Testing a Time Machine

I am currently in the process of writing a script for a film about time travel. The first draft is complete, I put it to one side, and am now redrafting. I know time travel is a much used narrative device, but I like to think mine is original. One of the major criticisms people have of time travel films is that there are too many paradoxes and plot holes. I’ve tried to avoid this, but there are certain puzzles to which I don’t have any answers. I don’t think, with a story about time travel, that it is possible to have zero defects, but it is important to bring it as close to zero as possible. I just thought it might be fun to discuss how a time travel machine would be tested.

So. Where do we begin? I’m keeping this vague, so there are no spoilers for my screenplay (should it ever be filmed).

As a time traveller
I want to be able to travel back in time
So that I can effect change

OK. So now, we need to drill down into the actual acceptance criteria.

Given that I have selected a time
When I initiate time travel
Then I should be taken back to the time in question

Bit vague here. As we all know, the earth spins, and whips round the Sun at an alarming rate. Let’s take travelling back 30 years. Where you are stood right now won’t be in the same place 30 years ago. I think. I’m not an astrophysicist. Also, if you were to calculate it so that you landed in the same place, the features of that place may well have changed. So, there is the possibility that between now and 30 years ago, a tree has been chopped down, a building knocked down, or number 28’s bin is still out. Doreen would get a right shock if she went to bring the bin in and there was a time traveller in there. With a banana peel on his head.

A big issue with time travel in films and other narrative media is that changes made in the past would have such an impact on the present that the conditions that led to the time travel are no longer possible. However, what test cases would you need to prevent this? There would be obvious ones, like not being able to kill yourself, or the inventor of the time machine. So, what cases would be required to stop this from happening? Are they too numerous to consider? As QAs/Testers, should we be telling the world that test coverage would be so low as to make time travel dangerous beyond acceptable levels? What would the test cases and acceptance criteria be?

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Well, I think you’d have to shift your testing effort left, because there are considerable issues with some of the basic assumptions you’re making about time machines.

First, you’re discounting the “many worlds” theory of quantum physics, where any decision taken causes an entire new universe to be spun off from the existing time-line. This gives the solution to the ‘grandfather paradox’ (what happens if you travel back in time and murder your grandfather; do you continue to exist under those circumstances? And if not, how then do you travel back in time in the first place?). The consequences of actually subjecting this to a direct test are pretty substantial, personally; you would need to have existing evidence as to whether the ‘many worlds’ hypothesis actually holds true in real-life applications. If you’ve got to the stage of actually testing a practical and working time machine, we have to assume that this issue has actually been settled by others. This application of quantum theory has become more commonplace in science fiction over the past twenty years or more, though Bob Shaw’s 1968 novel The Two-Timers is one of the earlier explorations of this issue.

The question of dislocation in space has to be addressed in the operation of a time machine. Dr. Who’s TARDIS achieves this by travelling in both time and space; the process appears automatic and seamless, but that is doubtless helped by the TARDIS being semi-sentient anyway. The plot device of spatial dislocation in temporal mechanics was used by Gregory Benford in his1980 award-winning novel Timescape, where this was the mechanism used to send a message back in time to try to correct catastrophic environmental change; I can’t just now call to mind any other work of science fiction where that has been directly taken into account; Benford’s Day Job as a bona fide Rocket Scientist gives him the necessary chops in this area.

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Location troubles me most. A bit like the star trek teleporter problem, what happens when the planet is actually moving relative to some other measurement point, which is why the Stargate implementation which solves that problem is the only one I find plausible. So testing it would really require quite lot of equipment, and probably a large desert , or better yet a lake for soft landings.

Good luck Christian

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For the benefit of those not so enlightened, @conrad.braam’s ‘Stargate solution’ refers to the feature film Stargate and its extension as a quietly successful tv franchise, Stargate SG-1 (and spin-offs). In that universe, easy travel to other planets is facilitated by a network of fixed portals, or Stargates; it was established early on that they incorporated a sophisticated mechanism to compensate for physical drift between portals, and indeed this was sometimes used as a plot element.

Occasionally, under very unique (and rather hand-wavy) circumstances, the Stargate could act as a time machine; but this was generally driven by the needs of that week’s plot.

