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Finding the Missing Link: How We Could Discover Interstellar Quantum Communications

Six decades of SETI have yet to produce a detection. Are there strategies we have missed? In today’s essay, Michael Hippke takes us into the realm of quantum communication, explaining how phenomena like ‘squeezed light’ can flag an artificial signal with no ambiguity. Quantum coherence, he argues, can be maintained over interstellar distances, and quantum methods offer advantages in efficiency and security that are compelling. Moreover, techniques exist with commercially available equipment to search for such communications. Hippke is a familiar face on Centauri Dreams, having explored topics from the unusual dimming of Boyajian’s Star to the detection of exomoons using what is known as the orbital sampling effect. He is best known for his Transit Least Squares (TLS) exoplanet detection method, which is now in wide use and has accounted for the discovery of ~ 100 new worlds. An astrophysics researcher at Sonneberg Observatory and visiting scholar for Breakthrough Listen at UC-Berkeley, Michael now introduces Quantum SETI.

by Michael Hippke

Almost all of today’s searches for extraterrestrial intelligence (SETI) are focused on radio waves. It would be possible to extend our search to include interstellar quantum communications.

Quite possibly, our Neanderthal ancestors around the bonfires of the Stone Age marveled at the night sky and scratched their heads. What are all these stars about? Are there other worlds out there which have equally delicious woolly mammoths? Much later, about 200 years ago, the great mathematician Carl Friedrich Gauß proposed to cut down large areas of Siberian forest, in the form of a triangle, to send a message to the inhabitants of the Moon. At the end of the 19th Century, many canals were built, including the Suez and Panama canals. Inspired by these engineering masterpieces, astronomers searched for similar signs of technology on other planets. The logic was clear: What the great human civilization can build must reflect what other civilizations will inevitably build.

Clearly, Martians must equally be in need of canals. Indeed, the Italian astronomer Giovanni Schiaparelli discovered “canali” on Mars in 1877. Other observers joined the effort, and Percival Lowell asserted that the canals exist and must be artificial in origin.

Something similar happened again a short time later when Guglielmo Marconi put the first radio into operation in December 1894. Just a few years later, Nikola Tesla searched for radio waves from Mars, and believed he had made a detection. It turned out to be a mistake, but the search for radio signals from space continued. The “Search for Extraterrestrial Intelligence,” or SETI for short, received a boost in 1960 from two publications in the prestigious journal Nature. For the first time, precise scientific descriptions were given for the frequencies and limits of interstellar communication using radio waves [https://www.nature.com/articles/184844a0] and optical light [https://www.nature.com/articles/190205a0]. Between 1960 and 2018, the SETI Institute recorded at least 104 experiments with radio telescopes [https://technosearch.seti.org/]. All unsuccessful so far, which is also true for searches in the optical domain, for X-rays, or infrared signatures.

Photons? Neutrinos? Higgs bosons?

Particle physics radically changed our view of the world in the 20th century: It was only through the understanding of elementary particles that discoveries such as nuclear fission (atomic weapons, nuclear power plants) became possible. Of the 37 elementary particles known today in the Standard Model, several are suitable for an interstellar communication link. I examined the pros and cons of all relevant particles in a 2018 research paper [https://arxiv.org/abs/1711.07962]. The known photons (light particles) were the “winners”, because they are massless and therefore energetically favorable. In addition, they travel at light speed, can be focused very well, and can carry several bits of information per particle.

Photons are not only known as light particles – they are also present in the electromagnetic spectrum as radio waves, and with higher particle energies than X-rays or gamma rays. In addition, there are other particles that can be more or less reasonably used for communication. For example, it has been demonstrated that neutrinos can be used to transmit data [https://arxiv.org/abs/1203.2847]. Neutrinos have the advantage that they effortlessly penetrate kilometer-thick rock. However, this is also one of their disadvantages: they are extremely difficult to detect, because they also penetrate (almost) every detector.

Incidentally, the particle that is the least suitable of all for long-distance communication is the Higgs boson. It was predicted by Peter Higgs in 1964, but was not observed for the first time until 2012 at the Large Hadron Collider (LHC) at CERN – it also won a Nobel Prize.

The Higgs boson decays after only 10-22 seconds. To keep it alive long enough to travel to the next star, it would have to be accelerated very strongly. Due to the Lorentz factor, its subjective time would then pass more slowly. In practice, however, this is impossible to achieve, because one would have to pump so much energy into the Higgs particle that it would become a black hole. It thus disqualifies itself as a data carrier.

