Jason T. Wright Roger Griffith, Steinn Sigurðsson Matthew Povich

Transcription

^
The G Search for
Extraterrestrial Civilizations
Jason T. Wright
Roger Griffith, Steinn Sigurðsson
Matthew Povich (Cal Poly Pomona)
Communication SETI
Communication SETI
Radio SETI
Communication SETI
Radio SETI
Kardashev (1963) calculated the optimum radio spectrum
for matching transmission bandwidth to background
astrophysical noise:
Communication SETI
Radio SETI
Communication SETI
Radio SETI
The
Kardashev scale
Classify extraterrestrial intelligences by their total power
supply (i.e. potential for transmission power)
Kardashev (1964)
The
Kardashev scale
Class I: Equal to humanity’s present energy supply
Kardashev (1964)
The
Kardashev scale
Class II : A solar luminosity
Kardashev (1964)
The
Kardashev scale
Class II : A solar luminosity
Class III: A galactic luminosity
Kardashev (1964)
Zubrin’s adaptation
Zubrin loosely applied this scheme to describe the extent and
approximate energy supply of a civilization:
K1: Any planet-spanning civilization (Humanity counts)
“Trantor from space” by Slawek Wojtowicz
K2: Any star-system-spanning civilization (with energy supply far
beyond the incident energy on a single planet)
Dyson (1960); image by Steve Bowers
K3: Any galaxy-spanning civilization (with energy supply far beyond
one stellar luminosity)
“Where is Everybody?”
• Enrico Fermi pointed out that
the time for ships to cross and
colonize the Galaxy is short
compared to its age
• Evidence of alien civilization in
the Milky Way is nonexistent.
This has been called “The
Great Silence” (Brin 1983)
Hart’s categories of solutions
• Michael Hart (1973) argued we are probably alone
• He divided other resolutions into 4 categories, which
he argued were unlikely or contrived:
• Physical: interstellar space travel is infeasible (even
for billion-year-old civilizations)
• Sociological: aliens lack the will/coordination to
colonize the Milky Way OR they are “hiding” from us
• Temporal: they haven’t had time to get here yet
• Temporal: they haven’t had time to get here yet
• We’re just missing them (would ants really recognize
us as a socially organized species?)
Physical: we built interstellar probes <80 years after
developing powered flight
Hart (1973)
Sociological: “Maybe the aliens are all...” hiding /
incurious / in virtual realities / shy / etc...
Hart (1973)
•
Unless all other species have a “hive mind,” we should expect many
subcultures in every species
Hart (1973)
•
•
Unless cultures never evolve, there will be a new set of subcultures every
few generations
Hart (1973)
•
•
few generations
•
Unless FTL communication is possible, we should expect no coordination
or common culture across the Galaxy
Hart (1973)
•
•
few generations
•
•
For most sociological solutions to work, the lack of will/coordination to
visit or contact us must apply to all aliens everywhere for billions of years
Hart (1973)
•
•
few generations
•
•
For most sociological solutions to work, the lack of will/coordination to
visit or contact us must apply to all aliens everywhere for billions of years
•
We dub this the “monocultural fallacy”
Hart (1973)
Temporal: It Takes <100 Myr to Colonize the Galaxy
Optimistic:
Optimistic:
Galaxies are ~30 kpc across, so at 0.1c crossing time is 1Myr
Optimistic:
Pessimistic:
Optimistic:
Pessimistic:
Gravity assists from:
Optimistic:
Pessimistic:
-3–10-2c (Dyson 1963)
binary
stars:
300–3,000
km/s
=
10
•
Optimistic:
Pessimistic:
-3–10-2c (Dyson 1963)
binary
stars:
300–3,000
km/s
=
10
•
-3c
halo
stars:
300
km/s
=
10
•
Optimistic:
Pessimistic:
-3–10-2c (Dyson 1963)
binary
stars:
300–3,000
km/s
=
10
•
-3c
halo
stars:
300
km/s
=
10
•
-4c
planets:
30
km/s
=
10
•
Optimistic:
Pessimistic:
-3–10-2c (Dyson 1963)
binary
stars:
300–3,000
km/s
=
10
•
-3c
halo
stars:
300
km/s
=
10
•
-4c
planets:
30
km/s
=
10
•
BUT, for slow ships, relevant velocity is not sped of spacecraft,
but the velocity dispersion of the stars
Model for minimum galaxy colonization time
Assume:
Assume:
Every colony launches a colony ship every 10,000 years
Assume:
Best cruise velocity is only 30 km/s
Assume:
Assume:
Zero technological advancement
Assume:
Colonize nearest unoccupied system out to 10pc
Assume:
Colonize nearest unoccupied system out to 10pc
No boosts from halo stars
Galactic Shear, halo stars, and velocity dispersion in
disk mix galaxies on rotational timescales
Result:
Credit: Elena D’Onghia, Mark Vogelsberger – Harvard FAS Supercomputer Odyssey, Thiago
ize – Scientific Computing and Imaging (SCI) Institute – University of Utah
Result:
Cruise time to nearest star is 100,000 years
Result:
In that time 10 colony ships launched
Result:
In that time 10 nearest stars have changed!
