Dimopoulos and others proposed in 1998 that gravity operates in extra dimensions of space beyond the three we can see, and that this dilutes its apparent strength in the everyday world. The idea came from string theory, the leading approach to quantum gravity, in which extra dimensions of space contain vibrating loops of "string" that give rise to all the familiar forces and particles. Trouble is, this idea can only be tested if we probe at suitably small scales - something that is technically difficult, to say the least.
Enter Kasevich, who has built up considerable expertise in a technique called atom interferometry, which he has used to measure the acceleration of atoms due to gravity, among other things. This high-precision method became feasible in the mid-1990s, thanks to major advances in the cooling of atoms. In 2002, Kasevich happened to run into Dimopoulos in the departmental coffee room, and the two hatched a plan to use the technique to push general relativity as far as it would go - and possibly discover new physics.
Zero in on gravity
Most tests of relativity have taken the form of observations of what goes on in outer space. This makes them difficult if not impossible to control. "You can't change the orbital velocity of Mercury," points out Jason Hogan, a graduate student in Kasevich's lab. With atom interferometry, more control is possible. "You can change the launch velocity of the atoms in our experiment," Hogan says. There are other inherent advantages too. "The atom is a very clean system, easily isolated from outside forces," Kasevich says. That enables the researchers to zero in on the effects of gravity.
Not that atoms are easy to work with. The experiment taking shape at Stanford blends the Tower of Pisa method with ultra-modern technology (
www.arxiv.org/gr-qc/0610047). In the interferometer shaft, a few million rubidium atoms will be cooled by lasers to a few millionths of a degree above absolute zero. Then they will be launched upwards by a precisely tuned blast of laser light from below. This is already impressive - imagine trying to kick a cloud - but the really mind-bending part of the story is what happens next.
According to the weird laws of quantum physics, each rubidium atom is in two places at once. If you tune the laser just right, "half" of the atom is launched upwards rapidly by the laser blast, and "half" of it is launched more slowly. While the atom is in this strange combination state, it is hit by a second blast of laser light that acts like the mirror image of the first: it excites the slow half of each atom's split personality, and slows down the fast half. After 1.3 seconds - the time it takes for the atom to rise to the top of the 10-metre vacuum chamber and fall back down again - the two halves catch up with each other and merge, thanks to a third, coordinating laser blast (see Diagram).
Except that they don't exactly merge - they interfere with each other, and hence the instrument is an interferometer. According to quantum physics, each atom behaves like a wave as well as a particle, and its history is encoded in its "phase", the exact timing of the wave's vibrations in space. When a wave interacts with itself after taking two different paths, it produces a distinctive pattern of bands called interference fringes.
Red balls, green balls
How will these show up in the experiment? Imagine, for simplicity, that each half of each rubidium atom's split personality is a ball of a different colour: green for the ones that started out fast and then slowed down, and red for those that started out slow and then speeded up. It looks as if you have launched a million red balls and a million green balls, and you might think you would see a million red balls and a million green balls coming back down. But that's not what happens. Each atom's quantum oscillations make it appear to change from red to green and back many times per second. Because of the interference, the researchers might detect two million red balls in one place, and two million green balls in another. According to general relativity, gravity will slow down these oscillations by millions of cycles over the course of one second. Any discrepancies due to violations of the equivalence principle, if they exist, will be much smaller - a shift of less than a millionth of a cycle - but still detectable.
What this means is that Kasevich and Dimopoulos can take two isotopes of rubidium with different atomic masses - rubidium-85 and rubidium-87, directly analogous to Galileo's light cannonball and heavy cannonball - and see whether gravity acts upon them in exactly the same way. If it doesn't, it would shake the foundations of relativity and perhaps usher in the era of quantum gravity. If the equivalence principle holds up, the researchers plan to test several unconfirmed predictions of relativity (see "What Einstein knew").
) happy with the concept of superposition - "each rubidium atom is in two places at once" doesn't really make sense to me....why 2 places? Why only 2 places? Are rubidium isotopes (or other radioactive isotopes) special cases? 
Please excuse my ignorance there, everyone. I must have been doing too much inertial frame dragging recently. It's kinda hard to lug the Universe around with you everywhere...
). It's only when people won't budge from those notions despite being shown where they break down that I ever get annoyed.