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

 

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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.