An ordinary camera flash lingers for hundredths of a second, but for atoms, that's an eternity. Inside molecules, electrons jump in attoseconds: billionths of a billionth of a second. To capture that, you need a light pulse shorter than the event itself.
Physicists take an electron beam in a linear accelerator and compress it so that individual bunches become thinner than a hair — in time. The beam then passes through a magnetic 'comb' that makes the electrons wiggle and emit X-rays. If the bunch is short enough, a single flash is born.
The new part is the compression method. Electrons are accelerated with a huge energy spread: the front runners sprint ahead, the back ones lag behind, like runners in a long-distance race. At the end of the track, this spread is carefully compensated, and the bunch collapses into a point. The result is a flash with a spectral width greater than 30 electronvolts — twice as wide as previously achieved.
You can then repeat the trick: compress the bunch after it has emitted, and make it produce a second, even more powerful flash. Or use the bunch's own electric field to 'push' atoms in a sample, like a sudden gust of wind shaking branches.
Attosecond X-ray flashes are a strobe light for the microworld. With them, you can see an electron hopping from orbit to orbit, or a chemical bond breaking. The very flash that freezes in a photographer's hand is here squeezed down to a scale where time nearly stands still.
In one attosecond, light travels a distance of three angstroms — about the size of an atom.