What If? By Randall Munroe

Serious Scientific Answers to Absurd Hypothetical Questions

This book contains answers to hypothetical questions. The author is the creator of xkcd, a stick-figure web-comic of romance, sarcasm, math and language. He worked at NASA for a while. He uses mathematics to answer questions. The questions were posted on his website.

Global Windstorm (Q: What would happen if the Earth and all terrestrial objects suddenly stopped spinning, but the atmosphere retained its velocity by Andrew Brown)

Nearly everyone would die. There would be super strong wind if the air was still moving. 85% of the population will experience supersonic winds. Buildings would be smashed. Hurricane usually killed by throwing other objects at high velocity at you. Those in basements might survive. There would be huge waves surging on the shore too. The sun would only rise and set once a year. Day would be 6 months long, and so would be night. The Earth might start spinning due to gravitational pull from the moon.

Relativistic Baseball (Q: What would happen if you tried to hit a baseball pitched at 90% the speed of light?)

Air molecules around would literally stop moving. The air that hits the ball would fuse with the atoms in the ball and release gamma rays and scattered particles. The ball would integrate and fragments would go in all directions. Everything would happen in nanoseconds. The plasma would swallow everyone. There would be a blinding light, followed by a fireball and then a mushroom cloud. Almost the whole town would be destroyed.

Spent Fuel Pool (Q: What if I took a swim in a typical nuclear fuel pool? Would I need to dive to actually experience a fatal amount of radiation? How long could I stay safely at the surface?)

A normal person can only thread water from anywhere between 10 to 40 hours. Every 7cm cuts the amount of radiation by half. If you touch the canister and swim out, you will die. But radiation in normal waters are lower than background radiation in the air. Spent fuel pools are serviced by divers.

(Q: Time travelling to back and then forward into the future)

A 1000 years ago in New York, it could possibly be inhabited by tribes from Europe. There would be larger predators and chestnut trees. 10000 years ago, the Earth was out of a cold period. 100000 years ago, the climate might have been stable like now. A million years ago, the world was fairly warm. A billion years ago, all the continents were fused and known as Rodinia. There were no plants or animals, with only single-cellular life. A million years forward, everyone would die. Few human artifacts will remain but non bio-degradable plastic could remain. In a billion years, the oceans will overheat and we will become a new Venus. Earth will then be consumed by the expanding sun.

Soul Mates. (Q: What if everyone actually had only one soul mate, a random person somewhere in the world?)

A hundred billion or so humans have ever lived, but only seven billion are alive now. We all have half a billion potential matches around the same age range. It would only be possible to find true love only in one lifetime out of 10,000.

Laser Pointer. (Q: If every person on Earth aimed a laser pointer at the Moon at the same time, would it change color?)

Regular laser pointers won’t. Some light would be dispersed in the air but most can reach the surface. Regular laser pointers have too little power. If you use a 1-watt laser, it won’t work too. It would be half a lux of illumination, compared to 130,000 from the sun. Even if you use the searchlights on helicopters, it would be no good. Even the most powerful searchlight in the world won’t cut it. If megawatt lasers were fitted everywhere on earth, the moon will lit up. It is possible to push the moon out of orbit too.

Periodic Wall of the Elements. (Q: What would happen if you made a periodic table out of cube-shaped bricks, where each brick was made of the corresponding element?)

30 of the 118 elements can be bought over the internet. Some of them are too radioactive to be used. The fifth row will kill you plus give you radiation. The first is hydrogen and helium. Lithium will tarnish, beryllium is toxic, fluorine is corrosive in nature and may form hydrofluoric acid if in contact with moisture. Phosphorus is difficult to handle. Onto row 4. Arsenic is scary to all forms of complex life, Potassium is also very reactive, selenium and bromine will react. The fifth row consists of technetium-99, a radioactive brick. The sixth row will kill you. Astatine is the nasty one and the building would be demolished instantly. The surrounding neighbourhoods would be uninhabitable. You will be cooked alive. The 7th row consists of transuranic elements. They can only be formed in particle accelerators. The cube will just decay like a nuclear explosion. Everything would be turned into plasma. There would be a mushroom cloud and a massive fallout. Collecting elements isn’t the smartest thing you can do.

Everybody jump. ‘Q: What would happen if everyone on Earth stood as close to each other as they could and jumped, everyone landing on the ground at the same instant?’

The Earth outweighs humans by a factor of ten trillion. The energy will be spread across the ground once the humans land. All the networks and transport systems will be jammed. People will starve and violence will be highly rampant.