Photons and quanta

Quanta, simply put, are discrete particles in a system that all have the same energy. For example, in 1905 Albert Einstein postulated that particles of light (photons) always have multiples of a smallest amount of energy. This gives rise to the field of quantum mechanics, which describes effects at the smallest level. The transition to the macroscopic, classical world is a grey area – quantum effects have also been demonstrated in fullerenes, which are spheres of 60 carbon atoms. So although quantum effects occur in all particles, it makes sense to focus on photons for interstellar communication because they are superior to other particles for this purpose.

Four advantages of quantum communication

1. Information efficiency

Classical communication with photons, over interstellar distances, can be well illustrated in the particle model. The transmitter generates a pulse of particles, and focuses them through a parabolic mirror into a beam whose minimum diameter is limited by diffraction. This means that the light beam expands over large distances.

For example, if an optical laser beam is focused through a telescope measuring one meter and sent across the 4 light years to Alpha Centauri, the light cone there is already as wide as the distance from the Earth to the Sun. So a receiver on a planet around Alpha Centauri receives only a small fraction of the emitted photons. The rest flies past the receiver into the depths of space. On the other hand, photons are quite cheap to buy: You already get about 1019 photons from a laser that shines with one watt for one second.

In the sum of these effects, every photon is precious in interstellar communication. Therefore, one wants to encode as many bits of information as possible into each transmitted photon. How to do that?

Photons (without directional information) have three degrees of freedom: their arrival time, their energy (= wavelength or frequency), and the polarization. Based on this, an alphabet can be agreed upon, so that, for example, a photon arriving at time 11:37 with wavelength 650 nm (“red”) and polarization “left” corresponds to the letter “A”. The number of bits, which can be encoded per degree of freedom, scales unfortunately only logarithmically: 1024 modes result in 10 bits per photon. In practice, one still has to take losses and noise into account, so that with this classical communication it is rarely possible to transmit more than on the order of 10 bits per photon.

Quantum communication, however, offers the possibility to increase the information density. There are several ways to realize this, but a good illustration is based on the fact that one can “squeeze” light (more on this later). Then, for example, the time of arrival can be measured more accurately (at the expense of other parameters). There are analytical models, and also already practical demonstrations, which show that the information content can be increased by up to 50 percent. In our simple example, about 15 bits per photon could be encoded instead of only 10 for the classical case.

2. Information security

Encryption of sensitive data during data transmission is an important issue for us humans. Of course, we don’t know if this is the case for other civilizations. But it is plausible that future colonies on Mars (or Alpha Centauri…) will also want to encrypt their communications with each other and with Earth. In this respect, encryption is quite relevant for transmissions through space.

Today’s encryption methods are mostly based on mathematical one-way functions. For example, it is easy to multiply two large numbers. However, if the secret key is missing, you have to go the other way around and calculate the two prime factors from the large number. This is much more difficult. However, the security of this and similar methods is “only” due to the fact that no one has yet found an effective method of calculation. We have in no case the mathematical proof available that such a calculation is not possible. There is always the danger that a clever algorithm will be found which cracks the encryption. Quantum computers could also be used in the future to attack some encryption methods.

In contrast, there is quantum cryptography. The best-known method uses a quantum key exchange, which has also been used in practice over long distances, for example via satellite. This is based on quantum mechanics and is unbreakable as long as no mistake is made during transmission – and as long as no one disproves quantum mechanics.

3. Gate-keeping

If there really is a galactic Internet, how to protect it from being spammed by uneducated civilizations? This problem has already occupied Mieczysław Subotowicz, a Polish professor of astrophysics, who wrote in a technical paper on neutrino communication in 1979 that it was: “so difficult that an advanced civilization could intentionally communicate only through it with aliens of its own level of development”.

Now, as mentioned above, neutrino communications are very inefficient. It would be much more elegant and energy efficient to use photons instead. As an entry barrier, it seems plausible not to allow classical photons, but to require quantum communications. This would leave out young technological civilizations like ours, though we would have a good chance of joining in the next few decades.

4. Quantum computing

Konrad Zuse built the Zuse Z3, the first Turing-complete computer, in his Berlin apartment in 1941. This was a single computing machine. It took several decades until the first computers were connected (networked together) in 1969 with the ARPANET. This gave rise to the Internet, in which billions of computers of all kinds are connected today: PCs, cell phones, washing machines, etc. All these devices are classical computers exchanging classical information (bits) on classical paths (for example via photons in optical fibers).

In the future, quantum computers may gain importance because they can solve a certain class of problems much more efficiently. This could give rise to a “quantum Internet” in which quantum computers exchange “qubits,” or entangled quantum bits. These could be intermediate results of simulations, or even observational data that are later superimposed on each other [https://arxiv.org/abs/2103.07590].

Likewise, it is conceivable that quantum-based observational data and intermediate results will be exchanged over larger distances. This is when interstellar quantum communication comes into play. If distant civilizations also use quantum computers, their communications will consist of entangled particles.

