Book summary “What If?”: summary

Introduction

You often wonder about the strange, impossible things that could happen if the laws of physics were pushed to their absolute limits. You might ask what would happen if you hit a baseball pitched at ninety percent the speed of light or if you tried to drain the oceans with a giant straw. These bizarre queries usually remain idle daydreams because you lack the mathematical tools and scientific frameworks to solve them. You need a way to bridge the gap between absurd imagination and rigorous academic inquiry.

Randall Munroe tackles these bizarre scenarios in What If? Serious Scientific Answers to Absurd Hypothetical Questions. The author argues that you can explore ridiculous premises through genuine physics, chemistry, and mathematics. You learn to strip away the humor and apply real equations to impossible problems. This method reveals that even the most nonsensical ideas yield concrete, devastating, and scientifically precise consequences.

The book takes you on a vast intellectual journey from the fabric of the cosmos down to the quirks of human biology. You examine cosmic scales, geological forces, thermodynamic laws, and fluid mechanics through extreme scenarios. You also confront the perils of solar proximity, biological growth, and everyday engineering pushed past the breaking point. The journey answers questions you might never have thought to ask, such as:

• What would happen if a trillion laser pointers were focused on the Moon?
• How many lightning bolts would you need to toast a piece of bread?
• Could you build a jetpack out of machine guns?
• What speed do you need to drive over a speed bump to survive?

1. Cosmic Scales and Planetary Destruction

Extreme astronomical scenarios reveal how physical laws dictate the catastrophic fate of planetary bodies and space systems.

Soupiter and Black Holes

Amelia, age 5, asked what would happen if the Solar System were filled with soup out to Jupiter. You would instantly create a massive black hole.

The enormous mass of the soup would exceed the threshold for gravitational collapse. This creates a black hole whose event horizon reaches the orbit of Uranus.

The system contains staggering quantities:

• 2 * 10^39 liters of soup;
• 10^42 calories worth of tomato soup;
• 1/500th the mass of the Milky Way.

Cosmic Road Trips

Sam H-H asked how long it would take to drive a car to the edge of the universe if expansion stopped. You would travel to a destination 270,000,000,000,000,000,000,000 miles away.

Due to astronomical distances, a vehicle traveling at 65 miles per hour speed would require immense fuel reserves. You would spend 4.8 * 10^17 years travel time on the road while trying to overcome astronomical vehicle crash rates.

Proton Earth and Electron Moon

An Earth made entirely of protons and a Moon made entirely of electrons would collapse under its own mass-energy into a massive black hole rather than forming a stable atomic or orbital system.

Noah Williams proposed a scenario where the Earth is made entirely of protons and the Moon entirely of electrons. Electrons repel each other with a force far stronger than gravity, and 10^52 electrons used to build a moon would push outward with immense energy.

According to string theorist Dr. Cindy Keeler, pushing energy per particle past the Planck energy enters unknown theoretical physics. General relativity dictates that the total potential energy involved exceeds normal bounds. You find that cosmic rays exceed LHC energy by a 10^6 factor, causing the system to form a massive black hole governed by Reissner-Nordström equations instead of remaining a naked singularity.

2. Extreme Human and Biological Feats

Human physical limits and biological requirements make seemingly simple impossible challenges vastly more complex.

Helicopter Blade Grip

Corban Blanset asked what would happen if someone hung on a helicopter blade by their hands and it was turned on. As the blade accelerates, centrifugal forces rapidly multiply your apparent body weight into tons of force, causing structural failure or detachment. The first full turn takes 10 to 15 seconds. After 20 seconds, the blade reaches a speed of 1 revolution per second.

T. Rex Calorie Intake

T. Schmitz asked how many humans a T. rex released in New York City would need to consume daily. A Tyrannosaurus rex would require approximately 40,000 calories per day, equivalent to about half an adult human or eighty hamburgers. Because large dinosaurs likely had lower metabolic rates similar to Komodo dragons rather than warm-blooded mammals, their daily food requirement is moderate relative to their size. An 80-kg human contains 110,000 calories.

Pigeon Lifting Power

Nick Evans asked how many pigeons are required to lift a person and launch chair to the height of Australia's Q1 skyscraper. Lifting a human to the top of a skyscraper using pigeons is mathematically impossible due to the exponential growth of carrying capacity stages. Because a pigeon can only carry a fraction of its body weight, a multi-stage lifting apparatus would quickly require more mass than exists in the galaxy.

The physical constraints involve exact bird capacities:

• an average pigeon weighs more than the load it can lift;
• the carrying limit is 25 percent of its body weight;
• that equals 124 grams carried by average pigeon;
• the target destination is the 322 meters high Q1 skyscraper.

3. Geological Power and Earth Scenarios

Planetary-scale geological forces and deliberate terraforming maneuvers yield violent, highly localized disasters.

