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I was thinking of a situation with humans finding a solar system, in which entropy decreases over time instead of increasing over time. In this solar system heat would flow from a cooler planet to the hotter star. Shattered rocks sometimes spontaneously unshatter before rising back to the top of a cliff. Sand will sometimes spontaneously fill a crater before a meteor rises up and goes into interplanetary space.

Could humans colonize some of the planets of this solar system?

Anders Gustafson
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    Time flowing backwards can be thought of as playing a film backwards. Humans would still remember the past, except the past is the future, and humans would not be the wiser. So if we're assuming time flows backwards, you should probably specify what doesn't flow backwards.. Perhaps beings from a place where time flows normally would be out of place in a solar system where time flows backwards? – Neil Jun 01 '17 at 06:24
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    Can you elaborate on "time flows backwards"? What would this look like to an outside observer? If I enter this system or land on one of the planets, does time flow backwards for me? – frodoskywalker Jun 01 '17 at 06:30
  • Short answer: no. As soon as they enter the place, time flows backwards for them as well, which means they would automatically leave the place with a backstep. Thus they aren't even able to get into it, let alone colonizing it. – Stegax Khenacc Jun 01 '17 at 07:32
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    I agree you are not specifying the concept well enough for an answer. See this Q, and read the novel The Arrows of Time for a detailed description of a visit! – JDługosz Jun 01 '17 at 08:08
  • For example, in the linked Q I wrote «You don't experience the environment to be anti-matter: as in Egan's story it would be anti-matter if your time arrows were aligned, so you have matching matter polarity but opposite entropy gradiants and (whatever that means if it's more than just entropy) opposite time directions.» – JDługosz Jun 01 '17 at 08:11
  • Do you need a science based answer (like a4android's below) or maybe you'd like to handwave some part of the explanation? – Yuriy S Jun 01 '17 at 08:24
  • @JDluglosz When I say that time would flow backwards I mean in terms of entropy so heat would tend to flow from the cooler planet to the hotter star and a mix of two liquids could spontaneously unmix and a shattered rock could spontaneously unshatter and rise back to the top of a cliff on a planet in this solar system. – Anders Gustafson Jun 01 '17 at 08:45
  • @AndersGustafson But this is a completely different quesiton – Raditz_35 Jun 01 '17 at 09:04
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    Is this system enclosed in some way? If not, how do these aberrations not affect the rest of the universe? If it is, how do you detect it,much less enter? – nzaman Jun 01 '17 at 09:47
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    @AndersGustafson you should [edit] the question to address concerns raised in the comments. Don't (just) answer with another comment! – JDługosz Jun 01 '17 at 10:42
  • Humans have already will have colonized it, right? – iAdjunct Jun 01 '17 at 13:00
  • What! You said "in which entropy decreases over time instead of decreasing over time" and in the title "in which time in terms of entropy is reversed". These different propositions. So, is time reversed or is entropy decreasing with time (going forward in time)? Also, rewriting your question disenfranchises answers posted prior to your changes. If you want to do that, you should asked a new question. – a4android Jun 02 '17 at 04:04
  • What we consider the direction of time is determined by, which direction entropy increases in so having entropy decrease with time is indistinguishable from time flowing backwards. – Anders Gustafson Jun 02 '17 at 06:35

2 Answers2

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No. It would be too dangerous. The danger doesn't arise from coping with dual causality situations on any of the planets. Matter existing backwards in time is antimatter.

As everybody knows when matter and antimatter meet a lot of gamma radiation happens very quickly. This will be fatal.

Potential colonists will soon be dispersed in the form of rapidly expanding plasma accompanied by large amounts of gamma radiation. This means colonization will be impossible.

There is a good chance no-one will realize the solar system is a place where time flows backwards because all the matter in this solar system will be antimatter. It will be classified as an antimatter solar system.

ADDENDUM:

The relationship between antimatter and time reversal is established by a derivation of the CPT Theorem. (Vide the Wikipedia entry linked above.)

This defines a CPT transformation if we adopt the Feynman-Stueckelberg interpretation of antiparticles as the corresponding particles traveling backwards in time. This interpretation requires a slight analytic continuation, which is well-defined only under the following assumptions:

The theory is Lorentz invariant;
The vacuum is Lorentz invariant;
The energy is bounded below.

