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Negative-Stiffness Vibration Isolation Aids Princeton’s South Pole Lab
By Jim McMahon T
o nullify gyroscopic pick-up of the Earth’s rotation while con- ducting Lorentz symmetry test-
ing, Princeton University’s Romalis group set up a field lab at the Amund- sen-Scott Station at the South Pole. The lab contains an ultra-precise atomic spin co-magnetometer in a vacuum, equipped with negative-stiff- ness vibration isolation. With the ef- fects of the Earth’s rotation almost completely suppressed, the group’s Lorentz symmetry test results im- proved by two orders of magnitude compared with testing at Princeton’s base facility in New Jersey. Lorentz invariance or symmetry,
a set of fundamental frameworks that underpin modern science and physics in particular, lies at the foundation of quantum field theory (QFT) and Ein- stein’s theory of general relativity, the two most successful theories in physics, which together describe the four fundamental forces of nature. In physics, particularly electro-
magnetism, the Lorentz force is the combination of electric and magnetic force on a point charge due to electro- magnetic fields. While modern Maxwellian equations demonstrate how electrically charged particles and currents, or moving charged par- ticles, give rise to electric and mag- netic fields, the Lorentz force law completes that picture by describing the force acting on a moving point charge in the presence of electromag- netic fields.
Lorentz Symmetry Testing The inability to incorporate
gravity, however, as described by general relativity into the QFT stan- dard model of particle physics has led to the development of alternative theories of quantum gravity. Since many of these theories break Lorentz symmetry at some small level, exper- imental searches for Lorentz-violat- ing effects could help shed light on new physics beyond the standard
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model, and provide clues as to the nature of quantum gravity. Some of the most precise tests re-
lating to Lorentz symmetry are being performed by the Romalis group at Princeton University. “Lorentz sym- metry underlies all of the known
In physics, particularly electromagnetism, the Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields.
forces of nature, providing one of the few links between gravity and quan- tum mechanics,” says Michael Roma- lis, Ph.D., professor of physics and head of the Romalis group. “It postu- lates that laws of physics are invariant under rotation, and remain the same in a moving reference frame. Lorentz symmetry is also closely connected to charge-parity-time (CPT) reversal symmetry that enforces the equiva- lence of particles and anti-particles.” Romalis uses ultra-high preci-
sion techniques involving polarized atomic spin to test Lorentz symme- try. “The presence of Lorentz viola- tion would appear as an effective field felt by the atoms. Presumably, this field acts as a cosmically fixed background which, from the point of view of our Earth bound experiment, fluctuates with a sidereal period as the Earth rotates,” he adds.
Effects of the Earth’s Rotation An alkali metal, noble gas co-
magnetometer, enclosed within a vacuum chamber, is used in the group’s experiment to very sensitive- ly measure fields that couple to atomic spin, while suppressing mag- netic field interactions. “Polarized atoms in the co-mag-
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netometer are extremely sensitive to rotations,” says Romalis. “At Prince- ton, we pick up a large background signal due to Earth’s rotation. At the South Pole, we can almost complete- ly eliminate that signal.” Vibrations in the range of few
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hertz to a few tens of hertz will influ- ence the testing. These internal and external influences primarily cause lower frequency vibrations which are transmitted through the structure, creating strong disturbances in sen- sitive equipment. Vibration within this range can
be caused by a multitude of factors. Every structure is transmitting noise. Within a building itself, the heating and ventilation system, fans, pumps and elevators are just some of the mechanical devices that create
vibration. External to the building, the
testing can be influenced by vibra- tions from vehicle movement, nearby construction, noise from aircraft, and even wind and other weather condi- tions can cause movement of the structure. Romalis selected a negative-
stiffness vibration isolator, cus- tomized to be only mildly magnetic, for its Lorentz symmetry testing, both at Princeton University and at the South Pole.
Negative-Stiffness Isolation Developed and patented by Mi-
nus K Technology, negative-stiffness isolators employ a completely me- chanical concept in low-frequency vi- bration isolation, with no air or elec- tricity required. What is advantageous about
negative-stiffness isolators is that they achieve a high level of isolation in multiple directions. Negative-stiff- ness isolators have the flexibility of custom tailoring resonant frequen- cies to 0.5 Hz vertically and horizon- tally, with some versions at 1.5 Hz horizontally). For an isolation system with a
0.5 Hz natural frequency, isolation begins at 0.7 Hz and improves with increase in the vibration frequency. The natural frequency is more com- monly used to describe the system performance.
Vertical-motion isolation is pro-
vided by a stiff spring that supports a weight load, combined with a Nega- tive-stiffness mechanism. The net vertical stiffness is made very low without affecting the static load-sup- porting capability of the spring. Beam-columns connected in series with the vertical-motion isolator pro- vide horizontal-motion isolation. A beam-column behaves as a spring combined with a negative-stiffness mechanism. Negative-stiffness isolators do
not require electricity or compressed air. There are no motors, pumps or chambers, and no maintenance, be- cause there is nothing to wear out. They operate purely in a passive me- chanical mode. If equipment can be isolated
from vibrations, without having to deal with compressed air or electrici- ty, then it makes for a system that is simpler to transport, and easier to set up and maintain. For the Romalis group’s Lorentz symmetry testing at the South Pole, this has proved ad-
vantageous. Contact: Minus K Technology,
Inc., 460 Hindry Avenue, Unit C, Inglewood, CA % 310-348-9656 fax: 310-348-9638 E-mail:
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