I'm working on a science fiction universe in which time travel exists, in a very limited form, but it's useless, at least it's the intention that it be useless. To that end time travel, on the rare occasions that it occurs at all, can violate apparent physical causality but not "Decisional Causality". This is, in the scheme of things, a minor detail but the concept creates some very interesting literary opportunities and rather strange problems within the world.

The idea of decisional causality is that a decision, once made, creates a point of departure that governs the events that follow; so rather than actions having to follow a purely cause-effect framework you can get cases of decision-effect-cause. What I'm struggling with is creating a hard and fast logical framework that defines a decision as having been made. In particular I'm interested in who can make a decision about dispatching a starship:

I've come up with several levels at which such a decision might occur:

Governmental level decisions are made by elected officials of one kind or another these are likely to be general.

Organisational level decisions are those made by admirals and CEOs. This is probably the highest level at which a specific ship will be named and assigned a target.

Command level is the captain of a ship. They will know exactly where they're going and it's their ship.

Execution level means the guy on the bridge who pushes the "go" button.

And there is also the matter of timing; since decisions can be made long before any action can be taken when is a decision then to be considered made? When the action is considered or when the person making the decision follows through and sets things in motion?

I know this is a bit metaphysical but what I'm asking is when and at what level should a binding decision logically sit within the causal narrative of the story?

  • Interesting premise. Are you saying that once a decision is made it stays made, so people can change history but it'll always come back to that decision being made somehow? So maybe in one timeline you interview and accept a job offer, and in another your interview is canceled out from under you but somebody there knows you and makes a direct offer that you accept -- so either way your decision to accept it stands? Something like that? Jul 25, 2018 at 15:25
  • @MonicaCellio It's more about trying to establish a point of decision that blocks any move further back in time, the short fiction Overdue is only one side of the coin, trips can be shorter than 0 time from point of departure but never take less than zero time from point of decision. The decision sets the limit for how far back you can go in time.
    – Ash
    Jul 25, 2018 at 15:32
  • This sounds like a really interesting system. I hope you'll consider writing about it somewhere. (The Worldbuilding blog would be one possible place, or maybe you have your own blog or some other place in mind.) Jul 25, 2018 at 15:40
  • @MonicaCellio I've been thinking about working up my setting notes into a rulebook of sorts for Federated Human Space so I may do a full write-up at some point, I'm working on a piece about the last jump of the McCarthy at the moment.
    – Ash
    Jul 25, 2018 at 15:45

5 Answers 5


Something irrevocable transpires.

I would say a decision is made when something irrevocable occurs. Words are spoken and heard, a button is pushed, a trigger is pulled, a letter is mailed, an email is sent.

Until then, it is only thoughts about a decision that might be made.

In Quantum Mechanics, we have this notion of a wave function, which describes all the possible ways that some set of sub-atomic particles can move and interact. However, some of these possible ways are mutually exclusive.

We also have the notion of wave function collapse, out of all these possible ways, if you try to measure some property of the particles, or any property of the particles, then instead of them being in all possible places with all kinds of energy, spin, momentum, etc, the wave function collapses and each particle is in a specific place, with a specifc potential energy, spin, momentum, etc.

Metaphorically speaking, decisions are like that. While the commander is mulling it over, he may mentally decide "Yes, I am going to launch at 5:00." But he doesn't tell anybody that, and this has no effect on the world, so in reality his wave function has not yet collapsed: He can change his mind and nobody would know, his earlier "decision" has not affected reality in any way, so it wasn't really a decision. An hour later something else occurs to him, and he tells his captain, "Launch now."

At that point something irrevocable has happened, he has changed the mind of the captain from "waiting for orders" to "orders received," and the captain is punching buttons and will launch. Even if the commander immediately regrets that order and rescinds it, that event cannot be undone with nobody knowing, the captain heard the launch order and his mind is forever changed by hearing it. Thus that decision was made. At that point, regrets and wanting to rescind that order would be thinking about making a decision; if it is voiced and the previous order rescinded, that was another decision, also heard by the captain, that cannot be undone.

