I have a time machine in my garage and it works. I put an apple in it, close the door, set the timer, open the door to an empty machine, wait five minutes and the apple reappears. (Still tastes great too! Gotta love Honeycrisps.) I haven't sent anything back in time because that's more risk than I want to take on just yet.

Help me out, please. At this point, I just want to test my machine to see that it works beyond trivial examples. (To use industry parlance, I've moved beyond unit testing and want to start user acceptance testing.)

Things to test

There's a couple of things I want to find out before I put my poor fragile body into this machine to see where it goes. I don't mind testing with live animals but want to avoid cruelty to animals if at all possible.

  • Is there a way for me to discover, non-destructively, how my universe will resolve or prevent any paradoxes? I'm aware that there's potential for "earth shattering kabooms" which I would like to avoid since I, and the people I care about, live here.
  • Is there a limit to how far forward or backward I can go?
  • Can I or anything else go backward?
  • How could I tell whether I live in a multiverse, single timeline or diverging timeline universe? (This is the subject of a previous question)
  • Is the past inviolate or can I remember more than one version?

I'm looking for a checklist that will help me test my time machine. I'll discover the results for myself, I just want to know how I should go about getting them. Recommendations to just destroy that machine will be flatly refused. What kind of a mad engineer do you think I am to build something and not test it? Seriously.

Note to those tempted to vote to close for "actions of a single person": While the question is written in first person, the underlying question is generic to all time-machines.

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    $\begingroup$ Are some of your questions covered in this other question? $\endgroup$
    – Samuel
    Commented Jan 9, 2017 at 22:57
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    $\begingroup$ Point four: Send out automatic cameras to the same time in the future, and see if they come back with different pictures. Last one: Send out an automatic camera to the same date in the future repeatedly. The camera that came back last time will change the future it captures next time. $\endgroup$
    – Karl
    Commented Jan 9, 2017 at 23:23
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    $\begingroup$ Are you sure your apple remembers what happened to him in the supposed past or future? ;-) $\endgroup$
    – Karl
    Commented Jan 9, 2017 at 23:24
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    $\begingroup$ Here's a fanfic chapter hpmor.com/chapter/17 that describes an interesting experiment that may give you some information on how your universe handles time loops. $\endgroup$
    – Peteris
    Commented Jan 9, 2017 at 23:37
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    $\begingroup$ You don't want to risk yourself? Send someone else $\endgroup$
    – Separatrix
    Commented Jan 10, 2017 at 9:29

5 Answers 5


What do you need is a series of experiments. Sending small, unconsequential things are best.

For the fourth point (multiverse or not), I have two experiments (there can be more). Both require generating a random digit sequence using random.org, writing it in a scrap of paper, and folding the paper.

  • Send the digit sequence a minute into the future. If the digit sequence received is different, definitely multiverse, otherwise inconclusive.

  • Make sure that the time machine is empty. Promise to yourself to generate and send, 1 minute in the future, a digit sequence 1 minute to the past. Check the time machine, pick the paper if there's one, but don't read it. Generate and send the digit sequence as promised. Now compare the digit sequences (sent and received). There are a few possible results:

    • There was no paper waiting for you. Either it was lost on the travel, or it created an alternative future to be sent into (multiverse valid).

    • There was a paper, and the digits were the same. Either single universe or divergence universe is valid.

    • There was a paper, and the digits were different. Definitely not single universe.

An alternative to the previous experiment is to generate the digits before promising to send them. I don't know what difference it makes, though.

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    $\begingroup$ Why can you not read the received digit sequence before sending? The random number isn't influenced by your knowledge. $\endgroup$
    – Samuel
    Commented Jan 9, 2017 at 23:08
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    $\begingroup$ @Samuel That's what you think... Muahahaha. But seriously though, merely the act of knowing that there's a message from the future in the machine could subtly alter the time at which you sent it and as true random numbers are time dependent (i.e. they change as time goes on) any change in the delay between telling yourself you're going to send a message and getting the random sequence will change the sequence you get so you probably could get a different sequence without A multiverse. The only way around this would be to get the random number first and then send it, dodging a paradox. $\endgroup$
    – Samwise
    Commented Jan 9, 2017 at 23:35
  • $\begingroup$ @Sam Wouldn't that time difference be accounted for in the number that came back from the future? $\endgroup$
    – Samuel
    Commented Jan 9, 2017 at 23:38
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    $\begingroup$ @Samuel but wouldn't that mean that the number would be different depending on whether you read it or not because you sent a number and then before you sent it you noticed something was different and that caused you to send a different number... time travel makes my head hurt. $\endgroup$
    – Samwise
    Commented Jan 9, 2017 at 23:46
  • $\begingroup$ @Sam Wait. I now remember having this conversation, back when I went by Sam. I remember that the valid argument was if you're not in a multiverse, whatever delays occur are in the history of the number you received in the past. Then I remember you/me agreeing to that. $\endgroup$
    – Samuel
    Commented Jan 9, 2017 at 23:51