Result:
Colonies now separating at ~30 km/s (disk stellar velocity dispersion)
Result:
Colonies now separating at ~30 km/s (disk stellar velocity dispersion)
Each colony has independent trajectory through galaxy
Result:
We should not expect Fermi “bubbles” or “voids”
Ize – Scientific Computing and Imaging (SCI) Institute – University of Utah
Result:
The chances of catching a galaxy in “transition” from K2 to K3 is <1%
Result:
Corollaries:
Result:
Corollaries:
• Nearly all galaxies should have zero spacefaring civilizations or be filled
with them
Result:
Corollaries:
• Nearly all galaxies should have zero spacefaring civilizations or be filled
with them
• Either the Milky Way is filled with spacefaring ETIs or we are the first
Hart’s analysis is optimistic!
Hart concluded that communication SETI is “probably a waste of
time and money,”
time and money,”
...but unless spacefaring life is unique to Earth...
time and money,”
If Hart is correct:
time and money,”
If Hart is correct:
Other galaxies should be filled with advanced civilizations.
A search for K3’s will eventually succeed
time and money,”
If Hart is correct:
If Hart is incorrect:
time and money,”
If Hart is correct:
The Milky Way should be filled with K2‘s.
time and money,”
If Hart is correct:
The Milky Way should be filled with K2‘s.
We should test Hart’s hypothesis by pursuing both routes
Artifact SETI
Arnold (2005) showed how giant artifacts could produce
clearly artificial transit signatures, and even communicate
low-bandwidth information
Artifact SETI
Artifact SETI
Artifact SETI
Artifact SETI
KIC 12557548 (Rappaport et al. 2012)
Artifact SETI
Dyson (1960)
Artifact SETI
Dyson (1960)
Energy is never “used up”, it is just converted to a lower
temperature
Artifact SETI
Dyson (1960)
temperature
If a civilization uses energy, that energy must emerge as
waste heat in the mid-infrared
Artifact SETI
Dyson (1960)
temperature
A civilization using most of its star’s energy would have little
optical luminosity but be a very bright infrared source.
Artifact SETI
Dyson (1960)
temperature
A civilization using most of its star’s energy would have little
optical luminosity but be a very bright infrared source.
This approach is totally general: any energy use by a
civilization would give a star a mid-infrared excess
Advantages to a waste
heat approach
Assumes only:
heat approach
Assumes only:
•Conservation of energy
heat approach
Assumes only:
•Laws of thermodynamics
heat approach
Assumes only:
•No “obvious” new physics
heat approach
Assumes only:
Makes no “sociological” assumptions about alien behavior or
communication willingness or methods
heat approach
Assumes only:
Makes no “sociological” assumptions about alien behavior or
communication willingness or methods
Can be applied out to very large (cosmological) distances
Disadvantages to waste
heat approach
heat approach
•Most easily applies to power supplies similar to stellar
luminosity
heat approach
luminosity
•For some classes of sources, need to deal with heavy
contamination from dusty objects (dust reprocesses starlight
into MIR radiation very similarly to Dyson spheres)
heat approach
luminosity
•For some classes of sources, need to deal with heavy
contamination from dusty objects (dust reprocesses starlight
into MIR radiation very similarly to Dyson spheres)
•Long road to prove source is of alien origin
A Philosophical
Challenge of SETI
“Any sufficiently advanced technology is indistinguishable from
magic” (Clarke’s 3rd Law)
A Philosophical
Challenge of SETI
“Magic” is the suspension of Natural Law
A Philosophical
Challenge of SETI
Science assumes the Universe is governed by Natural Law
A Philosophical
Challenge of SETI
If Clarke is correct, SETI risks being inherently unscientific, since it
assumes we can identify apparent violations of Natural Law
A Philosophical
Challenge of SETI
Unless care is taken, SETI can lead to an “aliens of the gaps”
approach
A Philosophical
Challenge of SETI
Unless care is taken, SETI can lead to an “aliens of the gaps”
approach
Communication SETI resolves this: find a signal of unambiguously
intelligent origin — no naturalistic interpretation is possible
Communication and Artifact
SETI are complementary
Artifact SETI
hard-pressed to prove
phenomena cannot be
natural
Communication SETI
must cast an impossibly wide
net
(frequencies, duty cycles,
bandwidth, power, targets...)
