What-If Science Guide: How Hypotheticals Work

Some of the most useful questions in science start with two words: "what if". What if the Moon disappeared? What if Earth stopped spinning? What if antibiotics stopped working? None of these is a forecast. Each one is a controlled change to a system we already understand, followed by careful reasoning about what the rest of the system would do in response.

Thought experiment. This guide and the articles it links to explore hypothetical scenarios for educational purposes. They are not predictions and should not be read as warnings, advocacy or instructions.

This page is the hub of the scihub101 what-if collection. It explains how scientists reason about counterfactuals, groups the most common scenarios into families, walks through a few short worked examples, and shows how to read any what-if article critically. In each section you will find the deep-dive article we think answers that family's questions best.

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Why "What-If" Questions Matter

Hypothetical scenarios are not just entertainment. They serve at least four real purposes.

1. They test theories at the edges. A theory that only works in ordinary conditions is a weak theory. Pushing it to extremes — enormous gravity, near-light speeds, the removal of an entire kingdom of life — shows where it holds and where it needs help.
2. They expose causal structure. Asking "what would change if X were gone?" forces you to identify everything X actually does. That is why removing one variable is such a good teaching tool: you learn about the Moon by imagining Earth without it.
3. They support planning for rare events. Some what-ifs, such as asteroid impacts, pandemics or large volcanic eruptions, are low-probability but real. Agencies that plan for them use the same scenario reasoning described here.
4. They make abstract ideas concrete. "Tidal forces scale with the inverse cube of distance" is hard to feel. "Here is what happens to your body near a black hole" is not.

Randall Munroe's What If? books popularised the format of an absurd question answered with real physics. The approach on this site is similar, with one addition: every article should say clearly which parts are established science, which are modelling, and which are speculation.

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How Scientists Reason About Counterfactuals

A good what-if answer is built from a small set of tools. Knowing them lets you judge any scenario article, including ours.

1. Change one thing, and say exactly what it is

The first step is to define the change precisely. "What if gravity were stronger?" is too vague to answer. "What if Earth's surface gravity doubled while its size stayed the same?" can be answered, because it tells you which quantities move and which stay fixed. Many of the most confused popular answers come from leaving this step out, so that different parts of the answer quietly assume different changes.

2. Fermi estimation: get the order of magnitude right

Physicist Enrico Fermi was known for producing surprisingly good estimates from rough inputs, and "Fermi problems" are named after him. The method is to break a big unknown into smaller pieces you can estimate, multiply them, and aim to be right within a factor of ten rather than to three decimal places.

For a what-if, order of magnitude is usually all you need. If an effect is a millionth of what would be noticeable, the precise value does not matter; if it is a thousand times larger than anything survivable, the precise value does not matter either. Fermi estimation tells you which of those regimes you are in before you do any detailed work.

3. Use established models, not new physics

The safest what-if answers plug a changed input into a model that has already been tested against reality. Orbital mechanics, the ideal gas law, energy conservation, population dynamics and climate models are all examples. The question changes; the equations do not. When an article invents new physics to make a scenario work, it is no longer a thought experiment about our universe.

4. Look for real-world analogues

Nature has run many experiments for us. Mars shows what happens to a planet that lost most of its atmosphere and its global magnetic field. Large historical volcanic eruptions show how stratospheric aerosols cool the climate. The fossil record shows what happens after mass extinctions. Island ecosystems show what happens when a single species is removed or introduced. An analogue is not proof — no past event matches a hypothetical exactly — but it anchors an estimate in something that actually happened.

5. Know where extrapolation breaks

Every model has a range where it has been tested. Newtonian gravity works superbly for spacecraft but fails near black holes. Economic models fitted to ordinary decades are unreliable for a world without the internet. Biological scaling laws stop applying once an animal is far larger than anything that has lived. The honest move is to state where you are extrapolating and to widen the uncertainty accordingly. If an article gives a precise number for something nobody can measure — such as the exact year a civilisation would recover from a hypothetical collapse — treat that number as illustration, not result.

