When the Artemis II came home, it didn’t look like engineering— it looked like the sky was trying to burn the spacecraft right out of existence.

The Orion capsule hit the upper atmosphere and was quickly wrapped in a glowing sheath — the video from inside, looking out, was terrifying. It was orange and white, and flickering like it was alive. When my physics students see that kind of thing, they go right to friction. Friction opposes forward motion and produces heat. What we saw must have been due to friction.

That’s a good “gut” answer, but it’s not a good “head” answer. It feels right, but it doesn’t quite feel right when physics enters the chat.

The capsule wasn’t burning — the air was breaking.

The Orion was moving through the upper atmosphere at about 11 km per second (close to 25,000 mph). That’s so fast that air can’t act like a fluid and flow around it. The capsule was smashing through the air, colliding hard with air molecules (nitrogen, oxygen, a few trace gases). They don’t have time to get out of the way.

The air couldn’t flow, so it compressed. It got squeezed so that the same amount of air occupied less volume.

Air molecules typically move at a few hundred meters per second (around 340 m/s at room temperature). Take them, and push them into a smaller space; they’ll move faster and hit each other more often. At the atomic level, the speed of a particle is directly related to how much heat energy it has, so squeeze a gas, you get a hot gas. That’s partly why a hand pump heats up when you’re pumping up a bike tire — you’re compressing gas over and over again.

But the air in front of the Orion capsule was way hotter than a bike pump. It was heading toward 5000 degrees C, about half the surface temperature of the sun. At that temperature, particles stop being nice and start getting real. The temperature is so high that oxygen and nitrogen, normally in the air as molecules (O₂ and N₂), break apart into individual atoms.

And things keep heating up. Particles keep hitting each other. The hits get so powerful that they knock electrons off their atoms. Now, instead of a mix of atomic gases, you’ve got electrons along with positive ions (charged particles that have lost electrons). Get enough ions into the mix, and you no longer have gas — you’ve got plasma. That’s a cool name for a soup of electrons and charged particles. And it’s really, really hot.

It’s also what was glowing. Electrons will grab onto positive ions, get excited, then fall back down to their normal energy levels, and everyone will be smacking into each other. All of that creates visible light. The light of atoms being torn apart and partially rebuilding themselves, and getting torn apart again.

Quick sidebar: The communication blackout during reentry? Thank the plasma. Radio waves are blocked because the free electrons in the plasma can move fast enough to absorb and re-radiate them, preventing them from traveling through. The plasma around Orion doesn’t block all radiation—it blocks the kind we use to communicate. The capsule doesn’t disappear. It just becomes unreachable.

Everything you just read is real — we just watched it happen. The atmosphere turned into plasma because something moved fast enough that the air couldn’t behave like air anymore.

Now imagine realizing that applies to you.

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The Fastest Man Alive

That’s exactly what Ryan North does in his take on The Flash.

He lets the Flash (Wally West) reflect on his own powers—not like a superhero, but like a smart guy who’s studied up and gleefully points out where things don’t add up. And how that requires a hall pass from the laws of physics.

Let’s see how Wally explains it. The following sequence is from The Flash #31, written by North, with art by Gavin Guidry and Adriano Lucas.

Reaction time? He’s 100% right. Human reaction time is about 0.2-0.25 seconds. That’s the time it takes the brain to register sensory input (sight, hearing, touch, and taste) and trigger a motor response. Hearing and touch are a little quicker, but at the speed Wally’s running at, it doesn’t matter. By the time his brain registered that something was in his way, he would already have crashed into it. So there’s that.

Wally’s call on friction? Perfectly understandable. My students would’ve said that—but think back to the Orion capsule. Wally’s moving faster. Maybe not light speed, but a percentage of it, which is still stupid fast. Like the capsule, Wally’s not rubbing his way through the air (which is what would cause friction), rather he’s slamming into the air—the individual molecules and atoms. The air in front of Wally? Just like the air in front of the Orion, gets compressed and the temperature spikes, molecules fall apart, electrons are knocked free from atoms, and you get plasma.

Friction occurs at normal speeds and under normal conditions. The Orion capsule? The Flash? Not normal. But I love what Wally is thinking — he’s like a student who’s just realized something is wrong… but hasn’t yet followed the physics all the way down.

Short sidebar: There might be some friction as Wally initially accelerates from a standstill, but given the final speed he reaches, the time when friction is the major issue is vanishingly small.

Let’s continue on:

Alright — here we go. Now Wally brings the physics in and separates himself from the Orion. We (normies) can move fast enough that air acts like a fluid and flows around us. The Orion on reentry was moving so fast that it was compressing the air in front of it, causing it to heat up. The Flash? He’s moving so fast that the air can’t get out of the way, and before it can compress, he’s hitting it. Physically smacking into the molecules. That’s not good, as Wally explains above.

The plasma from Orion? That’s at lower speeds, not Flash speeds.

Now, the air particles cannot get out of the way, and compression occurs even more violently. We’re now in conditions where collisions become extreme enough to even consider fusion. In the Flash’s case, it’s atomic nuclei—positively charged because of the protons in the nucleus—getting close enough to stick together. And they really don’t want to do that. You need absurd energy for that to happen. It happens in the Sun. That level of absurd.

At Flash speeds? Maybe. In the most extreme collisions, some tiny fraction of nuclei might actually fuse. And if they do, you’d get exactly what he describes—high-energy particles, gamma rays, radiation you definitely don’t want to be anywhere near.

But again, let’s back away from the scary fusion for a moment. In our world, at Flash speeds, there would already be radiation present.

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Radiation isn’t some special nuclear thing. It’s just energy moving as waves. Light is radiation. X-rays are radiation. Gamma rays are radiation. Same family, different energy levels.

So the real question isn’t “is there fusion?” It’s: are there charged particles getting thrown around? The moment the answer is yes, you’re going to get radiation. And the second the air turns into plasma—which we already walked through—you’ve got exactly that situation.

You’ve got free electrons, positive ions, and everything colliding, changing direction, speeding up, and slowing down. It’s chaos at the particle level. And charged particles that accelerate give off radiation.

Not “sometimes.” Not “if conditions are right” — always.

Electrons get knocked loose → radiation.
Electrons slam into ions → radiation.
Electrons get captured again and drop to a lower energy state → radiation.

And to answer your radiation question, yes—in our world, this is dangerous. Not in a vague way, but in a very specific, physics-has-names-for-this way. You’d be producing X-rays from electrons slamming to a stop (bremsstrahlung), visible and ultraviolet light from electrons dropping back into atoms, and at higher energies, even gamma rays. In other words: not just heat, but radiation that can ionize, damage, and destroy matter—including you. Bad bad bad.

So by the time the Flash is even thinking about fusion—nuclei sticking together, which is actually very hard to do—the situation is already out of control. The air is plasma, the particles are colliding, and radiation is already pouring out of the system.

Fusion isn’t step one.

Fusion is what you get if you somehow make an already catastrophic situation even worse, which is kind of the running theme here. You don’t need nuclear reactions to make this deadly.

You just need to take ordinary matter, hit it hard enough, and let physics take over. Things get bad very quickly.

Mr. Fusion

So let’s go with Flash’s claim that fusion is—or should be—starting to happen. Thankfully, it’s not in the DC Universe (because the DC Universe has magic. More on that in a second). That’s very cool from the physics side of things, but very, very bad from the “continuing to live” side of things.

Fusion is what happens when atomic nuclei—the dense, positively charged centers of atoms—are forced close enough together that they stick and form a new nucleus. The problem is that those nuclei repel each other. Strongly. So getting fusion requires collisions energetic enough to overcome that repulsion and push them into contact long enough for the strong nuclear force to take over. That’s why fusion is hard in stars and even harder in labs. You don’t get it just because things are hot—you get it because things are absurdly energetic.

