Hey it's professor Dave, let's talk about special relativity. Remember before how we said that the Newtonian paradigm breaks down in the realm of the very small and the very fast? We learned about quantum mechanics for the small, but what about the fast? To answer this, let's recall that in classical physics we learned about relative motion. Galileo developed the concept of an inertial reference frame, stating that measurements of velocity depend on the reference frame you adopt. But given that it was the 17th century, there were limitations as to the experiments he could perform. When young Einstein got his hands on Galilean relativity, he gave it a huge facelift. The part that stayed the same was the importance of assigning inertial reference frames, which we pretend are motionless while everything else is moving relative to that frame, which could be the earth, or a person, or a train. Each inertial reference frame has its own set of axes and a clock, whether real or imaginary, to measure time. But Einstein wondered what would happen if you assigned an object moving very close to the speed of light as the inertial reference frame, or even a beam of light itself. What would happen then? As it turns out, some pretty strange things. Special relativity is comprised of just two postulates, which at first glance seem very simple, but we quickly realize that in order for them to be true we have to completely restructure our understanding of space and time. First let's recall that an inertial reference frame is one where no acceleration is taking place. It must have some constant velocity, including zero if at rest, and we typically approximate the earth as an inertial reference frame. The first postulate states that the laws of physics are the same in every inertial reference frame. Whether you are standing still on earth or traveling with constant velocity in a car or plane or spaceship, the same laws of physics always apply. But the speed of light, represented by the letter c, is a law of physics. It is a constant that is used in numerous equations, so the second postulate states that the speed of light in a vacuum will be the same in every inertial reference frame. Before we brush this statement aside, let's understand how incredible it is. If you are standing still on the ground and you see a car go by at 100 kilometers an hour, you will measure the car's speed as being 100 kilometers an hour. But if you are in another car going 90 kilometers an hour on the same road and that first car passes you, you will measure the car's speed as being 10 kilometers an hour, because it is only moving 10 kilometers an hour with respect to the inertial reference frame of your car. The car has a different speed depending on which inertial reference frame you adopt, just like Galileo said. But Einstein said that light doesn't work this way. If you are standing still on earth, you will measure the speed of light as being 300 million meters per second. If you are in a plane you will reach the same conclusion. If you are in an ultra-fast spaceship moving 299 million meters per second, you will still measure the speed of light as being 300 million meters per second. No matter what you do, it's the same. Experiments have verified this, because when comparing two objects emitting light, one stationary and one in motion, they always yield the same value for the speed of light. But how can this be possible? How is c always the same and why can't we catch up to it? It's not just because we don't have the technology to go so fast, it's because the speed of light is a fundamental law of physics. It is the universal speed limit. The problem arises when we now have to try to account for these different reference frames. With the earlier example, the person on the ground and the person in the slower car measure different speeds for the faster car, and this agrees with our everyday experience. But in order for the person on the ground and the person in the spaceship moving near the speed of light to measure the same speed for light, they must be experiencing time in different ways. This is the first incredible conclusion we can derive from special relativity. Time is not some rigid detached parameter as Newton envisioned. It does not flow at an absolute rate. Time is relative. It flows at different rates for different observers. As much as this sounds like science fiction, special relativity has been verified experimentally countless times, and to remarkable degrees of precision, so this is no scam. This is how the universe works. Let's move forward and learn all about special relativity. Thanks for watching, guys. Subscribe to my channel for more tutorials, support me on patreon so I can keep making content, and as always feel free to email me:
