Luyện nói tiếng Anh bằng Shadowing qua video: Why Gravity Is A Lie, explained in Zero G

This plane is about to send me into zero gravity  for 10 minutes. And I'm going to try to explain why gravity is a lie while I go without it for  the very first time... "Gravity. Gravity. Gravity ain't real. We don't actually need gravity. Why  is there gravity? Why does it work?" Turns out that gravity is one of the most controversial  subjects in all of physics. This is not some philosophical debate. A huge amount of the tech  that you use every day all depend on our current limited understanding of gravity. If we can  understand this a little bit better, we can build more things that are huge if true. So, what  really is gravity? In this video, we're going deep to learn how it truly works together. And we're  going to do this in four levels starting now!

Right now, I'm in France and I'm  about to get on that special plane.

I've been invited by the European Space Agency  as they run experiments for their real upcoming space missions. They do like two of these a  year. They invite one civilian each time. It's a really big honor. I'm also really nervous.  It's going to be great. They call this plane the "Vomit Comet." I'm really hoping that I don't  get sick. And as we're getting ready to take off, I am realizing how little I know about this thing  that's been holding me down my whole life. Here's what I thought I knew about gravity from high  school physics. We'll call this level one. So, what I learned in school is that gravity is  what holds me down on Earth, but it's a force that objects with mass exert on each other. So,  the Earth pulls me toward it, and I in some tiny way pull back on the Earth, too. Same thing with  the sun. The sun pulls on the earth and the earth in some little way pulls back on the sun too. And  the math that describes this is beautiful. It's just the relationship between the mass of one  thing times the mass of the other thing divided by the distance between them squared. And g is a  constant. Don't worry about it. So this equation tells you how strong that pole will be. But until  today, Earth has always pulled me right down into the ground which stops me. So to experience  the raw unfiltered effects of Earth's gravity, I need to go into freefall, which is exactly  what this plane does. It's going to rise up and then plummet toward the Earth. This is the part  that's making me nervous. And as it does that, I'm going to be falling. And the plane is going  to be falling around me. And during that fall, I should feel like I'm floating,  like a skydiver or an astronaut, which is why real astronauts train and run  experiments on these planes. But hang on, why is rapidly falling anything like peacefully  floating? Think about it this way. So the Earth's gravity creates a pull at 9.8 m/s squared or 1g.  If you're in a rocket out in space and that rocket is accelerating at exactly 9.8 m/s squared, you  wouldn't be able to tell the difference between standing on the ground and flying in that rocket.  Einstein called this the equivalence principle.

And honestly, it sounded pretty intuitive to me  at first. And then the more I thought about this, the more it started to just break my brain.  Because if gravity and acceleration are the same, it means that being in freefall without air  resistance is the same as floating in space because they're both zerog, right? Yeah. Remember  this idea. It's going to be important later.

But right now, I don't totally understand this  yet. So, I'm going to go try it. Let's do this.

Okay, so we just pulled up. I'm now feeling  increased gravity. You might be able to tell in my face. Woah! This feels like being in a car and you're accelerating as we shoot up. The acceleration that I'm feeling is  about the same as being on a planet with twice Earth's gravity or 2g. I'm feeling squished on the  floor. And then when we hit this point, oh my god.

Tell us how you feel. I feel I all of  my thoughts just left my head. I feel zero G feels so weird. It's not just your  outsides, it's your insides, too. So, your stomach is moving around. And your  blood isn't getting pulled down from your head like normal. So, you feel like there's  more pressure inside of your skull. And then touching something will move you way farther  than you think. Lay back down. Oh my god.

It goes so fast. Keep your head still. Sit up.  That felt unbelievable. I don't even know what just happened. It's like nothing I've ever  experienced before in my life. Did you enjoy it? I think so. I'd like to try it again. Would  you like to get out? No. I'd like to do it again, please. We did it again. 1 G. 2 G. 0 G 2G 1g's  again and again and again. To see this happening, just watch what happens when I throw this  ball. This is 2G. I'm about to go into zero G.

This is Einstein's equivalence principle  in action. I know that I'm falling in this plane. But I feel nothing. I don't feel like I'm  falling. I feel like I'm floating. But here's the craziest part. It's not just like I'm floating  in space. It is exactly the same thing. Yeah.

