
Is is universally acknowledged that the speed of ...
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2answers
168 views
Excluding big bang itself, does spacetime have a boundary?
My understanding of big bang cosmology and General Relativity is that both matter and spacetime emerged together (I'm not considering time zero where there was a singularity).
Does this mean that ...
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3answers
257 views
Is spacetime moving in general relativity?
Is spacetime moving in general relativity?
If not, how does spacetime retain its past, while moving toward future?
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3answers
217 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 ...
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3answers
201 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 ...
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1answer
339 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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1answer
170 views
Is Space-Time Quantisation necessary or even meaningful?
It is believed among people working on Quantum Gravity, that space-time must be quantised at the Planck scale. Although it is very hard to verify such proposition, it is interesting from a ...
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1answer
70 views
Looking backwards in time at yourself
If a person on Earth today is looking at a star, say, 10 billion light years away, is it possible that some of the atoms he is looking at will eventually go on to make him?
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2answers
248 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
79 views
Points in Spacetime
Assume there are two points in spacetime $a=(t,x,y,z)$ and $a'=(t',x',y',z')$. Let's say that the first one is in the origin of spacetime i.e. $a=(0,0,0,0)$. The point $a'$ has two possibilities
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4answers
109 views
How can the big bang occur mathematically?
As we know time began with the big bang. Before that there was no time, no laws, nothing. Mathematically how can an event take place when no time passes by? How did the big bang took place when there ...
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2answers
137 views
How to find a curvature of the space-time by having $g^{\alpha \beta}$ in the following case without cumbersome calculations?
The metric tensor for Fock-Lorentz space-time,
$$
\mathbf r_{||}{'} = \frac{\gamma (u)(\mathbf r_{||} - \mathbf u t)}{\lambda \gamma (u) (\mathbf u \cdot \mathbf r) + \lambda c^{2} (1 - \gamma (u))t + ...
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1answer
138 views
Is there a relation between the number of dimensions of space time and the number of fundamental forces?
Is there a relation between the number of dimensions of space time and the number of fundamental forces?
Also, did the universe always have 4 space time dimensions?
And could there exist a world ...
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2answers
214 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 ...
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3answers
137 views
About gravity through space time curvature
Is it possible to produce virtual gravity? I mean gravity without the help of mass by curving spacetime with other effects like fast rotating objects?
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1answer
70 views
Can the fuzzball conjecture be applied to microscopically explain the entropy of a region beyond the gravitational observer horizon?
In this article discussing this and related papers, it is explained among other things, how the neighborhood of an observer's worldline can be approximated by a region of Minkowsky spacetime.
If I ...
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1answer
74 views
Could light travel more slowly than the “universal speed limit”? Could this imply quantization of spacetime?
One description of relativistic effects that I've heard/read goes something like this:
Everything moves through spacetime at a constant speed. An object's direction of travel through spacetime can ...
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2answers
72 views
Why can certain functions be absorbed into the Schwarzschild metric, while others can't?
Another question about the Schwarzschild solution of General Relativity:
In the derivation (shown below) of the Schwarzschild metric from the vacuum Einstein Equation, at the step marked "HERE," we ...
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1answer
93 views
The status / acceptance of block time?
What is the current status or acceptance of block time as it relates to Einstein's theory of relativity? Has quantum mechanics ruled it out or is it still the favored view of the world? Perhaps there ...
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2answers
193 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 ...
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1answer
86 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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1answer
58 views
Are branes in 4D-spacetime moving, or are they static?
Given that a worldline, worldsheet, worldvolume, are representation in a 4D-spacetime of a point particle, a string or a brane, respectively, I was wondering if those objects necessarily have to be ...
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1answer
129 views
Symmetries of spacetime and objects over it
I guess according to mathematical didactic, we first think of spacetime as a set and we reason about elements of its topology and then it's furthermore equipped with a metric. Appearently it is this ...
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1answer
209 views
What is the general relativistic calculation of travel time to Proxima Centauri?
