Last month Mobius asked a question about a world where time can flow both forward and backward. I suggested a particular solution to the problem he was looking at, and I would like to open up a follow up question as part of the fortnightly challenge.

The world of two streams

In the world I am looking at, time flows in two counterflowing "streams" with eddies connecting them. In the "live" stream, we see physics like usual. Quantum mechanics, general relativity, take your pick from modern science, we see these behaviors in this stream. The other stream is the "dead" stream, and it is where we go when we die. The physics of this stream will be constrained shortly, but the most important trait of this stream is that entities within this stream travel in the opposite direction of time. Any entity can enter this reversed stream simply by dying. Any entity within the dead stream may re-enter the "live" stream at any point [in the past] that they please, simply by wishing it so when they get there.

The physics of the dead stream are open for interpretation, but they are constrained in a way that guarantees consistency. The dead stream is ruled by knowledge and information. As you travel back in time along the dead stream, you are obliged by the natural laws to forget enough information such that, if you return to the world of the living, you have forgotten too much information to allow you to force a paradox to occur. For example, if you traveled back in time to kill your grandfather, you may be forced to forget your own last name. Or you might be forced to forget that you wanted to kill them. Or you might simply forget a key detail of your grandfather's personality that allows him to dodge the murder attempt at the last moment.

Feel free to set the rules for this forgetfulness in any way that supports your answer, with one caveat. The forgetting occurs inside the dead realm, not in the transition from dead to alive. The dead must constantly be forgetting enough to ensure that they have no opportunity to create a paradox. If they avoid a "cusp" where a paradox could have occurred, but they elected not to cross back over to the living at that point, they still have forgotten the information that could have been used to cause a paradox. The universe is overcautious in that respect.

This also means that the path one takes in the dead stream matters. If one chooses to observe cusp after cusp before arriving at a location, they may have had to forget more than if one chooses to sidestep cusps entirely, arriving at the same location. A dead entity which wishes to avoid forgetting would be wise to avoid sensitive areas of spacetime on its trip backwards through time.

Religion of two streams

For this question, I am looking for what traits we would see in religions which are not seen in our simpler unidirectional "arrow of time" approach used in the modern world today. Consider that a dying individual could travel back in time so far as to get "lost," forgetting all that made that entity an individual (but a Buddha, which theoretically knows only eternally true facts, could travel backwards forever without change). Self interaction is allowed in this world, so a religious individual could go back in time to teach his or her younger self, as long as that teaching did not invoke a causality paradox (if it would invoke such a paradox, the older version would simply find that they forgot too many details to cause the paradox). Obviously, in this world, life after death is a known thing, but there might be whispers of a "true death" that goes beyond simply entering the dead stream.

What traits would be found in religions of this world?

Judging rules:

  • I am interested in traits of religion which do not tend to appear in our world. I am looking for traits which are specialized to this particular universal structure. At the very least, if the trait exists in our world's religions too, the structure of this universe should dramatically amplify its importance.
    • The best answers are those which demonstrate a religious trait so profound that its mere presence will shape the world we build around a character. I'm looking for something that changes the world, not something which can be kept isolated inside a monastery.
  • The physics of the dead stream are open for interpretation. However, the universe should support a dualist viewpoint: in the live stream, body and mind/soul are attached. Moving to the dead stream separates the body and mind/soul, allowing the mind/soul to move backwards. (You are free to define a physical version of the dead stream, if you so prefer, but I should be able to treat the answer as though there was a separation between body and mind/soul, and you'll have to ensure information theory proves that your world prevents paradoxes)
    • The effect of a mind/soul trying to enter the living world in a location where there is no body for it to inhabit are up to you. It could be a "true death," or it could turn you into a ghost to haunt for all time. You might possess someone nearby, or you might simply enqueue to be given the "next available body nearby." Feel free to use imagination if it helps build your story for the religious traits offered as your answer.
  • Small and profound is better. It is best if a single trait explains multiple facets of people's world-view within a world. To use the example I already gave, one can define Nirvana within Buddhism within this particular universe. That definition also starts to define "ghosts," in that they are souls that forgot who they are along the way.
    • I expect the religious traits to be very dependent on how the universe goes about choosing what information the dead forget. Do not be afraid to specify details about this process. Just make sure that those details do not leave loopholes which would allow paradoxes to occur.
  • Answers should leverage the atypical nature of the dead stream. Obviously any solution along the lines of Novikov's self-consistency principle can trivially be proven to meet these rules, but they are well explored answers and lead to less interesting outcomes. Novikov started from the assumption that the physics of spacetime which contains spacetime loops is identical to that of physics of normal spacetime (that which has no loops). I am explicitly starting from the assumption that there are regions where the physics differs in some portions of the loops.

