Help:Evolving knowledge
Evolving knowledge is knowledge that changes over time.
There are at least two main reasons that knowledge evolves. The first one is that the world changes. For example, the capital of Brazil (Q155) was once Rio de Janeiro (Q8678) 
, but it is now Brasília (Q2844). The second one is that what was once believed to be true is nowadays totally obsolete. For example, spontaneous generation (Q178263) was once a legitimate scientific theory about the apparition of living organisms every day, but it is now abandoned. Wikidata has mechanisms to deal with both kinds of knowledge evolution in its datamodel, use of Ranks and community defined qualifiers.
Evolving world edit
Changes of the state of the world, like the change of the capital of Brazil, are dealt with both using qualifiers, to express the dates and periods at which a statement is considered valid, and using ranks to highlight the most current value.
Time period in which a statement is valid edit
Qualifiers are used to indicate the period of time the statement is valid. We can express that a politician was president of a country between a certain date or another one thanks to qualifiers start time (P580) and end time (P582) , and we can express that Pangaea (Q4398) was a continent of eras Carboniferous (Q133738), Permian (Q76402) and Triassic (Q47158) thanks to qualifiers like valid in period (P1264) .
Examples:
Values that are valid now edit
The capital of Brazil is (most likely at the time you read this line) Brasilia. A user that would want to know the capital of Brazil is likely to want the answer valid at the time he asks the question, and not a historical value. Ranks is a Wikibase data model feature intended to allow that. Historical values are supposed to be handled with "normal" rank statements, while the value(s) valid at present time is (are) supposed to be ranked "preferred".
"Preferred" rank statements are likely not to have an end date.
Here is the example of the previous section, where the appropriate ranks have been added (in 2018):
Outdated values with no replacement edit
Sometimes a property had historically a value for a statement, but nowadays there is no real relevant replacement for the value. For example a city could have been briefly in history the capital of a state, but now is not really a capital for anything. As it’s not really a valid value anymore that could appear in the results of a query with valid ranks nonetheless, a solution is to add a statement with no value Help and preferred rank for this property. That way it will not appear in queries that do not want historical values.
An example is the Okiato (Q7081990)
Alone, it will appear in any query as it’s the better ranked statement and is valid and so normal rank. To remove it, just add a statement like the following
Obsolete theories or mistakes edit
In the case of dismissed beliefs, or known mistakes, the approach of Wikidata is not to delete the statements, but to use the "deprecated" rank. After all, it is still true that the references used to source the statements state them, isn't it? It’s just that what they say is believed or known to be false. This allows for example to document mistakes for them not to be re-imported into Wikidata again, or to say something about falsified theories.
Deprecated statements are kept in the database but are more difficult to find. For example, the query service will not find them unless the query explicitly asks for them, and the Scribunto and Templates Wikimedia APIs that make it possible to use Wikidata on Wikipedia and its sister projects won’t by default use them.