Geophysical tests of gravitational physics with superconducting gravimeters

1
Geophysical tests of gravitational physics with
superconducting gravimeters

• Sachie Shiomi
• Space Geodesy Laboratory,
• Department of Civil Engineering,
• National Chiao Tung University,
• Hsinchu, Taiwan

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Contents

• Introduction
• Superconducting Gravimeters (SGs)
• Examples of applications to gravitational physics
• The global network of SGs
• Application 1 Testing the universality of
  free-fall
• Application 2 Searching for dilatonic waves
• Summary

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Superconducting gravimeter (SG)

• Sensitive and stable at low frequencies

1 m
Hsinchu SG, operated by NCTU and CMS
4
Working principle
Gravimeter Sensing Unit

• Sphere is levitated by magnetic fields induced
  by currents in the superconducting levitation
  coils.
• Motion of the sphere is monitored by capacitor
  plates.

F 2.5 cm
4.2 K
J. M. Goodkind, Review of Scientific Instruments
70 (1999)
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Applications to gravitational physics

• Earliest work (1976)
• Search for evidence of a preference frame
• (Warburton and Goodkind, Astrophysical Journal,
  1976.)
• More recent works, e.g.
• Test of the inverse-square law
• (Goodkind et al PRD 1993, Baldi et al PRD,
  2001)
• Measurement of gravitational constant G
• (Baldi et al PRD, 2005)

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The GGP network

• The Global Geodynamics Project (GGP) network of
  superconducting gravimeters (1997) about 25
  operating sites.
• To study geophysical signals in global nature,
  i.e. oscillation of the inner core, polar motion
  and wobbles.

D. Crossley, Journal of Geodynamics 38 (2004)
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D. Crossley
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Applications of the global network to
gravitational physics

• We investigate possible applications of the GGP
  network to study gravitational physics.
• One of such applications is testing the
  universality of free-fall.

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Testing the universality of free-fall
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Universality of free-fall

• Every material (point mass) in a given external
  gravitational field falls at the same rate.

http//www.endex.com/gf/buildings/ltpisa/ltpnews/p
hysnews1.htm
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Motivations of testing the universality

• Fundamental principle
• It should be tested as precisely as possible.
• New physics?
• Theories towards the unification of the four
  fundamental forces predict new interactions that
  violate the principle.

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Proposed new forces (spin-independent)
Motivated by Bosons (Spin) Mass Charge References (year)
Conservation of baryon charge Vector bosons (1) massless B Lee and Yang (1955)
Supersymmetry U-bosons (1) Massive (very light) B, Iz Fayet(1986)
String theory Dilatons(0) Dilatons(0) Moduli(0) massive massless massive (millimeter range) Ordinary matter B, Iz, E Ordinary matter Fujii(1971)Taylor and Veneziano (1988) Damour and Polyakov(1994) Dimopoulos and Giudice(1996)
Composition dependent
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Modification of Newtons law
V(r)
Yukawa-potential type
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Current limits
Fischbach and Talmadge, The Search for
Non-Newtonian Gravity (Springer-Verlag, 1999)
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The necessity of variety in experimental
approaches

• The universality has to be tested for various
  putative charges, using different kinds of test
  bodies, at different ranges.
• To confirm experimental results, it should be
  tested by at least two different experimental
  methods.

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The concept
Sun

• If the universality were violated, the inner core
  would move relative to the rest part of the
  Earth.
• Surface gravity changes

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Test bodies inner core and the rest

• Chemical composition
• Density kg m-3
• Inner core (iron, nickel) 13000
• The rest (silicon oxides) 5400
• Gravitational binding energy
• Inner core -3.7 10-11
• The rest -4.2 10-10

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Best observation points

• In Spring and Autumnal equinox points on the
  equator
• In Summer and Winter solstices on Tropic of
  Cancer or Capricorn

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Equation of motion of the inner core
Gravitational stiffness
Damping effect
Violation effect
S. Shiomi, Physical Review D 74, 027101(2006)
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Forced oscillation
When the damping coefficient (k) is sufficiently
small
Surface gravity changes
10-12 ms-2
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Expected sensitivity
Current limits a few parts in 1013 nearly four
orders of magnitude improvement is necessary.
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Improving the sensitivity (1)

• Carrying out coincidence measurements at two
  observatories located opposite side of the Earth
  near the equators.

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Improving the sensitivity (2)

• Development of the data analysis method to
  extract weak signals.
• e.g.
• Non-Linear Damped Harmonic Analysis method (S.
  Rosat et al, J. Geodyn. in press)

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Summary expected signals
Direction along the Earth-Sun line
Frequency once per day once per year
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Future works

• Improving noise reduction methods
• Identification of environmental noise
• Data analyses to extract weak signals
• Figuring out the optimum scheme of global
  observations
• e.g. coincidence measurements
• Improvements of the sensitivity of SGs
• Application of elaborate Earth models

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Conclusions

• The universality of free-fall can be tested using
  a superconducting gravimeter installed near the
  equator to 10-9.
• Some improvements can be expected from global
  observations and applications of advanced data
  analysis methods.

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Search for composition-dependent dilatonic waves
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Introduction

• String theory predicts the existence of relic
  background of the dilaton (a scalar partner of
  the graviton).
• Ordinary macroscopic test masses have dilatonic
  charges, which depend on their internal
  compositions.
• The response of the test masses to dilatonic
  waves is non geodesic.

M. Gasperini, Phys. Lett. B 470, 67 (1999)
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The concept
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Estimation of upper limits (1)
Spectrum of the displacement ( l ) at resonance
Dimensionless energy density for massless dilaton
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Estimation of upper limits (2)
From residual gravity data, an upper limit on the
displacement is 1.1 10-4 m Hz-1/2 at 7 10-5
Hz. (S. Rosat et al, J. of Geodyn. 38 (2004))
Effective viscosity 10-3 1012 Pa s (R.A.
Secco, A Handbook of Physical Constants)
S. Shiomi, Geophysical search for dilatonic
waves (submitted in 2007)
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• Nucleosynthesis and measurements of cosmic
  microwave background
• ?h2100 10-5
• To reach this limit, the effective viscosity has
  to be smaller than 2 106 Pa s.

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Conclusions

• Dilatonic waves can be searched for using
  superconducting gravimeters.
• The sensitivity is currently limited by the
  uncertainty in the Earth model.
• If the effective viscosity were determined to be
  smaller than 2 106 Pa s, this method would
  provide an upper limit better than the
  astrophysical limits.

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Other possible future applications

• Improved tests of the existence of a preference
  frame and an anisotropy of the gravitational
  constant.
• Direct detection of gravitational waves using the
  Earth as the receiver.
• Measurement of G
• Tests for a distant dependence, time dependence
  and a spatial anisotropy.
• Improved tests of the inverse square law.

J. M. Goodkind, Review of Scientific Instruments
70 (1999)
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Summary

• Superconducting gravimeters have been proved to
  be stable and sensitive in geophysical studies
  and also they have been used to study
  gravitational physics since 1970s.
• The global network of superconducting gravimeters
  has been developed to study the Earths interior.
  By using the Earth as the test body, we
  investigate possible applications of the network
  to gravitational physics.
• We have discussed the geophysical test of the
  universality of free-fall and the search for
  dilatonic waves.

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Thank you.

• Any new ideas for possible applications of SGs
  are welcome.