E is for... Einstein, Albert

The Doctor met physicist Albert Einstein on a number of occasions. The Fourth Doctor claimed to have already met him when he was searching for the third segment of the Key to Time. A later encounter occurred when the newly regenerated Seventh Doctor was captured by the Rani on the planet Lakertya. She was attempting to create a Time Manipulator - a huge brain that would allow her to change the history of the universe. To create it, she needed to harness the minds of many scientists. Amongst those whom she abducted was Einstein. Once the Rani had been defeated, the Doctor took the genii home in the TARDIS, and found he had to drag Einstein away from the ship's controls so that he did not learn too much about time travel.

Einstein was also a guest at Frank Sinatra's hunting lodge one Christmas, when the Doctor met and got married to Marilyn Monroe.
In 1945, Einstein experimented with a time machine of his own, which caused him to suddenly appear in the Doctor's TARDIS. The Doctor's new fez had originally been Einstein's. He had with him a flask of special liquid which he thought was a bionic fusion liquid. However, when he drank it he was temporarily transformed into an Ood. The Doctor helped him change back, though the scientist's hair now stood on end. The Doctor recommended he keep it that way as it looked more "sciencey".
When the Truth Monks took over the Earth, one of the false histories they created was having helped Einstein with his famous equation of E=MC squared.

Played by: Tom O'Leary, Nickolas Grace. Appearances: Time and the Rani (1987), Death is the Only Answer (mini-episode, 2011).

• Death is the Only Answer was the first of two mini episodes devised for school children, where they had to write a brief scene featuring only the Doctor and Amy, plus one character from history and one monster, to be filmed on the TARDIS set. The winners were the children of Oakley Junior School. It was broadcast on BBC 3 as part of the Doctor Who Confidential accompanying the Series 6 finale - The Wedding of River Song.