Time can only go forwards,
It may not pause;
It may not retrace its steps;
It may not speed up;
It may not slow down;
It is a one-dimensional direction;
To travel backwards in time
is a paradox, and impossible;
Likewise to travel faster than it,
Likewise slower,
Likewise without it,
Likewise independent of it;
To travel within time would require another time,
Such as we have not;
Therefore it is impossible.
We are constrained by time,
Likewise by space,
Likewise by our own bodies.
Time permeates creation,
As does space.
Time seems to us to be infinite;
It may be,
But yet probably is not,
No more than space.
We cannot imagine
(in our waking hours)
A timeless space,
Nor yet a spaceless time,
Nor timeless time,
Nor yet a spaceless place;
For we think but as men, and not as God.
There may be stuff not in this universe,
Not material, or perhaps so.
Yet there is only one universe.
Hereafter is no time,
And perhaps no space.
We have not much time here.

(Sunday,20th March,2005) .

David Mitchell

http://www.poemhunter.com/poem/time-13/