It’s 2015. We got pictures of Pluto that look like this. Doc Brown and Marty have arrived from 1985. And it’s the 100th anniversary of general relativity! Welcome to the Science Asylum. I am Nick Lucid. In a previous video, I gave you an overview
of what Einstein had to say about gravity and you told me three and a half minutes wasn’t long enough. Remember, you asked for this. General Relativity says massive things will curve paths that would normally be straight lines, but that can be really confusing if you’ve never seen anything like this before. So let’s start with a simpler question: What’s relativity? To understand that, we really need to go over some history. It all starts in 1632 with a crazy Italian guy named Galileo. While he was busy sticking it to the man a.k.a. the Vatican, he suggested, amongst other things, that all physical laws should be the same in every inertial reference frame or IRF. What’s a reference frame? I guess that does sound like some complicated science thing, but it really isn’t. Typically, physics uses a coordinate system or frame to refer to measurements. So your reference frame is just the coordinate system attached to your body. It’s your physical point of view. So what’s it mean for that to be inertial? Physically, the word “inertial” just means that motion stays the same, so an inertial reference frame is just a coordinate system that moves with a steady speed in a straight line, but let’s give this some context. Here, we have AwkwardM standing on the ground and Chauffeur Clone driving a car. Both have coordinate systems with steady motion and both completely agree about what forces are involved. The laws of physics are the same. This has a weird consequence though. They don’t agree on who is moving. AwkwardM would say the car is moving, but Chauffeur Clone would say AwkwardM is moving. No no no, AwkwardM is on the ground, so she’s the one that’s sitting still, right? False! The ground is moving due East at 770 miles per hour since the Earth is rotating. The Earth also goes around the Sun at 65,000 miles per hour. I could go on, but I think you get the point. The important thing to understand here: It’s not just that you can’t tell who’s moving and who’s sitting still. Inertial motion is relative so both AwkwardM and Chauffeur Clone are correct. All measurements of motion are correct even if they disagree. That’s relativity! And it’s not just limited to the motion of the frames. If Chauffeur Clone is driving at 20 miles per hour and then throws a ball at 30 miles per hour, AwkwardM will see the ball moving at 50 miles per hour. But the clone only sees the ball moving at 30 miles per hour. But how does Einstein fit into all of this? OK, this kind of relativity is known as Galilean relativity. It’s another 273 years before we get to Einstein. There’s way more timeline to go through. The story is important, OK?! He solved a major problem 273 years later. Back to the timeline, crazies! So, 1632, relativity was born and then Pope Urban the 8th put Galileo under house arrest for the rest of his life. From 1665-1667, Cambridge closed due to an outbreak of bubonic plague and a rebellious undergrad college student named Isaac Newton was sent home. That gave him the free time to develop his laws of motion, calculus, some optics stuff, and his universal law of gravity; which was far from universal even then. He published none of it until 1687 because he didn’t want anyone else to know how to do it. Selfish doo-doo-head! Anyway, everything was going just fine until some jerk-face named James Maxwell showed up in 1865! He discovered that, not only was light an electromagnetic wave, but also, that this meant the speed of light was a universal constant. What what?! Yeah, this is a serious problem. How is that a problem? Sure. Maybe if you just think about it for a second, you’re like: Meh. Whatever. But we don’t do any of that shallow thinking here in the asylum. Asylum Crazies think deep! Remember AwkwardM and Chauffeur Clone? AwkwardM would say the car is moving, but Chauffeur Clone would say AwkwardM is moving. Remember, both are correct. That’s relativity. and it makes perfect sense if we're talking about a ball. But what happens if we try this with light? Say you’re in a spaceship zipping past my space station at half the speed of light and then you turn on its headlights. You would see the light move forward at the speed of light, but do I see it move forward 1.5 times the speed of light? No! I also see it move forward at the speed of light. The speed of light is a universal constant, so everybody has to measure it exactly the same, no matter where they are or what they’re doing. Houston. We have a problem. Galileo says “All motion is relative.” And then, 230 years later, Maxwell’s all like “Well, not for light.” So now we have two competing theories of motion. Is Galileo right? or is Maxwell right? The debate raged on for four decades. Suggestions of universal reference frames, measurements of the speed of light in multiple directions, and then finally, in 1905, Albert Einstein showed up on the scene with 5 papers: Measurement of the Mole, Brownian Motion, Photoelectric Effect, E equals M C squared, and a crazy but brilliant suggestion for our problem: What if Galileo and Maxwell are right? The only way they can both be right is if we’re wrong about everything else. Completely wrong? No no, just a little wrong. If the speed of light is the same for everyone, then nearly every other measurement must be relative: Time, Distance, Length, Energy, Momentum. They must all depend on your point of view. We call this “Einstein’s Special Theory of Relativity” or “Special Relativity” for short because it only works in a special case: When you’re taking measurements in an inertial reference frame. If you want to take measurements