Which one? Einstein's? That's only general relativity, and is only the least complex way to describe what we can observe, proving my point. Einstein's relativity has not been reconciled with quantum relativity, either. Also, physically defined as what exactly? Either way, there is no proof whatsoever for any existence of causality outside of the mind (if it even exists inside the mind). Any other claim results in circular reasoning: causality exists because you can observe it, and you can observe causality because it exists. This doesn't mean causality can't exist independently of any observer, only that it would be foolish to assume that whatever you think is causality a) is causality, b) exists, independently or dependently. I do suggest to read up on the different theories on what causality could be like (foregoing whether it actually exists), and on how all of them suffer from unsolveable problems. Just to make things clear, however: https://www.youtube.com/watch?v=nX7CeTXoxyU