Is time travel possible, and can you go back in time? This covers the atomic clocks flown around the world in 1971 and the correction GPS makes every day, the loops in Einstein's equations and Hawking's chronology protection, why time only goes forward, where the arrow of time comes from and what Ilya Prigogine thought, why you can't undo a tipping point, whether parallel universes are real, and why it's so hard to let go of the past, with my teacher, the biologist Dr. Bob Melamede, in his own voice from a 2017 talk.
The full answer
Is time travel possible?
Is time travel possible? Forward, yes, a tiny bit, and it has been measured. Backward, nobody has found a way, and everything we know about the arrow of time says you can't go back. I'm Justin Hartfield, and I've been selling cannabis for twenty-five years. The biologist who taught me most of what I know about life, Bob Melamede, built his whole view of it on that arrow.
Can you travel forward in time?
Can you travel forward in time? Everyone does, at one second per second, and the rate can change. In 1971, two physicists flew atomic clocks around the world on passenger jets. When they landed, the clocks no longer agreed with the ones left on the ground. The ones flown east had lost fifty-nine billionths of a second, and the ones flown west had gained two hundred and seventy-three, close to what Einstein's relativity predicts. GPS satellites live with the same effect every day: their clocks run about thirty-eight millionths of a second a day fast compared with ours, and the system only works because it corrects for that.
Can you go back in time?
Can you go back in time? Einstein's equations allow a few strange shapes of space and time where a path loops back on itself. Kurt Gödel found one in 1949, in a universe that rotates. Others need a wormhole held open by matter with negative energy, which nobody has found in any usable amount. Stephen Hawking suspected the laws of physics forbid all of them, and he called the idea chronology protection. Nobody has proven it either way. What we can explain is why everything you have ever seen runs only one way.
Why does time only go forward?
Pour cream into coffee and it swirls in. Wait all day, and it never swirls back out. Nothing strictly forbids it. It's just so unlikely that it wouldn't happen once in the whole life of the universe. Almost every law of physics works just as well played backward. The second law of thermodynamics doesn't. Overall, disorder goes up. That one-way street is what physicists call the arrow of time. Bob had a simpler way to say it.
Dr. Bob Melamede, in his own words
The physics that we're taught in our schools says time can go backwards. Well, are you getting younger? When's the last time you drove your car and the gas tank filled up? You know, those things don't happen. There is an arrow to time.
Where does the arrow of time come from?
So where does the arrow come from? Most physicists say: from the beginning. The universe started out extraordinarily ordered, and everything since, stars, cells, you, is part of its long run downhill. The chemist Ilya Prigogine, who won the Nobel Prize in 1977, thought that was only part of it. He argued the arrow is written into the laws themselves, and that far from equilibrium, time doesn't just wear things down. It builds them.
Why can't you undo a tipping point?
When a system tips, what decides which way it goes? Sometimes a nudge too small to measure. Heat a thin layer of oil the same way twice, and you get the same kind of cells, but never in the same places. Prigogine called this order through fluctuations. At a fork like that, more than one future is possible. Only one of them happens, and everything after it builds on that one. And you can't simply back out of a fork. When a lake flips from clear to green, cutting the pollution back to where it was before the flip usually isn't enough. You have to cut much further to flip it back. The way in isn't the way out.
Are parallel universes real?
Quantum physics has a fork of its own. In Erwin Schrödinger's famous thought experiment, a cat sits in a sealed box with a poison that's released if a single atom decays. Until someone looks, the math holds both outcomes at once. In 1957, a Princeton graduate student named Hugh Everett proposed that both outcomes happen. The universe splits, and each branch carries on without the other. It's called many worlds. Take it all the way, and there's a version of you for every choice you didn't make. Can physics rule that out? No. Nothing we know of can see another branch. And even in many worlds, once branches split, they don't come back together. The arrow holds there too. Prigogine bet the other way. To him, the future isn't sitting out there with every version already written. It's being made, one step at a time, and no step can be taken back. Nobody has settled who's right. But either way, you only ever get to live one branch.
Why is it so hard to let go of the past?
Your body works the same way. It can't un-eat a meal or un-live a fight. What it can do is let go and update, and a system you were born with, your endocannabinoid system, helps it do that. In 2002, researchers trained mice to find a platform hidden in a pool, then moved it. Mice born without the system's main lock, CB1, learned the first spot just as well, but kept swimming back to where the platform used to be.
What does that mean for you?
Here's what I take from all of it. There's no rewind. And if there's another version of me out there in another universe, we'll never compare notes. That can sound bleak. I think it's the opposite. It's the reason choices count. Lots of futures were possible. One of them is happening right now, and part of it is made of what you do next.
You can come back to a place. You can never come back to a moment. The whole film is on my site, linked below. Next: what is Bryan Johnson's Don't Die?
What the studies show
Every claim in this answer, the study behind it, who was studied, and who paid for the research. “None listed” means the paper names no funder.