Communication and Artifact
SETI are complementary
Artifact SETI
provides candidates
Communication SETI
identifies candidates as alien
identifies other
technological signatures
provides candidates
^
G: Glimpsing Heat from
Alien Technologies
Phase I: K3 search of resolved galaxies
Phase II: K2 search of point sources
Phase III: K3 search of unresolved galaxies
We can use conservation of energy to parameterize a
civilization’s energy budget
α
power collected from starlight
ε
power generated by other means
(fossil fuel, nuclear, direct mass-to-energy)
γ
power expelled as thermal photons
ν
all other power expelled (radio waves, neutrinos,
KE, gravity waves, etc.)
Twaste
typical temperature of waste heat radiators
α+ε=γ+ ν
Artifact
SETI
α
ε
γ
ν
Twaste
α+ε=γ+ ν
Artifact
SETI
α
ε
γ
ν
Twaste
Communication
SETI
α+ε=γ+ ν
α
ε
γ
power expelled as photon waste heat
ν
all other power expelled
We normalize these parameters by:
•Starlight striking the planet (for a K1)
•Total stellar luminosity (for a K2)
•Total galactic luminosity (for a K3)
Humanity’s current parameterization:
α
~10-7 (counting only photovoltaic energy)
ε
~10-4 (mostly fossil fuels, some nuclear)
γ
~10-4
ν
~0
Twaste
~300 K
The Earth has 0.01% excess mid-infrared radiation because
of human activity
What are realistic values for Twaste for a many-billion
year-old civilization?
•Waste heat temperature set by the maximum thermodynamic efficiency
of a civilization η=1-Twaste/T★
4) to be conserved
Conservation
of
energy
requires
(L
=
A
T
waste
•
(A=area of radiating surfaces, L=luminosity)
4) to be conserved
Conservation
of
energy
requires
(L
=
A
T
waste
•
•Increasing efficiency from η=90% to η=99% requires a 104 increase in A
4) to be conserved
Conservation
of
energy
requires
(L
=
A
T
waste
•
2 surface area
Optimal
values
of
A
are
uncertain,
but
likely
roughly
1
AU
•
(Twaste = 150–600 K) constrained by available mass
4) to be conserved
Conservation
of
energy
requires
(L
=
A
T
waste
•
2 surface area
Optimal
values
of
A
are
uncertain,
but
likely
roughly
1
AU
•
(Twaste = 150–600 K) constrained by available mass
•Peak emission is then at 5-30 microns
What are realistic values for α for a many-billion yearold K2 civilization?
There is as much energy available in sunlight as in any
other source in the Solar System:
Sunlight over 10 Gyr
1051 ergs
Mass-energy of all planets
(40%/0.01% efficiency)
1050 / 1048 ergs
Chemical energy in planets
1042 ergs
Kinetic & gravitational
potential energy of planets
1042 ergs
Only superior source of energy is mass-energy of sun itself. Efficient massto-energy conversion probably requires a black hole.
Credit: Tom Murphy “Galactic Scale Energy” http://physics.ucsd.edu/do-the-math/2011/07/galactic-scale-energy/
Power (Watts)
24
10
22
10
1020
1018
1016
1014
0
200 400 600 800 1000
Time from now (years)
Credit: © Mafic Studios, Inc Credit: © Mafic Studios, Inc Credit: JAXA
Credit: © Mafic Studios, Inc Credit: © Mafic Studios, Inc What are realistic values for α for a many-billion yearold K2 civilization?