If we’re willing to say the Flash is moving fast enough to reach that regime, then what’s happening at the leading edge of his motion isn’t just plasma anymore. It’s a region where bare nuclei—nitrogen and oxygen nuclei from the air—are colliding at energies high enough that, occasionally, a pair doesn’t bounce apart. They fuse.

And when they do, something important happens.

The new nucleus that forms has slightly less mass than the two nuclei you started with. That “missing” mass hasn’t vanished—it’s been converted into energy. This is the part you’ve heard before, but it matters here in a very literal way:

E=mc²

Yep = the energy contained in matter is equal to the matter’s mass multiplied by the speed of light (3.0 x 10⁸ meters per second), squared.

Because the speed of light (“c”) squared is such a huge number, even a tiny loss of mass turns into a large release of energy. Not in a vague, hand-wavy sense—very specifically, and very violently. Collision by collision. And there can be many collisions.

That energy doesn’t come out as a gentle warming of the air. It comes out as high-energy radiation and fast-moving particles. Gamma rays. Subatomic fragments. Nuclei and protons kicked away at high speed. In other words, every fusion event is a tiny burst of energy that doesn’t stay put—it punches outward into whatever is nearby.

And that’s where this stops being a single reaction and becomes a problem.

Because those particles and that radiation don’t just leave the system. They interact with it. They slam into the surrounding plasma, dumping more energy into it, driving more collisions, raising temperatures, and increasing the likelihood that more nuclei will collide with enough energy to fuse. You don’t get one neat fusion event at the front of the Flash. You get a cascade—small, sporadic nuclear reactions feeding energy back into an already violent environment.

So if you take the comic seriously on this point, what the Flash should be carrying with him isn’t just heat, or even just plasma. It’s a moving region where nuclear reactions occasionally occur, where energy is released as radiation and particle showers, and where that energy continuously feeds back into the system.

At that point, the situation isn’t about “running fast” anymore. It’s about driving the atmosphere into a regime where the nuclei themselves start to stick, and every time they do, they release energy in the worst possible way—outward, into everything around them, including him.

Fusion doesn’t replace the plasma problem.

It sits on top of it—and makes it violently worse.

And the Flash actually explains that pretty well, too.

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The End of the Flash?

The Flash finishes his reflection on his powers in the worst possible way.

If all of that is happening—if the air has gone from gas to plasma, if collisions are dumping energy into the system faster than it can dissipate, and if even a fraction of those collisions are crossing into fusion—then there isn’t really a “runner” anymore.

There’s just energy being released along a path.

Wally’s line about nanoseconds matters. At these speeds, everything we’ve been talking about—compression, ionization, radiation, even the possibility of fusion—is happening incredibly fast. Not over seconds. Not even over milliseconds. Over billionths of a second. There’s no time for the system to stabilize. No time for heat to spread out. No time for anything to behave gently.

Energy gets dumped into the air all at once. When you release a large amount of energy into a small region in a very short time, physics has a name for that.

Explosion.

Not metaphorically. Not “like an explosion.”

An explosion.

The “mushroom cloud” line isn’t comic book exaggeration—it’s a rough description of what happens when hot, energized material expands outward rapidly and drives a shockwave through the air. That’s exactly what you’d expect if you took everything we’ve just walked through and let it run to completion.

So the panel lands the conclusion, even if it takes a slightly messy path to get there:

At those speeds, you don’t stay a person moving through the atmosphere. You become a moving release of energy.

That’s the point where the character stops being about speed and starts being about survival. Because if this is the physics—and it is, just pushed to an extreme—then there are only two options: either the Flash doesn’t exist,

or the rules do not apply to him.

In the comics, that answer has a name: the Speed Force. It’s not a boost or an upgrade. It’s a workaround.

Created by Mark Waid in The Flash (vol 2) #91 in 1994, the Speed Force is just that—a little wink, a nod, and an awe-inspiring sense of the DC Universe’s unknown, and possibly unknowable cosmos. Waid — no physics slouch—knew what he was doing when he created it. It’s not just a “magic” get-out-of-jail-free card—it was created as something more, and became even more than that.

The Speed Force is a way to run without turning the air into plasma, without dumping his kinetic energy into the world, without becoming the explosion that physics insists he should be. Without it, the Flash doesn’t outrun anything.

Given the quasi-spiritual nature of the Speed Force, a member of the “Flash Family” in the DC Universe that somehow rejects the protection it grants and can still run at “Flash” speeds has just signed their death warrant—and that for thousands of people around them when they take their final run.

Thankfully, Wally isn’t down with that.

Of Spacecraft and Superheroes

We just watched a spacecraft survive this.

That’s the part that’s easy to miss.

When Artemis II’s Orion capsule came back to Earth, it didn’t glide through the atmosphere. It didn’t slip past the air. It slammed into it, turned it into plasma, and then survived by managing the energy involved as carefully as humanly possible. Heat shields. Ablation. Angles of entry are calculated to within a degree or less. Everything about that moment was engineered around one idea:

Don’t let the physics win all at once.

And even then, it’s not comfortable. It’s not gentle. It’s a controlled argument with the atmosphere that the spacecraft only barely wins. Now take that same process and scale it up.

Faster speeds. More energy. Less time for anything to spread out or dissipate. Strip away the heat shield. Strip away the careful entry profile. Replace it with a person taking a step.

That’s the Flash.

Not the lightning. Not the blur. Not the cool pose mid-stride.

The physics. The part where the air stops behaving like air, where matter starts coming apart, where energy stops being something you carry and starts being something you release. The realization—quietly sitting inside those panels—is that this isn’t a problem you solve by trying harder.

You don’t train your way past it. You don’t build better muscles or faster reflexes.

You don’t win.

Unless you change the rules.

So the story does exactly that. It gives him something the Orion capsule doesn’t have and never could: not better engineering, but an escape hatch. A way to run without triggering everything we just walked through.

A way to move without turning the atmosphere into plasma, without dumping his kinetic energy into the world, without becoming the explosion physics insists he should be.

It gives him the Speed Force.

Not a cheat, a requirement. Because the universe already lets things move that fast. We see it in particle accelerators, in cosmic rays, in the jets of distant black holes.

It just never lets anything survive it.

Orion survives by respecting that boundary.

The Flash survives by stepping outside it.

And if you watch that reentry footage again—the glow, the plasma, the silence—you can almost see the moment where the real world stops…

…and the Speed Force would have to take over.

Final sidenote: for another version of this, that both The Flash and this article give a relativistic hat tip to, check out the GOAT, Randal Munro’s explanation of what would happen if a baseball were pitched at near light speed. Spoilers - same as here. Nothing good.

In Project Hail Mary, Astrophage is both the miracle and the problem.

It eats starlight—that’s the name: astro, “star,” and phage, “eater”—stores absurd amounts of energy, and somehow turns that energy into propulsion and survival. It’s the central problem in the story—and at the same time, the only reason any of it works.

Science tends to do that. The same idea can be both the problem and the solution, depending on where you’re standing. So let’s get into the science of Astrophage.

The quick version is simple: if Astrophage were real, it would change everything. It does inside the world of Project Hail Mary. So instead of treating it like fiction we hand-wave past, we’re going to take it seriously—real physics seriously—and see how far that gets us.

Out of the gate, it’s tempting to compare Astrophage to a plant. It eats sunlight. It reproduces. Is the Sun just being overgrown by space kudzu?

Not quite.

A better starting point is this: Astrophage behaves more like a microscopic space whale.

Whales don’t stay in one place. They feed in one part of the ocean, migrate across vast distances, and reproduce somewhere else entirely. Their lives are defined by moving between environments that provide different resources.

Astrophage is doing the same thing. Just replace oceans with space, plankton with starlight, and breeding grounds with Venus. That analogy gets us in the door—but it doesn’t go far enough.

Astrophage isn’t just biology. It’s a system that sits at the boundary of biology, chemistry, and physics—one that has access to energy on a scale that looks less like life on Earth and more like a particle accelerator.

More CERN than a cell.

Let’s build it up.

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Astrophage: Physical Characteristics

Astrophage is a life form that exists in the extreme environment of stellar surfaces, making it hyperthermophilic, capable of thriving at temperatures that would destroy most life on Earth. Astrophage cells are on the order of 10 micrometers (microns), which places them in the same range as Earth-bound eukaryotic cells, and roughly the same size as human cells. As Grace finds out in Project Hail Mary, Astrophage is extremely similar to Earth-based eukaryotic cells (cells with a nucleus surrounded by a nuclear membrane and specialized organelles), strong enough that panspermia is proposed as an explanation for the origin of life on Earth.

A Life on Earth Aside: Panspermia is the idea that life didn’t start on Earth; it arrived here. Not fully formed organisms, but the raw ingredients or early microbial life, hitching rides on comets, asteroids, or interstellar dust. Space, in this view, isn’t empty. It’s a delivery system. Rocks get blasted off one world by impacts, drift for millions of years, and—if the physics lines up—land somewhere else with their biological cargo still intact. Panspermia doesn’t explain how life begins. It just punts the question elsewhere.

What it does say is that life might spread. It might be resilient enough to survive vacuum, radiation, and time—and if that’s true, then biology isn’t a local accident. It’s something that can leak from world to world.

Like life on Earth, Astrophage appears to be carbon-based, contains water, and has very similar organelles.

While alive, Astrophage cells are effectively opaque across the electromagnetic spectrum and have a super-strong outer coating capable of withstanding the vacuum of space and the stellar corona. Despite its strength, the Astrophage coating can be pierced by something very sharp, such as the nanoneedle Grace used, which unexpectedly killed the cell. When Astrophage dies, they become translucent or clear, revealing their internal structure.

For Astrophage, that line between life and death comes down to one thing: energy.

Astrophage: Diet

Astrophage derives its energy from starlight. Rather than just the visible light that Earth’s plants absorb, Astrophage absorb across the entire electromagnetic spectrum. The preferred habitat of Astrophage is a star’s corona, giving it direct access to the energy being released. It’s the “infection” of the opaque Astrophage in the outer layers of the star, which blocks energy from the sun, causing it to dim. It’s like a mold growing across a clean window - as it grows, more and more light is blocked, and less light can enter.

The “how” of Astrophage eating all the energy produced by a star is the result of a property referred to as “super cross-sectionality.” And it’s poorly understood— that is, it shouldn’t work, but it does. Thanks to this property, nothing — nothing — can pass through Astrophage, not even neutrinos (which normally pass through matter almost without interacting) or subatomic particles that should be able to quantum tunnel through it. Weirdly, Astrophage also blocks wavelengths of light longer than its own size.

Light Interaction Aside: This is the diffraction limit, the reason why light microscopes only work up to a point. If an object is smaller than the wavelength of light, the light diffracts around it, instead of reflecting off of it, making it functionally invisible to that light. At 10 microns, Astrophage are larger than visible wavelengths (Grace can see them with a standard light microscope). Longer-wavelength radiation, such as infrared and radio waves, is also stopped by Astrophage, which is impossible according to physics as we know it. How? Super cross-sectionality. In short, nothing gets through, which is why Astrophage makes excellent heat and radiation shielding, and kind of a problem when they’re dimming your star.

And to make one thing clear: if Astrophage follows biological rules, we can ratchet back some of the hyperbole about what it does. Astrophage isn’t a “star-killer.” It doesn’t extinguish a star; it just turns it down. The Astrophage population in a system will stabilize at whatever level the star can support, given incoming energy, reproduction rate, and losses to predation and other unknowns.

So, like so many biological systems, Astrophage won’t destroy the system. It finds a balance point and holds.

Astrophage: Diet, part 2

What does Astrophage do with the energy it absorbs? Here’s where things get really cool.

Astrophage converts energy into mass. No other form of life we know does this. Not plants, not us, not chemotrophs living near deep-sea vents. This makes them unique. The core idea is simple. Start with the equation that everyone knows from Einstein: E = mc². That’s what underlies atomic energy, nuclear weapons, and a lot of subatomic physics. But that’s not what Astrophage does. They reverse it.

Just solve that equation for mass. That is, m = E/c². That’s it.

In theory.

Prior to their discovery in Project Hail Mary, the idea of energy-to-mass conversion was full-on bonkers, both that it could happen at all and that it could happen in something alive.

The “how” of it all going on inside Astrophage goes pretty deep into particle physics. We’ll try to stay shallow…

Astrophage are basically tiny biological batteries—but they don’t store energy the way anything on Earth does. They have a “sweet spot” temperature: about 96.4°C. That’s their critical temperature. At that exact temperature, something wild happens—when two protons (just hydrogen nuclei) slam into each other, their motion can turn into neutrinos, in a way that shouldn’t be possible under known physics. Neutrinos are tiny, almost massless particles that barely interact with anything.

Now here’s the trick: if the Astrophage gets hotter than that, instead of heating up more like normal matter would, it just keeps converting that extra heat into more neutrinos. So the temperature stays locked right around that same point. It’s like a built-in thermostat—but instead of turning off a heater, it converts heat directly into mass.

So even sitting near a star, where temperatures should keep rising, Astrophage just… doesn’t get hotter. It absorbs the energy and packs it away. And it packs a lot.

A single Astrophage cell can store about 1.5 megajoules of energy—an enormous amount of energy for something microscopic. Then, when it needs that energy back, two of those neutrinos collide and turn into light—specifically a very precise kind of infrared light called the Petrova wavelength.

That light does three jobs: it can power the Astrophage’s internal processes, warm it back up if it cools down, and, most importantly, be fired out the back of the cell like exhaust, propelling it forward.

Astrophage’s diet is directly connected to its motion. There are no flagella, cilia, or pseudopods to move them — they’d be useless in space anyway — there’s nothing to push back against. They use rockets. “Toot to scoot,” as Grace put it. Astrophage converts stored energy into light and blasts it out, which pushes them forward. And because the energy density is so high, they can accelerate to ridiculous speeds—up to about 92% the speed of light (2.76 x 10⁸ m/s, or about 617,000,000 miles per hour).

Light Propulsion Aside: Astrophage is essentially a biological version of laser propulsion. Photons don’t have mass, but they carry energy—and that energy carries momentum. Laser propulsion leans on that. Shine a powerful laser at a spacecraft, and the light hitting it delivers a tiny push. It’s small, but continuous, so over time it adds up and accelerates the craft. The problem is that the energy required is enormous and expensive. Astrophage is doing the same thing, with no external fuel cost, just converting its own stored mass into energy. Instead of an external laser, it generates its own light and fires it out in a tight beam. That beam carries momentum away, and the Astrophage gets pushed in the opposite direction. It’s basically a self-contained laser drive: no traditional propellant mass. Just light doing the pushing.

Astrophage is wild because it’s not running on the kind of chemistry life on Earth depends on. No sugars, no ATP, no familiar metabolic pathways. Instead, it’s tapping directly into mass–energy conversion—the kind of physics we usually only see in stars and nuclear reactions.

Life on Earth lives in the shallow end of the energy pool, moving electrons around. Astrophage is down in the deep end, where energy and mass are interchangeable.