Excursus: The (im)possible magic Pandora quantum box

The idea of using quantum entanglement to transmit information instantaneously (without loss of time) over long distances is a frequent motif in science fiction literature. For example, in the famous novel The Three Body Problem by Chinese author Liu Cixin, the “Trisolarans” use quantum entangled protons to communicate instantaneously.

This method sounds too good to be true – and unfortunately it actually contains three fundamental flaws. The first is the impossibility of exchanging information faster than the speed of light. If that were possible, there would be a causality violation: one could transmit the information before an event happens, thus causing paradoxes (“grandfather paradox” [https://arxiv.org/abs/1505.07489]). Second, quantum entanglement does not work this way: one cannot change one of two entangled particles, thereby causing an influence on the state of the partner. As soon as one of the particles is changed, this process destroys the entanglement (“no communication theorem”).

Third, an information transfer without particles (no particle flies from A to B) is impossible. Information is always bound to mass (or energy) in our universe, and does not exist detached from it. There are still open questions here, for example when and how information that flew in with matter comes out of a black hole again. But this does not change the fact that the communication by quantum entanglement, and without particle exchange, is impossible.

But wait a minute – before we throw away the “magic box of the entangled photons”, we should once more examine the idea. For there is, despite all the nonsense that is written about it, an actually sensible and physically undisputed possibility of use: known under the term “pre-shared entanglement” [https://arxiv.org/abs/quant-ph/0106052].

To perform this operation, we must first assume that we can entangle and store a large number of photons. This is not so easy: the current world record for a quantum memory preserves entanglement for only six hours. And even that requires considerable effort: It uses a ground-state hyperfine transition of europium ion dopants in yttrium orthosilicate using optically detected nuclear magnetic resonance techniques [https://www.nature.com/articles/nature14025]. But it is conceivable that technological advances will make longer storage possible. Conditions are particularly good for interstellar travel, because space is dark and cold, which slows decoherence caused by particle interactions.

So let’s assume such a quantum memory is available – what do we do with it? We take one half of the magic box on board a spaceship! And the counterpart remains on earth. Now the spaceship flies far away, and wants to communicate home. The trick is then not to send the bits of the information transmission simply on a photon letter to the earth, but to superpose each classical signal photon first with one (or more) stored entangled photons. The result is one classical photon per superposition, which is then sent “totally normally” to the receiver (for example the earth). Upon arrival, the receivers opens their own magic box and bring their part of the entangled particles with it to superposition. This allows the original message to be reconstructed.

The advantage of this procedure is increased information content: The amount of information (in bits per photon) increases by the factor log2(M), where M is the ratio of the entangled to the signal photons. Even a very large magic box is therefore of limited use, because unfortunately log2(1024), for example, is only 10. Losses and interference (due to noise, for example) also have a negative effect on the amount of encodable information. Nevertheless, “pre-shared entanglement” is a method that can be considered, because it is physically accepted – in contrast to most other ideas in popular literature.

Quantum communication in practice

But what does quantum communication look like in practice? Is there even a light source for it on earth? Yes, for a few years now this has actually been the case! When gravitational waves from merging black holes were detected for the first time at the Laser Interferometer Gravitational-wave Observatory (LIGO) in 2016, “squeezed light” was used. This is laser light traveling through a very precisely controlled crystal (an “OPO” for “Optical Parametric Oscillator”). This converts one green photon into two entangled red photons, to what is called a squeezed vacuum. This reduces phase uncertainty at the expense of amplitude fluctuation. And it is the former that matters: One would like to measure the arrival time of the photons very precisely in order to compare the length of the path with and without gravitational waves. The brightness of the photons is not important.

Such a squeezed light, with lower fluctuations compared to classical light, also improves interstellar communication. It still remains unresolved what is the best way to modulate the actual data. Signal strength is also still low, with just a few watts of squeezed light in use at LIGO. By comparison, there are classical lasers in the megawatt range. So the development of quantum light is several decades behind classical light. But more powerful quantum light sources in the kilowatt range are already planned for next-generation gravitational wave detectors. This would also mark the entry threshold for meaningful interstellar quantum communications.

Detection of quantum communication

Entangled photons are also just photons – shouldn’t they already be detectable in optical SETI experiments anyway? In principle this is correct, because for a single photon it is in principle not determinable who or what has generated it. If it falls on the detector at 11:37 a.m. with a wavelength of 650 nm (color red), we cannot possibly say whether it came from a star or from the laser cannon of the Death Star.

However, a photon rarely comes alone. If we receive one thousand photons with 650 nm within one nanosecond from the direction of Alpha Centauri in our one-meter mirror telescope, then we can be sure that they do not come from the star itself (the star sends only about 32 photons of all wavelengths per nanosecond into our telescope). Classical optical SETI is based on this search assumption. It is thus very sensitive to strong laser pulses, but also very insensitive to broadband sources.

