If I shot an infinitely strong laser beam into the sky at a random point, how much damage would it do?

Garrett D.

A lot of the time, if a question includes the word "infinity," the answer is "an infinite amount"—when there's an answer at all.

An infinitely strong laser pointer would deliver an infinite amount of energy to the air in its path, which would in turn radiate an infinite amount of energy in all directions, which would destroy everything. For more on this, see What If #13.

Except that's not really the right answer to your question. Most of our equations don't really work when you put "infinity" in them. So the right answer is "an infinitely strong laser beam isn't a real thing."

But if you made a laser pointer stronger and stronger, the result would look more and more like what I described. This is sort of like the mathematical idea of taking a limit; you can't say what happens at infinity, but you can see how it acts as you get closer and closer to it.

Since we looked at the "destroying the world" consequence in question #13, let's look at the other part of your question: The random aiming.

First of all, if you aimed in a truly random direction, you would have an almost 50% chance of hitting the Earth.

Almost 50%, but not quite. If we assume you're on a spherical Earth, ignoring hills and trees,[1]And ignoring refraction. you have a slightly better chance of missing the ground than hitting it—because you're holding the laser above the surface, and the Earth curves away from you:

If you miss the Earth, 89,999 times out of 90,000, your beam will pass right out of the galaxy without hitting anything. When it does hit something, it will almost always be the Sun or the Moon.

The first time I met astronomer Phil Plait in person, he mentioned a clever trick for calculating the "area" of the sky:[2]"Learn This One Weird Trick,"" he said, "Invented By a Local Schoolteacher!"‌[3]He didn't say that.

The area of a sphere is 4πr². Ok, great, but what's the "radius" of the sky? Well, if the sky is a sphere around us, the radius is "one radian", because that's the radius of anything. But a radian is also 57.3 degrees, which means the sky is 57.3 degrees "away" from us. 4 times pi times 57.3 = 41,253 square degrees.

The Moon and the Sun each take up about 0.2 square degrees, and so the chance of hitting either one of them is about 1 in 180,000. Those aren't great odds, but they're better than the odds of hitting anything else.

To calculate the odds of hitting something else, we can use this handy angular size reference as a shortcut, dividing the rough angular size of the object in the chart by the Earth's total area to see how much of the sky it covers. The odds of hitting one of Jupiter's moons, for example, are on the order of one in a trillion.

Stars are even worse. Your odds of hitting any star at all on your way out of the galaxy are almost zero, even if you aim for the core.

This is good, though! If your odds of hitting a star were high, it would mean the galaxy would be opaque. Since most "lines of sight" would terminate on a star, it would be very hard or impossible to see past our galaxy.[4]There are only a few stars in our galaxy in the Hubble Ultra-Deep Field image. The fact that the night sky is dark is at the core of Olbers' Paradox.[5]I was so tempted to vandalize this article to put a [citation needed] after every claim that the night sky was dark.

If you kept shooting long enough, eventually you'd hit a planet. Neptune would be the hardest to hit, followed by Uranus and Mercury. Pluto would be the hardest, but it might be worth it. An impossibly powerful laser would at least settle that debate.

Could a person walk the entire city of NY in their lifetime? (including inside apartments)

Asaf Shamir

Like the answer to Paint the Earth, the answer to the first part of this question is pretty straightforward to look up.

But what if it weren't? Can we figure out the answer from things we already know? Let's look at a few ways of estimating it.

First of all, how wide is a street? I've never seen one of those flashing crosswalk countdowns signs start with less than 10 seconds; if people walk at a meter per second, most roads are probably at least 10 meters wide.[1]There's a table from the 1892 World Almanac listing the widths of all avenues and streets in Manhattan, as well as the lengths of all the blocks, confirming that even in 1892 the streets were at least 20 meters wide. I found a copy of the table over at the blog Stuff Nobody Cares About.

Most people wouldn't have trouble walking 10 kilometers (6 miles) in a day. If the city were covered in kilometer-long streets laid down edge-to-edge, with no space between them, you could fit a thousand roads side-by-side in 10 kilometers. That means a person could walk back and forth across an entire 10km by 10km grid in, at most, 3 years.[2]On the streets. You have to add another 3 years for the avenues.

I don't know how many 10 km square swatches it takes to cover New York City, but it's probably not very many.[3]Turns out it's a little more than 1 to cover the land and water. And since NYC has some space not occupied by streets, this tells us that the answer to the first part of Asaf's question is almost certainly "yes"—purely from a geometry standpoint.[4]Another way to come at this calculation is to remember that Manhattan streets are numbered, and you never see four-digit numbers.

Here's another approach: I happen to remember that the US Postal Service employs about half a million people. NYC's population is almost 10 million people,[5]The city itself is about 8.5 million, and the metro area is about 20 million. so almost 1 out of every 35 Americans lives there.[6]I remember seeing some California politician boast that California had 14% of the country's millionaires. But 1 in 8 Americans live in California, so that's pretty close to what you'd expect. If New York also has 1 out of every 35 postal employees, that's about 15,000 people.

If all those employees were letter carriers, and they visit every address in the city every workday, that would mean it takes a total 15,000 x 8 hours = 14 person-years to traverse the city—much less than a lifetime! Since lots of postal employees are not letter carriers, and real letter carriers stop frequently, this estimate is probably still much higher than the reality.

Another way: Imagine that each person lives alone in a square room measuring 10 meters by 10 meters, which is about the size of a typical two-bedroom apartment. Furthermore, let's assume that everyone's apartment is on the ground floor with at least one side facing a street. In that case, at a walking speed of 2.5 mph, it would take only 2.4 years to walk past every apartment—which Wolfram|Alpha helpfully points out is roughly 1.4 elephant gestation periods.

Any way we come at this problem, it looks like the answer is "yes"—you can walk down all the streets in New York City. And, indeed, it turns out there are 6,074 miles of road in NYC, which would take a total of a little over 100 days of walking.

Now, what about the second part of Asaf's question—walking through all the apartments?

This one is trickier. As a rule of thumb, a household is overcrowded if it has more people than rooms.[7]There are a bunch of definitions for different family sizes and methodologies, but they all end up in a pretty narrow range. But at the same time, most households don't have more than two rooms per person. Let's assume all households have 1.5 rooms per person.

Let's assume it takes 20 seconds to get from the door of a room to the door of the next non-visited room. (Most of the time it will be much less, but sometimes the next non-visited room is on another floor or down the stairs, so it's good to give ourselves some extra time.)

If it takes 5 seconds to walk into a room and back out, then you can visit every room in New York City in 10 years. Even if you only visit rooms for eight hours a day, that's totally plausible to fit into one lifetime.

However, a word of warning to Asaf:

Under NY Penal Code §104.15, entering a dwelling without permission is a class A misdemeanor punishable by up to a year in prison.

So while it might take only 30 years to visit every apartment in New York City ...

... it could take you 2,000 millennia to serve out the resulting prison sentence.

People sometimes say "If I had all the money in the world ..." in order to discuss what they would do if they had no financial constraints. I'm curious, though, what would happen if one person had all of the world's money?

Daniel Pino

So you've somehow found a way to gather all the world's money. We won't worry about how you did it—let's just assume you invented some kind of money-summoning magic spell.

Physical currency—coins and bills—represents just a small percentage of the world's wealth. In theory, you could edit all the property records on Earth to say that you own all the land and edit all the banking records to say you own all the money. But everyone else would disagree with those records, and they would edit them back or ignore them. Money is an idea, and you can't make the entire world respect your idea.

Getting all the world's cash, on the other hand, is much more straightforward. There's a certain amount of cash in the world—it's about $4 trillion—and you want it all.

It won't necessarily do anything for you, since—without the cooperation of the outside world—you probably won't be able to spend it. But maybe you can swim around in it, like Scrooge McDuck in his giant room full of gold.

So you cast your magic spell and summon all the money.

The pile of cash is the size of the Empire State Building, but heavier. You probably don't want to be standing under it, so let's assume you're standing over here, off to the side:

The vast majority of the weight in the pile is coins, and the biggest single contributor is the US penny. Despite periodic efforts to kill off the penny, the US Mint keeps producing more of them.

There are probably 150 billion pennies[1]A 1996 GAO report mentioned a US Mint estimate of the number of pennies in circulation: 132 billion. If you're excited by phrases like "General Accounting Office", "auditing standards", and "Subcommittee on Domestic and International Monetary Policy", you can read the report here.

If you're so excited about penny statistics that the GAO report isn't enough for you, you're in luck: There's another way to derive this number.

We can get an idea of the number of pennies in circulation by looking at all the change in our pockets, counting the dates, and using that to estimate a loss rate. In 1999, some statisticians recorded the dates on 1,000 pennies they had lying around, and published the data in the book Workshop Statistics: Discovery with Data and Fathom, page 389. By plotting the frequency with which they saw pennies of various ages against the frequency we'd expect from US Mint production numbers, we can estimate the rate at which older pennies drop out of circulation per year. Using their numbers, I come up with an estimate of about 190,000,000,000 pennies in circulation.

We can then apply the same formula to previous dates, and come up with a number for 1996, to check against the GAO number. (I have a lot of spare time on my hands.) Surprisingly, this produces an estimate that's still about 190,000,000,000. Either number (190 billion or 132 billion) is probably a reasonable estimate, but if you're serious about your penny statistics—and if you've read this far, you must be—you should probably go with the US Mint guess. After all, they presumably know a thing or two about counting pennies. currently in circulation, for a total weight of over 300,000 tons. In total, US coins and bills are responsible for about 30% of the pile's weight, while the European Union—which has barely been minting coins for a decade—contributes 15%.

Unfortunately for you, the pile doesn't stay a pile for long,[2]When you pour lots of loose material in a heap, it tends to form a cone. Different materials will form cones with different slopes; the steepness of the slope for a given material is called its angle of repose.

