OSR: Rules of Thumb for High-Energy Physics

This post was written while moderately feverish. You've been warned.

It's good to have handy approximations for D&D purposes. You're about 5' tall. If you stick your arms up, you're 10' tall. A story of a building is around 14', so if you fall off a 10 story building that's 140'. Etc, etc. All GMs eventually come up with their own set of unstated mental yardsticks.

Here are a few handy tips for high-energy physics.

Wizards break rules all the time. Fall off a 10 story building? Hah! Try falling from 40,000' in the air thanks to a botched teleport spell. Use a sphere of annihilation to delete half a portable hole. Loot an ancient nuclear battery and cover it with lead. You know, the usual wizarding things.

1. Munroe's Third Law

If two objects of approximately equal density collide at high speed, the results will always resemble a ripe tomato thrown very hard at a cake.

We're going to use density a lot, so let's figure out some mental yardsticks.

Humans are approximately as dense as water. In fact, anything that's made of meat or plants is approximately as dense as water. Stone is 3 times as dense as water. Iron is about 8 times as dense as water. Lead (or gold) is about 10 times as dense. We're not going for accuracy, we're going for speed.

So if you fire a tree at a dragon at a bullet-speed? Cake and ripe tomato. Fire a stone cannonball at a stone tower? Cake and ripe tomato. But what happens if you fire an iron dart at a dragon?

2. Newton's Depth Approximation

This is one of those rules that seems really obvious once it's pointed out.

Momentum must be conserved. m1v1 = m2v2. So if you've got an object of mass m and velocity v heading towards a stationary object, and v is very fast, then all the object can push is the stuff directly in front of it. Once it's moved its own mass "out of the way", it'll stop.

An object's impact depth, in a material of equal density, can therefore be approximated as the object's length.

That's super neat! How deep is the impact crater? About as deep as the object.

How far will bullets go through water? Well, lead is 10 times as dense as water, so a bullet will go only ten times its own length. That's not very far.

And of course this ignores a ton of additional considerations, but it's a very handy rule if you start trying to run through walls or explode bunkers.

Wikipedia article

3. When In Doubt, Check The '50s

Air-to-air. Unguided. Nuclear. Weapons.

Look, the past is a different country, but some parts of that country are ludicrously dangerous. I'm sure very sensible papers and sober papers were written at the time on the absolute necessity of unguided air-to-air nuclear weapons... but... really?

Anyway, people (mostly the incomparable Alex Wellerstein) have created tools for visualizing various nuclear scenarios. Playing around with Nukemap will give you a few mental yardsticks for any potential nuclear disasters in your RPGs.

Fallout-style nuclear cars, alien power sources, ghost-busting rayguns, or other high-tech but portable devices can be approximated as the adorably murderous Davy Crockett portable nuclear device. Should said device detonate in the hand of a hapless PC, One large building disappears, a city block is leveled, around 3 blocks are scorched and shattered, and 9 blocks are irradiated to the point of medical consequences.

4. At Sufficiently High Velocities, Everything Behaves Like A Fluid

"Fluid" is a bit like "fish", in that it makes some degree of intuitive sense but falls apart when examined. Still, if you're dealing with velocities faster than the eye can follow, using your intuition towards pudding is probably not a bad idea.

5. You Don't Have Time To Un-Fuck Up

High-energy phyiscs, in general, doesn't deal with round-by-round D&D very well. Round 1, you're a sauropod eating plants in a shallow swamp. Round 1.000001, you and everything you've ever seen is plasma. Fast extinction-type asteroids don't go skittering across the sky like meteors. They go from space to ground level between refresh cycles on your monitor. There's less of a shockwave and more of a shockwall. It's a nice sunny day until everything suddenly goes white.

Take some radioactive material from subcritical to critical? Your reflexes aren't faster than a speeding neutron. Decide to rewire your bathroom while the wires are live? Your reflexes aren't faster than AC. Etc.

The OSR generally supports the notion that you can make one really bad choice, then a whole string of good ones, and still be utterly fucked over when that bad choice hits maturity. So does high-energy physics.


  1. Laughed hard, which was needed. Thanks, mate.

  2. Because you specifically mention the problem of teleporting into the stratosphere, it's worth noting that downwards force is determined by mass, while air resistance is determined by the area of the object's surface or cross-section. And because of the square-cube law, the latter scales up more slowly. The consequence of this is that larger objects tend to have higher terminal velocities.

    Or to phrase it as a handy rule of thumb:

    "You can drop a mouse down a thousand-yard mine shaft and, on arriving at the bottom, it gets a slight shock and walks away. A rat is killed, a man is broken, a horse splashes."
    — J.B.S. Haldane

    Whether this means that shrink spells can be used as emergency parachutes is left as an exercise to individual GMs.

  3. Get well soon! Also, excellent post.

  4. To be fair, an air-to-air nuke doesn't really need to be guided. Just getting close will really ruin your target's whole day.

  5. Blivets were an interesting idea in air warfare (and were developed by both sides of the Cold War). Especially given the limitations of long range detection and guidance of AA missiles at the time. Best chance of eliminating enemy bombers raining nuclear death down on your people. Remember this was also the time they were suggesting using nuclears for massive land clearance projects. Scientists were quite familiar with the deterministic (immediate) effects of radiation exposure, but still rather hazy on the stochastic (long term) effects which is what affects the radiation exposure limits we have today.

    Don't forget this was also the era of the nuclear-powered turbojet (designed so a bomber could remain on station until the food ran out), and the nuclear-powered cruise missile.

    The thermonuclear hand grenade has long been a staple in fantasy gaming circles (with players trying to engineer some solution where it might actually be of use. It was a very Paranoesque device. Awesome killing power, but only if you were feeling suicidal. Much like the Head of Vecna.

    Then again I do prefer Archimedean physics in my D&D games. A lot simpler. Rocks fall faster because they have a greater love of the Earth which attracts them. Feathers don't because they have a love of the Air, which is why birds have feathers. Helps them fly. It does mean that Reverse Gravity is a spell that is mostly unknown since gravity and other fields don't exist. On the other hand wizards might know the spell Slam that has the same effect - but can also be used sideways.

  6. Compared with this device https://en.wikipedia.org/wiki/Davy_Crockett_(nuclear_device) using a nuclear tipped air to air missile to take out a formation of nuclear armed bombers is sane and reasonable given the (then) recent experiences in Europe. Or would you prefer to be carpet bombed with Hiroshima level nukes?

    1. True, and the entire plane does act as a human-operated guidance system up to a point... but missile technology in the '50s was not exactly the peak of reliability. Firing a Gremlin and having it cruise neatly into a formation of bombers would be ideal. The chances of it just falling off the rail, sailing through a formation, or spluttering out too short are high, so of course you'd need to fire a few missiles at the formation...

    2. It wasn't even the experience of the War in Europe, that made this weapon seem a good idea.

      Since the twenties the prevailing idea was that "the bomber always comes through." You can not, under any circustance, allow that, if the bomber carries one or more fission or fusion bombs.

      Except for that, thanks for this great intuition pump.

    3. ah, but what about *pigeon* guided missiles? https://en.wikipedia.org/wiki/Project_Pigeon