# What if xkcd pdf

3 See “Leap Seconds,” medical-site.info, for an explanation of why this happens. RELATIVISTIC BASEBALL Q. What would happen if you tried to hit a. sonic wind zone, but the winds there would still be twice as strong as those in the most powerful tornadoes. Buildings, from sheds to skyscrapers, would be. From the creator of the wildly popular webcomic xkcd, hilarious and informative answers to important questions you probably never thought to ask. Millions of people visit xkcd: I'm a Car each week to read Randall Munroe’s iconic webcomic. Fans of xkcd ask Munroe a lot of strange.

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What If?: Serious Scientific Answers to Absurd Hypothetical Questions is a collection of many of the blog's most popular answers, along with brand new weird. A Mole of Moles. July 24, Robot Apocalypse. July 31, Glass Half Empty. August 7, Everybody Out. August 14, Everybody Jump. Q. What would happen if you made a periodic table out of cube-shaped bricks, where each brick was made of the corresponding element?—Andy Connolly.

My son 5y asked me today: If there were a kind of a fireman's pole from the Moon down to the Earth, how long would it take to slide all the way from the Moon to the Earth? In real life, we can't put a metal pole between the Earth and the Moon. The end of the pole near the Moon would be pulled toward the Moon by the Moon's gravity, and the rest of it would be pulled back down to the Earth by the Earth's gravity. The pole would be torn in half. Another problem with this plan. The Earth's surface spins faster than the Moon goes around, so the end that dangled down to the Earth would break off if you tried to connect it to the ground:. There's one more problem: The Moon doesn't always stay the same distance from Earth. Its orbit takes it closer and farther away. It's not a big difference, [3] You may occasionally see people get excited about the " supermoon ," a full Moon that appears slightly larger because it happens at the time of the month when the Moon is closest to Earth. But really, the full Moon always looks surprisingly large and pretty when it's near the horizon, thanks to the Moon illusion.

The top of the plume would reach up through the stratosphere, buoyed by its own heat. Entire regions would be devastated; the cleanup would stretch on for centuries. While collecting things is certainly fun, when it comes to chemical elements, you do not want to collect them all.

O2 and N2. They cover the kinematics pretty well. This crowd takes up an area the size of Rhode Island. At the stroke of noon, everyone jumps. Earth outweighs us by a factor of over ten trillion. On average, we humans can vertically jump maybe half a meter on a good day. Next, everyone falls back to the ground. A slight pulse of pressure spreads through the North American continental crust and dissipates with little effect.

The sound of all those feet hitting the ground creates a loud, drawn-out roar lasting many seconds. Eventually, the air grows quiet. Seconds pass. Everyone looks around.

There are a lot of uncomfortable glances. Someone coughs. A cell phone comes out of a pocket.

## Earth-Moon Fire Pole

Outside Rhode Island, abandoned machinery begins grinding to a halt. The T. Assuming they got things organized including sending out scouting missions to retrieve fuel , they could run at percent capacity for years without making a dent in the crowd.

Moments later, I, I, and I become the sites of the largest traffic jam in the history of the planet. Some make it past New York or Boston before running out of fuel. All the cops are in Rhode Island. The edge of the crowd spreads outward into southern Massachusetts and Connecticut. Any two people who meet are unlikely to have a language in common, and almost nobody knows the area.

Violence is common. Everybody is hungry and thirsty. Grocery stores are emptied. Within weeks, Rhode Island is a graveyard of billions. Our species staggers on, but our population has been greatly reduced.

But at least now we know. First, some definitions. A mole is a unit. A mole is also a type of burrowing mammal. A mole the animal is small enough for me to pick up and throw. One pound is 1 kilogram. I happen to remember that a trillion trillion kilograms is how much a planet weighs. An eastern mole Scalopus aquaticus weighs about 75 grams, which means a mole of moles weighs: Mammals are largely water. A kilogram of water takes up a liter of volume, so if the moles weigh 4.

The cube root of 4. So doing this on Earth is definitely not an option. Gravitational attraction would pull them into a sphere. But this is where it gets weird. The mole planet would be a giant sphere of meat. Normally, when organic matter decomposes, it releases much of that energy as heat. Closer to the surface, where the pressure would be lower, there would be another obstacle to decomposition— the interior of a mole planet would be low in oxygen.

While inefficient, this anaerobic decomposition can unlock quite a bit of heat. If continued unchecked, it would heat the planet to a boil.

But the decomposition would be self-limiting. Throughout the planet, the mole bodies would gradually break down into kerogen, a mush of organic matter that would—if the planet were hotter—eventually form oil.

