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Name:  Katrina C.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: May 2001


Question:
Why does gravity pull? Why doesn't gravity push?


Replies:
On all the sizes and distances overwhich gravity has been measured, it always pulls, although very weakly compared to other forces like electrical or magnetic. I do not think anyone has an answer to the question: WHY?

Some very recent astrophysical observations and theory seen to "need" to have gravity be repulsive on the scale of the Universe. It seems that some of the very furthest/oldest objects in the Universe are expanding faster than they "should". One idea put forth is that on the Cosmic scale gravity is also repulsive, but so far as I know it is not the only possible explanation.

Vince Calder


Good question! But it's more philosophy than physics. If the gravitational force were repulsive instead of attractive, life as we know it would, of course, not be possible. Matter would not congregate in stars, planets, etc., but would be rather uniformly spread throughout space.

Electrical charges of like sign do, as you know, repel, but thank goodness there are two signs of charge and opposites attract. This allows neutral atoms to attract one another and causes matter to form hard pieces (like iron) and crystals (like diamonds).

It is interesting to note that gravity has only one charge and always attracts, electricity has two charges (+ and -) and so can repel or attract. The strong force that keeps nuclei together seems to have basically three charges and is complicated.

On the other hand, not having done all possible experiments, we cannot be sure that there isn't some kind of gravity somewhere in the universe that repels (pushes). You may have heard that Einstein found an equation that predicted that, but he thought that made no sense and rejected the idea. Now, however, there is some experimental evidence for such an effect and it may even help explain why the universe is expanding!!!!

Keep thinking up good questions!

Best, Dick Plano...

Richard J. Plano


Katrina -

An excellent question. Others have asked this question, too. I don't have an answer for you, but rather a further question. If gravity pulls should it not also push? Magnets do. Static charges do. Could there be two poles or polarities for gravity also? Keep thinking.

Larry Krengel


Katrina,

We do not know why gravity pulls. We do not even know why gravity must exist. The universe we live in seems to have a force that pulls objects together. The heavier the objects are, the harder they pull together. Scientists decided to call it gravity. Experiments were done to figure out how it worked. We do not even know if our ideas about it are perfectly correct. It is something that agrees with real-life experience.

All of science is like this. First, there is reality. Then, there are scientists trying to figure out how reality works. They come up with experiments to test reality, and with ideas (usually called theories) that agree with the experiments. New scientists try new experiments to test these theories. If they agree, nothing happens. If they disagree, the theories are changed.

The science we learn in school is the set of theories that work best. They could be correct, or incorrect. They are the best we have. Some very basic theories agree with more advanced theories. The basic ones can be called the reasons for the others. Still, the reality is the reason for the science: the science is just a possible description of how reality works. Science cannot tell us why reality happens to be as it is.

Dr. Ken Mellendorf
Illinois Central College



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