Are there any experiments which test special relativity? I know of the Kennedy–Thorndike experiment, but it involves light. I am curious whether a different kind of experiments showing relativistic effects like time dilation or length contraction have been performed (I'm thinking of experiments involving mechanical devices like mechanic clocks, rather than light clocks). If there are none, why should we accept that relativistic effects occur for other kinds of moving devices (like mechanical clocks), besides those using light (like light clocks). I'm not questioning SR, I just don't know the answer.