Humanity, with modern science, has established that the Big Bang happened 13.8 billion years ago. The universe appeared in a low-entropy state, and for billions of years has been expanding and moving to a high-entropy state. We await, and dread, the heat death ahead of us... and the end of all life.

One day, we discover a civilization that perceives time in the opposite direction. They have, with modern science, established that the heat death happened 10100 years ago. For them, the universe appeared in an extremely random and large state, and they're terrified of the Big Bang coming up in just 13.8 billion years.

Is this realistic? I understand that physical laws are valid in both directions of time, and that's why I'm asking this question. I can't imagine how a biological system could have memory of the future. Even with the computing tools we have today, it's very difficult to study the future, whereas it's relatively easy to study the past (e.g. we know much about the history of life thanks to paleontology, but we know almost nothing about its future evolution).

I'm also wondering how humans would communicate with this kind of life. What we see as "first contact" would be, for them, the end of contact. Before we even meet them, the aliens would know everything about us, and our entire shared history. Including how we watched them devolve, technologically and biologically. Standing on even footing with them, at any point in time, would be incredibly difficult.

EDIT: I think some people may have misunderstood the question. To clarify, these aliens see the universe as contracting, as entropy in an isolated system going down, and as room-temperature water spontaneously freezing into ice cubes. This is because they have memories of the future, not the past; there is no "reverse time flow". This takes place in our universe, and time works exactly as it does now, on Earth.

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    $\begingroup$ Physical laws are valid in both directions EXCEPT thermodynamics $\endgroup$ – L.Dutch - Reinstate Monica Dec 15 '18 at 5:07
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    $\begingroup$ Having memories of the future is not the same as seeing the universe contracting. If they have memories of the future they just see the universe expanding while knowing what will happen $\endgroup$ – L.Dutch - Reinstate Monica Dec 15 '18 at 6:32
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    $\begingroup$ This is shy of a full answer, but it is an open question why we perceive time flowing in the direction of entropy increasing. Not only is it reasonable to go the other way, its so frustratingly reasonable that we can't disprove anything about it! $\endgroup$ – Cort Ammon Dec 15 '18 at 6:39
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    $\begingroup$ "I understand that physical laws are valid in both directions of time": Messrs. Sadi Carnot, Rudolf Clausius, Ludwig Bolzmann and Willard Gibbs would like a word. The arrow of time is for real; while time is indeed symmetrical on the microscopic (= quantum) scale, this is not the case on the macroscopic scale. $\endgroup$ – AlexP Dec 15 '18 at 7:19
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    $\begingroup$ @AlexP From what I understand, "the arrow of time is for real" may be a stronger statement than scientists are willing to back. It's an open question. $\endgroup$ – Cort Ammon Dec 15 '18 at 7:30

The aliens are machines.

They reached a very singular singularity (pun unintended), in which they were able to precompute all that there was to precompute until the end of the universe. This is a massive task. The issue is that such a feat comes at a cost, and their hardware is slowly fading. Every moment that passes their immense memory storage experiences a loss and a piece of their knowledge gets lost. These singularly intelligent machines, however, have foreseen this as well and decided to arrange their memory storage so that the loss will never lose information about what is to come. They eventually reach a stationary state in which every knowledge of anything before the present instant is lost.

How do they perceive time?

Past, present and future are our concepts. We perceive time due to memory. We can't have a perception of time unless we can recall a time before the present. Further, we associate with the concept of future "the unknown direction", the one that has not been fixed by the flow of time.

Let's therefore consider a different time-dimension of "known" and "unknown". The known is fixed and immutable. The unknown is open and harbor of possibilities. The machines in the question have only a memory of the future. Hence, the future is "known" and immutable. The past on the other hand is unknown. It is hard for us to imagine, but for them, the flow from the "known" towards the "unknown" actually goes towards the past. Yes, we can argue that it is a bogus argument, because none of us is capable of perceiving it that way. That is exactly the point: it is not ours, it is their perception.

