Chapitre 1
When Curiosity Meets Physics: A Journey Through Impossible Questions
What if the Earth suddenly stopped spinning? What if you tried to hit a baseball traveling at 90% the speed of light? These are the kinds of questions that keep physicists up at night-or at least one physicist in particular: Randall Munroe. In his cult classic book "What If?", the former NASA roboticist and creator of the beloved webcomic xkcd takes readers on an intellectual adventure through absurd hypothetical scenarios, analyzing them with rigorous scientific principles and a healthy dose of humor. Since its publication, the book has become a staple on the bookshelves of science enthusiasts, earning praise from figures like astronaut Chris Hadfield and physicist Brian Greene. Its unique blend of accessibility and scientific depth has made it required reading in many physics classrooms, where it serves as a gateway to complex concepts through the power of "what if" thinking-the same approach that led Einstein to his theory of relativity. As Munroe himself puts it, "I still don't know whether there are more hard or soft things in the world, but I've learned a lot of other stuff along the way."
Chapitre 2
Apocalypse by Physics: Earth's Rotation and Relativistic Baseball
If Earth suddenly stopped spinning while the atmosphere maintained its velocity, the consequences would be catastrophic. At the equator, where Earth's surface moves at 470 meters per second, the resulting thousand-mile-per-hour winds would flatten nearly all human structures within minutes. Even places like Boston would experience winds twice as strong as the most powerful tornadoes. Only polar research stations and underground facilities might offer shelter, though even hardened bunkers would face threats from debris impacts. The survivors-those in deep basements, subway tunnels, and polar stations-would emerge to find a transformed world with extreme heat, oceanic upheaval, and a new day-night cycle lasting a full year.
The atmosphere's kinetic energy would convert to heat, creating scorching temperatures and global thunderstorms. Oceans would lose their surface definition as wind churned cold water from the depths, causing both marine life blooms from nutrient upwelling and massive die-offs from low-oxygen water. With Earth no longer rotating, each hemisphere would experience six months of continuous daylight followed by six months of darkness, creating extreme temperature differentials and massive storms. Eventually, the Moon's gravity would gradually restart Earth's rotation through tidal forces.
Similarly catastrophic would be a baseball traveling at 90% the speed of light. At 600 million miles per hour, the ball would perceive air molecules as stationary obstacles. Rather than flowing around the ball, air atoms would fuse with atoms in the ball's surface, releasing gamma rays and particles that create an expanding bubble of incandescent plasma. After about 70 nanoseconds, the partially disintegrated ball-now a bullet-shaped cloud of plasma-would reach home plate accompanied by a shell of x-rays, instantly disintegrating the batter, catcher, and plate. The explosion would level everything within a mile of the park and engulf the surrounding city in a firestorm. According to MLB Rule 6.08(b), the batter would technically be considered "hit by pitch" and eligible to advance to first base-if they still existed.
Chapitre 3
Radiation Realities: Swimming in Nuclear Waters
Could you swim in a spent nuclear fuel pool? Surprisingly, yes-at least for a while. Assuming you're a reasonably good swimmer, you could probably tread water for 10 to 40 hours before drowning from fatigue, just like in any regular pool. The water temperature (25-35C) wouldn't be problematic-warmer than most pools but cooler than a hot tub. These pools are typically about 40 feet deep and contain anywhere from 300,000 to 600,000 gallons of heavily purified water to ensure optimal visibility for handling nuclear materials.
What about radiation? Every 7 centimeters of water cuts radiation in half, creating an exponential decrease in exposure as depth increases. Swimming near the surface would actually give you less radiation than walking around outside, as water shields background radiation effectively. At the surface, you'd receive only about 0.055 mSv/hour, less than what you'd get from a chest X-ray. The real danger would come from swimming to the bottom and touching fresh fuel canisters, which would likely be fatal. These canisters can emit up to 1,000 rem per hour at close range - enough to cause severe radiation sickness or death within hours.
