[MUSIC] If you decide to hold a party, you need to tell your guests where and when they should arrive. It's not enough to simply tell your friends where the party is without telling them when it occurs, or vice versa. Therefore, when we use the word event, we are using it to describe where and when something is happening. Events can describe things like your arrival at a party, the time you spilled your drink, or even something as simple as snapping your fingers. Can be an event. I could say that I snapped my fingers about five seconds ago at this location right here. So an event must include details of both the position and the time. Our universe has three spatial dimensions and so a defined event could look something like x, y, z and t, where x, y and z define the position and t defines the time. A good example of this occurs in the popular TV show Big Bang Theory. In an episode entitled the Cushion Saturation, Dr. Sheldon Cooper explains the first time he sat on his favorite spot on the couch as follows. In another changing world, it is a single point of consistency. If my life were expressed as a function on a 4 dimensional cartesian coordinate system, that spot at that moment, I first sat on it would be 0, 0, 0, 0. Sheldon feels most comfortable and at home at z, y, z coordinates of his spot, 0, 0, 0, 0. Although he can return to the spot on the couch many times, and he does, he can never return to the exact moment when he first sat on the couch to experience that event again. The reason for this is that the fourth coordinate in the event is time, t and is always increasing as time passes. Suppose he originally sat down to enjoy a 40 minute episode of Star Trek. We can describe the end of the show as an event that happened at 0, 0, 0, 40 minutes. The only way to return to the original event would be to use a time machine such as the one Leonard Hofstadter took a ride in Big Bang Theory episode, the Nerdvana Annihilation. If we were to return to Sheldon's first time on the couch and saw Penny riding a skateboard past the scene. Sheldon would see her in his reference frame. Would Penny see the first moment he sits in that spot followed by a 40 minute episode of Star Trek in the same order Sheldon experiences it? Considering our previous discussion about slicing up space time, do you think all observers see the two events in the same order, or with the same time interval between the events? Let's explore this further using one of Sheldon's favorite objects, trains. Sheldon is preparing for a trip on the Napa Valley Wine Train in his favorite 1915 Pullman standard lounge car. However, the Physicist, Sheldon would like to conduct an experiment to test the consequences of a constant speed of light. To do this, he set up a light source in the center train car and has two of his friends standing in his light detectors in the caboose and the engine at either ends of the train. While the train is in the station, we say that it is at rest. Here in the station, Sheldon conducts his first experiment by flashing the light and recording the arrival times that his friends measure at each end of the train. Since the distance to the light source is the same from both friends, they each detect the light at the same time. We call this a simultaneous detection. The train then leaves the station and begins its journey. Traveling at a constant speed through the mountains. Sheldon prepares the experiment again, this time with the train in motion. Since the train is traveling at a constant speed, once again, the flash of light arrives at each of his friends at precisely the same time. In both the stationary case, and the one with the moving train, the observers are at rest with respect to Sheldon and the light source. As a result, they observe the pulses arriving simultaneously on each occasion. Sheldon wants to know what his experiment would look like if he were stationary with respect to a moving train. So he gets off at the next stop. He gets ahead of the train and get set up to redo his experiment as the train passes at a constant velocity. This time, the light source flushes the moment that it passes Sheldon. Since the train is moving from left to right, Sheldon observes the light pulse arrive at the caboose of the train first. Followed by the arrival of the pulse at the engine of the train second. Sheldon is puzzled, he observed that the light pulse arrive at the back of the train first, while his friends on board report that the light pulse arrives simultaneously. In Sheldon's frame of reference, the light pulses do not arrive simultaneously. Even though his friends' frame of reference they do. This disagreement between observers is a result of light traveling at a constant speed, no matter how quickly the source of light moves. This is called the relativity of simultaneity. And it describes that stationary and moving observers will report the order of events differently depending on their proper motion with respect to one another. As strange as it seems, both Sheldon and his friends on the train are right. In some cases, the order of events depends on the motion of the observer. Einstein explained this through the concepts of length contraction and time dilation.