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Astrophage: Migration

Astrophage has effectively unlimited food coming from a star, a mechanism that allows it to convert all of that energy into mass, as well as a mechanism for keeping its body temperature at a comfortable (for it) 96.415^oC. Why would it ever want to go anywhere else?

Two reasons: reproduction and colonization.

The first — that’s the easiest one to understand. Once Astrophage is full of energy (there is an upper limit - about 1.5 megajoules of energy, about 17 nanograms of neutrinos), it’s stuffed, and time to reproduce.

Energy Aside: “mega” = millions, so a single Astrophage cell can hold 1.5 million joules. “Joules” is the unit of energy (it underlies all energy measurements, even the kilowatt hours of electricity that electric companies bill us for), and 1.5 million joules is considerable. For instance, at 1.5 megajoules, a single Astrophage cell is carrying about 360 food calories of energy. That’s a full meal. Not a snack—an actual, sit-down, plate-of-food meal. Or, if you want to talk water, a single Astrophage cell can store enough energy to raise roughly a gallon of water from room temperature to boiling.

That is an absolutely stupid amount of energy for something microscopic to be carrying around.

Okay - so it’s full of energy, and it’s time to reproduce. There’s a catch: while an Astrophage converts energy into mass, it still requires specific elements to replicate through mitosis. Over millions of years, Astrophage has evolved to seek out planets with abundant carbon. Again, an Astrophage may be a space-bound organism, but it’s similar to us and, like us, it’s carbon-based.

Using our solar system as an example, Astrophage migrates to Venus, which has an atmosphere made of 96.5% carbon dioxide and 3.5% nitrogen. Getting there, no problem. But “seeing” Venus? That’s a trick.

Astrophage doesn’t “see” as we do. No eyes, no images. What they can detect is the spectrum of light, which wavelengths are present and which are missing. When light passes through a planet’s atmosphere, certain gases absorb very specific wavelengths. Carbon dioxide, for example, absorbs particular parts of the infrared spectrum. So what’s left in the light carries a fingerprint.

A CO₂-rich world leaves behind a very distinct pattern—a set of “missing” wavelengths in the light coming off that planet or its star. Astrophage can detect that pattern. It’s like they’re reading the barcode of the atmosphere.

Once spotted, it’s time to go, and boy, do they go. Millions, billions follow the same path to get to the planet, all jetting along at near-light speed, so it’s a short trip. At full burn, Astrophage could cross the distance from the Sun to Venus in about six to seven minutes.

As they propel themselves through space, the Astrophage’s “jets” (toot to scoot) release infrared radiation at a wavelength of exactly 25.984 microns. That’s what was discovered by Dr. Irina Petrova, the Russian scientist who first spotted the infrared “line” connecting the sun and Venus. The Petrova line is the signal of Astrophage infection in a star system.

The quick travel time between the Sun and Venus assumes Astrophage is at top speed the whole way. Realistically, you’d have a short acceleration phase—but given how absurd their acceleration is, it barely changes the answer. And they don’t slow down when they get to their target. They air-brake. Astrophage rely on the planet’s atmosphere to slow them down— they hit the atmosphere. Hard.

Traveling at such high speed and hitting the atmosphere would destroy any human-sized ship, but for the microscopic Astrophage, it’s no big thing. Compressing air in front of them heats it to ridiculous levels, and they just gobble that energy, turning it into mass, replacing the mass converted to energy for their flight to the planet. With atmospheric deceleration, Astrophage stops when the air pressure is 0.02 atmospheres. Once there, it takes in carbon dioxide to produce biomass for the daughter cells and undergoes mitosis, splitting into two cells.

Astrophage: Migration, 2

Astrophage’s other large-scale movement comes from cells presumably forming spores and traveling to other, uninfected stars. The limit of this travel seems to be about eight light-years, the distance between nearby stars. The reason for this behavior is unknown, but, like Astrophage itself, most likely just biology doing biology.

When conditions are good, life grows. When things get crowded or uncertain, it spreads. On Earth, that looks like spores, seeds, bacteria drifting on air currents—millions launched, most failing, a few landing somewhere that works. That’s bet-hedging. You don’t need all of them to survive. You just need enough.

Astrophage follows the same strategy, just scaled up. A star is a finite resource. Upon finding a star, the population of Astrophage grows exponentially and reaches some carrying capacity. This would— if biology is universal— trigger some fraction of the population to break off, launch itself into space, looking for the next star. Not randomly—they follow light signatures, the spectral “fingerprints” that tell them where conditions might work. It’s not an invasion or even action undertaken with an intention. It’s reproduction, dispersal, and gradient-following, the same playbook life uses here, just written across stars instead of soil.

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Astrophage: Ecology

That spread we just talked about? Astrophage isn’t evil. It just is biology doing biology. Life being all life-y. Its home system, the place where it evolved, is Tau Ceti, where Grace and Rocky discover each other. In that system, Astrophage feeds off of energy from Tau Ceti and reproduces on Tau Ceti E (“Adrian”). But what Grace and Rocky discover is that Adrian is home to a rich biosphere, including Astrophage’s natural predator — the Taumoeba.

Star Course Aside: If Astrophage is the problem, then Tau Ceti looked like the place where Astrophage was either behaving differently or being actively reduced. The Petrova line is the fingerprint of Astrophage metabolism, and Tau Ceti’s strong signal meant that its metabolism is being pushed hard or disrupted. That’s exactly where you look for a solution. The Hail Mary wasn’t sent to Tau Ceti because it was close, interesting, or habitable. It was sent there because, in a sky full of stars being drained, Tau Ceti looked like it might be fighting back.

On Adrian, the Taumoeba keeps the Astrophage population in check by eating it. The presence of a predator is the strongest signal that the system is Astrophage’s home - predators don’t show up before prey. Taumoeba don’t show up unless there’s a long-standing, stable food source to evolve with. Predators are a response to the presence of prey.

Find a creature that specifically hunts Astrophage? Two things are almost certainly true:

First, Astrophage have been present there for a long time—long enough for natural selection to shape something that can find them, catch them, and survive off them. Second, the environment consistently supports that interaction. Not a one-off arrival. A system.

Mirror it to Earth.

Specialized predators don’t show up for something that washed up on the island last week. They show up where the prey has been long enough and abundant enough to matter. That kind of specialization takes time. Taumoeba tells you Astrophage weren’t just passing through the Tau Ceti system. They were native, persistent, and abundant enough for evolution to build around them.

And that’s the real point: Taumoeba isn’t the headline, Astrophage is. From an evolutionary perspective, Astrophage isn’t just an organism. It’s a massive, reliable energy source. And life does not ignore energy like that.

So in the Tau Ceti system, Astrophage is pretty benign. The system, including Astrophage on the star, is stable. Astrophage doesn’t grow unchecked, dimming the star and upsetting the energy balance on Adrian and other planets of the system where life probably exists.

Seen through this lens, Astrophage is an invasive species. A really good one at that. And in Earth’s system as well as Erid’s, Astrophage found an environment with no natural predator, and its population increased at a predictable exponential rate.

Earth version? Think wolves and deer. When deer have plenty of food and no predators, their population explodes. They overgraze, strip the environment, and start to crash under their own success. Introduce wolves, and everything changes.

The wolves don’t wipe out the deer. They keep them in check. The deer population drops to a sustainable level, vegetation recovers, and the whole system stabilizes. Predator and prey rise and fall together, but neither takes over completely. That’s the Astrophage–Taumoeba relationship.

Without the predator, the system gets overwhelmed. With it, the system finds a steady state.

Same biology. Just scaled from forests… to stars.

First — I teach physics.

Ask me what I love most about Andy Weir’s Project Hail Mary? The moment I realized I was never going to be without a copy of it?