6. Separate the physical answer from the social one

Many scenarios have two layers. The physical layer (how much energy, how much ice, how fast) can often be calculated with confidence. The human layer (how governments, markets and people would respond) is far less predictable. Good articles keep these layers visibly apart.

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The Main Scenario Families

The what-if collection falls into six broad families. The table summarises them; the sections that follow explain each one and point to the strongest deep dives.

Family	Example question	Key science	How plausible	Flagship article
Cosmic disasters	What if a gamma-ray burst hit Earth?	Radiation, atmospheric chemistry, orbital mechanics	Rare but physically possible	Gamma-ray burst scenario
Physics edge cases	What if the speed of light were slower?	Relativity, electromagnetism, fundamental constants	Not possible in our universe; purely a teaching device	A slower speed of light
Planetary counterfactuals	What if Earth had no Moon?	Tides, axial stability, orbital dynamics	Not possible now; useful for understanding Earth	Earth without its Moon
Biology counterfactuals	What if all insects disappeared?	Ecology, food webs, evolution	Total loss implausible; partial declines are real concerns	A world without insects
Technology and AI	What if quantum computers broke all encryption?	Computer science, cryptography, systems engineering	Some partly plausible; timelines uncertain	Quantum computers and encryption
Climate and Earth systems	What if the North Atlantic current stopped?	Ocean circulation, ice sheets, climate feedbacks	Studied as real risks, with uncertain probabilities	Stopping the North Atlantic current

Plausibility here is a rough guide, not a probability. "Not possible" means the scenario requires changing a law of nature or an unchangeable fact about Earth's history; it can still be an excellent way to learn.

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Family 1: Cosmic Disasters

These scenarios take an event that astronomy says can happen somewhere in the universe and place it near Earth. Their value is that the underlying physics is well understood, so the consequences can be estimated with some confidence even though the trigger is rare.

Impacts. The best-studied cosmic disaster is an asteroid strike, because Earth carries the evidence. The Chicxulub impact about 66 million years ago, made by an object generally estimated at around 10 kilometres across, is widely linked to the mass extinction at the end of the Cretaceous. That single analogue anchors almost every impact scenario. Our deep dive on what would happen if an asteroid hit Earth tomorrow works through how outcomes change with the size of the object, and why most near-Earth objects are far smaller than Chicxulub.

Radiation from space. Gamma-ray bursts are the most energetic explosions known, and the main danger from a nearby one would be to the ozone layer rather than direct heating. The gamma-ray burst article explains why distance and beam direction matter so much.

Visitors from outside. Rogue planets — worlds not bound to any star — are thought to be common in the galaxy. Our piece on a rogue planet entering the solar system shows that the main hazard is gravitational: a close pass could disturb planetary orbits long before any collision.

Changing the Sun. Light from the Sun takes about 8 minutes and 20 seconds to reach Earth across an average distance of about 150 million kilometres, according to the Smithsonian's National Air and Space Museum. That delay is the starting point for the scenario of the Sun going dark for one week: Earth would keep receiving light for those minutes and keep orbiting where the Sun's gravity last pointed it for the same interval, then cool steadily. A gentler version, a Sun 10% dimmer, is a good lesson in how sensitive climate is to incoming energy.

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Family 2: Physics Edge Cases

Here the question changes a constant or pushes an object into a regime ordinary experience never reaches. These are the purest thought experiments: nobody expects them to happen, and that is the point.

Black holes. The classic is falling into one. The black hole fall article explains spaghettification — the stretching caused by the difference in gravity between your head and feet — and why it happens outside the event horizon for small black holes but only deep inside for very large ones. That counter-intuitive result is a good example of why you calculate rather than guess.

