These days, schools across the nation teach a scientific theory that has been known for decades to be fundamentally wrong, while the much more accurate theories that supplanted it are generally only mentioned briefly outside advanced courses aimed at specialists. How can this scandal go on?
I'm speaking, of course, of Newtonian mechanics, with its tendency of a body at rest to remain at rest and an equal and opposite reaction for every action. The crowning achievements of 20th-century physics, namely relativity and quantum mechanics (QM), arose from discoveries that the predictions of Newtonian mechanics simply don't hold under certain conditions (now if only they could be made to play nicely with each other).
Granted, the conditions in question are extreme. Quantum effects generally don't matter outside submicroscopic scales, and relativistic effects are only easily noticeable at extremely high speeds or in extremely strong gravitational fields. Nonetheless, the effects are of more than purely scientific interest. The GPS system, for example, could not have been built without knowledge of quantum effects (for the chips in the satellites and receivers for example) and relativistic effects (which cause the highly accurate clocks, needed in order to pinpoint location, to run slightly faster in orbit than on the ground).
So why do we cling to this outmoded theory? Simple. It gives the right answers in the kind of cases most people will encounter. How fast would that car have to have been going to have skidded for the distance it did? Newton can handle that one. What are the stresses on the deck of that bridge? Newton can deal with it. Why do the daily tides rise and fall? Newton himself did the numbers on that one. Why does the orbit of Mercury precess just a wee bit more than it ought? Um, actually you need general relativity for that one.
For a new theory to take hold, it has to be more than new. However much its mechanisms may differ from those of the old theory -- and QM and relativity differ radically from Newton in that respect -- it must still explain the same facts that the old theory explained. Thus the correspondence principle of QM, which states that QM and classical (Newtonian) mechanics give essentially the same results when large enough numbers are involved. Given that there are stupefyingly many atoms in anything we can actually see or touch, it's not hard to encounter numbers large enough for the correspondence to hold. In fact, it generally takes work to narrow things to the point that QM comes to the fore.
A new theory also has to explain some things better than the old theory. For example, QM explains why subatomic particles don't behave completely like ideal Newtonian particles and relativity explains why planets don't quite exactly follow the paths that classical mechanics predicts.
New theories typically keep most of the concepts of the theories they replace, but often generalize them or interpret them in the new ways. For example, the conservation laws concerning quantities such as energy, momentum and angular momentum, which were derived from Newton's laws as classical physics developed, play a central role in QM. Newton's idea that bodies travel in straight lines in the absence of outside forces becomes Einstein's idea that a body in orbit, for example, is traveling in a path that is (locally) straight, but in curved space-time.
Many concepts make it through unchanged, for example the concept of things having mass, or charge, or being able to move. In fact, most concepts will have to remain unchanged. The whole field of physics assumes that there is a physical world with space and time, that it's possible to conduct experiments and get reproducible results, and so forth. These might seem too trivial to mention, but given the sort of things that QM and relativity do revisit, for example to what extent things can have a definite location or whether it's possible to say two things happened at the same time, no concept seems to trivial to count.
Even at its most radical, science is fundamentally conservative. An established theory, even one with known problems, is assumed to hold until there is compelling reason to adopt a new one, and even in that case the old theory may well remain useful. I've used physics here as a running example, but the same holds true in any scientific field.
So why do we continually hear about revolutionary advances and theories being overturned? There are probably several reasons:
- The press needs good, dramatic story lines because that's what we its audience want.
- It's natural to focus on what's changed as opposed to what's still the same. Even an incremental change at the margins is a dramatic change if you only focus on the margins.
- There's a lot of science going on at any moment.
- Every once in a while something big really does come along.
All of this seems mostly harmless, so long as it doesn't give the impression that the world at large is liable to change drastically overnight.
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