Well... if we mean Alien UFOs, as I understand space travel, the vastness of our universe, these Aliens would have to have some serious technology to see any ROI.
As I understand it, the ability to travel into space and get anywhere in any reasonable time requires propulsion technology well beyond what we have today. Further more, let's say that these aliens can travel at a very high rate of speed to come see this planet. Unless they have the ability to travel back in time, their own home planet would experience time at a much greater rate than the aliens making the trek. This means that if the aliens traveled back to their home planet, they would be arriving back at a time well into the future. Unless... they had the ability to travel back in time through some unknown means, worm holes, or other sci-fi tech. Not saying it isn't possible as these aliens could be billions of years older than us and have that technology, but if not, more than likely, it's a one way trip to earth for those aliens.
With regard to the idea of time travel, this from another thread here;
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There's this Law of physical science to effect that there is a finite amount of energy and matter in this Universe. A sum total of both and while energy could be changed into matter, or matter into energy, the sum total of both is a constant amount.
So let's say you hope into your time traveling spaceship and want to go back just 10 months into the past. And before you hoped in, you had a hamburger sandwich for lunch.
Ten months ago the wheat that became the bun of your burger was seeds on stalks growing in a field in Nebraska. The meat for the hamburger patty was still flesh of the cow grazing in pasture (or feed lot).
Once back ten months in time ...
Is that wheat still grains on the stalks or in your stomach?
Since matter can't be in two places at the same point in time, that 'wheat' is either 'grain on the stalk' or you have some holes in your stomach ~ GI track.
As for the meat patty, is it still part of the flesh of the still alive cow or in your GI track, or digested already in your blood stream/cells? It can only be one of those two places so does the cow now have some holes in it or do you have holes in you?
We could do a similar mind experiment where the aluminum used in the structure of your 'time&space travel machine', which has now gone back to this time in 1944; was part of the structure of a B-17 bomber and question is; does that B-17 bomber now have holes in it for the aluminum in your device, or does your device have the holes?
If your device has the holes, are they in critical parts that have now made your device inoperative?
"Time travel" might be a fun device for science fiction authors to toy with, but if doing realistic science, then "time travel", the "lever pulling, button pushing" sort, is just plain impossible
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Post #8 ...
I did not think of this but a brilliant man did The scene when they are being chased by a Muslim terrorist and go back to 1955 ?? Do you see the flaw here ?? The doctor never takes into consideration where the earths path is around the sun ?? So if you just went back to February of 1922...
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