I've never found the exact angle of repose for coins—or had enough of them to test it—but a forum poster with the username Master of Coin (who claims to be well-acquainted with "how piles of coins 'slish' about") says that it's probably no more than 5 or 10 degrees. and what seems like a safe distance isn't so safe.

In 1919, a tank of molasses in Boston collapsed. Molasses is thick, so you might think it would flow out slowly, but it didn't. The wave of molasses swept down the streets too fast to outrun, demolishing buildings and killing 21 people.

Something similar happens with the pile of coins. As it collapses, the pile spreads outward, a wave of money carrying a staggering amount of momentum. The pennies, quarters, loonies, and euros scour the landscape in an expanding ring. Within seconds, the wave of coins engulfs you and you die.

There are ways to avoid this. You could, say, build a wall around the coins to contain them. Unfortunately, then you might face a problem worse than death:

Building code violations.

Heavy skyscrapers need ground strong enough to support them. Places with large skyscrapers, like those in Manhattan, need bedrock sturdy enough to hold them up.[3]For a long time, people said that this was why there was a gap in Manhattan's skyline—the bedrock wasn't strong enough to support skyscrapers in the middle. However, a 2011 paper in the Journal of Economic History says that this is a myth, and the bedrock had little influence on the location of skyscrapers. A search through this giant PDF of NYC building codes suggests that if we went ahead with this plan, we would be in serious danger of violating section 1804 ("ALLOWABLE BEARING PRESSURES").

Which makes me wonder: Did Scrooge McDuck ever had to worry about this stuff?

My 12-year-old daughter is proposing an interesting project. She is planning to attach a number of helium balloons to a chair, which in turn would be tethered by means of a rope to a Ferrari. Her 13-year-old friend would then drive the Ferrari around, while she sits in the chair enjoying uninterrupted views of the countryside. Leaving aside the legal and insurance difficulties, my daughter is keen to know the maximum speed that she could expect to attain, and how many helium balloons would be required.

Phil Rodgers, Cambridge, UK

Thanks for getting your dad to send in this question! He said not to worry about the "legal and insurance difficulties," so I think it's safe to assume he's taken care of all that.

Note to police: If you've recently taken into custody two unidentified underage drivers, a stolen Ferrari, and a bunch of helium balloons, the person you're looking for is Phil Rodgers in Cambridge, UK.

Okay, on to your question:

Have you ever run with a balloon? It doesn't point straight up. The air rushing past you pushes it down:

How high the balloon goes depends on which force is stronger--the balloon's buoyancy pulling upward, or the wind dragging the balloon backward. If the drag is too strong, the balloon will stay low to the ground and you won't get a good view.

To figure out how fast you can go, let's first figure out how big our cluster of balloons (or one big balloon, which is probably easier) needs to be to lift you.

People your age weigh an average of 43 kilograms, which means you need a balloon 4 meters wide to lift you—that's about the size of a car. (If you don't weigh 43 kilograms, you can put your weight into this formula.)

A 4-meter balloon will be large enough to cancel out your weight. But that's not enough. It just means you wouldn't fall or float—so you'd be towed along the ground behind the car.

To float upward, you need a bigger balloon. A 5 meter balloon will produce 71 kilograms of lift[1]Usually, physics people will make a big deal about how weight and force are different from mass, but in this case, I'm going to resist the urge, because it's easy to just think of everything in terms of weight.​ (here's the formula!). That's enough to cancel out your 43-kilogram weight, plus a few kilograms for the chair and balloon itself.

The balloon will be dragged backward by the air. The faster your friend drives, the more the air will drag the balloon back. You can use this formula to figure out how much "weight" will pull backward on the balloon for different speeds and sizes. Just change the "20 mph" (driving speed) and "5 meters" (balloon size) in the formula.

If the upward pull from the helium is stronger than the backward pull from the wind, the balloon will float at a high angle. If the backward pull is stronger than the upward pull, the balloon will float at a low angle. If you're using a 5-meter balloon, even if you drive only 10 mph, the balloon will float pretty low behind you.

Fortunately, there's a solution: You can make the balloon bigger. As you make the balloon bigger, the buoyancy starts to win out over the drag.[2]The reason is that the buoyancy equation uses diameter^3 but the drag equation uses diameter^2, so if you make diameter bigger, the buoyancy equation grows more.

If you use a 10 meter balloon, the buoyancy is strong enough that you can drive at 20 or 25 mph and still stay pretty high off the ground. A 15 meter balloon is even better; it would let you go 30 mph while still getting a good view.[3]You could make the cable longer, so that even a low angle still gets you high off the ground. But the cable won't be straight; it makes a curve called a catenary. At a low enough angle, making the cable longer would just mean part of it would drag on the ground.

Unfortunately, there's a problem with using larger and larger balloons.

A 15-meter helium balloon plus a 12-year-old can lift 1,895 kilograms. But a Ferrari 458 (plus a 13-year-old) only weighs 1,532 kilograms.

The solution to all this is to ditch the helium. You don't need a balloon. All you need is a kite or a parachute—a surface to act as a wing and redirect that incoming air to push you upward.

In other words, see if your dad will take you parasailing.

How fast could you visit all 50 states?

—as discussed by Stephen Von Worley on Data Pointed

This week's article is a little different. Instead of answering one of your questions, I'm going to look at someone else's answer to a question, and how thinking about that answer raised some new questions of my own. Eventually, the whole thing sucked me down a rabbit hole of calculations from which I barely escaped.

In the summer of 2012, the blog Twelve Mile Circle posted an article about the search for a 24-hour-long Google Maps route that visits as many US states as possible. They found that the maximum was about 19 or 20 states.

If you can visit 19 or 20 states in 24 hours, how long would it take to visit all 50? Stephen Von Worley read the article and did some calculating. He came up with a 6,813-mile route that visited the contiguous 48 states, then wrote an article on Data Pointed discussing how long the journey would take using different types of transportation.

His conclusion:

• • 160 hours by car (plus airline flights to Alaska and Hawaii)
• • 39 hours by private jet (landing in each state)
• • 18 hours by F-22 fighter jet and helicopter (landing in each state)

And he stopped there.

Recently, someone sent me Stephen's article. I enjoyed it, but I got curious: Were there faster ways?

First of all, there are technically faster planes than the F-22. The SR-71 Blackbird is, by some measures,[1]Rocket planes are faster, but only over short distances, and usually don't take off on their own. Orbital rockets are much faster because getting to space is mainly a problem of going as fast as possible.​[2]The X-15 rocket plane, which was about twice as fast as the SR-71, was an example of the rockets discussed in last week's article—it originally used alcohol as its fuel. They later switched to ammonia. the fastest plane. It holds the record for the fastest trip from New York to London. It's fast enough that if you fly it along the Equator going west, even with pauses to refuel, you'll see the Sun rise in the west and set in the east.[3]I just came across this positively stunning firsthand account by Bill Weaver, an SR-71 test pilot. In 1966, Weaver was flying an SR-71 at full speed, Mach 3.18, when it abruptly and catastrophically disintegrated. Somehow, he survived the breakup. He didn't eject; the plane just tore itself apart around him and scattered in all directions. In other words, he suddenly found himself flying along at Mach 3.18 ... without his plane. It's a mind-boggling story. If you don't relax the requirement to land—so you just need to pass over the borders into the state—an SR-71 using aerial refueling could fly Stephen's route—plus trips to Juneau and Honolulu—in about 7 hours.[4]Or possibly more. It couldn't fly his route exactly, since at full speed the plane's turning radius is something like 100 miles.

And if we're not bothering to land in each state—if we're just trying to through the state's airspace—some new possibilities open up. This is where I got thoroughly nerd-sniped.

Satellites in orbit are an order of magnitude faster than even the SR-71. An object in low Earth orbit can cross the entire US in minutes. Furthermore, a satellite in a polar orbit will eventually pass over every state, since the Earth turns slowly under its orbital path, but hitting all 50 states this way would take many days.

I started to wonder how many orbits were required, and whether a satellite doing carefully planned course corrections could pass over all 50 states faster than the 7+ hours needed by an aircraft.

If you allow the satellite to change course an unlimited number of times, it can just forget about orbits completely, following a twisty course that stays over the US. At that point, it simply becomes a question of how much fuel you're allowing it to have.

Instead, I started considering another version of the problem: What if your satellite that had to coast while near the US, but could fire thrusters on the far side of the Earth once per orbit, putting it on a new course for each pass? How many passes would be required to visit every state then?

I had previously written some code for analyzing airplane routes to help me answer the question in "Flyover States" chapter in the What If book. I repurposed this code to tackle my satellite question.

For a while, the best my math could come up with was a set of six orbits that crossed all 50 states:

I decided that 6 was probably the limit; I just couldn't figure out a way to do it with 5. But I left my computer churning on the problem for an evening, searching through combinations of orbits, and yesterday morning ...

... it came up with a solution that does it in 5.

Those 5 orbits cross over all 50 states ... and DC, for good measure. They're all slightly curved, since the Earth is turning under the satellites, but it turns out that this arrangement of lines also works for the much simpler version of the question that ignores orbital motion: "How many straight (great-circle) lines does it take to intersect every state?" For both versions of the question, my best answer is a version of the arrangement above.

I don't know for sure that 5 is the absolute minimum; it's possible there's a way to do it with four, but my guess is that there isn't. Perhaps there's a way to get just the 48 contiguous states with 4 lines, but I haven't found it yet.

If you want to play with arrangements of lines, you can use the Google Earth path-drawing tool. It's a little clumsy, but it works. If anyone finds a way (or proof that it's impossible) I'd love to see it!

Bringing things back to Stephen's original question, the 5 orbits (four, really, since you could start and end over on the US side) would take just over six hours to complete, including the three maneuvers over the Indian Ocean. In other words, in a spacecraft, you could beat even the fastest airplane.

Of course, those right-angle turns would take a lot of fuel—and, as mentioned before—if you were really trying to set this record, you would just need to get as much fuel as possible and fly a space figure-8 that stayed above the country.[5]Or in a hyperloop, I guess.

