DOW-UAP-D140, AAWSAP DIRD, High-Frequency Gravitational Wave Communications, April 2010

This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD examines whether high-frequency gravitational waves could serve as a communications medium while avoiding the attenuation that limits radio-frequency systems. The report surveys proposed transmitter and detector concepts, argues that gravitational-wave communications could support secure point-to-point links, timing standards, and interplanetary navigation, and gives particular attention to laboratory generator concepts and the Li-Baker detector as possible building blocks for such a system. The document makes clear, however, that the entire concept depends on capabilities that had not been demonstrated in practice, including laboratory-scale generation and reliable detection of usable high-frequency gravitational-wave signals. It is an exploratory systems-oriented review built around a future communications concept.

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Defense
Intelligence
Reference
Document
Acquisition Threat Support
6 April 2010
ICOD: 1 December 2009
DIA-08-1004-005
High-Frequency Gravitational
Wave Communications
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High-Frequency Gravitational Wave Communications
Prepared by:
Acquisition Support Division (DW0-3)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Author:
AAP Person 77
Administrative Note
COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized.
This product is one in a series of advanced technology reports produced in FY 2009
under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace
Weapon System Applications (AAWSA Pro ram. Comments or uestions pertaining to
this document should be addressed t AAP Person 1 AAWSA Program
Manager, Defense Intelligence Agency, ATTN: CLAR/ DWO-3, Bldg 6000, Washington,
DC 20340-5100.
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Contents
Summary................................................................................................................. v
1.0 Introduction ..................................................................................................... 1
1.1 Introduction ................................................................................................. 1
1.2 Definition of High-Frequency Gravitational Waves ....................................... 1
2.0 HFGW Communications .................................................................................... 2
2.1 HFGW Generators (Transmitters) ................................................................. 2
2.1.1 HFGW Generator Concepts .................................................................... 2
2.1.2 Alternative Approaches ......................................................................... 6
2.1.3 Piezoelectric Approach .......................................................................... 6
2.1.4 Infrared-Excited Molecules Approach .................................................... 7
2.2 HFGW Detectors (Receivers) ...................................................................... 12
2.2.1 Alternative Approaches ....................................................................... 12
2.2.2 Concept (Li-Effect) .............................................................................. 14
2.2.3 Quantum Back-Action Limit................................................................. 16
2.2.4 Li-Baker HFGW Detector...................................................................... 20
3.0 Operational Concerns ..................................................................................... 22
3.1 Link Budget ................................................................................................ 22
3.1.1 Signal-to-Noise Ratio .......................................................................... 22
3.1.2 Link Budget Considerations ................................................................. 23
3.2 Bandwidth .................................................................................................. 25
3.3 Frequency and Time Standard .................................................................... 25
3.3.1 Improvements Accruing from a HFGW Time Standard......................... 27
3.3.2 Search Space Improvement Accruing From HFGW FTS ........................ 28
3.3.3 The Impact of Phase Noise Improvements on Phase Shift Encoding ... 29
3.3.4 The Impact of Frequency Noise Improvements on FDMA and FHSS..... 30
3.4 Possible Future Upgrades to the FTS Devices ............................................. 30
3.4.1 Propagating Signals From Optical Lattice Clocks for Timing ................ 31
3.4.2 In Navigating and Mapping Interplanetary Geoids .............................. 31
4.0 Future Potential ............................................................................................. 32
4.1 Developmental Roadmap............................................................................ 32
4.2 HFGW Communications Predictions to 2050 ............................................... 33
4.3 Interplanetary Navigation and Geoid Mapping to 2050 .............................. 34
4.4 Other Possible HFGW Applications ............................................................. 36
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4.5 2050 and Beyond ....................................................................................... 37
