The Encyclopedia Galactica has much to say on the theory and practice of time travel, most of which is incomprehensible to anyone who hasn't spent at least four lifetimes studying advanced hypermathematics, and since it was impossible to do this before time travel was invented, there is a certain amount of confusion as to how the idea was arrived at in the first place. One rationalization of this problem states that time travel was, by its very nature, discovered simultaneously at all periods of history, but this is clearly bunk.
The trouble is that a lot of history is now quite clearly bunk as well.
Here is an example. It may not seem to be an important one to some people, but to others it is crucial. It is certainly significant in that it was the single event which caused the Campaign for Real Time to be set up in the first place (or is it last? It depends which way round you see history as happening, and this too is now an increasingly vexed question).
There is, or was, a poet. His name was Lallafa, and he wrote what are widely regarded throughout the Galaxy as being the finest poems in existence, the Songs of the Long Land.
They are/were unspeakably wonderful. That is to say, you couldn't speak very much of them at once without being so overcome with emotion, truth and a sense of wholeness and oneness of things that you wouldn't pretty soon need a brisk walk round the block, possibly pausing at a bar on the way back for a quick glass of perspective and soda. They were that good.
Lallafa had lived in the forests of the Long Lands of Effa. He lived there, and he wrote his poems there. He wrote them on pages made of dried habra leaves, without the benefit of education or correcting fluid. He wrote about the light in the forest and what he thought about that. He wrote about the darkness in the forest, and what he thought about that. He wrote about the girl who had left him and precisely what he thought about that.
Long after his death his poems were found and wondered over. News of them spread like morning sunlight. For centuries they illuminated and watered the lives of many people whose lives might otherwise have been darker and drier.
Then, shortly after the invention of time travel, some major correcting fluid manufacturers wondered whether his poems might have been better still if he had had access to some high-quality correcting fluid, and whether he might be persuaded to say a few words on that effect.
They travelled the time waves, they found him, they explained the situation --- with some difficulty --- to him, and did indeed persuade him. In fact they persuaded him to such an effect that he became extremely rich at their hands, and the girl about whom he was otherwise destined to write which such precision never got around to leaving him, and in fact they moved out of the forest to a rather nice pad in town and he frequently commuted to the future to do chat shows, on which he sparkled wittily.
He never got around to writing the poems, of course, which was a problem, but an easily solved one. The manufacturers of correcting fluid simply packed him off for a week somewhere with a copy of a later edition of his book and a stack of dried habra leaves to copy them out on to, making the odd deliberate mistake and correction on the way.
Many people now say that the poems are suddenly worthless. Others argue that they are exactly the same as they always were, so what's changed? The first people say that that isn't the point. They aren't quite sure what the point is, but they are quite sure that that isn't it. They set up the Campaign for Real Time to try to stop this sort of thing going on. Their case was considerably strengthened by the fact that a week after they had set themselves up, news broke that not only had the great Cathedral of Chalesm been pulled down in order to build a new ion refinery, but that the construction of the refinery had taken so long, and had had to extend so far back into the past in order to allow ion production to start on time, that the Cathedral of Chalesm had now never been built in the first place. Picture postcards of the cathedral suddenly became immensely valuable.
So a lot of history is now gone for ever. The Campaign for Real Timers claim that just as easy travel eroded the differences between one country and another, and between one world and another, so time travel is now eroding the differences between one age and another. ``The past,'' they say, ``is now truly like a foreign country. They do things exactly the same there.''