I'm attending the "Testing Gravity 2025" conference
https://www.sfu.ca/physics/cosmology/TestingGravity2025/
and I thought I'd post a few highlights.

My favorite talk today was was from M. P. Ross of the University of
Washington 'E\"{o}tWash' group. This group
https://www.npl.washington.edu/eotwash/node/1
is world-renowned for their work on high-precision tests of Newtonian gravitation and the equivalence principle. (The group name is a portmaneu
of 'E\"otv\"os' (the Hungarian physicist who did pioneering experiments
of this type in the late 1800s) and 'Washington'.) Ross described a
number of beautiful experiments the E\"otWash group have done, using
torsion pendulums to test whether various objects have the same free-fall acceleration.

The basic idea here is to have a torsion pendulum where the suspended
mass is a dipole of two different materials. In the Earth reference
frame, the rest of the universe rotates about the Earth at a frequency
of 1 cycle per siderial day (about 23hr 56min), so if the two materials
have different long-range (typically gravitational) interactions with
the rest of the unverse, the torsion pendulum will experience a
time-varying torque at the siderial rotation frequency.

A baisc preinciple of high-precision measurements is that it's ungood
to try to accurately measure a DC or slowly-varying signal, because
real-world sensors, amplifiers, and other electronics often have
time-varying drifts in zero points (this is "just" a special case of
1/f noise), and it's far to easy for such drifts to mimic a DC or slowly-varying science signal. Instead, it's much better (i.e., you
can get much lower systematic errors and much more robustness against
drifting sensors/electronics/etc) if you can somehow modulate your
signal at some reasonable frequency, so that the "accurate measurement"
is of the *AC* signal at at the modulation frequency.

To apply this principle to a torsion pendulum, the E\"otWash group
mounts the entire apparatus (torsion pendulum and readout equipment)
on a air-bearing rotating turntable, rotating with respect to the
laboratory at a frequency of (typically) about 2 cycles per hour.
This way any external torques are modulated by the combination of
the turntable rotation frequency and the siderial rotation frequency.
See arXiv:1207.2442 for more background on (and results from) this
technique.

Ross described a very nice recent experiment (arXiv:2407.21232) to
test whether superconductors, and their Cooper pairs of electrons,
have the same free-fall acceleration as "ordinary" matter. I didn't
know this before hearing this talk, but it has been suggested that
the non-local quantum-mechanical properties of Cooper pairs in a
superconductor may lead to violations of the weak equialence principle,
i.e., might lead to the Cooper pairs -- and hence the superconductor
-- *not* having the same free-fall acceleration as "ordinary" matter.
In this recent experiment, the E\"{o}tWash group) were able to show
that copper and superconducting niobium have the same free-fall
acceleration to about 2 parts in 10^9. Assuming (consistent with
other experiments) that the rest of the niobium has the same
free-fall acceleration as copper, this result implies that the
niobium's Cooper pairs have the same free-fall acceleration as
copper to within about 1 part in 1000.

Ross also described an experiment they have in progress to test how
various masses free-fall in the gravitational field of dark matter,
in particular, the dark matter halo around the center of our galaxy.
For this experiment they wanted maximum sensitivity to possible
differences in coupling to baryons vs leptons, so they chose aluminum
and beryllium (which have significantly different fractions of their
rest masses composed of baryons/leptons) for the torsion-pendulum
dipole. The experiment is still running, but they already have enough
data to show that Al and Be free-fall at the same rate in the
dark-matter gravitational field to within (I think I got this number
right) about 2.5 parts in 10^5. Given the different baryon/lepton
fractions in Al and Be, this should let them put an interesting
constraint on how dark matter interacts (gravitationally) with
baryons vs leptons.

ciao,

--
-- "Jonathan Thornburg [remove -color to reply]" <dr.j.thornburg@gmail-pink.com>
on the west coast of Canada
The Three Laws of Thermodynamics:
1) You can't win, only lose or break even.
2) You can only break even at absolute zero.
3) You can't get to absolute zero.

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