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When astronomers first detected long-predicted gravitational waves in 2015, it opened a whole new window on the universe. Until then, astronomy had depended on observing light at all its wavelengths. We also use light to communicate, mostly radio waves. Could we use gravitational waves to communicate?
The idea is intriguing, but it’s beyond our capabilities at the moment. That said, there’s value in exploring the hypothetical, as the future may come sooner than we sometimes think. A new study explores this idea and how it might be applied in the future. It’s called “Gravitational Communication: Fundamentals, Cutting-Edge Technologies, and Visions for the Future” and is available on the preprint site arxiv.org.
Authors: Houtianfu Wang and Ozgur B. Akan. Wang and Akan work in the Internet of Everything Group in the Department of Engineering at the University of Cambridge, UK.
“Gravitational waves can maintain consistent signal quality over vast distances, making them suitable for missions beyond the solar system.” Houtianfu Wang and Ozgur B. Akan.
"The discovery of gravitational waves has opened a new observational window for astronomy and physics, offering a unique approach to exploring the depths of the Universe and extreme astrophysical phenomena. In addition to their impact on astronomical research, gravitational waves have also attracted widespread attention as a new communication paradigm," the authors explain.
Traditional electromagnetic communication has certain drawbacks and limitations. Signals become weaker with distance, which limits the range. Atmospheric effects can interfere with radio communication, scattering and distorting it. There are also line-of-sight limitations, solar weather and space activity can also interfere. Gravitational-wave communication (GWC) is promising in that it can overcome these problems.
GWC is robust in extreme conditions and loses minimal energy over extremely long distances. It also eliminates electromagnetic coupling (EMC) problems such as diffusion, distortion and reflection. There is also the intriguing possibility of using naturally generated GWs, which means reducing the energy required to create them.
"Gravitational communication, also known as gravitational wave communication, promises to overcome the limitations of traditional electromagnetic communication by enabling reliable transmission in extreme environments and over long distances," the authors note.
To develop the technology, researchers need to create artificial gravitational waves (GWs) in the laboratory. This is one of the main goals of GW research. GWs are extremely weak, and only huge masses moving quickly can create them. Even the GWs we have discovered, which come from merging supermassive black holes (SMBHs), which can be billions of solar masses, produce only small effects that require incredibly sensitive instruments like LIGO to detect. A necessary first step is to create GWs strong enough to be detected.
“The generation of gravitational waves is key to the development of gravitational communication, but it remains one of the main challenges of modern technological development,” the authors write. “Researchers have explored various innovative methods to achieve this, including mechanical resonance and rotating devices, superconducting materials and particle beam collisions, as well as methods involving powerful lasers and electromagnetic fields.”
There is a lot of theoretical work behind GWCs, but less practical work. The paper points out where research should be headed to bridge the gap between the two. It is clear that it is impossible to recreate such an amazing event as a black hole merger in the laboratory. But it is amazing that researchers were considering this problem as early as 1960, long before we even discovered GWs.
One of the first attempts involved rotating masses. However, the rotation speed needed to create GWs was impossible to achieve, in part because the materials were not strong enough. Other attempts and proposals involved piezoelectric crystals, superfluids, particle beams, and even powerful lasers. The problem with these attempts is that while physicists understand the theory behind them, they don't yet have the necessary materials. Scientists believe that some attempts have produced GWs, but they are not strong enough to be detected.
“High-frequency gravitational waves, often generated by smaller masses or scales, are possible for artificial production in the laboratory. But they remain undetected due to low amplitudes and mismatches with current detector sensitivities,” the authors explain. More advanced detection technologies are needed, or some method of matching the generated GWs with existing detection capabilities. Existing technologies are aimed at detecting GWs from astrophysical events.
The authors explain that “research should focus on developing detectors capable of operating over a wider range of frequencies and amplitudes.” While GWs avoid some of the problems faced by EM communications, they are not without their challenges. Because they can travel long distances, GWCs face the problems of attenuation, phase distortions, and polarization shifts from interactions with things like dense matter, cosmic structures, magnetic fields, and interstellar matter. This can not only degrade signal quality, but also make decoding more difficult.
There are also unique noise sources to consider, including thermal gravitational noise, background radiation, and overlapping GW signals.
“Developing comprehensive channel models is essential to ensure reliable and efficient detection in these environments,” the authors write.
To ever use GWs, we also need to figure out how to modulate them. Signal modulation is crucial to communication. Look at any car radio and you'll see "AM" and "FM." AM stands for "amplitude modulation" and FM stands for "frequency modulation." How can we modulate GWs and convert them into meaningful information?
“Recent studies have explored a variety of methods, including amplitude modulation (AM), dark matter-induced frequency modulation (FM), manipulation of superconducting material, and theoretical approaches not based on astrophysical phenomena,” the authors write.
Each is promising, but also fraught with obstacles. For example, we can theorize about using dark matter to modulate GW signals, but we don't even know what dark matter is.
“Frequency modulation involving ultralight scalar dark matter (ULDM) depends on uncertain assumptions about the properties and distribution of dark matter,” the authors write, addressing the elephant in the room.
GWC may seem unattainable, but it holds so much promise that scientists are reluctant to give it up. In deep space, electromagnetic communication is hampered by vast distances and interference from cosmic phenomena. GWC offers a solution to these obstacles.
A better way to communicate over long distances is crucial for deep space exploration, and GWC is exactly what we need. “Gravitational waves can maintain consistent signal quality over vast distances, making them suitable for missions beyond the solar system,” the authors write. Practical communication using gravitational waves is still a long way off. However, what was once only theoretical is gradually becoming practical.
“Gravitational communication, as a cutting-edge research area with significant potential, is gradually moving from theoretical research to practical application,” Wang and Akan write in their conclusion. This will depend on hard work and future breakthroughs.
The pair of researchers know that a lot of work needs to be done to advance the idea. Their paper is very detailed and comprehensive, and they hope it will be a catalyst for that work.
"While a fully practical gravitational wave communication system remains unfeasible, we aim to use this survey to highlight its potential and stimulate further research and innovation, particularly for space communication scenarios," they conclude.
