Re: When does a theory become a law?

Entire volumes have been and are being written on this particular subject, which is more in the realm of philosophy than science. A theory can be either a very limited or a very broad explanation of a particular set of facts, an explanation that describes the relationship between previously unexplained or noncohesive facts and ideas. It unifies, into a usually brief statement, how something works. It often specifies the mechanism of how some process of nature is working, although mechanism may only be a part of the overall phenomenon. If a hypothesis is a proposed explanation of how some process works, then a theory is a hypothesis for which some substantial supporting data has been obtained. However, as noted above, a theory can be very narrow (how a single type of small process works) or very broad (e.g., the theory of evolution, a very complicated process indeed).

A law is, to my mind, simply a very, very broad and very very important theory for which we have very very good data and which applies to many, many aspects of our world. Thus the Laws of Thermodynamics or in chemistry the Laws of Mass Action (notice the capital letters, which laws tend to be stated in, for emphasis), apply not only to physical chemistry and chemistry in general, but to physics, biology, perhaps even economics. We also have very good data that these laws are obeyed universally and have detailed knowledge of their mechanisms. In contrast, a theory, though widely accepted may lack some crucial details of mechanism. For example, although the evidence that evolution has and is occurring is incontrovertible, the actual mechanism by which evolution occurs is under intense debate, and indeed, evolution probably occurs via multiple interacting mechanisms.

Put succinctly, a theory is a Law that hasn't been around long enough or doesn't yet have enough data to become a Law.

Hope this helps

-- Dr. Michael E. Maguire
Department of Pharmacology
School of Medicine
Case Western Reserve University