EDIT: There was a question about what form dead mind/souls return to the world of the living in. The answer is up to you. The intent is to allow one of these souls to "recycle" into a newborn baby body (at the moment of ensourcelement), thus providing the mind/soul with a new body to use. However, to avoid over-specifying the system, I allow the dead to return at any time and place. This may turn them in to ghosts or shadows, or it may simply scatter their mind across all space time. It could also lead to superstitions like saying "bless you" after the sneeze. The origin of that ritual is that it was believed a demon could sneak in up your nose after you sneeze. Perhaps that myth is not as far from the truth as we think today. Perhaps you drop your guard when you sneeze, giving a dead mind/soul an opportunity to partially hijack a fully grown body.

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    $\begingroup$ You've described how the dead stream works; how much of that is known to inhabitants of the live stream? $\endgroup$ Commented Jun 28, 2015 at 19:06
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    $\begingroup$ The information known by the live stream is limited only by the ability to come back without causing paradoxes. The answer may depend on the particular nature of the forgetfulness process one chooses, but I intended to assume that sufficient individuals have returned from the dead stream with sufficient information to form religious effects. $\endgroup$
    – Cort Ammon
    Commented Jun 28, 2015 at 20:49
  • $\begingroup$ I find it hard to consieve of any scenario where a paradox won't occur if the (already experienced) future must stay exactly fixed. If it doesn't have to stay exactly fixed there are no paradoxes at all $\endgroup$ Commented Jun 29, 2015 at 17:36
  • $\begingroup$ In what form do the dead return to the living? Ghost/Shadow? Human like any other? But then what age? Sum of time spend in live and dead streams, age of death, age one HAD at that time in the live stream? $\endgroup$ Commented Jul 1, 2015 at 14:51
  • $\begingroup$ I did not read carefully enough, you had some information about that: but it is vague, you might want to specify. Indeed the form of coming back is highly relevant to the religious feeling, as probably the people coming back ARE the source of information about that effect. $\endgroup$ Commented Jul 1, 2015 at 14:54

2 Answers 2



Ok, yes, some of today's religions claim certainty too -- certainty about what God wants, about what happens to you after you die, and other matters. But none of it can be proven; it's a matter of faith. On the other hand, the dead stream gives us the possibility of evidence.

In our world some believe in reincarnation and that they can identify their past lives. You might find a past-life idea that resonates with you and that might bring you some peace. That's nothing compared to meeting your future self, who can provide identifying information. (On that: we should expect that there will be certain categories of information that people now guard carefully, things that only you and future-you would know, so that these checks can be performed.)

Meeting your future self provides some reassurance about your future. Your future self can't tell you tomorrow's lottery number or anything like that, and probably can't even tell you when you're going to die, but you know that you have a future, at least a little. Bed-time prayers ("now I lay me down to sleep" etc) will probably change.

Back to certainty. Enough people meeting their future forms and learning about the dead stream will validate some religious ideas and challenge others. There will still be differences of interpretation, but I would expect this common, shared body of knowledge to push people toward a common body of baseline beliefs. For one thing, we're pretty sure now that there is an afterlife; you don't just die and get stuck in the ground and that's it. Instead of trying to make the best life for yourself here and that's it, you now have a whole afterlife to contemplate too.

And uncertainty

So, what does it mean that you've never been visited by your future self? Did something bad happen? Did your future self die horribly and wants to avoid you lest you find out? Did your future self mess up and "overshoot", and he's now back in your grandparents' time? (Presumably not proceeding to be his own grandpa, because we're avoiding paradoxes here.) Or did he become one of those phantoms that people sometimes talk about?

There will be people who never knowingly have an encounter with someone coming back through the dead stream, and this will lead some of them to conclude that people don't always come back. So if they don't come back, something else must happen to them, right? So the idea mentioned in the question of ghosts and phantoms will be a real belief, even if it's not actually true. It's part of human nature, I think, to let our imaginations fill in what we can't verify -- both good and bad.

