# What is space time exactly?

#### benzun_1999

hi all,

I am confuced. what is space time excatly?

-benzun

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When Einstein introduced his special relativity in 1905, it said that if somebody is moving at a constant speed relative to you, you will measure his lengths as shorter than yours and also his time durations will be shorter than yours. Not only that, but the equations to convert from your coordinates into his, or vice versa, were almost symmetric in the way space coordinates and time coordinates came into them. There was just one rather easy to grasp sign difference.

Pondering on this, the mathematician Minkowski saw that "In the future physics, time and space will lose their individuality". Every appearance of a space coordinate was matched by a time coordinate, and vice versa, in all the basic laws of physics, after relativity.

So Minkowski visualized a four dimensional continuum, having the three dimensions of space, plus a time dimension, carefully distinguished from the others by a sign difference that followed it around in all formulas. Minkowski called this continuum spacetime. After some hesitation, Einstein accepted the new idea.

And when Einstein got to thinking about this new spacetime, he saw that some other physics ideas he had, like the equivalence principle, and maybe Mach's principle, could imply that spacetime curved. So he went to a mathematical friend of his and asked, "Do the mathematicians have any theories about curved continua?" And the friend, by chance, knew the answer. "Yes, they are called Riemannian manifolds, and the Italian school of mathematics around Professor Ricci has developed a detainled theory of them." And that's how Einstein got started on his general theory of relativity.

#### benzun_1999

a curve in spacetime causes gravity. so how will a solid, liquid and gas look in space time? how will a field be in spacetime??

#### turin

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Originally posted by benzun_1999
a curve in spacetime causes gravity.
I would like to refine this statement a bit. I would say that curvature of spacetime causes the tidal force. Stress energy causes the curvature. Solids, liquids and gases have stress energy, so they cause curvature. But, to understand how these things look in space-time (I'm assuming you want the 4-D intution), first, consider a 2-D version. Then, extrude that in a third dimension. And motion of the solid, liquid or gas will show up as variations in the extrusion. You can't really do the 2-D version for gravity. It would be kind of like a fan out in the raidal direction and a convergence in the azimuthal direction.

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#### hey.like

The time-space structure isn't permit or not to measurement and action

Naturely , the time- space is family enough and secret enough.
the one idea is space and time is permit limit in some rule. like mass against some action and mass in same time and space point.
What is the right rule in some surface time and space structure?
It's to permit or not ? What is the unit rule in space and time.Naturely it is a enough space and time now.The god gifts or nature.

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