Either way, one thing's for sure: In the time I spent doing all that calculation, I probably could have just visited all those states by walking.

But the calculating was fun.

What if everything was antimatter, EXCEPT Earth?

Sean Gallagher

This one doesn't end well for us. But—unlike most scenarios involving the word "antimatter"—the end is surprisingly slow and drawn-out.

The whole universe is matter, as far as we can tell. No one is sure why there's more matter than antimatter, since the laws of physics are pretty symmetrical, and so there's no reason to expect there to be more of one than the other.[1]Although when it comes down to it, there's no reason to expect anything at all.

It's possible that galaxies are made of antimatter, and we just haven't noticed because we haven't tried to touch them. This is a cool idea, but if there are zones of matter and zones of antimatter, we should see a telltale gamma-ray glow from the boundary between the zones. So far, we haven't seen that, although another telescope might help.

If the rest of the universe were swapped out for antimatter, we'd be in trouble. Outer space isn't really "space";[2]As far as I know, it really is "outer", for what that's worth. it's full of a thin gas.[3]Technically, plasma.​[4]Technically, there's also a substantial quantity of solid grains of dust.​[5]Look, there's a bunch of little bits that are hard to see, ok?.​[6]Ok, they're not always hard to see.

The Earth's magnetic field protects us from the solar wind, and would protect us from an anti-solar wind, too. A tiny fraction of the particles from the Sun do reach the Earth, funneled down by our magnetic field, and create the aurora. In this scenario, the aurora would get a lot brighter, but most of the time not bright enough to really cause problems.

Meteorites would be the real problem.

The Earth sweeps up space dust as it travels around its orbit.[7]Unfortunately for us, antimatter is probably attracted to matter by gravity. About 100 tons of dust per day enters the atmosphere in the form of tiny grains, most weighing about 10^-5 grams. An additional similar average per-day amount arrives in giant clumps all at once.

This inflow of antimatter dust would collide with the top of our atmosphere and be annihilated. The interactions between the nuclei and antinuclei and protons and antiprotons would be complex,[8]A lot of the energy would be carried away by neutrinos. but the end result would be a lot of gamma rays, which would turn into a lot of heat. This steady flow of material (which would be worst around dawn, when your house was facing in the direction of Earth's motion).

The heat and light added by the antimatter would most likely be enough to tip the Earth into a "runaway greenhouse" scenario, turning the Earth into something resembling Venus.

But the big asteroids would get us first. Even a relatively small object like the Chelyabinsk meteor would deliver as much energy as the meteor that killed the dinosaurs.[9]Although it would deliver it to the top of the atmosphere, so in some ways it wouldn't be as bad. Fairly large asteroids enter the atmosphere every few months—mostly unnoticed. If they were all antimatter, each one would trigger a tremendous pulse of energy in the sky and ignite a massive firestorm.[10]If an antimatter meteor is large enough, encountering a cloud could launch some of it backward without completely destroying it. However, it's hard to come up with a practical scenario in which a meteor would exhibit this effect in Earth's atmosphere—unless it were so large that it would have basically destroyed the planet anyway.

Right now, it's still an open question whether any significant percentage of the stuff in the sky is made of antimatter. It's probably not, but we'd need to build another orbiting gamma-ray telescope to really be sure.

However, it's easy to use a telescope to rule out one possibility: That everything in the sky is antimatter.

If you have a telescope, maybe you can get that result published.

When I was about 8 years old, shoveling snow on a freezing day in Colorado, I wished that I could be instantly transported to the surface of the Sun, just for a nanosecond, then instantly transported back. I figured this would be long enough to warm me up but not long enough to harm me. What would actually happen?

AJ, Kansas City

Believe it or not, this wouldn't even warm you.

The temperature of the surface of the Sun is about 5,800 K,[1]Or °C. When temperatures start having many digits in them, it doesn't really matter. give or take. If you stayed there for a while, you'd be cooked to a cinder, but a nanosecond is not very long—it's enough time for light to travel almost exactly a foot.[2]A light-nanosecond is 11.8 inches (0.29981 meters), which is annoyingly close to a foot. I think it would be nice to redefine the foot as exactly 1 light nanosecond. Because we don't have enough unit confusion in the world already.

This raises some obvious questions, like "Do we redefine the mile to keep it at 5,280 feet?" and "Do we redefine the inch?" and "Wait, why are we doing this?" But I figure other people can sort that out. I'm just the idea guy here.

I'm going to assume you're facing toward the Sun. In general, you should avoid looking directly at the Sun, but it's hard to avoid when it takes up a full 180 degrees of your view.

In that nanosecond, about a microjoule of energy would enter your eye.

A microjoule of light is not a lot. If you stare at a computer monitor with your eyes closed, then open them and shut them quickly, your eye will take in about as much light from the screen during your reverse blink[3]Is there a word for that? There should be a word for that. as it would during a nanosecond on the Sun's surface.

During the nanosecond on the Sun, photons from the Sun would flood into your eye and strike your retinal cells. Then, at the end of the nanosecond, you'd jump back home. At this point, the retinal cells wouldn't even have begun responding. Over the next few million nanoseconds (milliseconds) the retinal cells—having absorbed a bunch of light energy—would get into gear and start signaling your brain that something had happened.

You would spend one nanosecond on the Sun, but it would take 30,000,000 nanoseconds for your brain to notice. From your point of view, all you would see was a flash. The flash would seem to last much longer than your time on the Sun, only fading as your retinal cells quieted down.

The energy absorbed by your skin would be minor—about 10^-5 joules per cm² of exposed skin. For comparison, according to the IEEE P1584 standard (as quoted on ArcAdvisor.com), holding your finger in the blue flame of a butane lighter for one second delivers about 5 joules per cm² to the skin, which is roughly the threshold for receiving a second-degree burn. The heat during your Sun visit would be five orders of magnitude weaker. Other than the dim flash in your eyes, you wouldn't even notice.

But what if you got the coordinates wrong?

The Sun's surface is relatively cool. It's hotter than, like, Phoenix,^{[citation needed]} but compared to the interior, it's downright chilly. The surface is a few thousand degrees, but the interior is a few million degrees.[4]The corona, the thin gas high above the surface, is also several million degrees, and no one knows why. What if you spent a nanosecond there?

The Stefan-Boltzmann law lets us calculate how much heat you'd be exposed to while inside the Sun.[5]There's also direct pressure from the heavy particles, protons and stuff, bouncing around, but the radiation turns out to be the dominant component.

I'm going to hijack this note to ask another question: How does this transporter work, anyway?

When you teleport somewhere, presumably it does gets rid of the matter that was in the way, so you don't end up combining yourself with whatever was there. A simple solution is to have the teleporters swap matter between the two locations. Kirk gets teleported down to the planet, a Kirk-sized chunk of air gets teleported up to the Enterprise.

So what would happen if an AJ-shaped chunk of Sun-interior gets teleported to snowy Colorado, then we just left it there?

The protons inside the Sun bounce around at speeds of about 350 km/s (about half of the Sun's escape velocity at that depth, for weird and deep reasons.) Freed from their crushingly hot neighborhood, the whole collection of protons would burst outward, pouring light and heat energy into their surroundings. The energy released would be somewhere between a large bomb and a small nuclear weapon. It's not good. You would exceed the IEEE P1584B standard for second-degree burns after one femtosecond in the Sun.[6]Although it wouldn't be a second-degree burn until many picoseconds later, since the definition of a second-degree burn is one which damages some of the underlying layers of tissue—and in the first few femtoseconds, light wouldn't have time to reach the underlying tissue. A nanosecond—the time you're spending there—is 1,000,000 femtoseconds. This does not end well for you.

There's some good news: Deep in the Sun, the photons carrying energy around have very short wavelengths—they're mostly a mix of what we'd consider hard and soft X-rays.[7]<what_if_book_reference>I wonder if there are more soft or hard x-ray photons in the universe.</what_if_book_reference> This means they penetrate your body to various depths, heating your internal organs and also ionizing your DNA, causing irreversible damage before they even start burning you. Looking back, I notice that I started this paragraph with "there's some good news." I don't know why I did that.

In Greek legend, Icarus flew too close to the Sun, and the heat melted his wings and he fell to his death. But "melting" is a phase change which is a function of temperature, a measure of internal energy, which is the integral of incident power flux over time. His wings didn't melt because he flew too close to the Sun, they melted because he spent too much time there.

Visit briefly, in little hops, and you can go anywhere.

If you stripped away all the rules of car racing and had a contest which was simply to get a human being around a track 200 times as fast as possible, what strategy would win? Let's say the racer has to survive.

Hunter Freyer

The best you'll be able to do is about 90 minutes.

There are lots of ways you could build your vehicle—an electric car,[1]With wheels designed to dig into the pavement on turns. a rocket sled, or a carriage that runs along a rail on the track—but in each case, it's pretty easy to develop the design to the point where the human is the weakest part.

The problem is acceleration. On the curved parts of the track, drivers will feel powerful G forces.[2]Which you can broadly call either "centrifugal" or "centripetal" forces, depending on exactly which type of pedant you want to annoy. The Daytona Speedway in Florida has two main curves, and if the vehicles go around them too fast, the drivers will die from the acceleration alone.

For extremely brief periods, such as during car accidents, people can experience hundreds of Gs and survive. (One G is the pull you feel when standing on the ground under Earth's gravity.) Fighter pilots can experience up to 10 Gs during maneuvers, and—perhaps because of that—10 Gs is often used as a rough limit for what people can handle. However, fighter pilots only experience 10 Gs very briefly. Our driver would be experiencing them, in pulses, for minutes and probably hours.

There's a good NASA document on the physical effects of acceleration here, and a particularly helpful chart in Figure 5 here.

But the most fun data comes from John Paul Stapp. Stapp was an Air Force officer who strapped himself into a rocket sled and pushed his body to the limit, taking careful notes after every run. You can read a great essay about him on the Ejection Site. The whole story is fascinating, but my favorite line is, "... Stapp was promoted to the rank of major [and] reminded of the 18 G limit of human survivability ..."

