I know it’s a random subject but bear with me. This is a little thought experiment on the subject of time travel and, if it were miraculously invented today, would be totally pointless and deadly, at least for humans.
First of all, let’s look at what I mean by “time travel”. Basically, my definition is that it is travel forwards of backwards in the fourth dimension of time by popping out of space-time in one temporal location and popping back in another. i.e. there is no spacial movement. This assumes that there is actually a fundamental space reference frame within the Universe which would probably change size and shape caused inflation and local gravitational field effects.
So, why do I postulate that it would be pointless today?
Well, let’s think about this. We are not stationary. We have velocity relative to everything except the few things around us which happen to be enjoying the same small area as we are. Even objects you can see a few feet away from you have a very slightly different trajectory to you because of the curvature of the Earth and its spin and the orbit of the planet around the Sun and also the orbit of the Solar System around the galaxy. So, if you were to just pop out of space-time just for a few minutes you’d find yourself either high in the sky, possibly even in space, travelling in an awkward direction or, even worse, suddenly underground squished by the planet you’ve hit at a couple of thousand miles per hour. Ooops! Paraphrasing Douglas Adams, not a naturally tenable position for a human.
This means, of course, that before you can use your new time-travel machine first you need to build yourself a nice inter-planetary capable space craft. At least you’d not have to worry about getting off the planet but you would have to make sure that your time jump was long enough to make sure nothing was in the way when you popped back into reality.
Well, this would be fine as long as you had a decent inter-planetary drive and enough time to travel back to Earth. The problem comes when you want to travel for more than just a few hours, or maybe days. You see, the further you travel backwards or forwards in time the further you will be from Earth when you arrive. After a few days you’ll be finding yourself popping back into existence outside the Solar System so you’ll need not just a fast inter-planetary ship but an inter-stellar one. If a fast inter-planetary craft is hard to come by at the moment then a nice inter-stellar one is even rarer.
Of course, there are other problems when you start moving very far from where you began, the most notable of which would be the change in the distortion of space-time due to gravitational field differences. Goodness knows what the effect would be when popping out of the high gravitational field distortion close to the Sun, Moon and Earth and then popping into a far “flatter” field in inter-stellar space. Would the instant change in your own space-time bubble be detrimental to your survival or physical integrity? I don’t know. Of course, this effect would be amplified greatly if you made large time jumps as space-time itself would have changed size due to the expansion of the Universe.
Overall then, time travel, other than the usual one second per second forward, is probably not a good idea, and likely to be a tad unhealthy at that. It’s probably why you’ve not met any time travellers yet.