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April 2019, ScientificAmerican.com 51

this [there] at a later time,” says Miles Blencowe, a

theoretical physicist at Dartmouth College. “Where

do you start if you don’t even have a time parameter

or a distance parameter?” Lajos Diósi, a theoretical

physicist at the Wigner Research Center for Physics

in Budapest, sums up the conundrum this way: “We

don’t know what will be there, but we know for sure

that there will be a total scrambling of the spacetime

continuity if you go down to the Planck scale.”

Unfortunately for physicists, there is no way to

observe phenomena on the Planck scale and thus no

way to check the predictions of various theories of

quantum gravity to see which of them might be right.

“The situation is not that we do not have theories of

quantum gravity,” says Carlo Rovelli, a theoretical

physicist at Aix-Marseille University in France. “We

do. The problem is that we have more than one.”

In physics, the higher the energy scale of your ex -

periment, the smaller the distance you can probe.

And probing the Planck scale directly would require

a machine more than 15 orders of magnitude more

powerful than CERN’s Large Hadron Collider (LHC)

near Geneva, the largest particle accelerator ever built,

with a circumference of 27 kilometers. As one physi-

cist says, such an accelerator would need to be rough-

ly the size of our galaxy. Machines such as the LHC

bash particles together at nearly the speed of light,

and physicists hope something new will emerge from

the debris. The basic approach is not much different

from blowing up a safe to find out what is inside. The

practitioners of tabletop physics aim to replace brute

force with finesse, like safecrackers listening to the

tumblers of a lock clicking into place. “You’re trading

high energy for high precision is the way I look at it,”

says Eric Adelberger, a physicist at the University of

Washington. “There’s the energy frontier, and there’s

the precision frontier. If you can measure something

really, really well, you can test physics that’s going on

at some really high-energy scale.” Now at least three

groups, including Aspelmeyer’s, are designing exper-

iments to do just that. The scientists are optimistic

that these projects will finally reach the levels of pre-

cision needed to probe into the realm where gravity

goes quantum.

A THOUGHT EXPERIMENT

to understand wHy preCision allows physicists to

indirectly access higher energies, and thus smaller

scales, consider a historical analogue: Brownian

motion. In a paper published in 1905, Einstein

showed that the puzzling random movements of pol-

len grains in a jar of water could be explained by col-

lisions with water molecules, even though the mole-

cules themselves were many orders of magnitude

too small to be observed directly. Aspelmeyer and

other physicists are betting that the unobservably

SUPERCONDUCTING

CIRCUITS ( 1 ) aid the

levitation experiment.

Re searchers are also

trying to measure the

gravitational fields of

millimeter-wide gold

spheres ( 2 ) to observe

gravity closer to the

quantum realm.

1 2

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