Standing on top of Old Faithful during an eruption would subject you to thousands of pounds of upward rocket-like thrust. The geyser ejects massive amounts of water and steam at high velocities, generating enough momentum to launch a human hundreds of feet into the air.

Catherine McGrath asked about the launch speed and injuries from standing on top of Old Faithful. The conditions involve extreme output:

• half a ton of water per second;
• 70 meters per second exit speed.

An asteroid hitting Earth at a slow speed would not cause a global mass extinction, but its impact mechanism would trigger massive soil liquefaction and catastrophic local tsunamis. A slow-moving impactor would not vaporize a crater via extreme kinetic energy. Instead, because many asteroids are loose piles of rubble resembling giant sandcastles, the impactor would undergo supersonic soil liquefaction and spread outward as an omnidirectional landslide.

If an impact occurred near the original Chicxulub site in Mexico, a giant landslide and ensuing tsunami would bury the city of Mérida and destroy Gulf coast cities. The impact involves distinct physical parameters:

• 3 mph as a hypothetical slow impact speed;
• 66 million years ago as the time of the actual Chicxulub impact;
• 30 or 40 miles as the distance the resulting landslide would spread from the impact point.

Speeding up the Earth's rotation so a day lasts only one second would instantly tear the crust apart via centrifugal force, launching a relativistic debris disk across the Solar System. At a one-second rotation period, equatorial speeds would exceed 10 percent of the speed of light, overcoming gravity and flinging the Earth's crust outward into expanding building-sized fragments that act like an expanding buzz saw.

If this acceleration happened when the Moon crosses the Earth's equator every two weeks, the impact would turn the Moon into a high-energy comet rocketing out of the Solar System. The timeline of destruction unfolds rapidly:

• 40 milliseconds until the International Space Station is struck by the expanding atmosphere;
• 1.5 seconds until the debris reaches geostationary satellites;
• 10 seconds for the disk to pass the Moon.

4. Physics in Everyday Engineering

Engineering structures and everyday objects face strict physical boundaries when scaled to absurd proportions.

Swing Set Limitations

Playground swings of 10 to 15 feet are already optimally sized. Taller swings become much less efficient to pump and suffer from severe air resistance. Pumping a swing relies on shifting body weight to twist the swing against gravity relative to the crossbar.

On taller swings, rotation angles per pump shrink drastically. You need hundreds of pumps and extreme time to reach decent heights, while air drag at higher speeds negates any gains.

Visitors at Moses Mabhida Stadium in Durban, South Africa, can ride a 200-foot rope swing hanging from scaffolding, but momentum is quickly lost due to air drag. A standard swing sits at 8 feet and requires 45 pumps for a 45-degree arc. A 30-foot swing requires 640 pumps to reach 45 degrees.

Airliner Ground Catapults

Launching commercial airliners with ground catapults to save fuel is impractical due to low takeoff fuel burn, low-altitude air drag, and the extreme runway lengths required. While airliners burn extra fuel during takeoff, it represents a minor fraction of total flight consumption.

Accelerating a plane up to near-cruising speed on the ground would require runways miles long. This eco-climb would cut straight through cities and would quickly be slowed down by dense surface air drag.

A normal airliner experiences a forward acceleration of 0.2g to 0.3g during takeoff. You would need a minimum runway length of 4.5 miles to accelerate an airliner to near cruising speed. Extending a Washington, DC, airport runway to handle this launch would cross the National Mall between the Lincoln Memorial and the Washington Monument, which weighs 80,000 tons in its aboveground portion.

Vacuum Tube Smartphones

Building a modern smartphone out of old vacuum tubes would result in a massive device that generates enough heat to melt through the Earth's crust. Transistors and vacuum tubes perform the same logical switching functions, but vacuum tubes are vastly larger and less energy-efficient.

An iPhone 12 contains 11.8 billion transistors in a tiny case. Replacing them with vacuum tubes packed at early computer densities would result in a phone the size of multiple city blocks and power consumption exceeding ten billion watts.

ENIAC, the first programmable computer built with vacuum tubes, was taller than a person and 30 meters long. The heat produced by an equivalent vacuum-tube smartphone would reach a melting temperature of 1,780 degrees Celsius according to the Stefan-Boltzmann law and the thermal properties of the 7AK7 vacuum tube.

5. Optics, Laws, and Thermodynamic Limits

Thermodynamic laws and astronomical realities strictly forbid violations like magnifying moonlight or free energy.

Earth-Sized Eyes

An Earth-sized eyeball would possess immense resolving power limited by diffraction and quantum fluctuations in space. Due to its massive pupil and lens, an Earth-sized eye would have a theoretical resolution half a billion times sharper than a normal eye. You could read text on the Moon or view exoplanets.