•Available mass is not a limitation: The volume of Jupiter is equivalent to
a 1 AU sphere 5.5 m thick
•Does not require such a large, solid structure, just a swarm of
collectors close to the star
•All Malthusian limits can be
overcome through application
of energy and time
•All Malthusian limits can be
overcome through application
of energy and time
•α<<1 inconsistent with old
civilizations and exponential
power supply growth
What are realistic values for ε for a many-billion yearold civilization?
Do alternative power supplies exist?
(new physics, direct matter-to-energy, Penrose process with black
holes, zero point energy)
•If so, then (α+ε) >> 1 is possible
•If so, then (α+ε) >> 1 is possible
•If not, then α >> ε, but (α+ε) ~ 1 possible
What are realistic values for ν and γ for a many-billion
Thermodynamics requires that waste heat be emitted, based on the
thermodynamic efficiency of a civilization, η=1-Twaste/T★
•Maximum emission possible is ν = (α + ε) η
•Minimum waste heat possible is γ =(α + ε) Twaste/T★
But humanity disposes nearly all of its energy supply as waste heat, so
•ν~0
•ν~0
• γ ~ (α + ε)
A toy model illustrates plausibility
Assume that for large civilizations: ν~0
F ∝ [(1-α) B(T★) + γ B(Twaste) (T★ / Twaste)4] / d2
Order-of-magnitude estimate in mid-infrared (Rayleigh-Jeans limit) for
T★ >> Twaste, any α:
FMIR = (FMIR,no civ) [γ (T★ / Twaste)3]
For T★ = 4000K, Twaste=250K, γ=1, F/Fno civ = 4000
(9 magnitudes MIR excess)
For T★ = 4000K, Twaste=250K, γ=0.1, F/Fno civ = 400
(6.5 magnitudes MIR excess)
WISE
10000
Normalized SED F
99% Old Elliptical + 1% Blackbody
1000
100
300K Blackbody
10
𝛾 =1%
Old elliptical
1
1
10
Wavelength (µ)
100
SED from Silva et al. (1998)
Normalized SED F
WISE
10
6
10
5
10
4
10
3
10
2
10
1
10
0
Sd spiral
300K Blackbody
Old elliptical
𝛾 =0.1
1
10
Wavelength (µ)
100
SEDs from Silva et al. (1998)
Normalized SED F
WISE
10
6
10
5
10
4
10
3
10
2
10
1
10
0
Sd spiral
300K Blackbody
Old elliptical
𝛾 =0.1
1
10
Wavelength (µ)
100
Normalized SED F
WISE
10
6
10
5
Molecular features
10
4
10
3
10
2
10
1
10
0
CO, etc.
Sd spiral
300K Blackbody
Old elliptical
𝛾 =0.1
1
10
Wavelength (µ)
100
Normalized SED F
WISE
10
6
10
5
Molecular features
10
4
10
3
10
2
10
1
10
0
CO, etc.
Sd spiral
6 mags
(200x)
300K Blackbody
Old elliptical
𝛾 =0.1
1
10
Wavelength (µ)
100
Normalized SED F
WISE
10
6
10
5
10
4
10
3
10
2
10
1
10
0
Arp 220 (ULIRG/starburst)
250K Blackbody
𝛾 =0.9
1
Old elliptical
10
Wavelength (µ)
100
Normalized SED F
WISE
10
6
10
5
10
4
10
3
10
2
10
1
10
0
Arp 220 (ULIRG/starburst)
250K Blackbody
𝛾 =0.9
1
Old elliptical
10
Wavelength (µ)
100
Dyson “spheres” will be difficult to distinguish from
“natural” Galactic sources without spectra
GAIA will eventually provide distances (and so
luminosities) for most candidates
•Zero parallax: Extragalactic sources —follow up as galaxies
•Giant star luminosity: Likely dusty giant star, low priority
•Stellar luminosity: Excellent candidate for follow-up:
•Mid-infrared spectra to check for dust
•Mid-infrared spectra to check for dust
•Position in galaxy to check for association with star forming
region
Resolved galaxy-spannning civilizations will be easier
to uniquely identify
Resolved galaxy-spannning civilizations will be easier
to uniquely identify
We can distinguish K3’s from most other sources because the
will be above the Galactic plane, and they will be resolved by
WISE if they are galaxies in the local universe. This rules out
most Galactic sources and cosmological (redshifted) sources
Morphology can distinguish dust from K3’s
Morphology can distinguish dust from K3’s
Credit: NASA
4
T dwarfs
W1−W2
3
ULIRGs/LINERs
Obscured AGN
2
QSOs
1
0
Seyferts ULIRGs
LINERs
Starbursts
Stars and
Ellipticals
0
Spirals
2
LIRGs
4