Quantum SETI extends the search horizon by additional features. If we receive a group of photons, they no longer have to correspond to a specific wavelength, or arrive in a narrow time interval, for us to assume an artificial origin. Instead, we can check for quantum properties, such as the presence (or absence) of squeezed light. Indeed, there is no (known) natural process that produces squeezed light. If we receive such, it would be extremely interesting in any case. And there are indeed tests for squeezed light that can be done with existing telescopes and detectors. In the simplest case, one tests the intensity and its variance for a nonlinear (squared) correlation, which requires only a good CCD sensor [https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.113602].

There are numerous other tests for quantum properties of light that are applicable to starlight. For faint sources from which only a few photons are received, one can measure their temporal separation. Chaotic starlight is temporally clustered, so it is very likely to reach us in small groups. Classical coherent light, i.e. laser light, is much more uniform. For light with photon “antibunching”, in the extreme case, the distance between every two photons is identical – so their arrival times are perfectly uncorrelated. This quantum mechanical effect can never occur in natural light sources, and is thus a sure sign of a technical origin. The technique is used from time to time because it is useful for determining stellar diameters (“intensity interferometry”).

For a few stars we can already deduce on the basis of existing data that they are of natural origin: Arcturus, Procyon and Pollux [https://academic.oup.com/mnras/article/472/4/4126/4344853]. In the future, however, the method can be applied to a large number of “strange” objects to test them for an artificial origin: impossible triple stars [https://academic.oup.com/mnras/article/445/1/309/988488], hyperfast globular clusters [https://iopscience.iop.org/article/10.1088/2041-8205/787/1/L11], or generally all interesting objects listed in the “Exotica” catalog by Brian Lacki (Breakthrough Listen) [https://arxiv.org/abs/2006.11304].

Current status and outlook

The idea to extend SETI by quantum effects is still quite new. However, one can fall back on known search procedures and must adapt these only slightly. Thus, dubious light sources can be effectively checked for an artificial origin in the future. We can be curious what the next observations will show, and ask the question: “Dear photon, are you artificially produced?”

The paper is Hippke, “Searching for interstellar quantum communications,” in press at the Astronomical Journal (preprint). See also the video “Searching for Interstellar Quantum Communications,” available at https://www.youtube.com/watch?v=Kwue4L8m2Vs.


Comments on this entry are closed.

  • charlie June 18, 2021, 12:33

    ” … the great mathematician Carl Friedrich Gauß … ”

    u mean Gauss ????

    • Paul Gilster June 18, 2021, 15:15

      Same person, Charlie. Gauß is a German form, and Michael is German. You’ll find the same character used in the word for street: straße. The ß is a special character, just as the German umlaut is.

  • Michael June 18, 2021, 14:33

    Paul, I think the decay of the Higgs boson is to the minus 22 power.

    • Paul Gilster June 18, 2021, 15:18

      Exactly so. My mistake.

  • Thomas R Mazanec June 18, 2021, 14:51

    The Higgs boson decays after only 1022 seconds. To keep it alive long enough to travel to the next star, it would have to be accelerated very strongly. Due to the Lorentz factor, its subjective time would then pass more slowly. In practice, however, this is impossible to achieve, because one would have to pump so much energy into the Higgs particle that it would become a black hole. It thus disqualifies itself as a data carrier.

    Unless my math is grossly off this is about 300 million million years. Did you mean 10^-22 sec?

    • Paul Gilster June 18, 2021, 15:18

      Yes, proofreader error, and the proofreader was me. Now fixed.

  • Henry Cordova June 18, 2021, 15:30

    I think its always been understood, by all SETI enthusiasts, that there may very well be technical means of communication (besides EM radiation) across interstellar distances that we simply do not know about because we’ve been in the electronics business for such a short time. We’ve all wondered if maybe there is some hypothetical Q-ray we’re simply too primitive to know about, and that the ether is simply buzzing with ET Q-grams we don’t know how to tune in on.

    Be that as it may, the early SETI pioneers were not unwise to suggest radio waves as a means of communication. Even if the hypothetical Q-ray is common to all or most civilized species, there are still very good reasons to listen, and transmit, in the radio (and especially, the microwave) spectrum. The reasoning behind the Waterhole is still valid, and it is to be expected other species with astronomical interests, even those who have mastered the elusive Q-ray, will be monitoring the EM spectrum for other reasons besides communications, and they will deduce potential correspondents are doing the same.

    However, this line of thought does bring up a rather exciting (or possibly disturbing) possibility. Suppose we discover the Q-ray in the near future, and the possibility of quick, or even instantaneous interstellar communication becomes a possibility. And let us further assume that this new communications technology is simple and cheap and effective enough that it is reasonable to assume any civilization more advanced than ours has had it for a long time.