Because the moles form a literal fur coat, when frozen they would insulate the interior of the planet and slow the loss of heat to space. However, the flow of heat in the liquid interior would be dominated by convection. Eventually, after centuries or millennia of turmoil, the planet would calm and cool enough that it would begin to freeze all the way through.

The deep interior would be under such high pressure that as it cooled, the water would crystallize out into exotic forms of ice such as ice III and ice V, and eventually ice II and ice IX. There might be a billion habitable planets in our galaxy. If you want a mole of moles, build a spaceship.

All watts have to go somewhere. This is true of any device that uses power, which is a handy thing to know. Are they right? This is true of almost any powered device. At that temperature, the box will be losing heat to the outside as fast as the hair dryer is adding it inside, and the system will be in equilibrium.

If the box is made of metal, it will be hot enough to burn your hand if you touch it for more than five seconds. The temperature it reaches will depend on the thickness of the box wall; the thicker and more insulating the wall, the higher the temperature. I wonder how high this dial goes. Two megawatts pumped into a laser is enough to destroy missiles. One more notch. Now 18 megawatts are flowing into the box.

If it were steel, it would have melted by now. The floor is made of lava. Before it can burn its way through the floor, someone throws a water balloon under it. The burst of steam launches the box out the front door and onto the sidewalk.

According to Back to the Future, the hair dryer is now drawing enough power to travel back in time. It sits in the middle of a growing pool of lava. Anything within 50— meters bursts into flame. A column of heat and smoke rise high into the air. Periodic explosions of gas beneath the box launch it into the air, and it starts fires and forms a new lava pool where it lands.

We keep turning the dial. At In , H. Wells imagined devices like this in his book The World Set Free. The story eerily foreshadowed the development, 30 years later, of nuclear weapons. The box is now soaring through the air. Each time it nears the ground, it superheats the surface, and the plume of expanding air hurls it back into the sky. The outpouring of 1.

A trail of firestorms —massive conflagrations that sustain themselves by creating their own wind systems —winds its way across the landscape. A new milestone: The box, soaring high above the surface, is putting out energy equivalent to three Trinity tests every second. At this point, the pattern is obvious. This thing is going to skip around the atmosphere until it destroys the planet. We turn the dial to zero as the box is passing over northern Canada.

Rapidly cooling, it plummets to Earth, landing in Great Bear Lake with a plume of steam. And then. A brief story: When the 1- kiloton nuke went off below, the facility effectively became a nuclear potato cannon, giving the cap a gigantic kick. The cap was never found. When we turn it back on, our reactivated hair dryer box, bobbing in lake water, undergoes a similar process. The heated steam below it expands outward, and as the box rises into the air, the entire surface of the lake turns to steam.

It exits the atmosphere and continues away, slowly fading from second sun to dim star. Much of the Northwest Territories is burning, but the Earth has survived. If a charger is connected to something, like a smartphone or laptop, power can be flowing from the wall through the charger into the device. However, neither of them answered this particular question. Without people, there would be less demand for power, but our thermostats would still be running.

As coal and oil plants started shutting down in the first few hours, other plants would need to take up the slack. This kind of situation is difficult to handle even with human guidance. However, plenty of electricity comes from sources not tied to the major power grids. These can continue to operate until they run out of fuel, which in most cases could be anywhere from days to months. Wind turbines People relying on wind power would be in better shape than most.

Some windmills can run for a long time without human intervention. Modern turbines are typically rated to run for 30, hours three years without servicing, and there are no doubt some that would run for decades. One of them would no doubt have at least a status LED in it somewhere. Their gearboxes would seize up. Hydroelectric dams Generators that convert falling water into electricity will keep working for quite a while. The dam would probably succumb to either clogged intakes or the same kind of mechanical failure that would hit the wind turbines and geothermal plants.

Even without anything using their power, batteries gradually self-discharge. Some types last longer than others, but even batteries advertised as having long shelf lives typically hold their charge only for a decade or two. There are a few exceptions.

Nobody knows exactly what kind of batteries it uses because nobody wants to take it apart to figure it out. Nuclear reactors Nuclear reactors are a little tricky. As a certain webcomic put it: As soon as something went wrong, the core would go into automatic shutdown. This would happen quickly; many things can trigger it, but the most likely culprit would be a loss of external power. Space probes Out of all human artifacts, our spacecraft might be the longest-lasting.

Within centuries, our Mars rovers will be buried by dust. GPS satellites, in distant orbits, will last longer, but in time, even the most stable orbits will be disrupted by the Moon and Sun. Many spacecraft are powered by solar panels, and others by radioactive decay. Eventually the voltage will drop too low to keep the rover operating, but other parts will probably wear out before that happens. So Curiosity looks promising.

With no human instructions, it will have no reason to turn them on. Solar power Emergency call boxes, often found along the side of the road in remote locations, are frequently solar-powered.

