00:00Imagine you built a time machine. You go back in time to stop your grandma from ever meeting your
00:05grandpa. Maybe you have your reasons. Let's say it works. But then something impossible happens.
00:09You're still here. This is called the grandfather paradox. Physicists have been arguing about it
00:14for decades. And yeah, a surprising number of them have seriously thought through the logic
00:19of going back in time and making their own existence impossible. Most of them ended up
00:23concluding that this paradox proves time travel can't be real. But some scientists started
00:28wondering, what if the paradox just fixes itself? Spoiler, the universe probably won't let you do
00:34it. Lucky for grandpa. In classical physics, Isaac Newton had a pretty simple idea of time. It flows
00:42the same for everyone. Like one giant cosmic clock. Then in 1905, Einstein showed up and said, yeah,
00:50actually, no. In his theory, space and time are one single thing. Space-time. And time doesn't tick
00:56the same for everyone. If you are moving really fast, close to the speed of light, your clock runs
01:01slower. This is called time dilation. And it's not sci-fi. We measure it every single day. Take GPS
01:06satellites. Their clocks have to be constantly corrected. Because of their speed, they run a
01:11little slow. 7 microseconds per day. But because they are higher up with weaker gravity, they run a
01:16little fast. 35 microseconds per day. Net result? About 38 microseconds every day. If you didn't account for
01:23this, your GPS would be off by miles. So yeah, every time you use Google Maps to find the nearest
01:28coffee
01:28shop, you are relying on Einstein's theory of relativity. Okay, going forward in time. Easy.
01:36Just wait. One second. Boom. You are in the future. But can you go backward? Physics actually allows for
01:43some pretty weird possibilities. The most famous one. Wormholes.
01:49In 1988, physicist Kip Thorne, Michael Morris, and Yulvi Yurtsever showed that Einstein equations allow for
01:56tunnels through space-time. And here's a fun fact. That paper happened because of Carl Sagan. He
02:01wrote the novel Contact and sent the manuscript to Thorne. In the story, the main character travels
02:06through a black hole. Thorne read it and said, the physics doesn't work. He suggested a wormhole instead.
02:12Think of it like a piece of paper. Two points of it are far apart. But if you fold the
02:16paper,
02:17they're right next to each other. Now poke a hole through both layers. You've got a shortcut. That's
02:22basically a wormhole. A tunnel through space-time. The problem? These tunnels are incredibly unstable
02:27and would collapse instantly. To hold one open, you'd need something called exotic matter. This
02:33already sounds suspicious. Hypothetical stuff with negative energy. Quantum physics hints this might be
02:38possible at tiny scales, like in the Kazimir effect. But to hold a wormhole big enough for a
02:43human, you'd need more of that energy than we can ever imagine collecting from the entire visible universe.
02:50There's an even stranger possibility. Close time-like curves. Sounds like a pro-crock band name. But it's
02:56real thing in Einstein's equations. Simple version. Imagine a road. You're walking forward, but the road
03:01curves. And eventually loops back on itself. From your perspective, you're always moving forward. But you end up back at
03:08the
03:08same point in time. This brings us back to the grandfather paradox. You build a time machine,
03:13go back, find your grandpa before he met your grandma, and make a very bad decision. If they never
03:18met, you're never born. But if you're never born, who went back in the first place? In the Back to
03:23the
03:23Future, they handled this by having Marty slowly fade away. Which honestly doesn't solve the paradox at all.
