
What Is Spacetime? - artsandsci
https://www.scientificamerican.com/article/what-is-spacetime/
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chasing
Ironically, spacetime contains no actual space nor any time. It's actually
mostly whole grain wheat and malted barley flour.

Edit: Wait, that's Grape-Nuts. Not spacetime. Anyway, my point still holds.

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Koshkin
Indeed, this is similar to calling a plane "X-Y plane" \- coordinate axes are
a part of a reference frame and not of the plane itself.

~~~
raattgift
The tangent vectors at each point in the 2d flat plane are always positive,
while those in the flat 3d (Lorentzian) spacetime can be negative, null, or
positive. By extension curves can also be classified when the tangent vectors
at every point along them are all negative, all null, or all positive. You
can't have a negative or null curve in the 2d flat plane, but you can in 3d
flat spacetime; likewise, you can't have a negative or null curve in the 3d
Euclidean space, but you can in 4d Minkowski spacetime.

Strictly speaking, "time" in "spacetime" is superfluous, but it's often useful
in physics to distinguish between a generic space and a Lorentzian spacetime
-- for example, one frequently decomposes a spacetime into a set of spaces
(e.g. in the Hamiltonian formulation of General Relativity) where each space
is treated as having evolved causally from its neighbour. Maybe the choice of
"time" in spacetime (and "timelike" for negative curves) is just physics
chauvinism rather than something deeper, but if so does it really matter?

I think you know the following, but for anyone else reading, "time" is tacked
on to "space" giving spacetime much the same way that the differently-signed
dimension is tacked on to the line element. In Cartesian coordinates, with
mostly-plus eigenvalue signs, for flat space --> flat spacetime:

dx^2 + dy^2 --> dx^2 + dy^2 - c^2 dt^2

dx^2 + dt^2 + dz^2 --> dx^2 + dy^2 + dz^2 - c^2 dt^2

where c is some arbitrary constant, and x, y, z, and t are the labelled
orthogonal axes. The line element between two arbitrarily close points in the
space cases will be positive, but in the spacetime cases can be positive,
null, or negative.

There is enormous freedom to use different systems of coordinates and
different slicings, but there's no escaping a change in sign and constant
factor for one of the components of the line element in a (Lorentzian)
spacetime of dimension n > 1 compared to the components of the line element in
an n-1 dimensioned spacelike slice of it.

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nyc111
> In physics and, more generally, in the natural sciences, space and time are
> the foundation of all theories.

My opinion is that this is not true. If we assume that what he calls "natural
sciences" model nature by using equations and these equations correctly
describe nature and; we believe by definition that the terms in these
equations -and only those terms- describe nature -if so- then "space and time"
are not "the foundation of all theories." Simply because t and d in those
equations do not refer to "time" and "space" but to "interval" and "distance".
And even this is not exactly correct because what I called "interval" is also
distance. Because only distance can be measured, nothing else can be measured.
Time or t, is just the unit distance marked on an oscillator and used to count
how many times this unit distance occurs in d, the distance to be measured. Of
course after necessary unit conversions. Reading the term "t" as "time"
doesn't make it the philosophical time.

Of course, this can only be true if we believe that we can only know what we
can measure. So if we measure distance we say we measured distance. But this
is not always the case, someone may say "t is called time and measures
time"... I can only say that this is also an acceptable philosophical stand.
So I correct myself and say "depending on your philosophical stand; the
statement, 'space and time are the foundation of all theories' may or may not
be true."

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amai
See also:
[https://en.wikipedia.org/wiki/ER%3DEPR](https://en.wikipedia.org/wiki/ER%3DEPR)

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icc97
I was expecting this to actually describe what spacetime is, but from what I
can tell it just talks about interesting things around spacetime. I was
expecting more along the lines of this video [0]

[0]:
[https://www.youtube.com/watch?v=sryrZwYguRQ](https://www.youtube.com/watch?v=sryrZwYguRQ)

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stcredzero
This is PBS SpaceTime:
[https://www.youtube.com/watch?v=X5rAGfjPSWE](https://www.youtube.com/watch?v=X5rAGfjPSWE)

