Okay so spacetime curves and light follows that curve. So how are Globular Clusters “above” our plane? And then is there anything DIRECTLY below the Globular Cluster? How does that work unless it’s 4D but the 4th D is time and time follows “causality?” and doesn’t go up? Help my brain hurts a little
In: Physics
You are having confusion with the “plane” of our galaxy. It really has little to do with 4D spacetime and is just a natural result of gravity and momentum.
Our galaxy is roughly flat. This is the plane that you are talking about. But this is only *roughly* true. There is nothing that says that parts of our galaxy must lie exactly on this plane. There are many many galaxies in the universe and they are in all directions with all orientations of their own “planes” and I’m sure some galaxies are round rather than flat and thus don’t have a plane.
Spacetime is not a fabric as in a 2D sheet/plane. We call it a fabric because it can stretch and that is easy to show with textiles. Space-time has three physical dimensions and if you wanted you could just fly your rocket ship above the plane of our galaxy to visit the globular clusters of stars you are talking about.
Time is a fabric, a structure that moves, like a clock. From outside we see the entirety of the clock, see it moving and can know where it is, has been and will be. If you lived on the point of one hand it would seem that it always moved forward and you may not be able to see past and future.
To add, a change in time affects the entire structure at once. Looking back to the clock, if I took a marker and drew a smiley at the 6, it would just be there, not in a far away time but instantly. The person living on the moving hand though would not understand this, and see it as something that comes, is and goes away.
Space is three dimensional, which allows things to be “above”, “below”, etc; that’s what defines the words such as “above” and “below”. Spacetime, is by definition four dimensional (adding the dimension of time onto three of space). When mass curves spacetime, it curves it locally, but that doesn’t mean that any of the dimensions are lost – even in locally curved spacetime around say Earth, you still have three spatial dimensions clearly evident.
So, I think where you might be getting confused is the classic: “imagine spacetime is a big rubber sheet” demonstration. [Something like this](https://i.stack.imgur.com/pYYeY.gif). When you place an object with mass onto the sheet, it creates a divot in the fabric that holds other objects on the sheet trapped in its “gravity well”.
This demonstration is good, but it (along with every other analogy) is not perfect. It’s a simplified version of reality, because it demonstrates objects sitting on a 2-dimensional surface. When you place an object with mass onto the 2D rubber sheet, it curves* into a third dimension.* In other words, the rubber sheet has *length* and *width*, and then the curvature has *height*.
However, space, obviously, is 3-dimensional. The actual “fabric” of space isn’t a flat sheet, but a 3D one, with length, width, *and* height. You can travel up, down, left and right (and anywhere in-between) in space. That’s why you can have stars “above” and “below” each other, like you were saying.
So, where the 2D rubber sheet expands into a 3rd dimension when it is stretched by some mass, an object in our 3D universe stretches the “fabric” into a 4th *spatial* dimension. This higher spatial dimension is often called “the bulk”.
Since our human brains are incapable of imagining what a 4th spatial dimension would look like, we simplify things by collapsing our 3D world into a 2D one (i.e. a flat rubber sheet) so that the 4th dimension becomes the easier-to-comprehend 3rd dimension.
One of the common ways to introduce space-time is to present it as a flat 2d plane, like a fabric blanket stretched taut. If a heavy object is placed on the blanket, it’s easy to see how the mass will distort the shape of the fabric. This metaphor is where the term “space-time fabric” comes from.
However, the intuitive understanding it provide starts falling apart as we move in 3-dimensions.
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