Ph.D. is not given for one's knowledge... you need to at least have knowledge of it. It is given for one's contribution in the advancement of knowledge. So it is not that it needs to be refreshed every 5 years or so. The contribution made is alreay done and it will not vanish in the next 5 years!
Lucky this is not a few hunderd years back where some one could have been hanged for such a stupidity... Words seems to be okay!
