[jrussell (OP)]

a) space and time can no longer be thought of as seperate entities, and it is far more accurate to combine the two into a space-time

b) there is no fixed point in space, ergo, no fixed point in space time

c) it is reasonable to suggest, then, that each moment of 'time' - now spacetime, should be able to be co-ordinated as a point in space and a point in time, albeit one that is moving

my question is this; if we know the speed and distance that has passed between two points in spacetime, and presuming one can break the cosmic speed limit, if we used the co-ordinates of a point in space and time, would we find that moment in time?

further points -

d) an outside observer at some distance could be observing the french revolution as though that were the present, of course if they set out to visit byt the time they got here that would be the past. is it that we could only observe the co-ordinates and never visit, even breaking the cosmic speed limit?

e) if we can visit those co-ordinates, why do satellites not show a continual stream of moments in time and space?

hope that makes sense



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[flr]

b) there is no fixed point in space, ergo, no fixed point in space time

What do you mean by that? Would you give an example?

[jrussell (OP)]

well, space is not static, it is expanding, and not simply from the edge of the univers, if such a thing exists, but the space in-between objects in space is expanding; therefore there can be no fixed point in space, there is no one 'address' if you will that will be the same at any two points in time

[graham.d]

Models of the universe can be complicated, and recent ones are using 5 dimensions rather than 4, but there is always (I think) some coordinate system. If we take your 4-D space-time example, which is commonly accepted and consistent with Special and General relativity, it has 4 coordinates which uniquely define positions in space-time. These are not the same as locations in space because of the extra time dimension but are referred to as "events". The 4D equivalent to distance is called an "interval". In flat (Minkowski) space, assumed in special relativity, the 4 axes are orthogonal. In GR spacetime can be curved and the maths (and visualisation)becomes a bit hard, but the principle of a coordinate system remains.

[jrussell (OP)]

i appreciate that. however, i do believe that if we accept space and time as one and not two factors, then each 'moment' can be defined by co-ordinates of its point in space and time, and perhaps velocity and direction in time. my confusion comes from the conflicting idea, at least conflicting to me, that space is not fixed and is expanding; this is why it is believed there is no 'centre' of the universe - it expands constantly and would have to constantly move to adjust.

so, surely any co-ordinates must be incorrect?

[PhysBang]

There is a way to use whatever coordinate system you wish. However, doing physics in many coordinate systems is incredibly difficult.

[JP]

i appreciate that. however, i do believe that if we accept space and time as one and not two factors, then each 'moment' can be defined by co-ordinates of its point in space and time, and perhaps velocity and direction in time. my confusion comes from the conflicting idea, at least conflicting to me, that space is not fixed and is expanding; this is why it is believed there is no 'centre' of the universe - it expands constantly and would have to constantly move to adjust.

so, surely any co-ordinates must be incorrect?

I think the confusion might be that you're concerned with calculating how things move within space-time.  The equations of general relativity also tell you how space-time itself moves (and expands), and keep track of how things move within this expanding space-time.  So in theory, you should be able to calculate how something moves within space-time while the space-time itself is expanding.  Defining coordinates for something could be done in that case, since you know how the coordinates have to move as well as knowing how the object moves within those moving coordinates.

In practice, these equations are often going to be incredibly complex, so doing the calculations might not be practical.