Of course decisions at time X set one future in motion, a decision at time X+epsilon can set another future in motion.

But you have not really redefined cause and effect, the decisions are the causes of actions that change the world, however infinitesimally. And there will always be causes that have nothing to do with anybody's decision. One of the reasons we humans evolved is a ten mile wide asteroid crashed into the Earth and caused a nuclear winter that wiped out the dinosaurs, but our burrowing, scavenging ancestors survived. No decision of any sentient being on Earth caused an asteroid to crash into the Earth, and there are entirely plausible gravitational mechanics that allow random asteroids to be flung about in random directions within our solar system; and gravity is not "making decisions."

  • Cool that answers at least half the question and I already had a leaning towards ships' Captains on the other part of the issue so I'll go with that.
    – Ash
    Jun 4, 2018 at 16:49
  • It isn't entirely ships' Captains, though. When the commander issues the order, s/he made a decision to do that. If the Captain is not an automaton (i.e. without choice), s/he makes the decision to follow the order, that decision is made when s/he does something irrevocable; like push the launch button.
    – Amadeus
    Jun 4, 2018 at 17:10
  • Sorry Captain and pilot are two different things, the Commander is generally the Captain, but the pilot who flies the ship is usually a subordinate I'm thinking I'll give final authority to the ranking officer.
    – Ash
    Jun 4, 2018 at 17:18
  • Do the titles make any difference? The final authority makes no difference either, if a person is commanded to do something, they do not automatically do it. They are not machines. They must also make a decision to follow an order, or disobey an order, and that decision, too, is not made until they take an irrevocable action: Like deciding on their own and against orders to refuse to launch. You asked for a philosophical basis and this is it: Decisions have nothing to do with designated authority, all human brains are equal when it comes to deciding whether or not to obey an order.
    – Amadeus
    Jun 4, 2018 at 17:26
  • So you're advocating for an execution level decision, cool I can respect the logic. That combination makes the time travel mechanism super awkward but still workable.
    – Ash
    Jun 4, 2018 at 17:36

Have you considered having the act of time-travel be your decision-locking event? Under normal conditions, while all of us are riding along with the current of time, it doesn't matter whether recently past events are locked and immutable. We are all moving forward at a rate of one-second per second, so the past can safely be left in a free, unfettered state.

Only when a time traveler manages to slip out of the forward flow and jump backwards to a past decision point, does the universe have to defend itself against paradoxes.

So, in the interest of minimizing wasted energy with unnecessary defenses, the universe only locks the past as a traveler starts their backwards journey and only locks those moments which the traveler passes through on their way to their temporal destination. Upon arrival, any actions taken from the new current moment up to the moment of the traveler's departure from what is now the future, are locked and unchangeable.

So the traveler goes back in time but is powerless to change anything. He experiences the repeat passage of time exactly the same as when he first lived through it, with the possible exception that he might be able to think different thoughts while his body flawlessly repeats its original motions. The predestination would bind him right up to the moment when he originally pressed the button to start the trip. Then the defense mechanisms would destroy the time machine just to keep it from starting an eternal time loop. The time traveler is now back in the present, sitting in a smoldering time machine, but the fun doesn't have to stop there...

As an additional wrist-slap, the temporal defenses might punish the offending timeline in the hopes of discouraging future journeys. Any decisions made up to the moment of the traveler's departure could also be locked, even if their associated actions had not yet been performed in the traveler's original timeline. By aggravating the temporal defenses, the traveler not only forced himself to relive recent events without any hope of changing them; he has also forced everyone from his original timeline to experience a proximal future based on the strongly held intentions of decision makers from the moment of his departure. The traveler's attempt to change the past thus temporarily suspends everyone's normal ability to change the future.

This would not only make time travel useless, it would make it extremely dangerous for both the traveler and those left behind.

  • While this gives me some fascinating material for writing some time travel adventures in a completely different setting it doesn't help me with this particular setting and it's framework since I'm looking to establish an event threshold as a basis for the occurrence of time travel that by definition cannot cause paradox.
    – Ash
    Jun 4, 2018 at 16:42

To shift causality from the action to the decision to take action, you need to eliminate the possibility of disobedience.