It happens in the first chapter, and most readers probably glide right past it, because…math.

It starts with a problem I’ve used on physics tests for years — one that always makes my students look at me and ask, “When would this ever happen?” I feel so very validated. Thanks, Andy. Because suddenly that ridiculous physics-test question had a real-world use.

In the opening pages of the novel, Ryland Grace wakes up unsure of where he is — or even who he is — but aware enough to notice that something feels off. Following up on those instincts, he does a physics experiment.

In chapter freakin’ 1.

AMAZE!

Let me just grab a quick (non-copyright violation) taste of that sweet, nerdy angle, Ryland Grace:

In quick order, Grace drops a test tube off a table that’s 91 centimeters off the floor.

But he doesn’t trust a single measurement. Grace repeats the drop twenty times and averages the results. That’s not just good science fiction — it’s good science. When your stopwatch reaction time might be two-tenths of a second, and the fall only lasts three-tenths of a second, repeating the experiment is the only way to get a reliable number.

Grace writes all the times on his arm (he hasn’t found paper yet), and finds that, when the measurements are averaged, it takes 0.348 seconds for the test tube to hit the floor. He’s got data, and the data supports the “off” feeling he has.

Now he can get to work.

But before that, just a quick shout-out for this entire scene. Grace’s memory is gone, but his scientific habits survived. And that’s actually one of the smartest character signals in the entire opening scene, and one that many of us with decades of science training and teaching know too well. It tells us something important before the plot even starts moving: the man may not remember who he is, but he’s still a scientist.

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How Grace’s Body “Knew”

Before the math shows up, Grace’s body already knew something was wrong. To understand that, we have to sidestep physics for just a second.

Humans evolved inside a very particular environment: one planet, one atmosphere, and one gravitational field pulling downward at about 9.8 meters per second squared (32.15 feet/second²). Every movement you make—standing up, catching a ball, dropping a set of keys—happens inside that invisible constant. Everything speeds up downward at that rate — about 9.8 meters per second faster every second.

Over a lifetime of countless observations, your nervous system builds a model of it.

Your inner ear, deep inside the skull, is constantly measuring acceleration. Tiny fluid-filled canals shift when your head moves, letting your brain know when you start, stop, or change direction. Your muscles and tendons are doing their own measurements at the same time, sensing force and balance. And your brain is always predicting what should happen next—how fast something should fall, how hard you should land, how much force it takes to lift something.

Most of the time, you never notice this system running — it’s automatic.

But it means that when gravity changes—even a little—your body notices immediately.

• Drop something, and it hangs in the air a fraction longer than it should.

• Jump, and you rise just a little too easily.

• Your sense of weight shifts.

You don’t reach for a calculator. You just feel that something is off.

The universe is no longer behaving the way it always has.

It’s why the footage of astronauts on the moon looks weird (not faked, it just looks weird). The moon’s gravity is 1.6 m/s². Literally, things fall more slowly on the moon. There’s less “pull” downward on them than there is on Earth. Watching astronauts move across the lunar surface can feel eerie — something in our brains tells us that’s not how things should be.

That’s the moment Ryland Grace hits in the opening pages of Project Hail Mary. Before he remembers who he is, before he understands where he is, his brain notices the mismatch. The falling object takes too long, and that tiny delay is enough to trigger the most basic scientific response there is: that’s weird.

In a minute, we’re going to run the exact same experiment Grace does, but to get back to the story — Grace hasn’t recovered his memory yet, but the part of him that understands gravity is already awake.

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Back to the Math

Once Grace notices the fall feels wrong, the next step is the same one scientists have been doing for centuries: estimate first, calculate second. The physics of falling objects is about as simple as it gets. If something drops under gravity alone, the distance it falls is given by the formula:

d = ½gt²

Every intro physics class on Earth eventually meets this equation.

That is, the distance (d) something falls is equal to one-half multiplied by the gravitational acceleration ( g = 9.8 m/s² on Earth) multiplied by the time it takes the object to fall, squared. In other words, the distance an object falls depends on how strong gravity is, and how long the object has been falling.

In most physics classes, we solve this equation for the distance, not for g. But Grace isn’t in a physics class. If you want to solve for gravity instead, you just rearrange the equation:

That’s the version Grace is working with.

Now plug in what he observes. The object falls about 3 feet, which is roughly 0.91 meters. And the fall takes about 0.348 seconds.

Put those numbers into the equation and you get a gravitational acceleration of about 15 m/s².

Again, on Earth, gravity is about 9.8 meters per second squared.

So wherever Grace has woken up, the gravity is only about one and a half times that of Earth.

That’s a huge difference. In a place like that, everything would feel heavier. Grace couldn’t jump as high, and objects would fall faster (and hit harder, as Grace learned). Movements would feel heavier and more deliberate. Anything that depends on gravity would happen faster.

Grace takes this result and reaches one of two possibilities: he’s either somewhere with stronger gravity than Earth, or — thanks to Einstein’s equivalence principle — he’s inside a sealed chamber accelerating at 15 m/s².

Don’t worry — I didn’t ruin either the book or the upcoming movie for you. There’s a lot more for Grace to figure out before he fully realizes where he is. This happens in the first few pages.

Try It Yourself

You can do the same experiment Grace did here on Earth — just duplicate his method. Drop something a bunch of times. Aim for at least a meter of drop height, because your reaction time (how quickly you can start and stop a stopwatch) is locked in at about 0.250 seconds. Grace’s 91 centimeters? Right at the edge of the sweet spot.

Take lots of measurements — 20 repetitions wasn’t overkill, it was a best practice, given that human reaction time was involved. Drop something that’s streamlined — a test tube was perfect. You can use a pen or something similar. Do the same math as above, and see what you get.

And Just In Case…

If you’re feeling like my old students and thinking this is all very contrived…shhh. Just read the book (you have a little bit before the movie comes out). It all makes perfect sense.

Drop something and see what gravity says.

One Final Note

Just in case you’re a STEM Teacher, I did run this question by my physics class, and they loved the heck out of it, particularly the question of “logically, what can Ryland Grace conclude from his finding?”

They all answered it pretty quickly, and pretty well. I’ve got a room full of would-be saviors of the earth in first period.

In Hulk Smash Everything #1, the Leader comes up with a simple solution to his Hulk problem: remove him from the present entirely. Instead of fighting Hulk directly, he throws him backward through time—far enough that the Hulk simply can’t get back.

Where does he land?

About 66 million years ago, at the very end of the Cretaceous Period.

The Hulk has always had strong ties to science—and because of that, he’s long had an outsized pull on scientists and (ahem) science teachers. This series, Hulk Smash Science, takes a closer look at the real science behind those stories.

Let’s go!

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The Worst Place the Leader Could Send the Hulk(*)

In Hulk Smash Everything #1, the Leader comes up with a simple solution to his Hulk problem: remove him from the present entirely. Instead of fighting Hulk directly, he throws him backward through time—far enough that the Hulk simply can’t get back.

Where does he land?

About 66 million years ago, at the very end of the Cretaceous Period.

Geologists call this moment the Maastrichtian, the final stage of a world dominated by dinosaurs for more than 160 million years.

It’s a good choice on the surface. The Leader didn’t just send Hulk somewhere vaguely in the distant past, like a villain with access to a time machine but no clear grasp of history. Nope—the Leader dropped Hulk into one of the most dangerous moments in Earth’s history: the last quiet minutes before the asteroid arrives.

Before we get to that asteroid, though, we need to understand just how much history has already happened before Hulk arrives.

To do that, we need to talk about deep time.

The History Beneath Hulk’s Feet

Scientists use the phrase deep time to describe the enormous span of Earth’s history. Human history covers thousands of years. Recorded civilization spans perhaps ten thousand.