Stapp aside, the data shows that for periods on the order of an hour, normal humans can only handle 3-6 Gs of acceleration. If we limit our vehicle to 4 Gs, its top speed on the turns at Daytona will be about 240 mph. At this speed, the course will take about 2 hours to complete—which is definitely faster than anyone has driven it in an actual car, but not by that much.

But wait! What about the straightaways? The vehicle will be accelerating during the turns, but coasting on the straightaways. We could instead accelerate the vehicle up to a higher speed while on straight segments, then decelerate it back down when approaching the end. This would result in a speed profile like this:

This has the additional advantage that—with some clever back-and-forth maneuvering on the track—the driver can be kept at a relatively constant acceleration through the whole trip, hopefully making the forces easier to endure.

Keep in mind that the direction of the acceleration will keep changing. Humans can survive acceleration best if they're accelerated forward, in the direction of their chest, like a driver accelerating forward. The body is least capable of being accelerated downward toward the feet, which causes blood to pile up in the head. To keep our driver alive, we'll need to swivel them around so they're always being pressed against their back. (But we have to be careful not to change direction too fast, or the centrifᵫtal[3]Splitting the difference. force from the swiveling of the seat will itself become deadly!)

The fastest modern Daytona racers take about 3 hours to finish the 200 laps. If limited to 4 Gs, our driver will finish the course in a little under an hour and 45 minutes. If we raise the limit to 6 Gs, the time drops to an hour and 20. At 10 Gs—well past human tolerability—it would still take an hour. (It would also involve breaking the sound barrier on the backstretch.)

So, barring dubious concepts like liquid breathing, human biology limits us to Daytona finishing times over an hour. What if we drop the "survive" requirement? How fast can we get the vehicle to go around the track?

Imagine a "vehicle" anchored with Kevlar straps to a pivot in the center, reinforced with a counterweight on the other side. In effect, this is a giant centrifuge. This lets us apply one of my favorite weird equations,[4]See footnote [8] in article #86. which says that the edge of a spinning disc can't go faster than the square root of the specific strength[5](tensile strength divided by density) of the material it's made of. For strong materials like Kevlar, this speed is 1-2 km/s. At those speeds, a capsule could conceivably finish the race in about 10 minutes—although definitely not with a living driver inside.

Ok, forget the centrifuge. What if we build a solid chute, like a bobsled course, and send a ball bearing (our "vehicle") rocketing down it? Sadly, the disc equation strikes again—the ball bearing can't roll faster than a couple km/s or it will be spinning too fast and will tear itself apart.

Instead of making it roll, what if we make it slide? We could imagine a diamond cube sliding along a smooth diamond chute. Since it doesn't need to rotate, it could potentially survive more accelerations than a rolling ball bearing. However, the sliding would result in substantially more friction than the ball bearing example, and our diamond might catch fire.

To defeat friction, we could levitate the capsule with magnetic fields, and make it progressively smaller and lighter to accelerate and steer it more easily. Oops—we've accidentally built a particle accelerator.

And while it doesn't exactly fit the criteria in Hunter's question, a particle accelerator makes for a neat comparison. The particles in the LHC's beam go very close to the speed of light. At that speed, they complete 500 miles (30 laps) in 2.7 milliseconds.

Wikipedia lists about 850 motor racing tracks. The LHC beam could run the equivalent of a full Daytona 500 on each of those 850 tracks, one after another, in about 2 seconds, before the drivers had made it to the first turn.

And that's really as fast as you can go.

What is the farthest from Earth that any Earth thing has died?

—Amy from NZ

With Halloween approaching, I guess it's the season for death-related questions.

The farthest from Earth that any human has died is about 167 kilometers,[1]Plus or minus a kilometer. when three cosmonauts on Soyuz 11—Vladislav Volkov, Viktor Patsayev, and Georgi Dobrovolsky—suffered a depressurization accident while returning from Earth. They were moving at about 7,755 meters per second at the time, which is also the highest forward speed at which any human has ever died.

Volkov, Patsayev, and Dobrovolsky are the only humans who have died in space. Every other fatal space accident—and, for that matter, every other human death of any kind—happened within 70 kilometers of the surface.[2]Morbid list from my notes: The crew of Columbia died at just over 60 km, Pyotr Dolgov died at roughly 24 km, James Zwayer died at 23 km, Michael J. Adams died at 20 km, Ying Chin Wang died at 20 km, and Rudolf Anderson died at between 18 and 23 km. Jack Weeks presumably died somewhere between 20 and 0 km the ocean surface.

But humans don't hold this record.

For starters, there are plenty of test animals which have died in space. But, to be honest, I can't bring myself to collect statistics about them. I mean, at least the human pilots who died had all volunteered and understood what was happening to them. So instead, I'm going to skip straight to the organisms that are the real answer to Amy's question: Microbes.

Spacecraft carry bacteria, although we do our best to sterilize them before and during launch. This sterilization is important, because we don't want to contaminate another planet or Moon with Earth bacteria. There are two big reasons for this—one ethical and one practical. The ethical one is that we don't want to accidentally introduce Earth life that disrupts and/or destroys a native ecosystem. The practical one is that if we find life on some other planet, we don't want to have to struggle to figure out whether it was contamination from one of our probes.

But sterilizing spacecraft is hard. NASA has an employee specifically assigned to this task, and she has possibly the best job title of all time: Planetary Protection Officer.[3]Another competitor for this title is Philip M. Breedlove, who has the job title Supreme Allied Commander.

The Planetary Protection Officer is responsible for avoiding spacecraft contamination, although there are occasionally problems.

A 2008 study of lunar missions estimated that spacecraft carried 1.98x10¹¹ viable microorganisms per vehicle. Spacecraft such as the Voyagers and Pioneers, which were ultimately headed for deep space, were also not fully sterilized—the official planetary protection strategy was "try not to hit any planets."

Voyager certainly carries lots of bacterial spores. If we take the number from the 2008 paper as a (very rough) estimate of the number of microbes Voyager might carry, we can try to figure out how many might still be alive.

Some microorganisms can survive for a long time in a vacuum. One study found that the majority of bacteria that spent six years in space survived—though only if a shade protected them from the Sun's UV light. Other studies have agreed that radiation is the main thing to worry about, and the radiation environment inside a spacecraft is complex. The bottom line is that we just don't know for sure how long bacteria can survive in deep space.

But we can still give part of an answer Amy's question. If we assume that 1 in 1,000 bacterial spores on Voyager were of a space-tolerant variety, and 1 in 10 of those is somewhere on the craft where UV light doesn't reach it, then that still leaves on the order of 10 million viable bacterial spores traveling on Voyager.

If they suffer a death rate of 30% per six years, as in one of the studies, then there would still be a million of them alive after 50 years, dying at a rate of 1 every 10 minutes. On the other hand, the author of the 2008 study speculated that microbes could avoid hits from cosmic radiation for extremely long time periods, and other sources have speculated about survival for thousands or even millions of years. But no one really knows.

For our Voyager bacteria, there's a higher death rate at first, for spores in more exposed positions, and a much lower one for the more protected ones. Today, it's quite possible there thousands of bacterial spores still alive on Voyager 1 and 2, lurking quietly in the dead of space. Every few hours, days, or months, one of them degrades enough to no longer be viable.

And each one sets a new record for the most distant Earth thing to die.

What if people's incomes appeared around them as cash in real time? How much would you need to make to be in real trouble?

Julia Anderson, Albuquerque, NM

First, let's think about coins.

The US federal minimum wage in the US is \$7.25/hour, which is about \$15,000/year for a full-time job. If you earn the minimum wage, you make a penny every 5 seconds during work hours, or every 20 seconds if you average it over your whole week.

Someone making minimum wage in pennies would earn about 30 lbs of pennies per workday. Two weeks' worth of pay in pennies would fill a small carry-on suitcase. At 150 lbs, the suitcase would probably be too heavy to pick up. However, if they were paid in quarters instead of pennies, two weeks' pay would only weigh about 3 lbs.

If paid in a typical mix of loose change,[1]The makeup of a "mix of loose change" depends on your spending habits. Lots of people have calculated this to estimate the amounts of change in a jar. One of the more careful theoretical calculation I've seen, which considers prices and sales tax, comes from Dan Kozikowski.

Since I've spent way too much time on this question over the years, for the record: If you're ever trying to guess the value of coins in a jar, my suggestion would be \$13.05/lb and \$468/gallon if the person doesn't use quarters for laundry or parking, \$9.53/lb and \$336/gallon if they do, and \$16.75/lb and \$611/gallon if they discard pennies. the US federal minimum wage is about one plastic water bottle full of coins per workday, and the median household earns three or four water bottles per workday.

A CEO of a large company might make \$40,000 per workday. Assuming an 8-hour workday, that's 130 pennies per second. If paid in a mix of loose change, the CEO would earn about a water bottle of coins per minute, and a duffel bag full of loose change every hour, or 600 water bottles per workday:

If you're a CEO working in a 300 ft² office, the change would accumulate on the ground at a rate of about only half an inch per day. (If it were pennies, it would be more like 3" per day). It would mean frequently getting up to adjust your desk so it stayed on top of the pile, and I can't imagine your chair would be too stable, but it seems like it might be manageable.

Lugging those 150-kg duffel bags to CoinStar every hour would be a hassle. If we allowed paper money, things would get easier. A dollar bill has a volume of about 1.55 mL, which means that a CEO paid in dollar bills would only need one large-ish duffel bag for a day's pay.

If paid in \$100 bills, a CEO would only need a couple of duffel bags to carry home their year's salary.

In either case, at 60-70 lbs, a duffel bag full of bills would be a little on the heavy side.

Now, there are people who make a lot more than "typical CEOs".

Mark Zuckerberg, Warren Buffett, and Bill Gates all made in the neighborhood of \$10 billion in 2013. During the workday, that's a duffel bag full of \$100 bills every half-hour.