A normal human eye can reliably see as far as 3 million light-years away. Just as atmospheric turbulence blurs conventional telescopes, a 2015 study by astronomer Eric Steinbring suggests quantum fluctuations in space might blur the vision of such a massive eye.

Moonlight Fire Impossibility

Rogier asked if you can use a magnifying glass and moonlight to light a fire. It is physically impossible to start a fire using moonlight and a magnifying glass, no matter how large the lens. Lenses cannot make something hotter than the light source itself without violating the laws of thermodynamics.

Lenses do not consume energy to operate. Making heat flow to a spot hotter than the light source would create heat flow from a colder to a hotter place without external energy, violating the second law of thermodynamics. Lenses concentrate light into an area rather than a point.

The conservation of étendue prevents increasing the brightness of a light source beyond its original surface temperature.

• The Sun is 400,000 times brighter than the Moon.
• A conceptually required magnifying glass would span 400,000 square inches.
• The Sun reaches a temperature of 5,000°C.
• The Moon's sunlit surface reaches 100°C.
• There are 10^26 watts of available solar energy.

Reading Every Law

Keith Yearman asked how many pages of governing documents apply to a person. A citizen attempting to read all federal, state, and local governing documents, regulations, treaties, and case law applicable to them would face millions of pages requiring decades of reading time.

Constitutions, statutes, and local laws alone span vast quantities.

• The US Constitution contains 26 pages.
• Federal laws fill 82,000 pages.
• The Massachusetts constitution contains 122 pages.
• Massachusetts state laws fill 63,000 pages.
• Town laws add 450 pages.
• Federal regulations consume 295,000 pages.
• The 2020 US State Department list of active treaties contains 570 pages covering 7,700 total treaties averaging 33 pages each.
• Federal case law contributes 12.3 million pages.

Reading all applicable legal sources requires 45 years of total reading time. According to Harvard Law Library director Jonathan Zittrain, complex and unintuitive laws empower the state and individuals who can afford legal representation by enabling selective and potentially discriminatory enforcement.

6. Fluid Mechanics and High-Velocity Hazards

Fluids forced through extreme constraints generate velocities and pressures capable of planetary destruction.

Mariana Trench Tubes

6. Fluid Mechanics and High-Velocity Hazards

Fluids forced through extreme constraints generate velocities and pressures capable of planetary destruction.

Mariana Trench Tubes

You could build a 20-meter-wide glass tube extending down to Challenger Deep. The high pressure inside the tube is caused solely by being far below sea level. Air pressure doubles every six kilometers, making it nearly four times higher at a depth of ten kilometers.

Oceanographer Henry Stommel suggested in 1956 that connecting the surface and deep ocean via a tube could create a continuous water flow. If opened completely, the water column would rush up at Mach 1.3. A 2003 experiment with a PVC tube over the Mariana Trench confirmed that temperature and salinity imbalances could cause a slow exchange of water.

Niagara Falls Straw Flow

David Gwizdala asked what would happen if someone tried to funnel Niagara Falls through a straw. The Niagara River provides an average supply of 292,000 cubic feet per second, with a minimum flow rate mandated by law. Dividing this massive flow rate by the cross-sectional area of a typical 7mm straw yields a water speed equal to one-quarter of the speed of light. At these energies, water molecules undergo nuclear reactions and turn into plasma.

The physical system faces severe limits:

• cavitation;
• choked flow.

The recoil destroys the apparatus, and the total power output matches a small star, boiling away Earth's oceans.

Suction Aquariums

Inverse aquariums rely on atmospheric pressure pushing down on the surrounding ocean to support the elevated water column inside an inverted sealed container. However, air pressure cannot support water columns higher than roughly 10 meters, at which point the water boils into a vapor.

Marine biologists observed blacktip sharks fleeing shallow coastal waters ahead of a 2001 Florida tropical cyclone by sensing falling barometric pressure. This limit prevents you from maintaining tall structures using suction alone, compared with the 35-foot depth of the Open Sea tank at the Monterey Bay Aquarium.

7. Celestial Remnants and Solar Proximity

Approaching the Sun or interacting with dead stellar remnants exposes objects to lethal radiation and gravity.

Nanosecond on the Sun

AJ from Kansas City wondered about being instantly transported to the Sun for a nanosecond while shoveling snow at age 8. Visiting the surface of the Sun for a single nanosecond would not even warm you up, causing only a brief flash in your eye. A nanosecond is the time it takes light to travel about a foot. Only a microjoule of energy would enter the eye, and neural processing delays mean your brain would take 30,000,000 nanoseconds just to notice the flash.

Walking on White Dwarfs

Jabari Garland asked when the Sun will be cool enough to touch after becoming a white dwarf. After the Sun becomes a white dwarf and cools to room temperature over billions of years, touching it with your bare hand remains impossible. The stellar remnant will retain half the Sun's original mass packed into an Earth-sized volume.

