A handwritten letter by Albert Einstein goes on auction tomorrow on eBay. The online auction site has set the opening bid at a staggering $3 million. That’s more than seven times as much as the letter fetched just four years ago. “This is the most historic and significant piece we have listed on eBay,” said Eric Gazin, president of Auction Cause, the Los Angles-based agency consigned to sell Einstein’s two-pager. So what makes this particular letter by the 20th century’s most renowned physicist so much more valuable than any other missive he hand wrote? Did he scribble his famous formula...