    If suddenly we start listening and we hear nothing,, it brings up a new version of the Fermi Paradox to worry about. And this one will not be so easily dismissed.

    • Robin Datta June 20, 2021, 4:32

      Presuming the Q-wave to be “instantaneous”, the question arises whether with electromagnetic including light radiation WYSIWYG with Q-waves which one doesn’t see. What we see of the other side of the galaxy is nearly 100,000 years in the past, and the next big galaxy is a couple of million years in the past. Can hear what they’re saying “today”, but can’t see what they look like “today”.

      • Henry Cordova June 20, 2021, 12:09

        I don’t know enough physics to say whether or not “instantaneous” transmission of data is possible. Most practicing physicists tell us it violates the rules and assumptions of Einstein’s Relativity, and it may very well be that the entire concept of “simultaneity” is untenable. But science has been wrong before, so I try to keep an open mind. Still, I must confess, I would much rather live in a universe where FTL speeds are possible. And remember, if you can send a message, you can also send an image, audio or even a movie.

        As to your question, consider how contemporary sound dubbing of old war newsreel films allows you to hear the explosion of the U-boat’s torpedo, or the bomber’s ordnance, at the same instant you see the image of the detonation. Even those of us who know sound travels much slower than light intuitively accept this illusion.

        And remember how ground-breaking Stanley Kubrick’s refusal to use sound effects was when the action on the screen was taking place in vacuum. (2001: A Space Odyssey).

        Hopefully, perhaps there is some compromise allowed by nature. Ursula K Leguin speculates about the fictional device called an “ansible” that allows for instantaneous communication across interstellar distances (probably based on some kind of speculative quantum entanglement principle). However, the ansible must be physically carried across the intervening distance by an astronaut riding in an FTL vehicle. In “Game of Thrones”, the characters communicate over long distances using messages physically attached to crows, like homing pigeons. The plot would not make any sense if the news had to travel on horseback.

        Still, we have little choice but to restrict our speculations to whatever our science tells us is possible today.

      • Alex Tolley June 20, 2021, 13:01

        A very good explanation. It also implies that civs communicating with Q-waves can only communicate but not know where they are in the light cone. They may never be able to meet, just convey what to each is contemporary information, images, video etc. But Q-waves imply FTL communication which appears to create time paradoxes – receiving a reply from a civ before sending out the message.

        • Henry Cordova June 21, 2021, 21:21

          Why paradoxes?

          I understand that the speed of light is finite, and that is the only way we can get information about an event occurring somewhere else. But why should that imply time paradoxes? I’ve never understood why that should be.

          Suppose a star exactly 10 light years away goes supernova. There is no way I can know for sure it has blown up without waiting for ten years until the light from the explosion reaches me. But I know that exactly ten years ago I was buying my new car.

          Why can’t I say, or know, or believe that I bought my car at the exact same time that star died? I understand why I would have to wait exactly ten years to say that, it takes that long for the information to reach me, but that doesn’t mean the two events can’t be simultaneous. Where’s the paradox?

          Granted, we can’t make statements about events we haven’t become aware of, but that doesn’t mean that they haven’t happened.

          • Ron S. June 22, 2021, 8:12

            There are no universal clocks. It has been said (I don’t recall who I’m quoting) that the first cherished notion you must abandon in relativity is that of simultaneity. It doesn’t exist. Also, there is nothing special about light — its behavior is determined by the structure of spacetime. What it isn’t doing is traveling at a “finite speed” in a Newtonian universe.

            I recommend that you buy a good introductory text on spacetime physics and study it.

          • Alex Tolley June 22, 2021, 11:08

            I thought we discussed this a while back. There are articles on the web using math/space-time diagrams to explain the causality violation. My mental shortcut is to accept the authority of physicists who understand this a lot better than I and who state that causality violation would occur with FTL communication. Mentally, it is like pruning the search tree and accepting that this particular branch ends with “FTL communication and travel is not possible without causality violation. [Danger! Causality violation will have all sorts of strange reality ramifications].”

            • charlie June 27, 2021, 19:21

              “My mental shortcut is to accept the authority of physicists who understand this a lot better than I and who state that causality violation would occur with FTL communication. Mentally, it is like pruning the search tree and accepting that this particular branch ends with “FTL communication and travel is not possible without causality violation. [Danger! Causality violation will have all sorts of strange reality ramifications].””

              Alex, perhaps it is you who should possibly do some ‘mental pruning’ and take a fresh look at the equations of special relativity. I suggest that you should open your mind to the possibility that the equations of special relativity have nothing to do with either FTL motion or communication. To whit:

              We agree that Einstein’s SR equations (the Lorentz transform) isn’t intended to be used with any FTL speeds. If we presume that FTL exists, and apply the equations, we end up with crazy results, such as the causality violation.

