One of the great imagined paradoxes of time travel is the question of what happens to you if you back in time and kill your grandfather before the date of your parent’s birth. Do you remember a past that no longer exists? Do you “wink” out of existence?
This sort of paradox is actually a very serious question in terms of thinking about the continuity of reality if it can be shown that there are particles that are capable of traveling backwards in time. (At least on a poetic level it’s well known that anti-matter might be a candidate for that, since in terms of a Fermi diagram description an antiparticle propagates exactly like the archetypal particle but with its arrow of time reversed.)
Relativity theory actually allows for the existence of Closed Timelike curves (CTC) that would, in principle allow a normal particle the possibility of going back in time. (You’ve seen versions of the idea in any number of Star Trek episodes.)
There’s a team of physicists based in Italy and Japan who have taken the idea, fleshed it out theoretically and even developed and performed an experimental test:
“In the new theory, CTCs are required to behave like ideal quantum channels of the sort involved in teleportation. In this theory, self-consistent CTCs (those that don’t result in paradoxes) are postselected, and are called ‘P-CTCs.’ As the scientists explain, this theory differs from the widely accepted quantum theory of CTCs proposed by physicist David Deutsch, in which a time traveler maintains self-consistency by traveling back into a different past than the one she remembers. In the P-CTC formulation, time travelers must travel to the past they remember.
Although postselecting CTCs may seem complicated, it can actually be investigated experimentally in laboratory simulations. By sending a ‘living’ qubit (i.e., a bit in the state 1) a few billionths of a second back in time to try to ‘kill’ its former self (i.e., flip to the state 0), the scientists show that only photons that don’t kill themselves can make the journey.
[…]To demonstrate, the scientists stored two qubits in a single photon, one of which represents the forward-traveling qubit, and one of which represents the backward-traveling qubit. The backward-traveling qubit can teleport through a quantum channel (CTC) only if the CTC ends by projecting the two entangled qubits into the same state.”
Read the full article here.
In other words, the only particles that can go back in time are ones that already have gone back in time – there can be no change to the continuity of history.
One of the key objections to time travel is that if it were possible, we’d be awash in “time tourists”. Sort of a Fermi paradox for time travel.
But if the only allowed travelers are the ones that weren’t here already… then that explains why my business plan that caters to the discerning time traveler isn’t getting funded from the future. Ah ha!