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In an effort to work out a believable form of faster-than-light communications for my setting, I've ruled out a few things: wormholes (which are in use, but are too random in where they lead, to be used for this) and quantum entanglement (which flat out doesn't work for information transfer).

In my research, the only natural phenomenon that even hints at being capable of this is the hypothetical tachyon particle. The problem is, they're believed to not be capable of interacting with anything, if they do exist.

Does anyone possibly have a suggestion for a minimal-handwaving way to acceptably explain away how they might detect these, in light of this issue?

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    $\begingroup$ Even as an avid SF reader, I didn't know tachyons aren't supposed to interact with anything. Handwave it with something as simple as "we now can detect tachyons", or "tachyons generate their own particles that can be detected". $\endgroup$ – kikirex May 7 at 12:23
  • $\begingroup$ Can you add a citation for the claim that tachyons are believed to not interact with anything? That sounds more like neutrinos (which do interact, albeit with extremely small cross-sections) than tachyons. $\endgroup$ – HDE 226868 May 7 at 13:03
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    $\begingroup$ I'd use a tachyon detector. $\endgroup$ – axsvl77 May 7 at 17:04
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    $\begingroup$ One classical citation: localized tachyon disturbances are subluminal and superluminal disturbances are nonlocal $\endgroup$ – user62842 May 7 at 22:32
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Minimal handwaving is done with a single big lie rather than little ones.

If you try to tell the little lie, there's constant follow up, but what about this, but what about that, but what about the other.

The big lie tells not about the technology but about the timeline, not about how it was done but about who did it. Make it a story about the person who did it, where they were in their career, what country they were in. The war they'd just survived, when they emigrated from small war-torn country to large technological nation with nothing to their name. Acceptance to legendary institution, the great breakthrough, the implications and celebrations, but never how the technology actually works.

That way you have to wave your hand precisely once, rather than over and over again in lots of little ways.

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  • $\begingroup$ Take a look at "Let There Be Light" and "Solution Unsatisfactory" , both shorts by Heinlein, for some really good description of research and scientific progress, with a really hand-wavy description of how the technology actually works. He gives enough science to make the story work, without actually telling you anything. $\endgroup$ – Chris Cudmore May 7 at 15:33
  • $\begingroup$ Sorry, "Blowups Happen", not "Solution Unsatisfactory" $\endgroup$ – Chris Cudmore May 7 at 15:45
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    $\begingroup$ This is a great answer. It points out a common problem with new authors: getting stuck in the details. The captain screams, "contact Starfleet!" and the comm officer send the message "via subspace." What's more important to the story? The narrative value of the message or the scientific details behind "subspace?" Hint to new authors: it isn't the latter. In the eternal words of Captain Picard... "Make it so!" $\endgroup$ – JBH May 7 at 17:55
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    $\begingroup$ There's a secondary part to my answer which I haven't put in for now because it's more of a yarn. I remember a story about time travel, it was a classic of the "ants take over the world" form and would still be just as good today, if the author hadn't gone on about valves and how fragile they were. In that excessive detail on such a period specific technology, and thinking they'd be valid for hundreds of years, the author had taken the edge of something that could have been a story for the ages and made it a story of an age. $\endgroup$ – Separatrix May 8 at 7:11
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Tachyons are detectable.

Fortunately, I believe your question is based on a mistaken premise. Tachyons, if they exist, would likely indeed be detectable. In fact, since they were initially theorized, there have been several experimental searches for tachyons, though very few in recent years. I'll talk about a couple experiments noted in Status of experimental searches for tachyons? They're important because they represent a couple main avenues of detection, and you should be able to base your communication system on them.