              Another interpretation is to say no, this is a Reductio Ad Absurdum, we have proven that FTL is not possible *because* it would enable causality violations. Any further work on the soliton proposal would be a waste of time.”

              Yes as a matter fact, there is both agreement and disagreement here. As I said before if you look at Einstein’s original paper of 1905 you can see that incorporation of frames that move with velocities greater than light can result in negative times which could be interpreted as time reversals. As applied to his original equations you would fail to be able to synchronize clocks as is needed by his original formulation.

              However, to declare that because there could be causality violations, and that in of itself, would of necessity declare the death of FTL transit or communication, I continue to believe is not true and I have included here in a link relating the paper written by research scientist who are far above my pay grade who state that you can have FTL scenarios without time reversal. You can look at your leisure to see what you can make of it, personally for me I find it a difficult read but I’m not steeped in depth in relativity theory.

              However I can say one thing which I believe is absolutely certain: if you have FTL signals which propagate with infinite velocity then you will have guaranteed no framework which you can devise which can cause time reversal. Just by definition all observers must be moving in forward light cones toward the future. Here’s the link to the paper that I promised you below:


              • Ron S. June 27, 2021, 21:45

                That paper was introduced and demolished in the comment thread of a previous article. It hasn’t gotten any better.

                • charlie June 28, 2021, 14:39

                  actually that paper wasn’t demolished; it was hand waved away …

                • Ron S. June 29, 2021, 9:52

                  Only your hands have been waving. Specific technical comments refuting the paper were presented. Then as now, you neither countered those arguments (by me and especially David) nor did you demonstrate in what way the paper was correct. The same goes for the other reference your provided.

                  Unless you now have a concrete argument to offer there is no reason to take this further.

                  • charlie June 29, 2021, 18:39

                    @Ron S.

                    “David Byrden March 30, 2021, 4:52

                    Don’t assume that a scientific paper is worth the paper it’s printed on. Einstein himself proved that FTL enables backwards time travel, so when these guys contradict him, we should check their math.

                    And I did. And it’s terrible.

                    Here’s the executive summary: they define “time travel” as returning to a point in spacetime that you visited before. That’s a useful and clear definition.
                    So you need make a return trip, a closed loop in spacetime, to Time Travel. They claim it’s impossible and they can prove it.

                    So they set up a simple 2-dimensional spacetime. Normal travel is going upwards, while FTL travel is going Right or Left. Your challenge, as I said, is to travel in a loop. They intend to prove that you can’t.

                    And THEN they say :
                    “we may assume, without loss of generality, that the future quadrant of … FTL observers is right”

                    They won’t ALLOW you to go left ! You can’t make a closed loop now ! The game is rigged !

                    How did that paper ever pass the peer review?”

                    ” … And THEN they say :
                    “we may assume, without loss of generality, that the future quadrant of … FTL observers is right”

                    The game is rigged !You can’t make a closed loop now ! The game is rigged ! … ”

                    The game is rigged ! The game is rigged ! So they set up a simple 2-dimensional spacetime. ….

                    EXCEPT … (from the paper , it says clearly )
                    ” … The proof of Theorem 1
                    gives us an extension of the standard model of special relativity with FTL observers on all spacelike lines, in which Einstein’s Principle of Relativity holds but time travel is not possible. We then generalize this model to include each (n+1)-dimensional spacetime (n = 1, 2, 3, . . .), thereby showing (Theorem 2) that FTL motion does not in itself introduce the possibility of time travel after all.”

                    … We then generalize this model to include each (n+1)-dimensional spacetime (n = 1, 2, 3, . . .) …

                    (n+1)-dimensional spacetime (n = 1, 2, 3, . . .) . In other words all 3 dimensions of space. You can turn left (or in any other direction u wish …)

            • paolo June 28, 2021, 17:13

              Microcausality happens all the time in the world of elementary particles, the macro one is, of course illegal; let’s say you can borrow $100M with the interest of 1% for 0.1 second but you can’t do that for a period of 1 hr or 1 day.

          • charlie June 27, 2021, 19:31

            Yes Henry, I agree with you completely on your analysis and I believe that the idea that there is no definition of simultaneity is perhaps an illusion, especially in light of the fact that they have proven beyond a shadow of a doubt that quantum entangled particles have interactions which vastly exceed the velocity of light (perhaps infinite?). At this point there is no one who can say that that is not true.

