"Once" they are invented (meaning it's all by inevitable)? Then where are all the tourists from the future .
I'm not being snippy or sarcastic. I really do wonder this often: Where on earth are all the tourists, historians, (even evil cyborgs) etc. from the future ?
I've said this many times before, and will do so many more times.
Imagine that a time machine, rather than being a machine that actually travels through time, is a machine within which a person can travel through time (in the same way that railway lines stay where they are, but people can travel back and forth upon them).
So once the first working time machine gets invented and is switched on, it'll be possible to travel within that machine to any point in time during that machine's existence - so if it breaks down after fifty years then anyone can use it to visit any time during that fifty years.
With me so far?
Now imagine that the inventor knows (from having used the machine) that he can't travel more than fifty years into the future. He can travel to the time just before the machine breaks down (or a year before, or whatever) and build a brand new time machine right next to it, an upgraded model which might last twice as long.
Now you have two time machines (let's call them One and Two) side by side, like two elevators in the Empire State Building (neither of which travels the full height of the building, in case you didn't know).
The inventor (or anyone at all, of course) can use One to travel from his own time to fifty years in the future, then change from One to Two and travel forward another hundred years, at which point he can build time machine number Three, which might operate for two hundred years. And so on.
All of these machines go nowhere themself, just like a railway line that stays where it is. Hence we see no future tourist etc yet.
Ultimately, of course, time travel using machines in this way would become as common as train travel is to us. (Just remember less than two centuries ago there were no trains at all). Many people will be interested in travelling as far back as possible, and so most likely the first thing that will happen when that first time machine is switched on is that somebody from the future will immediately step out of it, closely followed by his agent, manager, bodyguards, papparazi and historians.
This is where you say "tl;dr" and I give up until next time.