They usually have lights on them, which provide illumination every night. If we follow a strict definition of lighting, solar- powered lights in remote locations could conceivably be the last surviving human light source. Watch dials used to be coated in radium, which made them glow. Over the years, the paint has broken down. Although the watch dials are still radioactive, they no longer glow.

Watch dials, however, are not our only radioactive light source. In the dark, these glass blocks glow blue. And thus, we arrive at our answer: Centuries from now, deep in concrete vaults, the light from our most toxic waste will still be shining. In case something went wrong, next to the railing was stationed a distinguished physicist with an axe.

The principle here is pretty simple. If you fire a bullet forward, the recoil pushes you back. So if you fire downward, the recoil should push you up. The Saturn V had a takeoff thrust-to-weight ratio of about 1. As it turns out, the AK has a thrust-to-weight ratio of around 2.

This means if you stood it on end and somehow taped down the trigger, it would rise into the air while firing. Thrust is the product of these two amounts: If an AK fires ten 8- gram bullets per second at meters per second, its thrust is: Since the AK weighs only In practice, the actual thrust would turn out to be up to around 30 percent higher.

The amount of extra force this adds varies by gun and cartridge. The overall efficiency also depends on whether you eject the shell casings out of the vehicle or carry them with you.

I asked my Texan acquaintances if they could weigh some shell casings for my calculations. We can try using multiple guns. If you fire two guns at the ground, it creates twice the thrust. If each gun can lift 5 pounds more than its own weight, two can lift You will not go to space today.

An AK magazine holds 30 rounds. We can improve this with a larger magazine—but only up to a point. The reason for this is a fundamental and central problem in rocket science: Fuel makes you heavier. If we added more than about rounds, the AK would be too heavy to take off. The largest versions of this craft could accelerate upward to vertical speeds approaching meters per second, climbing over half a kilometer into the air.

With enough machine guns, you could fly. Can we do better? My Texas friends suggested a series of machine guns, and I ran the numbers on each one. Some did pretty well; the MG, a heavier machine gun, had a marginally higher thrust-to-weight ratio than the AK Then we went bigger.

To put it another way: If I mounted a GAU-8 on my car, put the car in neutral, and started firing backward from a standstill, I would be breaking the interstate speed limit in less than three seconds. Its thrust-to- weight ratio approaches 40, which means if you pointed one at the ground and fired, not only would it take off in a rapidly expanding spray of deadly metal fragments, but you would experience 40 gees of acceleration.

This is way too much. Landing lights almost always broke after firing. Or something else? Fortunately, your body handles air pressure changes like that all the time.

Air pressure changes quickly with height. If your phone has a barometer in it, as a lot of modern phones do, you can download an app and actually see the pressure difference between your head and your feet. At about two hours and two kilometers, the temperature would drop below freezing.

The wind would also, most likely, be picking up. If you have any exposed skin, this is where frostbite would start to become a concern. However, unless you had a warm coat, the temperature would be a bigger problem. Over the next two hours, the air would drop to below- zero temperatures. But when? The scholarly authorities on freezing to death seem to be, unsurprisingly, Canadians. According to their model, the main factor in the cause of death would be your clothes.

Above meters—above the tops of all but the highest mountains—the oxygen content in the air is too low to support human life. Near this zone, you would experience a range of symptoms, possibly including confusion, dizziness, clumsiness, impaired vision, and nausea. Your veins are supposed to bring low-oxygen blood back to your lungs to be refilled with oxygen. This would happen around the seven-hour mark; the chances are very slim that you would make it to eight.

She died as she lived—rising at a foot per second. I mean, as she lived for the last few hours. And two million years later, your frozen body, still moving along steadily at a foot per second, would pass through the heliopause into interstellar space. Clyde Tombaugh, the astronomer who discovered Pluto, died in It can, of course, vary quite a bit.

The hull would likely be airtight. There may be a few specialized one- way valves that would let air out, but in all likelihood, the submarine would remain sealed. The big problem the crew would face would be the obvious one: Everyone knows that space is very cold. The ocean is colder than space. Interstellar space is very cold, but space near the Sun —and near Earth—is actually incredibly hot!

When I was a kid, my dad had a machine shop in our basement, and I remember watching him use a metal grinder. Without a warm environment around you radiating heat back to you, you lose heat by radiation much faster than normal.

Without rockets, it has no way to do this. Okay—technically, a submarine does have rockets. Unfortunately, the rockets are pointing the wrong way to give the submarine a push.

Rockets are self-propelling, which means they have very little recoil. With a rocket, you just light it and let go. But not launching them could. Remember to disable the detonators on the missiles. It means the warmth of sunlight in winter. Since there are 7. Your extra two million bills a year would barely be enough to notice.

