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LISA Pathfinder: the complexity of free fall - Michele Armano
1. lisa pathfinder
Michele Armano - LISA Pathfinder Scientist
for the LISA Pathfinder collaboration
2017.09.06 Lake Como School of Advanced Studies “Let’s Face Complexity”
LISA Pathfinder:
the complexity of free fall
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Appetiser: astronomy and cosmology of the EM field
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Astronomy and cosmology of curvature and gravity
•Final form of field
equations of
General Relativity
(GR)
•November 25,
1915 Academy
of Science of
Prussia
•Article
published on
December 2,
1915
geometry of
space-time
contents of mass,
energy and stress
~
causes the form
determines the motion
Rµ⌫
1
2
Rgµ⌫ + ⇤gµ⌫ =
8⇡G
c4
Tµ⌫
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Strain to equivalent acceleration spectrum
1 hour 1 minute
1 second
Sets the minimum detectable signal for a given frequency
How long does the phenomenon last?
Graph per “second of observation” - valid for any physical quantity
Sensitivity
curve
Visible
(signal > noise)
Invisible
(noise > signal)
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What are we discussing today?
•Gravity, GR and gravitational
waves
•production of waves
•the problem of noise
•Observatories
•ground-based: LIGO
•space-based: LISA
• Science and technology of free
fall and noise reduction: LISA
Pathfinder
• From first principles to
observables
• Experiments
• Ground segment and
operations
• Results
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Gravitational Waves?
•GR (perturbations of
Riemann tensor): tidal
waves travelling at the
speed of light
•Matter displacement or
equivalent energy and
momentum change
causes variation of space-
time curvature
•quadrupole+ waves
(momentum
conservation nulls
dipole)
•only 2 polarisations in
pure tensor theories
!2
= G
m1 + m2
l3
h(r, t) ' A(r) cos 2!
⇣r
c
t
⌘
A(r) =
4Gµ!2
l2
c4r
=
8G2
m1m2
c4rl
/
1
r
Binary systems
are prototypes:
plane waves
far from source
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Placing beads or gains of sand in space, at a GW passing…
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How to detect/observe/measure them?
• Ring of particles
• waves give particles
“tidal” accelerations:
simultaneous
stretching and
shrinking of
orthogonal axes of
polarisation
• precision
measurement of
time of vibration
• Maximise rest
distance L to
maximise signal
}
Δx(t)
}
Δy(t)
h =
L
L
⇠
R0j0k
!2
x(t) y(t)
L
⇠
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Why free fall?
•Does it cancel static
gravity?
•Yes
•Why?
•All bodies fall
likewise towards
one another
(and to Earth)
principle of
equivalence
•Tidal effect is visible
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Tides
•When in free-fall
tide is the only
detectable effect
•Who is moving
versus whom?
•Earth tides are
the detectable
effect of Moon
on Earth in the
field of the Sun
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That’s where interferometers come handy!
LIGO (Hanford,WA)
4km 4km
1. suspended mirrors (free fall)
2. laser comes and goes in arms, packets of photons
forming a ruler in space with proper time
3. difference in flight time ideally only due to
dynamic gravitational perturbation
4. distance variation is incredibly
small, hence L made as big as
possible
h ⇠
L
L
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Moreover: observe GW from fluctuation of test masses
Dynamic warping of space-time
Animation courtesy
of S.Vitale
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Lots of forces conjure to the same effect!
e.g. impact with gas molecules
Animation courtesy
of S.Vitale
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It’s also said that we all fall the same way… more or less!
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GWs can be classified w/r to their origin (strain plot)
Pulsar
Timing
SKA LIGOeLISA
Planck
Bicep2
cuerdas cósmicas
y otros bichos raros
Radio mm FIR IRVIS VUV X-ray γ-ray
Audio band
1Hz-10kHz
Ground-based
~ optical astronomy
Space observation
~ radio astronomy
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Hitting a GW: pattern matching
• Model of the physical system
• BH/NS/…: orbital parameters, mass…
• CMB/…: statistical mechanics of
particles species
• Solution of the problem in GR
• energy, impulse, geometry and
propagation
• deduction of waveforms far from the
source
• Waveform
• zoology of sources
• pattern matching
Example of “chirp” at
collapse of BH/NS
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This “chirp” is not that unusual…
Architectural chirping:
windows in perspective
Chirping of birds songs:
luscinia luscinia/thrush nightingale
luscinia megarhynchos/common nightingale
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More complicated waveforms: simulation of orbit and waves
http://adsabs.harvard.edu/abs/2007CQGra..24R.113A
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From theory to Nature
• Collapse of 2 BH of 29
and 36 M☉ at ~1.3
billions of light-years from
Earth
• gravity propagates per
waves carrying
information from the
source
• During the 0.5 s of the
impact: released 50+
more energy of the light
equivalent of visible stars
• Final BH mass = 62
M☉, ΔW in GWs =
2 M☉
Observatory
LIGO Hanford
Observatory
LIGO Livingston
Delay
~7ms
14th September 2015 10:50:45 CET: the
LIGO experiments in USA reveal GWs
for the first time in the history of Homo
Sapiens
MA turning 40
13th September 2015:
Thanks LIGO, what a present!
