In the Quantum Suicide Thought Experiment, You Never Die

The laws of the quantum world are so bizarre that if you follow them to their logical conclusions, you get some very strange results. That's why quantum physics is so full of thought experiments. You may have heard of Schrödinger's cat, for example: If you put a cat in a box with a vial of poison that has a 50/50 chance of killing the cat, the cat is both alive and dead — in a superposition of states, you might say — until you open the box. Well, try the quantum suicide thought experiment on for size: In that scenario, you're the cat — except you never die.

Quantum Suicide, Explained

The quantum suicide thought experiment was first posed by Max Tegmark in 1997, and it goes something like this: Imagine a gun is hooked up to a machine that measures the spin of a quantum particle every time the trigger is pulled. If the particle is measured as spinning clockwise, the gun will fire; if it's spinning counter-clockwise, it won't. A man points the gun at a sandbag and pulls the trigger 10 times. The gun goes off seemingly at random: "bang-click-bang-bang-bang-click-click-bang-click-click." Then, the man points the gun at his own head and attempts to pull the trigger 10 more times. What does he hear? "Click-click-click-click-click-click-click-click-click-click." He could keep on pulling the trigger for eternity, and the gun would never fire. How is that possible?

Now let's go back in time to the first moment he pointed the gun at his head. He pulls the trigger, and the gun fires. The man is dead. How can that happen when we already know the gun never fired? It's because every time he pulls the trigger, the universe splits into separate timelines: one where the gun fired, one where it didn't. When he was shooting the sandbag, he existed in the timelines created by that series of bangs and clicks. But when he aimed the gun at himself, the only timelines he could exist in were the ones where he survived — and thus, the ones where the gun didn't go off.

I Shot the Sheriff, but Only in This Timeline

This way of thinking is known as the many-worlds interpretation of quantum physics, which says that our reality is just one in an infinite web of infinite timelines. It's controversial but cool to think about nonetheless. With the many-worlds interpretation, every time you do anything, you cause a split in the universe. You're reading these words, but there's another timeline where you closed the article. You got out of bed at a certain time this morning, but there's another timeline where you slept later. You chose one career, but there's another timeline where you chose something wildly different. In each case, all you know is the timeline you're in.

In his 1994 paper, Max Tegmark pointed out an ironic twist to the many-worlds interpretation, which he calls the MWI: "Many physicists would undoubtedly rejoice if an omniscient genie appeared at their death bed, and as a reward for life-long curiosity granted them the answer to a physics question of their choice. But would they be as happy if the genie forbade them from telling anybody else? Perhaps the greatest irony of quantum mechanics is that if the MWI is correct, then the situation is quite analogous if once you feel ready to die, you repeatedly attempt quantum suicide: you will experimentally convince yourself that the MWI is correct, but you can never convince anyone else!"

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For more quantum weirdness, check out physicist Brian Greene's national bestseller "The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory." We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Written by Ashley Hamer May 31, 2017

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