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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 

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:   

- Doctor Mitteldorf, The Math Forum   
Associated Topics:
High School Physics/Chemistry

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