Changing constants. If light were much slower, relativistic effects would show up at everyday speeds. Our article on a slower speed of light is a careful introduction to why this also changes chemistry and electromagnetism, not just travel times. A related piece asks what if gravity became 10% weaker, which is a useful exercise in tracing one change through planetary orbits, atmospheres and the Sun itself.

Hypothetical particles and spacetime. Physicists have searched for magnetic monopoles for decades without finding one; the article on what if magnetic monopoles existed explains what would change in Maxwell's equations. Time travel and wormholes appear as mathematical solutions in general relativity but require conditions, such as exotic negative energy, that no experiment has produced.

Open questions, not hypotheticals. Some what-ifs touch genuine unsolved research. What if dark matter doesn't exist compares modified-gravity theories with the dark-matter model, and the multiverse article separates the versions physicists actually discuss from the science-fiction version. These are the cases where a good article says most clearly that the answer is not yet known.

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Family 3: Planetary Counterfactuals

These scenarios change something about Earth itself. They are often the most educational, because they reveal how much of everyday life depends on features we never notice.

The Moon. The Moon raises most of Earth's ocean tides and is widely thought to help stabilise the tilt of Earth's axis. Removing it in a thought experiment teaches both points. Start with what if Earth had no Moon, which covers tides, axial stability and night-time light.

Rotation and tilt. Stopping Earth's spin is one of the most-asked questions in the genre. The surface at the equator moves eastward at well over a thousand kilometres per hour, so a sudden stop would leave everything not anchored to bedrock still moving. Our Earth-stops-rotating article separates the sudden-stop case from a slow stop, which would instead redistribute the oceans toward the poles. Tilt is a quieter variable: Earth without axial tilt shows that seasons would largely vanish, leaving fixed climate bands.

Atmosphere and magnetic field. Mars is the analogue here. Earth with no atmosphere covers what the air does for temperature, pressure and protection. A flip of Earth's magnetic field is a case where the geological record helps: reversals have happened many times, and the article explains what the evidence does and does not show about effects on life.

Size, oceans and continents. Earth doubled in size is a scaling exercise — mass, surface gravity and atmosphere retention all change together. Pangaea never breaking up shows how geography shapes climate and evolution.

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Family 4: Biology Counterfactuals

Biological what-ifs come in two kinds: removing something that exists, and adding a capability that does not. The first kind can be grounded in real ecological data; the second requires care about what evolution can and cannot build.

Removal scenarios. Insects pollinate many crops and wild plants, break down dead material and feed birds, fish and mammals. Our article on all insects disappearing follows those dependencies outward. A narrower version, all bees going extinct, is useful because it separates crops that depend on animal pollination from staples such as wheat and rice that are wind-pollinated. Earth losing all fungi highlights decomposition and the root partnerships most land plants rely on.

Medical counterfactuals. Antibiotics no longer working is the closest of all biology what-ifs to a real-world concern, since antimicrobial resistance is already tracked by health agencies. It is a good example of a scenario that shows what modern medicine quietly depends on, such as safe surgery and cancer treatment.

Added abilities. Humans regrowing limbs like axolotls explains what regeneration requires at the cellular level. Humans photosynthesising is a classic Fermi problem: compare the energy sunlight on human skin could supply with the energy a human body uses, and the gap settles the question.

Deep-history biology. Dinosaurs never going extinct is speculative by nature, because evolution is contingent. The best treatment explains which outcomes are constrained by physics and ecology and which are open.

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Family 5: Technology, AI and Society

These scenarios ask how systems built by people would respond to a shock or a new capability. The physical parts can be estimated; the human parts carry much more uncertainty, and good articles say so.

Infrastructure shocks. The internet going down for a week traces dependencies in payments, logistics and communications. It pairs well with AI controlling the power grid, which looks at automation, failure modes and the value of human oversight.

Security and computing. Quantum computers breaking encryption explains which kinds of cryptography are vulnerable to known quantum algorithms, and why standards bodies have been preparing post-quantum replacements.

