Doing research into my time braking world I was referred to Split Second by Douglas E. Richards. It turns out he conceived of the exact same machine that I did, however he handled the paradoxes differently, and uses a slightly different explanation involving “dark matter”, turning his machine into a fancy copying machine to make an evil baddie rich. But here is the explanation I am trying to describe in my world, which presents a language paradox that is the goal of this question. So this question is hoping to find technically correct language to describe the effect:

“So Aaron’s question is this: Say I decide that in an hour from now, I’m going to send my phone back in time an hour. The hour passes and I press the button, as planned. Because I did this, an hour earlier, a cell phone magically appears in front of my earlier self. To make this as easy as possible to picture, imagine the phone appears right next to the earlier version of itself. Cool, the me in the past thinks when the phone appears. I must have sent it back from the future like I was planning. Now I have two phones.”

Ok, so now we have a cell phone that has appeared because you will in the future send one back. But here is where it seems to leave science and enter magic:

Knight arched an eyebrow. “But now, what if the me in the past changes his mind? What if he now decides not to send it back, after all? Now what happens? Does the second phone disappear the moment he makes this decision, like a photo of Marty McFly and his siblings? Or does it stick around? And what if the hour passes and I really don’t send my phone back? Do I still have two phones? And if so, how is this possible? After all, in this version of reality, I never sent it back, so how is it still there?”

“Which would be the single timeline theory of time travel,” said Jenna. “The one in which this type of paradox is possible. However, if a version of the chronology protection conjecture were operating, the phone would never appear in the past in the first place, as long as the universe knew you were going to change your mind and not send it. Or, if it did appear, you would send it back after an hour passed. Nothing could prevent you from doing this, including changing your mind.” Knight shook his head in wonder. “I have to say your grasp of time travel theory is truly impressive.”

“So what’s the answer?” said Jenna, ignoring him. “What happens?”

“What happens is that you can change your mind,” said Knight. “And the second phone remains anyway, even if you never send it back.”

Now mine works a little bit differently but this paradox is still there, I need to contend with the vernacular. So here’s the chronology that I’m trying to put into intelligent terms:

  • t = 0:00 A time machine exists, or it will when you plan to use one.

  • t = 0:01 You “decide” you want to send your phone back in time.

  • t = 0:10 A phone appears next to you, it’s the same phone that is in your pocket. Not a copy - the same phone.

  • t = 2:30 You took a nap, got up and made a sandwich completely forgetting to send your phone back in time. Oh, shoot! You actually forgot to build the thing! But, in your pocket, two phones ring reminding you to send the phone back in time. Fair enough.

In the end, you have two phones by sheer force of “opinion” (because will is the wrong word). You never used a single milliwatt of electricity. You never even turned the time machine on. In fact, if you decided to blow the machine up before you even used it, you still have two phones.

The event is called “retro-causality” or “backward causation” on Wikipedia. But the bottom line is that an effect simply happened because it was thought about. There is no escaping the fact that this also fits the definition of supernatural: departing from what is usual or normal especially so as to appear to transcend the laws of nature

The laws of nature, in any sane interpretation, do not allow things to exist “because you thought about someday using a time machine.”

To be clear, this is NOT the Stueckelberg interpretation of the positron as an electron moving backward in time as a negative-energy solution to the Dirac equation. In that quantum physics equation, the electron moves forward, becoming a positron moving backward. It’s not “the same phone.”

What sane scientific terminology exists, if any, which describes “the existence of something only because at one point in time you wanted it and somewhere, a time machine may exist to send it to you”, and what are the “copies” technically called?

To me, it’s “God” or “Magic.”

But “Retrocausality” is NOT the technically correct term, as that only applies to time-symmetric systems (which are mathematical models). This scenario is obviously not time-symmetric (the two versions are traveling the same direction in time, in different [x,y,z] coordinates, and do NOT converge to a causal event later).

Is there even a scientific word for the supernatural/paradoxical phenomenon described here? The phones are not “clones” or “twins,” they are the same. Is this an “entangled” pair?

