Time is a rainbow with color in both directions, but you cannot see either end. Time has meaning to everyone, but you cannot travel through it, except the usual way of one second per second. Time is often said to be wasted, but every finite resource is wasted.
If time is energy, speaking of waste, then how could one be efficient and improve the use of time, as though it were a fuel or a commodity? Especially when one must factor in the fact that humans need rest and relaxation, and probably meditation too.
Some method could be devised where the brain sleeps in shifts, lobes of the brain resting at a time. There really isn’t a left brain/right brain split. Both sides handle both aspects of creativity and intelligence. Would that mean genetic modification, or drugs? Who knows. It would extend the perception of time, since humans would have access to one third more time than they once did. But it wouldn’t extend lives. It wouldn’t extend absolute time, though. Wouldn’t extend absolute years.
That leaves an invention or device, and most estimates have time travel being theoretically possible, but requiring massive amounts of energy – amounts that humans cannot maintain, control or manipulate yet.
There is a theory that if time travel were possible, we would have seen time travelers by now. This is a terrible idea. First of all, if time travel takes large amounts of energy, it would be in the domain of governments and/or extremely rich people, and would be classified and limited. Secondly, if it is limited, then we may have seen time travelers who were just cautiously hidden in plain sight. They may have a disguise, or may have some other method of hiding, such as cloaking or phase shifting. And third, maybe time travelers do not consider the times we live in, these times of recording devices, to be especially interesting in the grand scheme of history. I think we need not have seen time travelers for time travel to exist.
Most of us have not seen a great many things but we still accept that they exist. Some are imaginary and cannot ever be proven, such as deities, but some we accept because we believe that we could prove them, if necessary. Most of us have not been to Easter Island, but we have no reason to doubt the existence of the island or the statues that reside there. Perhaps there is a simple explanation for time travel that we haven’t uncovered yet, just as the existence of Easter Island was unsupported until a seagoing craft landed there.
I think time travel is possible, and will be practical in the future, no pun intended. Even if someone were to bring a device here, it is possible that it relies on so much other technology that it is impractical. Imagine if you took a cell phone to the 1600s. It would be useless for most things. You could still play games and look at pictures, but there would be no calling, texts, or internet, and no GPS, no satellites, no service, no printing of anything, no looking anything up, because none of those other things existed in the 1600s. Eventually even the few things you could do would be impossible because the phone could not be charged. It would become a flat box that was slightly reflective, and if smashed would be found to contain incomprehensible technology.
Maybe that is the simple explanation.