Gravity's just a theory, too.
Creationists and those who firmly believe climate change isn't driven
by humans miss the point: science isn't about providing certainty. It's
about providing uncertainty.
Take gravity. Gravity is something that we can observe pretty easily
just by dropping an apple. We can note correlations (massier planets
seem to have larger gravitational fields, for example). We can guess
that, since the flux per unit area through the surface of a sphere
decreases as the inverse square of the sphere's radius, gravity is
subject to the inverse square law. We can even posit underlying
mechanisms linking gravity to a specific particle, like the Higgs
boson. What we can't do is prove that we understand how gravity
works, except in terms of other theories (like particle theory and
general relativity). We also can't guarantee that gravity functions
the same way (or at all!) in places out of our direct experimental
reach -- we can just show that the cosmological motions we see match
our expectations were gravity to work the same.
These are the same objections that people bring to evolution and
climatology: we don't understand much about the underlying mechanisms
in either area. We can't show that the same rules that we see
operating today are the rules that operated 2,000 or 4,000 or 500,000
years ago. We can say that what we see in the fossil record
and among living organisms today strongly suggests a single common
ancestor for all life on earth; but we can't rule out the theory that
God created the earth 6,000 years ago, because we don't have any
objective observers from that time. We certainly can't demonstrate
that human activity has caused climate warming, although there do seem
to be significant correlations between human activity and climate
change. (Note that correlation does not imply causation, though.)
So, why is gravity undisputed (except by Flat Earth people)? And
why are climate change and evolution such hot topics? I'm not sure,
but I can suggest a few reasons.
Gravity is undisputed today partly because no religion has made the
precise mechanism a point of recent dispute. It used to be in
dispute, though; remember Galileo?
That, ultimately, was a dispute about gravity on the scale of our
solar system. Yet no edicts about the Higgs boson, or general
relativity, have emanated from the Catholic Church, and Bush doesn't
seem to care about gravity.
Another reason that people don't argue much about gravity is that the
theory of gravity is predictive. Given a comet's position and
momentum, we can tell you pretty much where it's going to go. It's a
little harder in atmosphere, but we do it very well -- think ballistic
missiles, for example. This predictive power goes a long way towards
quieting dissent with the theory, because if you can predict something
people will generally believe you understand it pretty well. (We'll
come back to this.)
Evolution, for better or for worse, is not in the same position. It's
a major point of dispute in at least a few places, and it's not
predictive in the least. Even worse, it can't be very specific
in predictions, because it's a stochastic theory that is subject to
historical contingency. We will never be able to predict what mutations
will arise randomly, and we will probably never be able to predict
what effect those mutations will have on ecosystems. We might be able
to predict general trends, but that is still far away from being an
Climatology is a much younger science than either the physics of
gravity or the study of evolution. Like evolution, and unlike
gravity, it seems to be very sensitive to certain kinds of
perturbations -- that is, it's "chaotic". Very small changes may have
large effects elsewhere. Moreover we don't understand many of the
basic processes very well, and we don't have good ways to measure
even relatively simple things like energy input from the sun, much
less complicated things like CO2 consumption. Climatology is certainly
not a predictive science in general, although some things can be
predicted, just like in evolution: if you know where a hurricane is
today, you can guess pretty well where it's going to be tomorrow.
Climatology is also a big point of contention for economic reasons:
global warming, in particular. Corporations don't want to reduce
the emissions of greenhouse gasses because they believe that it will
have a negative economic impact on them. Therefore they (or their
proxies) attack global warming as an unproven theory, in order to
undermine its impact on public policy. As with the religiously
motivated attacks on evolution, this is definitely bad for science.
If we could predict climate, or predict the effects of evolution,
presumably people would regard these theories as being more credible
than they are now. Unfortunately it's impossible to turn evolution
into a predictive theory, and it's going to be a while before we get a
predictive handle on climatology. So both theories are amenable to
attack on the charges of being "unproven".
And here we come to the nut: the scientific method can't prove
anything, in general. It is is much, much better at
disproving theories than it is at confirming them; any working
scientist will agree with that! All that an honest scientist can say
about gravity, or evolution, or global warming, is that they haven't
been disproven yet. There are reasons to believe that gravity
and evolution are pretty good theories, scientifically speaking, because
they've withstood the test of time. I'm not very knowledgeable about
climatology but I do know it's quite a bit shakier in its underpinnings.
But attacking any of these theories for not having provided proof
is missing the whole point of science, which is to disprove as much as
People -- even many intelligent people who should know better --
frequently get this wrong. Michael Crichton, the prolific author of
(among other books)
Jurassic Park, gave an interesting lecture at Caltech where he
talked about scientist's involvement in political debates on public
policy. Nuclear winter and global warming were two examples where a
strongly biased view has been pushed strongly and publicly by a
relatively small cadre of scientists. Crichton's view seemed to be
that scientists were no less fallible than anyone else, which is
undeniable (though unpopular among scientists ;). What he missed, and
what I think many scientists fail to emphasize, is that thus far the
scientific method -- with objective measurements and peer review, in
particular -- is the only proven method of discovery known to
mankind. We ignore it at our peril.
Scientists can do their part by proudly admitting ignorance. It's not
pleasant, but it's undeniable: did you know, for example, that the
underlying mechanism by which evolutionary novelty arises is still in
dispute? Yep! We still don't really understand how new traits arise!
And did you know that the precise reflectivity of the earth -- which
is a major determinant of energy input into our climate, and is
directly linked to the "greenhouse effect" -- is still not easily
measurable? Yep! No long-term trends available! And these are just
two things I've worked on -- I'm sure there's an ocean of ignorance
out there, just waiting to be publicized. That's science!
The flip side of the coin is that those who critically examine
scientific theories should apply the same level of critical analysis
to their own beliefs. This applies to postmodern lit-crit as much as
it applies to religious believers -- and I think it's as important as
science is, as a method for making public policy.
Note to readers: I've been thinking about writing something like
this for a while. It's an ongoing project, so please e-mail
me at email@example.com if you have thoughts, criticisms, or suggestions.