The man who invented the theory behind quantum immortality died at 51 of a heart attack. He was legally drunk. He'd spent years chain-smoking and drinking, never saw a doctor, and his weight ballooned. He believed his own theory meant he couldn't really die.
His name was Hugh Everett III. In 1957, as a Princeton grad student, he came up with an idea that split physics in half. His theory says that whenever something happens at the tiniest level of reality, the universe doesn't pick just one result. It splits into copies. Both results happen, each in its own version of the world.
Quantum immortality takes that and stretches it. If reality always splits, then every time you face death, there's at least one version where you survive. And since dead people can't experience anything, the only "you" that ever knows what happened is the surviving one. So from your perspective, you can never die.
But MIT physicist Max Tegmark, who popularized this idea, walked back on it years ago. His problem: dying isn't like flipping a light switch. Your brain doesn't jump from on to off. It fades. Cells break down, and awareness gets foggy. The thought experiment only works if death hits faster than a single thought. Real death is almost always slow, and that breaks the whole thing.
There's a math problem on top of it. Every time reality splits, and you survive, the slice of "you" that exists in the equations gets cut in half. Survive 10 rounds, and the remaining you hold less than 0.1% of its original presence. Tegmark now says you should just expect totally normal odds of dying.
Sean Carroll, a physicist who actually believes in Many Worlds, flat out calls quantum immortality "a bad idea." When reality splits, future versions of you become separate people. The you that dies is a real person who really died. Having a copy survive somewhere else doesn't undo that.
Everett never wavered. His biographer says he was fully convinced his theory guaranteed him immortality. He died in his bed in July 1982. His teenage son Mark found the body and later said it was the first time he'd ever touched his father.
Everett asked for his ashes to be thrown in the garbage.
Fourteen years later, his daughter Elizabeth took her own life. She left a note asking for her ashes to be thrown out, too, so she could end up in the right parallel universe to meet up with Daddy.
Philosopher David Lewis considered what quantum immortality would mean if true. You'd survive. But the theory says nothing about surviving healthy. What you'd get is a slow, endless decline. Body falling apart, always barely alive, but never quite crossing into death. Like Tithonus from Greek mythology, who got eternal life but nobody thought to include eternal youth. He just aged. Forever.