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The signal Time
Relativeamplitudeh=ΔL/L
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Conversion to sound
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More news from LIGO
Estimate of mass and distance of BH
Estimate limit mass
of gravitons!
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The observatory in space: LISA
•Mirrors/test-masses (TM) in assisted free
fall on spacecrafts separated by millions of
km (signal ~ L)
•Removal of zero-point static g of Earth
•Removal of unwanted phenomena
•Measurement of proper distance (flight
time of photons) in the arms (as in LIGO)
captures the tidal perturbation of GWs
•Combination (TDI) of 3x2 connexions:
•TM to SC
•SC to SC
•SC toTM
SC to TM SC to TM
TM to TM
Sun
1AU
Sol
1 AU
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From the Mission Concept Document to the selection by the SPC
• Submitted on January
13th, 2017
• The LISA
Consortium: 12
EU Member
States plus the
US!
• Selected by the
Science Programme
Committee for an
“L3” launch in 2030+
• LISA will be
reality!
https://www.lisamission.org/proposal/LISA.pdf
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LISA: Sources,Astrophysics, Cosmology and Fundamental Physics
• Massive BH - 1E4-1E8 M☉ -
Formation? Evolution? Role in
structure formation? Luminosity,
redshift, mass of Graviton?
• Extreme Mass Ratio Inspirals,
(EMRIs) - 1-10 M☉ into 1E4-5E6
M☉ - Stellar dynamics in galactic
nuclei? Kerr Metric OK? Exotic
objects? GR OK?
• Ultra-Compact Binaries in the
Milky Way - Explosion
mechanism of type-Ia
supernovae. Formation, merger
rates of compact binaries
• Stochastic Signals - What before
decoupling of CMB? Phase
transitions? Higgs?
Supersymmetry? Warped extra-
dims? Branes?Topological defects?
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Can it be done? Trustworthy science?
Where? Dynamics Orthogonality
Control and
feed-back
Instrument
displacement
Instrument
forces
How? ? ? ? ? ?
Fakes GW? ? ? ? ? ?
Blinds GW? ? ? ? ? ?
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Can it be done? LISA Pathfinder (LPF)
•A high precision
geodesy device and
gradiometer
•free-fall (δa / ⍵c ≪
h) via high-precision
rockets & drag-free
•shielded + low-
noise sensing (δF /
m⍵c, ⍵ δx ≪ h)
•NOT a detector for
GWs (L is too small)
•1arm of LISA to 38
cm!
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The Optical Metrology System on LISA Pathfinder
Goal is to measure changes at picometre level
1,000,000,000,000th of a metre
Assess relative acceleration of test masses
by measuring their relative motion
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Actuators
Sensors
30
Sensors, injectors and controllers
Charge
handling
IS
IS
TM2TM1
Drag-Free &
Micro-
Propulsion
Attitude
Control
Systems
Measurement
processing
Controllers
OB
StarTrackers
Thrusters
Thermo-meters
Particle
detectors
Displacement/rotation
inputs
+
Thrusters
direct
input +
Capacitors
direct
input
+
Force/
torque
inputs
+
Heaters +
Magneto-coils +
UV-lamps +
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C(o + oi) + gi + gn = Dq
o = Sq + on
31
Towards the EOM
ISs & IFO
D-1- C
Thrusters
oi+
gi
+
S
qo
on
gn
TMs reaction
SC reaction
Dynamics ⟂ Control
and feed-backD S C ongn Noise
oi gi Inputs
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= gn + DS-1
onCoi gi + (DS-1
+ C)o
32
Models and residuals
acceleration noiseF, Δx inputs
D SC ongn
g
S
Fit of coefficients in noise
Where? Dynamics Orthogonality
Control and
feed-back
Instrument
displacement
Instrument
forces
How?
Non-null
couplings (1st
order ~Hooke)
Reference errors Control errors Interferometers
Capacitors,
thrusters
Fakes GW? Glitches Jitter
Blinds GW?
