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There was an interesting question about the maximum speed of a character, with an even more interesting answer that a properly optimized character can break the speed of light. (So, isn't general relativity present in D&D, and is there no time dilation, etc.?)

If someone can reach such ludicrous speeds, is would be certainly possible to also throw objects with nearly the speed of light. This would lead to an interesting phenomena, as you could turn any piece of rock into a nuclear warhead.

Would it then be according to the rules, that any character being able to reach such accelerations and speeds, could destroy huge cities just by throwing pebbles at it with 0.9 c?

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I am not sure this qualifies as "practical, answerable questions based on actual problems that you face" and may be off topic for this SE. –  mxyzplk Dec 6 '12 at 12:30
This seems to be making simulationist assumptions about gamist loopholes, which is just incorrect as a premise. (Given a contradiction as an assumption, anything can be proven "true".) –  SevenSidedDie Dec 6 '12 at 16:25
Remember, this would have to be in a vaccum. Throwing something that fast in the atmosphere would bring a detonation far closer to the character than the expected target. Unless they cast Protection from Atmospheric Friction, of course. –  GrandmasterB Dec 6 '12 at 17:23
Why do people assume Newtonian or relativistic physics in a game where a ziggurat of proper size behaves like a perfect half sphere? –  psr Dec 6 '12 at 19:12
@SevenSidedDie The referenced question is 3.5, but I'm not sure this one is necessarily 3.5-specific. –  okeefe Dec 6 '12 at 21:24
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closed as not a real question by SevenSidedDie, wax eagle, mxyzplk Dec 6 '12 at 22:18

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5 Answers

My opinion is that the basic Law of Magic, is :

The energy required for a magic spell is directly proportional to the lethality of the resulting effect

Magic is somewhat a wrapper on physics with different rules. For example, things like teleportation, or matter/energy creation from nothing (which is very difficult in physics, just to use an euphemism) are easy enough, but accelerating something, like a bullet, to 1000m/s (like a .50 sniper bullet) is very hard.

So, you could accelerate something to near-light speeds, but: if it simply goes from A to B without damaging anything, it's relatively easy. if it goes from A to B destroying anything inbetween, it's very very difficult, if not impossible.

Approaching the infinite damage, would require a near infinite energy expenditure, so spells going toward infinite damange start to fail, or revert to another form with a lower damage state.

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You die from fusion induced plasma disintegration before you know what happened, as noted in your link to XKCD.

If you actually managed to send this any appreciable distance you've probably cooked the atmosphere. Time to dust off the SpellJammer or Gamma World rules.

As far as your own game goes, don't worry too much about physics. Light speed is irrelevant in a game where the sun is actually carried in the back of a chariot by a god. Magic is all but required for these speeds and the magic can include the appropriate safety features to allow you to do it without having irreversible effects on the global ecosystem.

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Wouldn't pebbles be melted by the friction with air which happens until you reach 0.9c ?

Just think about how large meteorites burn up in the atmosphere only small chunks hitting earth, and they're not going at 0.9 c like you're intending.

Your link starts with "ignore how you can pitch a baseball at such speeds in the first place" I don't think that's a honest approach in your game you either "go full physics" or you don't.

Also the friction also applies to your character's skin.

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While I do not agree with the "full physics or no physics", then I consent to the notion that you shouldn't include the parts of physics beneficial to you damage output and forget about the parts concerning the means and consequences. Good answer though. –  Undreren Dec 6 '12 at 9:33
I was thinking more like "go full physics or just use the game rules". –  George Bora Dec 6 '12 at 9:46
Agreed. Otherwise they might as well just "cheat". –  Undreren Dec 6 '12 at 9:48
IANAPhysicist, but as long as we're splitting hairs I understand that the heat experienced at high speeds in atmosphere comes not from friction but from the compression of air in front of the traveler. –  Gregory Avery-Weir Dec 6 '12 at 14:25
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I do not believe that there is a "true answer" to this question, since it is very dependent on what you wish to achieve. If "real world emulation" is a goal, then I guess you can fire nukes all over the place. If you do not wish this to be possible, it ain't.

It doesn't really depend on system either. While these kinds of optimization questions regarding speed are all very interesting, then it doesn't really matter what the "real world physics" says; if you and your fellow players want it, you can have it.

However: If physics simulation is a desired goal, then I would just like to inform you, that as your speed goes towards the speed of light, then your weight goes towards infinity, meaning that you have to produce an infinite amount of energy to move that fast. Assuming this is possible in your D&D world, then since your weight is infinite, the entire cosmos would collapse upon you since your gravitational attraction is infinitely large.

"But I'm not trying to move at the speed of light, only almost!" you might say. While this might be true, then if you want to emulate physics enough to allow a rock to turn into a nuke by throwing it at enormous speeds, you might want to consider the fact that the sheer amount of energy needed to throw it this fast will destroy your entire body in the process. To a molecular level. Actually to an atomic level because of fission. I don't think this will translate very well into "damage".

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The damage would quickly explode into ridiculously large numbers –  Maurycy Zarzycki Dec 6 '12 at 9:09
@MaurycyZarzycki I think "arbitrarily large" is a better descriptor than "ridiculously large". While trying to actually calculate the damage might be fun, if at all possible, then it is ultimately futile. There is no way to survive the gazillion dice of damage, unless you are Pun-pun. –  Undreren Dec 6 '12 at 9:20
I used 'ridiculous' not as "fun to calculate" but "the numbers will be so big that it will be stupid in the funny way". At least that's what this word always meant to me. –  Maurycy Zarzycki Dec 6 '12 at 14:45
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D&D doesn't operate under real world physics. Attempting to graft real world physics onto D&D isn't necessarily any more crazy than what magic can do, though.

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+1 for short and concise. Besides, I like your comparison with magic. Magic in D&D can achieve much more violent effects than a thousand nukes ever could anyway. –  Undreren Dec 6 '12 at 8:45
This isn't answer. –  wax eagle Dec 6 '12 at 16:39
This is a reasonable comment but N/A isn't really a valid answer. –  C. Ross Dec 6 '12 at 16:42
I think it follows pretty clearly that if there's no real world physics in D&D, then there is no maximum speed nor are there relativistic effects. –  okeefe Dec 6 '12 at 17:31
I think this answer perfectly points out the faulty premise under which the question was asked, and thus qualifies as an answer. –  Ernir Dec 6 '12 at 19:54
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