Speaker: Michael Hippke Affiliation: Sonneberg Observatory Title: ''Interstellar communication: What works (not so) well and why? " Abstract: Our nearest neighbor star, Alpha Centauri C, has a planet in the habitable zone. The Russian billionaire Yuri Milnor funds research to send an exploration probe there. Obtaining remote observational data from such a probe is not trivial because of minimal instrumentation (gram scale) and large distances (pc). Together, we follow the long journey of a photon from the transmitter beam through the interstellar dust and gas, atmospheric turbulence, and other obstacles, before finally arriving in the receiver on Earth. We discuss wavelength/frequency choices, gravitational lensing, and particles other than photons, such as Neutrinos. We conclude by asking the "big picture" question: Only when we understand how to communicate efficiently over interstellar communications, we can hope to learn how to receive such communications from other civilizations, if they exist. Are radio waves the best choice?

1. Interstellar
communication

2. • Gentleman scientist affiliated with Sonneberg Observatory
• Everyday job in business / senior management
• Career in consulting (McKinsey & Company)
• Active as a hobbyist in astrophysics since 2015
• 29 publications (19 first author, 15 in major journals)
• Main interests:
• Interstellar communication (photons, Neutrinos, probes, …)
• Interstellar travel: How to decelerate a light sail (ApJL, AJ, MNRAS)
• Boyajian‘s Star (ApJL, 2x ApJ)
• Exomoons and Exotrojans (A&A, 3x ApJ)
• Variable stars (Physical Review Letters, ApJ, MNRAS, Physica D)
About Michael Hippke

3. Communication papers Deceleration
papers

4. • Why do we need that?
• Deep Dive optical/IR (laser beam, 𝜆 ∼ 𝜇𝑚)
• Why not radio?
• Extinction (interstellar dust and gas)
• Absorption (Earth‘s atmosphere)
• Dispersion and scattering
• How many bits per photon?
• Other Methods
• Lensing (the sun as a gravitational lens)
• Inscribed matter (write in stone, throw stone)
• Exotics (Neutrinos, Neutrons, gravitational waves, …)
• Next steps
Interstellar communication

5. Entdeckung eines Planeten mit einer Erdmasse um α Centauri C
(Proxima) im Jahre 2016

6. Tsiolkovsky equation prevents chemical interstellar rockets
Sail 1 m2, 1 gram (incl. payload)
Laser power 100 GW from 1x1 km array
Acceleration 10,000 gee
 in 2 minutes to 20% c = 60,000 km/sec
Breakthrough Starshot: Send a probe to Alpha Cen
and get photos

8. Use photon (radiation) pressure, magnetic field lines, and a triple gravity slingshot (3 papers 2017: ApJ, AJ, MNRAS)
Can we decelerate?

9. Looks easy. Is complicated.
α Cen B
α Cen A
α Cen C (Proxima)

10. Triple photo-grav-mag slingshot much better than
fly-by

11. Launch window every 80 years
Must do full 3D simulation

12. Vorbeiflug: numerische Simulation
• (Video Kervella)

13. Better deceleration for
• Less mass (𝑀)
• Larger size (𝐴)
• Closer to the star (heat resistance)
First approximation: 𝑣∝ 𝑀/𝐴
Temperature <100 C° for distances>5 𝑅⋆ (with good reflectivity)
Best known material: Graphen
𝐴 = 1m2
M = 8 × 10−4
g
M/A = 8 × 10−4 g/m2
Limit: Broadband reflectance at low weight (technical)
𝜈∞,max = 17050 km/s at 𝜃 = 19°
(8800 km/s at Cen A, 8400 km/s at Cen B)
Flight time 75 years + 46 years to Proxima
How well does it work?

14. Things get ugly quickly when the sail technology is
not that great

15. We made it! Now send these photos back home

16. Free space loss
𝐷𝑡
𝐷𝑟
𝑑
𝑃𝑡 power
𝑄 ~1.22
𝜆 = 𝑐/𝑓
h Planck′
s constant

17. To get a feeling for the numbers involved, consider a toy example without any losses
• Transmitter size 1 m
• Power 1 kW (optimistic for a 1 m2 1g probe)
• Receiver size 39 m (optical), 100 m (radio)
• Distance 1.3 pc (Alpha Cen) = 1017 m
• Capacity 1 bit per photon
At 𝜆 = 1𝜇m, we receive 1 kBit/s
At 𝜆 = 20 cm (𝑓 ∼ GHz), we receive 0.1 bit/s (factor 10-5)
 No way to get radio competitive
Just use radio?

18. Interstellar extinction – big picture

19. Interstellar extinction – optical and infrared

20. Atmospheric absorption – optical and infrared

21. Atmospheric absorption – details

22. Detailed models (Noll et al. 2012; Jones et al. 2013) with resolution R=106
• scattered moonlight
• Starlight
• zodiacal light
• thermal emissions by the telescope and sensor
• molecular emissions from the lower atmosphere
• sky emission lines of the upper atmosphere
• airglow continuum.
Site: VLT Cerro Paranal, altitude of 2640 m
Example value in good conditions: At 𝜆 = 1.064𝜇m, the total sky radiance is ~1 photons/s/nm/m2/arcsec2
Total noise: Sources