Going back to our earlier metric, that's 25 water bottles full of change—one minimum-wage worker's daily salary—per second of the workday.

If randomly dispensed from the ceiling in the form of loose change, Mark Zuckerberg's income would pile up at an inch every minute. There would be so many coins hitting the ground per second that it would meld into white noise.[2]The Mark Zuckerberg Money Pump would require over a kilowatt of power just to hoist the coins up to his ceiling and drop them. But that's not a problem. Even a penny is worth enough to pay for the electricity to hoist itself several thousand miles upward. The downward force from the coins falling on Mark's head and shoulders would certainly be annoying, but not debilitating. It would be possible for him to walk around, but if he sat still for an hour, he'd be buried.

It does sound awful.

Stopping rain from falling on something with an umbrella or a tent is boring. What if you tried to stop rain with a laser that targeted and vaporized each incoming droplet before it could come within ten feet of the ground?

Zach Wheeler

Stopping rain with a laser is one of those ideas that sounds totally reasonable, but if you—

While the idea of a laser umbrella might be appealing, it—

Ok. The idea of stopping rain with a laser is a thing we're currently talking about.

It's not a very practical idea.

First, let's look at the basic energy requirements. Vaporizing a liter of water takes about 2.6 megajoules,[1]It takes more energy if the water is colder, but not much more. Heating the water up to the edge of boiling only takes a little of the 2.6 megajoules. Most of it goes into pushing it over the threshold from 100°C water to 100°C vapor. and a big rainstorm might drop half an inch of rain per hour. This is one of those places where the equation isn't complicated—you just multiply the 2.6 megajoules per liter by the rainfall rate and you get laser umbrella power requirement (watts per square meter protected). It's weird when units work out so straightforwardly: \[2.6\tfrac{\text{megajoules}}{\text{liter}}\times0.5\tfrac{\text{inches}}{\text{hour}}=9200\tfrac{\text{watts}}{\text{square meter}}\] 9 kilowatts per square meter is an order of magnitude more power than is delivered to the surface by sunlight, so your surroundings are going to heat up pretty fast. In effect, you're creating a cloud of steam around yourself, into which you're pumping more and more laser energy.

In other words, you'd be building a human-sized autoclave. Needless to say, autoclaves are not really a popular place to live.

But it gets worse! Vaporizing a droplet of water with a laser is more complicated than it sounds.[2]And to be honest, it sounds pretty complicated. There are many, many, many papers on this subject,[3]Quote from the article Explosion of a Water Droplet by Pulsed Laser Heating by J. C. Carls and J. R. Brock: "... in practice, heating a droplet to very high temperatures before substantial motion occurs ... might be difficult."

Other, out-of-context quotes from that same paper: "The droplet seems to maintain its basic shape and does not appear to be shattering", "The particles formed previously will probably be vaporized.", "By acting strangely, the equation of state is saying that all is not well", """Avalanche breakdown", "extremely low and sometimes negative pressures", "the most dynamic response possible", and "Notice the very high temperatures". and the general gist is that it takes a lot of energy—delivered fast—to vaporize the droplet without just splattering it apart into little droplets.

Here's a video of a droplet getting zapped by a laser pulse; you can see that it splatters the droplet, more than vaporizing it. The upshot is that cleanly vaporizing a droplet would probably take more than the already-unreasonable amounts we were considering.

Then there's the problem of targeting. In theory, this is probably solvable. Adaptive optics allow for extremely fast and precise control of beams of light. Covering an area of 100 square meters (which Zach also asked about in his full letter) would require something like 50,000 pulses per second. This is slow enough that you wouldn't run into any direct problems with relativity, but the device would—at minimum—need to be a lot more complicated than just a laser pointer on a swiveling base.

It might seem easier to forget about targeting completely and just fire lasers in random directions.[4] If you aim a laser beam in a random direction, how far will it go before it hits a drop? This is a pretty easy question to answer; it's the same as asking how far you can see in the rain, and the answer is at least several hundred meters. Unless you're trying to protect your whole neighborhood, firing powerful lasers in random directions probably won't help.

And, honestly, if you are trying to protect your whole neighborhood ...

... firing powerful lasers in random directions definitely won't help.

Dispatches from a horrifying alternate universe

This week: Excerpts from What If articles written in a world which, thankfully, is not the one we live in:

... assuming American football has an average death rate of 1.2 players per game. This is slightly higher than the average for association football (soccer), but an order of magnitude lower than the rates found in volleyball.

... while the idea that a human-sized mirror could offer protection from moonlight makes intuitive sense, in effect it serves only to create a second moon. The interaction between any pair of moons—in this case, the real Moon and the mirror-image "virtual" Moon—takes place at the geometric halfway point between them.

Here, that point is located at the surface of the mirror you are carrying, which makes it clear why this is such a bad ...

What would happen if all of the rivers in the US were instantly frozen in the middle of the summer?

Zoe Cutler

This is another question that turns out even worse than I expected.

For starters, pretty much every animal in the water would die. This would wipe out several fish species completely (this site lists a number of species found only in the Mississippi and its tributaries) and seriously damage many others.

Then all the ice would start to melt. Melting ice takes a lot of heat energy, but the ice in this scenario would be spread out in thin strands across the country, so it would all melt pretty fast.[1]Strangely, solid ice usually melts faster than snow—not only in terms of weight, but in terms of inches melted per day.[2]

Or[3]Whoa, I can nest footnotes! centimeters. This seems a little counterintuitive, since ice is a lot denser than snow, so it requires a lot more energy to melt a given volume. But snow reflects a lot more light than solid ice, keeping it from absorbing energy, and its air pockets help insulate it.

As ice melts, if it stays in contact with water, the water speeds up the melting, since water can transfer heat more efficiently than air. This can be demonstrated by a simple household experiment. Snow is partly protected from this effect because it can't conduct heat in from the edges as easily. This would cause some localized cooling of the air, but we're talking about a relatively small amount of heat energy compared to the environment's overall energy budget.[4]To estimating the river volume, I found a book that collected estimates of the total world surface water volume in rivers (Chapter 2). It didn't have a breakdown of that number by continent, but it had total discharge rates by continent, so I just used that as a proxy to figure out how to apportion the water between the continents and came up with 16.5% for North America. To get a rough idea, we can apportion that water by surface area (I found a page saying Canada has 9% of the world's freshwater discharge, which seems consistent, although using two numbers from different sources like this creates large potential for error. That leaves 7.5% for the rest of the continent. Dividing up that 7.5% by land area between the US and Central America, the US has about 5%, which works out to 77 km³ of water.

We can use data on iceberg melt rates to estimate how quickly individual rivers would melt. Small streams would melt in days in all but the coldest areas, while ice in larger rivers would take longer.

But before that could happen, things would get weird.

To help me out with this question, I talked to Charlie Hohn, a Vermont field naturalist with an interest in river behavior. I brought Zoe's question to him, and he made a lot of interesting observations.

He pointed out that the water feeding into the rivers—from tiny rivulets and raindrops and melting snow all over the watershed—would suddenly find its path blocked by a wall of ice.

But all that water still has to go somewhere. And if the riverbed is full, it will go somewhere else.

"Any place with rain or snowmelt would have horrific flooding because the water would have nowhere to go," Charlie told me. "Water would shoot down narrow canyons, and once it hit the floodplain it would probably jump into old river channels where they exist, but in many areas that would mean towns and farms."

The problems would get worse as the ice melted and broke up, starting upstream and progressing down the river. The floating ice would pile up, damming the water and creating huge lakes.

Charlie notes that even a small ice dam can cause terrible flooding. When the Winooski river in Vermont was clogged by an ice dam in 1992, the water spilled over the banks and flooded Montpelier in a matter of minutes. And that's just a tiny river, he says. "The Winooski here is narrow enough to throw a rock across, and usually only a few feet deep. In a dry year you could wade across in summer in shorts without getting them wet. So scale that up to the Mississippi and 12-foot-thick ice."

As part of its normal meandering, the Mississippi has been trying for 150 years to jump its banks and flow down the Atchafalaya river's route. The Atchafalaya offers a steeper path to the Gulf of Mexico, but if it captured the Mississippi's flow, it would leave the major ports of New Orleans and Baton Rouge high and dry. The Army Corps of Engineers has been trying to stop it.

Charlie says that if the Mississippi froze solid, "you'd have the Atchafalaya disaster, without a doubt." And the problems wouldn't be limited to the Mississippi. "The California delta system would also flood, causing a collapse of most of California's water system—though if the aqueducts and reservoirs froze, it wouldn't matter that you couldn't get water from the delta."

If man-made reservoirs do freeze in this scenario (which I guess depends whether they count as "rivers") the ice could damage the dams by expanding in the turbines. Whether or not the reservoir froze,[5]We spent a while debating whether, if Lake Meade froze (and expanded), the Hoover Dam would burst due to the expansion. My gut feeling is that it wouldn't, but I'm really not sure. I think we should just try it. the dams could collapse, causing cataclysmic flooding. And where there aren't dams to collapse, the ice would create them ... and when ice dams break up, the result can be pretty spectacular.

According to Charlie, the vegetation around the rivers would handle the sudden freeze pretty well, but the effect on the broader ecosystem would be huge and complex. "If there were salmon in the rivers, the bears would feast on them when they thawed. But later, if the fishery crashed, there would be a bunch of desperate hungry bears unleashed on the environment."

He mentions that salmon spend part of their life cycle in the ocean, so some of them would survive the rivers freezing. "But if their food were gone, the fishery would still fail. Then again, with all the dams destroyed, maybe it would be a net benefit for salmon ... which is kind of sad."

Although agriculture would take a serious hit, humans would probably make it through ok. We wouldn't run out of drinking water; we get a lot of it from underground aquifers, and there would be plenty of quickly-melting ice lying around in most areas. In the worst case scenario, people could just use hair dryers to melt ice to make drinking water for only pennies per person per day. We'd probably limp through the scenario tattered but alive, buying food overseas.