Earth’s story unfolds across billions. The planet formed about 4.54 billion years ago, when dust and rock in the early solar system collided and merged to form the young Earth.

Not long after the planet cooled enough for oceans to exist, life appeared. The earliest evidence dates to roughly 4.28 billion years ago, when microscopic organisms were already thriving in ancient seas. It took about 628 million years for life to appear on the newly formed Earth. Through the lens of deep time, that’s practically an eyeblink.

For most of Earth’s history, life was microscopic. Tiny organisms dominated the planet for billions of years, gradually reshaping the chemistry of the oceans and atmosphere.

One of the most important transformations happened around 2.4 billion years ago, when photosynthetic microbes began releasing large amounts of oxygen into the atmosphere. This event—known as the Great Oxidation Event—fundamentally changed Earth’s environment and helped make more complex life possible.

Much later, around 540 million years ago, animals with shells, skeletons, and complex body plans began appearing in the fossil record during the Cambrian Explosion. From that point forward, ecosystems became increasingly complex.

Over the hundreds of millions of years that followed, continents drifted, oceans opened and closed, and ecosystems rose and vanished again and again.

By the time the Hulk arrives 66 million years ago, Earth has already hosted life for more than 4 billion years.

The ground beneath his feet contains the remains of countless earlier worlds. Layer upon layer of ancient rock preserves organisms that lived hundreds of millions of years ago. Fossils from long-vanished oceans, forests, and coastlines lie buried deep beneath the surface.

Even the oil and natural gas deposits we rely on today began forming long before Hulk arrived. Petroleum originates when enormous quantities of microscopic marine organisms sink to the seafloor and become buried under sediments. Over tens of millions of years, heat and pressure transform that buried organic material into hydrocarbons (which will return for a cameo next time).

Beneath Hulk’s feet lies a stack of ancient worlds—oceans, forests, and reefs that lived and died hundreds of millions of years before dinosaurs ever appeared. So when Hulk lands in the Late Cretaceous, he isn’t stepping into the beginning of Earth’s story.

He’s arriving very late in it.

And the world he sees is only one more chapter in a very long history.

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North America, 66 Million Years Ago

The next surprise Hulk would notice is that the continent doesn’t look the way it does today.

The asteroid that will soon strike Earth will hit near the modern Yucatán Peninsula, at the location now known as the Chicxulub crater. Today, that region is covered by dry tropical forest.

But 66 million years ago, it looked more like coastal Florida or the Bahamas.

Sea levels were much higher than today, and the Yucatán region sat on a warm, shallow marine platform filled with lagoons, reefs, and low limestone islands.

Most evidence suggests the asteroid struck shallow ocean water, roughly 100–300 meters deep. So Hulk probably wouldn’t be standing in dense inland jungle near the impact site. Instead, he’d likely be somewhere along a tropical shoreline, surrounded by coastal wetlands and warm, shallow seas.

In fact, issue #1 seems to place Hulk in almost exactly the right environment. The panels show him fighting small raptor-like dinosaurs along a tropical coastline dotted with palm-like trees and low rocky outcrops. That setting lines up well with what scientists think the Yucatán region looked like at the end of the Cretaceous—a warm coastal landscape along the edge of a shallow sea.

Zoom out further, and we see Hulk fighting on a peninsula-like stretch of land jutting into the ocean—something that closely resembles reconstructions of the ancient carbonate platforms that once formed much of the Yucatán region.

If that interpretation is right, Hulk wasn’t placed randomly in the dinosaur world.

He’s standing very close to one of the worst possible places on Earth to be 66 million years ago.

But the continent’s most important geographic feature at this time lay farther north. Instead of a single continent, Hulk would be looking at a North America split nearly in half by a warm inland ocean.

For tens of millions of years, a vast inland sea called the Western Interior Seaway stretched from the Gulf of Mexico to the Arctic Ocean, covering areas that are now Kansas, Nebraska, South Dakota, and eastern Colorado.

By the time Hulk arrives, the sea is retreating—but its influence (and its salt, which we still mine today) is everywhere. The continent remains divided into two regions:

To the west lies Laramidia, a narrow strip of land squeezed between the rising Rocky Mountains and the inland sea.

To the east lies Appalachia, a much older landscape built around the worn-down Appalachian Mountains.

Laramidia is where most of the famous dinosaur fossils are found today.

The environment there would feel surprisingly familiar to modern Americans. Imagine a prehistoric version of the Mississippi River basin or the Gulf Coast wetlands—broad river valleys, seasonal flooding, dense vegetation, and enormous floodplains stretching for hundreds of miles.

These landscapes supported vast herds of plant-eating animals.

And where there are herds of herbivores, there are predators.

The Dinosaurs of This Ecosystem

By the end of the Cretaceous, dinosaur ecosystems were highly developed and diverse.

Large herbivores dominated the floodplains. Among the most common were duck-billed dinosaurs like Edmontosaurus, animals up to 12 meters long that grazed on vegetation using hundreds of tightly packed grinding teeth known as dental batteries.

Nearby moved horned dinosaurs like Triceratops, massive animals roughly the size of elephants but equipped with three horns and enormous bony frills protecting their necks.

At the top of this ecosystem stood Tyrannosaurus rex.

At roughly 12 meters long and weighing several tons, T. rex was one of the largest land predators that ever lived. Biomechanical studies suggest its bite force exceeded 30,000 newtons (about 3-4 times the weight of a typical car), powerful enough not only to slice flesh but also to crush bone.

Sharing the environment were smaller, feathered predators such as Dakotaraptor, agile hunters armed with curved sickle claws on their feet. Overhead flew birds—small feathered descendants of earlier dinosaur groups, because birds didn’t replace dinosaurs.

Birds are dinosaurs.

By the time Hulk arrives, these ecosystems have been stable for millions of years.

Nothing about the world suggests it’s about to change.

The Hulk’s Sparring Partners

One of the fun surprises in Hulk Smash Everything #1 is that the dinosaurs Hulk encounters are actually pretty grounded in real Late Cretaceous biology. Ryan North and artist Vincenzo Carratu clearly did their homework.

Let’s play who’s who.

For help, we brought in a specialist: paleontologist Jimmy Waldron from Dinosaurs Will Always Be Awesome.

So let’s ID some dinos.

Hulk completes his time jump next to the carcass of a much larger dinosaur that’s already had its tastier bits eaten by smaller predators. From the image, it looks like a classic long-necked sauropod—the kind of enormous plant-eating dinosaur made famous by Apatosaurus and Brontosaurus. But there’s a catch.

“There was a period of time in the early days of the Late Cretaceous that we call the Sauropod Hiatus,” Waldron explains. “It was a time when we simply don’t find very many sauropod bones in North American rocks. We don’t really know why yet, and it’s very strange.”

Brontosaurus and Apatosaurus lived earlier in the Jurassic, but sauropods as a group persisted into the Late Cretaceous as titanosaurs. The panel leans into the visual shorthand of a gigantic long-necked herbivore to show just how immense the animals of this world could be.

“The only real sauropod known from Late Cretaceous North America is Alamosaurus,” Waldron says. “An absolute unit of a titanosaur, living between Utah, New Mexico, Texas, and northern Mexico.”

Given that the range of Alamosaurus may have been larger than what we’ve discovered so far—and considering its size—the half-eaten dinosaur Hulk encounters could plausibly be an Alamosaurus.

But before Hulk can get his bearings—or get us a positive ID—he’s surrounded by a group of small, fast-moving predators.

These are the raptors we were all expecting: versions of the creatures that have been haunting people’s nightmares ever since Jurassic Park arrived in 1993.

