First published on the Steem blockchain by @remlaps on December 19, 2022. View the original post.
"Time travel offends our sense of cause and effect—but maybe the universe doesn’t insist on cause and effect." -[Edward M. Lerner](https://www.tor.com/2009/10/16/tropes-and-mundanity)
Introduction
Pixabay license from Genty at source. |
Today, Science Alert published the article A Physicist Came Up With Math That Shows 'Paradox-Free' Time Travel Is Plausible, which is an update of a 2020 article on the same topic. The article summarizes the 2020 paper, Reversible dynamics with closed time-like curves and freedom of choice by Germain Tobar and supervised by Fabio Costa. This is an Open Access paper (Creative Commons Attribution 4.0) in the journal, Classical and Quantum Gravity.
According to the paper, Einstein's theory of general relativity opened the door for the possibility of closed time-like curves (CTCs), which are paths through spacetime where an observer can interact with past or future versions of themself. In our intuition, however, this idea is challenged by the notion of the grandfather paradox.
The grandfather paradox is a potential logical problem that would arise if a person were to travel to a past time. The name comes from the idea that if a person travels to a time before their grandfather had children, and kills him, it would make their own birth impossible. So, if time travel is possible, it somehow must avoid such a contradiction.
Classical physics, and human intuition, both suggest that time moves in a single direction where past events determine the current state of the system, and where all parts of the system experience time in the same way. Relativity, however, argues that an event in a CTC can be in its own past and future.
One way to bridge the gap is to suggest modifications to quantum physics that could make CTCs possible, but this approach commonly leads to a "radical departure from ordinary physics." A second quantum physics approach has been the use of process matrix formalism, which describes events with an indefinite causal path. Remarkably, this technique has even discovered events in classical physics that are incompatible with a causal order between events.
A classical approach to dealing with the grandfather paradox has been to divide a model of space-time into distinct regions and examine the CTCs that cross between regions. Each CTC that passes through a region receives an incoming state, performs some transformation on it, and sends it out in a new state. Before the work by Tobar and Costa, it had been shown that time travel was logically permitted for up to three regions.
In the 2020 work, Tobar derives the math to show that the same is true for an arbitrary number of regions. In the paper, he summarizes the accomplishment as follows:
Our results show that CTCs are not only compatible with determinism and with the local 'free choice' of operations, but also with a rich and diverse range of scenarios and dynamical processes.
As explained by space.com, the grandfather paradox may arise as a misunderstanding about the nature of time and space. We tend to think of time as a series events traveling in a straight line with the past on one side, the present in the middle, and the future on the other end. We also assume that the same timeline is experienced by the entire universe.
As a result of general relativity and the possibility of CTCs, however, time could take other shapes, maybe including loops and branches.
In the Science Alert article, Costa elaborates with this quote:
Try as you might to create a paradox, the events will always adjust themselves, to avoid any inconsistency.... The range of mathematical processes we discovered show that time travel with free will is logically possible in our Universe without any paradox.
I didn't review all of the complex mathematics, but if I'm understanding their work right, this suggests that I can go back in time and kill my grandfather before he was born, but if I do, events would somehow rearrange themselves to avoid the paradox (I'm still not really clear on how... maybe by forking off to a new future or by providing me with a new set of grandparents?)
This news release offered another scenario:
In the coronavirus patient zero example, you might try and stop patient zero from becoming infected, but in doing so you would catch the virus and become patient zero, or someone else would
It should be noted that the paper closes with this, "Further studies will be necessary to find genuine physical scenarios realising the acausal processes we have discovered." So, for now, the finding remains theoretical.
As an aside, all of this reminded me of a 2022 series on Amazon Prime, The Peripheral, by the creators of West World. I don't want to give away any spoilers, but the series explores some interesting metaphysical concepts including concepts of time travel and also of reality as a simulation. In The Peripheral, the creators resolved the grandfather paradox with the creation of new "stub" timelines that fork off whenever someone travels through time and alters the past. My wife and I enjoyed the 1st season this fall, and I heartily recommend it - both as an entertaining story, and as a thought provoking consideration of time travel in science fiction.
Here is the trailer:
First published on the Steem blockchain by @remlaps on December 19, 2022. View the original post.
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