Question

Which of following statements is true about scientific knowledge?
Scientific ideas are not subject to change, and so knowledge stays the same.
Scientific knowledge is constantly evolving as new observations are made.
While scientific knowledge increases over time, old knowledge never changes.
Theories can change frequently, but scientific knowledge is absolute.

Answer 1

Scientific knowledge is constantly evolving as new observations are made. :)

Answer 2

2000 years ago, everyone "knew" the earth was the center of the universe. 500 years ago, everyone "knew" the earth was flat.

Answer 3

It depends a lot on what you mean by "knowledge." If you mean measureable facts, then generally of course these don't change. But it sounds like you mean scientific theories -- ideas about how to interpret facts.

The distinction is key: noticing that your bank balance has dropped by $500 is a fact. Deciding that this is because (a) you spent a wild weekend in Vegas you don't fully remember, or (b) somebody stole your ATM card and has stolen $500 from you, are very different theories that both explain the observed fact.

In science it is crucial to distinguish measureable facts, which do not change, from theories that interpret patterns of facts, which can and do change quite a lot. For example, JFraser points out that a long time ago there was a theory that the Earth was the center of the universe. This theory explained the measureable facts of the time quite well: by looking at the sky you can see that the Sun, Moon, stars and planets all circle the Earth in a regular way. The simplest explanation is that the Earth is the center of the universe.

Later, more accurate observations, as well as additional facts (e.g. that the Earth is round) suggested a better explanation: that the Earth rotates, but so slowly that we don't feel that motion. This explains why the Sun, Moon, planets and stars appear to circle the Earth. But it also explains how the peculiar path of certain planets (Mars in particular) in the sky can come about even with very simple orbital motion on their part. (They're orbiting the Sun, not the Earth.)

Similarly, there used to be a theory of combustion in which heat was a fluid (phlogiston) contained in substances, which was released by rubbing or chemical reaction (fire). It fit all the observations of the day. But later scientists noted that some substances when burned gained weight instead of lost it, that the mass of nearby air decreased when things burned, and that you could produce heat indefinitely by mechanical motion. All these observations led to a revision of the theories of heat and combustion, and now we interpret combustion as the chemical combination of substances with oxygen in the air, and heat as the microscopic mechanical motion of atoms. Again, as observations become more numerous and sophisticated, we revise our theories to better fit them.

On the whole, I'd say your second statement is more satisfying.