Since people, it appears, enter the dead stream with the memories they had in life, the deaths of the very young will be seen as especially tragic. Before one is old enough to think and reason and have a basic body of knowledge, one won't know enough to be able to come out again. When babies die they're gone, or so many will believe, because how would they know enough to "steer" in the dead stream? At the other end, people who die with dementia are probably gone too. It is possible that the latter will lead to a higher suicide rate among the elderly -- better to go now while the brain still works than take your chances, some might say. We should expect some religious views that say this is not only ok but divine will, and others that say that it's terrible and that you're not supposed to "live" forever through the dead stream.

Some will wonder what happens if you decide not to come out of the dead stream. Can you go back to the neolithic age? To the formation of the earth? To the big bang? There will be much philosophy about this but no one will be able to find out. Possibly a focus on the "before-life" (so to speak) will replace the focus some have today on the afterlife.

So there'll still be plenty of uncertainty, but the key difference is that people will know that you have some control over your future path, and therefore people will see meaning in all contacts -- and non-contacts -- from the future.

  • $\begingroup$ I could see some really great pranks happening if you had two friends that looked rather similar. "Hello past self, I just wanted to let you know that [fill in female] is actually attracted to your buddy" $\endgroup$ Commented Aug 20, 2015 at 15:00

I've just discovered this, so apologies for such a long delay! So, some thoughts:

1) information has to be encoded in something (e.g. spirit matter, energy patterns etc.) and its degradation would be, almost by definition, entropy.

2) Hence, would people try to limit the entropy as much as possible, once they worked out that the universe worked in this way? perhaps by encoding a specific memory in multiple ways (so at least one way would survive the trip back). Think, for example, of Thomas Edison's experiments to contact the dead, and Spiritualism as a whole, and some of the Buddhist meditation practices which are about surviving aspects of the afterlife , and so forth. In essence, people will attempt to 'hack' the system and seek a way around the problem. If at any point in time, the physics of 'energy patterns/spirit matter/whatever mechanism' are deduced, then a technology may be derived to minimise entropy, or force some edge-case in the physics which would allow a breaking of the rule... Perhaps.

3) Maybe I missed it in what you said, but if people can come back at any time, can they not come back at a time prior to their forgetting too much? Can they come back as their own re-incarnation, thereby immortalising themselves within a time loop? Could the go back to their living self, and retrieve what they have lost from their living former self? And then keep replenishing their memories?

As for effects on religion: 1) prophecy would be big - and done right, more reliable; here's how my religion would do it: A) Supply a message you want to send to the past to a lot of people. B) Kill them all. C) They report to your former selves, but although entropy means they've lost information, they've probably not all lost the same information in the same manner, so each will have a small piece of the message. D) Apply statistical methods to put the fragments together and get some idea of the future.

2) Deceit via prophecy can be done in the same way - just get you're murdered population of death-travellers to tell lies (or lie to them all)...

3) What about temporal edge cases? Can you die/come back at the very edges of Time itself? The start or end of the universe? If you could retrieve any information from such times, you'd have clear stories about how the universe starts and ends.

4) There'll be stories in the myths of people from the 'future' meeting the heroes of the stories. Perhaps advising them etc.

5) Mostly, however, I'm not sure religion will be that different, because it already claims all these things. The only difference may be if there was a scientific means of demonstrating, verifiably, that the universe works this way... And given that we already live in a world where there are consistencies in the stories of people who have Near Death Experiences, where stories (cases) of supposed re-incarnation exists, prophecies etc.. I suspect it'll have the same scepticism we have now, which is kinda interesting, I think...

6) I think it all hinges on getting accurate, concrete factual information out of the dead/re-incarnated by some means. If that can be done, and repeatedly, then there's the scientifically verifiable proof one needs to turn religion into science and fact. Deceit aside, no more doubts about the afterlife, or the origins and end of the universe. Scientists will look for ways to exploit this for time travel, space travel etc. Governments and anyone with resources will try to gain prophecy as a means of power. Religion will die, and be replaced by the Science of the Hereafter.

So my answer would be this: If no reliable information can be garnered, it'll be identical to what we have now. Otherwise... huge differences, but religion will be dead,replaced by a fascinating new science. Science if what you get when you put religion and magic under the microscope - Consider Newton - he spent more time searching for secret messages in the bible than formulating physics as we now know it. He studied alchemy. All this was because in his time, there wasn't a distinction between magic and science - science was born of magic once you worked out what the rules of the universe really were, and Newton etc. were merely trying to understand how the universe worked (by seeking underlying rules to phenomenon, such as reactions in alchemy, or 'magical' processes, and finding them wanting or verifying an effect which could be studied).

Hope that helps!


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