At least, most of us would.

I think the most horrifying part of this summer freeze wouldn't be the large-scale, long-term devastation. It would be the immediate consequences.

As a kid, I spent summers swimming in the James River in Virginia. Now, thanks to Zoe, every time I set foot in a river ...

... I'm going to feel a faint chill.

What if I made a lava lamp out of real lava? What could I use as a clear medium? How close could I stand to watch it?

Kathy Johnstone, 6th Grade Teacher (via a student)

This is a surprisingly reasonable idea, by What If standards.

I mean, it's not that reasonable. At the very least, I'm guessing you would lose your teaching license, and possibly some of the students in the front row. But you could do it.

Just a warning: I'm going to be linking to a lot of videos of lava flowing and people poking it with sticks, so you may have a hard time getting to the end of the article without getting sidetracked into watching a bunch of them like I did while writing it.

You have a few choices for transparent materials that could hold the lava without rupturing and splattering half the classroom with red-hot droplets. Fused quartz glass would be a great choice. It's the same stuff they use in high-intensity lamp bulbs, the surface of which can easily get up to mid-range lava temperatures.[1]This bulb, for example, can supposedly handle bulb temperatures of up to 1000°C, which is hotter than many types of lava. Another possibility is sapphire, which stays solid up to 2,000°C, and is commonly used as a window into high-temperature chambers.[2]That link wasn't a lava video, but this is.

The question of what to use for the clear medium is trickier. Let's say we find a transparent glass that melts[3]Some people say glass is a liquid that flows very slowly. Other people smugly point out that this is actually wrong. Then another group of people dissects how we know it's wrong, and where this incorrect idea got started. And then at the end of the chain, a Metafilter user steps back and asks some supremely insightful questions about what's really going on here as we variously repeat and debunk these kinds of factoids. at low temperatures. Even if we ignore the impurities from the hot lava that would probably cloud the glass, we're going to have a problem.[4]And later, when the school board finds out, we'll have another.

Molten glass is transparent. So why doesn't it look transparent?[5]Which sounds sorta contradictory. "This music is loud, but it doesn't sound loud." The answer is simple: It glows. Hot objects give off blackbody radiation; molten glass glows just like molten lava does, and for the same reason.

So the problem with a lava lamp is that both halves of it will be equally bright, and it will be hard to see the lava. We could try having nothing in the top half of the lamp—after all, when it's hot enough, lava bubbles pretty well on its own. Unfortunately, the lamp itself would also be in contact with the lava. Sapphire might not melt easily, but it will glow, making it hard to see whatever the lava was doing inside.

Unless you hooked it up to a really bright bulb, this lava lamp would cool down quickly. Just like individual blobs of lava, the lamp would solidify and stop glowing within the first minute, and by the end of the class period you'd probably be able to touch it without being burned.

A solidified lava lamp is just about the most boring thing in the world. But the scenario made me wonder: If making a lamp out of molten lava wouldn't be very exciting, then what about a volcano made of lamps?

This is probably the most useless calculation I've ever done,[6]Ok, there's no way that's true. but ... what if Mount Saint Helens erupted again today, but instead of tephra,[7]The technical term for "whatever crap comes out of a volcano." it spewed compact fluorescent bulbs?

Well, if it did, the mercury released into the atmosphere would be several orders of magnitude larger than all manmade emissions combined.[8]45% of which come from gold mining.

All in all, I think making a lava lamp out of lava would be kind of anticlimactic, and would much rather go find some actual lava and poke it with a stick. I also think that it's probably good that Mount Saint Helens didn't erupt compact fluorescent bulbs. And I think that if I were in Ms. Johnstone's class, I'd try to sit toward the back of the room.

Lastly, for old time's sake, I'd like to share one final link with you: The music video for Rick Astley's "Never Gonna Give You Up."

How many fairies would fly around, if each fairy is born from the first laugh of a child and fairies were immortal?

—Mira Kühn, Germany

"There are always a lot of young ones," explained Wendy, who was now quite an authority, "because you see when a new baby laughs for the first time a new fairy is born, and as there are always new babies there are always new fairies. They live in nests on the tops of trees; and the mauve ones are boys and the white ones are girls, and the blue ones are just little sillies who are not sure what they are."
           —Peter Pan

Interestingly, fairies in the Peter Pan mythology definitely don't live forever. At the end of the book, Peter comes to visit Wendy a year after their adventures. Wendy asks about Tinker Bell, and Peter says he can't remember her and assumes she's died, and Wendy observes that fairies don't live very long.[1]When she expressed a doubtful hope that Tinker Bell would be glad to see her he said, "Who is Tinker Bell?"

"O Peter," she said, shocked; but even when she explained he could not remember.

"There are such a lot of them," he said. "I expect she is no more."

I expect he was right, for fairies don't live long, but they are so little that a short time seems a good while to them. This suggests that at any given time, there are probably be far fewer fairies than humans in the Peter Pan universe, since they have the same birth rate and shorter lifespans.

But Mira's scenario assumes immortal fairies, so let's talk about immortal fairy demographics.

A fairy is created by every[2]Approximately, anyway, but we'll round up by assuming that all babies laugh. newborn baby. The total number of humans who have ever lived is somewhere around 100-120 billion,[3]There are various groups who have estimated this, and they all tend to come up with a number around in this range. If you take a set of estimates for ancient human population (Wikipedia has a table of them) and assume a birth rate near the biological maximum before the 20th century (35 to 45 births per 1,000, according to this paper), you can derive a similar number yourself. meaning 100-120 billion fairies have been created.

How much do all those fairies weigh? Tinker Bell, the main fairy from the Peter Pan universe, seems to be a little under six inches tall. In a scene in Hook, Tinker Bell (played by the 5 foot 9 inch Julia Roberts) fits comfortably in a 1/12 scale dollhouse, suggesting a height of 5.75 inches. As another data point, the statue of Tinker Bell at Madam Tussaud's wax museum is 5.5 inches. This suggests that fairies in Peter Pan are pretty close to 1/12th scale.

If fairies are 1/12th the size of humans, then they weigh \( \left( \tfrac{1}{12\text{th}}\right) ^3=\tfrac{1}{1728\text{th}} \) as much as us, which means Julia Roberts's Tinker Bell is probably around 35 grams.[4](two mice) As of 2015, total fairy biomass would be about 4 million tons. That's less than humans or horses, and probably comparable to the total mass of all humpback whales.[5]Fairies also probably outweigh wild birds.

At these numbers, fairies would be a minor piece of the ecosystem, although possibly a pretty annoying one.

But it wasn't always the case. In the early days of our species, our high birth rates (and death rates) plus our low population mean that we would have accumulated fairies quickly. The exact numbers depend on when the modern human (fairy-generating) species developed, but by the time the last ice age was over, the accumulated fairies could have outweighed our tiny living human population 10 to 1.

Once our population started growing following the agricultural revolution, we would have quickly outstripped fairies in terms of weight. In 2015, fairy biomass would be down to about 1.2% of human biomass, and by the mid-21st-century, they'd bottom out at less than 1%.

But as long as humans keep reproducing, the fairy population would keep growing. If we assume the human population will level off at around 9 billion partway through this century, then the share of mass occupied by fairies would continue rising steadily.

If our population stays at a stable 9 billion indefinitely, by 2100, they'll be back above 1% of human biomass, and they'll reach 2% by the year 3000. In the year 100,000, if our species is somehow still around, they'll outweigh us.

This makes things interesting. Let's assume fairies need to eat. If fairies weigh about as much as us, they'd presumably be consuming a similar share of food and water. So even if the Earth can support 400 million tons of human, it may not be able to support 400 million tons of human and another 400 million tons of fairy. This suggests that, sometime in the next hundred millennia, the growing fairy population would start to crowd out the human population.

But if fairies crowd out humans, that would in turn reduce the growth rate of the fairies, slowing down the replacement process. The end result (in this idealized model) would be that the fairy population growth would taper off as the human population declined. The situation would never quite reach a stable equilibrium, because every 1,728 human births would create one human's weight in fairies, reducing the Earth's total human carrying capacity by 1. But since the rate of fairy creation would slow down as the human population shrank, the process would stretch out for a very long time.

This odd situation would only exist because fairies—in Mira's scenario—are immortal. The scenario would change dramatically if we introduced a fairy death rate. Perhaps fairies don't age or experience natural deaths, but can still die from other causes.

What would kill the fairies? Who knows.[6]J. M. Barrie introduces a fairy-killing mechanism in Peter Pan; any time anyone says "I don't believe in fairies", a random fairy dies. In a world of immortal fairies, this could serve as an effective feedback. If no one has heard of fairies, no one will say they don't believe in them, and their population will grow. As fairies start to be common enough to be noticed, people will have a reason to say they don't believe in them, and their population will drop.

Eventually, civilization would start documenting the existence of fairies, and then no one would have any reason to disbelieve their existence, and the feedback loop would break down. But ecosystems aren't static. If you mess with one part, the other parts change in response. Fairies could become the dominant species, but the system they learned to dominate wouldn't be there forever.

If fairies represent most of the biomass in the ecosystem, eventually, given enough time ...

... something will learn to eat them.

What if there was a lake on the Moon? What would it be like to swim in it? Presuming that it is sheltered in a regular atmosphere, in some giant dome or something.

Kim Holder

This would be so cool.

In fact, I honestly think it's cool enough that it gives us a pretty good reason to go to the Moon in the first place. At the very least, it's better than the one Kennedy gave.

Floating would feel about the same on the Moon as on Earth, since how high in the water you float depends only on your body's density compared to the water's, not the strength of gravity.

Swimming underwater would also feel pretty similar. The inertia of the water is the main source of drag when swimming, and inertia is a property of matter[1]♬ BILL NYE THE SCIENCE GUY ♬ independent of gravity. The top speed of a submerged swimmer would be about the same on the Moon as here—about 2 meters/second.

Everything else would be different and way cooler. The waves would be bigger, the splash fights more intense, and swimmers would be able to jump out of the water like dolphins.

