why spacetime

[mdl]

hi,
why do we need to have additional complexity to 3D space?

we know that space holds information.
but i'm not aware of any information in the universe which is held by "time dimension"..

isn't there only changing space and our brain does the "time job" with memorizing and sorting states of space?

[zhermes]

we know that space holds information.
but i'm not aware of any information in the universe which is held by "time dimension"..

The ancient greeks devised the 'arrow paradox,' in which: if an arrow frozen in time is stationary, at any given time; how does it get between stationary points?

How does space-alone contain velocity information? I.e. by only knowing where something is, how can you tell how fast it is moving.

The are lots of examples like this; certain things are innately tied to the temporal dimension---energy is another example, it only has meaning with respect to time.

There are other, more complex, situations in which the spacial dimensions and time become 'mixed together,' leading to the notion of space-time.

[mdl]

The ancient greeks devised the 'arrow paradox,' in which: if an arrow frozen in time is stationary, at any given time; how does it get between stationary points?

How does space-alone contain velocity information? I.e. by only knowing where something is, how can you tell how fast it is moving.

thank you for your reply,

but does the time dimension really solve any of these problems?

if space dimensions can't store information about where a particle/wave/whatever will be in next iteration (future), then the time dimension can't store that kind of information about *now*. so you can't tell whether *now* is moving forward or back or if it's moving at all.

or is time only for historical data? if so, it's not a dimension as most people believe, but only several states which generate future states.

[Fredrik]

Without the "time dimension", how would we assign coordinates to events? The event where I clicked the "new reply" button happened at spatial coordinates (0,0,0) (in a coordinate system that measures position relative to me), but the event where I clicked the "preview post" button the first time has the same spatial coordinates. We need a fourth coordinate just to distinguish between these two events. This doesn't even have anything to do with relativity.

However, it's in special relativity that spacetime gets really interesting, because the invariance of the speed of light implies that the coordinate systems that are the most natural to associate with the motion of an inertial observer "slice" spacetime into 3-dimensional hypersurfaces that we can think of as space at different times, in very different ways. So two events that you consider separated only in space, will be separated both in space and time by anyone who's moving relative to you (in a direction that's not perpendicular to the line connecting those events).

I don't know what you're talking about when you're talking about "storing information". Information is stored by physical systems, not by "dimensions", and the word "store" doesn't make sense if there's no time.