They look very much like dromaeosaurs, the same general family as Velociraptor. The long tails, narrow snouts, and raised sickle claws on their feet match what paleontologists recognize as classic raptor anatomy.

And then there are the feathers—a detail that reflects what paleontology has learned since Jurassic Park. For decades, dinosaurs were depicted as scaly reptiles, but fossil discoveries now show that many theropods—including raptor-like predators—were feathered.

In other words, the “giant murder turkey” aesthetic scientists sometimes joke about is actually pretty close to reality.

“Our raptor friends here are very Jurassified,” Waldron says, “based largely on the Velociraptors of Jurassic Park in their design and determination to rattle Hulk like a spoonful of green Jell-O.”

However, the real Velociraptor lived in Mongolia and was long extinct by this time.

“With their size and habitat range, these are likely Utahraptor,” Waldron continues. “And yes, spielbergensis was almost the species name—it ended up being Utahraptor ostrommaysi, named after two prominent raptor researchers.”

Fun fact #1: spielbergensis literally means “of Spielberg,” a nod to the director of Jurassic Park—and a shorthand scientists sometimes use for slightly fictionalized movie raptors.

Fun Fact #2: The sauropod skeleton in the Jurassic Park visitor’s center in the first film? That’s our buddy, Alamosaurus!

Fun fact #2: Dromaeosaurs and their close relatives, the troodontids, were a Late Cretaceous success story. These dinosaurs filled ecological niches across the planet, and many paleontologists think they may already have been developing flocking or pack-like behavior.

Some evolutionary biologists, including Jonathan Losos, have even speculated that if the asteroid had missed Earth—or struck harmlessly in the Pacific—raptor-like dinosaurs might have continued evolving into the planet’s dominant intelligent life.

Also notice the body plan: long balancing tails and grasping arms, exactly what you’d expect for agile predators in Late Cretaceous ecosystems.

None of the animals are drawn perfectly to scale or with museum-level anatomical precision—this is still a Hulk comic, after all. But the overall choices are impressively close to what paleontologists think actually lived in North America at the end of the Cretaceous.

So if you dropped a very large green superhero onto Earth 66 million years ago and waited for the Chicxulub asteroid to finish the job…

“Well,” Waldron concludes, “considering the geography near the impact, the giant sauropod, and the large North American dromaeosaurs, my estimation is that Dr. Banner is somewhere in central Texas—66 million years ago—smashing Utahraptors and avenging Alamosaurus.”

The Light in the Sky

And as that final panel shows, far out in space, a mountain-sized rock is already on its way.

The asteroid that will strike Earth is roughly 10 kilometers wide—about the size of Mount Everest laid on its side. It’s traveling at roughly 20 kilometers per second, or about 45,000 miles per hour.

When it strikes near the Yucatán coast, the energy released will equal billions of nuclear bombs detonating at once.

The impact will blast out a crater nearly 200 kilometers wide, launch molten rock high into the atmosphere, and trigger massive earthquakes and tsunamis.

Debris blasted into space will rain back through the atmosphere, turning the sky itself into an oven and igniting wildfires across entire continents.

Dust and aerosols will block sunlight for months to years, collapsing ecosystems worldwide.

And the world Hulk has just stepped into—this stable, thriving planet—will begin to unravel.

Not instantly.

But permanently.

Next time: the impact itself—and what happened after—in Hulk Smash Science #2.

Hulk Smash Science is a four-part series exploring the real science behind Marvel’s Hulk Smash Everything.

(*) - or so we thought! Just wait until issue #2!

All comic images © Marvel Comics, used for commentary and review.

Superman (2025) trailer © DC Studios / Warner Bros. Used under fair use.

So cool. Superman fighting a giant monster.

But a stupid voice in my head reminded me — we don’t have giant monsters in our world.

Sometimes the little kid in me shakes his fist at the universe.

The reasons why are locked in stone, as far as life as we know it on earth, and it all boils down to something called the cube-square law.

Friggin’ Laws.

As far as physics and biology go, the cube-square law is pretty straightforward: if you make an animal larger, the surface area increases by the square of the growth factor, while the volume (and mass) increases by the cube of that factor.

For instance, say you want to double (2x) the size of your pet dog. Your dog weighs 10 kg.

Your new, larger pet’s surface area would be about 4 times (2² = 4) its original size, while its mass (volume) would be 8 times (2³ = 8) more.

So far, so good. That’s just the physics – a square vs. a cube.

Now, let’s bring in some more physics and some biology.

You’re Gonna Need Thicc Legs

As everything has been getting bigger on your dog, we start to run into real problems with their legs.

Bigger muscles (to move the bigger animal) have more strength, sure, but that strength is related to the cross-sectional area (think the area of a circle, A = πr²) of the muscles and bones. Again, the mass increases by the cube, but the strength of the muscle and the bones (together = “leg strength”) increases by the square of the cross-sectional area.

It’s why elephants have uniform, thick legs.

But we’re not talking about elephants, we’re talking about your 10 kg dog. Let’s give them legs that are 2 cm in diameter (so a radius of 1 cm). Ready for some math? So am I!

Cross-sectional area of the leg:

A = πr²
A = π(1)²
A = 3.14 cm²

Your dog has a mass of 10 kg, so each leg is supporting one-quarter of that mass, in other words, 2.5 kg per leg. Per square cm, then —

2.5 kg/3.13 cm² = 0.79 kg/cm².

That’s the ratio that works for this dog, given its normal, doggy dimensions. Goof that ratio up, and things won’t work. It must be 0.79 kg/cm². That’s the stress it can handle, per leg. Yeah, dogs walking on two legs might be cool to see on TikTok, but they can’t do it for long, given this and other structural reasons (they’re not built that way).

Okay, so let’s double that dog’s size (and cube its mass).

New mass = 80 kg (10 kg x 8)

Now, each leg has to support 80/4 = 20 kg. Will that work?

Spoilers – no.

Scale up the leg size, doubling all its dimensions, making it twice as tall and twice as thick, with legs that are twice the diameter of the original. They were 2 cm, so now they’re 4 cm; therefore, our new radius is 2 cm. Running that through the same calculation as above:

A = πr²
A = 12.57 cm²
Each leg now supports 80/4 = 20 kg.

Our original ratio (for our normal-sized pup) was 0.79 kg/cm2. That’s what works for the proportions of the animal, large or small. When we run through the mass/area ratio after doubling…

20 kg/12.57 cm² = 1.59 kg/cm².

No bueno. Given the dog’s proportions, it can only handle 0.79 kg/cm². We need to pull that ratio down by making those legs thicc. How much thicker?

Well, algebra:

We divided the mass by the cross-sectional area to get the stress load. Let’s just rearrange:

mass/area = stress load.

How about —

mass/stress load = area

20 kg/0.79 kg/cm² = 25.32 cm².

That’s the area, so pull that back to get our radius from:

A = πr²
25.32 cm² = πr²
25.32/π = r²

r = 2.84 cm

diameter = 2 x radius = 5.7 cm.

Compare that to our original leg diameter (2 cm) at our original mass (10 kg). To stand up, our double dog’s legs must be 43% thicker than their original diameter. That’s just the legs – the rest of the double-sized doggo will be in proportion (which brings more problems), but those legs would be noticeably thicker. Your dog won’t look like a normal, proportionate dog anymore. It will look like there was a rhinoceros somewhere in its family tree.

The Kaiju in Superman has pretty thick legs (and big feets that need a pedi ASAP). Superman (2025) trailer © DC Studios / Warner Bros. Used under fair use

And remember, that’s the thickness of the legs for standing on all four. If our double doggo wants to stand on two legs, now that’s 80 kg of mass spread over two legs, so 40 kg per leg. A quick check of the math, and we’re now looking at legs that have to be 8.03 cm in diameter, or otherwise, they’d break from the stress load on them.

