00:00Scott Kelly is a lot of things.
00:04An American engineer, a retired astronaut, and...
00:08a time traveler. Wait, what?
00:12Kelly commanded the International Space Station on several missions.
00:16On one of those trips, he spent 340 days
00:20aboard the giant, floating science lab orbiting Earth.
00:24While he was up there, improving the water recycling system,
00:28testing heat transfer in microgravity, and doing all the
00:32other genius-level stuff astronauts do, he was also aging.
00:36But not like you, me, or even his
00:40identical twin brother, Mark. He was actually aging slower
00:44than all of us. Okay, so,
00:48both brothers are astronauts and have been to the International Space Station.
00:52But Scott spent way more time up there. About 10 times
00:56as long, actually.
00:58And here's where things get more interesting.
01:00Even though Mark was born
01:02six minutes before Scott,
01:04he is now six minutes and five milliseconds older.
01:08That happened because
01:10Scott aged a little more slowly
01:12while he was orbiting the Earth.
01:14And astronauts age slower because
01:16they're moving much faster than us,
01:18at 17,500 miles per hour.
01:22Basically, the faster you go,
01:24the more time stretches out.
01:26So the closer you get to traveling
01:28at the speed of light, the slower you age.
01:32From Scott Kelly's perspective,
01:34nothing felt unusual up there.
01:36Time passed like normal for him.
01:38But he actually aged less
01:40than his twin brother over the same
01:42period of Earth time.
01:44So when he returned, we can say
01:46that he had essentially leapt forward in time, relative to us.
01:52So, if traveling close to the speed of light
01:54lets us leap forward in time,
01:56that means astronauts have already
01:58kind of achieved time travel.
02:00Sure, it's a bit of a letdown
02:02that it doesn't happen like back to the future,
02:04you know, hopping into a cool futuristic car
02:07and zipping into another timeline.
02:09But this is real.
02:11At least, according to some scientists.
02:15Real time travel isn't about jumping
02:18from one moment in time to another,
02:20like going back to the age of the dinosaurs,
02:22then hitting a button to fast forward
02:24to the year 2100.
02:26The time travel that's possible today
02:29is more about actually living into the future.
02:32It's about moving faster than one second per second,
02:36just like Scott Kelly did.
02:38To explain the theory behind this wild idea,
02:42we have to bring a celebrity into the conversation.
02:45Albert Einstein.
02:47Think about your childhood and your retirement.
02:50Whether it's in your past or your future,
02:52time always ticks at the same steady rate.
02:56But then along comes Einstein saying,
02:59sure, but it's not that simple.
03:02He introduced the idea that time is relative.
03:05In other words, the amount of time you experience compared to someone else
03:09depends on what you're both doing.
03:12And where exactly you are in the universe.
03:15Time is not the same for everyone.
03:18Here's an example.
03:20You're sitting on a bench holding a melting ice cream cone.
03:24Meanwhile, your friend zooms past in a super fast spaceship.
03:28For you, maybe the ice cream melts in five minutes.
03:32But for your friend flying at a crazy speed,
03:35this same scene took less time on his watch.
03:38That's time dilation.
03:40Time actually moves slower for things that are moving faster.
03:44That's the heart of what's known as special relativity.
03:50We actually proved Einstein's theory back in the 70s.
03:54Two scientists took super-accurate atomic clocks
03:57and flew them around the world on planes.
04:00First going east, then west.
04:02They compared those clocks to a third one that stayed on the ground.
04:06When all three were brought back together,
04:08they showed different times.
04:10The clock that flew east in the direction of Earth's rotation lost 59 nanoseconds,
04:17and it was moving faster relative to the ground.
04:20The one that flew west against Earth's rotation was moving slower,
04:25so it gained 237 nanoseconds.
04:29Now, think about those astronauts living in space moving way faster than we are.
04:35They travel at about 5 miles per second,
04:38orbiting Earth roughly every 90 minutes.
04:41If you do the math,
04:43spending 1,000 days up there lets you skip about 0.027 seconds into the future.
04:50I know that doesn't sound like much,
04:53but that's because the speed of the ISS is tiny compared to the speed of light.
04:58Still, if we keep going with that idea,
05:01it pretty much makes the astronaut Oleg Kononenko
05:04humanity's greatest time traveler.
05:06In 2024, he set the world record for the most time spent in space,
05:11878 days in orbit.
05:15Okay, so we now know that Kelly and Kononenko are, in a way, living in the future.
05:22But what about traveling backward in time?
05:25Well, I'm sorry to say, traveling back to your glorious high school days is a whole lot trickier.
05:32In theory, we could.
05:34But in practice, it's pretty much impossible.
05:37Imagine space and time as this big, stretchy fabric.
05:42And gravity is what bends it.
05:44Kind of like when you put a heavy bowling ball on a trampoline.
05:48In theory, if you could bend it just right,
05:51you might create a shortcut through space and time called a wormhole.
05:55A tunnel that connects two different points.
05:58Let's say the present and the past.
06:01Now, usually, gravity pulls things inward.
06:06It wants to collapse tunnels, not keep them open.
06:09So the walls of this tunnel are constantly under pressure from the universe,
06:13trying to squish them shut.
06:15To stop that from happening, and to keep the tunnel open long enough for something,
06:20or someone, to go through and get to the past,
06:23you'd need something that pushes outward, against the collapse.
06:28And that's where negative mass comes in.
06:32Negative mass is a weird theoretical idea in physics.
06:36Unlike regular mass, which pulls things together with gravity,
06:40negative mass would actually repel things.
06:43It would push away instead of pulling in.
06:46It's like using anti-gravity to keep the tunnel from closing.
06:50It would act like braces holding the tunnel open from the inside.
06:56But here's the problem.
06:58We have never actually seen negative mass.
07:01It's just something that might exist on paper, in math.
07:05But no one's ever found any in real life.
07:08Plus, there's something called the self-consistency principle.
07:12Basically, it says that time travelers can't create time paradoxes,
07:17like those crazy scenarios in the Butterfly Effect movie.
07:22Let's say you go back in time to stop your granddad from meeting your grandmother.
07:27But here's the problem.
07:29If they never met, that means your parents were never born.
07:32And if they weren't born, well, how do you exist?
07:37That's a time paradox.
07:40Something that causes all sorts of inconsistencies in the universe,
07:44making everything a total mess.
07:47And according to the self-consistency principle, that's impossible.
07:51It just can't happen.
07:53Some specialists, though, think they have an answer to the grandfather paradox.
07:58All we need to do is think about time as an endless loop with no starting point.
08:03In a closed time-like curve, as this theory is called,
08:07the past and present blend together in an eternal time loop.
08:11There's no way out. There's no changing things.
08:15It would all look the same if played in reverse.
08:18For example, what did you do yesterday?
08:21Let's say you woke up, brushed your teeth, worked all day, had your meals, and then went to bed again.
08:29Following the closed time-like curve theory, if you could go back to yesterday,
08:34you'd do those exact same things in the same order, no matter how hard you try to take the day off.
08:42Because in a closed time-like curve, you're attracting not just your steps,
08:47but every single second of reality itself.
08:51And that's the only way time travel could be possible.
08:54So, even if we somehow manage to build a time machine,
08:59and even if we give it our best shot to change the past,
09:03nature will always find a way to prevent contradictions.
09:07In the end, everything would have to align perfectly to keep our history from becoming a mess.
09:15That's it for today.
09:16So hey, if you pacified your curiosity, then give the video a like and share it with your friends.
09:21Or if you want more, just click on these videos and stay on the Bright Side!
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