This extreme transformation involves several key milestones:

• 5 billion years until hydrogen depletion;
• 20 billion years for the Sun to cool to room temperature;
• 10 times Earth's gravity at the former surface location;
• 1 percent of the speed of light impact velocity.

These forces generate immense gravity. The environment would disintegrate any falling object or crush structures larger than a few inches, making direct touching completely unfeasible.

Earth-Moon Fire Poles

A 5-year-old named Ramon Schönborn asked how long it would take to slide down a firemen's pole from the Moon. Sliding down a hypothetical firemen's pole from the Moon to Earth is physically impossible and practically grueling. A real metal pole would snap under competing gravity and centrifugal forces. It would break off due to the Earth's faster rotation, which reaches 1,000 miles per hour at the equator.

If a magical pole existed, you would face daunting navigation hurdles:

• spending years climbing up near the Moon until reaching the L1 Lagrange point;
• pushing and gliding toward the planet;
• battling extreme supersonic winds and frictional heating upon atmospheric entry.

You would encounter an escape velocity of 11 km/s at the top of the atmosphere. The Moon's orbit moves at a ground speed of 390 to 450 m/s, contrasting with a 35 mph average ground speed relative to the center.

8. Absurd Biological Exponential Growth

Unchecked exponential biological growth scenarios quickly consume planetary mass and trigger relativistic collapse.

Griffin calculated a hypothetical scenario where one out of every four people owns a dog that reproduces yearly. Beginning with 2 billion initial dogs among 8 billion humans on Earth, the population would undergo explosive exponential growth.

Dog Overload Spheres

You would see 10 billion puppies born in the first year. The population would reach 6.022 x 10 to the 23rd power, or 1 mol, at 65 years.

• biomass would rival all other animal species combined;
• dogs would displace the atmosphere entirely;
• mass would exceed the Moon and Earth.

Reaching 10 to the 30th power dog population at 110 years would cause relativistic gravitational collapse into a black hole.

Global Snowfall Accumulation

Jed Scott's 7-year-old son Owen asked how many snowflakes it would take to cover the world in six feet of snow. All the clouds on Earth combined hold 13 trillion tons of water. Settling, compaction, and ocean distribution mean that a global six-foot snowfall requires 10 to the 23rd power snowflakes. Weather service measuring guidelines use a snow-measuring board. You would need immense quantities of water to match historical records like 28.6 inches of snow measured at the Baltimore airport, spread across 70 million kids in the United States.

Lemon Drop Rains

Shuo Peskoe-Yang asked what would happen if all raindrops were lemon drops and gumdrops. Replacing global rainwater with an equivalent mass of candy would bury cities into Candyland Pompeiis. A lemon drop falls with a terminal velocity of 10 meters per second. Flat roofs hold only 20 to 60 pounds per square foot of weight capacity before structural collapse occurs.

The massive influx of sugar oxidized by bacteria or fire would skyrocket atmospheric carbon dioxide levels. Current CO2 levels sit at 0.042 percent, but sugar decay would raise this to 5 to 10 percent. This change triggers a runaway greenhouse effect, boiling away the oceans and destroying all global agriculture.

10 Key Ideas

1. Filling the Solar System with soup out to Jupiter would instantly create a massive black hole whose event horizon reaches the orbit of Uranus.
2. Hanging on an accelerating helicopter blade by your hands rapidly multiplies your apparent body weight into tons of force, causing structural failure or detachment.
3. A Tyrannosaurus rex released in New York City would require approximately 40,000 calories per day, equivalent to about half an adult human or eighty hamburgers.
4. Speeding up the Earth's rotation so a day lasts only one second would instantly tear the crust apart via centrifugal force, launching a relativistic debris disk across the Solar System.
5. Replacing a modern smartphone's components with early computer-density vacuum tubes would result in a device the size of multiple city blocks that generates enough heat to melt through the Earth's crust.
6. It is physically impossible to start a fire using moonlight and a magnifying glass because lenses cannot make something hotter than the light source itself without violating the laws of thermodynamics.
7. Funneling the Niagara River through a 7mm straw would force water to travel at one-quarter of the speed of light, causing water molecules to undergo nuclear reactions and turn into plasma that destroys the apparatus.
8. Sliding down a hypothetical firemen's pole from the Moon to Earth is physically impossible because a real metal pole would snap under competing gravity, centrifugal forces, and the Earth's faster rotation.
9. An unchecked exponential population growth of dogs starting with 2 billion initial pets would reach a mass exceeding the Earth and Moon, eventually causing relativistic gravitational collapse into a black hole at 110 years.
10. Replacing global rainwater with lemon drops and gumdrops would bury cities into Candyland Pompeiis while sugar decay triggers a runaway greenhouse effect that boils away the oceans.