So I've designed a si-fi rpg with my friends that uses a mixture of magic and tech. One of my players is magically a teleporter and wants to know if his teleports would maintain his current velocity or if it would disperse it. The example he gave is "if he's in free fall, could he use his teleports to save himself?" I've designed the spell he's using to be pretty intuitive, since it's magic, and doesn't allow him to do things like teleport into solid rock (although he does take psychic damage if he tries) or teleport into another creatures space (instead materializing in an empty space 5ft away). This is why I'm tempted to give him the choice on whether or not he maintains momentum or at least determine the direction that momentum is heading. My thought was that in the event of freefall, he could teleport to the ground with his momentum headed in an upward direction (have you ever teleported upside down?) and while that might not save his life as he'd still be hurled up into the air, once he started to fall back down he could teleport again to the ground with very little momentum to spare. I guess I'm just looking for some thoughts on all this and if you guys have any ideas or suggestions.

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    $\begingroup$ "Maintain his momentum" ... relative to what? Momentum is not an absolute quantity, it depends on the frame of reference. For example, if the teleporter teleports from New York to Beijing (with no initial velocity relative to the New York local frame of reference) and maintains his momentum with respect to an inertial frame of reference he will find himself travelling at about 1400 miles per hour (2200 km/h) with respect to the Beijing local frame of reference. $\endgroup$ – AlexP Aug 4 at 15:48
  • $\begingroup$ Welcome to the site, Hannah. Please note that the Worldbuilding SE is dedicated to providing detailed answers to specific questions you have while building your fictional world. While we do help refine and improve magic systems, you're asking us to define one for you, which is outside our scope. We don't define the rules for your magic; only you can do that. Answers to that type of question are purely subjective. Feel free to take the tour and check out our site culture to get a better understanding of the site. $\endgroup$ – Frostfyre Aug 4 at 16:15
  • $\begingroup$ @Frostfyre I apologize if it came across as asking people to build my game for me, but I assure you that I was just seeking people opinions on mechanics I'm thinking of implementing based on their past experiences and maybe even their home brew creations. Also thanks, It's good to be here. $\endgroup$ – Hannah Aug 4 at 16:26
  • $\begingroup$ "seeking people opinions" -- Unfortunately, this is the problem. The StackExchange model is built on the idea of "one question, one best answer." Asking for opinions necessarily defies this expectation and is why there's an explicit vote for closure for this reason. $\endgroup$ – Frostfyre Aug 4 at 16:51
  • $\begingroup$ I think it is fine to look for rules to make fantastic stuff workable and consistent within a game. There are various ways to do it and one might be the simplest or most workable or most awesome or most realistic - whatever the op is seeking. There have been several previous looking for telekinesis rules; similar principle and not closed. $\endgroup$ – Willk Aug 4 at 17:18

Unless he is in space there will always be matter at the target site.

Gas is mass too! Here are some rules suitable for an RPG.

1: Mass currently occupying site determines suitability for teleport arrival. It must be below a certain mass or he cannot manifest in the site.

2: Any mass at arrival site (probably gas mass) switches places with mass at departure site. So there is not a vacuum or sudden wind on arrival or departure.

3: Kinetic energy of arriving atoms is normalized to kinetic energy of surrounding atoms. So his atoms are matched with those of his surroundings. He might be high in the air but on arrival he is not moving relative to the gas around him.

I think the issue of "where does the kinetic energy go" is subsumed within the larger "Teleportation???" question.

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When you want to allow teleportation over longer distances, then you necessarily need to take momentum into account due to the rotation of the Earth. When you teleport one quarter around the world eastwards, then your radial velocity which usually makes you move with the surface of the Earth is now an upward velocity.

When we are talking about short-range teleportation, then the momentum question mostly becomes relevant when teleporting from or onto/into moving objects. Like teleporting from one moving car to another.

But simply allowing the teleporter to decide their velocity however they want when they materialize would also open up a lot of other exploits. When they can stop themselves from falling that way, then they can also do the opposite: catapulting themselves in the air by choosing to materialize with a high upward velocity.

Possible compromises you could take here:

  • Only allow the teleporter to teleport onto a solid surface. They take on the velocity of the surface they teleport onto.
  • Allowing them to change their velocity, but it reduces their teleportation range accordingly
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