  • Clay 1988: Cosmic ray showers. As high-energy particles plow through the upper atmosphere, they decay into a number of lighter particles which are detectable by humans on Earth. The products of the decay of the initial particle all travel extremely close to the speed of light. Clay notes that the first scientist to exploit this phenomenon was Ramana Murthy, who proposed looking for particles that arrive even sooner, before the first photons from the event.
  • Alvager & Kreisler 1968: Cherenkov radiation. The effective speed of light is different in different mediums; photons interact with atoms in particles in a substance, effectively slowing them down. If a charged massive particle travels faster than this effective speed of light, it should emit photons called Cherenkov radiation, which is a well-studied phenomenon for massive particles. As tachyons travel faster than the speed of light in a vacuum (and thus faster than the speed of light in any medium), they should produce Cherenkov radiation, and would in fact be the only particles to produce such a signal in a vacuum.
  • Alvager & Erman 1965: Mass-energy and momentum. We can use special relativity to calculate the magnitude of a particle's energy $E$ and momentum $p$. For normal massive particles, we expect $|E|>|p|$; for tachyons, we should see $|p|>|E|$. I am still trying to find out more details of their experiment; the pair monitored an isotope of thulium, $^{170}\text{Tm}$. Thulium-170 usually transitions to Ytterbium-170 via $\beta^-$ decay, but it appears that tachyons could play a role in more complicated processes.
  • Baltay et al. 1970: Missing energy. Even in the case of tachyons that don't interact (or don't interact strongly) with detectors, we should still be able to see them indirectly. In particular, some unstable particles might have decay chains involving tachyons, and if these chains are observed and some energy remains unaccounted for, it could be a sign of tachyons. Neutrinos, incidentally, were originally detected basically the same way.

The basic point is, tachyons can be detected directly (e.g. as products of cosmic rays or atomic decays) and indirectly (e.g. through Cherenkov radiation and missing energy from meson decay).

Applying this to communication

These experiments are, to be frank, not very useful for communication. Most involve observing tachyons produced naturally, instead of by humans, and at effectively uncontrollable rates. We can rule out most of them for your use, but I think the most promising is Alvager & Kreisler's method of Cherenkov radiation. Let me talk about their idea in slightly more detail.

The pair's setup involved two parallel plates with a static electric field between them. Tachyons should gain energy traveling through the field while losing energy via radiation, and it should be possible to tweak the field's parameters such that this total energy changer is zero - which they did. The tachyon should, over the course of traveling through the detector, travel through a potential difference of $\sim9\text{ kV}$ and gain corresponding energy based on its charge; the Cherenkov photons would have energies in the range $0\text{-}3.8\text{ eV}$. It was expected that 12% of the produced photons would be detected (although no doubt we could, today, increase that percentage). Tachyons with charges from $0.1\text{-}2e$ could be observed.

A diagram of the tachyon detector
Figure 1, Alvager & Kreisler 1968. A diagram of the duo's detector.

I would guess that this setup could be scaled up such that detecting tachyons traveling over interstellar distances would be feasible. Presumably, information would be coded in the number of tachyons detected, and therefore the amount of energy produced in the form of photons. Furthermore, of course, you are perfectly able to change the parameters of the device and the properties (e.g. charge) or your tachyons, so you can optimize the process as you wish.

Suspension of disbelief, handwaving, and all that

Separatrix's answer, which I think also makes your assumption of undetectability, argues that you should do as little handwaving as possible - quite true - by avoiding discussing the details of the technology. This can be quite effective, and it definitely should not be ignored. I could stand to make use of it more.

That said, the basic idea behind the Alvager & Kreisler detector is simple enough that I believe this issue is not very important. If you wish to go slightly in depth when describing the device - or if you want one character to use something akin to jargon while talking to another - simply mention the electric fields used, or the potentials, or the energy range. I'm not a fan of using random (and irrelevant) jargon in writing, but in this case, it's quite relevant indeed, and the detector is simple enough that you're not as likely to alienate readers as you might think.

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  • $\begingroup$ Kilovolts are a unit of electric potential, not of energy. Did they mean kiloelectronvolts? $\endgroup$ – John Dvorak May 7 at 15:05
  • $\begingroup$ @JohnDvorak It's unclear to me; they note that the field has a magnitude of $3\text{ kV/cm}$ and that the detector is $3\text{ cm}$ long, meaning that they're really talking about a potential difference of $9\text{ kV}$. I'll look for a less ambiguous (or less wrong!) way to phrase that. $\endgroup$ – HDE 226868 May 7 at 15:09
  • $\begingroup$ @JohnDvorak I've edited it to be a bit clearer. I guess even listing the number might be superfluous, honestly. $\endgroup$ – HDE 226868 May 7 at 15:46
  • $\begingroup$ Perhaps the information I read of them being non-interactive was outdated, then. I happily stand corrected. Thanks! $\endgroup$ – Cereza May 8 at 3:52
  • $\begingroup$ I must correct you, I don't assume undetectability, I very carefully don't mention the particles at all. Consider the telephone, we mention Alexander Graham Bell, we argue the differences between iPhone and Android, but we effectively never talk about how a telephone actually works. It's just not a thing we do with any such device outside a very narrow professional setting. Hence discussing how to detect tachyons should be outside the scope of a story, even if technology within it is based on them. $\endgroup$ – Separatrix May 8 at 14:50
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Tachyons aren't known to exist. But if they did, they could absolutely interact with conventional matter. That being said, you'd probably have trouble sending tachyon pulses around the galaxy. Not because of any tachyon-specific issues, but just because galaxies are huge. Right now there are probes about 20 million miles from Earth, and picking up their radio signals is a huge to-do. A galaxy is a billion times bigger than that distance, which means that the energy is diffused a billion billion (a quintillion) times more.

