Space, the final frontier; and, Time the wibbly wobbly stuff. Anyone would tell you that these are two unrelated notions. That you can move in space, and that time is a construct of the human mind used to differentiate events (a view of time that I wholly disagree with). To differentiate cause and effect, like the semiclassic eggdropping thought experiment.
This multipart series is meant to explain what we know about space and time, along with their connection with gravity. This is not directly related to my research, but the notion of spacetime and its relation to gravity (and vice versa!). I’m not sure how long this series will last and what its full scope will be, but I will enjoy the entire journey. So, why don’t we get started?
The Equivalence Principle
Today’s topic is the equivalence principle. Rather, today’s topic preview is the equivalence principle. I shall state and illustrate it, though briefly. The next part of the series will be a more indepth, up close and personal look at it.
There are multiple versions of the equivalence principle. The two of interest in this preview are Newton’s and Einstein’s. Generally speaking, Newton’s principle states that mass is proportional to weight. Moreover, ignoring other forces, the trajectory of a freely falling object is independent of what it’s made of [see: Living Reviews in Relativity]. Newton’s principle is sometimes called the Weak Equivalence Principle (WEP).
Einstein’s version states that

WEP is valid.

The outcome of any local nongravitational experiment is independent of the velocity of the freelyfalling reference frame in which it is performed.

The outcome of any local nongravitational experiment is independent of where and when in the universe it is performed.
[These were directly taken from the above link.] The meanings and consequences of these three points will be explained in the next part of the series. There is still plenty of background information that needs to be told (for instance, what is a “freelyfalling reference frame”?).
For now, I will end by stating the punchline: if the equivalence principle holds (which experiments insist it does) then spacetime (the union of space and time, of course) is curved.
[By the way, the title of this series is not at all related to the topic at hand. I just simply liked the analogy, inspired by the Transistor OST; specifically, the title was inspired by The Spine. It’s an amazing song that I recommend.]
+Milo