Somewhere in a comment I can no longer find, in response to a previous article, Charles “Chuck” Endicott posited that you
can’t travel back [in time]; all the energy used to create [the] past has dissipated so [it is] no longer there. Energy for the future has yet to be created so where are you really going?
At the time I suggested that an article would be needed to address the issue; this is that article.
He objects to time travel in principle, on the grounds that those “times” do not exist as substantial destinations. The matter and energy which was (or will be) “there” “then” is “now” occupied “here”, and thus that it is not and cannot be in the configuration it had or will have at a different moment in time.
As to whether the past and future exist in any sense of the word “now”, quite a few scientists believe the “fixed time” theory, holding that all events past, present, and future, exist in a static continuum and we only experience them in temporal order; Stephen Hawking has considered that possibility and not rejected it. Thus the issue for these physicists is not whether the past and future exist, but whether they can be altered by time travel. Next week we hope to begin a second series on temporal theory (Temporal theory 102) which will discuss the nature of time in this regard, but it is for the moment sufficient that the concept of the future and past existing in some form “now” is not incompatible with modern physics.
For literary purposes, though, in approaching a story it is necessary to accept those assumptions the book makes which are necessary to its story and neither patently absurd nor known to be false (excepting, of course, those books that are intended to be patently absurd). The effort of analyzing time travel films has been to determine which ones do and do not work under such rules for time travel as seem rationally likely, given that time travel is possible and persons have free choice to act to guide events based on their knowledge, with due consideration given to whether other plausible theories of time might explain them. For example, Source Code is clearly a multiple dimension theory story.
If the objection is fundamentally theological (that the kind of time God created exists only as a present moment), somewhere in the Psalms (and I wish I had noted the spot) the author comments that to God his life is like a book (or scroll?), such that God can turn to the final words and read how it ends–and that can only be so if the future already exists even as we are living in the past. This certainly does not prove that we can go to that future (or return to the past), but it does suggest that however it is we experience time, the future already exists and the past still exists, at least in the sense that it can be observed by God.
Ultimately the objection appears to apply to all time travel stories that do not involve multiple dimensions in some way, and thus that discussing what “would” happen “if” someone could travel through time is moot. Yet people write time travel stories, and try to make them rationally consistent with our expectations. I do not regard parallel or divergent dimension theories to be time travel, because the traveler is in someone else’s past; fixed time theory requires a rigorous determinism that eliminates not only free choice but often the integrity of causal chains. Thus I work with a form of replacement theory as rationally the most probable theory of the effects of time travel.
These issues will be further elucidated in the upcoming theory series.