Depending on your setting, it might be reasonable to send the message to the nearest outpost, and wormhole it to some other outpost, and then tachyon it to your friend.

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    $\begingroup$ OP is asking how to handwave tachyons detection. I am not sure you are answering that question $\endgroup$ – L.Dutch May 7 at 15:29
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    $\begingroup$ Hmmm... your assertion that tachyons would need to interact with (let's call it "other") matter is fundamentally true, but it appears you're assuming that what we can detect today is the end-all of detection. The OP is literally asking, "what believable detection solution can I use?" You seem to be restating his problem statement rather than answering his question. $\endgroup$ – JBH May 7 at 17:11
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Mary particle.

Mary

Tachyons always move faster than light, and so also backwards in time. It is hard to imagine how to interact with something like that from our standpoint in the kitchen with a coffee. Sort of like interacting with God. God is so Godly.

But what about things like massless particles - our familiar friend the photon, and his weird cousins gluon and graviton? Those things always move at the speed of light. What does a tachyon look like from the perspective of a massless particle? From that perspective what is the tachyon up to? Mary is familiar and motherly, and less intimidating than God - the glorious intermediary. So too your particle.

In your fiction, you can discover that tachyons can be detected through their (time-backwards) interactions with massless particles. To keep it squarely in fiction you can invoke the little known graviton since it is so mysterious you can assert what you like. People might call you out if you ascribe new properties to the photon. But you could.

It hurts my head some to think about what interactions between a light speed (?timeless) particle and a superluminal time backwards particle would look like. Good luck!

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    $\begingroup$ I am immediately thinking of the fatline: Dan Simmons *the Fall of Hyperion" $\endgroup$ – nigel222 May 7 at 14:47
  • $\begingroup$ Tachyons only imply you could send information back in time if you assume they behave the same way in every inertial reference frame, which due to the relativity of simultaneity between different frames in relativity implies the possibility of something like a tachyonic antitelephone. Some physicists who speculate about tachyons suggest a way around this is to posit that tachyons have a preferred reference frame. $\endgroup$ – Hypnosifl May 12 at 15:57
  • $\begingroup$ @Hypnosifl It's amazing the hoops people will jump through to protect the postulates of SR rather than recognize that things like the Tachyonic Antitelephone and the paradoxes that arise from it are perhaps indicators that SR might be wrong about some things. Perhaps there is in fact a privileged reference frame. Maybe time isn't actually a dimension. $\endgroup$ – Corey May 15 at 0:00
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    $\begingroup$ @Corey - It would be hard to explain why so many known physical phenomena are observed to obey Lorentz symmetry if there is actually a preferred reference frame, and "time is a dimension" is more of a way of conceptualizing the math of relativity than an actual physics claim. A simpler way to avoid paradoxes like the tachyonic antitelephone is just to posit that "tachyons" (in the sense of particles that can transmit information faster than light) don't exist, especially since existing quantum field theories don't allow them. $\endgroup$ – Hypnosifl May 15 at 2:28
  • $\begingroup$ @Hypnosifl Except that if time is not a true dimension then a) Relativity of Simultaneity is wrong and b) the Tachyonic Antitelephone is impossible. Not because tachyons don't exist, but because the math is predicated on time being an actual dimension. Assuming that time is not actually dimensional then all apparent time travel effects must be invalid. Clearly the TA couldn't actually result in information travelling through time since the tachyons cannot move through time. The math is wrong... if time is not a dimension. $\endgroup$ – Corey May 16 at 0:26
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If tachyons indeed don't interact with the universe, then they look an awful lot like dark matter. That doesn't really interact with anything either except gravitationally.

So just imagine there's multiple types of dark matter, which includes tachyons. Anything that doesn't or barely interacts with the conventional universe will be some type or another of dark matter. Your tachyons could interact with some of this dark matter, allowing you to detect the passage of a tachyon by detecting changes in the dark matter.

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