Book · 2014
What If?: Serious Scientific Answers to Absurd Hypothetical Questions
by Randall Munroe
Randall Munroe takes absurd physics questions — what if a baseball traveled at 90% the speed of light? — and answers them with actual math.
What does Reddit think of What If?: Serious Scientific Answers to Absurd Hypothetical Questions?
All 12 mentions live in r/askscience, which tells you something. This is not a book people bring up in reading communities; it comes up when someone asks a genuine science question and another commenter says, essentially, "Munroe already did this one." The ↑268 top comment recommends the book directly to a curious asker. The ↑219 response quotes the supernova neutrino scenario mid-thread as supporting evidence. The function is reference text, not leisure read. Sentiment across all 12 unique commenters registers as unscored — not because people are ambivalent, but because the mentions are descriptive citations rather than evaluative takes. Nobody argues about whether What If? is good. One ↑48 commenter compares their own science-answering work to Munroe's and calls the comparison "high praise." The mentions span 2019 through 2025, steady rather than spiked, which suggests it stays in rotation without needing a promotional moment.
Community feedback & reader fit
Themes
- · Fermi estimation and order-of-magnitude reasoning
- · Absurd hypotheticals answered with real physics
- · Science communication through humor
- · Thought experiments as teaching tools
- · Radiation and nuclear physics explainers
Common praise
- + r/askscience commenters drop it as a citation mid-thread, treating it the way physicists cite papers.
- + The supernova neutrino entry (↑219) gets quoted verbatim years after publication as the clearest available explanation of that phenomenon.
- + The nuclear reactor swimming question (↑56) resurfaces repeatedly because the counterintuitive answer — you'd probably survive — sticks in a way that straightforward textbook answers don't.
- + Commenters recommend it to library borrowers and buyers in the same breath, suggesting they trust most readers will want to own it after sampling.
Common criticism
- − The 12-mention sample is too small to surface any real dissent — no one in r/askscience argues with Munroe's conclusions, which may mean critics simply don't show up in this community.
- − One ↑63 comment asks whether a sequel is coming, implying the original felt finite rather than exhaustive.
- − Mentions cluster around a handful of high-profile entries (supernova, reactor, diamond meteor) rather than spreading across the book, suggesting the full catalog doesn't get equal traction.
Who it's for
If you already spend time in r/askscience reading the long answers rather than skipping to the top comment, this is the book version of that habit. Physics-curious readers who don't have a degree but aren't intimidated by unit conversions will find it works better than a textbook because the questions are genuinely stupid in the best way. Skip it if you need narrative structure — there isn't one. Each chapter is self-contained, which makes it good for a commute but thin as a cover-to-cover read.
Mentions over time
Top subreddits
Which Reddit comments matter for What If?: Serious Scientific Answers to Absurd Hypothetical Questions?
Top-upvoted quotes across the subreddits where this book is mentioned. Click through to read the full thread.
“There's a book called "What If?" by author Randall Munroe (author of xkcd.com comics) where people submitted crazy or silly scientific questions and he provided in depth, serious answers. You should check it out from a library or just go buy it if you like to wonder about these k…
“Same with Supernovas. Every time I try to figure out how massive the numbers are I'm reminded of the XKCD What If that points out that if you were 1 AU away from a star undergoing core collapse in a supernova, you'd die from the sheer number of neutrinos slamming into you (in addition to everything …
“A cavitating hypersonic whale would be a sight to see. Especially if it impacted anything at that speed... Does anyone know if Randal Munroe is doing another What If book soon?
“There's a book by XKCD creator, either "How to" or "what if" and it asks a question: what would happen if you swam in the water pool used to cool a nuclear reactor. It was very interesting, and iirc the basic answer was "nothing really, depending on how close to the reactor you were" Apparently wat…
“I've been answering science questions in this sort of format for a bit longer than Mr. Munroe has been running the What If? part of XKCD, but I think he's significantly better at it! The comparison is high praise indeed, thanks! Part of the reason, and I know Randall gives this justification too, i…
“The term 'near' means very little when talking about the speed of light, but others have pointed that out already. Given that you asked the question, I thought you might enjoy these two articles on XKCD What If! There's one where he tries to figure out what happens to a diamond meteor that hits the…
“This is the subject of an amazing XKCD ‘What If’ post. Here’s the link
“>...yeah, sure, but if you're lost in the forest, you likely know which hemisphere you're in, whether you're upside down, and whether there's an eclipse at that moment. These are all excellent topics to contemplate. They might not answer your immediate question of "how to get out of this forest". …
Convinced? Pick up What If?: Serious Scientific Answers to Absurd Hypothetical Questions
What else does r/askscience read?
Other books mentioned in the same sub, ranked. Shared-sub overlap with this title breaks ties.
The Selfish Gene
Richard Dawkins
Richard Dawkins argues that evolution makes more sense when you stop rooting for the organism and start watching the gene.
The Emperor of All Maladies: A Biography of Cancer
Siddhartha Mukherjee
Siddhartha Mukherjee traces cancer from ancient papyrus to targeted therapy, framing it as a 4,000-year war medicine is still losing on some fronts.
Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body
Neil Shubin
A paleontologist finds a fish fossil in the Arctic and uses it to trace the evolutionary origin of your hands, ears, and hernias.
A Brief History of Time
Stephen Hawking
Stephen Hawking explains black holes, the Big Bang, and the edge of time to people who never took a physics class.
Immune: A Journey into the Mysterious System That Keeps You Alive
Philipp Dettmer
Philipp Dettmer of Kurzgesagt turns the immune system into a 300-page illustrated war story, written for the person who watched every YouTube video and still wanted more.
Cosmos
Carl Sagan
Carl Sagan's 1980 tour of 15 billion years of cosmic history, written for anyone who once looked up and wanted a real answer.
What If?: Serious Scientific Answers to Absurd Hypothetical Questions — frequently asked
What does Reddit actually use What If? for?+
Mostly as a citation. All 12 mentions are in r/askscience, where commenters drop it mid-thread to support a physics claim — the supernova neutrino scenario (↑219) and the nuclear reactor swimming question (↑56) get quoted as evidence rather than recommended as entertainment. It functions less like a book people discuss and more like a reference they trust.
Is What If? by Randall Munroe actually accurate, or just entertaining?+
r/askscience treats it as accurate enough to cite in science threads, which is a reasonable signal. One commenter with their own science-answering background called the Munroe comparison 'high praise' (↑48). No one in the 12-comment sample disputes his math. That said, the community using it is self-selecting: people who already know the physics are the ones recommending it.
Does What If? hold up years after it came out?+
The mention spread — 2019, 2020, 2021, 2022, 2024, 2025 — says yes. It keeps getting cited without any new edition or press moment driving traffic. The supernova and reactor entries specifically come up across multiple years as the go-to explanations for those phenomena, which suggests the core scenarios haven't been superseded.
Should I read What If? or just browse the xkcd What If website?+
Depends on format preference. The ↑268 top comment recommends both the library copy and buying outright, implying the book version earns its place. The website and book cover overlapping but not identical ground. If you've already read the online entries that went viral, the book has additional scenarios not published there — but the Reddit data doesn't quantify how much overlap exists.