I was looking at how our solar system works and see that essentially the curvature from space and gravity or, lack of creates the movement of our planetary systems. I couldn’t seem to make sense of the details of how space is similar to a fabric and can be shaped in some way.
The example used was the age old blanket with a bowling ball in the center creating a wide curvature leading to the edges of the blanket.
How is this possible but can’t be seen, nor does it cause friction?
In: 2411
Because spacetime isn’t a substance, it isn’t a thing. Spacetime is hard to describe as anything other than a fabric, but it’s not a literal fabric of material that bends. If you imagine instead the universe as a play, the particles are the actors, the fundamental forces are the words and the script, but spacetime is the stage. Spacetime is the medium in which things exist, and it can just curve and bend. Why? It just can.
And slightly off-topic, a physical thing can be completely invisible if it doesn’t interact with light at all. An example of this is the neutrino. Every second, over a trillion neutrinos pass through your body. But they can’t be seen, and they can’t do any harm, because they don’t interact with light. They are literally as invisible as anything can ever be.
The idea is that we *do* see it: we see objects with no other forces acting upon them seeming to change direction, and we see light bend as if curved spacetime acts like a lens. The reason planets don’t lose momentum to friction as they move through curved spacetime is the same reason objects in motion remain in motion traveling in straight lines through flat spacetime: the objects aren’t rubbing against anything, and they’re not being deflected either.
If you take a piece of graph paper and draw the graph for Y=X you’ll get a straight diagonal line. If you pick that paper up and roll it, now it looks like your line is going around in a spiral. But from the line’s perspective, it hasn’t changed direction: it’s just following the same straight line, only now on curled paper.
Spacetime is bent by gravity. Gravity is really weak compared to the other fundamental forces.
You ever seen one of those spinning levitating magnetic tops? The magnets in the top and in the base, which you can hold in your hand, are able to cancel out the downward pull from *the entire planet and everything on it*. That’s how much stronger electromagnetism is than gravity.
Therefore, the bending effect of spacetime is too small to measure or detect unless you have something incredibly massive (like a black hole). Even with some of the world’s most precise measuring devices, we can barely detect the ripples from crazy violent events like neutron stars merging.
You CAN see it in extreme cases (or, at least its effect) – like light bending around a black hole or gravitational lensing around a cluster of galaxies.
You can see it, albeit indirectly. In order to “see” anything, you’re looking at light that has interacted with the thing you’re trying to observe in some way.
Light has no mass, so according to your classical gravity force equation you know that gravity isn’t exerting any force on it. And that’s true – light just follows what’s called its “geodesic” path through spacetime – or the trajectory of an object if it simply moves forward in spacetime with no forces being acted on it.
Despite that, light still bends around massive objects in the universe, a phenomenon known as [gravitational lensing](https://esahubble.org/wordbank/gravitational-lensing/#:~:text=Gravitational%20lensing%20occurs%20when%20a,accordingly%20called%20a%20gravitational%20lens.). This is light simply moving forward through spacetime, still not experiencing a force – but the spacetime itself is curved.
In that way, you’re “seeing” the curvature of spacetime. You’re just looking at the effect it has on light, which makes sense because, well, that’s how you see things
I think you are thinking to much in the 2d sense. A good analogy is spacetime is the 3d extension to the outside surface of a balloon. This shape exists in 3 spatial dimensions (it requires height, length, and width) but is a 2 dimensional shape with no depth. Spacetime is this but in 3d,it has depth, length and width and is the outside of the 4d balloon. With that analogy, let’s look back again at the 2d balloon. An ant living on the outside of this balloon only sees length and width, even though the space surrounds 3 dimensions. We are the same but in our dimension. When we add a weight to this balloon, it bends the space and a lighter weight would fall in the cavity formed, but the ant, who only experiences the x and y plane, doesn’t see that cavity but does see the effect on other weights. Applying this to our spacetime. A planet bends the space in the dimension we don’t experience or perceive but we still see the gravitational attraction
The fact is that space-time isn’t similar to a fabric.
First of all, fabric is two-dimensional in this example (the thickness is negligible). Space-time is three-dimensional (actually more, but for all practical purposes let’s stay at 3D). And you don’t observe space-time from outside, you’re in it, actually part of it. And so are the celestial bodies.
So stars and planets don’t “sink” into a space-time plane. They bend the space all around them. Including the space you’re occupying – as opposed to being outside of the “fabric” that you’re observing.
And you can’t normally see this because space-time isn’t visible. And it’s very, very weak. You can measure it with some instruments. Or you can observe the effects it has on matter (well, actually on energy but let’s not digress). You can’t really see things being “bent” around Earth because it’s too small and the effect is weak and your POV is bad.
You can observe the gravitational lens effect of distant black holes, which are comparatively very strong and can bend light enough that you can see the distortion effect. Although this is not exactly the same phenomenon, it is the closest you can get to seeing this effect with your eyes (well, actually through specific instruments once again).
Because it doesn’t bend very much nearby us, we need to look a long way away.
Then we see it in many places.
But you need a telescope to see that far.
The obvious one is seeing the ring lines that show space is working as a giant magnifying glass.
You can’t see spacetime. What you see is it’s effect on other objects. Think of wind. You can’t see the actual wind but you can see leaves and trees being blown by it , and you can feel it.
Imagine that the Earth was perfectly smooth and that you and a friend were standing side by side along the equator. Look around. The world looks flat doesn’t it?
Now both of you face north and start walking. If the world was truly flat, you guys would walk side by side forever. Instead, you two will eventually come together as you approach the North Pole. Nothing is pushing you together. You naturally come together because the world you’re in is curved even though it appears flat. How soon you come together depend on how curved your world is. E.g. on the Moon, you and your friend wouldn’t have to walk as far to come together because the Moon is more curved.
That’s what gravity is. For a long time, people thought that it was a force that pulls things together. But no, it is evidence that the true shape of the world is curved.
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