Today, Sabine Hossenfelder, "my favorite quantum physicist", had an article about an academic paper in which the author proposed that aliens using really advanced technologies might be able to modulate the quantum properties of photons in order to carry information. (Note: this is not the SFal idea of using entangled particles to communicate faster than light, which - sorry - is not possible; there is no way to modulate the effect to carry information.) Such a modulation scheme could carry a lot more information than our current schemes that modulate properties like amplitude, frequency, phase, etc. A communication beam with quantum modulation would have to be extremely narrowly focused, lest it run into some other matter which would cause the quantum properties to decohere, losing all the information content.
The author wrote a possible approach would be to use frequencies in the infrared, which would require antennas on each end about one hundred kilometers across, which is, actually, not a completely crazy idea. Dr. Hossenfelder also mentioned that because of the decoherence issue, the aliens would be careful NOT to aim the beam near any planet, like, you know, ours. But even if we did receive it, we would have no clue how to demodulate it. I got to thinking about this. (That's why I follow Dr. Hossenfelder, and even support her work on Patreon.)
I recently spent three days at the NIST Time & Frequency Seminar held at the National Institute for Standards and Technology (NIST) Boulder laboratories. A big part of that seminar were demonstrations on how to measure and characterize noise in precision frequency sources. Precision frequency sources like the NIST-F2 cesium fountain clock, shown below, which is the principal frequency reference for the definition of UTC(NIST), the United State's contribution to the international definition of Universal Coordinated Time or UTC. This noise measurement and characterization is not completely removed from measuring noise in communications systems, which, by the way, depend on precision frequency references to work. (A big Thank You to Dr. Jeffrey Sherman, below, for the tour!)
Along the commuter train line from our neighborhood to downtown Denver there is an old AT&T Long Lines tower with the giant microwave horn antennas that used be the backbone of the long distance telephone system. This was before fiber optic cables were run along every railroad track - because the railroads owned the right of ways (an effort which gave the telecommunications company SPRINT its name: "Southern Pacific Railroad Internal Networking Telephony").
The Spousal Unit is so very tired of me telling the story - which I do virtually every time we ride the train (sorry) - of the two Bell Labs engineers who were tasked with figuring out and eliminating the source of the noise in the early models of these microwave antennas (which were big enough you could easily stand up inside of them). Alas, they ultimately weren't able to eliminate it: they determined the noise was the Cosmic Background Microwave Radiation that was the result of the Big Bang. They were picking up the noise from the birth of the Universe. As a consolation, however, they did win a Nobel Prize in physics. And invented the entire realm of radio astronomy. (I eventually took a little motorcycle ride and found that Long Lines tower, shown below.)
Noise exists in every communication link, whether it's radio, wire, optical, etc. You can't get rid of it completely. Eventually maybe you give up and just declare it's "cosmic background", or "thermal noise", or "electrical noise from other equipment in the room". But noise in communication systems is no small problem; given enough, it can jam your GPS, your WiFi, your mobile phone, etc., or just make your vintage vinyl albums sound bad.
I very dimly recall a result from Information Theory that says something like: the output of a theoretically optimal data compression algorithm is indistinguishable from noise. That is: there is no statistical test that can tell you whether the data stream you're looking at is just noise, or is optimally compressed data. (That's not quite the same as saying, however, that it is random.)
What if the aliens have a nearly optimal compression algorithm? (A perfectly optimal one is impossible.) Sending data from one star system to another is bound to be really expensive, not to mention take a long time. So they would be highly incentivized to use such an algorithm. What if part of the noise we see and hear and receive every day in our own radio communications systems is really alien data transmissions?
We could be awash in extraterrestrial data communications and not even know it.
