We can address the 'why' question two different ways:
- Utility: Gravity provides a functional relationship for all the mass of the universe.
- Teleologicaly: Because God designed it to work that way.
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Chicago Bob say: To keep us from drifting off into Space.!!
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Jeff say: Because cheeseburgers would not remain on your plate!
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The tin man say: To counteract frivolity and levity
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geessewereabove say: Because our Earth is spinning around, just like all other stars, planets, and moons! Look at how in outer space, papers that astronauts have tossed from their ships, have stayed close to the ship!
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quaz say: why is a physics question in R&S ?
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YKhan say: The actual answer is not known yet, but our current best hypothesis is that gravity exists to balance out energy concentrations in the universe. If you look at energy like a fluid, then a fluid always wants to even out in its concentration over its entire volume. If you have a little bit of extra concentration of fluid in one part, then that means that there is less concentration in the areas immediately around it. So other things are going to want to come in to fill the void in the areas of low concentration.
Now, another thing we've found out over the past century is that matter and energy are really the same thing. Matter is just a very concentrated & trapped form of energy. So if matter is a concentrated form of energy, then the space around it has a lower concentration of energy. This creates a well which attracts other energy to come settle into it. Usually what this means is that matter starts attracting other matter to fill the void around it. But all matter develops its own low-energy void around it, so if matter attracts other matter, they just join and become an even bigger concentration of energy and they have an even bigger void to fill around their combined mass.
So basically gravity is the void or the energy well around matter, which creates an attraction by wanting to be filled.
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