Alternate worlds … with the power of PHYSICS!
This is just me playing with an idea.
I’m sure you’re all familiar with the ‘generic fantasy story plotline #1’, which involves people from our world travelling to someone else’s. It’s never usually too clear ‘where’ exactly this ‘other’ world is, but the implication is generally that it’s somewhere external to our universe. (That is to say, it’s located in a region of spacetime such that cosmological expansion will never bring inside the visible region of space, even after infinite time has elapsed.)
Usually, perhaps with a few exceptions, these ‘worlds’ at least appear to have the same sort of physics that we have here on Earth.
Consider. There are relatively few fantasy stories (that I’m aware of, anyway) which do without things like (say) gravity. Thrown objects usually come back down again. Arrows work. Momentum transfer (i.e., swords and lances) seems to behave itself. There’s weather, which implies the presence of some sort of water cycle. There are usually sunlight and seasons, which also tends to imply some sort of astronomy – although the question of whether or not the thing in question is a ‘planet’ as such is rarely addressed. (A rare example is in Michael Moorcock’s ‘Corum’ series, where the Earth transitions from being a Chaotic world to a Lawful one. One side-effect of this is stuffed-up weather patterns – the prevailing wind-patterns worked properly when the planet was still flat. They work rather less well when it suddenly becomes spherical … leading to heavy snowfall in Cornwall in summer!)
But where does this leave all this gadding back and forth between worlds?
The transferences are often presented as very limited, finite events. Sometimes this is even to the point of being one-offs. The implication is almost always some sort of ‘magical’ process, which is thus beyond any understanding or limitation, of course.
Except that this is complete crap.
One end of these processes has to be grounded in our world, and a basic principle of our natural law is essentially that if something can happen once, it can happen again. Also, an unlimited and un-understandable magickal wiggit risks becoming a rampant, plot-wrecking deus ex machina. If at all possible, I think it’s much better narrative technique to ground any ‘magical’ events in some sort of limitations.
So … suppose we have two worlds, A and B. We want people to go from A to B, but not too often and in a way that stops the transference process from wrecking the plot. (If you can bounce back and forth between planes at will, then you’re unlikely to ever be in much danger…) We also want it to be vaguely consistent with natural law in both A and B.
Asking a lot, you might say. How might we do this?
Well, here’s one idea. We’re assuming an inhabitable planet in both A and B – but that doesn’t mean they have to be identical. If Planets A and B are slightly different sizes, say, or have slightly different orbits … Well, in that case it’s perfectly possible that they won’t always be ‘overlapping’ each other. Sure, you can make the jump from A any time you like, but if Planet B is several million miles away from you when you arrive… Yeah. Obviously not the best of plans there!
So, in this case, when can you make the transition?
Ideally, you need to do it at the moment when Planets A and B are occupying the same equivalent positions (presumably relative to the local suns). That way, you just step across, and there’s ground under your feet at both ends. However, that’s only going to be a very brief instant. It’s also presumably not going to happen any more often than once a year (unless the orbits are very odd indeed). It may well be much rarer, depending on the local kinematic arrangements. Also, the Coriolis effect could give you a nasty bump, as is pointed out in Terry Pratchet’s Discworld series.
Also, if things go wrong, you risk arriving either hundreds of feet up in the air or miles below the ground. Neither of those is really very viable!
However, you may actually be best off waiting until when you’d arrive up in B’s air. You see, the atmosphere has depth – you have some room for error. And if you have a parachute strapped to your back, arriving in mid-air might not be such a problem…
Of course, there’s an additional issue to consider. They’re called ‘conservation laws‘. The really big ones on the human scale are conservation of energy (mass is effectively a subset of this, thanks to Einstein), conservation of momentum and conservation of charge. These laws essentially say the same thing – that the total amount of the quantity in question can’t change. You can shift it around, you can transmute it a bit, but the fundamental thing itself isn’t going anywhere. (Also note that these are some of the most robustly-supported physics that we have – if you want to argue with these three, you need a damned good intellectual saving-throw.)
So how do these relate to going from A to B?
Well, we’ll ignore conservation of charge for the moment – people are usually pretty neutral, electrically. But if you’re moving a mass out of the Universe, well, there’s a pretty obvious conservation of energy issue there!
One way around it, of course, is moving an equivalent mass back, at the same time. (70 kg of adult person goes A to B, 70 kg of something else goes from B to A. They wave at each other in the middle.) Obviously this might be difficult to arrange in practise (but we want travel between worlds to be difficult, don’t we?).
Okay, so maybe CoE is vaguely under control – sort of. Umm. (Okay, it’s not at all, but let’s just siddle away from the transit-duration issue for the second here, shall we? If we’re very, very quiet, it might not see us…)
The really sticky issue is going to be conservation of momentum. You can possibly control for differing planetary rotation speeds and differing orbital velocities, as we touched on above. But, ensuring that the mass exchange also has the correct momentum exchange … that’s going to be extremely tricky indeed.
Of course, you can sidestep a lot of these issues if you make one small assumption: that you’re not actually leaving our universe. If your fantasy-world is actually an exoplanet, then no momentum or energy have been vacuumed out of our cosmos.
However, this will create a secondary set of narrative issues. Also, as a plot-revelation it would have sold better in the 70s or the 80s. (SF used to outsell fantasy, weird as that may sound now. This is why a lot of ‘old’ fantasy novels have carefully-planted hints that allow them to be read as SF rather than fantasy. Consider for example the ruins in the Shannara series, and the description of what sounded suspiciously like an ancient atomic war.) Also then you have the big issue that if Planet B has a human population as well, then Mr Darwin may ask you, ‘Seriously dude, WTF?’
(Although, you are assuming that going from A to B is at least possible, and if it can happen once, it can happen again…)
Ironically, this one thing is less of a problem for the ‘alternate universe’ scenario. Some estimates suggest around ten to the twentieth power random combinations in humanity’s evolutionary history, which obviously leads to a very, very, very low random-repeat probability. But if you assume more than 10^20 universes … well, even the horrifically-unlikely will happen eventually. Hurray for the long tail on the Poissonian curve.
…So, in summary, where have we found ourselves? What do you need when you’re packing before catching the flight to B?
Item #1 is, of course, a parachute.
Item #2 is a set of bathroom scales. (You need to know how much you mass…)
and Item #3 is a damned good watch. (You really, really don’t want to be late.)
Item #4 is a well-sprung net to catch whatever gets slung back B to A – but only if you like your neighbours and don’t want to put their house at the bottom of a crater.