At the execution level, the operator decides to press the button but has a heart attack and dies before his hand moves. His body has disobeyed the decision by dying.

At the command level, a captain announces an order, but the crew mutinies and chooses to do something else with the ship. The crew has disobeyed the decision of the captain.

All the way up through the levels, the opportunity for subordinates to disobey orders potentially breaks the link between decision and effect. Therefore in all of these cases, causality remains with the action, not with the mind which decided to act.

To change this, we need to change the nature of the actors. If an operator's body can fail to obey a mind's order, get rid of the body. Have the ship's operator be an artificial or digitized consciousness in the ship's main computer. If a crew can disobey a captain, get rid of the crew. Have the consciousness which runs the ship be that of the captain. Crew, when present have no ability to disrupt or subvert the ship's function. Since human captains can disobey their admiralty, lobotomize the captain's consciousness to the point that it is an absolutely obedient slave and as long as you are at it, fire the admiralty. Let the Emperor himself directly command his slave star ships by direct brain-to-ship download. The moment the Emperor decides to take action, his ships receive their orders and start obeying them. Now finally, we have a situation where the Emperor's decisions are causal to the effects of his fleet.

Now your time traveler can jump ahead of the approaching armada and even board the flag ship before its departure from the Emperor's space dock; but she still can't change the destination or actions of the ship, because in this specific case...

The Emperor's commands are physical law.


The simplest logical framework to achieve time travel which is effectively useless and very useless is one which avoids causality violations.

Sequences of events cannot be undone because they have happened. This essentially builds a degree of determinism into your fictional universe. Time travel will create causal anomalies, but no violations of causal sequences or chains and no meeting your earlier selves. No revisiting past events you have visited previously.

This will provide a framework for useless, limited time travel. It is can have a resemblance to your proposed decisional causality, but avoids the metaphysics of deciding what a decision is in any given set of circumstances.

  • But I like the metaphysics it creates interesting possibilities.
    – Ash
    Jul 25, 2018 at 14:14
  • 1
    @Ash There's always someone who does. The possibilities become more interesting when the metaphysics is better defined. Causality violation free time travel has its own suite of interesting possibilities. Plus many interesting challenges for the SF writer.
    – a4android
    Jul 26, 2018 at 4:43

So my solution to this problem was the rule that can be boiled down to the following: The Past is set in stone, the future is clay. The present is the kiln.

Basically, knowledge of a future event will automatically make anyone privy to that knowledge unable to change the past... those still ignorant of things to come may however change the past... and while very difficult, someone with knowledge of the future can lead those without on a course that will cement things differently. Essentially this led to a story where my time traveler, had to play the role of the villain and lead the heroes to discovering on their own the the cause of the troubled future and eliminating that threat without telling them. It's stressed that this is very difficult as his actions may only indirectly change the past... if there's a big red button that will destroy the world, he cannot destroy the button, no matter how hard he tries. He can only cause the button to activate... BUT it's perfectly legal for him to let the hero believe he will press the button and set up a situation where he is going to press the button, which the hero concludes is bad and destroys it before the time traveler has a chance to press it.

This also relies on a concept that time is somewhat aware of the violation of forward motion that is a time traveler and will become somewhat hostile to the traveler. This could be something that seems like a string of bad luck that gets progressively worse as the traveler gets near the decision point... this could also be just as simple as the traveler was there during the orignal event, and will always be there, but no matter what, is unable to stop the event... no matter how many times history plays out, it's always the same way. The traveler goes back, gets blocked by the crowd on the grassy knoll, causes a ruckas, the cops detain him, he is unable to get into the book depository, and Kennedy is shot. Always... every single time...

Then of course there is a comedy sketch I wrote where we learn that after about the third time a time traveler appears and tries to kill him, Hitler installs a guard who's job it is to stop time travelers from killing Hitler. He even sets up a number dispenser, has them all line up, and calls out "Number One... Number One..." until the traveler with the ticket steps forward... at which point he is shot and the guard begins calling for Number 2.

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