# Reverse entropy, reverse causality and physical laws

I do not have any scientific education and I am far away from understanding relativity, quantum mechanics and such.

I am just wondering about entropy, causality and if we could reverse these in a thought experiment without violating the physical laws of the "universe".

Is it possible to look at the "universe" disregarding our perception of the arrow of time and assume that entropy, causality, laws of thermodynamics are relative without violating the laws of physics?

Could we assume that the laws of nature such as entropy and causality are reversed? Eg. the arrow of time is relative and entropy increases over time, cause and effect are reversed - the universe didnt "start" with the big bang, but rather started as a system in a state of high entropy?

Do some laws of physics forbid to think of the universe in such a model?

Im aware this might appear like a stupid question and i dont expect huge detailed answers, but if someone could give me a link to some research or thought experiment that goes this direction?

## 1 Answer

How do you distinguish "reverse causality" from "ordinary causality"? That is, if you say X is the cause of Y, and Y (at some previous time) is the effect, how do you prevent me from simply redesignating Y as actually the cause of X, and redesignating X as the effect (since it happens after Y)?

Even in some sort of "looped causality" situation (where people imagine sending the lottery numbers back in time, for example, and then moving forward in time with them), we would always experience the effects (of winning the lottery) before their causes (the sending of the numbers back), so the future would be set in iron (since the effect, having already occurred, would require its cause to later be consummated, and new effects could not be introduced into the past, because we would have seen them already and they would already have had their effect).

Indeed, winning the lottery and then feeding those numbers into the machine after the win in order to "send them back", would just appear as a quaint eccentricity - it would not be clear whether you were causing the win that had already occurred, or doing nothing at all and expressing a disturbance of mind!

If you think these things through rigorously, you realise that you're abandoning any known laws of nature as we know them. That's not to say sci-fi never became science, but it's pointless asking scientists what they think about sci-fi (in terms of today's science, and in their capacity as scientific experts, rather than as artistic or literary critics).

• Thanks for your answer! My question wasnt meant to be about possible problems for "time travel" and causality regarding living things - especially humans. – user183630 Feb 3 '18 at 16:50
• Thanks for your answer! My question wasnt meant to be about possible problems for "time travel" and causality regarding living things - especially humans. It's more like this: lets assume theres a hypothetical entity that is not confined to the arrow of time, nor dependent on causality laws. Could this entity (if observing our universe) assume that thermodynamics, entropy and causality works in the opposite order of what humans perceive? Hypothetically, could this entity oberserve different arrows of time without violating physical laws? sorry, new to this and made a mistake with commenting – user183630 Feb 3 '18 at 16:57
• No problem. I've only added the human time travel aspect for additional substance - the essence of it is simply that "reverse causality" is conceptually and observationally indistinguishable from "causality". – Steve Feb 3 '18 at 16:58
• Let me recast your question. You are asking "if we disapply certain fundamental laws of physics, can things do allsorts that they can't if all the laws of physics are applied?". The answer is yes - this thing can go backwards in time, teleport, leap all buildings in a single bound, create effects with no cause, and so on. Imagination is the only limit to what can be done in this world into which you have led me. – Steve Feb 3 '18 at 17:15
• My intention was not to disapply any fundamental laws of physics, but rather to ask if the laws of entropy and causality are relative. I wanted to display this intention with that weird idea of the hypothetical observer who basically doesnt care about an "arrow of time". – user183630 Feb 3 '18 at 17:20