Three complete answers in the form. Read them before drafting. They are the target — match their rhythm, not a description of their rhythm.
All three are original. None of these questions appears in the What If? archive, and no text here is copied from it.
- Heating a house with laptops — domestic, thermodynamic
- Eating your body weight daily — biological, scaling
- Every cloud falling at once — geophysical, deflationary
What if I heated my house entirely with laptops? — Devin
It works. That's the irritating part.
Every watt that goes into a laptop comes back out as heat. Not most of it — all of it. The screen makes light, the light hits your walls, the walls get warm. The fan makes noise, the noise dies in the carpet, the carpet gets warm. The processor spends six hours computing something, and the answer has no mass and no temperature and goes nowhere, so that becomes heat too. A laptop is a space heater that has been persuaded to do arithmetic on the way through.
So the only question is how many.
A reasonably insulated house in a cold climate sheds heat at something like 10 kilowatts on a bad February night.[1] A laptop under moderate load pulls about 50 watts. That's two hundred laptops.
Two hundred is not an unimaginable number. It's four pallets.[2] You could stack them in the spare room, and the room would be a fire inspection problem, but the house would be warm.
The difficulty is keeping them awake. Idle, a laptop draws around 10 watts, so a sleeping fleet has to be a thousand strong. To get your 50 you have to load every processor, continuously, all winter. You need something for two hundred computers to think about at four in the morning in February, and it has to be something you don't mind the answer to.
[drawing: a stick figure in a parka sits cross-legged on a mattress, ringed by two hundred open laptops, all rendering the same frame of the same video]
You could go bigger per unit. A gaming desktop under full load will do 400 watts, which brings you to twenty-five machines. A rack-mounted server will do one to two kilowatts, which brings you to about eight. Keep climbing and you stop buying computers and start buying a data centre, at which point the heating problem inverts and becomes the only problem you have.
That inversion is the part worth keeping.
Large data centres don't worry about heat the way you worry about heat. They worry about it the way a city worries about sewage — as a volume that has to go somewhere, urgently, forever. And a few of them have noticed that sewage is only sewage until somebody wants it. Meta's data centre in Odense, Denmark, feeds its waste heat into the local district heating network and warms thousands of homes with it. The servers were going to make that heat anyway. Someone put a pipe on the end.
The tidy version of this answer leaves something out, which is that heat has to actually be where you are. Two hundred laptops in a spare room make a spare room at 40°C and a house that is still cold, because nothing is moving the air. You'd need ducting and fans, and the fans would need electricity, and that electricity would also turn into heat — which is at least a problem that solves itself.
[drawing: the same figure asleep in the warm room, while frost forms on the inside of the kitchen window down the hall]
And it is the wrong appliance. Electric heating of any kind — laptop, toaster, baseboard, hairdryer — is exactly 100% efficient, which sounds like a ceiling because it is one. A heat pump is not 100% efficient. It runs at about 350%, because it doesn't make heat at all; it goes outside and fetches heat that already exists and carries it indoors.[3] Your two hundred laptops and one heat pump warm the house equally well. The heat pump does it on 2.9 kilowatts instead of 10.[4]
So yes: you can heat your house entirely with laptops, at three and a half times the running cost, about sixty thousand dollars up front, and the constant sound of two hundred fans.
You were already heating the house with a laptop. Just the one.
[1] Wildly dependent on the house. A draughty Victorian terrace can double this; a well-sealed new build can halve it. The real number for your house is on your gas bill.
[2] They are not, as a rule, sold by the pallet. I checked.
[3] Coefficient of performance around 3.5 in mild cold, falling as it gets colder outside, which is precisely when you would prefer it didn't.
[4] The heat has to come from somewhere, and "somewhere" is the freezing outdoors, which bothers people more than it should. Your fridge works the same way and nobody is upset about the fridge.
What if I had to eat my own body weight in food every day, like a shrew? — Priya
You'd cook.
Not from the eating. The eating turns out to be survivable, which is the genuinely surprising part. It's the afterwards that gets you.
Start with the mechanics, because they sound impossible and aren't. Seventy kilograms of ordinary mixed food carries something like 140,000 calories, against a normal day's two thousand.[1] Getting that in sounds like the hard part, so it's worth checking against the fastest sustained human ingestion anyone has bothered to measure, which is competitive eating. The record pace runs to roughly 11 kilograms in ten minutes.[2]
Hold that pace and seventy kilograms takes about an hour.
An hour. That's the whole meal. You could do it before work.
[drawing: a stick figure at a kitchen table facing a wall of food that extends off the edge of the panel, checking a wristwatch]
Your stomach objects first. It holds about a litre comfortably and maybe four once it's been argued with, so seventy kilograms means emptying and refilling it around eighteen times a day, every day, forever. Your jaw would go. Your teeth would go faster. But none of that is what kills you.
What kills you is that 140,000 calories a day is 6,800 watts.
That's the number that matters.
A resting human runs at about 100 watts — an old-fashioned bright lightbulb, which is why a crowded room gets warm and stays warm. At 6,800 watts you are four electric kettles, boiling, permanently. And you have roughly 1.8 square metres of skin to get rid of it through.
Run the radiation and your skin has to sit somewhere around 235°C to shed heat that fast.[3] You would not starve. You wouldn't even get indigestion. You would glow faintly and set fire to the chair.
The tidy version of that leaves something out, which is that you'd never get near 235°C. Protein starts coming apart at about 40. You'd be dead of heatstroke inside the hour, a couple of hundred degrees short of the answer, with most of breakfast still queued in the hallway.
So how does the shrew manage?
