This argument isn't _that_ compelling because: send today's tech back a century, use that as your aliens in case 'c'. They would 100% be able to see that tech. They wouldn't know what the hell they're looking at, or be able to do much about it, but they'd see it.
If we’re talking about aircraft, the combination of modern radar mitigation and modern sensor packages would allow a time traveling plane or drone to be effectively invisible in 1925.
Sure they’re not going to bend light around themselves, but they can fly outside of visual range and 1925 radar technology won’t stand a chance of detecting them.
Maybe this is a stupid question, but aren't they still going to be loud as fuck, and quite visible? How high do you need to be before you're not audible or visible? I guess go at night, sure, but...isn't all that crap more about being hard to precisely target than it is about being literally undetected?
Not a stupid question at all. I’m not sure about piloted aircraft, but drones currently operate at altitudes where they can’t be seen or heard from the ground.
It depends where you send it / why. There's lots of places you can send it where there's just nobody to see it. We still occasionally find an uncontacted tribes out there after all, so if someone didn't want to be seen (or even just seen in a place full of cameras), it would be trivial.
Sending the tech from 100 years in the future to today is not directly comparable to sending today's tech 100 years back.
By 2125, military aircraft will probably be silent, able to rapidly ascend to 100,000 feet (out of visible sight), and maybe even invisible. So people today, faced with properly-done future technology, can't see it at all.