A Mole of Moles. ‘Q: What would happen if you were to gather a mole of moles in one place.’

A mole is 6 * 10^23. A mole is also an animal that burrow. It would weigh about the same mass as the moon. We might end up with a mole planet. It would be a lump of meat. Without oxygen, they won’t decompose.

Hair Dryer. ‘Q: What would happen if a hair dryer with continuous power were turned on and put in an airtight 1*1*1 metre box?’

There will be 1875 watt of heat flowing out. The system will lose heat to the surroundings and form an equilibrium with the outside. The inside of the box will be like an oven. If the hairdryer is indestructible and we can increase the power, the surface of the box will be 200 degrees. Now, 2 megawatts is pumped in. The box has a temperature of 1300 deg. Now, 18 megawatt of power are in the box. Steel would have melted too. Now, it is powered to 1.875 gigawatts. The box is blinding bright. At higher powers, it will surge up into the air.

The last Human Light. ‘Q: If every human somehow simply disappeared from the face of the Earth, how long would it be before the last artificial light source would go out?’

The fossil fuel plants would stop as no humans are running it. Diesel generators can operate for a few months. Geothermal plants can run for a few years. Windmills may run for many years on end. Hydroelectric dams will experience the same fate. Batteries might last up to a decade, but no more. Space probes might last for much longer. Solar panels might last for a century. Radioactivity has the potential to last even longer.

Machine-Gun Jetpacks (Q: Is it possible to build a jetpack using downward-firing machine guns?)

The AK47 has a thrust to weight ratio of 2. This is not true for all machine guns though. Thrust is mass ejection rate * Speed of ejection. It could take off but it won’t lift anything more than the squirrel and the gun itself. The Russians invented a gun where the recoil was too huge and would damage the aircraft.

Rising Steadily (Q: If you suddenly began rising steadily at 1 foot per second, how exactly would you die? Would you freeze or suffocate first? Or something else?)

After a minute, you would be above the trees. After 25 minutes you would be above the empire state building. At two hours in, the temperature will drop below freezing. Frostbite will happen. Now, the air pressure will drop too. One will succumb to hypothermia. If you were nude, you might succumb to it at the 5 hour (5km mark). At 8km, oxygen is too little to support human life. You will lose consciousness and chances are that you won’t make it to 8km. Two million years later, your body will enter interstellar space.

Orbital Submarine. (Q: How long could a nuclear submarine last in orbit?)

The crew would die. The submarine can withstand the pressure. It could remain sealed. The ocean is colder than space. If there is no air around you, the heat you radiate won’t be lost be soon.

(Q: If my printer could literally print out money, would it have that big an effect on the world?)

You can print $200m a year, thus it will have a negligible impact on the Earth. There are 7.8 billion $100 bills in circulation.

(Q: Lightning)

How does lightning pick its targets? It targets in 60m spheres. Anything like treetops, fence posts, golfers are likely to get hit. Is it dangerous to be in a pool during a thunderstorm? Answer is quite dangerous.

Little Planet. (Q: If an asteroid was very small but supermassive, could you really live on it like the Little Prince?)

It is possible to reach escape velocity for a planet that is so small. Tidal forces will screw you over and you won’t survive.

Steak Drop (Q: From what height would you need to drop a steak for it to be cooked when it hit the ground?)

Compression heating happens above Mach 2. Felix Baumgartner hit Mach 1 at around 30km. The author performed much research in this area. It always takes 6 to 7mins to fall from 25km to the ground. It won’t be appreciably heated. You can drop it from 50km to break the sound barrier. From 100 km, it will travel at Mach 2. But the heat generated will be too little compared to the freezing outside temperature.

Hockey Puck (Q: How hard would a puck have to be shot to be able to knock the goalie himself backward into the net?)

Hockey players are massive while pucks are not. The answer would be between Mach 2 and Mach 8. It is possible to use a hypersonic gas gun. The surface of the puck would melt first.

Common Cold (Q: If everyone on the planet stayed away from each other for a couple of weeks, wouldn’t the common cold be wiped out?)

It might not be worth as the economy will come to a standstill. A lot of us would be standing in the desert etc. The idea is someone feasible. Will the rhinovirus die out? For those with weak immune systems, the rhinoviruses will stay in their body for much longer. It is more practical not to do it as it is extremely costly due to disruptions on the economy.

Alien Astronomers (Q: Let’s assume there’s life on the nearest habitable exoplanet and that they have technology comparable to ours. If they looked at our star right now, what would they see?)

