# Q & A: a fair and balanced acccount of gravity

Q:
I found this website, seems a metaphor or sarcasm: http://www.bringyou.to/apologetics/p67.htm But can you answer why gravity theory is true? "Secondly, school textbooks routinely make false statements. For example, "the moon goes around the earth." If the theory of gravity were true, it would show that the sun’s gravitational force on the moon is much stronger than the earth’s gravitational force on the moon, so the moon would go around the sun. Anybody can look up at night and see the obvious gaps in gravity theory. The existence of tides is often taken as a proof of gravity, but this is logically flawed. Because if the moon’s "gravity" were responsible for a bulge underneath it, then how can anyone explain a high tide on the opposite side of the earth at the same time? Anyone can observe that there are 2–not 1–high tides every day. It is far more likely that tides were given us by an Intelligent Creator long ago and they have been with us ever since. In any case, two high tides falsifies gravity. There are numerous other flaws. For example, astronomers, who seem to have a fetish for gravity, tell us that the moon rotates on its axis but at the same time it always presents the same face to the earth. This is patently absurd. Moreover, if gravity were working on the early earth, then earth would have been bombarded out of existence by falling asteroids, meteors, comets, and other space junk. Furthermore, gravity theory suggests that the planets have been moving in orderly orbits for millions and millions of years, which wholly contradicts the Second Law of Thermodynamics. Since everything in the Universe tends to disorder according to the 2nd Law, orderly orbits are impossible." Thank you.
- arlene (age 17.5)
MA
A:
That web site is a riot, a great satire. Thanks for steering us to it.

I'm not sure what you're asking when you ask why gravity theory is true. At some level, we don't know the reasons why our deepest theories are true, but just have to take nature as we find it. For our current theory of gravity (General Relativity, which isn't exactly the same as Newton's theory), we actually suspect that it will ultimately be found not to be exactly true, because it doesn't combine properly with quantum mechanics, our other deep theory. Meanwhile, however, it works just fine for all sorts of purposes, to extreme accuracy.
I suspect that maybe that's what you're asking- not why is it true but why do we believe it. The reason is that it is simple, precise, and makes lots of different predictions which are exactly right, so far as anyone can measure. Of course, the various phrases thrown around on that satirical site are just fancy-sounding gibberish, modeled after Creationist propaganda.

Mike W.

I like the satire of the biology textbook sticker at the top. I've seen it argued (very nicely), that everything we believe is just theory and not fact. After all, even if we have some pretty good models of how the world works that predict the outcomes of future experiments very well, they could still turn out someday to require some modification. Newton's laws needed to be modified when we learned of special and general relativity, but that doesn't nullify their usefulness. As Mike says, general relativity may someday need modification. One could deny the existence of "facts" altogether, because any statement about the workings of the universe may be found someday to require some small modification to make it better. It all makes little difference, because we base our behavior on the best theories we have about the outcomes of our actions, and usually a good approximation is good enough.

We cannot use the argument that our current theories may turn out to need some tweaking someday as new evidence is accumulated to justify radically different theories which conflict with evidence. Or theories that cannot be tested because of the way they are constructed.

Some statements are true by construction or definition. 2+2=4 is a construction, and not really a statement about nature. Here's a great quote from Albert Einstein (I don't entirely agree with it, however, but it is food for thought)

"As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality. " -- A. Einstein

Tom

(published on 10/22/2007)