Last night I had the realisation that time travel backwards is impossible. The idea came from my animation software, Argus, which needs to calculate each frame and can easily move forwards but struggles to move backwards (I essentially have to go back to the start and recalculate everything). My music software has this problem too, and so does digital video - try playing a film backwards on your computer and the problem will become tangible. These things find it easy to move forwards but struggle going backwards because more information is needed.
A celluloid film strip can move forwards or backwards with equal ease, but the film itself is an extra source of information, a duplicate 'buffer' which is not strictly necessary, it is, after all, a second copy of what happened in real life and this second copy amounts to extra energy and information. This is the extreme amount of the extra data needed for 'frictionless' backwards time travel, a complete record, but even a tiny amount of moving backwards takes some extra data, extra information and/or energy (information and energy are synonymous), and this itself is not efficient to store if we are content with moving forwards.
When moving forwards, things can be calculated step by step, but, occasionally, information is overwritten or corrupted or lost. To move backwards, this would need to be restored. It should be possible, in localised cases, to allocate this data; set aside a 'buffer' for the information and so permit backwards time travel in a small region, but this will have some expense of energy, and this expense makes it impossible for the whole universe to move backwards. The key part is that even if it takes any energy at all, it is more difficult to move backwards than forwards, there is a sort of 'temporal resistance' to move backwards.
We simple humans may have a memory of the past, but it is flawed and damaged, imperfect. A film camera too stores imperfectly (it can only see what is in the frame, and perhaps with added grain), and its film deteriorates. Digital data also deteriorates, machines can be damaged or lost, and those too can't perfectly capture the universe. Every aspect of our knowledge of the past is imperfect; it's not possible to store a perfect copy of the past, the whole past, clearly, as this 'magic photograph' would take as much space as the universe itself to store. Anything less would be an imperfect copy. All of these things ensure that the past can't fully exist once we have experienced it, and that as it decays it will become more and more obscure. It is this information lack that prevents the restoration of past data which would be necessary for backwards time travel.
To travel backwards is, after all, an experience of lost data. We could travel to ancient Rome and see how the city looked then in its perfect finery, but where is that perfect finery stored? It may have been there once, but if we are to visit it, it would need to be restored for us, yet, that data is lost. The past itself is only as real as its present level of decay.
Now, perhaps, as I do with Argus, we could re-run the universe from the start and re-calculate everything. This might be possible, but, of course, it would take the same energy for this second run as the first, and the perhaps same time too. If we wanted to see Rome in its finery, we might have to start with the big bang and wait a few billion years for that glimpse.
Forwards time travel is quite possible, we could, for example, freeze ourselves and be thawed out in 1000 years, yet, naturally, never coming back.
One day I must work out the complete implications of this.