W2−W3
6
Underlying figure reproduced from Lake
et al. (2012) and Jarrett et al. (2011)
4
T dwarfs
300K
200K
W1−W2
3
ULIRGs/LINERs
Obscured AGN
2
QSOs
1
0
Seyferts ULIRGs
LINERs
Starbursts
Stars and
Ellipticals
0
Spirals
2
LIRGs
4
W2−W3
6
4
T dwarfs
300K
200K
W1−W2
3
ULIRGs/LINERs
Obscured AGN
2
QSOs
1
0
Seyferts ULIRGs
LINERs
Starbursts
Stars and
Ellipticals
0
Spirals
2
LIRGs
4
W2−W3
6
Several science outputs of such
a survey
a survey
•A null detection puts an upper limit on the abundance of
advanced civilizations occupying some volume of (d-L-α -γTwaste) phase space
a survey
•A thorough analysis of the categories of reddest objects in
the near- and mid-IR
a survey
•A comprehensive follow-up strategy to identify dust or
other confounders, using radio or IR spectroscopy
a survey
•Guidelines for design and follow-up for future searches
a survey
•Potentially new classes of astrophysical objects
a survey
•Potentially new classes of astrophysical objects
•Aliens?!
What does a null result
imply?
imply?
•Civilizations with large energy supplies do not exist:
imply?
•They are logistically impossible
imply?
•They are physically impossible
imply?
•They inevitably stop growing
imply?
•They are inevitably short lived
imply?
•New physics (violation of conservation of energy, laws of
thermodynamics, FTL communication) is employed by all or most
spacefaring civilizations
imply?
•New physics (violation of conservation of energy, laws of
thermodynamics, FTL communication) is employed by all or most
spacefaring civilizations
•Civilizations inevitably operate at low values of Twaste , or minimal
values of γ, beyond our detection or confusion threshold
Conclusions
•Communication and Artifact SETI are
complementary
•Hart’s analysis argues a either a K2 or K3 search
should eventually succeed
•Alien waste heat is likely emitted in the MIR
•WISE and Spitzer enable sensitive upper limits on
alien energy supplies around stars and across
galaxies
•Null result can imply new physics, or uniqueness of
humanity.
^
The G Search for
Extraterrestrial Civilizations
Jason T. Wright
Roger Griffith, Steinn Sigurðsson
Matthew Povich (Cal Poly Pomona)
A hidden assumption
“Sociological” explanations for finite lifetimes of civilizations must
be universal and inevitable to explain absence of galaxy-spanning
civilizations in other galaxies
A hidden assumption
Every galaxy is essentially an independent test of resolutions to the
Fermi paradox
A hidden assumption
Fermi paradox
This assumption of independence is only valid if superluminal travel
is not possible. “Warp drives” make coordination (and extinction)
possible across (or among!) galaxies.
A hidden assumption
Fermi paradox
This assumption of independence is only valid if superluminal travel
is not possible. “Warp drives” make coordination (and extinction)
possible across (or among!) galaxies.
If FTL communication is possible, the monocultural fallacy is not
fallacious.
What about finite
civilization lifetimes?
We can imagine how we might extinguish ourselves (nuclear
holocaust, radial climate change, biowarfare)
What about finite
Chances of extinction drop significantly with existence of selfsufficient Moon/Mars colonies
What about finite
Chances drop much further with colony orbiting other star
What about finite
Light travel time between stars makes coordination,
contamination, or warfare virtually impossible
What about finite
Light travel time between stars makes coordination,
contamination, or warfare virtually impossible
First interstellar colonies imply K3 status on Hart-Fermi
timescales, and “immortal” “supercivilizations”

Jason T. Wright Roger Griffith, Steinn Sigurðsson Matthew Povich

Transcription

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