5.0 Acknowledgements ........................................................................................ 37
6.0 References ..................................................................................................... 37
Appendix A: Nomenclature................................................................................... 44
Appendix B: Li-Baker HFGW Detector................................................................... 45
Appendix C: Perturbative Photon Fluxes Generated By High-Frequency
Gravitational Waves and Their Physical Effects .................................... 52
Figures
Figure 1. Communication Link Block Diagram ........................................................ 2
Figure 2. Change in Centrifugal Force of Orbiting Masses, Afct, Replaced by Change
in Tangential Force, '1ft, to Achieve HFGW Radiation ............................... 3
Figure 3. Circular Resonator Geometry Using Infrared Excitation .......................... 8
Figure 4. Radiation Pattern Calculated by Landau and Lifshitz (1975) ................... 8
Figure 5. GW Flux Growth Analogous to Stack of N Orbital Planes ......................... 9
Figure 6. Stack of Circular-Wave-Guide Plates With Typical Molecule Jerks, Af's ... 9
Figure 7. Omni-Directional Nature of the HFGW Radiation Pattern....................... 10
Figure 8. Predicted Relic GW Energy Density as a Function of Frequency............. 11
Figure 9. Birmingham University HFGW Detector ................................................. 13
Figure 10. INFN Genoa HFGW Detector ................................................................ 13
Figure 11. The National Astronomical Observatory of Japan 100 MHz Detector ... 14
Figure 12. Detection Photons Sent to Locations that are Less Affected by Noise.. 15
Figure 13. Quantum Back Action as a Mechanism for Creating the Standard
Quantum Limit..................................................................................... 17
Figure 14. Schematic of Ultra-Sensitive HFGW Detector....................................... 21
Figure 15. Fractal Membrane Component of Li-Baker Detector Exhibited in Planar
Form .................................................................................................... 21
Figure 16. Conceptual SNR Fill Factors: Signal and Noise Components ................ 23
Figure 17. A Block Diagram of a Typical Link Budget............................................ 24
Figure 18. A Proposed Near Earth Distribution of Frequency Time Standard........ 26
Figure 19. HFGW Supplemented Remote Terminal Design.................................... 27
Figure 20. Acquisition Search Space Improvement Accruing From HFGW FTS ...... 28
Figure 21. The Impact of Phase Noise Improvements on Phase Shift Encoding ... 29
Figure 22. The Impact of Frequency Noise Improvements on FDMA and FHSS ..... 30
Figure 23. The Earth's Associated Lagrangian Points ........................................... 31
Figure 24. HFGW Com Space Application Development Roadmap, Estimated
Timeline .............................................................................................. 32
Figure 25. A GW Pair on Earth as Used by a Lunar Mission ................................... 34
Figure 26. A GW Pair on Earth and on the Moon, as Used by a Mission to Mars .... 35
Figure 27. A GW Pair on Earth and on Mars for an Outer Planetary Reference
Pair...................................................................................................... 35
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High-Frequency Gravitational Wave Communications
Summary
• Fourteen laboratory high-frequency gravitational wave (HFGW) generators
(or transmitters) have been proposed in the past 45 years in peer-reviewed
journal articles.
• The most promising laboratory HFGW generators are those that utilize very
large numbers of sub-microscopic radiation elements.
• The Piezoelectric Approach to HFGW generation is best for the proof-of­
concept test and the proposed IR-excited Molecules Approach is best for an
operational communications HFGW transmitter.
• Ten different HFGW detectors (or receivers) have been proposed since
1978 and reported in peer-reviewed journal articles.
• Several different HFGW receivers can be utilized for communication, but the
proposed Li-Baker detector (plans & specification development in Appendix
B) shows the most promise (underlying concept in Appendix C). The Li­
effect, upon which the Li-Baker detector is based, is not so new that it is
untested in the literature. At least nine peer-reviewed research publications
concerning the theory have appeared following the initial peer-reviewed
article by Li, Tang and Zhao (1992).
• Because HFGW communications are carried on an extremely narrow beam
directly through the Earth, there is a very low probability of interception.
• Theoretical results confirm that the Li-Baker detector is photon-signal
limited, not quantum-noise limited-that is, the Standard Quantum Limit is
so low that a properly designed Li-Baker detector can have sufficient
sensitivity to observe HFGWs of amplitude A:::;$ 10-32 m/m.
• Utilizing the IR-excited Molecules HFGW generator approach and the Li­
Baker detector, the theoretical information-transfer rate over 7,000 km of
distance, beamed directly through the Earth, is about 1.9 x 106 bits per
second.