Backdrop info (this content is not part of the question and does not change the validity of any answer)

What made me say “God” or “magic” is because this case is exactly what was written by three different people in the Bible - the miracle of the Feeding of The Multitude:

16Jesus replied, “They do not need to go away. You give them something to eat.”

17 “We have here only five loaves of bread and two fish,” they answered.

18 “Bring them here to me,” he said. 19 And he directed the people to sit down on the grass. Taking the five loaves and the two fish and looking up to heaven, he gave thanks and broke the loaves. Then he gave them to the disciples, and the disciples gave them to the people. 20 They all ate and were satisfied, and the disciples picked up twelve basketfuls of broken pieces that were left over. 21 The number of those who ate was about five thousand men, besides women and children.

(Mat 14:15-21)

It occurred to me that if at any point in history at all Jesus (or his Father) had access to some form of backward time travel, this is exactly what would happen. He could look up, think about sending some fish back, and reality itself would create the fish and bread. The time machine doesn’t need to be anywhere nearby or even exist at that time. Hence, the phone is also a “miracle.”

  • 3
    $\begingroup$ NTSC&TVP is well established in fiction (the Non-Time-Symetric-Causality and Thermodynamics-Violating-Paradox), multiple memos relating to it have just simultaneously turned up on my desk, twice. $\endgroup$ Nov 1, 2019 at 2:51
  • 2
    $\begingroup$ Since time mashines are at and beyond the edge of science (and far beyond practical science), I don't think any strict scientific terminology exists on this subject. $\endgroup$
    – ksbes
    Nov 1, 2019 at 12:09
  • $\begingroup$ This reminds me a lot of a question I answered on a PH.SE a while back. philosophy.stackexchange.com/questions/57093/…. While I don't know of a scientific term to describe it, Scenario A explains how and why you could do this without creating a paradox. $\endgroup$
    – Nosajimiki
    Nov 1, 2019 at 20:27

6 Answers 6


First of all this isn't, strictly speaking, a case of retro-causality, it's simply a standard piece of time travel. There are three possible general solutions to explain how this can happen. Each solution depends on the nature of time. And, yes, each solution has its own nature of time. You can think of them as different models of time and causality.

You have been thinking of using your time machine to send your phone back to yourself.

(1) The Deterministic Model of Time and Causality

At 09:00 am, a phone appears in your time machine. Your phone is still in your pocket. Now you have two phones. Each are exact copies other, except one phone believes its one hour ahead of the other.

At 10:00 am, you will place your phone, the one that believes it is 10:00 am, not the one that is at 11:00 am, into your time machine send it back to your earlier self.

Determinism means you have no choice. You will send your phone back in time one hour. Nothing you can try to do will prevent this sequence from occurring. If you take a nap, you will wake up and make it to the time machine to send the phone back.

This is a simplistic description of determinism. No causality violation is possible. The scientific term for this form of time travel, the nature of deterministic time, and causality is Novikov self-consistency principle.

The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture and Larry Niven's law of conservation of history, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s. Novikov intended it to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general relativity that contain what are known as closed timelike curves. The principle asserts that if an event exists that would cause a paradox or any "change" to the past whatsoever, then the probability of that event is zero. It would thus be impossible to create time paradoxes.

(2) Multiple timelines

The multiple timeline model also gets rid of any paradoxes.

09:00 am. The phone arrives from the future. This time the phone comes from the future of another identical (almost) timeline. Now you have two identical phones. Again the times they tell will be one hour different.

10:00 am. You stand in front of the time machine and decide not to send one of the phones to a version of your self in another version of your timeline. In fact, you could either no phones or both phones or any one of the two phones to that other version of yourself in another version of your timeline.

(3) Novikov Self-Consistency Revisited.

09:00 am. You receive your phone from the future. OK. It's not showing the right time. So you reset its clock to tell the right time in your current present. This time you keep track of which phone is which.

