Gravity Tested at 52 Micrometers (2020): No Extra Dimensions Yet

The 2020 Eöt-Wash experiment tested Newton’s law of gravity at separations down to 52 micrometers, about the width of a fine human hair, and found it still holds. J. G. Lee and colleagues at the University of Washington concluded that the largest extra dimension that gravity could feel must have a radius below 30 micrometers, the tightest laboratory limit of its kind.

What the paper found

• Newton still wins. Across separations from 52 micrometers to 3.0 millimeters, the measured gravitational torques matched Newtonian gravity. Adding a new force did not improve the fit appreciably.
• No new force above 38.6 micrometers. Any extra interaction as strong as gravity must have a range shorter than 38.6 micrometers, at 95% confidence.
• A limit on extra dimensions. That translates into a bound on the size of the largest extra dimension: its radius must be less than 30 micrometers. The authors describe these as the tightest existing laboratory constraints on “string inspired” new gravitational phenomena.
• Limits on hypothetical particles. The result also sets lower limits on the masses of proposed particles such as the dilaton or a heavy graviton (above 5.1 milli-electron volts).

How they did it

The Eöt-Wash group uses a torsion balance: a pendulum hanging from a thin fiber that twists in response to tiny forces. Below the pendulum, an attractor disk rotates slowly. Both carry thin test bodies cut from 50-micrometer tungsten foil, with patterns of holes arranged with 18-fold and 120-fold symmetry. As the attractor turns, gravity pulls the pendulum back and forth at frequencies set by those patterns, and the two patterns test the law at two length scales at once.

The researchers measured these twisting torques as they brought the two surfaces closer together, and compared them with predictions for Newtonian gravity and for gravity plus a short-range extra force. An electrostatic shield between the bodies blocked electrical forces. Vibrations from the environment prevented them from going below 52 micrometers.

Why it mattered

If gravity spreads into extra dimensions, as in the large extra dimensions model of 1998, it must get stronger than Newton’s law predicts once two masses are closer than the size of those dimensions. For two extra dimensions, ADD originally expected the change between about 100 micrometers and 1 millimeter. Torsion balances are the only instruments that can test that regime directly.

This experiment pushed the limit down from earlier Eöt-Wash work. In 2007, Kapner and colleagues had found that the inverse-square law holds to 56 micrometers and that an extra dimension must be smaller than 44 micrometers (Phys. Rev. Lett. 98, 021101). Both experiments also probe distances below the length scale of about 85 micrometers associated with the density of dark energy, where some theories expect new physics.

What we know now

As of October 2026, no laboratory test has found a deviation from Newton’s law at short range. The 30-micrometer bound is used by theorists to rule out or shape models. For example, the dark dimension proposal of 2023 cites this result and predicts a single extra dimension in the range of about 0.1 to 10 micrometers, just below what the experiment could reach. Pushing torsion balances to shorter distances is the most direct way to test it.

Short-range gravity tests complement collider searches such as the ATLAS monojet search and astrophysical tests such as GW170817’s limit on leaking gravitational waves. Together they define where extra dimensions can still hide; our explainer on the fourth dimension lays them side by side.

Who wrote it

The five authors, J. G. Lee, E. G. Adelberger, T. S. Cook, S. M. Fleischer and B. R. Heckel, work at the Center for Experimental Nuclear Physics and Astrophysics at the University of Washington in Seattle, home of the Eöt-Wash group.

Read the original

Lee, J. G., Adelberger, E. G., Cook, T. S., Fleischer, S. M., and Heckel, B. R. (2020). “New Test of the Gravitational 1/r² Law at Separations down to 52 μm.” Physical Review Letters 124, 101101. DOI: 10.1103/PhysRevLett.124.101101. Free to read on arXiv (2002.11761).