Note: this question uses third-party worlds and events to illustrate the point, but it is not in any way about them, therefore this question is not off-topic.

I have had an interest in time travel. The idea of sightseeing historical events, watching a T. Rex hunt some duckbills, snapping a selfie at the signing of the Declaration of Independence, maybe pop over to the Caribbean and sail on the Queen Anns Revenge with Black Beard pirate ship. But of course, stories involving such trips usually warn about the dangers of inadvertently changing the past and causing paradoxes.

But what if that is all a classical case of Hollywood nonsense?

This is one of the implications of what I call the “One More Ball” thought experiment. It was originally put forth by SFDebris in his review of Five Days to Midnight when he asked what exactly would break down if the past were to be changed. What would be the worst that could happen if one murdered man changed his fate? (You can watch it here https://sfdebris.com/videos/tv/fivedaystomidnight.php, part 4 is where the thought experiment is.). For those of you simply interested in the Thought Experiment, allow me to summarize it here.

Let’s imagine that a temporal portal has been created. Nothing too fancy, just a window to 30 seconds into the past. And above the portal stands a robotic arm holding a ball. enter image description here Now to an outside observer what happens next should look like this. enter image description here A new ball appears out from under the portal and 30 seconds later, the robot arm drops the ball into the portal and that ball disappears. The ball has traveled 30 seconds into the past. enter image description here Now, this is logically consistent with what we understand of time travel. The governments of the world aren’t going to collapse and galaxies aren’t going to explode. But here is where things get interesting. Let’s put a sensitive plate just beneath the portal, like so. enter image description here Now if anything touches that plate, it will shut off the robotic arm, making sure it can’t drop the ball. Now, what happens when a ball appears and drops onto the plate? enter image description here Congratulations we have just created a temporal paradox. If the ball was dropped and landed on the plate, it would shut off the robot arm, preventing the ball from being dropped, but the arm couldn’t be shut off if the ball never landed on the plate, meaning that the arm should be free to drop the ball, so that it can land on the plate, preventing the ball from being dropped.

Now those of you who are familiar with Time Travel and its many headache-inducing situations may recognize this as the Grandfather Paradox. The past has been changed in a way that would contradict the events that lead to the past being changed. But the interesting thing is this shows that free will is not a factor in the paradox, because none of these objects are thinking or aware of what’s happening and yet a paradox has occurred. We have violated causality by preventing the ball in the robot's hand from going back, and we have violated the conservation of matter by producing an extra ball from nowhere.

Now no doubt all of you are putting together an explanation that keeps the rules in place. Perhaps bring the Many Worlds theory into it by saying there is one universe where the ball was dropped and never reappeared, and another where the ball landed and prevented its counterpart from being dropped. enter image description here

But perhaps all of these are under a false assumption. That the universe gives a crap.

There is One More Ball in the universe than there was 30 seconds ago. Then for all intents and purposes, the universe might just shrug and say, “so what? Deal with it.”

Now I understand that this idea may be a little hard to wrap your head around, but thankfully modern Cinema has actually produced an easy-to-follow example of how this could be applied. In Avengers: Endgame. I find it interesting that a few people have bent over backwards trying to shove this movie into the “Stable Loop” or “Alternate Universe” camp when the film clearly states that it isn’t playing by those rules, and in fact provides proof that they are playing by “One More Ball” rules.

There is a variant Loki who would go on and break the multiverse and not get his neck snapped by Thanos, as well as a duplicate Nebula dead and buried under the rubble of Avengers HQ, with no real effect on the current repentant one we know and love, a new Gamora who never went to Vormir and…vanished into thin air as soon as the battle is over, now that I think about it. Not to mention the extra Thanos who is now ash in the wind four years before he collected the stones and get his head removed by Thor. Steve Rogers even gets in on the action by going back to have a life with Peggy Carter, while his past self waits to be thawed out in 2011.

All these characters are now Extra Balls that are given no explanation of how they can be, nor does the universe make any attempt to rectify the situation.

Even in the events of the Loki TV series, all the TVA does is collect the Extra Balls and sweep them under the rug.

So if someone can travel back in time, and change the past with zero consequences, then this thought experiment tosses the rules of how we think time travel works right out the window. As a world builder and hopfull story teller, this also opens up new possibilties for time travel stories in general.

So here is the question, what are the implications and consequences of the universe following the “One More Ball” thought experiment?

