The Science Case for STEP

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The Science Case for STEP
(Satellite Test of the Equivalence Principle)
Quantum to Cosmos 3, Airlie Center, 8 July 2008
James Overduin (Gravity Probe B, Stanford
University and Maryland Space Grant Consortium,
Johns Hopkins University)
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The equivalence principle

• Einstein's happiest thought (1907) the
  effects of gravity can be locally
• transformed away by accelerating along with
  the test body

• I.e., gravitation acceleration

• But is this equation exact? Is gravity
  perfectly geometrical?

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Testing equivalence

• Test by dropping different kinds of test
  materials in same gravitational field

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STEP concept

• Essentially an orbiting version of Galileo's
  drop-tower experiment

• More time for separation to build, and signal is
  periodic

• Larger driving acceleration and quieter
  seismic environment allow
• for improvement of 5-6 orders of magnitude over
  terrestrial tests

• Inherits key technologies from Gravity Probe B
  drag-free control,
• cryogenic SQUID readout, electrostatic
  positioning system, others

5
Discovery potential

• STEP tests the foundation---not a
  prediction---of Einstein's theory of
• general relativity, which holds up half of
  present-day physics

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Crisis and unification in physics

• Einstein's geometrical theory of gravity based
  on the Equivalence Principle
• cannot be unified with the rest of physics (the
  standard model)!

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What to drop?

• heavy vs. light elements?
• baryons vs. leptons?
• different particle generations?
• matter vs. antimatter?
• fermions vs. bosons?
• matter vs. dark matter?

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What might STEP find?
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Importance of a null result

• a null result closes the door on new fields of
  the kind generically predicted
• by unified field theories, implying that a
  gravity really is purely geometrical
• and a more radical approach is required to
  unify it with the standard model

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A win-win experiment

• Unification generically requires the existence
  of new fields

• We receive critical experimental guidance,
• whether EP violations are detected or not

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Complementarity
Fundamental physics triangle
EP violations

• LHC may find indirect evidence for new fields
  (missing-energy signature)

• SNAP may find direct evidence for new fields
  (dynamical dark energy)

• only STEP can tell us how these fields interact
  with everything else

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Summary

• STEP tests foundations (not predictions)
• of our current theory of gravity

• Points the way toward unification of
• general relativity and the standard model
• of particle physics, regardless of what it finds

• Predictions for violations of the Equivalence
  Principle fall within STEP's
• grasp and probably only STEP's grasp
  whether based on dimensional
• arguments, specific theories, or generic
  calculations involving scalar
• fields (as hinted at by unified-field theories
  and cosmology)

• STEP is complementary to high-energy particle
  physics experiments
• (LHC, ILC) and cosmological probes (SNAP, JDEM)