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I finally found the answer! After hundreds of white-boards filled, I am sure I can build the first time machine! ...Wow, it will be expensive, and time consuming... and actually pointless... Unless, maybe...

My character is about to build a time machine, she has solved every little issue that she would encounter, she has all the blueprints, but now she is in doubt...

According to the theory (which is acceptably true in the universe(s) she lives in):

  • You can not send mass in time, only waves, transporting only energy (e.g. encoded signal, so SMS and mails are allowed) and,
  • When one sends waves in time, it does not alter the universe one lives in, the wave travels to an alternate time line where this wave is received (multiverse theory?). For example, if she sends to herself an email containing lottery numbers, she is sure that the "she" from an other time line would become rich, but the actual she (who sent the email) would stay poor with a huge Internet bill. Same happen if she tries to send energy in the future (Paradigm 2 on this question)
  • Finally, she can not send signals to a date before the construction of the time machine. The time machine acts as a receiver as well as a transceiver.

The constraints are largely inspired by the anime Stein's Gate, if it could help to have a clearer view.

What features could be added to the time machine to make it beneficial knowing the time travel constraints stated before? Because, as stated before, sending back the lottery numbers would not make her richer.

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closed as off-topic by Mołot, Azuaron, MolbOrg, James, Paul TIKI Apr 18 '17 at 16:21

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    $\begingroup$ To my understanding energy is a part of matter. Without matter there would be no energy. $\endgroup$ – Mormacil Apr 18 '17 at 12:36
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    $\begingroup$ This sounds like a question about what a character would do, not about building a world. "Why would she spend (a lot of) time [...]?" is basically what you as the author should come up with. Maybe she just likes the idea of helping another version of herself? Maybe she hopes that another version of her thinks the same? $\endgroup$ – Sec SE - clear Monica's name Apr 18 '17 at 12:42
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    $\begingroup$ Matter and energy are the same thing... You cannot send energy without sending at least some photons or some neutrons or some other particles. Besides, you cannot send energy without affecting the actual universe -- some energy will disappear without trace. $\endgroup$ – AlexP Apr 18 '17 at 12:45
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    $\begingroup$ @AlexP From the universe's perspective that energy could have just swapped from a 0 to a 1, or whatever the correct term would be, as opposed to vanishing/emerging without a trace. In the receiver universe a 1 would have swapped to a 0, so your message could be encoded in that. $\endgroup$ – Callum Bradbury Apr 18 '17 at 14:29
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    $\begingroup$ I'm voting to close this question as off-topic because the question is seeking for plot device about timetravel $\endgroup$ – MolbOrg Apr 18 '17 at 15:24
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This is a modification of Josh King's Answer. Targeting a specific universe has problems, so let’s not introduce that.

dedicated link

Once a connection is made, it stays open and allows two-way communication. But, one is at an earlier time than the other, so how does she return the favor?

Make the time rate at each end variable and random. While initially 2 weeks behind, the other universe will go slower and faster, and will catch up and even pass up our time.

In The Proteous Operation the other universe ran faster at some power of the time difference, to make the gate “catch up”. Use the same idea, but with simple harmonic motion: the past gate will run faster, catch up and pass up our end, slowing its rate until it is nearly 2 weeks ahead of ours.

It’s like the 2 machines are connected by a spring, which will bounce up and down.

infinity

You send info to another, random, universe. Meanwhile, some other universe chose yours at random. So what you do to others, some other copy is probably doing to you.

This works because time travel doesn’t create a new universe, but just connects to a different existing one among the infinite. This still allows for cycles and things that would be paradoxical in a single time line, but still imposes some constraints.