AI scenarios. AI what-ifs work best when they stay concrete. Artificial general intelligence arriving tomorrow asks what would happen in the first days and weeks, not in a distant imagined future. Robots doing all human work is mostly an economics question about income, purpose and distribution.

Society and history. Counterfactual history is the least testable kind of what-if, because there is no second Earth to compare against. Humans never discovering fire is grounded in research on cooking, diet and human evolution. Language completely shaping thought examines the strong and weak versions of linguistic relativity and what experiments support.

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Family 6: Climate and Earth Systems

Climate what-ifs are different from the rest because several of them are active research topics with real-world relevance. They should be read as model-based scenarios, not prophecies.

Ice. Antarctica completely melting asks what the coastline would look like if the largest ice sheet on Earth were gone. Scientists do not expect full melting on human timescales; the scenario is a way to understand how much water the continent stores and why even partial losses matter.

Ocean circulation. The North Atlantic current stopping is one of the most-studied climate tipping-point questions. Researchers disagree about how likely a collapse is this century, and the article explains why the uncertainty is wide.

Volcanoes. The Lava Creek eruption that formed the present Yellowstone caldera happened about 631,000 years ago and produced roughly 1,000 cubic kilometres of material, according to the USGS Yellowstone Volcano Observatory. Our article on Yellowstone erupting tomorrow stresses that the most likely future eruption at Yellowstone is a lava flow or hydrothermal explosion, not a supereruption.

Human-driven scenarios. Burning all fossil fuels is an upper-bound exercise that climate scientists have modelled. Nuclear war and its effects on Earth draws on nuclear-winter research to explain how soot in the stratosphere could affect global harvests.

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Worked Examples

Short worked examples show the toolkit in action. Each one uses only well-established figures and states its simplifications.

Example 1: How long does Earth keep its daylight if the Sun vanishes?

• Define the change: the Sun ceases to exist instantly.
• Known input: sunlight takes about 8 minutes 20 seconds to reach us (Smithsonian National Air and Space Museum).
• Model: light and gravity both travel at the speed of light, so nothing changes on Earth for about eight minutes.
• Result: after eight minutes the sky goes dark and Earth moves off in a straight line rather than a curve. The oceans and ground cool over the following days and weeks, and the article on the Sun going dark follows that timeline.
• Limits: "a star vanishing" is not physically possible, so this is a teaching device about light-travel time, not a risk.

Example 2: Could a person run on sunlight?

• Fermi framing: a human body needs a steady supply of food energy every day. Skin exposed to the Sun intercepts only a modest area, and photosynthesis converts only a small fraction of sunlight into stored chemical energy.
• Result: the energy available falls far short of daily needs — by an order of magnitude or more — before you even account for clothing, shade or night.
• Lesson: you can reject the idea without precise numbers; the gap is too large for refinement to close it. That is the power of order-of-magnitude reasoning.

Example 3: How big must an impactor be to matter globally?

• Analogue: the Chicxulub impactor, about 10 kilometres across, coincided with a global mass extinction about 66 million years ago.
• Scaling idea: impact energy rises with the mass of the object, which rises with the cube of its diameter. A body one-tenth the width has roughly one-thousandth the mass and, at similar speed, roughly one-thousandth the energy.
• Result: most objects that strike Earth are small enough to burn up or cause local damage; only rare large bodies have global effects.
• Limits: composition, speed and angle all matter, so real assessments use detailed models rather than one scaling rule.

Example 4: What changes if Earth stops spinning — slowly?

• Define the change: rotation slows to zero over a long period, so there is no sudden jolt.
• Model: rotation makes Earth bulge at the equator and pulls the oceans outward there. Without it, water would flow toward the poles where the solid Earth sits relatively higher.
• Result: large redistribution of oceans and very long days and nights, as covered in the rotation deep dive.
• Limits: the rocky Earth would also slowly adjust its shape, which changes the details over geological time.