            The primary reason why people continue to resist the idea is that, if in fact, it could ever be possible to reinterpret the laws of physics through the lens of quantum entanglement that could possibly spell the death of (or at least revision of) relativity which must as its foundation DEMAND that there be no signal which propagates at a velocity greater than that of light. But, too much everyone’s chagrin there does in fact exist velocities in which particles communicate with each other. Of the fact that these communications are of a totally random nature in no way undermines the fact that they are, in fact, ‘communications’.

            There simply isn’t any other way to get around that fact and that is an embarrassment to the physics community which must backfill as a reason that these don’t represent any form of recognizable ‘signal’ which can act as an information conduit. I have a feeling that this particular appeal to such an idea is merely a way to avoid the embarrassment that they could possibly be wrong in something that they hold sacrosanct. I believe in the future we will find a way to use the quantum entangled state as a means to communicate. If such a way is found than the way it will be open up to the idea of a ‘universal clock’.

      • Deanna Shaw June 20, 2021, 15:25


        Any suggestion of information traveling faster than the speed of light?

        Gotta say “nope!” in our universe. Too many paradoxes.

        And after a few re-reads of this article, I’m just not seeing any serious competitor to low energy photons. Radio, optical laser, etc.

        So let’s focus to build a ginormous radio telescope on Luna’s far side and listen.

    • Michael Fidler June 22, 2021, 5:09

      Found something to make this even more confusing, I believe Bohm said that there is only one electron in the universe. Now see if you can wrap your mind around that!

  • Alex Tolley June 18, 2021, 16:19

    I found the youtube presentation more illuminating than this post.
    If I understood it correctly, the claim is that quantum photons for communication have advantages that EI (and humans) would use. Furthermore, we already have the capabilities to analyze light to determine whether it is an artificial (only possibility) source of quantum photons so we may as well start.

    The only objection I have is the statement at the end of the presentation, that we could “discover a message from space”. Yes, we could determine that it is an artificial signal, but unless it is targeted at us specifically with a simple code format, like a binary b/w image, we couldn’t read it. Under those circumstances, why wouldn’t ETI just send non-quantum photons? The assumption therefore is that ETI transmitters are sending communications in an interstellar internet that will be both prolific, possibly omnidirectional, but unbreakable encrypted (one advantage of the technique), that we will intercept by chance. I see all the usual arguments against omnidirectional beams would still apply, suggesting that communications would be tight beams, that we would be unlikely to intercept except by chance. This makes me think that the search will be as fruitless as conventional SETI whether with radio or optical sources. If the needed kit can be added to all-sky stellar surveys for a low cost, then it might be worth doing to test the light of millions of stars for quantum emissions. If that were to be done, how much searching with no results would be convincing enough to indicate that no signals are being sent, and to discontinue the analysis, perhaps to be replaced by another idea to test?

    • Ron S. June 18, 2021, 21:17

      The only thing that comes to mind is that these photons have a property that is easily distinguished (apart from frequency and polarity) that can be exploited to build a filter that improves the signal-to-noise ratio. Were that so the probability of detection is improved for the same signal amplitude. The author does speak to this as being accomplished with CCD something-or-other. I didn’t dig deeper to understand whether this is possible or feasible.

    • LuigiG June 19, 2021, 3:23

      Squeezed light may be just the tip of the iceberg in terms of the levels of steganography that can reliably be achieved by manipulating the above mentioned ‘quantum properties’ of photons, assuming that sender & receiver seek to conceal the very existence of the message. From a security standpoint, avoiding detection is preferable to encryption alone (even if ‘uncrackable’), a plausible reason why ET would want to go down that route.

  • Ron S. June 18, 2021, 16:39

    “one would have to pump so much energy into the Higgs particle that it would become a black hole.”

    This can’t be what was meant. If you pump energy “into” a particle, you are increasing its mass, but it isn’t being accelerated. If you use energy to accelerate a particle, you do not increase its mass.

    Also, how one could accomplish either with a Higgs is not at all clear to me from what I know of the particle’s properties.

  • Geoffrey Hillend June 18, 2021, 21:39

    The Higgs boson’s mass is created out of the quantum vacuum, so it is a virtual particle which is why it has short life. The quantum rule is that particles or bosons with high mass can only be exchanged short range which is why the Higgs boson (125 GeV) can’t be used to communicate. Photons and gravitons have no mass so they can travel long range.

    The problem with squeezed light is that is it not coherent light like a laser, so it would spread out over of distance. Single quantumly entangled particles would be too thin a beam. I suggest using quantumly entangled lasers linked to a quantum computer which uses qubits, so we could make hacking impossible unless the planet one was sending the QE laser beam also had a quantum computer. It seems to me though such communication might be used only by a very advanced civilization who did not want a less advanced one to hear it. Civilizations which have yet to find ETI on other world like us would not want to make it hard to understand the signal.