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    $\begingroup$ Way too much text, and at the end you are asking "What if X happened?", which is clearly a no fit for our community. $\endgroup$
    – L.Dutch
    Apr 8 at 7:58
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    $\begingroup$ Sounds a bit like zero point energy (or matter in this case). I like the thought experiment, but it doesn't seem to fit here. $\endgroup$ Apr 8 at 8:01
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    $\begingroup$ Essentially the question asks what would happen if basic laws of physics, such as the conservation of momentum and the conservation of total energy, do not hold. Not even fundamental laws of philosophy hold, such as the rule that every effect has a cause and the cause precedes the effect. The answer is that this is a world with fundamentally different physics than ours, and even with fundamentally different metaphysics. Everything is different. There is no point of similarity and no comparison is possible. $\endgroup$
    – AlexP
    Apr 8 at 8:11
  • $\begingroup$ @AlexP actually those "basic laws of physics" dont need to be true. Such as Black Holes potentially deleting information and with it conservation of energy laws. Quantum mechanics already broke many of the "basic laws" we used to have and we are rewriting them every day. For time travel to be a paradox you have to accept that it is a paradox. $\endgroup$
    – Demigan
    Apr 8 at 10:51
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    $\begingroup$ @Demigan: (1) As far as I know the conservation of momentum is not contradicted by any experiment. (2) Conservation of information may or may not be a fundamental law. I don't know anywhere near enough physics to have an opinion, except to say that physicists obviously must have a very different definition of information from other disciplines. $\endgroup$
    – AlexP
    Apr 8 at 12:06

5 Answers 5


I don't see what is hard to understand. The ball drops, a new ball is created, and stops the original ball from falling. The experimenter has two balls (insert salacious joke here). Matter was created. No Paradox.

A variant of the Many Worlds possibility removes all paradoxes. Time Travel is a one-way ticket to a new timeline at the point of re-entry; the end result in appearance is the creation of matter at the time of destination; that matter being the time machine and the travelers inside it, and the cessation of matter in the original timeline, thus the total matter is constant. In truth, matter was just transported from timeline #1 to timeline #2, and their histories are consistent.

Thus if I leave my timeline #1 and go back to 1880 I must be in timeline #2, because my "time travel" did not happen in timeline #1. So if I kill my young grandmother in timeline #2, I will never be born in timeline #2, but I will not cease to exist in timeline #2. And I will already have ceased to exist in timeline #1, by moving. I can live out my life in timeline #2, but it is unchangeably altered. Even if I could find my way back to timeline #1, nothing will have changed there, in that timeline my grandmother was not killed, history happened exactly as needed for me to be born.

That will implicitly require an infinite number of universes at every possible increment of time; i.e. running parallel to our universe are other universes that represent every possible moment of history. But who cares, we can't even wrap our heads around infinite space, an infinite past or infinite future. What does infinite parallel universes matter?

For your "One More Ball" scenario, there are two timelines. in Timeline #1 the ball drops and vanishes into the portal. End of story. In timeline #2, running 30 seconds behind timeline #1, The setup is complete, the ball is held by the arm, another ball spontaneously appears falling out of the portal, and turns off the machine. There are now zero balls in timeline #1, and two identical balls in timeline #2. The second ball only moved from timeline #1 to timeline #2. Nothing else happens.

Except in timeline #2 the ball rolls off the table and the cat that was napping under the table darts out to attack it, while in timeline #1, the cat continues her nap, bored with this experiment.

Edit in response to a question; "Demigan: What are the implications of this?"

You don't have to worry about paradoxes. There are none. The confusion comes from thinking the time-traveler's history has changed, but it has not. I time travel to an independent exact copy of my world in 1880, when my grandmother was an infant. This is not my past, just a perfect copy of it. When I accidentally kill my grandmother, it affects the future of the copy, it does not affect in any way my own unalterable history in my original universe.

