I've been reading on what current physics say about the possibility of time travel. There are three ways that you could create a closed-timelike-curve which would allow traveling back in time. One is a gravitational wormhole – made by combining a black hole and a white hole. Hard to do – because you need a LOT of negative energy to keep it open. We know negative energy can exist - we've MADE It in laboratories, but you'd need a LOT – for a warp-drive you'd need about 10-billion times hte mass of the universe worth (oops).
Then there is a magnetic wormhole – much easier to do (doesn't need negative energy) but it does need a shitload of magnetism and the length of the wormhole is inversely proportionate to how much you have. Any wormhole made with the magnets we got today would have to be about 150 lightyears long – and magnets all the way from one end to the other… expensive to build methinks. You could probably find short ones on neutron stars – where there is incredible magnetic forces, but since the gravity there will also squash you into a cube about 3mm on a side… not really a safe place to go for a time-trip :P
Option three is the most practical. Ronald Mallet discovered that light too can bend space-time, and light can be bent. Light at normal vaccuum speed needs a LOT of energy to bend, but we CAN slow light down, Harvard experiments have managed to bring it down to below the speed of sound using Bose-Einstein concentrate. True that exists near absolute zero – but it's already more practical.
A bent light time-machine is by far the simplest, make a circle out of it, and walk along the donut further and further into the past until you step out. It's still beyond current technology but out of all the options – the one which has the fewest practical problems to overcome, and most likely the closest to being practical to build.
All three however have one major problem – none of them will let you travel back any further than the moment of their construction. You can always go back AS FAR AS the time you first built it… but never earlier than that.
Bringing us to my point… I can't possibly be the only one who has noticed that this ENTIRELY solves the cumulative audience paradox and partially the grandfather paradox can I ? Why don't we have records of millions of time-travelers witnessing the birth of Christ ? Because time machines built say in 2020, can never go back to the birth of Christ. They are great if you keep them running for a while and you want to go back from 2065 to 2020 – but you can't GO back far enough. This solves the cumulative audience paradox for all historic events prior to their construction (and thus any historic events we can currently use to state that this paradox even exists).
As for the grandfather paradox – it rules it out entirely for the constructors (they are already conceived and born by the time they built it, they can't go back far enough to prevent it, and it only enters again if it's kept running enough for a future time traveler who was conceived AFTER it's construction to use it to prevent said conception happening. Of course, this may or may not be possible, either way to build and keep running such a device would in all cases be expensive and one can therefore imagine that is' use would be rather well regulated. Whoever i s paying ot keep it going over numerous generations will get to choose who can go back, how far, and for what purpose. Killing your grandfather probably won't get stamped by the managers who will take over
Still it does raise the less violent paradox version of a future traveler going back to right after construction and destroying the machine through sabotage, so it never ran long enough for him to have used it that far, so he couldn't have gone back in time to destroy it, but he did, so it did, so … solve that one !
Yeah, I really DO think about this sort of stuff for fun…