What is energy? It all comes down to the work of a patent clerk in Switzerland at the turn of the last century
To understand E=mc2 it helps to go back to the year 1904. Albert Einstein was then an unknown 25-year-old, who had offended his professors at university so much because of his lack of obedience that they had refused to write him letters of recommendation to get a good job. He had ended up as a patent clerk in Bern, Switzerland, where the town's one science library was closed on his day off, so he couldn't keep up with the latest research.
When he did try to work on his own ideas at the office, he had to stop whenever his supervisor got close, and slam his notes into a drawer - which he jokingly called his "department of theoretical physics" - so no one would see. And that was the ideal preparation for what he was about to achieve.
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For in 1904, everyone in science believed that the universe was divided into two great realms. On the one hand there was the realm of energy, where winds blew, coal burned and lightning crackled. On the other hand, there was the realm of mass, where trees and mountains and paperweights, and perhaps even irritating patent supervisors, existed.
Those two realms were thought to be entirely separate. There was no link between the two.
That's what Einstein managed to challenge. He was close enough to his university days that he was still up to date on technical tools, but he was far enough away - isolated in the patent office - that he wasn't locked into the consensus of other professional physicists.
There had been a few hints that something was wrong with the standard view that energy and mass were entirely separate. Marie Curie in Paris, for example, had found that certain metal-streaked ores - radium compounds especially - managed to send out glowing energy for hour after hour, month after month, without ever seeming to run out of power. It was such an odd finding that most people just ignored it.
As 1904 turned into 1905 and Einstein approached his 26th birthday, something clicked in his personality and raised his creativity to a higher level. There was, he gradually realised, a link after all between the two realms. Solid matter could explode apart and reveal hidden energy.
Realisation
No one had recognised this before. It was as if people had lived on a planet where ordinary wood never ignited. Even more, Einstein realised just how much energy is held concentrated within ordinary mass. The "c2" in his equation can be thought of as a huge number.
It's like the reading on a pressure valve in a complex plumbing system. Because c2 is so big, what Einstein found is more than just saying that burning a little piece of paper will release a little bit of energy. On the contrary.
The plumbing system is under enormous pressure. Ordinary mass is so dense and concentrated that when it is let "loose", a vast amount of energy gushes out.
In the bomb that exploded over Hiroshima in 1945, for example, only a few ounces of uranium were entirely transformed into energy. But it was enough to destroy a city.
The operations of Einstein's E=mc2 pervade our universe. The sun itself can be seen as a giant pumping station, floating in space. Every second, millions of tonnes of hydrogen within it vanish from existence. In its place, great amounts of energy emerge: enough to heat our planet, and glow on through the solar system. Our very existence stems from the equation, for it also operates in "reverse". Not only does it say that mass can explode apart into energy, but energy can be squeezed tight to end up as ordinary mass.
This means that if two beams of pure light are shined right at each other, solid particles can pop into existence where they collide. Ordinary torch beams won't be strong enough to do this. But when the universe was very young, it was filled only with light, carrying tremendous amounts of energy.
Where those light beams collided, they hit hard enough that the transformation the equation described did take place. Bits of light "disappeared", and solid mass appeared in its place. That led to atoms, stars, planets and, ultimately, even to one patient, whimsical patent clerk , wondering how mass and energy work.
· This article was amended on Tuesday May 13 2008. We originally referred to "millions of tonnes of helium within [the sun] vanish[ing] from existence"; we meant hydrogen, not helium. This has been corrected.