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)

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 Gravity on earth---or acceleration in space Falling on earth--- or floating in space Holds regardless of the mass or composition of the test body (not true for any other forces, e.g. magnetism) Implication: gravity is a property of spacetime, not matter (Einstein identified the relevant property of spacetime as its curvature) But is this equation exact? Is gravity perfectly geometrical?

Testing equivalence 10 STEP Test by dropping different kinds of test materials in same gravitational field 400 years of testing have established that Remember this range 10 -18 STEP STEP will improve this by 5 orders of magnitude to Galileo (1610) Eöt-Wash (Adelberger et al., 1990) LLR (Williams et al., 1996) Bessel (1832) Roll, Krotkov & Dicke (1964) Newton (1686) Eötvös (1922) Simon Stevin (1586)

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

Discovery potential STEP STEP tests the foundation---not a prediction---of Einstein's theory of general relativity, which holds up half of present-day physics: Fermilab, CERN, LHC, etc. Standard model Human progress Science & technology Physics (Theorist's view - not to scale) GPB, LIGO, LISA, etc. General relativity covariance General Equivalence principle STEP

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”)! Physics in crisis today: Gravity (Einstein's theory) Everything else (Standard model of particle physics) Electromagnetism (Maxwell's theory) Everything else (Newtonian mechanics) Physics in crisis, circa 1900: Constancy of the speed of light Equivalence principle Michelson-Morley experiment Crucial experiment: STEP Special relativity Resulting unified theory: Theory of everything?

What to drop? heavy vs. light elements? baryons vs. leptons? current STEP baseline in a first experiment, span widest possible region in parameter space (e.g. Blaser, Damour, others): nuclear electrostatic energy heavy vs. light elements? baryons vs. leptons? different particle generations? matter vs. antimatter? fermions vs. bosons? matter vs. dark matter? total baryon charge neutron/proton difference

What might STEP find? STEP cf. target DIMENSIONAL ARGUMENT: any attempt to combine gravity with quantum field theory generically predicts new effects above some characteristic energy scale E (Adler 2006): predicted = standard where and: STEP target cf. SPECIFIC THEORIES: those making quantitative predictions (e.g. Damour et al. runaway dilaton, Dimopoulos et al. “little strings”, Dvali et al. variable-a) all predict values of in this range

Importance of a null result GENERIC APPROACH: supplement standard model with simplest possible generic new field: one scalar field couples to standard-model matter via dimensionless couplings standard calculations (Carroll 1998 and others) show that if then: (current constraint) 10 -18 (STEP sensitivity) -9 dimensionless couplings as small as are natural – barely! (cf. ) couplings smaller than are not! (cf. “strong CP problem”) Ed Witten (2000): “It would be surprising if f exists and would not be detected in an experiment that improves bounds on EP violations by 6 orders of magnitude” 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

A win-win experiment Unification generically requires the existence of new fields if these couple to matter if they don't EP violation no EP violation but absence of coupling or some other explanation requires new protective symmetry new force of nature discovered path to unification We receive critical experimental guidance, whether EP violations are detected or not

Complementarity STEP: SNAP LHC EP violations Dark energy Fundamental physics triangle: Particle physics Cosmo- logy Gravity Dark energy (quintessence, variable a, ...) Scalar fields (dilatons, moduli, ...) STEP: 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

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)