I know that a symmetric matrix has a set of orthogonal eigenvectors if all its eigenvalues are distinct. As far as I know, their eigenvalues might not be distinct. However, some sources claim that symmetric matrices always have eigenbasis.
I know that we can get orthonormal basis using Gram Schmidt on the eigenvectors of each basis. However, I don't think that it guarantees the resultant vectors are still eigenvectors. Shouldn't the Gram Schmidt process turns the eigenvectors into non eigenvectors? If not how to prove that the resultant vectors are still eigenvectors?
So, my question then is if symmetric matrices always have eigenbasis regardless of whether its eigenvalues are distinct? Best if a calculus-free proof for whether or not symmetric matrices have eigenbasis can be given.