It manages because it's small, and small is a completely different physical proposition. Heat is made in volume and lost through surface, and when you shrink an animal the surface wins fast — halve its length and you keep a quarter of the skin but only an eighth of the body. A shrew is very nearly all surface. It sheds its heat without trying, because it never has to make anything remotely like 6,800 watts.
[drawing: a shrew and a stick figure drawn to scale, the shrew radiating small tidy heat lines, the figure radiating a great many]
Which turns the question inside out. The shrew isn't eating constantly because it's greedy. It's eating constantly because being that small costs that much per gram, and it has no way to stop. An Etruscan shrew's heart runs at up to 1,500 beats a minute, and it starves to death in a matter of hours if it can't find food.[4] Its whole life is spent paying a bill that arrives every ninety minutes.
There's a hard floor under how small a warm-blooded animal can be. Anatomy doesn't set it and evolution didn't run out of ideas — the floor is the point where the surface finally outruns the volume and the animal cannot eat fast enough to stay warm. The shrew is standing on that floor. There is nothing smaller with a heartbeat, and there can't be.[5]
You're at the other end of the same rule. That's why you get to sleep.
[1] Rough, and the roughness is unavoidable. Food energy density varies enormously — cooking oil is nine calories a gram, celery is close to a rounding error that you have to chew.
[2] Competitive hot dog records are the best-documented sustained ingestion rate in the species. Science takes the data it is given.
[3] Stefan-Boltzmann, ignoring sweat, convection, and the fact that you would be dead. Sweating buys you real headroom, but nothing remotely on this scale — you cannot perspire six kilowatts.
[4] Which is why nobody has ever photographed a relaxed shrew.
[5] Among mammals and birds, anyway. Cold-blooded animals opt out of the whole argument by declining to maintain a temperature, which is cheaper and, on current evidence, extremely successful.
What if every cloud in the sky fell as rain at the same instant? — Tom
Less than a millimetre. You'd barely notice.
Clouds look heavy, and that's the whole trouble with this question. A cumulus is the size of a mountain and drifts like something with mass, and your eye files it as a reservoir suspended over your head. It's mostly air.
Take a fair-weather cumulus a kilometre across. It holds about 500 tonnes of water, which sounds like a catastrophe waiting to happen to someone. Spread over the square kilometre of ground it's shading, 500 tonnes is half a millimetre.
Now do all of them. The liquid water suspended in cloud runs to something like a tenth of a kilogram for every square metre of ground beneath it.[1] Roughly two thirds of the Earth is under cloud at any given moment. Across 510 million square kilometres of planet that comes to about 34 trillion kilograms of water — a genuinely enormous number, and the reason this question feels like it ought to have a dramatic answer.
Spread it back over the planet it fell from and it is 0.067 millimetres.
That's the answer. It's a damp pavement.
I ran it three ways, because a number that small is exactly where a decimal point goes missing. Low-end cloud water gives 0.03 millimetres, high-end gives 0.13. All three land within the same order of magnitude, which is as much agreement as this sort of estimate ever gives you.[2]
[drawing: a stick figure stands outdoors under a completely clear sky, holding an open umbrella, entirely dry]
For scale: an ordinary thunderstorm drops 25 millimetres on the ground under it. That one storm, over its own small patch, delivers nearly four hundred times what every cloud on Earth is holding right now. A wet Tuesday in Manchester comfortably outperforms the entire global cloud inventory.[3]
If you meant all the water in the air, though — not just the visible part — the question changes character entirely. Wring every molecule of vapour out of the atmosphere and you get about 25 millimetres over the whole planet. A full day of hard rain, everywhere at once, with nothing left in the sky afterwards.
And that is still only about nine days of ordinary global rainfall. The entire atmosphere holds a bit over a week of rain. It simply keeps refilling.
Which means rain doesn't come from clouds. Not really.
This is the part worth keeping. The water that lands on you in a storm was never in the cloud you were looking at. The cloud is only the visible section of a process that keeps hauling new water vapour up from below, condensing it, dropping it, and hauling up more. There are about 25 millimetres of water sitting invisibly in the air column above your head as vapour — some 370 times the liquid held in all the clouds in the world — and a storm is a machine for converting that invisible stock into rain, a fraction at a time, for as long as the air keeps rising.
A cloud is not a bucket. It's the working end of a pump.
The tidy version above also assumes the water falls and stays fallen, which it wouldn't. A total depth of 0.067 millimetres, released all at once, is a fine mist, and most of a fine mist evaporates on the way down — the air underneath is unsaturated, which is precisely why it wasn't already raining.
So realistically, most of it never reaches the ground at all.[4] You'd get a smell.[5]
[drawing: the same figure, umbrella now closed, sniffing the air with mild interest]
Five hundred tonnes of cloud, and it can't wet a footpath.
[1] Liquid water path, and it varies by well over an order of magnitude between a thin stratus deck and a thunderhead. I've used a middling figure and checked both ends.
[2] A Fermi estimate is allowed to be wrong by a factor of two. It is not allowed to be wrong by a factor of a thousand, which is what one misplaced kilogram-per-square-metre will cost you, and did, on the first pass.
[3] Manchester's reputation is statistically unfair. It is wetter than London, but Cardiff and Glasgow both beat it and neither has to hear about it.
[4] Rain that evaporates before landing is called virga, and on a dry day you can watch it happen — the grey streaks that hang below a cloud and stop in mid-air.
[5] Petrichor: mostly plant oils, plus a bacterial compound called geosmin. Your nose picks up geosmin at a few parts per trillion, which is better than most laboratory instruments manage.