And what were the dominant terrestrial species at said time? Would there even be any? I have zero idea of what’s the expected survival rate of an iPhone in the fossil record, but I like to imagine a giant sloth stepped on one once and we are none the wiser.
Alternative title:
“Assuming the timeline is stable and time travel is possible, but no evidence is ever found until its invention, what would be the earliest acceptable time period for time-travel tourism, and the latest cut-off point no one would be allowed to go to?”
Ah, yep. So more like they bring their multi-function communication / information / documentation / entertainment devices with them. Heck, they may even use those devices for the time-travel itself. And then when they get to the ancient past, they pay no attention to the scenery and just watch reruns.