This[2]Not this one. The other one.​[3]The simplest approach, which gives us an approximate answer, is to treat the swimmer as a simple projectile. The formula for the height of a projectile is:

\( \frac{\text{speed}^2}{2\times\text{gravity}} \)

... which tells us that a champion swimmer moving at 2 meters per second (4.5 mph) would only have enough kinetic energy to lift their body about 20 centimeters against gravity.

That's not totally accurate, although it's enough to tell us that dolphin jumps on Earth probably aren't in the cards for us. But to get a more accurate answer (and an equation we can apply to the Moon), we need to account for a few other things.

When a swimmer first breaks the surface, they don't have to lift their full weight; they're partially supported by buoyancy. As more of their body leaves the water, the force of buoyancy decreases, since their body is displacing less water. Since the force of gravity isn't changing, their net weight increases.

You can calculate how much potential energy is required to lift a body vertically through the surface to a certain height, but it's a complicated integral (you integrate the displacement of the submerged portion of their body over the vertical distance they travel) and depends on their body shape. For a human body moving fast enough to jump most of the way out of the water, this effect probably adds about half a torso-length to their final height—and less if they're not able to make it all the way out.

The other effect we have to account for is the fact that a swimmer can continue kicking as they start to leave the water. When a swimmer is submerged and moving at top speed, the drag from the water is equal to the thrust they generate by kicking and ... whatever the gerund form of the verb is for the things your arms do while swimming. My first thought was "stroking," but it's definitely not that.

Anyway, once the jumping swimmer breaks the surface, the drag almost vanishes, but they can keep kicking for a few moments. To figure out how much energy this adds, you can multiply the thrust from kicking by the distance over which they're kicking after breaking the surface, since energy equals force times distance. The distance is most of a body length, or 1 to 1.5 meters. As for the force from kicking, random Google results for a search for lifeguard qualifications suggest that good swimmers might be able to carry 10 lbs over their heads for a short distance, which means they're generating a little more than 10 pounds-force (50+ N) of kicking thrust.

We can combine all these together into a big ol' equation:

\[ \text{Jump height}=\left(\frac{\tfrac{1}{2}\times\text{body mass}\times\left(\text{top speed}\right)^2+\text{kick force}\times\text{torso length}}{\text{Earth gravity}\times\text{body mass}}\right)+\left(\text{buoyancy correction} \right) \] footnote contains some detail on the math behind a dolphin jump. Calculating the height a swimmer can jump out of the water requires taking several different things into account, but the bottom line is that a normal swimmer on the Moon could probably launch themselves a full meter out of the water, and Michael Phelps may well be able to manage 2 or 3.

The numbers get even more exciting when we introduce fins.

Swimmers wearing fins can go substantially faster than regular swimmers without them (although the fastest swimmer wearing flippers will still lose to a runner, even if the runner is also wearing flippers and jumping over hurdles). Champion finswimmers can go almost 3.2 m/s wearing a monofin, which is fast enough for some pretty impressive jumps—even on Earth.

Data on swimfin top speeds and thrusts[4]This paper provides some sample data. suggest that a champion finswimmer could probably launch themselves as high as 4 or 5 meters into the air. In other words, on the Moon, you could conceivably do a high dive in reverse.

But it gets even better. A 2012 paper in PLoS ONE, titled Humans Running in Place on Water at Simulated Reduced Gravity, concluded that while humans can't run on the surface of water on Earth,[5]They actually provide a citation for this statement, which is delightful. they might just barely be able to do so on the Moon. (I highly recommend reading their paper, if only for the hilarious experimental setup illustration on page 2.)

Because of the reduced gravity on the Moon, the water would be launched upward more easily, just like the swimmers. The result would be larger waves and more flying droplets. In technical terms, a pool on the Moon would be more "splashy".[6]The SI unit of splashiness is the splashypant.

To avoid splashing all the water out, you'd want to design the deck so water drains quickly back into the pool. You could just make the rim higher, but then you'd spoil one of the key joys of a pool on the Moon—exiting via Slip 'N Slide:

I 100% support this idea. If we ever build a Moon base, I think we should absolutely build a big swimming pool there. Sure, sending a swimming pool's worth of water (135 horses) to the Moon's surface would be expensive.[7]If you decided to bundle a backyard pool into individual two-liter bottles, and sent them in 3,000 batches of 10 each via the startup Astrobotic, it would cost you $72 billion (according to their website's calculator). But on the other hand, this lunar base is going to have people on it, so you need to send some water anyway.[8]Sending a supply of water and a filter system is probably cheaper than sending a replacement astronaut every 3 or 4 days, although I encourage NASA to run the numbers on that to be sure.

And it's really not impossible. A large backyard swimming pool weighs about as much as four Apollo lunar landers. A next-generation[9](or, heck, previous-generation) heavy-lift rocket, like Boeing's NASA SLS or Elon Musk's SpaceX Falcon Heavy, would be able to deliver a good-sized pool to the Moon in not too many trips.

So maybe the next step, if you really want a swimming pool on the Moon, is to call Elon Musk and ask for a quote.

You are in a boat directly over the Mariana Trench. If you drop a 7kg bowling ball over the side, how long would it take to hit the bottom?

Doug Carter

It is a good thing you mentioned the weight, because of a very surprising fact:

Most bowling balls float.

It's true. Bowling balls all have about the same volume, so they all displace the same weight in seawater—12.13 lbs, or about 5.5 kg. But their weights vary substantially, from as little as 6 lbs to a max of 16. Only the balls that weigh more than 12.13 lbs will sink.

"Hang on a moment," the serious bowlers reading this are saying, "most bowling balls are at the heavy end of that range. The usual range is more like 14-16 lbs for men and 10-14 lbs for women. Maybe the average women's ball floats, but the overall average ball surely sinks."

This is definitely true for serious bowlers, but for casual bowlers, it might be better to define the "average" ball using the distribution on the rack of balls at the local bowling alley. These house balls often list the weight on them, so rather than go to a bowling alley, I just sifted through all the Flickr photos of bowling balls and tallied up all the visible weights. The average was 10½ lbs.

So at the very least, for most people, the average bowling ball they've picked up in their lives probably floats. If nothing else, this makes the popular simile "sank like a bowling ball"—which appears in a number of books—seem a little poorly chosen.[1]In defense of the authors of Volume 53 of North Dakota Quarterly, the simile they used is, "sank like a bowling ball in whipped cream," which is perfectly reasonable. On the other hand, How To Hook a Man (And a Baby) and The Year's Work in Lebowski Studies are on their own.

But Doug's bowling ball is 7 kg (15.5 lbs), making it plenty heavy enough to sink in the ocean. How fast will it fall?

Small falling spheres in viscous (goopy) liquids (like hair gel) exhibit weird behaviors and sometimes complex oscillation, but bowling balls falling through water are pretty straightforward. Their falling speed is determined by the drag equation and their weight (accounting for buoyancy). For Doug's bowling ball, that terminal velocity will be roughly 1.3 meters per second, which means it will take two hours and 20 minutes to reach the bottom. That's plenty of time to enjoy a two-hour movie.

A 13-pound bowling ball, which is much closer to neutrally buoyant in seawater, would take four and a half hours to reach the bottom. On the other hand, a bowling ball made of solid iron would reach the bottom in half an hour.

A bowling ball made of lead would reach the bottom in 23 minutes, and a bowling ball made of solid gold would make it in 17. However, a bowling ball made of solid gold would also weigh more than the average bowler.[2]At least, I think it would, but I can't think of a way to sample the weight of the average bowler without getting punched. A bowling ball made of solid osmium, the heaviest naturally-occurring element, would weigh 120 kilograms, and could sink to the bottom of the Mariana Trench in just 16 minutes.

But what if you got the weight wrong? What if the bowling ball you were using turned out to be 7 pounds, not 7 kilograms? Or maybe it's 10.5 pounds—the "average" bowling ball, at least according to a dubious Flickr sample.

In that case, the ball would never reach the bottom. Instead, it would drift sideways with the currents. We can track what course it would take using the Adrift.org.au ocean plastic tracking tool. The bowling ball would first drift west, past Luzon in the Philippines, then probably north along the coast of China, turning right at Japan and heading out over the Pacific.

Sometime in the summer of 2018, it would approach the coast of California. Most likely, it would follow the coast for a little while, then become swept up in the great Pacific garbage patch.

However, it's possible that it would be swept up on shore instead. If, during those three years, Doug took his boat from the Mariana islands to Los Angeles, found a nice, sheltered cove, and set up ten carefully weighted bowling pins just below the surface, then there is a tiny, tiny chance ...

... that he could succeed in bowling the single most improbable strike of all time.

If you made an elevator that would go to space (like the one you mentioned in the billion-story building) and built a staircase up (assuming regulated air pressure) about how long would it take to climb to the top?

—Ethan Annas

A week or two, if you're a champion stair-climber. Or 12 hours if you're on a motorcycle.

A tower to space would be very different from a space elevator. A space elevator would be about 100,000 kilometers tall, while a tower "to space" would only need to be 100 kilometers. As Ethan mentions, it would need to be pressurized, with an airlock every few miles.

A stairway to space[1]If there's a bustle in your hedgerow, well, uh, boy, I don't know what to tell you. I guess ask it to leave? would have about half a million steps. World-champion stair-climbers[2]or "towerrunners" like Christian Riedl or Kristin Frey can travel roughly a Mount Everest's height in a day; Riedl set the half-day record last October by climbing 13,145.65 meters in 12 hours. At that pace—taking the other twelve hours to rest, eat, and sleep each day—it would take him a little over a week to reach the top.

Climbing all those stairs would burn calories, which would mean you'd need to carry food. It turns out that the most efficient food you can carry, in terms of calories per pound, is butter—which is why Arctic explorers carry so much of it.

Suppose your backpack holds 9 liters. Climbing 10 stairs burns about a calorie, which means climbing all the way up to space will burn about 72,000 calories. If you fill your backpack with butter, it would hold almost enough calories to get you to the top.