Spent fuel pools are regularly serviced by human divers, though they must exercise extreme caution and follow strict protocols. Divers wear specialized radiation-resistant suits with dedicated air supplies and multiple dosimeters to monitor exposure levels. In 2010, a diver at Switzerland's Leibstadt reactor retrieved highly radioactive tubing from the pool floor, receiving a dangerous dose to his hand that measured over 500 millisieverts. Had he tucked it closer to his body, it could have killed him. This incident led to enhanced safety protocols and improved remote handling equipment.
The safety of these pools is taken very seriously, with multiple layers of security. When asked about swimming in their research reactor pool, one worker noted, "In our reactor? You'd die pretty quickly, before reaching the water, from gunshot wounds." This stark response highlights the intense security surrounding nuclear facilities, perhaps more immediately dangerous than the radiation itself. Nuclear facilities typically employ armed guards, motion sensors, reinforced barriers, and various other security measures that make unauthorized access virtually impossible.
The water itself requires careful maintenance, with sophisticated filtration systems removing both particulates and dissolved impurities. The blue glow often associated with these pools, known as Cherenkov radiation, occurs when particles travel faster than light can through water, creating a distinctive blue flash. While this effect is harmless, it serves as a visible reminder of the ongoing nuclear processes beneath the surface.
So while you'd probably be fine swimming at the surface of a spent fuel pool, the combination of security measures and the risk of accidentally touching highly radioactive materials makes this a thought experiment best left untested. The real-world consequences of attempting such a swim would likely involve arrest or worse, long before radiation exposure became a concern.
Chapitre 4
Time Travel Through Manhattan: A Journey Across Epochs
What would happen if you time-traveled from modern Times Square to various points in the past? Manhattan 1,000 years ago would have been inhabited by Native American tribes predating the Lenape people Europeans encountered in the 1600s. The landscape would have resembled the old-growth forests of the Northeast, but with notable differences: more large predators like wolves and mountain lions, abundant chestnut trees (later decimated by blight in the 20th century), and passenger pigeons. One surprising absence would be earthworms, which hadn't yet returned to New England after the ice age.
Going back 10,000 years, you'd find Earth just emerging from a deep cold period. The great ice sheets that once covered New England had retreated northward, leaving a landscape scoured to bedrock with life slowly returning. Melting chunks of ice left kettlehole ponds like Oakland Lake in Queens, while glacial erratics (transported boulders) can still be found in Central Park today. Rivers of meltwater created eskers-sand and gravel ridges forming unusual landforms including vertical U-shaped riverbeds.
At 100,000 years back, the world might look surprisingly similar to our own. During the Sangamon interglacial period with its stable, warm climate, the coastal geography would be completely different-without Staten Island, Long Island, or other islands pushed up by later ice advances. The forests would contain familiar animals like birds, squirrels, deer, and bears, but also fearsome predators like dire wolves, short-faced bears, and saber-toothed cats.
A million years ago, before the most recent glaciations, the climate was relatively warm and stable. The fearsome predators of later eras were joined by Chasmaporthetes, a long-limbed hyena resembling a wolf that had crossed from Asia into North America during periods of low sea levels.
At a billion years back, the continental plates were joined in the supercontinent Rodinia. In this ancient world without plants or animals, oceans contained only simple single-cellular life. Blue-green algae (cyanobacteria)-the first photosynthesizers-pumped oxygen into the atmosphere, causing the "oxygen catastrophe" that killed most other life forms.
Looking forward a million years, humans will likely be gone-either spreading to the stars, succumbing to disease and famine, or falling victim to some technological disaster. Climate change will delay the next glaciation, but eventually the ice will return. Our most lasting legacy will likely be the layer of plastic we've deposited across the planet. In the far future, the Sun will gradually brighten until, in a billion years, Earth's feedback loops will fail, oceans will boil away, and our planet will become a second Venus. Eventually, after several billion more years, the expanding Sun will consume Earth, blasting the molecules of Times Square outward as dust clouds that might someday form new stars and planets.
