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.
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
(published on 10/22/2007)