Originally Posted by Zubon

The difficulty may only be practical, but I have trouble imaging a meaningful instance of time travel that does not cause a significant impact on the time traveled to. Let me set aside time travel linearly forward, since we do that on a daily basis given a linear interpretation of time, and instead consider a simple jump backwards along a simple linear time stream

Moving backwards in time seems to necessarily involve interaction at the earlier point due to your simple existence there. You absorb and reflect light, you displace air molecules, etc. As I said, this may only be a practical difficulty to sending back an invisible, intangible observer, but I cannot imagine what such an observer could observe; if you are invisible, you cannot see anything, unless we have a new way for vision to work without absorbing light. I would also expect you to breathe, thus altering the composition of air between times, but hopefully we can use a self-contained breathing system to avoid, say, introducing modern TB into the 1800s and wiping out a few cities. Also, similar difficulties about picking up dirt, lint, etc. between times.

Perhaps none of those are going to be significant enough to cause any change, but if you cannot go back without causing the sort of time loops that MageB420666 refers to, it seems to me that either time travel (backwards along a linear stream) is impossible or time is not linear. Otherwise, we would already have created some loop and retroactively wiped out all of existence, or some such.

Semi-digression: how can we test if a time loop would unravel all of existence without risking unraveling all of existence? Kind of a big risk if time turns out to be linear in that way.

Or maybe I'm just off on my own here.