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Conservation of energy is a "universal" law, and would be counterintuitive to have someone travelling from a universe like ours - with our physics framework - into another universe that obeyed other laws of other-physics.

So, if we keep that premise that a parallel universe accessible from ours has to be under our same laws of physics framework, to have someone or something travelling from our universe into it (and back from it into ours) would break energy conservation law as matter/energy would disappear from our universe and "be created" in another parallel universe.

I thought about a mandatory constraint that would kill someone - or "someones" - into the destination universe, but that is bad because it makes the universe "sentient" somehow and then it turns to crazy pseudoscience and not some "possible science" based on quantum speculation. Also, how would the origination universe replenish the energy/mass lost when the traveler left it.

Then, I had the idea to have a forceful exchange between a traveler and its counterpart on the destination universe. That is an amazing thing for storytelling, but creates another problem, wormholes are one-way only conduits meaning there is only one direction through the central singularity from the black hole to the white hole. That makes impossible to exchange subjects without another wormhole opened from the destination universe.

I am hitting my head against this roadblock in my worldbuilding now, as parallel universe traveling is the basis of all my stories.

What are some ways you would suggest that would make it possible to keep energy conservation universal constraint when a given mass travels from a parallel universe to another through a wormhole?

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    $\begingroup$ Matter can't be created or destroyed. In this case you aren't doing either. You are moving it. It disappears from one place and appears in another, but it isn't be created or destroyed. $\endgroup$
    – Issel
    May 26 at 17:59

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Energy conservation is not a universal constraint. It is a consequence of a symmetry called time translation invariance and energy is not conserved if this symmetry is broken. In fact our own universe breaks this symmetry due to its expansion and as a result the energy of the universe is not conserved (though energy conservation still applies at smaller scales).

Without a precise description of the physics involved in travel between universes it is impossible to say whether the process will or won't conserve energy, but there is no reason why it must do so. Given this I think you are worrying unnecessarily.

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In our world we know that, because of thermodynamics, the entropy of a closed system can only increase. Yet we have systems where locally the entropy decrease. This does not violate the thermodynamic laws because the local decrease is compensated by a large increase outside the system, resulting in an overall increase according to the law. Examples of this are a fridge or a teenager cleaning their room.

Another example is the conservation of mass: since the times of Lavoisier chemists knew that the mass in a reaction is conserved. Then somebody came on stage with some strange (nuclear) reactions where mass is not conserved. The conservation was saved by changing it to the conservation of mass-energy.

Go along the same way: it's the energy of the whole multiverse which has to be conserved, not the one of the local universe. If up to a certain moment the inhabitant of your local universe have thought the conservation of energy was holding true it was only because they didn't have any experimental evidence of the contrary, like it was for nuclear reactions.

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    $\begingroup$ Exactly this. MEC can be sustained by defining the scope as the multiverse, rather than each individual universe contained within. $\endgroup$
    – GOATNine
    May 25 at 14:36
  • $\begingroup$ Alternatively, mess with the assumptions here: "because of thermodynamics, the entropy of a closed system can only increase" Once you add a second universe, the system is not closed. Asimov's The Gods Themselves explores this to an extent. $\endgroup$
    – Anon
    May 27 at 0:49
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When an object of mass $m$ enters a wormhole, the perceived mass of the entrance end increases by $m$ and the perceived mass of the exit end decreases by $m$. Energy is thus conserved locally on both ends, and globally in each universe.

This means wormholes have a finite number of uses, but if they're large enough, that won't interfere much.

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    $\begingroup$ Just adding some references for you: physics.stackexchange.com/questions/109649/… physics.stackexchange.com/questions/277727/… $\endgroup$
    – E Tam
    May 25 at 19:48
  • $\begingroup$ "This means wormholes have a finite number of uses" Or that you need to be careful about balancing the directional mass transfer across them, to prevent them from collapsing into black holes. $\endgroup$
    – nick012000
    May 26 at 7:35
  • $\begingroup$ @nick012000 that assumes wormholes are bidirectional. If they're one-way, then even if you have two wormholes to complete the path, it won't balance masses of each wormhole end. $\endgroup$ May 26 at 13:22
  • $\begingroup$ This is the correct answer. $\endgroup$
    – John Doty
    May 26 at 20:03
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The law of conservation of mass/energy says that within a closed system the total mass/energy can't increase or decrease.