Would the storm cell be immediately vaporized? It turns out the National Oceanic and Atmospheric Administration—the agency that runs the National Hurricane Center—gets it a lot, too. I recommend you read the whole thing,1 but I think the last sentence of the first paragraph says it all: Water turbines can be pretty efficient.

For those 42 minutes, our hypothetical house could generate up to watts of electricity, which might be enough to power everything inside it.

The stars are named Joe Biden. It works, but it feels so wrong. I bike to class sometimes. To increase the temperature of the air layer in front of your body by 20 degrees Celsius enough to go from freezing to room temperature , you would need to be biking at meters per second. Since drag increases with the square of the speed, this limit would be pretty hard to push any further.

How much physical space does the Internet take up? The storage industry produces in the neighborhood of million hard drives per year. If most of them are 3. So, by that measure, the Internet is smaller than an oil tanker.

I am not an authority on lightning safety. I am a guy who draws pictures on the Internet. With that out of the way. To answer the questions that follow, we need to get an idea of where lightning is likely to go. Roll an imaginary meter sphere across the landscape and look at where it touches.

They say lightning strikes the tallest thing around. I mean, not all lightning hits Mount Everest. But does it find the tallest person in a crowd? The tallest person I know is probably Ryan North. What about other reasons? So how does lightning pick its targets? The leader carries comparatively little current —on the order of amps.

This is the blinding flash you see. It races back up the channel at a significant fraction of the speed of light, covering the distance in under a millisecond. This is where the meter sphere comes in. To figure out where lightning is likely to hit, you roll the imaginary meter sphere across the landscape. Places the surface makes contact —treetops, fence posts, and golfers in fields—are potential lightning targets.

The shadow is the area where the leader is likely to hit the tall object instead of the ground around it: After the current hits the tall object, it flows out into the ground. Of the 28 people killed by lightning in the US in , 13 were standing under or near trees.

But lightning striking the water near you would still be bad. The 20, amps spread outward—mostly over the surface—but how much of a jolt it will give you at what distance is hard to calculate. What would happen if you were taking a shower when you were struck by lightning? Or standing under a waterfall? Or a submarine?

A boat with a closed cabin and a lightning protection system is about as safe as a car. Or what if you were doing a backflip? Or looking straight up at the bolt? The core of a lightning bolt is a few centimeters in diameter. A bullet fired from an AK is about 26 mm long and moves at about millimeters every millisecond. Copper is a fantastically good conductor of electricity, and much of the 20, amps could easily take a shortcut through the bullet.

Surprisingly, the bullet would handle it pretty well.

If it were sitting still, the current would quickly heat and melt the metal. It would continue on to its target relatively unaffected. This effect is similar to how when a traffic light turns green, the cars in front start moving, then the cars in back, so the movement appears to spread backward. On the other hand, apples are better. Humans, for example, are probably still far better at looking at a picture of a scene and guessing what just happened: To test this theory, I sent this picture to my mother and asked her what she thought had happened.

The cat knocked over the vase. The cat jumped out of the vase at the kid. The cat was mummified in the vase, but arose when the kid touched it with a magic rope. The vase exploded, attracting a child and a cat. The child put on the hat for protection from future explosions. The kid and cat are running around trying to catch a snake. According to computer scientist Hans Moravec, a human running through computer chip benchmark calculations by hand, using pencil and paper, can carry out the equivalent of one full instruction every minute and a half.

A new high-end desktop PC chip would increase that ratio to So, what year did a single typical desktop computer surpass the combined processing power of humanity? After all, these comparisons are one computer against all humans. How do all humans stack up against all computers?

This is tough to calculate.

It turns out that processors from the s and processors from today have a roughly similar ratio of transistors to MIPS —about 30 transistors per instruction per second, give or take an order of magnitude. It looks something like this: This tells us that a typical modern laptop, which has a benchmark score in the tens of thousands of MIPS, has more computing power than existed in the entire world in The complexity of neurons Again, making people do pencil-and-paper CPU benchmarks is a phenomenally silly way to measure human computing power.

There are projects that attempt to use supercomputers to fully simulate a brain at the level of individual synapses. The numbers from a run of the Japanese K supercomputer suggest a figure of transistors per human brain. One, the pencil-and-paper Dhrystone benchmark, asks humans to manually simulate individual operations on a computer chip, and finds humans perform about 0.

A slightly better approach might be to combine the two estimates. This actually makes a strange sort of sense. If we assume our computer programs are about as inefficient at simulating human brain activity as human brains are at simulating computer chip activity, then maybe a more fair brain power rating would be the geometric mean of the two numbers.

Wilson, there are to ants in the world. By comparison, in there were about transistors in the world, or tens of thousands of transistors per ant. Mere Machine to Transcendent Mind. Biology is tricky. I hope things are better in the future. Please figure out a way to come get us.