Chapitre 5
Soul Mates and Laser Pointers: Probability and Power
What a nightmare it would be if everyone had only one randomly assigned perfect soul mate. With 7 billion living humans out of 100 billion who've ever lived, 90% of soul mates would be long dead. Even limiting potential matches to people alive now and close to your age, you'd have about 500 million possible soul mates. If you make eye contact with dozens of strangers daily, you'd only find your soul mate in one lifetime out of 10,000.
Society might restructure around maximizing eye contact through conveyor belts or webcam services like "SoulMateRoulette," but most people would still never find true love. Many would fake finding their soul mate just to belong, hiding their relationship problems behind happy facades. The wealthy might sponsor charitable computer projects to get more people online, and jobs with high stranger interaction would become prized.
On a different note, what would happen if everyone on Earth aimed laser pointers at the Moon simultaneously? Regular laser pointers would not change the Moon's color noticeably. Even with 1-watt green lasers (at a total cost of $2 trillion), the effect would be imperceptible compared to sunlight's 10 petawatts. Nightsun searchlights would barely be visible on the dark side. IMAX projector arrays and even Luxor Hotel spotlights would make minimal difference.
Only military megawatt lasers could match sunlight's brightness, but would require double the world's electricity consumption. Theoretical 500-terawatt lasers would turn the atmosphere to plasma, make moonlight bright enough to boil Earth's oceans, and eventually push the Moon out of orbit through laser ablation, turning it into a dwarf planet on a potentially Earth-crossing orbit-clearly not worth the effort just to make the Moon change color.
Chapitre 6
The Periodic Table of Dangers: When Elements Attack
Element collectors try to gather samples of as many elements as possible, but building a physical periodic table with cube-shaped bricks of pure elements would be increasingly dangerous with each row. The first row (hydrogen and helium) would simply float away. The second row contains reactive lithium, toxic beryllium, and extremely corrosive fluorine gas that would cause spontaneous fires with almost any substance it contacts. According to organic chemist Derek Lowe, fluorine would form dangerous hydrofluoric acid upon contact with moisture, while leaving only neon untouched and maintaining "an armed truce" with chlorine.
As you move down the table, the dangers multiply. Row three introduces phosphorus that spontaneously ignites in air, while row four adds arsenic and other toxic elements. By row five, radioactive technetium appears. Row six contains several radioactive elements including astatine, which is so unstable it would vaporize itself and demolish the building with a superheated gas explosion.
The seventh row would trigger an actual nuclear explosion, creating fallout thousands of times worse than Chernobyl that would contaminate regions for centuries. When it comes to chemical elements, collecting them all is definitely not recommended-unless you have a death wish and no regard for the surrounding population.
This progression of danger illustrates a fascinating aspect of chemistry: as we move through the periodic table, we encounter increasingly unstable arrangements of protons and neutrons, with the heaviest elements existing only momentarily in laboratory conditions before decaying into lighter elements. The periodic table isn't just an academic organization system-it's a map of increasing instability and potential destruction.
Chapitre 7
Collective Human Experiments: Jumping Together and Hair Dryer Physics
What if everyone on Earth gathered in one place and jumped simultaneously? This popular question has been examined before, but let's look deeper. The entire human population would occupy an area roughly the size of Rhode Island. At noon, everyone jumps about half a meter high. The Earth, outweighing us by a factor of ten trillion, would barely notice-moving less than an atom's width. When everyone lands, the energy dissipates through the continental crust with minimal effect beyond a drawn-out roaring sound.
