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A Little Time Travel Thought Experiment

During the Back to the Future (BTTF) anniversary celebrations there was a lot of talk about how accurately they represented time travel. The consensus seemed to be that travelling back and appearing at the same physical location but in a different time was how time travel would likely work. This is where I got thinking.
The universe is an ever moving beast. Nothing sits still and this leads to some pretty big problems. To start with the Earth rotates on its axis every ~24 hours, so our traveller would need to arrive at approximately the same time of day in the past. He’s worked out time travel so that’s not going to be hard to do.
Next, the Earth orbits the sun every ~365.25 days. This one is a little more of a problem. If our traveller is in the heat of June and has decided to travel back because he wants to see an Xmas in the past he’s got a problem. The Earth will be on the other side of the Sun. So Marty needs to travel back to roughly the same day each year? Fair enough I suppose but it gets worse.
The Solar System itself orbits a super massive black hole in the Galactic Centre. A Galactic (Cosmic) year last between 225–250 million terrestrial years. To put that in perspective, humans started walking the Earth about 0.001 Galactic years ago (thanks Wikipedia). If our traveller wants to go back and visit Middle Ages England then when he reappears the whole Solar System is in another location altogether and he’s in deep space. If he has to travel back in units of Galactic Years in order for Earth to be back to the same location in space, and ignoring the fact that the terrestrial year would have to line up with this exactly, then the first period he can go to is the Permian-Triassic Extinction Event. So Marty is going to find out what happened to the dinosaurs but getting run over by his grandfathers car is becoming less of a concern.
We’re not done yet though, and this is the deal-breaker. The galaxies themselves are moving through space with increasing speed and, so far as we can tell, not in an orbit of anything. So by the time the Earth and Solar System have completed their respective orbits the whole thing has moved off anyway!
So what’s my point? Well, firstly Doc Brown had actually created a machine that can traverse both space and time in an instant, so well done for that. And secondly I am genuinely wondering if trying to time travel is even worth the effort? My two cents, the human race are explorers so I'll say yes but just don’t expect to actually see very much when we do.
Originally posted to Medium on May 8th 2016.

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