It was written in Even in our best telescopes, the largest asteroids were visible only as points of light. The Little Prince took this a step further, imagining an asteroid as a tiny planet with gravity, air, and a rose. If there really were a superdense asteroid with enough surface gravity to walk around on, it would have some pretty remarkable properties. If the asteroid had a radius of 1. This is roughly equal to the combined mass of every human on Earth.

It would feel like you were stretched out on a curved rubber ball, or were lying on a merry-go-round with your head near the center.

The escape velocity at the surface would be about 5 meters per second. That means you might be able to leave our asteroid by running horizontally and jumping off the end of a ramp.

Your orbital speed would be roughly 3 meters per second, which is a typical jogging speed. But this would be a weird orbit. Tidal forces would act on you in several ways. If you stretched your arm down toward the planet, it would be pulled much harder than the rest of you. And when you reach down with one arm, the rest of you gets pushed upward, which means other parts of your body feel even less gravity.

A large orbiting object under these kinds of tidal forces—say, a moon—will generally break apart into rings. However, your orbit would become chaotic and unstable.

Rugescu and Daniele Mortari. Their simulations showed that large, elongated objects follow strange paths around their central bodies. This type of analysis could actually have practical applications. There have been various proposals over the years to use long, whirling tethers to move cargo in and out of gravity wells—a sort of free-floating space elevator.

The inherent instability of many tether orbits poses a challenge for such a project. Mallory Ortberg, writing on the-toast. And you may need to defrost it after you pick it up.

Things get really hot when they come back from space. When skydiver Felix Baumgartner jumped from 39 kilometers, he hit Mach 1 at around 30 kilometers. There was no clear conclusion.

To try to get a better answer, I decided to run a series of simulations of a steak falling from various heights. Apparently, the US government was shoveling tons of money at anything even loosely related to weapons research. It took me a while to realize there was a much easier way to learn what combinations of time and temperature will effectively heat the various layers of a steak: Check a cookbook.

No matter how fast it was going when it reached the lower layers of the atmosphere, it would quickly slow down to terminal velocity. For much of those 25 kilometers, the air temperature is below freezing —which means the steak will spend six or seven minutes subjected to a relentless blast of subzero, hurricane-force winds. When the steak does finally hit the ground, it will be traveling at terminal velocity —about 30 meters per second. To get an idea of what this means, imagine a steak flung at the ground by a major-league pitcher.

## (PDF) medical-site.info | Claudiu Simon - medical-site.info

If the steak is even partially frozen, it could easily shatter. However, if it lands in the water, mud, or leaves, it will probably be fine. Steaks can probably survive breaking the sound barrier. In addition to Felix, pilots have ejected at supersonic speeds and lived to tell about it. We need to go higher. At supersonic and hypersonic speeds, a shockwave forms around the steak that helps protect it from the faster and faster winds.

I searched the literature, but was unable to find any research on this. However, this is little more than a wild guess.

If anyone puts a steak in a hypersonic wind tunnel to get better data on this, please, send me the video. In this scenario, the steak reaches a top speed of Mach 6, and the outer surface may even get pleasantly seared. The inside, unfortunately, is still uncooked. From higher altitudes, the heat starts to get really substantial. That is, it becomes charred. Charring is a normal consequence of dropping meat in a fire.

If the heat is high enough, it will simply blast the surface layer off as it flash-cooks it. If most of the steak makes it to the ground, the inside will still be raw. Which is why we should drop the steak over Pittsburgh. And the Andromeda Strain. Not necessarily fine to eat. The problem, in a nutshell, is that hockey players are heavy and pucks are not. A goalie in full gear outweighs a puck by a factor of about Even the fastest slap shot has less momentum than a ten- year-old skating along at a mile per hour.

It also suggests that if you started to slowly rotate a hockey rink, it could tilt up to 50 degrees before the players would all slide to one end.

Clearly, experiments are needed to confirm this. Firing an object at Mach 8 is not, in itself, very hard. One of the best methods for doing so is the aforementioned hypersonic gas gun, which is—at its core —the same mechanism a BB gun uses to fire BBs.

Imagine throwing a ripe tomato—as hard as you can—at a cake. After a few days, your immune system notices and destroys it,3 but not before you infect, on average, one other person. Could our immune systems then wipe out every copy of the virus? A global quarantine brings us to another question: How far apart can we actually get from one another? A lot of us would be stuck standing in the Sahara Desert,5 or central Antarctica.

That way, we could walk around and interact, even allowing some normal economic activity to continue: Would it work? To help figure out the answer, I talked to Professor Ian M. He said that rhinoviruses—and other RNA respiratory viruses —are completely eliminated from the body by the immune system; they do not linger after infection.

