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we're asking the reason alien civilizations might be rare is because most aliens are huge.

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I had a really good time travel joke to start the episode with, but it turned out, none of you liked it.

So today we start off our sixth year here on Science and Futurism with Isaac Arthur, and I'm

your host, Isaac Arthur. As we travel

from 2019 to 2020, time travel seemed like a good topic, and I figured we'd start by discussing

why we don't think it can work under known physics, then talk about what the implications

would be if it turned out we were wrong. Of course, the thing I always take as the biggest

proof time travel isn't possible is that nobody is offering packaged deals for temporal

vacations. If time travel were possible, we should see folks from the future and the past,

everywhere, and every win, and not just a few secretive folks with a time machine, like we see so often in

science fiction, people don't really seem to look much at what the extended and expanded

use of such a technology would be. While that may be forgivable when a story contains

such a technology having just been invented, so everything revolves around a prototype or its

first use.

This doesn't really work with time travel since it's being invented and used at any point

means it's basically been invented and used at all points in time.

If you manage to invent time travel, you ought to be able to walk out of the machine a moment

later and be welcomed by all your admirers and descendants from when you hopped

...