90% of the cosmology below is solid, 5% is "out-there" science and 5% is "out-there out-there" science.
So not impossible...
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Schwertberger's Multiverse
We are all time-travellers but don't know it.
Start by combining Linde's Eternal Inflation with Everett/Deutsch and the Multiverse - revealing a Swiss cheese honeycomb of bubble Universes making an infinitesimal Multiverse,
The Big Bang was not the "beginning".
Mersini-Houghton: "The moment you have a physical mechanism showing how the Universe was formed, you end up with a whole series of predictions of how that Universe may look at a later time."
Cosmologists now focus on realities prior to the Big Bang in a continuum of Big Bang, Big Crunch, Singularity, Big Bang, Big Crunch, Singularity... that might run to infinity in either "direction".
Incidentally, there will also be other Universes that slide to an "infinitesimally non-undulating spread of nothingness" - a bit like Yorkshire.
Deutsch is an Eternalist, with their slogan: "all points in time are equally "real"" (Kuipers) - a space realm containing every possible "now".
In relation to Eternalism stands the Cosmism of Fyodorov and Tsiolkovsky, and the Physics of Immortality of Tipler.
In the sense that Lewis meant it, there are a minimum of three options:
i) either "time" stretches infinitesimally in either "direction"?
ii) or there is the Mother of All Big Bangs and then on and on ad finitum?
iii) or it not only stretches infinitesimally in a continuum but that continuum is curved and joins up with itself in a loop?
Lewis would see i) and iii) as examples of Infinite Linear Chains...
...Universes that have lasted forever
But are they linear?
Is spacetime compatible with the "existence" of cause and effect?
In the 13.7 billion year history of our Universe, we were initially blown up by Inflation and nowadays all the galaxies are connected by a vast Cosmic Web.
With Eternal Inflation and the Multiverse, does a vast web of Universes cascade "outwards" joined by an Eternal Web?
And Tegmark theorises that there are four levels to this Multiverse...
...an Omniverse if you will - Multiverses in a Continuum.
Interference can exist at a quantum level between local Parallel Universes that are similar in form.
Hawking claims that there is a threshold on the micro-macro spectrum beyond which such interference cannot occur.
But there is nothing in Quantum Theory that suggests that we cannot interfere with our "other" selves elsewhere in the Multiverse.
Multiversal ripples between Parallel Universes.
And this would be time travel in the Everett/Deutsche sense - travelling between different parallel realities.
There are obvious links here with both Gödel's "any two points in history can be joined together" theory and the Quantum Theory of Entanglement which links the fates of two or more particles even if far apart.
So.
Instead of travelling back in time (in the Lewis sense) to aid Robespierre and Toussaint L'Ouverture, "create" a Parallel Universe where it is just so.
And, allegedly, there are more Universes in the Multiverse than there are atoms in our particular Universe.
So...
Time travel is most certainly logically possible in a range of ways.
In particular, we are all potentially time travellers between different Parallel Universes.
Haldane: "The Universe may not only be queerer than we suppose, but queerer than we can suppose."
Here on our little planet in our nearly flat, nearly fractal Universe, a gem in the Cosmic Web within the Eternal Cosmic Web, linked to the four levels of the Multiverse and the Omniverse...
What form might these interferences take?
Consciousness?
Omniversal Consciouness?
Horwich's coincidences?
And what is this time that we are attempting to travel anyway?
Julian Barbour suggests PlatonIa - "where all possible configurations of matter exist - there is no passage of time, merely a set of unconnected instants... Time doesn't exist. Time can't end."
Leibniz said time and space are relational and Universes are set apart by the nature of their spacetime.
Bousso/Ravelli would tell us that time is in our mind and not a basic physical entity.
Deutsch: "Objectively there is no present...Other times are just special cases of other Universes."
Time isn't a constant.
Time is invalid in random states.
Time prevents General Relativity and Quantum Mechanics combining.
There might be fractals of time across all different event horizons.
What about the Buddhist/Sophist "time as illusion"?
And how might tachyons "see" time?
Perhaps time. like mathematics, is only "true" across small slices of the Multiversal continuum and beyond there is spacetime and other complex contortions of reality in the Block Universe...?
Stuff happens...
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