However, since it would take you weeks to climb all those stairs, you'd also need your normal dietary allowance of 2,000 calories (three sticks of butter) per day. Combining that with the 72,000 calories just from climbing the stairs, and you'd probably need to upgrade to a more serious 16-liter backpack. If you fill that backpack with butter, it will let you carry around 110,000 calories,[3]Coincidentally, about the amount you get from eating a human body. which should be enough to get you to the top if you're really dedicated.

If you didn't want to eat 35 pounds of butter,[4]For whatever weird reason. you could try getting to the top by motorcycle. Based on how quickly this rider ascends 45 stairs, a motorcycle could conceivably make it to the top in a day.

Ok, so you got to the top. Now what? Getting up to space isn't that hard, after all—the hard part is getting into orbit, and the tower doesn't help you very much with that.

So what else could you do?

Michael Longuet-Higgins was a research professor at the University of Cambridge and an expert in fluid dynamics, bubbles, and unusual types of waves.[5]Given his apparent research interests, this video would blow his mind.​[6]Or this one, or this gadget, or this. In 1953, Dr. Longuet-Higgins was shown, by a colleague, an "interesting toy" which had recently appeared on the market. This toy, "Slinky," had some unusual properties. The professor immediately set to work analyzing it, and wrote up his results in a paper.

Dr. Longuet-Higgins first determined through mathematical modeling that the rate at which the Slinky descends steps should depend only on the properties of the spring itself, and not the size or shape of the stairs. He and his colleague conducted a series of experiments "on five different flights of stairs, of various dimensions, in Trinity College, Cambridge." Their conclusion: The Slinky descended a constant rate of about 0.8 seconds per step.[7]Except on some wide, flat stairs, where the Slinky "came to rest after three or four steps at most," which gives me a wonderful mental image of two disappointed British professors at the bottom of a staircase.​[8]Sadly, this was before the invention of the StairMaster. Fun fact: After a surprise StairMaster management shakeup in 2011, for some reason not a single newspaper ran the headline "StairMaster CEO steps down".

Dr. Longuet-Higgins determined that the Slinky quickly reached a constant descent rate after first few steps. This tells us that if you placed a Slinky (similar to his) at top of the stairway to space, and gave it just the right nudge ...

... it would make it back to the bottom in just over five days.

Or, to put that in more appetizing units:

Would it be possible for two teams in a tug-o-war to overcome the ultimate tensile strength of an iron rod and pull it apart? How big would the teams have to be?

—Markus Andersen

A couple dozen people could pull a half-inch iron rod apart.

Tug-of-war, a simple game in which two teams try to pull a rope in opposite directions, has a surprisingly bloody history.

I don't mean that there's some kind of gruesome historical forerunner of modern tug-of-war.[1]Although it's definitely an ancient sport, so I'm sure people have come up with all kinds of horrific variations over the centuries that I don't really want to spend hours reading about. Humans seem to be creative when it comes to that kind of thing. I mean that modern tug-of-war involves a lot more death and mutilation than you might expect—precisely because people underestimate how few people it takes to break "strong" things like heavy rope.

As detailed in a riveting article in Priceonomics, recent games of tug-of-war have resulted in hundreds of serious injuries and numerous deaths—all caused, one way or another, by ropes snapping. In particular, this seems to happen when large groups of students try to set a world record for largest tug-of-war game. When a rope under many tons of tension suddenly snaps, the recoiling ends can—and do—cause a terrifying variety of injuries.

Before we answer Markus's question, it's worth noting that the physics of tug-of-war can be a little tricky. It seems like common sense that the "stronger" team has an advantage, but that's not quite right. To win, you need to resist sliding forward better than the other team. If you can't resist sliding, then increasing your arm strength means you'll just pull yourself forward. Since sliding friction is often proportional to weight, tug-of-war on many surfaces is simply a contest over who's heavier.[2]Champion tug-of-war teams focus on body angle, footwork, digging into the ground, and timing pulls to throw off the other team. The strongest team in the world would lose a tug-of-war with a six-year-old and a sack of bricks, as long as the sack had a firm grip.

So, how much force can tug-of-war players exert?

A 2011 paper analyzing the immune systems of several "elite tug-of-war players"[3]The paper notes that "Few studies have been done to examine the effects of [the] tug-of-war sport on physiological responses," which seems likely enough to me. measured their average pull force (on a school gym floor) to be about 102.5 kilograms-force, or about 1.5x their body weight.

The ultimate tensile strength of cast iron is about 200 megapascals (MPa), so we can use a simple formula to figure out how many players would be needed to break one.

\[ \text{People required}=\frac{\pi\times\left(\tfrac{1}{4}\text{ inch} \right )^2\times200\text{ MPa}}{102.5\text{ kg}/\text{person}}\approx25\text{ people} \]

Two teams of 25 people[4]I originally wrote 25 people total, forgetting that two people pulling with 100 units of force each will produce 100 units of tension on the rope, not 200! Thank you for Gordon McDonough for pointing this out. could probably pull a half-inch iron bar apart. An inch-thick iron bar could be torn in half by teams of 101 people,[5]People often play tug-of-war with their dogs. Going by weight alone, 30 humans would probably be about evenly matched against 101 dalmatians. and a 2-inch-diameter bar would need over 400. It's hard to have a tug-of-war with something thicker than about 2 inches. Since you're not allowed to install handles on the rope,[6]Or wrap it around your hand, for reasons which will become clear if you read some of the articles on tug-of-war injuries. it has to be narrow enough to grip easily.

While "400 people" may be the limit for plain iron bars, there are much stronger substances out there. Common types of steel, for example, have a tensile strength about 10 times that of cast iron. Common half-inch rebar, for example, would in theory take teams of over 200 people to pull apart, compared to 25 for cast iron. Other substances are even stronger; a half-inch shaft made from high-grade steel or a polymer like Kevlar (or, theoretically, a solid silicon crystal) could handle the pulling force from teams of anywhere between 500 and 800 competitive tug-of-war players.

If we limit ourselves to a two-inch diameter rope, which seems to be about the maximum size for tug-of-wars,[7](William Safire returns from the grave to point out that it should really be tugs-of-war.) then the maximum number of tug-of-war players given a super-strong rope like Kevlar is in the neighborhood of 10,000.[8]Or several times that many, if they're not very athletic.

If we figured out how to manufacture large ropes out of graphene ribbons, which have tensile strengths over 10 times higher than existing materials, we could theoretically support a tug-of-war between teams of up to 100,000 players each. Such a rope would be over 200 miles long, and could stretch from New York to Washington.

If our experience with nylon ropes failing is any indication, when the graphene finally snapped, the death toll could be enormous among both players and bystanders. Lengths of graphene would crack across the landscape like bullwhips, slicing down forests and demolishing buildings.

In the end, trying to develop stronger ropes leads only to greater danger to everyone, both participants and bystanders. In the ultimate game of tug-of-war ...

... the only winning move is not to pull.

What in my pocket actually contains more energy, my Zippo or my smartphone? What would be the best way of getting the energy from one to the other? And since I am already feeling like Bilbo in this one, is there anything else in my pocket that would have unexpected amounts of stored energy?

—Ian Cummings

The Zippo lighter easily beats the phone, even though its fuel tank is barely half the size of a large phone's battery, because hydrocarbons are fantastic at storing energy. Gasoline, butane, alcohol, and fat contain a lot of chemical energy, which is why our bodies run on them.[1]I mean, the latter two, at least. You can't eat gasoline.​[2]As far as I know.​[3]Although technically swallowing gasoline may not kill you, according to Utah Poison Control specialist Brad Dahl. However, he cautions that you will find yourself "burping gasoline," which is "not real tasty." (Actual quote.)​[4]Also, if you don't rinse your throat afterward, it will give you chemical burns.

How much energy do they contain? Well, let's put it this way: A fully-charged car battery holds barely as much energy as a sandwich.

A container of butane the size of a phone battery could, in principle, power the phone about 13 times longer than the battery itself could.[5]In the case of my phone, that could give me as much as three hours. The obvious question, then, is "why doesn't my phone run on propane?"

The obvious answer is "because your phone would catch fire," but that's not quite it. See, lithium-ion batteries are also extremely flammable, and a huge amount of effort has gone into making Michael Bay scenarios less common.

The truth is more complicated. People have wanted to build various kinds of "fuel cell" batteries for almost as long as we've had portable electronics. The allure of hydrocarbon energy storage continues to this day—if you do a Google search for fuel cell phone charger, you'll find news stories about new products announced every year. Many of them are no longer available.

If you really want to power your phone with butane, the current hot project—as far as I can tell from a cursory search—seems to be the kraftwerk portable USB generator, which has made over a million dollars on Kickstarter with several weeks left in its campaign. Of course, a portable battery of the same size could do a lot of the same things, but there are certainly some use cases where the butane charger offers advantages. If you place a premium on reducing weight, or have to go a long time without contact with the power grid, it could be a good option. Let's put it this way: If the phrase "power your phone on butane", makes you think, "hey, that would solve a problem I have!" then go for it.

This gives us the answer to Ian's second question. The Zippo lighter has more energy, but getting it into the phone is a little difficult and requires the overhead of a fuel cell or generator. Getting the phone to start a fire, on the other hand, is quite reasonable, although it may require doing bad things to the battery.

Ian's third question was "what else in my pocket might contain more energy?" Like Gollum, I have no idea what's in your pocket,[6]Or whether you're happy to see me, for that matter. but I can guess that it might contain one thing with more energy than a battery: Your hand.

An adult man's hand weighs about a pound.[7]I wanted to put "citation needed" after that, but to my mild dismay I actually do have a citation. The hand isn't the fattiest part of the body, but if burned completely, it would probably give off about 500 watt-hours of energy, give or take. That's 50 times the energy content of the phone battery, and almost 10 times that of the Zippo. It's also about as much as a car battery.

And, for that matter, about as much as a sandwich.