How do multiple time dimensions work?

[Vector_Graphics]

Let's imagine 2 time dimensions, or even 3. How would that work, exactly? And how would a being in such a universe perceive the "passage" of time (if such a concept would exist)?

[steelpillow]

Good question. There is no one answer, one can propose various schemes - and speculative physicists sometimes do. Can one change dimension or do we move along all of them at once? The biggest issue is, how does cause-and-effect in one time dimension affect it in another? Can we enter paradoxical time loops which take us backwards in time?

For example, could you shift to a different one, have a grand adventure, and then return to your original one at the moment you left it? This was the model adopted by CS Lewis for Narnian time. In his earlier, unfinished novel The Dark Tower he had explored more complicated possibilities for "Othertime".

In some schemes you can hop dimensions so that when you return you are in your original timeline's past and can kill your grandmother. Stephen Hawking came up with the temporal protection conjecture, that such paradoxical time travel would of necessity break the laws of physics (whatever they might be) and so cannot exist. Shame, as it is a popular theme in SF stories.

Google for the "Time Cube" and you will find an incomprehensible piece of new-age foolery. And so on.

Physicists will always point to the maths. Are these dimensions real, imaginary or complex? How many spatial dimensions accompany them? Hawking favoured an imaginary dimension for Time at the Big Bang, rotating to become a real dimension today. He did not figure out the rotation, but mathematicians would expect it to take place in the complex domain. Meanwhile Nobel physics laureate Roger Penrose (a mathematician by profession) points out that the +++− metric signature of Relativity can easily be converted to −−−+, making space imaginary and time real, and it makes a lot of the maths easier that way. So maybe Hawking had it backwards. Others have proposed various mathematical models for a two-time physics, with varying responses to the causal paradoxes raised.

Mystics always point to the mind. Can our consciousness travel in other, perhaps higher, time dimensions? Schemes vary in the number proposed, according to their breakdown of spiritual levels. Ouspensky proposed a circular model, in which each cycle round connects to the next level up, so you end up spiralling through time like the spring out of a biro. JW Dunne proposed an infinite serial regression of levels of both time and consciousness, each demanding a higher level in which to experience time passing in the one below. None got their maths together, though Dunne at least tried to.

Please do not regard this ramble as exhaustive!

[Vector_Graphics]

> such paradoxical time travel would of necessity break the laws of physics (whatever they might be) and so cannot exist.

What laws of physics would they break, and what if we allow said laws of physics to be broken that way?

[steelpillow]

> such paradoxical time travel would of necessity break the laws of physics (whatever they might be) and so cannot exist.

What laws of physics would they break, and what if we allow said laws of physics to be broken that way?

Conservation of mass and energy would be broken for a start. A persistent wormhole might be able to pass a sustained charged particle stream, violating the conservation of electric charge too. The foundational principle of thermodynamics, that causality is defined as the effect following its cause in time, would be violated. The theorems in quantum physics that ban the passing of information outside the source's light cone would be violated. Hawking may have had others in mind too, though there is not a lot left to violate.

It is is a theorem of formal logic that: if we allow any inconsistency whatsoever (such as the grandmother paradox) then with a little ingenuity it is possible to allow anything at all - including the opposite of what you just allowed. There could be no laws of physics any more, the Universe would revert to primal chaos, which medieval theologists imagined was its state before God let there be light. Such broken systems are sometimes used to demonstrate the falsehood of some proposition, by using it to "prove" some absurdity - an argument known as reductio ad absurdum. Schrödinger's cat, Boltzmann brains, doppelgangers and the like are often raised as such absurdities, with much ensuing argument as to whose other assumptions are the broken ones here. So if you do break some law of physics, you really do need to supply another one - one which is consistent with all the others you have kept. Theoretical physics has rather a lot of that going on at the moment, thanks to the many inconsistencies and gaps between theory and observation which have arisen in recent years.