The laws of physics do not prohibit travel in either direction but to travel back you need a device to link to two times and so it is impossible to travel back to a time before the time machine was built. There is speculation about the use of wormholes as time machines though. Of course you could travel back in time if you travelled faster than light which is also not prohibited but you just can't there there as you would have to cross the unobtainable speed of light barrier to get there. Particles could come into existence above the speed of light (tachyons) and travel backwards in time and so could be used to pass information into the past but tachyons are only hypothetical as they are not prohibited by relativity.FarliesBirthday wrote:I believe you: and the answer to the mystery might be closer to home, but just as strange...
There's a UFO researcher called Jenny Randles who points out that, where UFOs have been tracked by radar, - and that does happen from time to time, - they often "vanish" just beyond the earth's atmosphere. Supposing, just supposing that in the distant future, mankind can master time travel, as least for probes...
This would explain why there are so many UFO sightings: otherwise, why would little green men bother to keep covering the immense distances just to watch people fish (!).
But for earth historians - what would we give for a glimpse of Walker at Redmire (to maintain the angling example)?
Now, what does the edge of space have to do with all this?
Randles points out that, for a time jump, it's no use doing it on the ground, because you might end up in the middle of a tree or a building, at the end of the time journey etc. But in relatively open space...? That makes sense; - far less risky.
Ah, but time travel is impossible? Well we travel forwards all the time, of course; and at the speed of light, we would travel forwards pretty rapidly, compared to earth time, and - after a high speed trip of just five years in a space craft, we would find that hundreds of years had passed by on earth. This is a fact, according to the theory of relativity.
Apart from the problem of the paradox, which is overcome if we accept that our travelling back in time would split some time lines, creating different versions of the future, most scientists today (Snape?) seem to accept that travelling back in time is not impossible, so far as we know.
I think a better view of space-time is to see it like a field you want to cross when out on your morning walk. The conventional view of time would be that there is a clear path crossing the field which represents past going to the future. In this model if you were to travel forward or back you travel along the path. This gives rise to the grandfather paradox when you go back and kill your grandfather and so are never born to go back in time. However, my preferred model is that when we stand on the edge of the field there are an infinite number of paths (routes) across the field and none are the same. The routes which are close together are very similar and those further apart are very different. When I am halfway across the field I can move forwards and backwards but I end up at different places in the field (space-time) and so if I go back I do not return to my exact past and from that point there are an infinite paths forwards again. This allows movement within space-time and no paradoxes as if I go back and kill my grandfather I am not on the path where I started and whichever path I now move forward on it does not connect to where I started and therefore from that perspective I am never born. This means if you leave the path you are on to travel forwards or backwards there is no coming back to the exact point where you started. However the whether it is practically possible to move around like this is a completely different matter.
What this brings rise to is the mind-gurgling realisation that all possible pasts exist and all possible futures exist but we are only conscious of the moment we call 'now' and our consciousness is weaving a path from the past to the future and we retain memories of the points along the way we call 'past'. We exist in all pasts from after our birth and all futures. Yesterday and all other days which have gone do still exist getting to them from here, however is another matter.
This leads to a realisation that 'time' per se has no actual existence. Just infinite 'nows'.
See Prof Julian Barbour