10:00 am. You place the phone you know you received from the future, in the time machine. It now shows the time is 10:00 am, and send it to yourself one hour earlier.

What we can call the "second phone" only exists along a causal loop where its existence is purely determined by the fact that when it first appears at 09:00 am and will disappear at 10:00 am when it is sent into the past. This phone exists along a closed causal loop.

Igor Novikov, who writes about such things, calls something like the closed causal phone of this example a "djinn". These are objects which only exist along closed causal loops. Novikov has much niftier example of an ancient spaceship and its looped fate.

Put simply, there are several models of time and causality. Each one of which yields different explanations to sequences of events involving time machines. But be warned, if you live in a deterministic universe things will become extremely bizarre to enforce the maintenance of a deterministic sequence of events. For example, nothing you can do can prevent your phone being sent back in time. The phone could spontaneously fly into the time machine and send itself back. I will leave other exotic possibilities as an exercise for your imagination.

  • 2
    $\begingroup$ The Legacy of Kain series has an interesting take on a deterministic model of time. Everything is predestined and you cannot escape fate. In fact "fate" is just the natural continuation of events. The easiest way to explain is is a tape - if you rewind back, the following segments are what one would normally call "fate", since it's all predestined/predefined. Even timetravel doesn't interfere with this. However, in few edge cases this timeline can be broken - it then readjusts itself to take the new course of actions into account and sets it in stone. $\endgroup$
    – VLAZ
    Nov 1, 2019 at 11:22
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    $\begingroup$ The "looped fate" runs into enthropic problems. That phone is not the same after existing for an hour. We think it looks the same, but we exist in a ridiculously steep entropic gradient where everything is "decaying" and recording everything it experiences. The paint on that phone sent back is slightly faded, compared to the one that arrived, as an example of this. You cannot cross the same river twice. $\endgroup$
    – Yakk
    Nov 1, 2019 at 19:42
  • $\begingroup$ @Yakk I would have agreed with your entropy argument. Recently I came across the proposition that entropy is reversible both in classical & quantum physics. Sending the phone into the past & it will arrive at lower state of entropy than its state when it was dispatched. It then progresses to that final state. The tricky bit is the phone only passes through the loop once. $\endgroup$
    – a4android
    Nov 1, 2019 at 23:08
  • $\begingroup$ nit: Modern cellphones can pick up the current time from cell towers. Whether or not this causes the displayed time to update is a different question (there are rare/difficult attacks available if the network time is blindly trusted). I have no idea what would happen to actual cell communications in such a situation. $\endgroup$ Nov 1, 2019 at 23:42
  • $\begingroup$ @Clockwork-Muse. Good point. That might save the time machine operator from resetting the time on the phone from the future. Thanks for that. $\endgroup$
    – a4android
    Nov 2, 2019 at 0:15

I believe that you're misunderstanding how this works. Or, misunderstanding how people understand how this work, I'm not sure there's a practical difference when discussing theoretical concepts like this which probably can't ever occur. Anyway, to quote you:

The event is called “retro-causality” or “backward causation” on Wikipedia. But the bottom line is that an effect simply happened because it was thought about. There is no escaping the fact that this also fits the definition of supernatural: departing from what is usual or normal especially so as to appear to transcend the laws of nature.

The laws of nature, in any sane interpretation, do not allow things to exist “because you thought about someday using a time machine.”

The event isn't happening because you though about it. That is, as you put it, not a sane interpretation of the law of nature. More accurately, the event isn't happening because you've thought about it - let's go through the chain of events.

  • t = 0:00 A time machine exists, or it will when you plan to use one.

  • t = 0:01 You “decide” you want to send your phone back in time.

  • t = 999:99 After making long and arduous processes, you've somehow developed a time machine. (Good for you!) You send your phone back in time.

< rewinding cassette noises >

  • t = 0:10 A phone appears next to you, it’s the same phone that is in your pocket. Not a copy - the same phone.

