A Summary and Critique of
Randall Munroe’s
What If?
Serious Scientific Answers to Absurd Hypothetical Questions

Randall Munroe became famous thanks to his website, xkcd. The idea of xkcd came from Munroe’s several days of teaching experience (the weekend course that he volunteered to teach was part of an MIT weekend program that relied on volunteers with no prior teaching experience to teach the classes).

Munroe received his degree in physics from Christopher Newport University in Virginia. During the final year of his study at the University, he wrote a thesis on robotics (as part of his study program) at NASA’s Langley Research Center, which was located nearby. During his last semester of study, his work on this thesis transformed into a full-time contract position with NASA that lasted for a few months. He continued working for NASA as a contractor for the summer following his graduation in 2006. His contract expired by the end of that summer, and NASA did not renew it. Then, Munroe began writing xkcd full-time.

On his website, Munroe uses cartoons, science, math and logical reasoning to answer unusual and sometimes weird questions submitted by his online followers. A couple of years ago, Munroe also started his “What if?” blog, providing “serious scientific answers” to “absurd hypothetical questions” asked by his readers. Munroe’s readers have been submitting to him “What If?” questions, which require more-or-less scientific answers, since 2012. Every week, Munroe selects the most interesting or the weirdest questions and posts his answers, accompanied by mathematical calculations and entertaining cartoons.

Many of these questions and Munroe’s answers have been included in his book What If?: Serious Scientific Answers to Absurd Hypothetical Questions. Munroe’s book clearly conveys his love for math and science. It educates, entertains, and helps readers learn more about science.

Overall, Munroe’s book What If?: Serious Scientific Answers to Absurd Hypothetical Questions is a compilation of questions and answers from Munroe’s website. Some questions answered in Munroe’s book are bizarre and do not have any helpful or thought-provoking information. However, some questions offer an opportunity to apply scientific concepts to the real world, to practice critical thinking and to encourage intellectual curiosity.

For example, what would happen if you attempted to build a physical model of the periodic table of the elements, and each element in the model was presented in a form of an actual brick of the substance? To answer this question, Munroe imagines how this model would be constructed row by row. He points out that the first two rows could be built without any serious problems. The third row would catch fire because of chemical reactions, and the fourth row would produce toxic gases that could kill you. The fifth row would emit a lethal dose of radiation, and the sixth row would explode violently. Munroe does not recommend building the seventh row at all.

The idea behind this question and its answer is rather simplistic: take chemical elements located next to each other in the periodic table of the elements and figure out how they would interact with each other. Nevertheless, it is a brief and entertaining high-school chemistry refresher.

Another question brings a highly hypothetical situation: what would happen if the Earth stopped spinning? Munroe replies that the air would keep moving, creating supersonic winds pretty much everywhere on the planet, except for its polar regions. In addition, half of the planet would freeze, and the other side of the planet would get overheated. Cold water would rise from the ocean depths, followed by tsunamis and global cooling. The Moon would stop moving away from the Earth, and its gravitational pull on the tidal bulges would make the Earth spin again.

Munroe does not discuss the fact that real winds are part of atmospheric circulation and they do affect the Earth’s rotation. I discuss the connection between a planet’s rotation and its winds in Part III of my book where I tell my readers how winds can really change the spin of our planet. I also discuss how earthquakes can affect rotation of the Earth.

Another question in Munroe’s book sounds rather unexpected: what would happen if you tried to hit a baseball that was moving at 90 percent of the speed of light? Munroe tells us that answering this question requires knowledge of certain nuclear tests, which implies that throwing such a ball is a rather dangerous endeavor. Munroe simply informs the reader that a baseball moving at the speed of light would produce an x-ray shockwave expanding outwards from a fireball of plasma. He points out that the fusion triggered at the front of the moving ball would destroy everything within about a mile of the park where the ball had been thrown.

The science behind Munroe’s answer to this question is too difficult for regular readers to understand. Munroe provides the readers with his way of getting an answer, but he does not help his readers grasp the scientific ideas supporting the answer. Which is why the answer might give readers a “pseudo-understanding” of science.

What would happen if you tried to fly an electrically powered airplane on Venus? Munroe answers that the airplane could fly in the Venusian atmosphere rather well, but it would be on fire during the flight. While answering this question, Munroe helps his readers look at the planet Venus from an interesting perspective and explore the challenges of surviving in the Venusian atmosphere, known for its extremely high atmospheric pressure, its high temperature (sufficient to melt lead), and its hostile chemical composition (carbon-dioxide rich air complemented with acid rains).

Munroe also uses math to figure out how heavy a mole of moles would be, if we gathered all the moles (small furry creatures) together somewhere in outer space, away from the Earth’s gravity. To remind our readers, a mole (not the animal) is a unit of measure in chemistry. That is, one mole equals Avogadro number (6.022 multiplied by 1023) of units of a substance. Munroe finds that an Avogadro number of moles would be sufficient to make a planet half the mass of the Moon.