Most of the answers to date have dealt with single timelines or a multiverse. Actually the multiverse concept needs further refining because there a variety of multiverse concepts. This answer will deal with what the OP calls the "diverging timeline universe". This is more correctly called the branching timeline universe. It's also the most difficult time-travel friendly universe to test and be assured time-travel is safe.

This form of time-travel forms a timeline whenever a time-traveller arrives anywhere in time. Past, future or present, it doesn't matter. History branches from the time-traveller has arrived and presumably continues onwards until the end of time.

Testing this time-machine by sending anything into the past or the future, it result in precisely nothing. No slips of paper with random numbers, lottery tickets, apples or mangoes (some examples were in alternative timeline versions of this question) will materialize before or after being dispatched by the time-machine.

This does mean the operator of a time-machine cannot distinguish things are travelling into branching timelines or what they thought was a time-machine is a perfect disintegration machine.

The only way a time-traveller can be sure they exist in branching timeline universe is to travel in their own time-machine and see where it takes them. if it works, the time-traveller will be visiting different past and futures to his merry content. Until he decides to go home.

One consequence of this kind of time-travel is that it is effectively impossible to return to your timeline of origin. Return to your initial present and a new branch timeline will be created. It may look like home, but it won't be, and people in that initial present will never see a returning time-traveller.

Yes the time-traveller will experience many alternative branching timelines and many alternative versions of his initial present. But anyone in his genuine initial present he never see him return. Also, those branching timelines may only visited by one time-traveller in their entire existence.

This is one form of time-ravel that can never be tested in advance of taking the plunge and the time-traveller does it himself. The positive side is that there is no possibility of any form of causality violation. Time-travellers can never their own actual past. They enter other versions of their own past and make whatever changes they like, but this won't violate their own history.

Time-travel in a branching timeline universe will be soon discontinued. Time-machines will leave and never return. The probability of time-travellers arriving from other timelines, branching or not, practically zero. There may be time-travellers wandering like transtemporal Flying Dutchmen (irrespective of gender) who never return to their actual points of origin.

This answer has addressed one form of time-travel neglected by commentators and other answers. This form of time-travel is highly resistant to testing. Time-travellers be warned! What look's like a time-machine may, in fact, be a very effective and practical disintegration machine. There's no way of knowing in advance. Prepared to take a chance?

  • $\begingroup$ I’m afraid that’s a long comment but does not lay out a plan for testing to determine anything, as asked. $\endgroup$
    – JDługosz
    Commented Jan 10, 2017 at 9:23
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    $\begingroup$ @JDługosz What does lay out is an argument that travel in a branching timeline universe is untestable -- except in the sense that time-travellers will disappear from their original time and never return. This led to my amused remark that this type of time-travel could be indistinguishable from a disintegrator. $\endgroup$
    – a4android
    Commented Jan 10, 2017 at 10:27
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    $\begingroup$ I disagree slightly with part of the reasoning, such as "Also, those branching timelines may only visited by one time-traveler in their entire existence.". If traveler A arrives to time-line X, then it forks and you have X (in which A never arrived) and X' (where A did arrive). At some point later, then Traveler B might arrive at X' forking it to X' and X''. Even though it's near zero chance, it's still a non zero chance. Given infinite number of timelines, it should happen at least once. And people in X' saw one appear (if they noticed) and in X'' two appeared, so they will send travelers. $\endgroup$
    – Mrkvička
    Commented Jan 10, 2017 at 15:40
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    $\begingroup$ However, "Time-machines will leave and never return" is false - let's assume that I travel 5 minutes to past. Yes, there will be one timeline where I enter the time machine and disappear, but there will be also one timeline where I arrive and meet myself 5 minutes before my departure - and in this timeline, time travel will flourish. In effect, each travel to the past will appear as successful in 50% of the timelines, and there will be a single timeline where all attempts are successful, where "the coin never landed on tails". $\endgroup$
    – Peteris
    Commented Jan 10, 2017 at 23:18
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    $\begingroup$ @Peteris "However, "Time-machines will leave and never return" is" true. Your argument applies to different model of time-travel from one I discussed. Yours is closer to an ensemble of parallel timelines. Mine basically branched everytime a time-traveller arrived. This applies to past, future & present. There is time-travel model where time-travel migrates sideways across parallel timelines, always in the same direction, this would look like the one you postulate. You haven't proved my argument false because it's not the model you assumed. $\endgroup$
    – a4android
    Commented Jan 11, 2017 at 4:25

