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What causes gravity?



In physics-C this week we started (re)learning about gravity, potential energy due to gravity, orbits and keplers laws, and escape velocity. I figured I would talk about the different popular modern physics theories and there views on what causes gravity.

We all know that gravity exsits. If you drop something, it falls to the floor. If you jump, you come back down. It's easy to see that gravity pulls you towards the center of earth. But WHY?

No one knows for sure! But, over the summer I read "A Brief History of Time" by Stephen Hawking, and it talked about many of the major modern physics theories, and here is some of the things I remember:

Based on Einstein's theory of relativity, gravity is caused on dips in space-time caused by mass. If you can imagine space-time in a two dimensional flat plane, then a large mass like the earth would cause a well in this plane, like a large ball resting in the middle of a trampoline. In 3-D, all these wells in space time cause a force towards the center of the earth. This model works great for large masses like planets. The theory holds very strong when dealing with the large scale. General Relativity is the current description of gravity in modern physics today. However, relativity doesnt hold up well on the small scale.

On the smaller scale, quantum mechanics is another possible theory that trys to describe gravity. Quantum mechanics has more to do with the behaivor of particles, so it trys to describe gravity in this way as well. It states that gravity is caused by an exchange of particles called gravitons by anything that has a mass. However, these particles are only hypothetical. If they exsist that would need to be completely massless, and have a spin of 2. If you know anything about the spin of particles, spin 2 means they have a rotational symmetry of 720 degrees... and if you dont know about the spin of particles, look it up! It is crazy stuff.

These are the two excepted theories that govern the universe. General relativity works great on the large scale, and quantum mechanics works great on the scale of particles. (Ofcourse - these theories go into alot more then just gravity, so if you are interested in learning about them more, read a book by Stephen Hawking!)

There have been many attempts to unify relatvity and quantum mechanics into a single theory that governs the universe and can explain gravity. Some of these include Quantum Gravity and String Theory. However, we like to call it String Guess because there is really no evidence that on the smallest scale possible, everything is governed by little strings that vibrate in different ways. The string theory view on gravity is that everything with mass has these little receptors, and strings run between them from one mass to another to look like really long H's. Ofcourse this is a little farfetched.

What do you think? What is your favorite theory on gravity?

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I'm still a fan of the femto-elves with pointed hats taking tethers and pulling objects with mass toward each other... but assuming that ISN'T really the case, there's a lot to be said for Einstein's general relativity making intuitive sense -- no supporting data, but it does seem to make intuitive sense and relate well to our understandings of the universe.

Now, having said that, my giant beef with string theory is the fact that there's no supporting evidence, yet it's become extremely popular because "the math works out well." Hypocritical? Absolutely... so we keep searching for answers!

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