The standard Muddle of Relativity
Einstein does not say that space and time are identical!
TBD
Einstein does not say "Everything is relative"!
TBD
Relativity is not Relativism
There is a loose, analogical resemblance between relativity and relativism, but it is no more than that.
To say that something is subjective, ie relative to persons, is to make the epistemological claim that
the truth of a statement legitimately depends on someone's psychology. Thus, aesthetic preferences
are subjective because everyone has their own taste. To say that something is is relative, in the Einsteinian
sense is to say that physical measurements made by obserervers will vary accoding to objective, physical characteristics
of observers, such as their relative velocity. Even the word 'observer' is misleading here as the same relativistic distortions and dilations would
be recorded by an automatic apparatus, such as a rocket-mounted video camera, travelling the same trajectory as a human observer. Some subjectivists try to evade this counterargument by claiming that a human oberserver is still needed to examine
the video footage (or whatever) -- presumably meaning that the footage is somehow ontologically indeterminate until a human looks at it. But for a human observer to generate the right data, in agreement with the theory, she would have to know the trajectory of the camera, and what evidence would she have for that except the tape itself. This manoeuvre is surely pretty desparate.