Gravity
(This column was first published in
the September 12, 2004 issue of The Buffalo Sunday News.)
As
an antidote to obsessing about contemporary politics, I have been thinking
about gravity. This is a good time to do so as our Cassini-Huygens probe is whirling around Saturn taking
spectacular pictures and the Messenger rocket has just set out on its 41-month
voyage to Mercury. Gravity is very much involved with those space trips and I
share with you some of my thoughts about it.
We
all "know" from junior high school science that Isaac Newton
"discovered" gravity back in the 17th century. Accepted wisdom tells
us that he did so when a falling apple konked him on the head.
Not
quite. It is simply not possible to discover something that is already known to
everyone. Apples are no different from anything else in this regard. You toss a
ball into the air and it is pulled back down. If you try to emulate Spiderman
or Cat Woman by jumping out of a window, you similarly fall to the ground. The
force that pulls apples and us down is gravity and it was the same force in and
before Newton's time. Aside from a few bemused youngsters who imitate those
comic strip characters by leaping from barn lofts, we all know now and knew
then about gravity.
Newton's
achievement was not somehow to discover something everyone knew but rather to
establish that the gravity that pulled the apple toward his head was a
universal force that governed space and in particular our solar system. Just as
gravity pulls down that apple or baseball, it also pulls the planets and comets
toward the sun and the moon toward the earth.
But
wait a minute. Consider the moon. If it is indeed being pulled toward the
earth, why doesn't it simply plop down and crush everybody in, say, Asia?
Clearly
something else is going on here. Something is balancing the gravity pulling the
moon to the earth and that balance is near perfect. The moon has continued its
monthly revolutions around the earth for ages and will go on doing so into the
distant future.
I
was a good junior high school science student as most of what we were taught
there seemed reasonably associated with common sense, but I recall not being
satisfied with our teacher's explanation of the moon's orbit. I recall Mr.
Connors talking about counteracting centripetal and centrifugal forces. He also
had a student come to the front of the room, hold one end of a four-foot string
with a ball tied to the other end and swing the ball around, turning as he did
so. "That's the way the moon is held in orbit," he told us and
everyone else seemed satisfied.
That
didn't work for me. There is no string hooking the moon to us. I didn't realize
at the time that the role of the string is replaced by the pull of gravity.
Okay,
I wasn't as smart as I thought I was. But there is another law involved here, a
law employed by Kepler before Newton. A moving object continues in its path
unless acted upon by other forces. This force called inertia in the moon's case
exquisitely balances gravity. Left to its own devices, the moon would simply
travel in a straight line through space, but it is pulled sidewise just enough
to turn around the earth. That's a bit oversimplified but it is essentially how
the moon stays in its obit. I wish it had been explained to me that way.
What
is wonderful about that gravity is how our space scientists are taking
advantage of it to maneuver our space probes. Consider Messenger. It has been
shot out into space only to be pulled back toward earth by gravity just close
enough to be swung, like the last kid in snap-the-whip, off again now much
faster. Its new path will take it near Venus where it will duplicate its two
visits to earth before it is again slingshot, this time finally toward Mercury.
All the little satellite must contribute to this remarkable voyage once it
rises initially is to make minor corrections to its various orbits. It would
never be able to make this trip without these gravity assists.
Unfortunately,
all this still leaves us with a problem. Gravity is a remarkable and all-powerful
force but how does it work? I am afraid that I have no answer to that question
and neither to my best knowledge does anyone else.
I
not only don't believe in mental telepathy or extra-sensory perception or
spiritualism or astrology but I consider them absolute nonsense. They all
require some kind of additional forces or actions that have never been
demonstrated. Unfortunately gravity, I must admit, is until now at least,
equally unexplainable.
Unlike
those others, however, gravity is demonstrated to us every single
day.-- Gerry Rising
_______
Note: For more information on the
Cassini-Huygens Saturn probe, see http://saturn.jpl.nasa.gov/home/index.cfm,
and for the Messenger probe, see
http://www.nasa.gov/mission_pages/messenger/main/index.html.
Shortly
after this column appeared in the newspaper, the following letter was sent to The
News by
a respected university colleague. The newspaper supplied the headline, Einstein
Brings Columnist Back Down to Earth.
Gerry Rising gave
an interesting account of gravity in the Sept. 12 News, as it was described 350 years ago with
Isaac Newton's discovery. Rising asked: "How does it work? I have no
answer and neither does anyone else." One person who admittedly did not
understand gravity was Newton, though he described the phenomenon correctly in
his day.
But Rising did
not seem to realize that Albert Einstein answered his question less than 100
years ago, with his discovery of the theory of general relativity - a theory
that revolutionized physics. Without going into details, Einstein did indeed
"explain" gravity in a way that would have satisfied Newton. His
theory led to a totally different basis for gravity than Newton's, that
duplicated all of Newton's correct results, in addition to other experimentally
proven gravitational facts not given by Newton's theory.
Thus, Einstein's
explanation of gravity did indeed supersede Newton's formulation of a theory of
gravity, and it provided a scientifically acceptable explanation of the
phenomenon of gravity.
Mendel
Sachs
UB professor
emeritus of physics
Williamsville
Professor
Sachs and I have talked about his point of view and I am currently working my
way through his interesting book on this topic, Relativity in Our Time: From
Physics to Human Relations. I am hoping that my reading will satisfy me that Einstein
did indeed explain gravity so that my claim about the lack of a satisfactory explanation
will be countered. Stay tuned.--Gerry Rising
I also received several e-mail messages claiming that gravity is not a force from Allen
C. Goodrich. His case is carefully spelled out at his website.