There was a question recently about gatekeepers controlling the flow of goods between universes. The gatekeepers became a religious order. The premise got me thinking, and I have come to a conundrum over one of the lateral associations.
String theory allows for, even proposes, an infinite number of universes. Infinity, of course, means the system is not closed. The universes may be closed individually, but collectively there would not be a closed system. String theory also allows for the hypothetical interaction and interplay, including collisions, between universes.
So, in a world where goods and energy COULD be transported between universes, through a system of gates, then it couldn't be said definitively that the individual universe was a closed system. Energy could still come in from outside - from another universe. If energy could be transferred between an infinite number of universes, then energy itself would be infinite.
What would entropy look like under such a system? What would happen to a universe that had an excess amount of energy brought in, and what would happen to the universe that lost the energy? The Laws of Thermodynamics apply only to a closed system.
Here is a reference that addresses the issue. Confronting the Multiverse: What 'Infinite Universes' Would Mean
'Is such a multiverse merely speculation? Certainly it is not as widely accepted by scientists as quantum physics or the Standard Model of particle physics. But it is motivated by real science, and it does follow from the equations of cosmology that optimally explain the origin and structure of our universe. In fact, in some of Linde's mathematical models, cosmic inflation must be expanding eternally and chaotically.'
This thread is somewhat addressed in the new sci-fi saga
Mission Beyond The Stars: Book #1 of "Saga Of The Lost Worlds" by Neely and Dobbs
A saga that addresses what happens if one universe starts appropriating energy from another universe, upsetting the multiverse balance and the entropy within a universe.
To reduce the scope of the question somewhat, I will eliminate the concept of cosmological inflation. Universes are not continuously appearing. Big Bangs do not occur randomly out of nothingness. What we have is what we've got. It's just that there is an infinite number of them. Sort of a static infinity. Since the answers so far apparently assume this already, it does not change any answer.
To further clarify the question, I should clarify my terms. Entropy has been used as a fudge factor variable, whose definition tends to change from situation to situation, the primary purpose of witch is to provide a reason for why something can not be done, in the absence of any other factual reason.
I am talking about the SI definition of entropy, (carnot?) with the units joule/kelvin. If the temperature is zero, division by zero presents a real problem. If energy (joules) keeps increasing, then so to does the maximum entropy level. If there is finite energy, then it would seem the limit of entropy is the maximum energy. Apparently, very bad things happen when this limit is reached. If entropy can never be destroyed (lowered), then at this limit this energy could not be used to generate more heat. Ostensibly, this is because there is no more energy potential difference - no more potential for energy to go from a higher level to a lower level. All available energy is equally distributed. Heat is produced when energy flows from higher potential to lower. Presumably, the temperature would therefore be close to, but not exactly, zero. More energy, obviously, raises the entropy limit. But also, obviously, only if it is available to produce heat through a potential difference. A warmer universe that can provide available heat for transfer would lower entropy. Heat and energy are not the same thing.
So a gate between universes at a different energy level would presumably produce an energy potential difference at the gate. If one universe were warmer than the other, heat could be transferred.
So, if energy were drawn from another universe, would its entropy drop? Recall that mass IS energy, and moving mass from one universe to another is moving energy. Would it end up cooling faster?
Would trade through these gates be one colossally huge application of a negative balance of trade? Could they make it up from another universe?
Could you set up a universe-to-universe Carnot engine, for example?