Some notes on time travel
John and Nick wonder, if time travel is possible, why haven't we seen any time travelers from the future?
Although physicists have imagined a couple different designs for a time machine, they all share a basic limitation: you can't travel to any date prior to the time machine's construction.
We tend to think of a time machine as something like the DeLorean in Back to the Future, just vanishing and reappearing at dates specified on a digital readout. Instead, the time machine envisioned by physicists can be thought of as a machine with two doors. If you walk into door A, you'll walk out of door B, say, tomorrow. If someone walks into door B tomorrow, they'll walk out of door A today. But you need to have built this machine first. Before the machine is built, there's no door for anyone to walk out of.
So, we shouldn't expect to see any time travelers right now, because no one's built a time machine yet. Only after we build the machine can we expect people to start walking out of it. And that means we'll never be able to go see the dinosaurs, unless we find a time machine that was built by aliens a hundred million years ago.
Although physicists have imagined a couple different designs for a time machine, they all share a basic limitation: you can't travel to any date prior to the time machine's construction.
We tend to think of a time machine as something like the DeLorean in Back to the Future, just vanishing and reappearing at dates specified on a digital readout. Instead, the time machine envisioned by physicists can be thought of as a machine with two doors. If you walk into door A, you'll walk out of door B, say, tomorrow. If someone walks into door B tomorrow, they'll walk out of door A today. But you need to have built this machine first. Before the machine is built, there's no door for anyone to walk out of.
So, we shouldn't expect to see any time travelers right now, because no one's built a time machine yet. Only after we build the machine can we expect people to start walking out of it. And that means we'll never be able to go see the dinosaurs, unless we find a time machine that was built by aliens a hundred million years ago.
posted by Ted at 2:48 PM
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