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

● Test by dropping different kinds of test materials in same gravitational field ● 400 years of testing have established that ● STEP will improve this by 5 orders of magnitude to

10 -18

Remember this range STEP

LLR (Williams

et al.

, 1996) Eöt-Wash (Adelberger

et al.

, 1990) Roll, Krotkov & Dicke (1964) Galileo (1610) Newton (1686) Bessel (1832) 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 tests the

foundation---not a prediction---

of Einstein's theory of general relativity, which holds up half of present-day physics:

GPB, LIGO, LISA, etc.

Human progress Science & technology

Fermilab, CERN, LHC, etc.

Physics General relativity Standard model (Theorist's view not to scale)

STEP

General covariance

Equivalence principle

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: ● Physics in crisis, circa 1900: Gravity ( Einstein's theory ) Everything else (Standard model of particle physics) Electromagnetism (Maxwell's theory) Everything else (Newtonian mechanics)

Equivalence principle

STEP Theory of everything?

Crucial experiment : Resulting unified theory:

Constancy of the speed of light

Michelson-Morley experiment Special relativity

What to drop?

current STEP baseline

● heavy vs. light elements?

● baryons vs. leptons?

● different particle generations?

● matter vs. antimatter?

● fermions vs. bosons?

● matter vs.

dark

matter?

● in a first experiment, span widest possible region in parameter space (e.g. Blaser, Damour, others):

What might STEP find?

●

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:

cf.

STEP

target

●

SPECIFIC THEORIES:

those making quantitative predictions (e.g. Damour

et al.

Dvali

et al.

runaway dilaton, Dimopoulos et al. “little strings”, 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:

10 -18

(current constraint)

10 -9

( STEP sensitivity)

● 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 new force of nature discovered but absence of coupling requires new protective symmetry or some other explanation

path to unification

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

Complementarity

LHC

Fundamental physics triangle: Particle physics Cosmo logy

Scalar fields

(dilatons, moduli, ...)

Gravity

SNAP

Dark energy

(quintessence, variable

a

, ...)

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)