Gravity "A very serious matter"
By Michael L. Aymar
Saint Augustine
once remarked: “What, then, is time?”
If no one asks of me, I know; if I
wish to explain to him who asks, I know not.” The
same thing could be said of gravity. If asked, most people would claim to know
what gravity is; if asked to explain, most would fail miserably. Why should this
be so? Why should the concept of “things fall” be so elusive? To help
understand this all too familiar phenomenon better, we must first undertake a
brief history.
FORCES
(4 Different Ones)
Scientific study shows that all of nature’s activity can be reduced to the operation of just four fundamental forces. These forces, or “interactions” as they are known to physicists, are the source of all change. Each force has similarities to and differences from the others. These four forces are:
1) Gravity
2) Electromagnetism
3) Strong force
4)
Weak force
The first two, gravity and electromagnetism, are infinite in range. The latter two operate within the confines of an atom. The strong force binds protons together against the repulsion caused by their electric charges. The weak force is the force behind radioactivity and certain nuclear reactions in the sun and other stars. With this in mind, we turn our attention back to gravity.
Law
of Gravitation
With Johannes Kepler’s work before him, Isaac Newton (1642-1727) formulated “the greatest generalization achieved by the human mind.” Newton’s law of gravitation states that two bodies exert a force upon each other which varies inversely as the square of the distance between them, and varies directly as the product of their masses. Stated mathematically, this law reads thus:
F=G
mm1
R2
The effects are isotropic. The earth’s gravitation (for example) has no edge. It becomes weaker and weaker with distance until it gets lost in the confusion of the strong fields of other stars. So the earth’s gravitational field never ends, but peters out slowly in a precise and careful law, to the end of the universe.
From Newton’s third law ( for
every action, an equal and opposite reaction) we can deduce that gravitational
force is mutual. The earth not only exerts a force on the moon, but is , in
turn, subjected to a gravitational
force from the moon. So then, any two objects (masses) affect each other.
These “influences” are determined by their masses and distance from each
other. Yet all we have described so far is “how” gravity works. Until Albert
Einstein (1879-1955) science would have nothing to say about “why” this
should be so.
Independent of our ability to
describe gravity (mathematically or otherwise) we all speak quite comfortably
about the “force” of gravity. But what do we mean by “force”? The
Caltech physicist, Richard Feynman, wrote in his famous
Lectures on Physics: “If
“If you insist upon a precise
definition of force, you will never get it!” Newton himself never really
understood what a force was. “Was this…something that spreads from place to place, affected
instant by instant, point by point; or was it a property given as a whole, an
interaction [existing] between bodies remote from each other?”
Let us agree then (incomplete though it may be) to simply define
“force” as an action exerted on a body in order to change its state.
If I hold out my
car keys and let them drop from my hand, they will
surely fall to the ground.
We would all agree that the “force”
of gravity pulled them down. What could be more obvious? Yet, by this logic, if
I extend my hand and retain my grasp on the keys, a
subject who passed his hand
under the keys should feel the “force” of gravity tugging on them; trying,
as it were, to free my keys. This is clearly not so. What then, is this
“force” we call gravity?
In Einstein’s general theory of Relativity, gravity is not really a force at all. Rather it is a manifestation of the curvature, or warping, of space-time. Bodies are not “forced” into curved paths by gravity; they simply follow the straightest, easiest path through a curved space-time. Thanks to the genius of Einstein, we now have a “why” to accompany our “how” of gravity. Gravity is simply … geometry ! How can space/time be warped? Perhaps an analogy will help visualize the point. Imagine outer space as an infinite sheet of rubber. If we place a bowling ball onto our rubber sheet, it will sink in, distorting the (formerly) flat surface. Now we place another, smaller object- a baseball, say, or a marble on the sheet. Placed far enough away, the second object will merely dimple our imaginary rubber surface. Placed too closely however, the smaller object will be “forced” to roll towards the bowling ball. The more massive object will distort the surface in such a way so as to appear to exert a force on the smaller one. So it is too with planets and stars. The more massive the object is, the more “warped” the surrounding space time becomes. Anything- from stars and planets/ comets and asteroids/ even light itself, will be thus affected.
Review
So…what have we learned? Certainly less than all, but hopefully more than nothing, as we teachers like to say. For myself, I remain as St. Augustine- comfortable in my knowledge unchallenged , less so when it is. Yet to discuss this profound mystery of the universe, even so incompetently, gives temporary respite from the inanities of pop culture and celebrity fascination. The only true liberation, remember, comes through books. As such, I leave those interested in pursuing the subject further to the following:
Cole,
K.C., Sympathetic Vibrations:
Reflections on Physics as a Way of Life ( Bantam New Age, 1984)
Davies,
Paul , Superforce: The Search for a Grand Unified Theory of
Nature ( Simon & Schuster, 1984)
Ferris,
Timothy, Coming of Age in the Milky Way ( Anchor Books/Doubleday, 1988)
Feynman,
Richard, The Character of Physical
Law ( MIT Press: Cambridge, Mass. 1982)
Hawking, Stephen, The Universe in a Nutshell ( Bantam Books, 2001)
SciSim@cubanology.com