in an accelerated reference frame, one that isn’t moving steadily in a straight line, then we’re going to need a more “General Theory of Relativity” or “General Relativity” for short. But we have a minor issue. Physics isn’t built on the things that are relative. It’s built on the things that stay the same, like the laws of physics between reference frames. We’re quickly running out of things that stay the same. Einstein solved this problem too, but it took him another decade. What happens in accelerated reference frames? Hmm, let’s take a look at one! Are you sure you've got this? Yes yes, I got this. Alright. Let’s say there’s a ball hanging from a car ceiling. When the car is sitting still, the ball hangs straight up and down. There’s a force of gravity pulling down and a force of tension pulling up. But if it accelerates, the ball swings toward the back of the car. AwkwardM standing outside doesn’t see anything all that strange. The car is accelerating forward and the extra tension is what also accelerates the ball forward, so everything makes sense. In the car we have a problem. Everything in there looks like it’s sitting still. There’s no acceleration. So the only explanation is there must be a new force on the ball. So what’s the new force? In all my merry-go-round videos, I called it a fictitious force because it doesn’t exist from all points of view. It exists inside the car, but not outside the car. A force is just an interaction between two or more objects. The tension is between the ball and the string, and gravity is between the ball and the Earth, But where does this new one come from? It doesn’t appear to come from anywhere, so we say it’s fictitious or fake. But is it really fake? Also, why are we treating inertial reference frames like they’re more real than accelerated ones. Shouldn’t everyone be on equal footing either way. These are very good questions and the same ones Einstein was asking himself. Back to the timeline! So, 1905, miracle year of physics! A couple years later, good ‘ol Al started really thinking about acceleration. It occurred to him that if you were inside a spaceship accelerating at 22 miles per hour per second in a shake-proof sound-proof room with no windows, it would feel exactly as if the spaceship was sitting on the ground. We call this the equivalence principle. More often than not in physics, if two things are indistinguishable, then they are exactly the same thing and it’s not just limited to spaceships. Remember this ball hanging in the accelerating car? Yeah, that fake force is actually gravity. There’s now a gravity pulling down and another gravity pulling backward. So the total gravity points this way. It kind of defines the new down. It isn’t any different than if the car was sitting on a hill. Same exact physics, therefore, same exact thing. Wait, you said that new force was fake. If it’s actually gravity, doesn't that make the gravity with the Earth is also fake? Yep! Gravity? Not a real force. So how does an acceleration in one frame look like a force in another? For that, back to the timeline! In 1908, a guy named Hermann Minkowski invented a 4D frame called spacetime. Einstein wasn’t a fan but, by 1912, he knew his own approach wasn’t working, so he had no choice but to accept Minkowski’s. The math was a bit beyond him though, so he consulted some math friends and, in 1915, he finally figured it out after collaborating with David Hilbert. General Relativity was finally a thing. The solution: some accelerated reference frames are inertial reference frames. Before you freak out, let’s review. Remember, physics isn’t built on the things that are relative. It’s built on things we call invariants. Those are the things that are the same from all points of view. Let’s make some lists. I love lists. Relative things: length, distance, time, mass, energy, motion, the list goes on. Invariant things: the speed of light, spacetime paths, electric charge and... I think that’s it. In Part 1 of this series, we saw a car moving with steady motion in a straight line. Let’s take a look its spacetime diagram. As it moves through space, it also moves forward in time. Represented by this slanted spacetime path. That path is invariant, so it’s measured the same by everyone, but, separately, space and time are relative. From the clone’s perspective, he’s sitting
still in space and only moving through time. On the spacetime diagram, you can’t change the path. It’s invariant, so you have to change the frame itself. The clone’s time axis lines up with his path, therefore, no moving through space. The accelerating clone from this video has to be treated the same way. His spacetime path looks like this, but he also thinks he’s stationary, so his time axis looks like this. It’s not straight. It’s curved. Unfortunately, we can’t see the curvature. We always see the frame as straight, even when it isn’t. This makes things look a little weird. And the equivalence principle said accelerations are exactly the same thing as gravity, so gravity isn’t a force at all. Gravity is just an illusion created by our perspective. It’s the result of a curved or warped frame. You think this squirrel is falling toward the Earth speeding up at 22 miles per hour per second? Nope, it’s moving at steady speed in a straight line through curved spacetime. You think the ISS is orbiting around the Earth in a circle? Nope, steady speed, straight line, curved spacetime. Hopefully, this cleared some things up for many of you and maybe made your brain hurt a little? It’s good for you every now and then. If you have any questions, please ask in the comments. I have a feeling you’re definitely going to need a comment response video for this. Thanks for liking, sharing, and subscribing. And until next time, remember, it’s OK to be a little crazy.