The real problems begin afterward. Seven billion people find themselves stranded in Rhode Island with no functioning cell networks as the unprecedented load collapses communications. Outside this human-packed state, abandoned machinery worldwide grinds to a halt. The local airport, even operating at 500% capacity, couldn't make a dent in evacuating the crowd for years. As people spread into surrounding areas, chaos ensues-language barriers prevent communication, nobody knows the area, violence erupts, and resources quickly deplete. Within weeks, Rhode Island becomes "a graveyard of billions." The survivors spread across the world to build a new civilization on the ruins of the old.
On a different scale of thought experiments, what happens when you put a powered hair dryer in an airtight box? At increasing power levels, the consequences become increasingly catastrophic. At 187,500 watts, the box glows dim red at 600C, hot enough to melt aluminum inside or lead outside, and certainly enough to set a wooden floor ablaze. At 18.7 megawatts, the box creates conditions similar to a space shuttle launch pad. At 1.875 terawatts, it's like a house-sized TNT explosion every second, creating firestorms across the landscape.
When the hair dryer reaches 11 petawatts, it becomes a weapon of planetary destruction. The box, placed in Great Bear Lake, turns the lake's surface to steam. The heated plasma accelerates the box upward through its own bubble of expanding plasma with little resistance. It exits the atmosphere and continues away, slowly fading from second sun to dim star, leaving much of the Northwest Territories burning.
Chapitre 8
When the Lights Go Out: The Last Human Illumination
If humans suddenly vanished, how long would artificial lights continue to shine? Most power grids would fail within hours due to cascade failures as coal and oil plants shut down. Various power sources would persist for different durations: diesel generators (days to months), geothermal plants (a few years until maintenance failures), wind turbines (some potentially lasting decades without servicing), and hydroelectric dams (several years on autopilot). Nuclear reactors would quickly go into automatic shutdown without human supervision, triggering a SCRAM when external power is lost.
Our spacecraft might be the longest-lasting human artifacts, with some orbits lasting millions of years, though their electrical power won't last as long. Mars rovers will eventually be buried by dust, satellites will fall back to Earth as orbits decay, and even GPS satellites in distant orbits will eventually be disrupted by the Moon and Sun. While some spacecraft like Curiosity have radioactive power sources that could last over a century, they don't keep lights continuously illuminated.
Solar-powered emergency call boxes along remote roads could be among the longest-lasting light sources. Built to be durable with minimal servicing requirements, solar panels in dry locations could continue providing power for a century if kept free of dust by occasional breezes or rain.
The final contender for last human light comes from an unexpected source: radioactive waste. When radioactive particles travel through materials like water or glass, they emit blue Cherenkov radiation-the distinctive glow seen in nuclear reactor cores. Radioactive waste products like cesium-137 (half-life of thirty years) are mixed with glass blocks that glow blue in the dark. Even centuries later, these blocks will still emit about 1% of their original radioactivity, maintaining the same blue color but at diminished brightness. Deep in concrete vaults, the light from our most toxic waste may be humanity's final illumination-a poetic end to our light-making legacy.
Chapitre 9
Computational Comparisons: Humans vs. Machines
How does human computational power compare to modern computers? Comparing humans and computers is like comparing apples and oranges (though apples are better). While humans still excel at certain tasks like interpreting visual scenes and making contextual inferences, computers vastly outperform humans at pure calculation tasks.
According to computer scientist Hans Moravec, a human performing computer chip benchmark calculations by hand can execute about one instruction every 90 seconds. By this measure, a midrange mobile phone calculates 70 times faster than the entire world population, while a high-end desktop PC outperforms humanity by 1,500 times. A single desktop computer first surpassed humanity's combined processing power in 1994, when Intel's Pentium chips achieved 70-80 MIPS compared to humanity's 65 MIPS.
When measuring by complexity rather than calculation speed, the human brain remains more sophisticated than any supercomputer. Based on neural simulation requirements, a single human brain has the complexity equivalent to about 10^15 transistors. By this measure, all the world's logic circuits didn't match a single brain until 1988, and computers won't surpass all human brains until approximately 2036.
