I have a PhD in theoretical physics, but little experience of GR. I have recently encountered the debate about eternalism v presentism, and I'm trying to determine whether it is purely metaphysical (and therefore probably futile) or whether it has physically testable implications. The particular question I would like to put to wiser members of the forum is as follows...
Assuming a 4-d block spacetime, the universe occupies some surface or time wise segment corresponding to the present. Supposing it were possible that other time-wise segments of time could likewise be occupied by matter- would the existence of that matter at another hyper-plane of time have any measurable impact on the segment of spacetime we occupy?
To make the scenario specific, suppose there were a parallel universe evolving through time ahead of ours. Would it leave any traces that could be detected? Would the size of the time lag between its existence and ours make a difference to our ability to detect it (eg if it were a billion years ahead of us or five minutes)?
I have convinced myself that we would be unable to detect light from a universe that either led or followed ours, regardless of the size of the time lag, as the light in that universe only exists in its version of the present (which by definition of the scenario is temporally separate from ours). However, I don't understand GR sufficiently to determine whether the warping effects of matter are confined to a present in the same way that EM radiation is, or whether they leave some kind of trail in the past. Does the theory have anything to say about that?
A clarification to my question in response to Ben...
If our universe currently occupies a space like surface at time T years (crudely speaking) then in a million years our universe will be at time T+1,000,000. I.E the space like surface will have advanced 1,000,000 years in a timelike direction. My point is this- could it be possible that the nature of time was such that another universe is already ahead of us at time T+1,000,000 years? It is not a future state of our universe as we understand it, but something separate at another space-like surface 1,000,000 years ahead of the one we occupy, and moving ahead through time 1,000,000 years ahead of us, so that by the time we get to T+1,000,000 years that other universe will be at T+2,000,000 years. In such a scenario we would never catch-up with the other surface, as it would propagate through time as our surface currently does.
In that scenario, the leading universe would have been at time T (ie the present for us now) when our universe was at time T-1,000,000 years.
If such a scenario were possible, would the universe at the surface ahead of ours leave behind it any effects on space that we could detect, or are the effects that mass and energy have on space confined, temporally, to the space-like surface in which the mass and energy exists?