The remote islands of St.

Kilda, far to the northwest of Scotland, for centuries hosted a population of about people. The exact cause of the outbreaks is unknown,8 but rhinoviruses were probably responsible for many of them. Every time a boat visited, it would introduce new strains of virus. These strains would sweep the islands, infecting virtually everyone. After several weeks, all the residents would have fresh immunity to those strains, and with nowhere to go, the viruses would die out. If all humans were isolated from one another, the St.

After a week or two, our colds would run their course, and healthy immune systems would have plenty of time to clear the viruses. The story is different for those with severely weakened immune systems. In transplant patients, for example, whose immune systems have been artificially suppressed, common infections —including rhinoviruses —can linger for weeks, months, or conceivably years. This small group of immunocompromised people would serve as safe havens for rhinoviruses.

The hope of eradicating them is slim; they would need to survive in only a few hosts in order to sweep out and retake the world. While colds are no fun, their absence might be worse. On the other hand, colds suck. And in addition to being unpleasant, some research says infections by these viruses also weaken our immune systems directly and can open us up to further infections.

If the average were less than one, the virus would die out. If it were more than one, eventually everyone would have a cold all the time. Kilda correctly identified the boats as the trigger for the outbreaks. But what if the empty half of the glass were actually empty—a vacuum? Which half is empty? For the first handful of microseconds, nothing happens. On this timescale, even the air molecules are nearly stationary.

For the most part, air molecules jiggle around at speeds of a few hundred meters per second. But at any given time, some happen to be moving faster than others. The fastest few are moving at over meters per second. These are the first to drift into the vacuum in the glass on the right. However, in the vacuum of the glasses, it does start to boil, slowly shedding water vapor into the empty space. While the water on the surface in both glasses starts to boil away, in the glass on the right, the air rushing in stops it before it really gets going.

The sides of the glass bulge slightly, but they contain the pressure and do not break. A shockwave reverberates through the water and back into the air, joining the turbulence already there.

The shockwave from the vacuum collapse takes about a millisecond to spread out through the other two glasses. The glass and water both flex slightly as the wave passes through them. Around this time, the glass on the left starts to visibly lift into the air. This is the force we think of as suction. The boiling water has filled the vacuum with a very small amount of water vapor. However, the glass and water are now moving too fast for the vapor buildup to matter.

Without a cushion of air between them —only a few wisps of vapor —the water smacks into the bottom of the glass like a hammer. The momentary force on the glass is immense, and it breaks. In our situation, the forces would be more than enough to destroy even the heaviest drinking glasses. The bottom is carried downward by the water and thunks against the table.

The water splashes around it, spraying droplets and glass shards in all directions. Meanwhile, the detached upper portion of the glass continues to rise. After half a second, the observers, hearing a pop, have begun to flinch. Their heads lift involuntarily to follow the rising movement of the glass.

The glass has just enough speed to bang against the ceiling, breaking into fragments. The lesson: If the optimist says the glass is half full, and the pessimist says the glass is half empty, the physicist ducks.

Radio transmissions Contact popularized the idea of aliens listening in on our broadcast media. Sadly, the odds are against it. Space is really big. The full picture is more complicated, but the bottom line is that as our technology has gotten better, less of our radio traffic has been leaking out into space. Even in the late 20th century, when we were using TV and radio to scream into the void at the top of our lungs, the signal probably faded to undetectability after a few light-years.

They were outshone by the beams from early- warning radar. But the same march of technological progress that made the TV broadcast towers obsolete has had the same effect on early- warning radar. This massive dish in Puerto Rico can function as a radar transmitter, bouncing a signal off nearby targets like Mercury and the asteroid belt.

However, it transmits only occasionally, and in a narrow beam. If an exoplanet happened to be caught in the beam, and they were lucky enough to be pointing a receiving antenna at our corner of the sky at the time, all they would pick up would be a brief pulse of radio energy, then silence. Visible light This is more promising. The Sun is really bright, [citation needed ] and its light illuminates the Earth. Both of these effects could potentially be detected from an exoplanet.

You could probably figure out what our water cycle looked like, and our oxygen-rich atmosphere would give you a hint that something weird was going on. So in the end, the clearest signal from Earth might not be from us at all. Heeeey, look at the time.

Gotta run. A radio transmission has the problem that they have to be paying attention when it arrives. Instead, we could make them pay attention. If we can figure out how to make a guidance system that survives the trip which would be tough , we could use it to steer toward any inhabited planet. But slowing down takes even more fuel. So maybe if those aliens looked toward our solar system, this is what they would see: There are easier ways to lose a third of a pound, including: This happens for two reasons: One, the Earth is shaped like this: When you stand, your muscles are constantly working to keep you upright.