23. Total noise – optical and IR

24. Total noise – details: Lots of trees in the forest

25. Interstellar dust and gas
• Pulse delay (dispersion) ~ps
• Scatter broadening <fs
Atmospheric effects
• Dispersion (broadening: ~ps), variable time delay (10 ns) correctable to ~ps
• Turbulence ~ps
• Scintillation (twinkle little star), ms timescale, 1-20% amplitude
Barycentering
• Earth moves at 30 km/s  kHz pulses smeared after 30 s
• Correction possible to ~ns over hours
Effects on pulse delay and shape (𝜆 ∼ 𝜇𝑚)

26. How many bits of information can a single photon
carry?

27. How many bits of information can a single photon
carry?Intuition says “one”, but this is incorrect
With an alphabet based on the
• photon’s time of arrival
• Energy (color/frequency/wavelength)
• and polarization
several bits can be encoded
The unwieldly anwer: Giovanetti 2004+ proved the case with noise as
𝐶𝑡ℎ = 𝑔(𝜂𝑀 + 1 − 𝜂 𝑁 𝑀) − 𝑔( 1 − 𝜂 𝑁 𝑀) (bits per photon)
where 𝑔 𝑥 = 1 + 𝑥 log2 1 + 𝑥 − 𝑥 log2 𝑥−1
so that 𝑔 𝑥 is a function of 𝜂 × 𝑀
𝜂: transmissivity. 𝑀: Photons per mode. 𝑁 𝑀: Noise photons per mode

28. How many bits of information can a single photon
carry?Much easier visually:
In brief: 1..10 with our technology
(for details, see the paper)
Reality: Our technology can not yet
simultaneously measure
• Polarization
• Energy (color/frequency/wavelength)
• Time of arrival
at an accuracy near the Heisenberg limit.
Homodyne/Heterodyne limited to a few bits
per photon for useable bit error rates

29. Physical limits for the number of bits per photon

30. Data rate: Putting it all together

31. Idea by Einstein (PRL, 1936)
Publicized by Eshleman (1979) and Maccone (many papers)
Doesn’t work <300 GHz due to coronal refraction
Point spread function and relevant details by Turyshev 2017
The sun as a gravitational lens:What is it?

32. What is it good for? Imaging!
Turyshev 2017+

33. What is it good for? Imaging!
Turyshev 2017+
Rotational deconvolution
Rotational deconvolution

34. Is it difficult? Coronal noise overlays the Einstein
ring

35. Aperture increase by ~109
Noise increase by ~103
Data rate increase by 106 (bits/sec to Mbits/sec)
Disadvantage: Have to fly to 550..2000 au
The sun as a gravitational lens: How large is the
effect?

36. What are the energy minima per bit of information, for photons versus matter?
In principle: Kinetic energy invested into accelerating a mass can (almost) be recovered during its deceleration
Inscribed matter:Throw the data

37. Let us assume you pay twice, acceleration + deceleration
Photons: 𝐶γ ∼∝ 𝜂𝑑−2 𝐷𝑡
2
𝐷𝑟
2 (bits J−1) with
𝜂: efficiency (good),
𝑑: distance (bad),
𝐷: apertures (good)
Matter: 𝐶rel ∼∝ 𝑆𝜂𝐿−1 𝑣−2 bits J−1 with
𝑆: information density (bits per gram) (good),
𝐿: relativistic Lorentz factor (irrelevant <0.2 c),
𝑣: velocity (bad)
Denser storage is good. DNA is 5 × 1021
bits g−1
plus structural overhead
Slower velocity is good. Assume something realistic (cf. Interstellar dust)  order 0.1 c
Energy per bit of information / photons versus
matter

38. • Matter is more energy efficient in any configuration after some critical distance d
• Trade-off between velocity and energy efficiency
• Energy equivalence for 100 pc, S~0.1 DNA, v=0.1 c
 Requires photon apertures of 100 km (optical) to 1000 km (radio)
Energy per bit of information / photons versus
matter

39. Hard to send:
• An ideal Neutrino accelerator („factory“) is based on Muons
• Neutrino can not be focused with ordinary matter – only gravity and
muon/electromagnetism works
• We have no black hole at our disposal
• Beam divergence is 𝜃𝜈 =
1
𝛾
=
10−4
𝐸
TeV where 𝛾 is the relativistic boost
factor of a muon
• For comparison, photon mirror: 𝜃optics = 1.22
𝜆
𝐷
• Unfortunately for Neutrinos, for D=1 m, the difference in beam
divergence is 1010
Hard to receive
• Small cross section: 1 km (of e.g., ice) has 1% detection rate (at most,
near Glashow resonance 6.3 PeV)
Neutrino beams

40. • Solar gravitational lensing is very different: Through the sun!
• Gain 1011 to in 1016 in detector mass
(-) Neutrino detector on Earth is isotropic
(-) Lensing is for point sources (beam width of km over pc distance)
Neutrino lensing?Yes!
(+) Focus in 23 au (500 au for photons)

41. The particle zoo – what is your favourite animal?

42. • A probe at Alpha Cen will be our first interstellar communication
• It will not use radio
• Should we search for Alien signals (SETI) primarily with radio waves?
Some learnings with a different vector

43. How would we signal to other planets given a big laser (100 GW)
• If we shine the beamer on Proxima, it would be visible in daylight
• Is there such a thing on our sky? What about IR?
• Looking for collaborators interested in optics (lasers etc.)
If we can send a probe to other systems…
• Can others send a probe to us?
• If a probe is in our solar system, where is it?
• Looking for collaborators interested in orbital mechanics
Beamer feedback loop
Interesting questions (for me) - looking for
collaborators

45. Michael Hippke
Sonneberg Observatory
www.hippke.org
michael@hippke.org