Time. Whether it is a physical field akin to gravity, electromagnetism, and dark energy, or simply a quirk of perception, is one of the greatest debates of our age. Studying time is very much like trying to grab air with our hands; there's SOMETHING there, but without the proper tools, we can not hold onto it long enough to really study it.
One of the biggest challenges of time is that all of our measurements are quite arbitrary at the largest — and smallest — scale. Seconds are nothing more than 9,192,631,770 cycles of electron fluctuations within caesium atoms — hardly a fundamental measurement. Minutes are merely 60 seconds, and hours 60 minutes — leftovers from the Babylonian base 60 numerical system. The division of days into 24 hours is also arbitrary in the sense that it reflects two cycles of twelve, and these cycles were originally created as reflections of the twelve lunar cycles in a year. That makes sense for Earth, but what about a world with no moon? Or many moons? Or a world that has one moon, but orbits its star further away or closer and therefore has more or less lunar cycles?
I believe that outside of human measurements, there exists a fundamental particle of time, and that time is also a field that permeates the universe. I am hardly the first one to come to this conclusion. In 1947, Chen Ning Yang put forth the theory that time is a quantum variable with a spectrum. In 1950, Henry Margenau created the term chronon — the time it takes for light to travel the classical radius of an electron. Unfortunately, Margenau was still referring to time as a product of something else (in this case, light and an electron), and not something completely independent.
In 1982, Sam Vaknin came up with the first cohesive idea that chronons are fundamental units, and that time is the product of chronons interacting with each other. He argued that there are time quarks with different properties that cancel each other out, thus creating the "arrow of time."
Of course, I've realized that if these time quarks cancelled each other out completely, then there would be no time. It would stand still. So some kind of chronon has to be dominating the others, a "forward" chronon, if you will.
If time truly is a field full of chronons, and if there is a "forward" chronon dominating the field, then the key to time travel is discovering the forward chronons and creating a way to neutralize them, or perhaps even "flip" them to become "backward" chronons. If enough of these chronons could be flipped, time would go backward instead of forward. Everything would reverse. So perhaps the nature of time travel isn't to travel through it classically, but to instead alter its very chemistry. And I believe dark energy is the key.
Dark energy may in actuality be the very field of chronons we seek. It's causing the expansion of the universe, even accelerating it. It is spread uniformly across the universe, just as time would be. Unfortunately, our knowledge regarding dark energy is extremely limited at the moment, but further study may reveal that it is made up of chronons.
In the far future, maybe 100 thousand years from now, our descendants will find a way to manipulate chronons. The ability to reverse the expansion of the universe will be still be far beyond even their means, but they will be able to create pockets of "reverse time" by flipping chronons from forwards to backwards within a set field. The problem is that anyone within that field would be subject to the backward chronons; they'd forget why they were in the field and even die because they'd be pulled back to a point before they existed. And since any light that went into the field would reverse, anyone inside would be essentially invisible to those outside. The field itself would be bright from the light bouncing constantly back and forth against its boundary. Or possibly it would appear similar to a black hole since no light within the field would come back out.
Our descendants will likely solve this problem by creating two fields in one. The person or object within the backward field would be in a smaller forward field, thus keeping themselves alive and sane. However, they'd still be subject to the forward chronons, and potentially die before they would get to their destination time. Even this could be circumvented, however, by accelerating the backwards chronons and speeding up the journey.
If our descendants could do this, though, why hasn't anyone seen them? The answer might be simple.
It may be impossible for anyone to actually be inside the field without dying; instead, the fields would merely act as "windows" to the past. Those in the future could observe, little more.
But it could be all they need.
100 thousand years from now, the Offshoots, Malanites, and Machines will have the ability to replicate brains down to the subatomic level. The rebuild will have the effect of making it seem like the person is living the same life twice. But since the pathways are being created exactly as they were before, the two "lives" will be seemingly simultaneous; the only clue being the occasional "déjà vu."
Once in possession of both time observation technology and brain replication tech, our descendants will realize they can observe any person or animal throughout history, scan their brains, and create an exact duplicate in their era. I may be missing a step or two, but the end result will be the same. Our descendants will be able to bring us from our time to theirs.
It won't only be the citizens of Earth who will benefit from this discovery. Any civilization in the galaxy lost to time will be able to be retrieved (this also means that if we go extinct before we can accomplish this feat, another civilization in the galaxy could potentially retrieve us).
The ramifications of this discovery will throw the galaxy into chaos. Billions of people and trillions (probably even quadrillions) of animals (to say nothing of any extinct alien species that may have existed) will suddenly enter a galaxy that will seem strange and even frightening. Some of our descendants will want to help the dead adjust and create rich new lives. Others will want to take advantage of our relative naïveté and use us for their own ends. Still others will want to keep us in the past, afraid that our backward views will contaminate the galactic status quo.
Despite the best efforts of the Galactic Demarchy and its allies such as the United Hindu Castes, the Wanderers of Buddha, the Malanite Kingdom, and elements of the Christ Divinity, Covenant of Stars, and Star Caliphate, war will break out. The forces of darkness (pardon the hyperbole) will attempt to wrest control of the Retrieval technology for themselves, while the forces of justice and peace strive to stop them.
The Retrieval Wars will last for decades, until the combatants finally come to a compromise. The Christ Divinity will retrieve Christians, the Star Caliphate will retrieve Muslims, the Covenant of Stars will retrieve Jewish people, and so on. The possibilities are almost limitless, since there will be civilizations based on all periods of history. Cavemen will be retrieved by those who have adopted a blend of technological advancement and caveman culture, for example. The Galactic Demarchy will retrieve everyone who does not fit into a certain group or those they think would not be happy in any of the other powers.
The Retrieval process itself will be implemented by machines, since they would never get bored or be swayed by emotions. Each machine would observe its subject from conception to death, recording the development of the subject's brain carefully. As the subject is studied, a chamber will grow a clone of the subject. The clone's brain will be developed in tandem with the original, until the moment of death. At that point, the clone will be awakened and carefully eased into his, her, or its new life.
I believe consciousness is nothing more than biology. I don't subscribe to the conceit that an exact duplicate — down to the subatomic level, down to the exact neural framework of my brain — of me isn't "me." The idea that consciousness is something to be lost despite exact duplication of the brain is as ludicrous to me as the idea that some invisible God will save my soul. Of course, if we each have a future machine chronicling our lives, ready to usher us into a new future life, I suppose that gives some credence to the idea that we will be saved by some invisible force. However, we will not get there unless we teach our children to value science over blind religion. We will not get there unless we ourselves work to keep our species going. I don't know for sure if any of this will come to pass.
But I can hope.