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What Space Does to the Human Body
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Pascal's Wager: The Famous Bet on God
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The Real Robinson Crusoe
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Grab your coffee and let’s dive in.

Scott Kelly went to space and came back about an inch and a half taller than his identical twin. For nearly a year aboard the International Space Station, with no gravity pressing down on him, the soft discs between his vertebrae swelled and his spine stretched out to its full, uncompressed length. Mark Kelly, his twin brother and a fellow astronaut, stayed on the ground and stayed the same height. It is the kind of party trick that sounds wonderful until you learn it comes with back pain and vanishes within days of landing, as gravity promptly squashes you back down to normal. The height gain is temporary and almost every astronaut experiences it, and it is the gentlest thing that happens to a body in orbit. The human machine was built across millions of years for one specific setting, the constant downward pull of Earth. When you switch that setting off, the machine starts misbehaving in ways that range from the comic to the genuinely alarming.

The real trouble starts with fluid. On Earth, gravity drags your blood and water down toward your feet, and your heart spends all day pumping it back up. Remove gravity and all that fluid drifts upward instead, pooling in the chest and skull, which gives astronauts the famous puffy "moon face" and strangely skinny "chicken legs". This is more than a bad look. The extra pressure pushes against the back of the eyeball and flattens it slightly, swelling the optic nerve and changing their vision. The condition affects most people who spend long stretches in orbit, and some, like astronaut John Phillips, have come home needing glasses they never required before. Meanwhile the bones are quietly bailing out. Without the daily work of holding up your weight, the big load-bearing bones in the hips and spine stop rebuilding themselves and shed around 1 to 1.5 percent of their density every month, roughly what an elderly person loses on Earth in a full year.

NASA had a once-in-a-generation experiment sitting right in front of it. Scott and Mark Kelly are genetically identical, so when Scott spent his year in orbit, scientists could compare him against a matched copy who had stayed home. The Twins Study turned up changes nobody had ordered, including shifts in which genes were switched on and off, alterations to Scott's immune system, and even changes to the telomeres capping the ends of his chromosomes. Most of it drifted back to normal within months of his return, though not all of it, which is a sobering thought for anyone planning a round trip to Mars. To fight the muscle and bone loss, astronauts strap themselves to treadmills and resistance machines for about two hours a day, exercising furiously just to arrive home merely weakened rather than wrecked. Even then the ground wins in the end. When Scott Kelly landed after 340 days in space, he stood about an inch and a half taller than his brother, but within about 48 hours the gap had closed, and the twins were the same height again.

The man who reduced belief in God to a gambling problem had helped invent the mathematics of gambling itself. Blaise Pascal was a French prodigy who built a working mechanical calculator as a teenager in the 1640s. Trading letters with Pierre de Fermat over a dispute about how to split the stakes in an interrupted game of chance, he worked out the foundations of probability theory. So when he turned to the question of God, he reached for the tools of a bettor rather than a priest. His argument survives only as rough notes, scribbled on scraps of paper and found after his death in 1662, later published as part of the Pensées. His opening move was a shrug. On whether God exists, Pascal said, human reason simply cannot decide, so you are left having to place a bet, and sitting the game out is itself a bet against.

The trick lies in how he loaded the stakes. Picture four outcomes on a grid. Bet that God exists and turn out right, and the payoff is infinite, an eternity of bliss. Bet on God and turn out wrong, and you have lost almost nothing, a few earthly pleasures skipped for no reason. Flip it over. Bet against God and turn out wrong, and you face infinite loss, while betting against God and turning out right wins you only a finite prize, the freedom to have lived as you pleased. Line those up and the arithmetic seems to shout one answer, because any wager offering even a sliver of a chance at an infinite reward beats any finite alternative. In the language of the expected-value math Pascal had pioneered, as long as the odds of God existing are anything above zero, the expected payoff of believing works out to infinity, and no amount of worldly fun on the other side can touch it.

The argument has holes, and the holes are the interesting part. The sharpest was pointed out by the philosopher Denis Diderot, who noted that a Muslim imam could run exactly the same calculation for his own God, as could a believer in any of the thousands of gods humans have worshipped. That turns Pascal's tidy two-column grid into chaos, since betting on the wrong deity might land you in someone else's hell. There is also the awkward fact that you cannot simply decide to believe something because the math is favorable, the way you choose a breakfast. Pascal saw this objection coming and offered advice that amounts to faking it, suggesting you start attending Mass and going through the motions until genuine belief creeps up on you. Whether or not it works as a proof of God, Pascal had stumbled onto something that outlived the theology, the logic of acting sensibly when you cannot know the odds. You are running his reasoning every time you buy insurance or wear a seatbelt on a short drive you are almost certain will be fine, betting a small sure cost against a tiny chance of catastrophe.