03:29If he was never born, he'd vanish instantly. But if he vanished, who stopped his own birth? It's a loop,
03:35but hey, it's a movie. People often say it's impossible to write a time-travel story without
03:39paradoxes. That's probably true. Sometimes writers use parallel universes to get around it. Which helps,
03:45kind of. But in 2020, physicists Germaine Tobar and Fabio Costa mathematically showed,
03:50even if closed time loops exist, events can be self-consistent. This is a model, not proof about
03:56real space-time. What does that mean? Not that the universe magically protects grandpa. It means that
04:01according to the equations, the chain of events would arrange itself so the paradox never actually
04:06happens. You gun jams. Grandpa runs. Or maybe it turns out he wasn't your actual grandpa. Or maybe
04:12grandpa shoots you first. Now we're getting into quantum physics, where your intuition starts to
04:19shock it. One of the most famous effects here is quantum entanglement, predicted by quantum mechanics
04:25back in 1935. And Einstein absolutely hated it. He called it spooky action at a distance. But today
04:31we know it's real. Quick disclaimer. Nothing I'm about to describe is actual time travel. It's more
04:36like an almost a statistical trick with quantum correlations that look like fixing the past. No
04:41information actually travels backward. The laws of physics don't allow it. Here's an analogy. You send a
04:46friend a gift on the first, so it arrives on the third. But on the second, you find out he
04:51needed
04:51something totally different. Normally, too late. But here's the quantum trick. You have two entangled
04:56particles, created together so these states are linked. You send one into an experiment. You keep
05:01the other. On the second, you measure the one you kept. And that measurement retroactively influences
05:06what you'll see in the results of the first one. Can you actually send information to the past?
05:11Short answer, no. Not with any known experiment. Causal T holds. But quantum theory shows that the
05:16relationships between time, information, and measurement is way more complicated than classical
05:21physics ever suggested. Maybe that's where new physics will show up someday. Maybe one day you'll
05:26be able to send your past self the winning lottery numbers. Speaking of, what would you send yourself?
05:30And would you do it knowing the consequences could be totally unpredictable?
05:36Let's try a simple experiment. Fill a glass of water. Drop in a little fruit die. At first it's a
05:41neat
05:41little drop. After a minute, it's pressed through the wall glass. Now imagine playing that video
05:45backward. The die pulls itself back into one spot. Your brain immediately says, nope, that doesn't
05:51happen. Here's what's wild. At the level of fundamental physics, it's not actually forbidden.
05:55The equations governing atoms are nearly timed symmetric, with rare exceptions in nuclear physics
06:01that don't affect everyday stuff. Swap time for its reverse in those equations, and they still work.
06:06Theoretically, if you could perfectly reverse the velocity of every single atom in that glass,
06:11the wall system would run backward. Physics allows it. But here's the problem. It's statistical. That
06:16glass has around 10 to the 23rd power water molecules. For the die to pull back together
06:21on its own, all of those molecules would have to randomly start moving in perfectly coordinated
06:26directions at the same moment. The odds are astronomically small that this basically can't
06:32happen in the entire lifetime of the universe. That's where entropy comes in. The number of possible
06:37microscopic arrangements a system can be in. Then the die is one drop. There's a very few arrangements
06:42that look like that. When it's spread out, there are astronomically more. So systems naturally move from
06:48less likely states toward more likely ones toward disorder. That's the arrow of time. This isn't hard
06:54law of mechanics. It's a statistical one, but it has a deep root. The universe started in a state of
06:59incredibly low entropy. That initial condition is what gives time its direction. We're moving from that
07:04uniquely ordered beginning toward increasingly probable states. Think of it this way. Imagine a
07:10box divided in half. Gas on the left, empty on the right. Remove the divider. Could the gas pull back
07:15to the left on its own? Mechanics doesn't forbid it. But there are astronomically fewer ways for all the
07:20gas to be on one side versus spread out. So almost any random movement leads to mixing.
07:28So what do we actually know about time travel in 2026? Forward in time? Yes. Already confirmed
07:34experimentally. Backward in time? Theoretically possible in certain models of space-time. But
07:39whether it can work in reality, we generally don't know. And it's possible the laws of nature are set
07:45up to protect causality. Or maybe time travel is possible. And if someone already went back, we'd have
07:50no way of knowing. Because for us, it would always have been part of history. Maybe there are parallel
07:55realities where you did send yourself that message and hit the check-board. But that's a story for
07:59another time. That's all for now. Bye.
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