    We also are not listening and transmitting on all frequencies and have to find the right one. The reason why a laser does not spread out over a long distance is all of the photons have the same wavelength which move together at the same energy, phase and direction which is called coherence.

    • AlexTru June 20, 2021, 9:13

      … Photons and gravitons have no mass so they can travel long range.

      Do you mean : the entity that has mass cannot travel long distances?
      It sounds like the new physics…

      • Geoffrey Hillend June 21, 2021, 16:13

        No it is not any new physics. Sorry for me not sighting a reference. Hawking, p. 90, A Brief History in Time. All bosons are the quanta of force fields and those with high mass can only be exchanged short range like the W bosons of the weak nuclear forces, they have only a very short range and life.

  • Michael June 19, 2021, 0:29

    I suppose using gamma rays would work, they have very low dispersion due to their wavelength. We could generate them with a free electron laser and we could perhaps catch them for reading with electrons. If we had a stream of electron flowing at relavativistic velocities parallel to the infomation flow they could interact with the gamma rays at a more normal energy and pick up the signals.

  • Andrei June 19, 2021, 7:06

    Thank you Michael Hippke, and the woolly mammoth comparison is indeed valid also today. Since we only have one example of life from here on Earth – while there might be endless alternative possibilities even for the very basics of encoding of genetic information.
    Popular presentation based on a press release here:
    And as for the main part of the subject here, regardless if any message is sent via radio or light it will face degradation, it’s well known fact and many SETI searches do assume that they only will be able to pick up the carrier wave, and not the information content itself.
    And thank you for reminding me of Mieczysław Subotowicz, who indeed before the advent of the Internet indeed did forsee the problems of ‘spam’, trolls and those that might have malicious intent even in interstellar communications.
    This possibility have later been mentioned in fiction such as in ‘Contact’ where it’s suspected that the device might be a doomsday machine, one such device could also be so expensive and made from such exotic materials that it intentionally could be meant to wreck a planetary economy of beings of less technological capacity. (Ok that one is far fetched yet cannot be ruled out).
    So there might means of communication that is done in such a way or using a technological firewall to keep the lowbies of the universe out from the network.

  • Andrew P June 19, 2021, 8:16

    For interstellar communications, one needs tachyons.

    • Michael June 19, 2021, 12:28

      Made from unobtanium…

  • Michael June 19, 2021, 12:31

    Wondering if we could entangle the magnetic or electric part of the photons to adjacent ones so it no longer undergoes dispersion i.e. effectively parallel for ever more.

  • charlie June 19, 2021, 17:42

    “Classical coherent light, i.e. laser light, is much more uniform. For light with photon “antibunching”, in the extreme case, the distance between every two photons is identical – so their arrival times are perfectly uncorrelated. ”

    “… – so their arrival times are perfectly uncorrelated. ” … – uncorrelated.

    Seems like the opposite would be true …

  • Mike Serfas June 19, 2021, 21:12

    There are problems with this: “in 1905 Albert Einstein postulated that particles of light (photons) always have multiples of a smallest amount of energy” First, photons *don’t* have multiples of a smallest amount of energy – so far as I know, the spectrum is continuous, velocity is continuous, and the energy of a photon can be changed by an arbitrarily small amount by changing your velocity by an arbitrarily slow speed. Second, the photoelectric effect was not about that, but only showing that the energy was arriving in particle form, so (for example) multiple photons of non-ionizing radiation don’t add up to ionizing radiation.

    • Michael June 20, 2021, 9:47

      The photoeffect theory was I believe that light was quantized i.e. in discreet packets. It’s is not dependant on the intensity of the arriving light i.e. a load of electromagnetic radiation of a wide band of wavelengths could arrive but only discreet, quantized, amounts can be absorbed or emitted by matter dependant on the material properties at the time.

  • Robin Datta June 20, 2021, 0:14

    Could a higher dimensional object or message projected into a linear form be reconstituted to extract the message? As in a three dimensional comb passing through a two dimensional plane would appear as a line of dots expanding to a row of ovals, merging into a long oval, shrinking to a line and disappearing. Could such signals form spontaneously and carry insights into the nature of the universe in other dimensions?

  • AlexTru June 20, 2021, 9:30

    … the information content can be increased by up to 50 percent. In our simple example, about 15 bits per photon could be encoded instead of only 10…

    Some problematic or wrongly formulated phrase here…
    5 additional bits does not mean 50% (or 1.5 times) of increase , but 3200% (or 32 times), i.e 2^5 …

  • Shimrra Shai June 20, 2021, 21:21

    The thing I wonder about is: wouldn’t the beam need to be aimed at us for us to pick this up? Why should we assume that we’re so lucky and/or important as to have such a beam sent this way? Of course, doesn’t mean don’t look, but I wouldn’t read too much into not finding anything if you do.