Originally Posted by Jay R
Most of the world follows all physical laws; it's just magic parts that bend the laws. I generally assume that fantasy worlds have laws of physics like my own except that magic is a source of energy and so on. And you know what? My assumptions have never been proved wrong.
The same set of physical laws that include a spectrum also prevent inter-species breeding, like dragons and humans, or owls and bears. They also prevent flying without wings or jet exhaust or being lighter than air.
Putting aside how you're assuming that owlbears and half-dragons are produced through normal reproduction (they are produced entirely or partly through magical means, respectively), and how you seem to think that "jet exhaust" is what makes planes fly...how are things things related?
When discussing universal laws, "Everything works exactly the same except when it doesn't" is semantically equal to "It doesn't work the same."
"There is a law of conservation of energy, except magic" is the same thing as "energy is not always conserved."
"There is a universal gravitational constant between two bodies unless magic changes it" means that there is no universal gravitational constant.
We don't consider something a physical law unless it's universal. Saying that mass & energy is conserved unless magic changes it is as meaningful as saying that an eggshell has never been cracked except once.
Ah, but mass isn't
always conserved. Nuclear reactions change the mass of substances all the time. Yet "conservation of mass" is still considered a meaningful physical law. Also, isn't Netwon's second law exactly
the kind of law you're saying is meaningless? "Everything maintains its velocity until something changes it."
If there's a well-defined set of instances where a physical law is violated, it can still be used as a meaningful scientific law (assuming those instances are properly accounted for). As it so happens, typical fantasy settings do
have (reasonably) well-defined sets of instances. They might not be defined well in the work itself, but a sufficiently dedicated thaumatologist could figure out how it works as well as a sufficiently dedicated physicist can figure out how, say, nuclear fission works.
In fact, I'd argue that the apparent violations of conservation of mass/energy/momentum are just the mass/energy/momentum coming from something unusual. Just because energy can be supplied by gods or souls or qi or draconic blood or multiversal space worms or whatever doesn't mean we can (or should) throw all physical laws out the window. And before you say something about nonphysical forms of energy not counting, I'd like to bring up potential energy and neutrinos. A rock at the top of a cliff is identical in every way to one at the bottom, but the former has more energy. Beta decay seemed to violate conservation of energy and momentum, but rather than throw the theory out the window, Pauli and others hypothesized that some unknown and then-undetectable particle was the source of the energy (and were proved right, of course)—so who's defining "physical," anyways?
One reason not to assume modern physical laws exist except when they don't is that the assumption serves no purpose. It doesn't help the game in any way, and it didn't even preserve the physical laws.
I heartily disagree. If we're free—let alone forced
—to set aside all assumptions of how the world works, it's impossible to start anywhere. You literally have to create an entire new set of physics before your players understand what they can and can't do...and if the end result isn't
just "It's like our world, but with magic," the players aren't going to be able to understand what's going on.
Another reason is to allow cool story ideas. In the same game, I introduced seven artifacts, in the hands of adventurers, called the Staves of the Wanderers. They turned out to be staves that each carried powers from the seven planets of the Ptolemaic system ("planetes asteroi" - the wandering stars). So they were themed to the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn. Note that I used the medieval assumption that the sun and moon are planets, and the earth is not. An unthinking assumption of modern physics would have prevented that entire adventure.