For a while. Amanita bisporigera is a species of mushroom found in eastern North America. Destroying angel is a small, white, inoccuous-looking mushroom. Amanita is the reason why. Then you start to feel better. Amanita mushrooms contain amatoxin, which binds to an enzyme that is used to read information from DNA.

Since most of your body is made of cells,4 this is bad. Death is generally caused by liver or kidney failure, since those are the first sensitive organs in which the toxin accumulates. The picture is even more vividly illustrated by two other examples of DNA damage: Some are more precisely targeted than others, but many simply interrupt cell division in general.

The reason that this selectively kills cancer cells, instead of harming the patient and the cancer equally, is that cancer cells are dividing all the time, whereas most normal cells divide only occasionally. Some human cells do divide constantly. The most rapidly dividing cells are found in the bone marrow, the factory that produces blood.

Without it, we lose the ability to produce white blood cells, and our immune system collapses. Chemotherapy causes damage to the immune system, which makes cancer patients vulnerable to stray infections. Doxorubicin, one of the most common and potent chemotherapy drugs, works by linking random segments of DNA to one another to tangle them.

## Earth-Moon Fire Pole

A loss of DNA would cause similar cell death, and probably similar symptoms. This is the period where the body is still working, but no new proteins can be synthesized and the immune system is collapsing. On the other hand, there would be at least one silver lining. If we ever end up in a dystopian future where Orwellian governments collect our genetic information and use it to track and control us.

I got one of your friends to sneak into your room with a microscope while you were sleeping and check. They stimulate white blood cell production by, in effect, tricking the body into thinking that it has a massive E. Instead, they physically dissolve the blood-brain barrier, resulting in rapid death from cerebral hemorrhage brain bleeding. Plants undo this by stripping the oxygen back out and pumping it into the air. Engines need oxygen in the air to run.

The Sun: To see what would happen to our aircraft on Mars, we turn to X-Plane. X-Plane is the most advanced flight simulator in the world. This makes it a valuable research tool, since it can accurately simulate entirely new aircraft designs —and new environments.

X-Plane tells us that flight on Mars is difficult, but not impossible. NASA knows this, and has considered surveying Mars by airplane. The tricky thing is that with so little atmosphere, to get any lift, you have to go fast. The X-Plane author compared piloting Martian aircraft to flying a supersonic ocean liner.

If dropped from 4 or 5 kilometers, it could gain enough speed to pull up into a glide—at over half the speed of sound. The landing would not be survivable. But physics calculations give us an idea of what flight there would be like. The upshot is: Your plane would fly pretty well, except it would be on fire the whole time, and then it would stop flying, and then stop being a plane. Unfortunately, that air is hot enough to melt lead. A much better bet would be to fly above the clouds.

You would need the wetsuit, though, to protect you from the sulfuric acid. Venus is a terrible place. The picture here is a little friendlier than on Jupiter. Uranus is a strange, uniform bluish orb.

It at least has some clouds to look at before you freeze to death or break apart from the turbulence. Its gravity—lower than that of the Moon—means that flying is easy. Our Cessna could get into the air under pedal power. A human in a hang glider could comfortably take off and cruise around powered by oversized swim-flipper boots —or even take off by flapping artificial wings. The power requirements are minimal—it would probably take no more effort than walking. Judging from some numbers on heating requirements for light aircraft, I estimate that the cabin of a Cessna on Titan would probably cool by about 2 degrees per minute.

The Huygens probe, which descended with batteries nearly drained, taking fascinating pictures as it fell, succumbed to the cold after only a few hours on the surface. If humans put on artificial wings to fly, we might become Titanian versions of the Icarus story—our wings could freeze, fall apart, and send us tumbling to our deaths.

The cold of Titan is just an engineering problem. With the right refitting, and the right heat sources, a Cessna could fly on Titan —and so could we. What is the total nutritional value calories, fat, vitamins, minerals, etc. How much Force power can Yoda output? First we need to know how heavy the ship was. Next, we need to know how fast it was rising. The front landing strut rises out of the water in about three and a half seconds, and I estimated the strut to be 1.

Lastly, we need to know the strength of gravity on Dagobah. Wookieepeedia has just such a catalog, and informs us that the surface gravity on Dagobah is 0.

Combining this with the X- wing mass and lift rate gives us our peak power output: But telekinesis is just one type of Force power. What about that lightning the Emperor used to zap Luke? So I spent the second half of the class just solving problems like that in front of them. And then I was like, "That was really fun. I want to keep doing it. That was where the idea came from.

I actually wrote the first couple entries quite some time ago, based on questions students asked me in that class -- and then on another couple questions that my friends had asked.

