A very popular quote attributed to Albert Einstein but did he really say it? If so, what is the original document containing the original explanation of it?
The quote "An alleged scientific discovery has no merit unless it can be explained to a barmaid." is popularly attributed to Lord Rutherford of Nelson in as stated in Einstein, the Man and His Achievement By G. J. Whitrow, Dover Press 1973. Einstein is unlikely to have said it since his theory of relativity was very abstract and based on sophisticated mathematics.
The short answer is: probably no he didn't say this quote. There is no citation supporting this claim.
Another unsourced variants:
You do not really understand something unless you can explain it to your grandmother.
If you can't explain something to a six-year-old, you really don't understand it yourself.
But if you open page 418 of Einstein: His Life and Times (1972) by Ronald W. Clark, it says that Louis de Broglie did attribute a similar statement to Einstein:
To de Broglie, Einstein revealed an instinctive reason for his inability to accept the purely statistical interpretation of wave mechanics. It was a reason which linked him with Rutherford, who used to state that "it should be possible to explain the laws of physics to a barmaid." Einstein, having a final discussion with de Broglie on the platform of the Gare du Nord in Paris, whence they had traveled from Brussels to attend the Fresnel centenary celebrations, said "that all physical theories, their mathematical expressions apart ought to lend themselves to so simple a description 'that even a child could understand them.' "
See all misattributed Einstein quotes here: http://en.wikiquote.org/wiki/Albert_Einstein#Misattributed.
Just to add two quotes that come close.
Peter Singer (2016):
There is a view in some philosophical circles that anything that can be understood by people who have not studied philosophy is not profound enough to be worth saying. To the contrary, I suspect that whatever cannot be said clearly is probably not being thought clearly either.
Attributed to Richard Feynman, by two of his colleagues at Caltech in 1989 (after his death):
Feynman was once asked by a Caltech faculty member to explain why spin 1/2 particles obey Fermi-Dirac statistics. He gauged his audience perfectly and said, "I’ll prepare a freshman lecture on it." But a few days later he returned and said, "You know, I couldn’t do it. I couldn’t reduce it to the freshman level. That means we really don’t understand it."
Daniel Dennett (2013):
if I can’t explain something I’m doing to a group of bright undergraduates, I don’t really understand it myself.