A Student Just Proved Paradox-Free Time Travel Is Possible

[Nevilledog]

https://www.popularmechanics.com/science/math/a34146674/paradox-free-time-travel-is-possible/

In a new peer-reviewed paper, a senior honors undergraduate says he has mathematically proven the physical feasibility of a specific kind of time travel. The paper appears in Classical and Quantum Gravity.

🤯 You love time travel. So do we. Let's nerd out over it together.

University of Queensland student Germain Tobar, who the university’s press release calls “prodigious,” worked with UQ physics professor Fabio Costa on this paper. In “Reversible dynamics with closed time-like curves and freedom of choice,” Tobar and Costa say they’ve found a middle ground in mathematics that solves a major logical paradox in one model of time travel. Let’s dig in.

The math itself is complex, but it boils down to something fairly simple. Time travel discussion focuses on closed time-like curves (CTCs), something Albert Einstein first posited. And Tobar and Costa say that as long as just two pieces of an entire scenario within a CTC are still in “causal order” when you leave, the rest is subject to local free will.

“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,” their paper concludes.

*snip*

[soothsayer]

1. Where should we go?? I mean when

[lagomorph777]

2. Some other place somewhere, some other time.

Some time after the Reptilican Party has gone extinct.

[snort]

16. This is a Sirius discussion

No Psychobabble.

[Nevilledog]

3. We should probably have a meeting and discuss it.

[ProfessorGAC]

6. I'm Going For Dinner In The 1920s

Men dressed up then, and I like dress hats!
When dinner is over, I'm coming back.
That no TV or computer thing would get old!

[Buckeye_Democrat]

4. Someone needs to go back to that damn escalator...

... that Trump descended a few years ago.

[smb]

7. Cut The Power; He'll Be Stuck There For Hours nt

[crickets]

9. 👆

[iemitsu]

5. Tobar, go back Tobar.

[Initech]

8. Can we go forward in time so we can find a cure for COVID?

[renate]

10. I think that's really interesting, and fun to think about, but I'm still puzzled...

At least as the authors explain it for the lay article, isn't there too much of a focus on macro events? I understand their example of how stopping the coronavirus infection of a specific Patient Zero could be possible, and that the time traveler (or someone else) would just end up being the new Patient Zero anyway because the pandemic was inevitably going to happen because history can't be changed by time travelers.

But what about the little micro events that they seem to dismiss that would still be changed? Why are they irrelevant just because they're small? I can think of two objections.

1) The previous-history Patient Zero would go on his or her merry way, not being sick, so they wouldn't be staying home on a certain day when, say, they gave a dying street dog their leftover meal. And that street dog lived, and reproduced, and a year later had a puppy that ran out into traffic and caused an accident that killed a woman who would otherwise have become the mother of the beloved teacher of a future president of China who would have taught her students about human rights and democracy, leading to a total change in global geopolitics.

2) Or maybe the previous-history Patient Zero was able to go to the grocery store and, because they had more energy to reach further, pick a certain apple rather than another one, leading to a whole other world in which Apple B was picked instead of Apple A. It's an insignificant difference to us, but a very real one. Even if the scale of the effects of this choice are tiny, there's still a world in which Apple B is chosen that is demonstrably, irrefutably different from a world in which Apple A is chosen.

It's only our (biased) assessment, not mathematics' or logic's, that determines whether choosing one apple over another is a smaller deal than who starts a pandemic. (To the apple, its being chosen or not is a bigger deal than that of who starts a human pandemic. The apple's worldview still matters, in terms of physics, even if it's tiny.) The start of a pandemic or the choice of an apple is not mathematically or logically more significant than the other when it comes to resolving a paradox. I don't expect myself to understand the mathematics in the paper--and I absolutely trust that the authors and peer reviewers are far wiser than I am--but I don't get how this new view of time travel could be anything other than a version of the many worlds theory of the cosmos but with a fun twist. Which doesn't really solve a paradox, since there's really no paradox involved in the many worlds theory.

Thank you for sharing this! I'm really interested in stuff like this and it's certainly fun to think about even though I'm still puzzled by what question this actually solves, if any. I'm sure my logic is riddled with holes so I'd love to read about other people's interpretations.

[Silent3]

11. I figured this out years ago, while working on an educational project...

...for teaching Einstein's Special Relativity.

All that was required was something that philosophically, but not mathematically, went against the central idea of relativity -- that you can't in any absolute sense tell how fast you're going, because speed is always relative to to a particular frame of reference, and frames of reference are supposed to be arbitrary.

But if you do have a special universal frame of reference anyway, and faster-than-light (FTL) travel or communication is tied to the special frame, then as long as cause-and-effect relationships always move forward in time in that one special frame, those CTSs you refer to (closed time-like curves) can't be formed.

In other frames of reference sometimes a cause will technically happen after its effect, but only when the cause and effect are so greatly separated that light can't cover the distance (more precisely, the "interval" ) between the two events quickly enough for causality paradoxes to become an issue.

[dslyahoo]

12. Kind of like...

The movie Sliding Doors with Gwyneth Paltrow.

[roamer65]

13. We already know u can travel forward in time.

Get near a black hole (but not too close) and stay awhile, then come back to Earth. You will get to see if humans actually survived or not.

I do believe that traveling back in time is near impossible.

[aidbo]

17. Im traveling forward in time right now. At a rate of 1 second per second.

[mshasta]

14. ...so go back and do nothing..just be an observer

...

[chriscan64]

15. Fred Trump is about to get a lot of company.

"Damn time travelers, knocking on the door every time I try to get busy with the wife!"

[tandem5]

18. Conceptually I think we've already known this for some time...

Absent of a true random choice that spawns a many worlds scenario, if the present you leave (to go back in time) is predicated on a past that exists because of a causality loop you created then not only is that possible it's requisite. The catch is you already know the impact of your past interference as a time traveler up until the present point that you leave to go back.

[intrepidity]

19. Didn't Netflix's "Dark" already prove this? nt

[Roland99]

20. One HUGE problem with time travel

If one managed to find a way to go BACK in time (Delorean or no), one would materialize in empty space

Without appropriately warping space to match the change in time, the Earth would be billions and billions of miles from where it was when the traveler left

[marie999]

21. I will considered it proven when someone does it.