This is where things start to get weird. You  will look around and everything will look like it's weightless. In reality, you're inside  a plane and you might think to yourself, "Well, this is some sort of an illusion." What Einstein  realized is that situation of you in the plane and that situation of an astronaut deep in space  are equivalent. They are identical. It's not that one looks like the other, but they are the same  situation. If we covered all of the windows, there is nothing that I could do, no test, no experiment  to prove that I'm falling in a plane and not floating in space. This is exactly what Einstein  was talking about. And actually, astronauts orbiting Earth aren't really floating. They're  falling too. They're literally falling towards the Earth all the time. And they're just cruising  so fast that they miss the horizon and they never crash into the earth. If you were to stop it from  cruising so fast, it would drop like a stone right down to the earth. So they are falling all the  time. They are experiencing weightlessness all the time. Okay, fine. So falling is floating. Why does  that actually matter? It matters because it means that this plane actually made gravity disappear.  Gravity is gone. It's not there. How did gravity disappear? Well, it was never really there to  begin with. Yeah, the gravity that you experience on Earth isn't what you think. We're about to  enter level two. But first, I've done six parabas up and down. I'm feeling a little bit nauseous.  I got to be honest. Just take some time, relax, sit against the wall, chill out. Okay, while I  do that in the plane, let's talk about level two.

That version of gravity that I learned in  school, the masses pulling on each other version, right? That came largely from Isaac Newton and  his way of thinking about the universe persisted, largely intact for hundreds of years. But then  Albert Einstein came along and he had a new thought that caused our entire view of space  and time to wobble. Look how cool this is.

But before I explain how, what I'm building right  now is from our sponsor, KiwiCo. I'm so excited because this is such a perfect fit for this  show. When I want to learn about something new, I always try to find a hands-on way to do  it. Whether it's going zero G in a plane or finding dinosaur bones or closer to home, building  something that actually helps me really understand and just have fun doing it. Like look, I got to  paint all of them. Here's my Earth. Here's my Mars. I really enjoyed trying to paint a ring on  Saturn. Cubic crates are packed with projects for kids that explore real STEM principles in a fun  and engaging way. Crates arrive monthly with each one diving into a different science or engineering  concept, so you're always building something new.

This one that I'm building right now is a  model of the solar system with, wait for it, that's awesome. This is perfect for that  tiny astronaut in your life who wants to learn about the solar system and the history  of space exploration. And wait, hold on. Here's another one that I'm really excited about that's  a bit more advanced where you wire up a set of real headphones and along the way you learn about  how circuits work and how sound actually travels.

It's a really good introduction to both physics  and engineering. Look how cool these look. Plus, I have a few more that I've set aside to give  as gifts to the kids in my life that I love. So, if you want to try it, go to the link in my  description or use my code cleo for 50% off your first monthly crate at kiwiode.com/cleo.  3 2 1. Now, how did Einstein make our whole understanding of the universe wobble? The  important thing to remember is that scientists, especially physicists, are lazy people. That's  the most important thing. And what I mean by that is that we want the minimal number of  equations to explain the most stuff. The holy grail here would be one equation or theory to  describe everything. But we're not there yet.

A few decades before lazy Einstein, lazy James  Clerk Maxwell had realized that he could distill all of electricity and magnetism down to a  single thing described in just four equations, electromagnetism. And those equations found that  light was an electromagnetic wave. And these waves come in different energies across a spectrum. And  weirdly, all light had the same constant speed.

And I think you can guess what that speed is. It's  the speed of light. So here we have an enormous uh profound implication that shocked everybody.  That's insane. It's nothing has a constant speed but makes no sense. But it's not just some  equations. We have tested this. This is incredibly widely tested. These are facts we can  experimentally confirm. Everybody was confused by this because up until then to measure something's  speed you divided how far it goes in the time it took to get there. But if light is always going  the same speed no matter the distance or the time then there's something wrong here. Maybe there's  something wrong with what we mean by distance and time. Or if not wrong, maybe there's something  relative. This was Einstein's huge if true moment.

He realized that if light is always the same  speed, then distance and time have to be able to change. Going back to our original Newtonian  description of gravity, this part was distance.

