What is Gravity?
Date: 05/06/2001 at 01:34:40 From: Matt Lewis Subject: What is gravity? My roommate, an Engineering student, says that the Earth's rotation is what causes gravity on Earth. I said no, all objects exert a gravitational pull on each other no matter how far apart they are, regardless of chemical make-up or rotation. However, he is confused about centrifugal force. Please explain this matter further for all the other confused engineers out there. Isn't it also generally accepted that we don't know what gravity is, and that we have no real proof of what causes it? (No, this isn't David Hume.)
Date: 05/06/2001 at 06:27:51 From: Doctor Mitteldorf Subject: Re: What is gravity? Dear Matt, Gravity here on Earth is not caused by the Earth's rotation, but by the Earth's mass. The rotation does cause a centrifugal force that makes gravity appear to be a bit weaker at the equator than at the poles. When Einstein put forward the (special) theory of relativity in 1905, he realized that energy is mass, and that gravitation both responds to mass and is a form of (potential) energy. So could it be that gravitational energy is a form of mass that creates more gravitational attraction? This conundrum left him scratching his head for twelve more years, at the end of which time he'd come up with the general theory of relativity, which is really a theory of gravity. In general relativity, gravity is not a force field like the electric force, but a deformation of geometry. 4-D space-time is actually slightly curved around a massive object, in a way that we can't visualize except by analogy with a 2-D surface that can be curved within a 3-D space. Then along came quantum mechanics. It became a general principle that on the smallest scale, all information is digital, not analog. Physicists over the years have come up with digital (quantum) theories for all the kinds of force that exist on the smallest scales of matter: the electromagnetic force, the strong and weak nuclear forces. But physics has yet to come up with a quantum theory of gravity. It is thought that this must involve some kind of digital space-time, where space itself is not a continuum but a very-closely-spaced lattice in which, say, a particle can exist at point A or point B, but there is no place "between" points A and B where it's possible for the particle to be. Stay tuned... For another discussion on gravity and special relativity, see "Adding Velocities in Relativity" from our Ask Dr. Math archives at: http://mathforum.org/dr.math/problems/bigari.07.12.99.html - Doctor Mitteldorf, The Math Forum http://mathforum.org/dr.math/
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