# Concept on Gravity

## Main Question or Discussion Point

Hi,
I have a conceptual problem about Gravity.

According to GR, gravity is simply a blending of space and it is NOT a force. However, I always hear that there is a so called gravitation wave. So, I am confused. Is it a wave? or a geometry blending??

thanks
Alex

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tiny-tim
Homework Helper
Welcome to PF!

Hi Alex! Welcome to PF!

(btw, it's "bending", not "blending" )

Gravity is curvature of space, and gravitational waves are ripples in the curvature.

Consider the ripples on a ponds surface, they propagate as a wave and they also warp the surface of the pond. You are essentially looking at the same thing from different viewpoints conceptually, describing the same thing in different ways. The value is that how we understand something and describe it allows us to see a set of characteristics, so if we can view the same thing in several ways and extract different characteristics from each view, we gain a deeper understanding of the underlying principles that govern that thing.

Thanks Tim for correcting me my word. I am a self-learner in physics and I am excited to come here. It is an informative forum.

LouMazero, so it means my understanding are valid for both bending and wave concept.

I like this question! It's said that the curvature of spacetime makes objects "roll" ("ripple"?)towards the source of the curvature, right? But does this really make gravity any more understood? Before, we just questioned why an apple rolls down a hill. Now we get to question why matter rolls down curved spacetime! Is this right or is there a way to understand this that I just haven't run across yet?

When I asked this question, I have thought another question.
Einstein's brilliant idea explains the gravity and it causes our planets move around the Sun.
However, what makes the planets start to move around so that the planet can move around the curve. By the way, I can also guess that our Solar system is contracting since planets will move to the source of curvature as Hoku said.

Sorry please correct me if I have anohter misunderstand.
Alex

I believe what started the planets orbiting the sun was simple momentum from the big bang. Everything was already in motion as the solar system formed.

I don't think the planets are getting closer to the sun. I think there's a precise ratio between the distance of a satellite from it's "host" and the velocity of the satellite that determines whether a satellite maintains orbit, leaves orbit or plummets directly towards the source. I think this ratio is *just so* with the satellites in our solar system that gravity actually helps maintain the satellites momentum, preventing a "collapse" in it's orbit. Is this right? But if the satellite get's to close, then it begins to "contract" towards the host until it has landed.

I guess planets get closer to Sun because of precession of the planets. Example: Mecury has 43 degree / 100 yrs and so other planets.

tiny-tim
Homework Helper
If you mean the precession "caused" by general relativity, no …

general relativity shows that a planet orbits in an ellipse of constant shape, at the same distances, but that the ellipse very slowly turns.

If you mean the precession "caused" by general relativity, no …

general relativity shows that a planet orbits in an ellipse of constant shape, at the same distances, but that the ellipse very slowly turns.

In fact, I have no idea why the precession occurs. I just guess there is an external force that caused this effect.