Currently, there are problems reconciling relativity and quantum theory (such as the problem that scientists noticed that the speed of light is a constant everywhere in every direction). The laws of physics that govern large objects do not seem to apply to objects on the quantum level, so that begs the question, "what is Space-Time?". Unfortunately, there is no answer, at least not for now. The reality is that the universe appears to have one time and three space dimensions. Space-time continues to be one of the great scientific mysteries of the universe.

The velocity addition problem
The velocity addition problem is a problem that has presented itself in the Newtonian models of space-time. Let’s say you have two cars, car A and car B. If, in your frame of reference, car A is standing still, but car B is moving at a constant rate. If car B throws a ball forward out of the car, to car A the velocity of the ball is the combined velocity of the thrown ball and car B’s velocity (for example if, to car A, car B is moving at 60 mph and throws a ball at 30 mph, relative to itself, then the speed of the ball relative to car A is 90 mph). If we replace the thrown ball with light, the story changes. If car B was able to move at half the speed of light relative to car A, and then turned on its headlights you would expect, based on the Newtonian model, that car A would see the light coming off of the headlights to be moving at one and a half times the speed of light. This is untrue, the light moves at exactly the same speed for both observers. Because we know that the distance doesn’t change, and the speed of light stays the same, we know that it must be time that changes, this effect is called time dilation.
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Time as a fourth dimension
In order to explain how time could change, in the same way that velocity could change, physicists proposed that time was really just another dimension. When most people think of dimensions, they think of the three geographic dimensions: length, width, and depth (the x, y, and z axis). Just like the other dimensions, time can be curved (the mechanism that curves time is acceleration, just like the mechanism that curves velocity). As one accelerates, one is not only changing how fast they change their change in position, but also how quickly they change their change in time.