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Original question on worldbuilding.stackexchange by JDĆugosz
We can consider factors of plot and literature, not just hard science. But I do intend for it to be hard-SF.
In Greg Eganâs âThe Arrows of Timeâ he had a crew visit a region where the thermodynac arrow of time was in the opposite direction. The ambient sunlight was not visible to them and they would unmake footprints.
But I think thatâs not right. Consider what would happen if you prepared such a region by somehow reversing the momentum of every particle. It is very precarious with every gas molecule having to be exactly right! As soon as you observe it, you peturb that delecate state and time runs normally from that point forward.
Consider an experiment where a gas cloud in a large isolated chamber will rush into a bottle, because the state of every molecule is just so. If you go into the chamber, it wonât work anymore because you mess up those perfect trajectories. (Unless the trajectories were chosen with knowledge of your exact effect in mind...!)
Suppose you had a wormhole that led to another part of the universe where time ran backwards or the orientation of the exit mouth were such that it dropped you into the time dimension facing the wrong way.
You donât experience the environment to be anti-matter: as in Eganâs 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 itâs more than just entropy) opposite time directions.
What happens? How?
Maybe you are like the gas bottle cloud and the large environment at your destination peturbs you. Does time do funny things? Do time and entropy break up into separate concepts? Could you go down in an elevator thatâs ascending, escaping in the same ride that the police use to arrive at the scene of the crime?
The title of your question and the question youâre asking seem to be two different things: In the title youâre asking what would happen at the interface, but in the body of the question you ask what it would be like to experience such a world.
I can answer both: You wouldnât be able to experience such a world because the interface canât happen. Or more to the point, if it did happen, nothing would be able to cross it.
Consider a person trying to cross the boundary. In our universe their muscles are pushing their flesh. In the other universe? Their muscles are following. On the boundary? Itâs one of two things: Either thereâs a continuous (but tiny) change from one set of thermodynamic laws to another, or thereâs a straight discontinuity where the two universes meet.
In case 1 itâs fairly obvious that the boundary is an area where time doesnât travel in either direction. If time isnât flowing: you canât cross the boundary, so you canât get to the other universe past the wall of particles that are undergoing an asymptotic temporal dilation as they approach the boundary.
In case 2 thereâs going to be an interesting moment when you cross the discontinuity. The cause of your skin crossing the boundary (the effect) will have to happen after the effect in the new universe. Sadly that canât happen, because the thing causing that effect (in this case a push from your muscles) is still in the wrong universe where time is going the other way. In this case the boundary causes causal violations on both sides, as particles try to cross but canât as they canât move over the boundary until theyâve already crossed on one side, and canât cross the boundary until theyâve moved over it on the other side. Confused? Not half as much as you would be at the boundary. By which I mean every quark in your body will be in a confused jumble floating back and forth at the boundary. They canât move back in this universe and canât move forward in the other one.
Essentially: Anything that has a temporal component canât work as it crosses the boundary. Sadly that includes everything (including spacetime. I canât even picture the Minkowski diagrams for this one.)
@JoeBloggs is right there are two scenarios for crossing the interface between two universes with opposite thermodynamic arrows of time. His case 2, where the interface is sharp discontinuity, requires anyone passing to do so quickly. Otherwise the person would be a mixture of matter and antimatter. The model of quantum tunneling through a soap film like wormhole would make for a safe passage.
What would this look like? This can be demonstrated with a thought experiment. Assume the interface lies at the mid-point of a tunnel. A small vehicle A is used to cross from one universe to another. It travels at one metre per second and the tunnel is twenty metres long. When the vehicle enters the tunnel its passengers we see an identical vehicle B enter the tunnel at the other end and moving towards them backwards. The people in the backwards will be waving hello to those in the vehicle from our universe.
The two vehicles approach each other at the mid-point and instead of colliding from the point of view of vehicle A vehicle B will vanish from in front of them while immediately reappearing behind them and heading backwards towards the entrance where vehicle A had originally entered.
Vehicle A continues forward and goes through the tunnel mouth leading the reversed universe as they do they wave goodbye to the people in vehicle B.
Obviously vehicles A and B are the same vehicle viewed at different parts of its worldline and in close proximity, spatially, to each other.
Case 1 is much more interesting. Here crossing the interface will happen gradually in small degrees. This reminded me of Kurt Godelâs rotating universe solution to general relativity where a spacecraft travelling through its spacetime will gradually have its light cone tipped over until its relationship to past and future are reversed. This reversal of time is relative to the starting point of the Godelian spaceship. To anyone on the spaceship time will still go forward as if nothing had changed.
This seems plausible. The interface between the two universes in case 2 would most likely be vast, perhaps several light years across. Perhaps, it might be as big as a universe in its own right. In that case, crossing between two universes with opposite directions in time will include an intermediate universe. Anyone travelling between the universes has to cross this intermediate universe before going from one universe to another. It would need something like this, and possibly as big as another universe to allow the progressive tipping of a traverserâs light cone in being able to cross the interface. In a SF story the writer can scale down the size of a Godelian spacetime as interface for the purposes of the plot.
There are no surprises here. Once in the time reversed universe everything seen exactly the same in their home universe. Time would appear to go forward in exactly the same way. Entropy increases as usual. If the traversers had a viewing devices to observe what was happening in their home universe it would appear to be running backwards. This omits any possible effects of relativity on the viewer. Things become more than difficult considering how this might work.
If the traversers could cross the interface back their home universe, there might be time travel. This is more likely with an abrupt discontinuity. Whereas with the case 1 huge interface the time taken to it might cancel out any possible time travel.