But if someone travels by wormhole or otherwise from Universe A to Universe B, neither Universe A nor Universe B can be a closed system. The smallest closed system will have to be Universe A and Universe B. And if there are also wormholes leading to Universe C, Universe D, etc., Universe C, Universe D, etc. will also be part of the same closed system as Universe A and Universe B.

And moving mass/energy from one part of a closed system to another part of a closed system does not violate the law of conservation of mass/energy. It is something which happens all the time.

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  • $\begingroup$ Universes are elastic. You move energy from uA to uB then uA gets slightly smaller and uB grows slightly larger. Move enough energy from uA to uB then uA simply collapses. $\endgroup$
    – Arluin
    May 25 at 18:49
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Suppose that energy is not conserved within a particular parallel universe. This could lead to an imbalance over time, but that imbalance could be interesting for narrative purposes. As an analogy, consider a lightning storm, where a large charge separation develops and there's eventually a large enough potential difference between the clouds and the ground to cause lightning strikes. If these parallel worlds are rubbing up against each other and sometimes exchanging energy via wormholes, then this energy imbalance might lead to interesting consequences in whatever physics powers your variant of multiverse. Perhaps the formation of wormholes in the first place is analogous to lightning strikes, openings forming as a result of an energy imbalance?

There's also nothing that says this has to be a simple tit-for-tat process. Your inter-dimensional weather, as it were, might be something that tolerates a fairly large imbalance up until it crosses a certain critical threshold, at which point wormholes start popping up all over the place. There's a lot of potential narrative there, especially if this starts happening in a universe that's been relatively low energy for a long time and suddenly has a ton of wormholes appearing from a neighboring high energy universe.

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  • $\begingroup$ ObSF: The Gods Themselves, Isaac Asimov, 1972. Doesn't actually say "wormhole", but the interuniversal transfer mechanism does generate the same kind of energy imbalance. $\endgroup$ May 26 at 13:25
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It is neither created nor destroyed

You already state that the premise is that any accessible parallel universe needs to be under the same laws. There is a free solution to your problem. If the accessible universes are under the same laws, it can be seen as one law for these universes.

When something is moved from one universe to another, you have the same amount of energy in all universes, allowing the law to be correct.

Different problem

Although this seems to solve the problem, it's not quite true. If one travels from a planet to the orbit of a planet on another universe, you still have some added potential energy! To correct this, you could add the constraint that extra power for transfer is needed in such cases. The energy required will automatically demand more or less energy, depending on the difference of (potential) energy the places have with each other. Energy free travel to outer space or the like!

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  • $\begingroup$ Interesting thought about potential energy being a problem. Momentum is another one... but harder to account for if we don't have a shared reference frame as a foundation, I guess. $\endgroup$
    – Corey
    May 26 at 3:12
  • $\begingroup$ It's not a problem. The exit mouth of the wormhole is in orbit and has the potential energy. When a ship exits the mouth, the mass of the exit mouth decreases and the potential energy of that mouth is transferred from the mouth to the ship. $\endgroup$ May 26 at 13:34
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If you open a portal between dimensions then the possibility exists for two-way movement of matter/energy.

It is possible that as the person/ship/whatever moves from one universe to another, an equal amount of energy/matter moves the other way in the form of subatomic particles/em waves virtual or otherwise. As you are writing the story you can choose the exact composition to suit.

Maybe the influx is detectable from far away or only close by. Maybe the effect is dangerous (think nuclear explosion), moderately dangerous (huge amount of heat/light/sound), innoculus (neutrinos).

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"Conservation of energy is a "universal" law, and would be counterintuitive to have someone travelling from a universe like ours - with our physics framework - into another universe that obeyed other laws of other-physics."

There's an easy way to do this, if you are willing to bend the meaning of "travel", which is to use an avatar. You convey information between universes so as to create a body in the other universe and control it remotely. What the body senses is conveyed across the interuniversal void and fed into your senses. Your chosen actions in response are conveyed across the void and acted on by the body.

Gods incarnate themselves on the mortal plane as avatars, demons steal control of somebody else's body as a posession, robots are used to give the remote-body experience in "telepresence", and of course computer games (especially with the VR headsets and haptics) allow us to explore an artificial game universe which has different laws of physics. And as in the film "The Matrix", the experience could in principle be so immersive and lifelike that the explorers don't even realise that the world they experience is artificial, and they are actually controlling their bodies from another universe with different physical laws.