  • $\begingroup$ After watching your reverse-video of sulfuric acid eating a marshmellow - presented as proof-of-concept that mere perception meets the OP's requirements - I took the time to read your post. It fails the OP's requirement as the machines will never perceive the universe as rushing toward a "big crunch" since they can always remember a future that includes the discovery of how the universe operates - which means they'll always understand the universe is expanding and rushing toward heat death. This answer fails the OP's expectations. $\endgroup$ – JBH Dec 16 '18 at 0:07
  • $\begingroup$ @JBH: Chill out, I think that the OP can speak by themselves. --- in any event, the answer to your comment is no: they forgot the moment in which they precomputed everything, which happened at the singularity. $\endgroup$ – NofP Dec 16 '18 at 9:23


Sorry. The reason we even came up with the theories and math supporting the universe's heat death was (among other things) noticing that everything is more-or-less expanding. For your society to legitimately believe as they do, they would need to view the universe as contracting.

Now, move backward in technological history long enough, say 1842 and the discovery of the Doppler Effect, and people wouldn't know the universe is expanding.

However, they also wouldn't know about the "big crunch theory." The Big Bang Theory was popularized after a BBC radio broadcast in 1949 based on a lot of prediction that isn't important here. What is important is that the theory developed after the discovery of the Doppler Effect.

Which means your species would not develop a "Big Crunch Theory" unless they lived in a universe that actually was contracting. Otherwise, they'd figure out it wasn't happening before anyone popularlized the idea in the first place.

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    $\begingroup$ Perception and reality may be very distinct. $\endgroup$ – NofP Dec 15 '18 at 12:53
  • $\begingroup$ @NofP, how do you perceive a doppler shif that's red as blue, and vice-versa? How do you perceive a chemical reaction as if it were unreacting, especially when your own hands are used to combine the chemicals, as if that's the way it should be - especially as time in the universe is actually moving forward? Remember, 5 minutes later the species must actually believe the chemistry undid itself, despite the fact that it actually did react. How should the species react when they drink what they believe is clear water, but it actually tastes like cherry Kool-aid? That's a mighty strong delusion. $\endgroup$ – JBH Dec 15 '18 at 15:17
  • $\begingroup$ nah, simpler... youtube.com/watch?v=7Mfl1Obmnv0 $\endgroup$ – NofP Dec 15 '18 at 22:49
  • $\begingroup$ @NofP, you're offering me a video played in reverse as proof of concept? $\endgroup$ – JBH Dec 15 '18 at 23:53

If by past/future you mean cause and effect, as in, what happened in the past shapes what happens in the future, a society that perceives the future before the past would essentialy become omniscient. Since their "place of origin" is the point in time where everything that could happen has already happened (I assume the end of time itself would be to them what the big bang is to us), whatever reality they're in at any time is a direct result of something that is yet to happen to them, and since they know how things are in their "present" they can extrapolate (pretty acuratelly I'd think) how thing will be their "future", e.g.: They know there is a burnt down forest therefore they know a forest fire is coming. Their whole reality would be self fulfilling prophecy.

As for the plausibility, I'm not sure it's anywhere near possible with our understanding of how time works. The "flow of time" is intimately connected to our movement through space, the faster you move the slower time passes (btw, that's been proven by experiment, which basically means we've alredy perfomed time travel, I personally think that's pretty awesome), so when you reach the "speed limit of the universe" aka the speed of light, time is completely still. However, it's called the speed of light because only light can reach that speed since light is currently the only "massless thing" we know, anything with mass could pottentially reach speeds close to but never quite light-speed (it has to do with how particles stay together, if they move too fast it all just breaks apart). All that means that to travel back in time you would need to, not only be mass-less, but have negative mass, right now we know of absolutely nothing like that (no, anti-matter ain't it). There are however theoretical particles called Tachyons that travel faster than light (therefore back in time) but they're just speculation.

Hope I could help, I'm not a physicist by any means, but that's what I understand of the topic at hand, if there's any physicist out there, please correct me on all the stuff I probably got wrong.

EDIT: Oh, by the way, if you're thinking of maybe using tachyons, something that is interesting to point out is that the energy to movement dynamic kinda works in reverse. With "normal" particles, the more energy you use, the faster it goes and to get it to light-speed you need infinite energy, with tachyons it's the other way arround, you would need infinite energy to get it to move slower than light. I'm not sure what happens when there's no energy (I'll need to research some more) but I'd assume an "energy-less" tachyon would have infinite speed(?), so it would kinda be omnipresent(??), maybe(???), I really don't know, just speculation from someone that, again, isn't close to being a physicist.


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