NB Einstein actually pondered over to call his theory the theory of the absolute because besides showing that space, time simultaneity, mass (and more) were relative he also showed what was absolute

e.g.  c  the velocity of light in vacuum and "interval " in spacetime

I don't know the sayings in English but in Sweden many people says "Allt är relativt" 

that is  "It's all relative"   But this is not what Einstein said!

History of electro-magnetism up to space-time History e-m to spacetime
The starting point for my idea of six-dimensional relativity

Links

Dr E A B Cole  mathematician who have published basic work on six dimensional relativity.

On his homepage you find all references and an article. Among else he has written about

Prediction of dark matter using six-dimensional special relativity

 Dr Matej Pavsic physicist who have published on generalised LT, three times and tachyons

He solve the causality paradox with tachyons using Everett's many worlds interpretation of QM.

As argued in my  abstract I think  Everett's interpretation is doubtful.

Prof Howard Wiseman have written an interesting history about   Einstein and Bell's theorem

and which to my mind shows how shaky it was when the Copenhagen interpretation  was put together.

And that Bell's theorem and Aspect like experiments force us to choose between non-local reality or solipsism. He cites Bell's and Popper's criticism of the latter.

My view is that to believe in solipsism is intellectual hubris,

I think Plato's ideals The True. The Beautiful and The Good are a valid base for all human activities including science