You can even interpret Cartesian Dualism - the idea that there are separate spirit worlds and material worlds, and our spirit-world "soul" occupies and controls our material-world body - as another instance of the same situation. How can immaterial spirits and souls occupy and interact with the world of matter? By "remote control".

If you don't like the idea of the traveller staying in their home universe while travelling, you can also use it as a temporary stage. You create a working copy of yourself in the other universe, fully autonomous, making appropriate changes to account for the different laws of physics (assuming they are Turing-complete), and then delete the original. Minds are software. So when an AI transfers from one computer to another - which may have different hardware and a different instruction set - you have to translate the software, but it does the same thing. It has the same memories, personality, behaviour, likes and dislikes as the original.

There are all sorts of interesting philosophical questions about personal identity if you can make multiple copies, if you can fork and merge personalities, or if you can distribute a mind across multiple universes, perhaps exploiting different rules of physics in one universe to be able to perform computations impossible in another. A quantum computer, it is said, is a computer spread across the multiverse, harnessing the distributed computational power of computers in billions of parallel realities to be able to do things no classical computer can do. All those computers follow the same laws of physics, but what could you do if you had access to universes with different laws?

Are they multiple instances of yourself (the same person) or are they more like your children (different persons)? Are they like a society of interacting individuals (many people) or more like a hive mind or individual neurones in a brain (a single person)? There are lots of options.

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    $\begingroup$ Absolutely brilliant first post. Welcome to Worldbuilding, please enjoy our tour and read-up in the help center about how we work. $\endgroup$ May 25 at 18:16
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You're assuming too much.

Quanta need not be identical in every aspect of their form (or, conceivably, in any aspect of their form) in order to satisfy the fungibility requirement: the relationship between mass and energy is fixed, but mass is "fluid" and energy is "fluid"; therefore the form required to balance the transfer is also fluid.

You're basically asking "If I remove from this universe quantity AB, and put that into a parallel universe, then how do I guarantee that "A" and "B" are transferred from that universe into this one?" It doesn't work that way:

The balance happens as a consequence of the transfer — so, there's nothing required on the part of the entity initiating the transfer.

Moreover, the balancing transfer can be either local or non-local (or both) — so, you might transfer a person from this universe to a parallel (and hopefully, compatible) neighboring universe, but both

(a) the balancing mechanism might involve countless other universes; and

(b) the points at which the balance in this universe is restored could be one or several or an incalculably great number, and the mass-energy distribution at any particular point could equal the minimal existential value.

  • Let's imagine E = 10 and m = 10: Em = 100.
  • Now let's imagine E = 25 and m = 4: Em = 100.
  • They're different, and yet they're the same.

The implications are many more than can be competently discussed in this forum, but I hope that I've helped you to understand better how to approach this issue.

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The wormhole, while it appears to exists in one location, actually stretches over the whole Universe. Thus when someone or something goes through the wormhole, the wormhole exerts a tiny force on everything on the other side. This mostly drags some unlucky hydrogen and helium atoms into the wormhole to balance the mass/energy of each Universe. Some scientists have speculated that this is where all the odd socks disappear too.

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Simply release the energy

If the trip is from a high potential energy to a lower potential energy, then the energy is released in pure form. This can result in pair production of photons, protons, neutrinos ... whatever you like. Think of it as a Big Bang in miniature. (Hmmm... was the original Big Bang from someone who tested your wormhole and found out the first step was a doozy?) There will be particles and antiparticles formed in equal numbers, so the traveller might be exposed to antimatter in some small amount, not to mention gamma rays. Shielding might handle some of that, though it released more energy to bring it through.

But what if you're going uphill? Same thing! Only now you release negative energy = negative mass, which is a sort of ridiculous concept of particles that are lighter than zero. (Some people once thought wormholes and warp drives needed to be made with it) Negative mass does funny things like run away with positive mass because the negative gravitational pull pushes on the positive but attracts the negative. But that's just another sort of radiation and eventually it should hit something and get cancelled out. The net effect from the traveller's perspective is that some of their subatomic particles go away without trace, leaving them with some minor radioactivity. Absurd, yes ... not as absurd as interuniversal travel with wormholes, so you should be fine.

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