Digital camera tests:

Use a digital camera take photos around your laboratory, house, and outside. Download copies and then send the camera forward. After you receive the camera 1 minute in the future, examine the photos side by side and look for any discrepancies. If there are any discrepancies, this means that your camera came from a divergent timeline and that your original camera was sent elsewhere. If there are no discrepancies, I suggest you repeat this test a few times with various photos from around the world, just to be certain. Maybe even sent a video forward.

Sending things backwards makes my brain hurt. You could send the camera back with a note saying "take a photo, save a copy and send the camera forward", but then you would remember receiving that message in the past. You would have to promise yourself to send the camera backwards whether you remember receiving it or not, and hope that a new photo appears on the camera after you put it in the time machine.

For you, no time will have passed at all, and there are a few possibilities for what your will find:

a) There is no photo on the camera and you don't remember receiving a note, meaning it didn't go backwards.

b) You remember the message, and there is a photo on the camera that matches the one you saved a copy of...proof that you sent the camera back to the same reality.

c) You remember the note, and there is a photo that does not match the photo you have saved, meaning it could be from another multiverse.

These are just the tests I can think of off the top of my head. You would probably want to do each test several times to ensure that you're absolutely positive of a non-divergent timeline. You probably would want to try sending the camera over increasing lengths of time to play with the possibility that things change more drastically over longer lengths of time.


Attend Stephen Hawking Time-Travellers Party. To test moving forward in time without possibility of destruction, take a 3-week-long holiday. Tell everyone you will go to the desert for 5 days without access to phone, internet, mail, humans or any form of communication (not even a telegraph). Then on the first day of the trip, travel those said 5 days.

There shouldn't be any repercussion, as you were supposed to be "missing" for those days anyway.

Now - if you go by "everything that is in history happened", then you can't change anything. If you go by the multiverse theory, you need to reconcile with the fact that anything/everything create/move you to another universe, so there is no coming back.

If you go by "one timeline to change as you wish" with the moment of inventing a working time machine, you can see what the changes would be. If anything changes rapidly around you, it means the machine is working and you or somebody else changed time. If nothing changes, then you can assume you died and no one else used or discovered a time machine.

But the most critical question is: Did you invent a time machine or a space-time machine?


Sigh. Your mad engineer had some actual reason to invent and build this contraption. He had some actual theory it is build on. And, lo and behold, since the machine works, the theory probably has some resemblance to the reality in your purely fictional universe.

So what your engineer would actually be doing would be to either work on achieving his actual goals or in fine tuning or falsifying his actual theory. The two might be the same, of course. So you need to start by working out a draft of the engineers goals and theories then plan his actions accordingly.

He might want to be rich. Buying two newspapers every morning and sending one of them back in time a set number of days would be a good start. A comparison of the differences between the copies would give you more information than random numbers can.

And if stuff like sports results or stock values stays similar, you should be quite lucky with your finances. If the future is mutable, it would be fairly trivial to become very rich with patent applications. Famous too. You might be able to significantly accelerate the rate of technological innovation by sending yourself scientific papers and patent applications from few years ahead. This kind of meddling would also enable you to see if the changes propagate ahead.

And yes, you can do this stuff with random numbers but why would you? Not only is actual information more valuable, but different kinds of information should have different "temporal inertia", so you'd get more information out of it.


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