A compromise between these extremes-the geometric mean of both measurements-places human brains at about 30,000 MIPS, comparable to a modern personal computer. By this balanced measure, Earth's digital complexity overtook its human neurological complexity around 2004.
Interestingly, the world's ant population has a combined neural complexity similar to that of all human brains. This suggests that while we often think of ourselves as the planet's dominant intelligence, we're not alone in our collective cognitive capacity. Ants, with their sophisticated colony behaviors and problem-solving abilities, represent a different but comparable form of distributed intelligence-one that has been successful for millions of years longer than human civilization has existed.
Chapitre 10
Space, Speed, and Steak: Physics at the Extremes
In space, the fundamental challenge isn't about height-it's about speed. Space is only about 100 kilometers away (closer than the sea for many inland cities), but staying there requires tremendous velocity. Getting to space is relatively easy-a rocket the size of a telephone pole could get someone there, and the X-15 aircraft reached space simply by going fast and steering upward.
The real challenge is staying in orbit. The Space Station experiences about 90% of Earth's surface gravity, and to avoid falling back into the atmosphere, spacecraft must travel sideways at about 8 kilometers per second. This orbital velocity, not the altitude, consumes the vast majority of a rocket's energy. Orbital speed is blisteringly fast-the ISS circles the entire Earth in just 90 minutes, moving so quickly that it could cross a football field before a rifle bullet travels 10 yards.
Speaking of speed, how high would you need to drop a steak for it to cook when hitting the ground? Despite what intuition might suggest, a steak dropped from any height would likely remain raw inside. Objects returning from space get hot because compressing air in front of them generates heat-a noticeable effect above Mach 2. When Felix Baumgartner jumped from 39 kilometers, he reached Mach 1 at around 30 kilometers, but the extremely cold air (around -40 degrees) prevented significant heating.
Simulations show that a falling steak would accelerate until reaching 30-50 kilometers altitude, then begin slowing as air thickens. Regardless of starting height, it would take 6-7 minutes to fall from 25 kilometers, during which the steak would be subjected to hurricane-force subzero winds-effectively freezing it even if it had been briefly heated earlier in the fall.
Even from low Earth orbit (250km), the steak reaches Mach 6 and might get a pleasant sear but remains raw inside. From higher altitudes, the extreme heat causes the steak's surface to char and ablate away repeatedly. No matter the height, you'll end up with a steak that's charred outside and raw inside-"Pittsburgh Rare" style-and you'd better be prepared to retrieve it quickly after impact.
Chapitre 11
Final Thoughts: The Power of Asking "What If?"
Throughout this journey through Randall Munroe's fascinating thought experiments, we've seen how seemingly absurd questions can lead to profound insights about physics, biology, astronomy, and human nature. From the catastrophic consequences of Earth stopping its rotation to the surprising longevity of emergency call boxes in a post-human world, these explorations reveal the hidden complexities of our universe.
What makes these questions valuable isn't just their entertainment value-though they certainly provide that-but how they force us to apply scientific principles to unusual scenarios, revealing the underlying rules that govern our existence. When we ask what would happen if everyone jumped at once, we're really exploring the relative masses of humans and Earth, the physics of energy transfer, and the fragility of human social systems.
The power of "what if" thinking lies in its ability to break us out of conventional patterns. By examining edge cases and extreme scenarios, we gain a deeper understanding of how things work under normal conditions. This approach has driven scientific discovery throughout history, from Galileo's thought experiments about falling objects to Einstein's imagined rides on light beams.
Munroe's work reminds us that curiosity-even when directed at seemingly ridiculous questions-is a powerful tool for understanding our world. So the next time you find yourself wondering about some impossible scenario, don't dismiss it. Instead, follow the thread of logic and see where it leads. You might just discover something profound about how the universe works, or at the very least, have a good story to tell about why we shouldn't try to build a bridge to New York out of Lego bricks.
