Amorphous liquids behave as mechanically
rigid solids on time scales below the relaxation time, which grows unbearably
long as the temperature is lowered or as the pressure is raised. These two
pathways to the glass transition are equivalent for the system of thermal hard
spheres. Dimensional analysis suggests that the relaxation time τ , made dimensionless
as τ (Pσd-2/m)
½ by pressure P, sphere
diameter σ, and sphere mass m, must depend only on the dimensionless ratio T/Pσd,
where T is temperature, and d is dimensionality. Thus, the dimensionless
relaxation time increases in exactly the same way whether T is lowered or P is
raised, and depends only on the ratio of the thermal energy to the
pressure-volume needed to open up a hole of the same order as the particle
We conducted experiments 
in which spheres roll stochastically due to turbulence in a uniform upflow of air. Here the relaxation time grows as the air
flow is decreased or pressure is raised.
The thermal energy of the beads is negligible compared to their kinetic
energy, which corresponds to a well-defined effective temperature Teff . Pressure is applied by tilting the sample,
and is the projected weight per length as required for hydrostatic equilibrium.
Remarkably, the dimensionless relaxation
time for this granular system depends only on the ratio of Teff to the pressure-volume work needed to open up a hole of
the order of the particle size, just as for thermal hard spheres. Thus, the
effect of pressure, a mechanical load, is directly related to the effect of
kinetic energy supplied by driving the system via air flow.
Figure 1. (a) Uncollapsed data for relaxation
time vs. pressure. (b) Collapsed data
for dimensionless relaxation time vs. inverse dimensionless pressure.
 Phys. Rev. Lett. 108,
 Phys. Rev. Lett. 101,