  • t = 2:30 You took a nap, got up and made a sandwich completely forgetting to send your phone back in time. Oh, shoot! You actually forgot to build the thing! But, in your pocket, two phones ring reminding you to send the phone back in time. Fair enough.

You have the second phone, not because you decided to do it by opinion, but because you actually built the dang time machine and sent it back. By do so, you've also erased the fact that you need to send the time machine. So your event in the future does two things: 1) Send the phone back and 2) Rewrite causality and stop the need from setting the chain of motions into events which sent the phone back. That is to say, merely deciding to send the phone back would do nothing without the fact that the decision would cause this event in the future. Hence, 'retrocausality'.

  • 1
    $\begingroup$ I love when the opening sentence answering a time paradox has the words, “it works”. But great point. Pondering... $\endgroup$
    – Vogon Poet
    Nov 1, 2019 at 11:45
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    $\begingroup$ This - just "deciding" to build a time machine doesn't mean I could. Even if I had the knowledge & the materials it may or may not work (ask any programmer who "decides" how to fix a bug). Somewhere/when in the multiTimeUniverse there would have to be an actual process of building, making mistakes, swearing, making corrections, crossing fingers etc $\endgroup$
    – Dragonel
    Nov 1, 2019 at 18:09

The short answer is that there isn't a sane scientific explanation to what you're proposing because it's impossible, even if time travel was indeed possible. These kinds of 'kill your grandfather' paradoxes would break everything we know about the universe to date. On the one hand, being able to do this would be good news because it means that the universe truly is non-deterministic, meaning free will is possible after all. The bad news is that the effect you're describing would mean that the non-determinism could create cause and effect chains in both directions of time which would make the universe so much more fluid and harder to understand than it is according to our current science.

That said; there is a concept that you might like to consider that sounds sciencey, but whose existence is prohibited by our current understanding of the universe - metatime.

Meta-time, or the time of time, is what's happening in the Marty McFly example; you are busy changing the past and your picture from the future changes in real time to reflect what you've done. This is not possible, at least in our experience, because to us the past is immutable (hence the Immutable Past theory) and therefore cannot be changed. From our own perspective, the past has already occurred so if you go and change the past, or at least interact with it, then you always did so and our memory of the past doesn't realign in real time so to speak; from our future self perspective, what happened always happened and therefore our interaction with the past perfectly aligns with our historical truth.

So; in such an environment, you can't kill your grandfather, or forget to send back the phone, because your historical perspective proves that an attempt to kill your grandfather, or not send back the phone, is destined to failure.

This single timeline model (immutable past), with the phone example you describe, actually is unsettling because it means that if time travel is possible, we ARE living in a deterministic universe in that backwards causality can exist and can no more be violated than the causality chains in the normal 1x forward progression through time we experience. If that is the case, then what we do in the future is as set every bit as much as what we did in the past, meaning no free will.

So, this is where the multiverse comes in. No paradoxes, because every deviation spins off a new universe in which things happened differently. Quantum physicists love this model (it tends to fit rather nicely with Brane theory) and means that we may well have free will but exercising it spins off new versions of the universe at every point at which we might make a different choice.

The problem is, if you got the second phone in your universe, then you sent it back in your universe. The causal chain means that there's no way to get the extra phone and then NOT send it back within your current version of the universe. There's no parallel universes in which you end up with two phones in one and none in the other.

So from a scientific perspective, no I can't see how this is possible at all, meaning that there is no sane way to explain it scientifically. But, the closest 'technobabble' term that may assist you with a viable explanation would be the meta-time example, where you somehow build a plausible narrative about meta-time changing between the item being received in the past and the actual transmission into the past. Personally though, my suspension of disbelief would be strained if I was reading it.


"Sufficiently advanced technology"

This looks to me like an example of Clarke's third law:

  1. When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
  2. The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
  3. Any sufficiently advanced technology is indistinguishable from magic.

Source: https://en.wikipedia.org/wiki/Clarke%27s_three_laws


As far as what would actually happen in the real universe, we cannot know without a working time machine to experiment with.