In words, matter moving backwards in time will be antimatter. Therefore, if there is a solar system where time flows backwards its matter will be antimatter.

a4android
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    "Matter existing backwards in time is antimatter." source? – Miguel Bartelsman Jun 01 '17 at 07:13
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    @MiguelBartelsman Dear me! What is physics education coming to these days? It's only time symmetry. OK. I'll mooch off and find a source. – a4android Jun 01 '17 at 07:17
  • @MiguelBartelsman I've done my mooching and a source has been provided. Richard Feynman came up with this concept back in the 1940s. Really a nice idea. – a4android Jun 01 '17 at 07:28
  • @a4android I had the same thing happen, here. Someone was surprised that I said it was well known/commonly taught how anti matter is time reversal. Oh, and Feynman related an anicdote; it wasn’t his idea about there being only one electron moving back and forth. – JDługosz Jun 01 '17 at 08:00
  • Hey, remember? You were a newbe! – JDługosz Jun 01 '17 at 08:13
  • @JDługosz That was universes with an interface. So backwards time was global. This question doesn't suggest, although this is one possible interpretation of the situation, that the backwards time isn't a property of the spacetime in the solar system. If it was, do I have an answer for you! Yes it was Feynman & in a phone conversation with his PhD supervisor John A Wheeler. They devised a solution for the arrow of time problem out of that discussion too. – a4android Jun 01 '17 at 08:56
  • @JDługosz This surprises me too. Matter being time-reversed antimatter and vice-versa seems so common place that it is remarkable it's not well known. The idea is so stunning in its simplicity and beauty. Once you hear, it's almost impossible to forget it. – a4android Jun 01 '17 at 09:00
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    We don't actually know that anti-matter is time-reversed regular matter, but its pretty well accepted that the two are pretty much indistinguishable. "Feynman Diagrams were constructed using Ernst Stueckelberg's interpretation of the positron as if it were an electron moving backward in time. Thus, antiparticles are represented as moving backward along the time axis in Feynman diagrams." Doess't mean its true, but it works. – Draco18s no longer trusts SE Jun 01 '17 at 15:23
  • @Draco18s I was well aware of this when I posted my answer. This is an old issue in the philosophy of science. If a model works, then is it true? Are solar systems where time runs backwards true? Well, if so, then time-reversed matter can be antimatter. Empirical validation will always be needed. – a4android Jun 02 '17 at 03:53
  • @a4android My comment was more addressed towards the other commenters here, rather than you. Not everyone realizes how similar antimatter and time-reversed matter are. Interestingly enough, I never thought about it until I read Harry Potter and the Methods of Rationality (for those who don't care for Potter or Fanfics, I advise trying to read up through chapter 13 first). – Draco18s no longer trusts SE Jun 02 '17 at 04:08
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    @Draco18s Your point was a good one. So I appreciated it. I included the three Stueckelberg assumptions in my quote from Wikipedia to indicate science isn't absolute truth. It may be better than other kinds of knowledge, but that isn't everything. Models and reality in science is always a vexed issue. I thought it was appropriate for backwards-time solar system. – a4android Jun 02 '17 at 04:27
  • @a4android no need to get like that. I just want to know what that's about because it's the first time I hear about it. As far as I was concerned antimatter was just matter with a different set of elemental particles. – Miguel Bartelsman Jun 03 '17 at 09:44
  • @MiguelBartelsman Sorry if I offended. If so, it was unintentional. To me this was so obvious I was amused I had to explain it, and that the joke was on me by assuming too much. JDługosz has had a similar experience. Antimatter is made of the corresponding antiparticles of any given piece of matter. Antihydrogen is an antiproton and a positron. Just enjoy the surprises nature plays on us mere fallible mortals. – a4android Jun 03 '17 at 12:50
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Above answers provide scientific explanation to why it's not possible. Regular sc-fi explanations would also yield the same answer but for a different reason.

In that solar system the colonization took place already. In that solar system YOU are the BttF Butch, you are the Connor (from Terminator 2&3), you are Sarah Connor (from Terminator). Depending on your chosen time travel rules. Those people travel back in time while you are going forward.

So for example, you can't colonize the system now because of the antimatter. But in 500 years they figured out how to overcome this and send a ship. And that ship is the one that started the world you've seen 500 years earlier.

Synergetic
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