  • $\begingroup$ And what are the implications of this? $\endgroup$
    – Demigan
    Apr 8 at 11:00
  • $\begingroup$ @Demigan You don't have to worry about paradoxes. There are none. The confusion comes from thinking the time-traveler's history has changed, but it has not. I time travel to an independent exact copy of my world in 1880, when my grandmother was an infant. This is not my past, just a perfect copy of it. When I accidentally kill my grandmother, it affects the future of the copy, it does not affect in any way my own unalterable history in my original universe. I will add this clarification to my answer. $\endgroup$
    – Amadeus
    Apr 8 at 11:06
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    $\begingroup$ Well, if Amadeus' reasoning holds, there are two outcomes: either, this forking into multiple realities only occurs as a result of time travel, or... all possible outcomes have their own pre-existing universe. In either case, time travelers "ride" a multiverse, without causing paradox issues and time travel is applicable. If a method is found.. $\endgroup$
    – Goodies
    Apr 8 at 11:26
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    $\begingroup$ @Separatrix If you like you can imagine the extra forking universes are already there as some five dimensional multiverse doohickey. They are not created. $\endgroup$
    – Daron
    Apr 8 at 14:04
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    $\begingroup$ @Separatrix The question of "do you have free will?" turns into "what is a you?" $\endgroup$
    – Daron
    Apr 8 at 14:13

First of all, time travel is a fantasy in any realistic universe.

Second, if you read enough time-travel science fiction, you realize that there are dozens of different versions of how time travel works. That is part of the fun.

Third, a major part of the fun, at least for me, is to work through the implications of how people fit into a particular time-travel scheme. Bad time-travel fiction is riddled with internal inconsistencies. Great time-travel fiction is internally consistent, intriguing, and surprising; the reader says, of course, if X is possible, then Y must also be possible.

You seem to dismiss efforts to "explain away the paradox." I think that this is misguided. There is an old adage among lawyers that says that the law is anything you can convince a judge of. My adage is that good writing is anything that you can convince a reader to enjoy.

  • $\begingroup$ But what are the implications of the technology? You dont seem to actually provide an answer, more an extended comment. $\endgroup$
    – Demigan
    Apr 8 at 11:50
  • $\begingroup$ yes, by design. The question is as posed unanswerable. $\endgroup$ Apr 8 at 15:41
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    $\begingroup$ That is not what answers are for. Especially not when you dont present your comment as such. And this paradoxless time travel has many interesting implications specifically because it can focus on the technical usage of time travel instead of the beaten to death paradoxes, many of which arent paradoxical unless you present it as if it is paradoxical $\endgroup$
    – Demigan
    Apr 8 at 16:04

Well the whole idea of "the universe gives a crap" is stupid anyway. The universe does not care if you kill your grandfather or if all you do is hang a single particle in deep space, the universe has already changed the moment you did. Why would it ever care more for a paradox than for any other change to it?

The simplest answer is "the universe will continue with the exact same physics as before, but with an extra ball and all the consequences of that ball".

Now anyone who wants to exploit it will simply use it to duplicate things, like fuel for whatever powerplant you are using and food, building materials, soon not to be rare minerals etc. By changing the sensitive plate to stop the robot arm from dropping it for 30.1 seconds, you get a new ball every 0.1 seconds in your time. The biggest problem would be when the balls essentially clip into one another if you drop them "too fast"*. Although that would be essentially the same as matter quantum tunneling into one another, which means you cam create the ultimate cold fusion and also create any matter you want by fusing other matter in the correct sequence. Ofcourse rather than a touch plate you use an optical sensor to detect if/when something passes through the time portal.

You could duplicate people as well, which causes moral issues but also useful things, like duplicating important researchers. Although it would be easier to set up a system where you send back the completed research documents so you can focus on something else (and repeat the process). Essentially you would be able to research everything to the end of time instantly as long as you can create a system where it can be guaranteed to be send back. Any information would be send back as light and sound to speed up transmission (assuming 30cm very long storage devices arent used instead).

If 30cm aperatures are the limit (even if you ask for people to research better) then there's still no problem. A single molecule of a new material (say Graphene 2.0) can still be send and then duplicated over and over again. Send them at the right moment and right position and you can molecularly bond them together to form sheets or entire blocks of the material 30cm in circumference and as long as you can make it.

With infinite resources and technology that moves faster than you can keep up with you need to think of population, so you place some of these time portals in space and send back smart matter. A single smart matter particle is enough, you can duplicate it easily. This will then start producing space stations like Oneil stations, with which you put into orbits in the solar system. Generation ships can also be build, which will simply keep building itself larger and larger as they progress through space, dumping anything they dont need because why recycle if your waste will likely not impact anyone else?