Electrostatics,
thermal,
magnetics, self-
gravity
Mixing, cross-talk
Motion
suppression,
electrostatics
Noisy readout Noisy forces
Enhancement by injection
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Experiments scenario
Acceleration
measurement
System
identification &
Cross-talk
Laser
Optics
Free-fall and
unassisted drift
Magnetic
perturbations
Thermal
perturbations
Radiation and
particles
monitoring
Dynamics
Physics zoo (HW and Environment couplings)
Charging
Parasitic
voltages and DC
patches
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Calibration of Interferometry
IS2IS1
OB
TM2TM1
o1o12
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Know thy (DC) potential!
• Patches of charge populate theTMs
surfaces
• Gradients of electric field are
created: noise and fluctuation
into the readout and more
“springs”!
• These potentials can be suppressed
by per-electrode voltage
compensation
• TheTMs potential and total charge
can be measured and varied via UV-
lamps beamingVTM
V1
V2
V3
V...
Vact
V1
V2
V...
Vact
V equiv
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Thermal experiments
IS2IS1
OB
TM1 TM2
Heater 1
1
Thermometers
Heater 3
Heater 2 Heater 4
2
3
4
7
8
5
6
Optical
Windows
heaters and
thermometers
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Thermals
•Contribution to acceleration noise:
•Measure and fit:
gn ! gn + GIS · TIS
= gn +
↵IS1 0
↵IS1 ↵IS2
T1
T2
0 2000 4000 6000 8000 10000
293
time [s]
0 2000 4000 6000 8000 10000
−0.2
−0.1
0
0.1
0.2
time [s]
ΔT[K]
Figure 7.12: Scheme of the absolute temperature of the EH walls and the di erential tem
the EH walls. The signals in this plot are very simplified, however, they g
about the temperature evolution during the test.
0 2000 4000 6000 8000 10000
−80
−60
−40
−20
0
20
40
60
80
displacement[nm]
time [s]
o
1
o
12
Figure 7.13: Displacement of the test masses when perturbing TM 2 with the tempera
shown in Figure 7.12.
0 2000 4000 6000 8000 10000
293
293.5
time [s]
T[K]
0 2000 4000 6000 8000 10000
−0.2
−0.1
0
0.1
0.2
time [s]
ΔT[K]
Figure 7.12: Scheme of the absolute temperature of the EH walls and the di erential tem
the EH walls. The signals in this plot are very simplified, however, they g
about the temperature evolution during the test.
0 2000 4000 6000 8000 10000
−80
−60
−40
−20
0
20
40
60
80
displacement[nm]
time [s]
o
1
o
12
↵IS(T, p) =
ATM
mTM
✓
16
3 c
T3
+
1
2
p
T
+
outgas
T2
◆
' T3
+
T
{a , T, T} : a (t) = T3
(t) T(t) +
T(t)
T(t)
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Magnetic experiments
IS2IS1
OB
TM1 TM2
Coil 1 Coil 2
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Magnetics
•Linear acceleration (x)
•Angular acceleration (y, z)
DDS-LTP
Ref. S2-IEC-TN-3
Version
Magnetic experiments on board the
LTP
Date 16/Jan/2
Page 15
0 500 1000 1500 2000 2500 3000 3500 4000
−10
−8
−6
−4
−2
0
2
4
6
8
10
time [s]
displacement[nm]
o1
o12
Figure 6.1: Relative distance variations between TM1 and TM2 (blue line) and variations o
distance between TM1 and the EH (red line) when a current of 1 mA at a frequ
of 1 Hz is fed to the coil next to TM1. The red line shows that the S/C mot
considerably noisier than that of the TMs.
If the TM magnetisation happens to be homogeneous throughout its volume then we
the simplified expression
1
o1 and o12 for 1 mA current at
1 Hz into Coil 1
N =
D
M ^ B0 + r ^ (M · r) B0+
µ0
r ^ (Benv · r) B0
E
V sin !0t
Fx =
⌧✓
M +
µ0
Benv
◆
· rB0x V sin !0t
+ hB0 · rB0xi
2µ0
V
hB0 · rB0xi
2µ0
V cos 2!0t
1⍵
DC
2⍵
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Planning, Downlink, Uplink
Science
Technology
Operations
Center
POR
2016
02
20
ICE
TM & DA
Mission timeline (set of Payload Operations Requests)
POR
2016
02
21
POR
2016
02
22
POR
2016
02
23
POR
2016
02
24
POR
2016
02
25
...
Full/HK
TM & DA
DataAnalysisandReplan
AOS
LOS
MOC
Remote
analysis
centers
•Telemetry validation
•Analysis with qualified pipelines
•Is the scientific objective met?
•Simulations? re-planning?