As for time travel as a fictional plot device, the best explicitly stated model I have seen is in Eliezer Yudkowsky's "Harry Potter and the Methods of Rationality", Chapter 17 "Locating the Hypothesis". (I hope it's okay to reference this, it's a not-for-profit fanfic which JK Rowling has said she is okay with.)

The summary is that only stable time loops are possible. The scenario "watch appears; subsequently decide not to send it back in time" is not a stable time loop, so it can't happen. Or perhaps it could but only for incredibly short itmescales bounded by Heisenberg's uncertainty principle (uncertainty in energy x uncertainty in time < Heisenberg's const.)?


It depends on the shape of space-time.

If space time is fully deterministic and time travel is a closed time-like loop, what you describe isn't possible. There are two ways for it to not be possible; one is good news, the other is bad.

If we don't have closed timelike loops from time travel, either the "sending" future exists independently of the past it broadcasts into, or it doesn't.

If it exists independently, anyone inventing a time machine doesn't notice it ever working. But each time they use it, a new timeline with things "just appearing" forks off. If they do this close enough to the time that they start working on a time machine and plan to send tests back, they see those tests arriving. But what they send back has no impact on their own past.

If it ceases to exist, anyone inventing a time machine ceases to exist after using it. People planning on inventing time machines end up with a pile of stuff arriving. The only future that persists is one where they never get around to using it, despite all of the things arriving.

For the closed timelike case, the good/bad news ones look a bit different.

In the good news ones, the universe is deterministic, and nothing can be sent back that wasn't already sent back, and nothing stops it from being sent back.

In the bad news ones, if you fail to send something back you picked up, your timeline doesn't exist. If you send something back you did not pick up, your timeline doesn't. Arbitrarily improbable events will occur to make the timeline be consistent; the timeline "solves" the problem much like physics "solves" orbital mechanics. This one is bad if you forget to build the time machine, because if you didn't send it back, something else caused it to appear.

Now, the "timelines get destroyed" is an interesting one.

You build a computer. It attempts a branch of an NP-complete problem, running for long enough that it will either prove the solution correct or fail. When it runs out of time and hasn't proven the solution, it sends a message back saying "try the next branch".

If it sees a "try the next branch" message, it follows it.

The only timeline that sees past the end of the computation is one where the computer found the solution on the first try.

Bad things happen if you program this wrong. Suppose you think there is a solution, but you screwed up, and there isn't one. Well, then all timelines where the computer runs end up terminating.

Only timelines where something happens to make the computer break before sending a message back can continue. So a bug in your program results in the computer breaking and failing to send a message back despite not finding the solution.

This could be caused by a particle engaging in alpha decay or whatever.

Now, if you make your computer more and more reliable at the hardware level, but still feed it unsolvable problems, we still run into the situation where the only future that persists is one where the computer fails to send the message back.

In effect, you have built an improbability bomb. The more reliable the device to send a message back in time the more unlikely the event that can reasonably occur to stop it.

If the probability that a civilization containing a functioning time travel device sends a message back is high enough, no civilization containing time travel exists long beyond the point of time travel creation.

I call this the "Skynet was actually the good guy" time travel model.

  • $\begingroup$ You would presumably get to the point where no matter how reliable the computer could be made, the least unlikely event is that some random event, no matter how unlikely, causes the computer not to send back the "try the next branch" message for a wrong answer. So even if you push the physical limits of how reliable a computing device can be made, if you feed it a problem without a solution, it just returns a random incorrect answer. $\endgroup$ Nov 4, 2019 at 11:16
  • $\begingroup$ @Adrian-JusticeforMonica With a sufficiently reliable computer, the "does not send it back" could be something like "the false vacuum collapses". Basically as you increase reliability, you can increase the likelyhood of any failure mode you didn't make more reliable. Sort of like how making car handling better doesn't reduce crash rates. ;) $\endgroup$
    – Yakk
    Nov 4, 2019 at 13:59

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