Einstein often used his imagination to learn. He imagined how it would feel surfing a light wave and came to the conclusion that space and time are two interchangeable aspects of a single continuum—the space-time continuum. He imagined the feeling of free-falling and came to the conclusion that gravity—the pull we feel toward the center of earth—is simply the bending of space-time by mass and energy. To turn these “happy thoughts” into a mathematically consistent theory Einstein spent 7 years calculating!
In a highly simplified way, you can get a feeling for this concept by asking two friends to hold a blanket taut and dropping a basketball in the middle of it. The blanket will bend downward and if you toss in any smaller balls afterward, those will “fall” toward the larger ball. The blanket is the “spacetime”, the balls are made of matter and possess mass and they change the “shape” of the blanket and the movement of objects around them.
Large masses change the shape of spacetime so much that even light is bent.
You can see large masses bending light in this picture from NASA:
https://www.spacetelescope.org/images/potw1506a/
Or you can simulate this in your kitchen! Draw a dot with a marker on a piece of graph paper and cover it with a wine glass. Look through the base and see how lines are bent and how the dots are deformed and split in mirror-images!
For a cartoon explanation of the core of general relativity thought experiments see:
http://spark.sciencemag.org/generalrelativity/http://spark.sciencemag.org/generalrelativity/
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