• A means of propagating a Frequency Time Standard may be one viable
early low-bandwidth application for HFGW communications.
• HFGW sources on the Earth, the Moon, and Mars may act as reference
standards for interplanetary navigation, with the advantage that they
cannot be shielded or shadowed by planetary masses. Plasma interference
seen at planetary entry would be eliminated, and precise charting of
Lagrangian points would be possible.
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1.0 Introduction
1.1 INTRODUCTION
Of the applications of high-frequency gravitational waves (HFGWs), communication
appears to be the most important and most immediate. Gravitational waves have a
very low cross section for absorption by normal matter, so high-frequency waves could,
in principle, carry significant information content with effectively no absorption unlike
electromagnetic (EM) waves. Multi-channel HFGW communications can be both point­
to-point (for example, to deeply submerged submarines) and point-to-multipoint, like
cell phones. HFGWs pass through all ordinary material things without attenuation and
represent the ultimate wireless system. One could communicate directly through the
Earth from Moscow in Russia to Caracas in Venezuela-without the need for fiber optic
cables, microwave relays, or satellite transponders. Antennas, cables, and phone lines
would be things of the past. A timing standard alone, provided by HFGW stations
around the globe, could result in a multi-billion dollar savings in conventional telecom
systems over ten years, according to the recent analysis of Harper and Stephenson
(2007). The communication and navigation needs of future magneto hydrodynamic
(MHD) aerospace vehicles, such as the MHD aerodyne (www.mhdprospects.com), which
is high in electromagnetic interference, similar to plasma interference seen at reentry,
would be another possible applications area for HFGW communications.
1.2 DEFINITION OF HIGH-FREQUENCY GRAVITATIONAL WAVES
Visualize the luffing of a sail as a sailboat comes about or tacks. The waves in the sail's
fabric are similar in many ways to gravitational waves (GWs), but instead of sailcloth
fabric, gravitational waves move through a "fabric" of space. Einstein called this fabric
the "space-time continuum" in his 1915 work known as General Relativity (GR).
Although his theory is very sophisticated, the concept is relatively simple. This fabric is
four-dimensional: it has the three usual dimensions of space-east-west, north-south,
and up-down-plus the fourth dimension of time. Here is an example: we define a
location on this "fabric" (Einstein, 1916) as 5th Street and Third Avenue on the fourth
floor at 9 AM. No one can see this "fabric," just as no one can see wind, sound, or
gravity. Nevertheless, those elements are real, and so is this "fabric." If one could
generate ripples in this space-time fabric, many applications would become available.
Much like radio waves can be used to transmit information through space, gravitational
waves could be used to perform analogous functions. Gravitational waves are the
subject of extensive current research, which so far has focused on low frequencies.
High-frequency gravitational waves, as defined by physicists Douglass and Braginsky
(1979), are gravitational waves having frequencies higher than 100 kHz. Low-frequency
gravitational waves (LFGWs), such as those detectable by interferometric GW detectors
(for example, the Laser Interferometer Gravitational Observatory, or UGO) are not
applicable to communications due to their very long wavelengths, often thousands of
kilometers in length and, even more importantly, the inability to generate them
effectively in the laboratory. Furthermore LFGW detectors cannot detect HFGWs
(Shawhan, P. S., 2004).
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2.0 HFGW Communications
Consider the case of a single point-to-point two station full duplex communication
system, as is represented in Figure 1. Such a system is often characterized as a single
data link, and requires two transmitters, one at each end, and two receivers, one at
each end. To avoid self-interference the link in one direction often uses a frequency of
radiation different than the link in the opposite direction.
Full Duplex Communication Link
Using Gravitational Wave Generators and Sensors
Station 1 Station 2
r------------------~ r------------------~
GW Generator '' II
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GW Sensor
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Sianal 1 + Source Noise
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-<-0-1- -. Xmit 1
Additional Link Noise
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Rcvr 2 -(-0-1--.
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GW Sensor II GW Generator
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Figure 1. Communication Link Block Diagram
If one were to apply the emerging technology of gravitational w