I find it unfortunate that, while answering this question, Munroe does not go into details about one interesting aspect. Namely, why exactly this “mole planet” would have a spherical shape and why the pressure would increase toward its center (this is because of its self-gravity), and how “small” the “mole planet” would have to be in order to lose its spherical shape. I further discuss how gravity affects the shape of a planet, as well as the height of its mountains and volcanoes, in Part II of my book.

Readers can learn about practical applications of Newton’s laws of motion while following Munroe, who tries to figure out how to build a jetpack equipped with downward-firing machine guns. Munroe calculates the thrust-to-weight ratio of an AK-47 in order to find out if this machine gun could be used for a jetpack. He determines that 300 AK-47s could make a jetpack that would lift a human.

Munroe uses a very creative approach when he discusses the effects of DNA-damaging mushrooms, radiation and chemotherapy in order to answer a very strange question: what would happen to a person whose entire DNA has suddenly disappeared? It turns out that a person without DNA would become very ill in much the same way as a person who becomes ill due to exposure to radiation during chemotherapy.

The book goes further and asks another DNA-related and genetics-related question. What if a woman had a child conceived using her own genetic material? Would this be safe for a child? Not according to modern science. Such unusual inbreeding would cause genetic damage and a drop of IQ in the following generations.

Munroe’s readers ask about other ways of endangering human life. For example, bringing a person very close to a supernova, which sends into space—among other things—neutrinos. Normally, neutrinos very weakly interact with regular matter and cannot harm a human. However, a supernova releases such a huge number of neutrinos that they can become lethal for someone who gets too close to it, say, at a distance equal to the distance between the Sun and Mars.

Munroe further finds that people indeed can swim in a pool of spent nuclear fuel rods, and their swim would not be fatal. They might get even less radiation than a person standing at the surface, as long as they do not dive and touch a spent nuclear fuel rod. Some people actually swim in pools of spent nuclear fuel rods. They are workers who service such pools. Again, while readers are provided with the scientific answer to this “radioactive” question, they do not learn much about radioactivity. This makes the answer more entertaining and less educational.

Meanwhile, a pool of spent nuclear fuel rods is not the only object that becomes heated by radioactivity. Our planet has been heated up partly by radioactive decay for a few billions of years. I discuss this question in Part III of my book.

Munroe makes his readers travel through time with this question: what would it be like if you traveled back in time, starting in Times Square, New York while remaining at the same place? He describes geological formations and tells readers what the flora and fauna would look like. He goes back to a time when all the continents were still together. I further investigate the question of time travel in Part IV of my book.

Instead of time travel, how about travel through space, away from the Earth? Munroe considers a person rising above the Earth’s surface at a speed of one foot per second. According to Munroe’s estimations, the person would die from lack of oxygen after reaching an altitude of 8,000 meters (about 5 miles).

For this question, I think it would be very interesting to discuss the properties of different layers of our atmosphere that the person’s body would be passing through during his or her upward journey. Munroe’s book does not include such a discussion. Munroe only very briefly mentions the altitude requirements for someone to be considered an astronaut. I discuss this matter in more details in Part I and Part III of my book. In Part III of my book, I also explain why we can’t fly from the Earth to Mars on a regular airplane, and how artificial satellites stay in orbit.

How about getting back to the Earth? Let us say a person jumps out of an airplane while holding 10 party-size tanks filled with helium and a balloon and tries to inflate the balloon during free-fall. (Free fall is the motion of an object that moves under the influence of one force only—the force of the planet’s gravity. If we neglect the air resistance, then the speed of the person in free fall will be increasing by 9.8 meters per second each second.) Munroe calculates that the person would need to fill the balloon with helium from the tanks really quickly in order to have a safe landing. This question is an excellent example of using approximate calculations while considering very technical issues.

Munroe tells more about free-fall motion when answering a question about a person who wishes to experience the longest free fall on our planet. Munroe recommends that an adventurous person jump from the top of Canada’s Mount Thor, the greatest vertical drop that can be found on the Earth. Munroe thinks that for a safe landing, the person should use a wingsuit, which is a combination of a parachute and parachute pants. Such an outfit would reduce the speed of the falling person.

How about sending a submarine into space? Nuclear submarines are designed to withstand a lot of pressure. There is one problem, though. Munroe finds that the crew of the submarine would run out of oxygen because submarines use devices to extract oxygen from seawater. In Part III of my book, I go further and answer another question about sending a submarine into space. That is, I discuss what would have happened to a submarine if it was launched into the Sun’s corona where the temperature exceeds 1 million degrees.

Munroe also wonders about the possibility of sending 7 billion humans away from the Earth. There are different ways to accomplish such a task: using a space elevator or nuclear pulse propulsion. Moreover, he considers a scenario of sending all the oceans away from the Earth to space through a circular hole with a radius of 10 meters. Eventually, such a “dehydration” of the Earth would cause mass extinctions, but some humans would be able to survive its devastating effects.

