Einstein's Big Idea homepage

(but it's not as simple as you think)

A good equation is not simply a formula for computation. Nor is it a balance scale confirming that two items you suspected were nearly equal really are the same. Instead, scientists started using the "=" symbol as something of a telescope for new ideas—a device for directing attention to fresh, unsuspected realms. This is how Einstein used the "=" in his 1905 equation. Einstein made a giant intellectual leap when he realized that mass and energy are interchangeable—that with the right "conversion factor" of c2, they can straddle an equal sign.

But where did the seemingly mundane symbol at the heart of Einstein's profound equation come from? It can be traced to an enterprising academic of the 1500s named Robert Recorde.

## Birth of the equal sign

Bibles of the 14th century often had text that looked much like telegrams:

IN THE BEGINNING GOD CREATED THE HEAVEN AND THE EARTH
AND THE EARTH WAS WITHOUT FORM AND VOID AND DARKNESS
WAS UPON THE FACE OF THE DEEP

Major typographic symbols were locked in rather quickly once printing began at the end of the 1400s. Texts began to be filled in with the old "?" symbols and the newer "!" marks. Minor symbols took longer.

Through the mid-1500s there was still space for entrepreneurs to set their own mark by establishing minor symbols. In 1543, Robert Recorde, a pioneering mathematics textbook writer in Great Britain, tried to promote the new-style "+" sign, which had achieved some popularity on the Continent. The book he wrote didn't make his fortune, so in the next decade he tried again, this time with a symbol, which probably had roots in old logic texts, that he was sure would take off.

In the best style of advertising hype everywhere, he even tried to give it a unique selling point: "...And to avoide the tediouse repetition of these woordes: is equalle to: I will sette ... a pair of parallels, or ... lines of one lengthe, thus: ====== bicause noe .2. thynges, can be moare equalle..."

It doesn't seem that Recorde gained from his innovation, for it remained in bitter competition with the equally plausible "//" and even with the bizarre "[;" symbol, which the powerful German printing houses were trying to promote. But by Shakespeare's time a generation later Recorde's victory was finally certain.

 Just imagine how Einstein's famous equation might have looked if one of these other proposed symbols for "equals" had triumphed.

 Robert Recorde wanted to make his math books clear and accessible. He was the first to write such books in English, rather than in the Latin or Greek of the educated elite.