Oh, and all of this also applies to other structural elements of bodies, including those that are external. Spiders, scorpions and all insects have to obey the cube-square law. Body mass and structure limits will always be partners in a well-choreographed dance.

No dog-sized spiders are allowed on Earth.

Thicc Legs Were Just the Start

A few more issues to quickly consider that are all related to the bigger picture:

• Energy: As that volume increases by the cube, its energy demands are also going to increase…by the cube. Big, big animals need to eat a lot to keep their fires burning. There are all kinds of environmental implications with this. An enormous Kaiju has the potential to decimate populations of prey and find itself really hungry.
  
  Food is eaten, chemical bonds broken, and energy released that the animal can use. On our scale, with our metabolism, it’s what gives us our 98-ish oF body temperature.

• Heat: Animals get rid of heat through their skin, or whatever surface is in contact with the air or water around them. The surface area of the skin increases by the square, but the stuff inside — the stuff making the heat — that increases by the cube. An animal’s metabolism is responsible for the heat on the inside, and the inside increases by the cube. So, apply that to animals big and small.
  
  The smaller the animal, the faster it will lose heat, therefore the faster its “engine” is running. Mice have seriously fast heart rates. Additionally, mice must eat constantly to maintain their metabolism. On the other hand, an elephant has a slower metabolism, loses its heat slowly; therefore, it doesn’t have to eat as often, and moves on the whole, slower than smaller animals (though they can move fast when needed).
  
  Following our rules (as we understand life), Kaiju, given their huge size, would rarely eat, move very slowly, and have a serious problem with heat, due to their large volume. They’d need some mechanism for getting rid of that excess heat, whether it’s radiators (fins, fans) or, now and again, blowing off jets of plasma.

It may not be mean, it may just need to get rid of heat… Superman (2025) trailer © DC Studios / Warner Bros. Used under fair use

• Movement: Mentioned above, the larger the animal, the slower the movement. Movement takes muscles, and remember, muscles scale up by the square, not the cube. Physically, a larger animal has less muscular force per mass than a smaller animal does.
  
  Thanks to that, it’s more difficult for a large animal to start, stop, or redirect any motion. The force provided by muscles in smaller animals just isn’t there for larger ones. Throw in things like momentum and inertia, and Kaiju on the whole would have a much harder time throwing (and stopping that swinging arm) a punch than a smaller animal. Again — force to start the motion and to stop the motion.

• Cells: That volume of the larger animal that increases by the cube? That volume is made up of cells, fluids, and living creature stuff, but let’s focus on cells. Cells are where the metabolism happens — that’s the unit that creates the heat, needs the food, and all the rest of the living stuff. Cells need oxygen — it’s why we breathe, and other organisms have figured out their own methods of getting the gas into their inner workings.
  
  Oxygen diffuses into cells from the blood (or whatever is carrying it through the body) – the bigger the cell, the longer it takes for the oxygen to get to the good stuff — cellular respiration. For small animals, this pathway is short, and the delivery can be quick. The larger the animal, the longer the path, and the more complex the circulatory system. Paths have limits on their lengths — too long, and the oxygen can’t reach the parts of the cell efficiently. And also, the larger you are, the bigger and stronger the pump you need to move that blood through the body without it stalling out.
  
  Kaiju — without invoking magic, this is another dealbreaker. There are ways around this that can be imagined, such as smaller cells, a more complex circulatory system, multiple hearts, or membranes to extract oxygen from the atmosphere… All plausible in their own way, but each adaptation to provide for the Kaiju’s system takes energy to build and maintain. More energy to maintain, higher metabolism.

Just hangry for a Big Belly Burger? Superman (2025) trailer © DC Studios / Warner Bros. Used under fair use

• Food: Been talking about metabolism and energy — larger animals need more energy, and we all get energy from food. Small animals with fast metabolisms eat constantly to replace the energy as its used up (and have relatively small poops per capita). Larger animals flip that – they eat less (but a lot of them’re vegetarians — grasses and feed vegetables have less energy than meat) because their slower metabolism allows for it. They also have relatively larger poops per capita. And given the size of stocks of what Kaiju would consider food, they would decimate entire regions of large animals, leaving only the small ones that could easily hide and quickly run away.
  
  Kaiju — as shown historically (and in the trailer), tend to move quickly given their size. That would be supported by a faster metabolism (gas exchange, and heat considerations too) that would require a lot of energy to support it. A lot. And Kaiju would have, per monster, epic poops.

Our “Kaiju”

Yes, we do have some big animals on Earth, but nothing approaching Superman’s Kaiju, Godzilla, Kong, or any of the others that hit screens. Unfortunately (shakes fist at universe), our earthly kaiju have to follow the cube-square law.

Our own “kaiju” are worth a look, largely because we have three distinctive environments: air, land, and water. The main player in how big our kaiju in these environments can grow is an oldie but a goodie — gravity.

Gravity determines weight. So while mass (the “stuff” inside a thing, molecules and atoms) doesn’t change, weight does. Less gravity, less weight. More gravity, more weight.

“Living” in the air gives a little help against gravity — wings can help provide lift, and lighter structures than land animals (birds’ hollow bones, for example) can assist with a size boost, but when birds land, they’ve got the same problems with mass that land animals do. So let’s just consider two environments: land and water.

On land, the current champion is the African bush elephant, weighing approximately 6,000 kg. It matches everything you’d think — slow, thick legs, low metabolism, eating infrequently (but as a vegetarian, eating a lot). We’ve had bigger, like the Argentinosaurus, which probably topped off at 100 metric tons (100,000 kg), but we’re unlikely to see anything that big again.

African bush elephant size comparison. By Stevoc86, Wikimedia Commons, used under CC BY 4.0

Our Kaiju can manage the sufficient skeletal structure and metabolism that Argentinosaurus must have had, but the giant dinosaur had a couple of other factors going for it — a stable food supply (lush forests and grasses), and a stable environment (both in terms of climate and predators). Given Earth’s gravity and environmental/climatic conditions, most researchers feel that the 100 metric tons of Argentinosaurus is at the practical limit for land creatures on Earth.

Argentinosaurus was big. Not just big. But BIG big. by Slate_Weasel Wikimedia Commons, used under CC BY 4.0

Jump into the ocean and float. A body’s buoyancy reduces the effects of gravity, so the water can allow larger animals to develop. The current champion is the Blue Whale, weighing approximately 200,000 kg. Water does help. As far as we know, there’s never been a challenger to the blue whale’s title as the biggest, although, to be fair, monstrous marine dinosaurs could have existed, but they tend to sink when they die.

It’s hard to grasp the size of blue whales — even with comparisons. Image (c)Orca Ireland

But the blue whale is strictly limited to the water. If one wanted to come up against the African bush elephant on land, it would immediately realize its mistake. Without the structural support of a skeleton made for living on land, the whale’s mass would crush it.

But…But…But

I get you — the “what about if…”s. Yeah — what if it was nuclear powered (looking at you, Godzilla), uses stronger stuff in its skeleton, has radiant fins on its back to get rid of heat, or has some other exotic biology that allowed it to grow to a massive size that we just don’t know about. All fine options…

in fiction. In our world, we’re stuck — the cube-square law wins the day.

But we can keep hope alive. Real Kaiju would be cool. Terrifying, but cool.

Or maybe you got here because of a link and now can’t find the article?

We’ve been at this since…I want to say 2014, so there’s a bit.

We’ll be bringing the good stuff over as we go. It’s going to take a minute, but it will be coming. Keep an eye out, and you’ll be back in touch with some old nerd science soon!

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