How about sending the Earth’s atmosphere into outer space? Actually, this happens all the time. You can learn more about the Earth’s escaping atmosphere from Part III of my book.

Munroe also suggests satisfying scientific curiosity by sending all the water from the Earth’s oceans to Mars. After some prolonged period of time, he tells the readers, the water would freeze and become covered by Martian dust. Considering that some questions already addressed time travel on the Earth, I think Munroe’s answer would be even more interesting if it discussed in details how Mars would look in the distant future, when its frozen water would become liquid again as the Sun gets hotter and hotter.

Munroe’s readers also think of having the Earth expanded. Munroe estimates that if the Earth was expanding at a rate of 1 centimeter per second, then the Earth’s gravity would keep increasing. It would destroy the Earth’s surface infrastructure in the course of 5 years. Mountains and trees would have collapsed in 40 years, when the Earth’s gravity would have tripled. Munroe expects that after some time the Earth would become a hypothetical black hole.

How else can we damage the Earth? We could fire a bullet at the Earth. The trick is that the bullet’s density is equal the density of a neutron star. Neutron stars are the extremely dense remnants of very massive stars. (Density is the amount of matter in 1 cubic meter.) One sugar cube of neutron star matter would weigh about 1 hundred million tons on the Earth. Munroe finds that the neutron bullet would make a crater on the surface of the Earth and then travel all the way to the Earth’s core. The bullet would remain stuck in the planet’s core, and the crater would fill in over time.

What if we turn the Sun off? What would happen if the Sun went dead? Munroe tells his readers that everything on the Earth would ultimately freeze and die. This is a very popular question, and the answer does not bring any surprises, considering that the Sun is the major source of light and heat in the Solar System.

In Part III of my book, I provide more calculations and discussions of solar energy reaching the Earth. I also discuss what would happen if all the energy generated by the Sun were sent directly to the Earth.

Munroe also wonders what would happen if everyone on the Earth pointed a laser pointer at the same spot on the Moon at the same time. He finds that people would have to use extremely powerful lasers in order to make the Moon look just a little brighter. In order to produce the effect of sunlight, they would have to use 50-trillion megawatt lasers, and the light reflected from the Moon’s surface would set the Earth on fire.

Munroe does a good job of educating readers about the nature of lightning. (Lightning, a sudden flow of atmospheric electricity, generates temperatures exceeding the temperature of the visible surface of the Sun, and it sends shockwaves in all directions, reminding us just how powerful nature is.) To help readers appreciate the extent of our atmospheric electricity, Munroe calculates that the power of all the lightning happening on the Earth just in 1 day is equivalent to the power of 2 atomic bombs.

Munroe recalls the concept if inertia, which is the tendency of a body to remain in its existing state of rest, when he answers the following question: what would happen if every person on the Earth jumped and landed at the same moment of time? He finds that the Earth has so much mass, which is the measure of inertia, that it could easily handle the “jumping” of humankind.

One of the questions in Munroe’s book refers his readers to the famous Star Wars movies, asking him to figure out how much horsepower Yoda needs to generate using the Force in order to lift a heavy object. In Part I of my book, you will learn how to measure your own horsepower.

In his book, Munroe answers a few questions about the “end-of-the-world” scenario. He tells readers what happens when the Sun dies. He also determines that a nuclear bomb will not destroy a hurricane, tornado or volcano. Munroe even considers the possibility of destroying our planet using a hair drier. The idea is to place a hair drier that runs uninterruptedly in an airtight box and see what happens. The conclusion is that if we could use an indestructible hair drier and let it run for a long time, it would destroy our planet.

Another end-of-the-world question: If all humans ceased to exist, when would the last human-made light go off? Munroe finds that power grids would stop working fairly soon because of lack of fuel or mechanical breakdowns; solar panels could last for about a century; and toxic radioactive waste would keep glowing for centuries.

In his book, Munroe refers to radioactivity in at least two questions without explaining the nature of radioactive decay. Our readers will find further discussion of radioactivity in Part III of my book.

Many answers in Munroe’s book include good examples of scientific reasoning. Unfortunately, some questions and answers in his book refer to serious scientific topics, which are beyond the grasp of an average reader, and his answers do not include detailed explanations of more complicated topics. Such answers play a role similar to that of special effects in movies: they can entertain, but readers cannot really learn from them.

It is rather unfortunate that Munroe did not organize questions in his book in any meaningful way so that readers cannot build their knowledge base and apply it while moving from one question to the next.

Aside from these drawbacks, Munroe’s book familiarizes his readers with some scientific skills. It demonstrates how to break down a problem, use data, make assumptions, model a solution, and investigate its consequences. Munroe does a good job when he shows readers how to use analogy and order-of-magnitude calculations and how to look at complex questions from different angles.

Munroe’s book also reminds us that real-life problems require cross-disciplinary scientific skills, as well as the ability to make scientific assumptions and guesses.  I recommend Munroe’s book to students, teachers and readers interested in science.

Go to References

Go Back to Table of Contents