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  1. So, in General Relativity a massive body bends, curves the spacetime continuum...

But WHAT exactly is it that "thing" that gets curved? What exactly is empty space? Does it have a real, physical existence? Surely, a massive body cannot bend nothingness, it has to bend "something." They say it is the fabric of spacetime... But what is that spacetime, and what exactly is that fabric? The luminiferous aether was abandoned a century ago, so what exactly is that fabric of spacetime that gets bent/curved?

  1. WHY does a massive body bend it? It seems to me that General Relativity merely replaced a classical force that caused matter to attract matter with an extremely mysterious force that causes matter to attract. what exactly? Emptiness? Nothingness? Time?
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  • $\begingroup$ Who are these 'They'? Talking about spacetime as a fabric is an useful analogy but it isn't a fabric. Spacetime is just a term for three dimensions of space and one of time, used as a tool for analysis of events. $\endgroup$ – OmG Apr 3 at 6:38
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I am pretty sure your question is a duplicate, but I lack the reputation to make comments

Phrases such as the fabric of spacetime are found in many popular science books, but you would not get away with writing them in a textbook, because as your question demonstrates, the notion of what spacetime actually is, is an open one.

We don't have a definite accepted definition of spacetime, time is what we measure by using a clock and space can be defined as the distance between objects, or on a more general level, as an indication of the relationship between objects. But that's as far as physics currently gets you.

I would suggest that instead of trying to develop a mental picture of spacetime, that instead you consider the path of an object in curved spacetime as compared to flat spacetime.

As to why mass influences that path of objects, we have a range of laws regarding on different scales, from Newton's laws to Einsteins Equations, but as to the exact how and why behind these laws, we may have to wait (and hope) that a testable theory of quantum gravity can be developed that can help us understand gravitational effects better than we do now.

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  • $\begingroup$ A comment after a vote (up or down, but especially down), would help make this a better site. $\endgroup$ – user185905 Mar 5 '18 at 23:18

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