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  • $\begingroup$ I really like the "Infinity" concept. It makes for an interesting plot/world point. $\endgroup$ – Ranger Apr 18 '17 at 16:07
  • $\begingroup$ How they know what information to pass which will be useful? Most information sent will be useless, because the future already know us and there is no guarantee that after you send the information, that past will branch to our timeline $\endgroup$ – Vylix Apr 18 '17 at 16:10
  • $\begingroup$ @NexTerren like Hilbert’s Hotel. $\endgroup$ – JDługosz Apr 18 '17 at 16:12
  • $\begingroup$ @AdiNugroho you choose when to send at random, or you send lottery results every week (even after you win yours). $\endgroup$ – JDługosz Apr 18 '17 at 16:13
  • $\begingroup$ Let's say a lottery. If you (and other yous) agreed to keep sending information, there is a guarantee of infinite number received by your machine, coming from infinite number of possible future. Which lottery number will you buy? There is a future where the 100000 wins, 100001, and so forth. $\endgroup$ – Vylix Apr 18 '17 at 16:18
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You just need to make a friend in another universe (or multiple universes) and use the machine to send beneficial information back and forth.

As in your example, send todays lottery numbers from your universe to an alternate you yesterday in Universe 2, you in U2 then wins the lottery and becomes fabulously rich. Along with the lottery numbers you send instructions to send lottery numbers back to you from next week, so U2 waits a week and sends back lottery numbers to you next week.

This doesn't seem to break causality anywhere as far as I can tell.

You may run into problems depending on how much effect you winning the lottery in U2 has on next weeks lottery numbers diverging between the two universes.

This would require your machine to be able to identify and sort between alternate universes to accurately sent messages back to you, or you could just general broadcast, or do a sort of chain letter making every alternate of you in a universe with similar lotteries rich, hopefully including you.

This also requires the alternate you to be altruistic enough to return the favor, but they just won the lottery so any cost associated with sending the message should be negligible to them.

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  • $\begingroup$ To sum up, the ability to dial a time line like when you use a phone must be added to my time machine, right? $\endgroup$ – EngelOfChipolata Apr 18 '17 at 15:18
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    $\begingroup$ This doesn't work given the model of time presented here. In this sort of model a person(1) sending a message back to another person(2) is fine, but person(2) can't send a message back to person(1) the universe that their message would reach is a person(3). $\endgroup$ – Yeshe Apr 18 '17 at 15:26
  • $\begingroup$ @Yeshe it's something to be added to the description to make it useful. $\endgroup$ – JDługosz Apr 18 '17 at 15:43
  • $\begingroup$ @Yeshe you are assuming that the multiverse branches at time travel events and communication is only possible to downstream branches; this may or may not be the case. $\endgroup$ – Josh King Apr 18 '17 at 15:44
  • $\begingroup$ @JoshKing being as that is what is alluded to, I don't see that as an assumption. Now if this were simply a question of communicating with parallel universes this would be a non-issue, but in a situation where a message is being sent to a parallel history, even if a message to "the future" is possible, once the message terminates the future that the message came from is no longer accessible from the receivers end. That future can't exist in a chronological sense. So, if there is ANOTHER device that communicates outside of time sure, but that wouldn't be the "time machine". $\endgroup$ – Yeshe Apr 18 '17 at 16:05
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This scenario reminds me of a short story 'I don't know, Timmy, being God is a big responsibility', in which researches build a computer that simulates their universe 100% accurately, right down to the simulation building a simulation of their universe, infinitely.

If they were to manipulate their simulations' past then it would mean that their existence would also have be manipulated in the same way.

Putting this into context of your time machine - if the machine was hard-wired to forward any message on, at the time the message was originally sent, then an infinite number of universes would be manipulated. The original Jenny may not get the winning lotto numbers, but every Jenny in the central finite curve (every reality with a time machine built by Jenny) would.

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  • $\begingroup$ Infinite chain is what I’m thinking too. But forward it on won’t work because it’s limited to the start time at which the machine was turned on. $\endgroup$ – JDługosz Apr 18 '17 at 16:15
  • $\begingroup$ The idea expressed in the web short story you pointed to, where each universe is influenced by the parent and eventually it reaches a point where each is influenced in the same way to do the same thing to its child, is called a fixed point. It's essential in the theory of functional programming, to have recursion. $\endgroup$ – JDługosz Apr 19 '17 at 0:21

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