So when Einstein thought about physics and what happens on moving platforms like a [UNKNOWN] rocket ship versus stationary platforms. He was able, because of his genius, to see something that no one else saw before. That these usual rules that we expect from moving, not very close to the speed of light have fooled us in some sense. And what Einstein really what he was really bothered by was he tried to imagine what it would be like to ride a light beam. You know we've, we've already seen light is a a, a wave of electric magnetic energy moving with the speed of light. And what he understood was, or what he found was there was a paradox. There was a paradox as he tried to imagine what it would be like to look at a light from a, from someone, standing on the ground. And watching light going by, or if you could ride the light wave. And it was, that is what led him to really change his and our understanding about the way space and time are coupled together, how they work together. In order to do this Einstein had to change physics such that he invented or came up with two new postulates. And out these postulates he was going to build an entirely different physics. So let's look at these postulates. The first one is that the laws of physics are the same in all, what are called inertial frames of reference. So, these are frames of reference with no accelerations occurring on them. So your rocket motor can't be on. You have to just be cruising through space, you know, the motor's turned off. You can't be orbiting either because in an orbiting frame of reference. You still are accelerating because, you're, you're orbiting. So what Einstein said was that, in these inertial frames, all the physics has to be the same. You shouldn't be able to tell the difference from one inertial frame to the other. Any laws of physics you have should be the same. Every observer, basically, can argue that he or she is at rest while all the other observers are in motion. So what this means is when you're driving in a car and the windows are blackened. You shouldn't be able to do any experiment that tells whether or not you're the one moving. Or whether it's the, the ground around you which is moving past you. The second one is particularly radical, which is that the speed of light should be the same for all observers. So no matter what you're doing, no matter how you're moving relative to everybody else. Your measurement of the speed of light should be the same as everybody else's measurement of the speed of light. And to understand why that's so radical, we have to go back to our example of adding velocities. Right? Remember we saw that the with the pellet gun. That the person standing on the ground as the rocket ship goes by measures the velocity to be the speed of the rocket plus the speed of the pellet as it leaves the muzzle of the gun, right. But what Einstein said is that if that was a laser beam rather than a pellet gun then everybody measure the same speed of light. That, the, if I shoot the laser on the rocket with the windows blackened, I will see the laser beam move away from me at the speed of light, which we call C. But the person on the ground will have to measure the exact same speed. There can be no indication that, there are no addition of velocities with, when it comes to light. Such that you have to add mind motion relative to the person on the ground plus the speed of light. And what that does is it changes physics dramatically. And in order to get the speed of light to be the same for everybody observing it, you have to allow space and time to be able to become distorted. They can become they are no longer absolute each in their own category in some sense. Because any measurement of speed is always a measurement of how far something has gone, over some duration, over some amount of time. And if you need the speed of light to always be the same, no matter who is measuring it. It means you have to allow space and time to contract or expand for each individual observer. In order to keep the speed of light, keep that measurement the same. And that is really the remarkable conclusion of what we call special relativity. This was Einstein's first step into relativity. That space and time are not absolute, in that they have to be joined together in something we call space/time. And that each person's measurements of space, or measurements of time will be different depending on how they're moving relative to everyone else. So for me, a second or a, a meter will be different than for you. What you measure to be a second or a meter. Depending on how we're moving relative to each other. So space and time individually become flexible, they become malleable And what doesn't change is this overall thing we now call space-time. So the change in time, the flow of time from one frame of reference to another is often called, referred to as time dilation. And the video that accompanies this lecture gives you nice example of the mechanics of time dilation, how The time, the flow of time in one frame of reference will be different from the flow of time in another frame of reference. But perhaps the best way anecdotally, so to speak, to talk about the effects of time dilation, is to discuss what's called the twin paradox. And this can be proven, you know, very explicitly using the mathematics of special relativity, which turns out to be actually not very complicated. But basically the idea is if you have twins who are born on Earth and, you know, of course they're aging together. And then when they're both 20 one of the twins gets into a spaceship and decides to fly off to a faraway planet. In a spaceship that is moving very close to the speed of light. Because none of these relativistic effects, show up unless your moving very close to the speed of light. What will happen is, is as the twin is flying to the the other planet, and then flying back, very close to the speed of light. Time for that twin will move more slowly, meaning, literally, the number of heartbeats that they have experienced, the ticking of their clocks. that, their time will flow more slowly relative to the twin on Earth. So when the twin lands, it will be perhaps three years that they will have experienced on the spaceship, whereas the twin who stayed home Will now be, will be 30 years for them. So the, the, the twin will get out of the space ship to greet their, their sibling and suddenly they see an old person. It'll be very shocking for them if they didn't know about time dilation. And this is the most dramatic example of how space and time have to distort The, for the twin on the spaceship, they didn't notice any difference. Time flowed for them just the way it always does. But it's only when they meet up with someone who wasn't traveling. Who was not in their frame of reference that this effect of time moving at a different speed becomes apparent. So really, what we learned from special relativity is that space and time alone are no longer absolute. That each can grow or shrink depending on how, people are moving relative to each other. And we know have to think about this new quantity called space time. Which is the most, which is really what is absolute. Which is really the ground of physical reality. So, with that, now we're able to take the next step forward in our understanding and begin to talk about General relativity. And general relativity is where Einstein went beyond just inertial frames and began to consider accelerations as well. [BLANK_AUDIO]