Radio transmission is a long shot. Most of these technology are obsolete now. Visible light might be detectable for an alien planet.

No more DNA (Q: This may be a bit gruesome, but… if someone’s DNA suddenly vanished, how long would that person last?)

You would weigh a third of a pound lighter. If you eat a destroying angel (a mushroom) it will bind an enzyme that is used for reading information on DNA. It will cause damage to whatever cells it collects in. Chemotherapy and radiation are other methods. Radiation will damage your DNA. It will result in death after a while.

Interplanetary Cessna (Q: What would happen if you tried to fly a normal Earth airplane above different solar system bodies)

Without atmosphere. The plane would crash. It would vaporize at the sun, flight on Mars is difficult as the plane must go very fast to get off the ground; the temperature is also too hot in Venus, Jupiter’s gravity is too strong. You will need a plane with three times the power. There is also no surface on Jupiter. Titan would be too cold, similar to that of liquid nitrogen.

Flyover States (Q: Which US state is actually flown over the most?)

The answer is Virginia. Most of Delta’s flights from Atlanta will fly through Virginia. The Toronto Pearson airport is also a big contributor. Delaware has the highest ratio of flights over to flights to.

Falling with Helium (Q: What if I jumped out of an airplane with a couple of tanks of helium and one huge, uninflated balloon? Then, while failing, I release the helium and fill the balloon. How long of a fall would I need in order for the balloon to slow me enough that I could land safely?)

A balloon would have to be at least 10 to 20m across. You will also need about 10 helium tanks and will be required to fill up your balloon very quickly.

Everybody Out (Q: Is there enough energy to move the entire human population off-planet?)

To move someone out of Earth and into space, it requires 1megawatt-hour or the household electrical energy for a month. The problem are that rockets need to lift their own fuel and the things onboard. One trick is the space elevator but we don’t have strong enough materials to build it. Another way is to ride on the shockwave of a nuclear explosion but remain shielded of course.

Self-fertilization (Q: If a woman were to have sperm cells made from her own stem cells and impregnate herself, what would be her relationship to her daughter?)

Stem cells could be used to produce sperm or eggs. No one has succeeded yet though. ‘One copy of the gene can make you stronger, but two copies – double multipliers – lead to a serious disorder.’ It is very risky to self-fertilize for complex organisms like humans. Only very few organisms reproduce asexually.

High Throw (Q: How high can a human throw something?)

There are other animals that can throw things too. Humans, however, are a lot more accurate and strong. We are better at throwing things forward rather than upward. An average person can throw a baseball at least 15 metres in the air. A strong person can go to 25 metres. A pitcher can reach up to 50 metres in height. The max height would be like 75 metres for the fastest pitcher on Earth. It might be easier to throw a golfball rather than a baseball, as it more aerodynamic in nature.

Lethal Neutrinos (Q: How close would you have to be to a supernova to get a lethal dose of neutrino radiation?)

They usually do not hit ordinary matter. Particle accelerators can detect neutrinos. Supernovas are incredibly bright and powerful. They are unimaginably huge. Even if you’re 1AU away, you could die from the radiation.

Speed Bump (Q: how fast can you hit a speed bump while driving and live)

Surprising fast. It could destroy your tires and suspension. It is possible to fracture your vertebra. The car suspension can protect you normally even if you’re moving fast. The jolt won’t be fatal but you might lose control. The faster you go, the more your car will start to lift.

Orbital Speed (Q: What if a spacecraft slowed down on re-entry to just a few miles per hour using rocket boosters like the Mars sky crane? Would it negate the need for a heat shield?)

It is difficult to get into orbit because you have to go so fast. Space is 100km away from us. It is easy to go up but difficult to stay there. Gravity there is as strong as on the surface. Therefore, you need to go sideways extremely fast. It is about 8km/s. It requires a lot of fuel to achieve this. Therefore, it is not practical to carry enough fuel on the return leg. It is a blistering fast speed at which the ISS is going.

Free Fall (Q: What place on Earth would allow you to free-fall the longest by jumping off it? What about using a squirrel suit?)

The largest purely vertical drop is at Mount Thor. A human has a terminal velocity of 55m per second. Wingsuits can slow you down. You could probably last a minute then. The record is 3min and 20 seconds.

Drain the Oceans (Q: how quickly would the oceans drain if a circular portal 10 metres in radius leading into space were created at the bottom of Challenger Deep? How would the Earth change as the water was being drained?)

You won’t get a whirlpool because the ocean is too deep. Some parts of land will be connected when the ocean is drained. Some bodies of water like the black sea are not connected to the ocean. Indonesia will become a big blob. There will be new islands that keep popping up. The ecosystem will collapse. It is unlikely that humans can survive it.

Drain the Oceans: Part 2 (Q: Supposing you did drain the oceans, and dumped the water on top of the Curiosity rover, how would Mars change as the water accumulated?)

There is water on Mars but it is frozen. It is possible to form a lake if water is dumped really fast. Mars has some really tall mountains that are over 10km tall and these won’t be covered by the water dumped in. Eventually the oceans will freeze.

Lego Bridge (Q: How many Lego bricks would it take to build a bridge capable of carrying traffic from London to new York? Have that many Lego bricks been manufactured?)

It would take 350m bricks to connect the two cities but it’s insufficient to form a bridge. There have been over 400 billion Lego pieces produced worldwide. But to support a car, you will need a lot more bricks. The problem is also to ensure that it doesn’t sink. The Lego bricks are surprisingly tough. However, the wind and waves will cause tension to the bridge.

Random Sneeze Call (Q: If you call a random phone number and say ‘God Bless You’, what are chances that the person who answers just sneezed?)

It is probably 1 in 40,000. An average person sneezes 200 times a year.

Expanding Earth (Q: How long would it take for people to notice their weight gain if the mean radius of the world expanded by 1cm every second?)

The earth is not currently expanding. At the end of the first day, you would weigh 70.01kg from 70kg previously. The problem is that GPS won’t work very well. Surface gravity is proportional to radius. If the mass increased by 1.2%, surface gravity would have increased by only 0.4%. After 1 year, gravity would have been stronger by 5% and there would be failures with roads, bridges etc. Air pressure will drop. After 5 years, gravity will be 25% stronger. The expanding ground will cause buildings to give way. After 49 years, gravity would have tripled. Only the strongest humans can walk the Earth. Breathing will be difficult now. There would be earthquakes and volcanos. After 100 years, the gravity would be 6g. The heart doesn’t have strength to pump oxygen to our brains. Small insects and sea animals will be fine. Humans can still survive in domes. After 300 years, the moon will gradually break apart because it will spiral inwards.

Weightless Arrow (Q: Assuming a zero-gravity environment with an atmosphere identical to Earth’s, how long would it take the friction of air to stop an arrow fired from a bow? Would it eventually come to a standstill and hover in mid-air?)

After ten seconds, the arrow can travel 400m, but the speed would have been reduced from 85m/s to 25m/s. After 5 minutes, it would have travelled a mile and it would be at walking speed. After a few hours, it would be moving very slowly before completely stopping. Air can act like honey. The arrow will eventually stop.

Updating a Printed Wikipedia (Q: If you had a printed version of the whole of English Wikipedia, how many printers would you need in order to keep up with the changes made to the live versions?)

You would require 6 printers that are being operated consistently. Wiki receives close to 100 edits from users per minute. Therefore, 6 printers can churn out about 15 pages each per minute. You will need to print 300 cubic metres every month and spend $500,000 per month to do that. The ink cartridges would be extremely costly.

Facebook of the Dead (Q: When, if ever, will Facebook contain more profiles of dead people than of living ones?)

It is either in the 2060s or 2130s. Based on the age breakdown, there should be about 10 to 20million people who created FB profiles who passed on already. About 290,000 FB users in the US died. The death rate will double and in 7 years, it will double again. If FB fails to attract new users, the dead will outnumber the living in 2065.

Stirring Tea (Q: I was absently stirring a cup of hot tea, when I got to thinking…”Aren’t I actually adding kinetic energy into this cup?” If I stir it fast enough, can I boil by stirring?’

Answer is no. The idea makes sense but heating water requires a lot of energy. You need a power source of about 700watts to boil water. To boil a cup of water, you have to drop it from higher than the top of the atmosphere. No matter how hard you stir your tea, it’s not going to get any warmer.

Raindrop (Q: What if a rainstorm dropped all of its water in a single giant drop?)

If all the rain is contained in a drop, it would a sphere over a km in diameter. Once the water hits the surface, you will be crushed to death by the pressure. The water will be forced sideways, creating a jet that destroys everything in its path for over 10km.

SAT Guessing (Q: What if everyone who took the SAT guessed on every multiple-choice question? How many perfect scores would there be?)

None. There are 158 multiple choice questions in SAT, with five options each. The answer is 1 in quinquatrigintillion (1/2.7*10^110).



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