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Thanks Robert for getting this machine test idea going.
I suppose my physical properties problem aside, the proof that the time travel accomplished the desired customer outcome (journey) starting at the UX experience , post-travel medical checks. I’m assuming this is a product intended for human time travel, not for goods and services, although those are far more likely. That’s just my take, I’d want to be testing it with things like horserace cards and small bets. Just uncovered an Assumption: Is travel back and forth supported, or is travel limited to into the future and then back, or at all?

I think I’m starting to touch on 2 things here.

  1. How do we know the travel was correct , repeatable and robust. And can we accurately measure it.
  2. Boundary and edge cases , does the device have limit checks of size, weight of goods, and types of goods. To prevent travel outside of safe bounds.
  3. On the killing the inventor thing: If the device is limited to travel forwards only or travel forwards with a return ticket to the same point, that case is covered. If not that’s really a infrastructure problem for business continuity and the accountants and not legally an engineering concern? Probably the toughest one to answer.
  4. On the commercial side: Is it easy to buy a one way ticket at discount - I’m assuming a service here, not a consumer equipment. Another assumption to test.

Mostly I suspect I’d be having a load of fun sending self powering atomic clocks back and forth in it. Or sending a taperecorder to this church ASLAP organ playing
https://universes.art/en/specials/john-cage-organ-project-halberstadt - or maybe not.

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  1. You need to be certain that the time travel was accurate, too. In Connie Willis’ novel Doomsday Book, where Oxford University uses a time machine to send history students to research their chosen eras for real, one student gets into serious trouble when a miscalculation sends her back to the time of the Black Death instead of fifty years earlier.
  2. Reminds me of the time displacement device in the Terminator franchise; it can only send back living tissue, but the terminators get around that by growing human tissue over a machine substrate. I’d view that as a definite edge case failure. (Doesn’t quite explain the T-2000 in T2, but when the plot demands it…)
  3. In his novel Dragon’s Egg, Robert L. Forward gave us technical appendices giving the theoretical bases for building time machines, though they only permit one-way travel into the future. (Their maths works out, but the physical requirements, of being able to manipulate and build constructs made out of neutronium, rather put a dent in the practicality user stories.) Ray Bradbury’s 1952 short story A Sound of Thunder, about tourists who travel back in time to hunt dinosaurs, explores some of the other issues of this scenario.
  4. Julian May’s Saga of Pliocene Exile (The Many-Coloured Land and sequels) looked at one-way time travel for people who wanted to Get Away From It All and take up a simpler way of life. By going back way into prehistory, it’s assumed that they cannot make any changes that would impact the future. Things go wrong, of course, but only for the exiles who quickly come to regret their decision, because the Pliocene is already occupied…

Yes, the Halberstadt John Cage performance would make an interesting test case…

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One of the thoughts I did have for my screenplay was that the travellers go back, then are put in cryogenic storage until they reach their departure time. Then I realised there’d be more than one of the person at the same time, one in storage, one not. Then I thought about whether there’d be some sort of psychological impact of this sort of pseudo-cloning (ooh, can I invent the term chronoclone?). And the moral question about what do do with the “clones”. And then I had an idea for another story where a time traveller starts feeling more and more drawn out until he finds out that this is what’s happening and there’s a huge storage facility full of versions of him. But that felt too close to Moon. So I’m going to write a story about a dog instead.

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I don’t think we intended to break your Time Travel theatre idea at all. The idea behind theatre is that it is an exploration much like software testing often is, that uncovers a truth. Worrying about whether people think your production is a copy of another is probably not the best way of overcoming writers block either I guess. We wish you luck with your dog centric plotline. Actualise!

You didn’t - this was just a fun exercise, and I wouldn’t bin an idea based off a conversation on a forum. The script is still there, and I’m part way through the second draft. I just kind of like the dog idea more at the moment…

Your scenario has been touched on by Algis Budrys’ novel Rogue Moon, where an alien artefact on the Moon is being explored via matter transmitter - except that the matter transmitter works by making a highly detailed scan of an individual and then recreating them on the Moon from local materials. (This was 1960, you understand.) The original person here on Earth is untouched.

This is not a problem, because the alien artefact is highly deadly and kills everyone who tries to explore it. It only becomes a dilemma at the end because two explorers actually manage to go into the artefact and come out the other side unharmed - at which point, the question is raised “What do we do next?”.

But given that the problem is only posed right at the end of the novel and is left hanging, there’s a lot of room to explore your ‘chronoclones’ idea further.

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