April 2019, ScientificAmerican.com 51this [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 fromthe 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 unobservablySUPERCONDUCTING
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© 2019 Scientific American © 2019 Scientific American