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# Gravity

Does anyone out there understand science enough to explain to me in simple terms how gravity works? How do objects attract each other? Is it some kind of wave or force field?

Is there a way to measure how objects pull on each other? How would an instrument like that work? Can you somehow interrupt your own gravity on other objects? If so, how?

If a planet ten billion miles away blows up, how long does it take for its gravity to stop effecting me?

These might be stupid questions, but I got to thinking about it over the weekend and I realized I don't have any idea how gravity actually works. Science books are probably all way over my head, so I thought heck I'll start here.

Gravity is one of the four fundamental forces/interactions in the universe. It's fundamental in the sense that it cannot be explained in terms of other forces/interactions; it's an inherent property of the universe. It is equivalent to a set of base axioms in mathematics. The other three fundamental forces are electromagnetism, the weak nuclear force, and the strong nuclear force. People have hypothesized the existence of a theoretical particle called the graviton that mediates gravitational interactions, but this is more a theoretical construct to make gravitational theory a little more in line with the rest of quantum theory.

There are multiple ways to measure gravitational interaction. (I know it's cheeky, but a scale does pretty well ) Seriously, there are lots of interesting experiments. Right now, one of the hot areas of research is trying to detect gravitational "waves" from distant galaxies, similar to waves in water.

You cannot interrupt your gravity on other objects, nor can you shield yourself from gravity, based on any known interpretation of physics.

Gravity travels at the speed of light according to the theory of relativity, so if a planet far away blows up, it's fairly straightforward for you to calculate how long it will take to affect you.

Those are all good questions by the way. If you go far enough in physics, there are all kinds of wonderful things to discover related to gravity and the other fundamental interactions. If you really want to blow your mind, look into quantum theory, particularly quantum entanglement, what Einstein called "spooky action at a distance."
I thought gravity had a lot to do with the rotation of the earth, and a little to do with the sun? Someone set me straight here..
No on both.
My understanding is that gravity is simply the objects in the universe following the curvature of space-time.

Think of it this way: Einstein's theory is that the universe is actually four dimensional. There are the three dimensions we are familiar with, and the fourth dimension is time. Space and time are related.

Since we live in three dimensions, we can see the curvature of a plane, but fundamentally lack the capability of observing three dimensional curvature. For example, if you lived in two dimensions, you could not see the curvature of the plane you lived on, because every way you move just looks flat, since you can only detect two dimensions. It's the same thing for us, only in three dimensions.

All objects naturally follow the curvature of space-time. For example, a gravitational field around a planet is space time curving around it. An object orbiting that planet is following the curve, like a marble rolling around and around in a bowl. It's somewhat difficult to visualize, but the theory is very well developed, although I don't know all of its finer points.

Anyway, that's just another perspective for you. For just about all intents and purposes, though, Newton's theory of gravity holds as a "great truth" in the universe. Einstein simply provided a way to explain how distant objects could apply forces on one another without any contact or exchange of particles or EM field.
Quote:
 Originally Posted by tennisets My understanding is that gravity is not technically a "force." Gravity is simply the objects in the universe following the curvature of space-time.
Your space-time comment is true, but gravity is a force. Specifically, it's the force which causes deformation of space-time in the presence of masses.
Gravity is one of the four fundamental forces of the universe [gravitational, weak nuclear, strong nuclear, and electromagnetic].

Think of gravity as a stretched out sheet of rubber or something. If you roll a ball across it, it depresses the material below it [i.e. a gravitational field]. Now bend your mind a little and imagine this as being 3D, so it bends everywhere around the object, instead of just "below" it.
Quote:
 Originally Posted by AlanY Your space-time comment is true, but gravity is a force. Specifically, it's the force which causes deformation of space-time in the presence of masses.
Thanks for the clarification. My post has been edited to reflect that.

On another note: I never thought I'd actually hit 1000 posts. It certainly takes a while when you average 1.2 posts/day.
Quote:
 Originally Posted by mr_baseball_08 I thought gravity had a lot to do with the rotation of the earth, and a little to do with the sun? Someone set me straight here..
You're probably getting gravity confused with centrifugal and centripetal force. These are really unrelated concepts to gravity. When an object travels around in a circular motion with a constant speed, there is a net acceleration towards the center of the circle. That just comes from Newton's third law (for every net action there is an equal and opposite reaction). There's no real external force involved.

The reason we don't fly off the Earth as it spins is because gravity is under us, providing that net centripetal force allowing all of us to continue moving in circular motion along with the surface of the Earth. Take the Earth away (hollow it out, throw the center away, leave just an ultra-thin crust rotating as it does now), and we would float away rather than being able to walk around normally, unless we all moved inside the rotating hollowed out sphere. You might have seen science-fiction movies like 2001 where a rotating space vehicle uses this principle to create artificial gravity, with people walking along the inner outside surface of the spinning craft.
Just remember 9.8m/s²
Quote:
 Originally Posted by tennisets Thanks for the clarification. My post has been edited to reflect that. On another note: I never thought I'd actually hit 1000 posts. It certainly takes a while when you average 1.2 posts/day.
Heh, I reached my 1,000th post yesterday... took me a while too. Ooops, now it's 1,001 dammit!
Quote:
 Originally Posted by smeggy Heh, I reached my 1,000th post yesterday... took me a while too. Ooops, now it's 1,001 dammit!
Interestingly, we also both registered at about the same time and also both have quite similar systems. Weird.
Actually, the spacetime picture is not clearly compatible with the graviton picture from quantum mechanics. This is one of the leading issue in contemporary foundations of physics.

The definition of force simply involves accelerating mass, so gravity is a force regardless of how the foundational issues sort out.

Great stuff!
Quote:
 Originally Posted by AlanY Gravity is one of the four fundamental forces/interactions in the universe. It's fundamental in the sense that it cannot be explained in terms of other forces/interactions; it's an inherent property of the universe. It is equivalent to a set of base axioms in mathematics. The other three fundamental forces are electromagnetism, the weak nuclear force, and the strong nuclear force. People have hypothesized the existence of a theoretical particle called the graviton that mediates gravitational interactions, but this is more a theoretical construct to make gravitational theory a little more in line with the rest of quantum theory. There are multiple ways to measure gravitational interaction. (I know it's cheeky, but a scale does pretty well ) Seriously, there are lots of interesting experiments. Right now, one of the hot areas of research is trying to detect gravitational "waves" from distant galaxies, similar to waves in water. You cannot interrupt your gravity on other objects, nor can you shield yourself from gravity, based on any known interpretation of physics. Gravity travels at the speed of light according to the theory of relativity, so if a planet far away blows up, it's fairly straightforward for you to calculate how long it will take to affect you. Those are all good questions by the way. If you go far enough in physics, there are all kinds of wonderful things to discover related to gravity and the other fundamental interactions. If you really want to blow your mind, look into quantum theory, particularly quantum entanglement, what Einstein called "spooky action at a distance."

Did a project on quantum entaglement and quantum erasers. That stuff is just nuts. Its like magic......but real
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