431 utenti della rete avevano questa curiosità: Spiegami: Why is the fabric of space bendable but also not visible by eye.
I was looking at how our solar system works and see that essentially the curvature from spaces and gravity or lack of creates the movement of our planetary systems. I couldn’t seem to make sense of the part detail 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?
Ed ecco le risposte:
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.
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.
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.
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).
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.