Spacetime is pretentious nonsense. Spacetime curvature is pretentious nonsense on steroids. To understand why, all we need is a rudimentary knowledge of commonly used terms.
According to the dictionary, space is the infinite three-dimensional realm in which everything exists. Time has been considered the fourth dimension of space. It signifies space at different times. For this extra dimension to be meaningful, space must change over time. This is not possible because space has no observable properties which can change, either physical or chemical. Space is immutable, always exactly the same, no matter what the time.
Space is infinite. If it were finite, it would have a boundary. What lies beyond that boundary? The very act of trying to visualize space as finite makes us see that it is not. Given this absence of a boundary, there is no way to identify a point in space with reference to distance from that boundary. And neither is such identification possible based on properties; space has none.
For space to curve, shrink, or expand, points in space would need to move. There is no way to prove or disprove that such a movement has occurred. If it is not possible to uniquely identify a point in space, then it follows that it is also not possible to measure the distance between two points in space.
Suppose we go to a blackboard in a classroom and mark an X on it. Can we use that point as a frame of reference? The answer is no, we cannot. That X identifies a point on a material object. It does not identify a point in space. The point in space which X occupies is constantly changing, moving several miles every second, given the spin of the Earth around its axis, its orbit around the Sun, and the Sun’s orbit around the center of the Milky Way galaxy, and so on. Material objects can curve, shrink, and expand, but not space.
Like space, time is also infinite and impossible to measure. If time had a beginning or an end, what happened before the beginning? What will happen after the end? The very act of trying to stipulate a point in time as the beginning or the end makes us see that it is not.
There is a famous story about Einstein getting on a tram and looking back at the tower of Bern. If the tram were moving at the speed of light, it is argued, then time would freeze, and the clock would continue to show the same time. There is an entire cottage industry built around this incident, with complex mathematical equations painstakingly demonstrating the significance of the frame of reference of the observer.
It would be useful here to go over a popular high school math problem. It has to do with two moving trains. Given their locations at a given point in time, and the direction and speed of their motion, the student is asked to calculate their locations at a point in time in the future.
The incident about the clock tower can be fully understood simply through this analogy. Instead of trains, we’re talking about light and a tram. If the tram started moving away at the speed of light from the clock tower when it showed 2PM, then an observer on the tram would continue to see it showing the time as 2PM. Any light which left the tower after that time would never catch up with the tram. However, what is frozen is the observer’s view of the clock tower, not time.
There have been a number of scientific experiments conducted showing that highly accurate atomic clocks run at different speeds depending on their elevation. This is taken to prove that gravity slows down time. This is erroneous; it is the clocks which run at different speeds at different elevations, not time. There is no way to measure time. We can use a clock to measure the length of an activity, or even the speed of another clock, but we cannot measure time itself.
Gravity has no effect on either space or time; this is because space and time do not have mass. Strangely, the same people who argue that gravity bends space and dilates time, implying that they both have mass, also postulate that photons have no mass. They are wrong on both counts.
There is one universal time in the entire infinite universe, and it moves forward at a constant pace. It has no beginning or end. And space is the infinite three-dimensional realm in which all matter exists. Matter moves through space and occupies different amounts of it over time. Matter reacts only with other matter, not with space or time. Space and time exist independently of each other and have no interaction.
Originally answered on Quora in 2017:
What is spacetime? What did Einstein mean when he said it was curved?