Travel to other stars is “merely” a problem of engineering and resources. We can already travel through space. The difference between going to the moon and going to Alpha Centauri is simply one of scale. No untested scientific theories need be used.
The possibility of time travel is only hypothetical. That is, scientists have devised ways it might be possible without violating tested theories. But, as far as I know, no time-travel hypothesis has been tested.
Time-travel hypotheses involving relativity typically use faster-then-light particles or negative-energy matter. Neither of those have been found. Hypotheses involving quantum mechanics put strict limits on the time travelled versus the energy travelled. And still not tested.
And then there is the fundamental flaw with supposing one could time travel without traveling in space–it’s not possible to do the first without the second. The Earth and the Solar system and the Milky Way are all moving. Anyone who travels in time, will necessarily have to then travel in space, because everything will be in a different place. And if your time-travel system lets you choose where you appear at your destination time, you have a space-travel system, too!
Exactly backwards. The problems of FTL travel and time travel are exactly equivalent: Any device, technique, or technology which could be used for one could also be used for the other. So far as we know, neither is possible: While some aspects of the mathematics of relativity and quantum mechanics can be said to describe time travel, those aspects all correspond to conditions which do not appear to occur in our world (things like exotic matter that has negative mass). On the other hand, while such presumed-impossible technology is necessary for time travel, it is not necessary for space travel: If you’re patient enough, you can travel between stars at slower-than-light speeds.
An eight-foot-long “millipede” isn’t going to be nearly as heavy as most top predators, and in any case is orders of magnitude smaller (in weight) than many terrestrial vertebrates. As I said, the exoskeleton places limits on how big a terrestrial arthropod can get, and this was the best they could do even under conditions of enhanced oxygen (which allowed them to get enough oxygen to their tissues even using a tracheal system).
Supposing that they are future humans, what would be the point of coming back to the here and now?
They’re archeologists or historians doing research? What is there to find out aboutus today that isn’t digitized on the internet and theoretically preserved forever?
Sociologists or timecops influencing our progress towards or away from a specific future outcome? The paradox possibilities are legion. Admitted, everything we presume to know about time travel is theories wrapped in guesses. Or guesses wrapped in theories. So maybe I can assassinate a young Hitler without causing a paradox loop. I’ll probably never know for sure.
Sampling genetic material? Is this really the best time period? Maybe if they’re tracking the evolution of a certain trait. But the more technologically advanced we become, the better the chance we’ll twig on the truth and/or capture something from uptime. If we haven’t already.
Arthropods have exoskeletons and they have an inefficient tracheal respiratory system, but the former doesn’t necessitate the later. Alien arthropoids might happen to evolve an efficient oxygen transport system and grow as big as you please. Maybe some arthropod on Earth will evolve such a system in the future.
True. However, even if they had more efficient respiration it may be impossible for animals with exoskeletons to become as large as those with endoskeletons. Because the exoskeleton is on the outside of the body, and the muscles that move it are on the inside, as the exoskeleton becomes thicker and heavier to support the body there is proportionately less room inside it for the muscles that move it.
In any case, my main point was not about what alien arthropods might look like, but that the small size of Earth arthropods was due to their inherited body plan.
Possible, but since the arthopods have been around for over 500 million years, evolving many millions of species in that time, and haven’t gotten around to it yet, my money is on it being very difficult.
In answer to the OP; if humans are evolving, we’re evolving slowly and erratically. This is because humans are constantly “moving the goalposts”; changing our environment to suit ourselves.
If we’re going to hypothesize about the future I think tech like genetic engineering, nanotechnology and biochemistry will play a much bigger role than natural selection.
<flight of fancy>
So, looking to the far future, I see two directions for mankind’s physical appearance:
In the first scenario, customising the body gradually becomes more acceptible and ambitious – we start out looking like RPG characters, then we’ll desire wings, more than 2 hands etc and we’ll progressively get weirder and weirder.
In the second scenario, we learn to modify our minds and the consciousness that we experience before scenario 1 can happen. In which case the body will actually be of little importance. But it’s hard to predict where this scenario would go.
</flight of fancy>
If technical progress continues, I seriously doubt evolutionary progress is what is going to define humans, as per the question. First off, a lot of these pressures are gone thanks to medicine, hygiene, nutrition, etc. A human from the future will look like a human like today the same way Julius Ceaser looked like a modern person.
What we’ll probably be seeing is some genetic engineering or even full blown cyborg replacements. Maintaining all this meat, dealing with cancer, aging, etc seem pretty counter-productive to a theoretical super-technical society. I doubt any of these scenarios will ever lead to looking like the X-files aliens. Theories into why folklore has piked neonatal looking things to represent aliens is abound, but there’s no science here. Its just fairy tales.