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Den 17.01.2025 22:20, skrev LaurenceClarkCrossen:It is a bad idea to give time a dimension of the same kind as space.On Thu, 16 Jan 2025 19:35:26 +0000, Paul.B.Andersen wrote:
>Den 15.01.2025 06:31, skrev LaurenceClarkCrossen:Euclidean geometry is for planes, and non-Euclidean geometry is for>
other surfaces, such as spheres. Space is not a curved surface.
One can but admire your sharp observation that space is not a surface.It is sad that you can't recognize that non-Euclidean geometry appliedThis rather funny statement of yours reveals that the only
to space is a reification fallacy because space is not a surface.
non-Euclidean geometry you know is Gaussian geometry.
Loosely explained, Gaussian geometry is about surfaces in 3-dimentinal
Euclidean space. The shape of the surface is defined by a function
f(x,y,z) where x,y,z are Cartesian coordinates.
Note that we must use three coordinates to describe a 2-dimentional
surface.
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Riemannian geometry is more general.
Loosely explained, Riemannian geometry is about manifolds (spaces)
of any dimensions. The "shape" of the manifold is described by
the metric.
The metric describes the length of a line element.
The metric describing a flat 2D surface is:
ds² = dx² + dy² (if Pythagoras is valid, the surface is flat)
The metric describing a 2D spherical surface is:
ds² = dθ² + sin²θ⋅dφ²
Note that only two coordinates are needed to describe the surface.
The coordinates are _in_ the surface, not in a 3D-space.
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The metric for a "flat 3D-space" (Euclidean space) is:
ds² = dx² + dy² + dy² (Pythagoras again!)
The metric for a 3D-sphere is:
ds² = dr² + r²dθ² + r²sin²θ⋅dφ²
Note that only three coordinates are needed to describe
the shape of a 3D space.
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In spacetime geometry there is a four dimensional manifold called
spacetime. The spacetime metric has four coordinates, one temporal
and four spatial.
The metric for a static flat spacetime is:
ds² = − (c⋅dt)² + dx² + dy² + dz²
If ds² is positive, the line element ds is space-like,
If ds² is negative, the line element ds is time-like.
In the latter case it is better to write the metric:
(c⋅dτ)² = (c⋅dt)² − dx² − dy² − dz²
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