You'll likely set up time farms, where thousands of time machines remain open with registered timeslots to prevent items arriving at the same time. These you use for the future to send technology towards so you dont have to make complicated promises. You yourself will periodically send that technology back too, including tech you just received as this will guarantee you will always receive material yourself too. Another time farm is for "now", letting you duplicate matter and create/fuse materials.

*alternatively the portal connects the two times, and the second ball will simply be blocked by the other ball, "resting" on it. However if a THIRD ball then tries to pass through that portal as the second ball tries to do the same, you still get a fusion.

  • $\begingroup$ That's closer to the H2G2 model where for obvious reasons, Time travel, by its very nature, was invented in all periods of history simultaneously. $\endgroup$
    – Separatrix
    Apr 8 at 13:48

You end up with time travellers but no time machines

What you describe is simply that the ball goes back and changes the past. In this case, the effect is that it doesn't go back - and that's not entirely coincidence. In order for your scenario to terminate, the time travel trips can't keep changing from one version of events to another. You only have a final draft of history when there's no trip.

The wider effect of this is that anybody who develops time travel will kick off a series of events that eventually ends with them not developing time travel, one way or another. Those events may involve countless people and machines arriving from the future and changing history - but if any of them go on to change their own history, you didn't see them in the first place ... so they won't.

You should be able to deduce a number of interesting things about future technology and even time travel itself. You might worry it will cause another time travel phenomenon but there's little point - if it will, then what you're doing now isn't actually happening anyway. Though some will argue that by not doing so, they reduce their risk of actually being in a temporary draft of events?


Conserve The Fundamentals

As long as energy as well as momentum is conserved there is no problem.

Make the process of sending something back to past very energy intensive. Say it takes some multiple orders of magnitude as much energy as the mass-energy equivalence of the object. You dont have to have such a highly inefficient machine but it would make it rare to use it so your story will still be tight.

Let Paradoxes Happen

Instead of worrying about a paradox turn it on its head. You can kill your grandfather in his childhood no problem (he he) you are already borned thats why you can do it.

Another Dimension Of Time

You can say universe has atleast one more dimension of time because of which such things are possible. Its like if I burn my razor after shaving my shaving is not undone. If universe had just one dimension of time then ofcourse one cannot go to past without undoing everything that comes in the way.

Note that you dont need multi-worlds for this.

Get Used To Things Appearing From Seemingly No-where

You also asked for effects. Well there may be stuff moving around in your universe that cannot be traced linearly back in time. People and objects coming out of seemingly no-where, disappearing to no-where (when moved forward in time).

Dont Travel Through Time

It wouldnt be like the famous weather effect and aging effect as shown in the latest Time Machine movie based on H G Wells novel. Its because as you showed in your ball experiment things dont move through time it just appear out of seemingly no-where.

You Can Duplicate But Only Once

You wouldnt be able to have more than two yous at any time. If you bring to now a you from a hour in future you cannot bring another you from two hours in future because there is no you then (you cut that possibility when you bring the you from one hour in future). If you make two time jumps in sequence you still cannot duplicate you more than once.

If you from 2010 jump to 2000 then you from 2005 also jump to 2000 then as soon as you from 2005 make the jump the you from 2010 cease to exist. Not just in 2000 but at any time in the linear dimension. The you from 2010 that went to 2000 may said to exist in the second dimension of time but it cannot be confirmed. Its after all a dimension of time not linked with a space dimension. Only after you from 2005 go back to his time the you in 2010 come back from the second dimension of time to our dimension of time.

The you in 2010 wouldnt remember his existence in second dimension. For him it would feel like nothing happened, time machine didnt work.

You Cannot Visit Same Place Twice

Soon you will find out that there are certain intervals of time you just cannot go to. Actually you did but was just bounced back. Since you didnt remember anything you think you didnt go. It would be intervals your earlier self has already visited using a time machine.

This is very important to keep the story existing. The you from 2005 can die in 2000. Then you from 2005 onwards dont exist, have just vanished. You dont even have to die to have that. You just cease to exist forward in time as soon as you jump to past, till you come back.

You Can Meet Yourself In Past

If you meet yourself in past nothing happen. You can shake hands and hug as much as you want.

Who Built That Time Machine Anyway?

You can bring information from future. No problem in that. You can bring back the sketches of time machine to your past self. He then make the time machine. Future changes. You no longer invent the time machine in future. Who then created the time machine? You ofcourse. When did you create it? In a time thats gone now. It still exist but in another dimension, the second dimension of time.


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