It has already been asked here how fast a probe would have to travel to reach Alpha Centauri within 60 years. NASA has done some research into a probe that would take 100 years to make the trip. But ...
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1answer
123 views
Is time the rate at which one moves through space
I'll start out with the cliche attempt in a protective shield of my dignity. I am a young highschool kid just eager to learn and understand. If I'm way off or this is already a known idea, or maybe ...
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1answer
117 views
Is it possible to describe the entire universe with the behavior of an $\mathbb{R}^n$ field?
Suppose every phenomena in this universe (of course most are reducible to some particular general ideal ones - basically I'm talking about those!) could be described as ...
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1answer
71 views
Is it best to look at light as a particle when trying to understand special relativity?
So my course about special relativity explains time dilation using a moving train, where one sends up (i.e. perpendicular to the direction of movement) a light pulse which gets reflected etc. (a ...
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1answer
116 views
A question about multiverse
I have a question that I would like to ask here since I'm not an expert or any near to that.
Actually I just came up with this question when I was watching The fabric of cosmos.
I couldn't find where ...
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1answer
109 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, ...
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2answers
190 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 ...
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3answers
225 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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1answer
82 views
Killing vector argument gone awry?
What has gone wrong with this argument?!
The original question
A space-time such that $$ds^2=-dt^2+t^2dx^2$$
has Killing vectors $(0,1),(-\exp(x),\frac{\exp(x)}{t}), ...
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0answers
64 views
Do we expect that the universe is simply-connected? [duplicate]
I heard recently that the universe is expected to be essentially flat. If this is true, I believe this means (by the 3d Poincare conjecture) that the universe cannot be simply-connected, since the ...
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0answers
66 views
What were Feynman's objection(s) to a cubic lattice universe? [duplicate]
In this video of Feynman discussing the scientific method, starting at around eight minutes and 30 seconds, Feynman describes the proposition that space consists of a cubic lattice of points (as ...
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0answers
46 views
When is spacetime homogenous and isotropic?
When is spacetime homogenous and isotropic?
For example, some metric $g_{\mu \nu}$ is homogeneous and isotropic. We now construct effective metric
$$n_{\mu \nu} ~\rightarrow~ g_{\mu \nu} + ...
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0answers
45 views
Naked singularity and null coordinates
I'm trying to understand the notion of a naked singularity on a more mathematical level (intuitively, it's a singularity "one can see and poke with a stick", but I'm having troubles on how to actually ...
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0answers
38 views
When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?
It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
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0answers
111 views
What happened in Philadelphia Experiment(1943 October 28)? [closed]
The Philadelphia Experiment was an alleged naval military experiment reported to have been carried out at the Philadelphia Naval Shipyard in Philadelphia, Pennsylvania, USA sometime around October 28, ...
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2answers
90 views
about the 1D singularity of black hole
I saw some responses here saying that the singularity into the black hole is one dimension object so my question is : is it possible that the singularity is simply a merger of the 4 dimensions of the ...
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0answers
93 views
Higher dimensions
1) How we determine whether the higher dimensions are Unstable or Unpredictable? Or on the basis of what assumption we make this prediction?
(Source of Image: Max Tegmark. See also this Wikipedia ...
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2answers
147 views
Special relativity, spacetime, velocity and units
We all know that space and time are the fundamental units, means no mathematical expression can express their relation to other variable fundamentally. But as we know that moving rod has a contracted ...
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1answer
353 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 ...
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1answer
78 views
Two definitions: 'semi-classical space-time' and 'supersymmetric Minkowski space'
By reading articles I ran several times into two terms, never being defined so I assume they must have well established definitions somewhere.
The first is semi-classical space-time. If I where to ...
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174 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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0answers
89 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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0answers
111 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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1answer
318 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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1answer
449 views
What is the 4th dimension? [closed]
I have heard before that the 4th dimension is time, however, another theory makes a lot more sense to me. This is that the 4th dimension is the third dimension stacked on top of each other in a ...
