What do they mean when they say space time is curved? I keep hearing a lot of talk about Space Time being the 4th dimension, and no matter how many trampoline examples I see, I just don’t get it.
In: Physics
A line is 1 dimensional, and can only be (edit) measured in one direction. For it to be curved, it requires two dimensions. If it goes North/South, any East/West movement is in another “dimension,” or direction of movement.
A plane figure (square, circle, whatever) is 2 dimensional, and requires three to curve it. Imagine a circle, and it slowly deforms into a bowl shape. It’s moving in another dimension, or direction, to curve it.
Now…a sphere or cube is 3 dimensional. The curve is in the 4th dimension. It’s not a dimension that we can see, because our whole lives have been experienced in 3 dimensions. It can be understood as a continuation of the above ideas…curving an object in one dimension higher than it exists in.
They mean what they say, that it’s curved.
You can see this around objects with high gravity fields. Best examples are so called gravitational lenses. This is where something like an entire galaxy bends the space around it to such an amount that you can see what is behind it. If the space wasn’t curved then the galaxy would block the light. By the space around the galaxy being curved the light from the objects behind it travel in a straight line, because that is what light does, but it’s travelling in a straight line through curved space. Sometimes this means you can see the same object multiple times.
If you’ve seen Interstellar you would have seen it with the black hole.
Example http://assets.cdn.astronomynow.com/wp-content/uploads/2015/02/02153356/Interstellar_black_hole_940x400.jpg
That’s a flat disc but the high gravity has bent the space around it so you can see behind it.
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