But if distance isn't stable, what is gravity  anymore? It was so beautiful and now the whole thing is breaking down. Hang with me here. I need  a little break. Remember when I said they called this the vomit comet? I did throw up. Didn't get  that on camera. Feeling much better. John, what did you just call what I just did? Tactical cher.  A tactical chuter. Just a tactical cher. Okay, back to my guy Einstein. So he goes back to the  drawing board to reformulate not just gravity but basically everything else. He spends the next  decade struggling with this relative nature of space and time and like Maxwell unified  electricity and magnetism. Einstein finally emerges with a new model that unifies space and  time. Spacetime. In Einstein's new equations, spacetime gets warped by stuff like our sun  or earth or a black hole or even you. In some infinite decimally small way, you're warping  space and time around you. I love that. Now, think about what that means. Let's say I give you  a little push and you go floating off into space.

If we zoom out and look at your path through  spaceime, we'd see that it's not actually flat.

There's all of this stuff warping spaceime around  you. So, you're going with the flow. And the main bit of stuff near you is Earth. And look, as  you follow your path, you're drifting toward its surface. It looks like you're experiencing  gravity. But crucially, there isn't any force pulling you in. It helps me to think about it  this way. You have two people at the equator and you tell them to walk north. They appear to come  together, but there's no actual force causing them to come together. It's just the simple fact that  the Earth is curved. That's all gravity is. Don't think about it as a force. Think about it as the  geometry of spacetime around stuff. And actually, this classic view of flat warped spacetime is  also wrong. This is all just a visualization, but I try to imagine it more like this. But hang  on. Is this just semantics? Why does it matter how I describe gravity? Like Newton's theory says that  gravity looks kind of like this and Einstein's theory says it looks more like this. But same same  but different, right? Why does it matter whether I think about gravity as a force that literally  pulls things or as the curving of spaceime? I think that's a great question. The reason why it  matters is because Newton's law which describes this mass pulling and that mass pulling as you  described, it's not exactly correct. Einstein's spacetime leads to genuinely different predictions  than Newton's forces. If we still thought about gravity like Newton, we wouldn't have accurate  satellites or planes or Uber. If you're calling Uber or Lyft and you're on a street corner,  you would be off by kilometers if you did not correct for the relativistic differences between  where the satellite is that's receiving and sending the signals and your location down on the  Earth. So, it has an impact in our daily lives.

Now I'm back. Your face whenever you land is  brilliant. After everything that we've learned, what is it that I'm feeling right now? Well,  Einstein would say that you're just on a space-time path. Imagine for a second that the  Earth isn't there, but somehow the distortion in spaceime is. So, you keep moving along your curved  path. You're experiencing the same phenomenon as me floating in the plane. Gravity has disappeared.  We're back to zero G. It turns out the only reason we ever feel gravity is when something stops us  in our space-time paths. So if the only thing that makes you feel gravity right now is Earth stopping  you, wouldn't that mean that gravity is sort of fictitious or fake? Is gravity a fictitious force?  Bingo. That's the important part. That's what you got it. This is why gravity was never there to  start with. It's it's it's fictitious in that sense. Yeah. Gravity is a lie. In that case,  I buy it. I'm I'm with you. Except if gravity is a fictitious force, how do you explain what's  happening in empty space between the sun and the Earth? Now, we're at level three. If the goal  for physicists is to use as few equations as possible to explain the most amount of stuff,  they've done a really good job boiling things down. If you go back to your textbook and you talk  about forces in the universe and the picture used to be really complicated. There's this force,  there's that force, but what we really know is you can boil everything down to four. The first  two mostly just operate within atomic nuclei.

The third is the force from charges attracting or  repelling each other. And together they make up almost every force that you experience in your  daily life. And scientists now agree that all three of these actually fit together. Physicists  have been able to basically write those forces in the same sort of mathematical language. It's  it's the language of quantum mechanics. Quantum mechanics is awesome. It is our most accurate  and successful model of the universe ever. It describes our universe as chunky with these tiny  discrete or quantized particles. But it still can't describe what's happening here because  which force is at work here? It's empty space.