•Send inputs to Mission
Operations Control/SC
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2015.11.12 Kourou
2015.11.20 Kourou
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2015.12.03 04:04 UTC
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LISA Pathfinder mission timeline
Date Milestone
3 Dec 2015 Launch, apogee-raising maneuvers, insertion into L1 orbit
11 Jan 2016 Activation of the LISA Technology Package
2 Feb 2016 Unlocking of the TMs and venting to space
15 & 16 Feb 2016 De-caging of the TMs: both TMs free-floating in controlled motion
18 Feb 2016 Alignment of the interferometer
22 Feb 2016 First entry into Science Mode: assisted free-fall
1 Mar 2016 Start of Science Operations
+30 dias
Acceleration measurement and calibration of the parameters of the system. Charge
measurement and first thermal experiments
+30 dias
Measurement of parasitic voltages on the TMs. Progressive reduction of control
authority. Discharge of TMs. Characterisation of the interferometer
+30 dias
Early magnetic experiments. More measurements of acceleration. Measurements of
charge with calibration tones. More characterisation of interferometry. More
thermals and magnetics. Free-fall with drift and jumps. Final calibration
29 Jun - 1 Dec 2016 Start of operations of the Disturbance Reduction System (NASA/JPL)
1 Dec 2016-30 Jun 2017 Mission extension
6 Apr 2017 Pulling-out of cruise orbit
18 Jul 2017 Passivation of the SC. End of the LPF mission
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LPF Operations Worldwide
Cebreros
Antenna
GSFC
(Greenbelt)
New Norcia
Perth
Malargüe
Antenna
JPL [DRS/ST7]
(Pasadena)
UTN
(Trento)
IEC
Barcelona
ICL
(London) AEI
(Hanover)
UGL
(Glasgow)
Operations Centre
PI Center / DA
Ground Station APC
(Paris)
ESTEC
(Noordwijk)
Kourou
ETH
(Zurich)
ESAC [STOC]
(Madrid)
ESOC [MOC, STOC]
(Darmstadt)
Rome Univ.
TorVergata
Urbino
Univ.
ESAC
Madrid
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LPF for your friends and families…
3E-16 kg:
e.g. Prochlorococcus cyanobacteria
the smallest and most diffuse
photosynthetic organism on Earth
https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)
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And we’ve made it!
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.231101
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LISA and LISA Pathfinder requirements
g(t)
.
= ¨x(t) + !2
2 x(t) + !2
12x1(t) gc(t) g⌦(t)
Strain to equivalent acceleration spectrum
10-14m s-2: 1 millionth
of a billionth of acceleration
due to gravity on Earth (Femto-g)
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First day of operation. March 1st, 2016
g(t)
.
= ¨x(t) + !2
2 x(t) + !2
12x1(t) gc(t) g⌦(t)
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April 8-14, 2016 - Phys. Rev. Lett. 116, 231101
Decreased: elapsed time and basic instrument optimisation
Removal of centrifugal forces induced by SC
quasi-static rotation and noisy star-trackers
For both curves: removal of SC
motion leaking into x by angular
contamination (eta, phi)
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The limiting disturbances
Einstein’s thermal noise: gas
molecules hitting the test-masses
Scales with residual gas pressure
Interferometer noise
No real test-mass motion
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May 16-18, 2016. Pressure gone further down.
System continuously vented to outer space
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The low frequency tail
Low frequency
extra noise
Found to decay
with time
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Explaining the noise: investigations ongoing
Low-frequency noise decay being investigated:
probably a mix of thermal and mechanical effects
thermo-mechanical system relaxation and/or pressure gradients
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Best results: February 2017
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This result boosted LISA!
Sensitivity curve of LISA from the LPF calibrated sensors/laser
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Black Hole Astronomy by 2030
aLIGO, aVIRGO,
KAGRA
SKA, Pulsar
Timing
Future EM Obs.
LSST, JWST, EELT
Mass [log M/M☉]à
RedshiftZà
ET (proposed)
Graphics courtesy
of K.Danzmann
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Black Hole Astronomy by 2030
SNR
LISA
RedshiftZà
Mass [log M/M☉]à
Graphics courtesy
of K.Danzmann
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This is still a school in physics! Recommended reading…
•Best course on GW I’ve seen: Caltech's
Physics 237-2002 - Gravitational Waves
(Thorne, Bondarescu, Chen) http://
elmer.tapir.caltech.edu/ph237/
•Misner,Thorne,Wheeler “Gravitation”,
Freeman
•Maggiore,“Gravitational Waves”, Oxford
•MA Ph.D.Thesis on LPF https://arxiv.org/
abs/1110.3031
•The top-science LPF publication http://
journals.aps.org/prl/abstract/10.1103/
PhysRevLett.116.231101