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How to Read What-If Articles Critically

What-if content is entertaining, and that makes it easy to over-trust. Use these checks on any scenario article.

1. Is the change stated precisely? If not, the conclusions may rest on hidden assumptions.
2. Are the numbers sourced? Figures such as eruption volumes, impact sizes or light-travel times should be traceable to agencies such as NASA, USGS or NOAA, or to peer-reviewed work.
3. Does it separate calculation from speculation? A strong article says "the physics gives this" and "the human response might be this" in different places.
4. Is there an analogue? Real events, past extinctions or other planets make an estimate far more credible.
5. Does it admit uncertainty? Precise dates and percentages for things nobody can measure are a warning sign.
6. Does the headline match the body? "Earth's apocalypse" in the title and "unlikely and survivable" in the conclusion means the headline is doing the selling.

Hypotheticals versus debunked claims

Some popular "what if" questions are really claims that have already been tested and rejected. Framing them as open questions can mislead readers into thinking the science is unsettled when it is not. Our articles on these topics are written as evidence reviews: they explain what the claim says, what the measurements show, and why the claim does not hold.

• What if the Earth were flat? walks through the observations that establish Earth's shape.
• What if the Moon landing was faked? uses physics to show why the evidence supports the landings.
• Why the climate change "hoax" claim doesn't hold up reviews the independent lines of evidence for warming.
• How we know vaccines are safe explains the monitoring systems behind vaccine safety.
• What science found about 5G and health explains why radio-frequency exposure at network levels is non-ionising and how exposure limits are set.

The distinction matters. "What if Earth had no Moon?" is an honest counterfactual. "What if a debunked claim were secretly true?" is not a thought experiment about our universe, so we treat those topics as explanations of the evidence instead.

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Where to Start

If you are new to the collection, three articles give a good sense of the range:

• A physics edge case: the black hole fall, linked above under Family 2.
• A planetary counterfactual: Earth without its Moon, linked under Family 3.
• A biology counterfactual: a world without insects, linked under Family 4.

From there, the life on Mars article is a good bridge between pure hypotheticals and real engineering questions about living on another planet.

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Frequently Asked Questions

Q: Are what-if articles predictions? A: No. They change one condition and follow known science to the consequences. Some scenarios, such as asteroid impacts or ocean-circulation changes, are related to real risks, but the articles describe mechanisms, not forecasts.

Q: What is a Fermi estimate? A: It is a rough calculation that breaks a big unknown into smaller pieces you can estimate, aiming to be correct within about a factor of ten. It is named after physicist Enrico Fermi and is the main tool for answering what-if questions quickly.

Q: How can a scenario that is physically impossible still be useful? A: Removing or changing something shows what it does. Imagining Earth without the Moon, for example, makes clear the Moon's role in tides and axial stability. The impossibility does not affect what you learn about the real system.

Q: Why do some what-if articles have very different answers to the same question? A: Usually because they define the change differently. A sudden stop of Earth's rotation and a slow stop produce very different results. Check what each article assumes before comparing conclusions.

Q: How do I know if a what-if article is reliable? A: Look for a precisely stated change, sourced numbers, a real-world analogue, clear separation of calculation from speculation, and honest statements of uncertainty. Precise figures for things nobody can measure are a warning sign.

Q: Why are some topics, like flat Earth, written as evidence reviews instead? A: Because they are claims that have already been tested and rejected, not open counterfactuals. Treating them as open questions would misrepresent the state of the evidence.

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Sources

• USGS Yellowstone Volcano Observatory, on the age and volume of the Lava Creek eruption.
• Smithsonian National Air and Space Museum, "Ask an Explainer", on sunlight travel time and the average Earth–Sun distance.
• Schulte, P. et al. (2010), "The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary", Science 327: 1214–1218, on the age of the impact and its link to the end-Cretaceous extinction.