What If You Could Travel Through Time?

The pocket watch in your hand is ticking down. One minute left. You're standing in a backyard in 1968, and a ten-year-old boy with a gap in his teeth keeps falling off his brand-new bike. The other kids are laughing. He's about to give up for good. And he has no idea that he's your grandpa. From now on, you can turn the hands of Grandpa's old silver pocket watch to any date, past or future, press the button, and be there for one hour. You watch the dinosaurs, the first airplane flight, and your dad's worst haircut in human history. You nap in yesterday afternoon and peek at next Friday's quiz. But the past is fragile, and stepping on a single flower in 1985 can turn your front door purple and rename your dog Gerald. We follow the consequences all the way: the fun, the clever uses, the butterfly effect, the day you almost erased your best friend, spoilers for everything, running into yourself, and what you miss when you're never really here. Plus the real science of time travel: Einstein's time dilation, the atomic clocks flown around the world in 1971, why GPS satellites have to correct for time, the cosmonaut who traveled 1/50 of a second into the future, the night sky as a window into the past, the grandfather paradox, and Stephen Hawking's party for time travelers. So... would you want to travel through time? Tell us in the comments. 00:00 1968: the boy on the bike 00:24 The birthday present 00:54 The rules 01:28 The dinosaurs 01:46 History, live 02:04 Your parents as kids 02:25 Extra hours and yesterday naps 02:38 Peeking at next week 02:54 100 years ahead 03:07 All of time is your playground 03:17 Where it goes wrong 03:21 One flower in 1985 03:37 Too careful 03:59 Spaghetti and all 04:06 No more surprises 04:21 Running into yourself 04:39 Never really here 05:01 Real science 05:15 Time dilation 05:28 Atomic clocks on jets 05:48 GPS and gravity 06:06 Astronaut time travelers 06:27 The night sky is the past 06:46 Paradoxes 07:05 A party for time travelers 07:21 Bending the rules 07:33 Back to 1968 07:43 Holding the seat 08:01 He's riding 08:17 The porch 08:43 Those exact words 08:53 Right here, right now 09:23 Would you? Sources: The Wright brothers' first powered flight lasting 12 seconds, December 17, 1903: Smithsonian National Air and Space Museum Time dilation: Albert Einstein, special relativity (1905) and general relativity (1915) Atomic clocks flown around the world on commercial jets: Hafele and Keating, "Around-the-World Atomic Clocks," Science 177 (1972); the measured offsets were tens to a few hundred nanoseconds GPS satellite clocks running about 38 microseconds a day fast relative to the ground, corrected in the system's design: see e.g. Neil Ashby, "Relativity in the Global Positioning System," Living Reviews in Relativity (2003) Sergei Krikalev, about 803 days in space, roughly 1/50 of a second "younger": a widely quoted estimate Sunlight taking about 8 minutes 20 seconds to reach Earth: NASA Andromeda about 2.5 million light-years away: NASA Stephen Hawking's party for time travelers, held June 28, 2009, with invitations sent out afterward: shown in his 2010 Discovery series "Into the Universe with Stephen Hawking" Next time: What if you could control the weather? #WhatIf #TimeTravel #Science #Animation #Einstein