1. 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)

2. 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?

3. 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)

4. 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
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

6. 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?

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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)