It was only recently that I finally managed to get around to starting it up as a blog. The variety of the topics you tackle is incredibly broad. How do you figure out the best way to explain all these different, complicated subjects to people? Part of what I'm doing is selectively looking for questions that I already know something interesting about.

Or I've stumbled across a paper recently that was really cool, and now I'll keep an eye out for a question that will let me bring it up -- something I can use as a springboard. So in a conversation, someone might say, "Money doesn't grow on trees. Our economy would collapse. On the other hand, we would switch to a new currency. It's complicated. What I like doing is finding the places in those questions where normal people -- or, people who have less spare time than I do -- think, "This is stupid," and stop.

I think the really cool and compelling thing about math and physics is that it opens up entry to all these hypotheticals -- or at least, it gives you the language to talk about them.

But at the same time, if a scenario is completely disconnected from reality, it's not all that interesting. So I like the questions that come back around to something in real life. And the great thing with this is that once someone asks me something good, I can't not figure out the answer, you know?

I get really serious, and I'll drop whatever I'm doing and work on that. One of the questions I recently answered was, "What if, when it rains, the rain came down in one drop? Why that need to answer? Is it because people are asking you -- because you want to help them out by answering the questions for them? Oh, no, no, no, there's nothing altruistic about it!

It's just like, once it gets in my brain, it keeps bugging me, and I don't know the answer, but I'm really curious. What really happens is: I have an idea for what the answer is, but then I want to figure out if I'm right or not.

So I have to keep working to find out. And oftentimes, in the process of learning I'm wrong, I'll run into something even cooler. And then once I find that, I just want to tell everyone about it. So I basically set up this blog to flatter all of my random impulses.

And it's been a lot of fun so far. And how do you decide which questions you ultimately commit to answering? For the first few entires I wrote, I just wanted to make sure this format made sense.

So I wrote a couple entries with questions just from my friends. And then when we put up the blog, we included an "Ask a Question" link. And since then, the volume of questions has been high enough that I don't think any set of two or three people could read them all. So I pretty much just sit down whenever I have a few spare hours and go through them and answer the questions that come in and try to see if there's anything that would make a good article.

Of the ones you've done, do you have a favorite so far? The one that I recently put up : "What would happen if the land masses of the world were rotated 90 degrees? And I kept on thinking, "Oh, what about this thing? But I also really enjoyed the first one that I posted , and that's been one of a lot of people's favorites: the one about the baseball thrown at 90 percent of the speed of light.

That one I have a soft spot for because it was the first one I put up. And I heard from people who know a lot more about these things than I do -- I got email from a bunch of physicists at MIT saying, "Hey, I saw your relativist baseball scenario, and I simulated it out on our machine, and I've got some corrections for you. It showed there were some effects that I hadn't even thought about. I'm probably going to do a follow-up at some point using all the material they sent me. I imagine, given all that, that the posts are incredibly labor-intensive.

How long would an average one take you? I'm still deciding how long to make them -- and part of that is just figuring out how long it takes to answer the average question. When I started out, I didn't really know what to expect from the questions, so I wasn't sure if I'd be able to answer them quickly or what. But I think, now, it's about a day of work in which I don't do much else.

That's it? I was figuring it'd take much longer! Well, that's a day of solid work -- I mean, most people don't actually work through a whole day. I certainly don't.

So in practice, it's a few days, because there's a lot of email checking, or having to go run an errand. Makes sense. And, design-wise, I love how each article stands on its own -- a single product on a single page.

Since that's the same structure you use for xkcd, I'm wondering: Why did you choose to repeat that format? Especially because I was so delayed in actually getting the site up , I had a lot of time to think about how I wanted it to look. Did I want to have individual entries, or did I want to do more of a blog format, or did I want to have a bunch of questions answered as they came in?

So we settled on the current format, and it seems to work pretty well. One of the things I've learned with doing xkcd is that you sort of give people, "Here's the thing, and here's the button you can press to get another thing. Do I really want another page like this? I think it's really annoying to want to read partway through, and then you navigate away, and can't get back.

Does What If, at this point, have a business model? When I first started xkcd, it was all stuff I drew during classes -- because I wasn't paying attention to the lecture. And then when I started drawing them from home, I found that I'd have a lot of trouble coming up with ideas. And then I'd get a project and start working on that -- and I found that, instead of it taking up more of my time, I had more comic ideas per day and was drawing more of them.

So they all reinforced each other. The format would definitely make sense as a book. But for the moment, it's just been so much fun to write and answer. My experience of the Internet has been that if you make something really cool, the neatness speaks for itself. And that's much more important than trying to make something marketable -- trying to make something into a product. So I just found that if I'm steadily trying to make cool things and putting them up, some of them, in some way or another, have a business opportunity.

Is there a direct relationship between xkcd and What If?