It's none of these three. So, physicists had no  choice but to describe what's happening here as a fourth force. Uh-oh, here comes gravity.  We've just had this long conversation about how gravity isn't really a force. And now  what you have done is you've stumbled into the biggest problem of 20th century and now  21st century physics. Hang with me here. We are getting right to the cutting edge. But there's  a really big problem because these two, they predict different stuff. They are in conflict.  Gravity would make predictions that say quantum mechanics is wrong. Quantum mechanics would make  predictions that says, you know, relativity is wrong. And that sucks because if we could just  combine the two, we could do so much more.

So why can't we just do that? What what is  the problem? I mean, why why can't I write gravity in the same way? Because unlike the  other forces, I can make gravity disappear, right? In the plane. Yeah. In the plane, gravity  disappears. With the other forces, you can't make them disappear. Part of the reason we can't unite  them is, do you remember those quantized particles in quantum mechanics? Gravity doesn't seem to  have one. We can't find any chunks of gravity. So, our situation right now is this. We have one set  of math for the little stuff or the quantized particles and we have another set of math for big  stuff that's smooth or continuous. But the holy grail for physicists is trying to combine these  two. This is the theory of everything. We're now at level four. Okay, here are the options that the  smartest people on Earth have come up with for the theory of everything explained in simple terms.  Option one, you basically shove gravity into quantum mechanics. That would probably require  finding that missing particle for gravity, a graviton. But unfortunately, it doesn't work. The  mathematics does not work. For one thing, even if you did find a particle, how would a particle just  disappear like gravity does? So most scientists are exploring option two. You stick them together.  The mathematics that people are trying has become really quite complicated to stick these things  together. There are a bunch of ways to try to stick these things together and you might have  heard their names over the years. This is string theory, M theory, loop quantum gravity. The most  important one to know about is this paper. This is the most cited paper in all of theoretical  physics. The author seems to have found a thread of a connection between the tiny quantum stuff  and the big continuous gravity stuff. And it's the first time that we've made this connection, but  it is so complicated that I couldn't understand it by reading the paper. So, I just called him  up. The objective I had when writing the paper was to understand gravity. What he came up with  is at the edge of theoretical physics. If you started this video at level one and you end just  knowing the name of this theory, that's awesome.

It's called ads CFT and the concept is in the  name. This part describes the big continuous gravitational forces in our universe and this part  describes the small chunky quantized forces. But what's really exciting to physicists is weirdly  this slash in the middle because what this slash represents is the fact that the two ideas  are actually connected here mathematically.

It looks like uh gravity has something to do with  the entanglement or correlations that exist in the in the system of particles that we are discussing.  This theory describes a thread that might connect the two. But it is very complex and makes some  simplifying assumptions and has parts of it that are still unsolved. But it's a glimpse of what  might become a theory of everything. But we don't yet understand it. If we understand it, we'll be  able to explain more aspects of the universe. And this is where I just start to dream big. Because  if we can fully understand gravity, who knows what we could build? I'm imagining flying cars and  close to lighteed travel and hoverboards. And I don't know, maybe it's farfetched to think  that one day we could manipulate gravity, but if we never understand it, we never will. So  physicists are trying to strengthen this thread to maybe one day be able to explain the entire  universe. That was incredible. You doing so well, you're a natural. I'm going to remember this for  the rest of my life. So we started with Newton and then Einstein and then quantum mechanics and  now the theory of everything. And with every step, we're getting just a little bit closer to  understanding both ourselves and everything around us. That's what this flight was really all about.  And yet, even with all of this incredible science, it feels like we're just at the beginning.  Solving this big mystery won't be easy. And it's not guaranteed. This is a physicist's dream.  And the universe doesn't have to play ball, right? We've had this feeling in our gut. Oh,  yeah. The universe at heart must be beautiful, must be simple. Maybe it's not. Maybe it's beyond  our ability to understand. I hope not. After all, it wasn't so long ago that humans figured out  planets and atoms and so much more. We can do this. And as you're watching this, thousands  of scientists here and all around the world are working to better understand our universe so that  we can all build what comes next. Huge if true.

And finally, we're making three more  videos that you might love. Taking you inside of the cockpit of this crazy plane  to explain how a zerog plane actually works, showing you the coolest experiments on this plane  with me and taking you behind the scenes into what I think might be the most interesting  experiment in the world to show you how we proved that gravity has waves. For those and more  optimistic science and tech stories, subscribe.