Gravity’s Equal Pull

Atoms Fall as Fast as Cannonballs 

A high-tech experiment has confirmed on an atomic level what Galileo knew 400 years ago.










 

ABC News SCIENCE  30 August 1999






By dropping atoms, physicists have measured the force 

of gravity to within a few parts per billion. 

by Kenneth Chang  ABCNEWS.com







Legend, perhaps only myth, is that Galileo dropped a musket ball and a 

cannonball off the Leaning tower of Pisa to show that a small, light 

object drops to the ground just as quickly as a bigger, heavier one. 



Scientists have now demonstrated that gravity is equally efficient at 

tugging objects a great deal smaller than musket balls: individual 

atoms. The experiment, described in the Aug. 26 issue of Nature, 

measures the force of gravity to a within a few parts per billion. 



"It’s the quantum equivalent of Galileo’s experiment," says Stanford University physicist Steve 

Chu, the paper’s senior author. "There was no reason to suspect that isolated atoms would fall 

differently than more massive objects. Nevertheless, you want to establish that." 



Counter to Everyday Experience

Galileo’s insight on falling objects ran counter to the prevailing thought, as well as to the 

everyday experience of people then and now.  After all, a rock drops much faster than a feather 

or a sheet of paper. However, that’s just because air molecules slow down a flat sheet or a 

soft feather more than a dense, smooth-surfaced rock.  In an airless environment — on the moon, 

for instance — the feather, the paper and the rock would soundlessly hit the ground at the same 

moment. 



If you dropped some atoms, they, too, would fall at the same rate.  In the Stanford experiment, 

Chu’s group trapped atoms of the element cesium within the light beams of six lasers and cooled 

them to within two-millionths of a degree above absolute zero, minus-459 degrees Fahrenheit, 

the point at which all movement stops. 



Altering the frequency of one of the downward-pointing lasers causes the cesium atoms to shoot 

upward at about 5 mph. "They then can be considered objects like a baseball tossed up and down,"

Chu says.  Measuring how quickly the atoms slow down during the upward arc gives a precise 

measurement of the acceleration due to gravity, which is approximately 32 feet per second 

squared at the Earth’s surface. 



Inconstant Gravity

The latest results agree with an older, equally precise technique involving shining lasers off 

falling glass prisms. That shows that gravity acts the same on big and small objects.  Chu has 

been performing similar experiments for years, but, "What he has done now is crossed all the t’s

and dotted the i’s," comments William Phillips, a physicist at the National Institute of 

Standards and Technology. Phillips shared the 1997 Nobel Prize for physics with Chu for the 

underlying laser-cooling technique. 



"The quality of these measurements is as good as the best measurement techniques of all other 

kinds that have been used up to this point,” Phillips says. “It has the potential of getting 

better still."  With accuracy of a few parts per billion, the answer jumps around almost 

constantly. 



The shifting waters of high and low tides nudge the force of gravity by a few parts per 

billion.  Shifting temperatures — with the resulting expansion and contraction of air density — 

can shift gravity’s pull by a similar amount. Moving the apparatus up or down a little can also 

change the answer. 



Chu’s ultra-accurate technique could lead to new instruments that "allow us to test things 

about the nature of the gravitational field," Phillips says. "Then probably it will result in 

changes in the way Einstein thought about gravity." 



While Albert Einstein’s theory of general relativity has been very successful in describing 

most of gravity’s properties, physicists have not yet been able to “unify” gravity with 

nature’s other fundamental forces, such as electromagnetism.  Another, more practical, 

potential benefit: the technique could lead to instruments that enable geologists to search 

more easily and quickly for oil. Oil-containing rocks tend to be less dense, with a weaker 

gravitational pull, than solid rocks. 



Even as gravity fluctuates, though, it is still an equal opportunity force. Left to gravity’s 

pull, even as it waxes and wanes, atoms still fall as fast as cannonballs.  



The Associated Press contributed to this report.

 



Clocking a Speeding Atom

How to measure the speed of an atom? A radar gun doesn’t suffice.  Chu and his collaborators 

rely on a bit of quantum mechanical trickery. They shine a laser pulse as the atoms shoot 

upward, which knocks each cesium atom into quantum schizophrenia; the original is joined by a 

nearly identical twin, and the two coexist in a half-and-half mix. After a moment, a second 

laser pulse causes the twin to merge into the original atom.  For these twinning, untwinning 

quantum metamorphoses to occur, the laser pulses must be precisely tuned to specific 

frequencies that depend on the atom’s speed. Without gravity, the frequency of the second 

pulse would be identical to the first.  With gravity, the atom’s speed slows slightly in the 

moment between laser pulses. That in turn changes the frequency needed for the second pulse. 

Figuring out that shift in frequency tells how much the atoms have slowed and thus, by 

calculation, the acceleration due to gravity.  Chu shared the 1997 Nobel Prize in physics for 

his laser-cooling invention dubbed “atomic fountains,” the same technology used in the 

atom-dropping experiment. It’s also used to improve the precision of atomic clocks.