# How would FTL travel based on tachyons most likely work?

So, I am trying to design an FTL system for my sci-fi setting. I find the concept of tachyons to be intriguing, and want to incorporate it into the system, but I'm not that familiar with how they work. So, I want to check myself so I don't get called out for bad science.

For my setting, I'm taking more of a Star Trek approach: it's soft sci fi space opera, but the technology in it is at least theoretically possible.

From what I understand, a tachyon is a particle that can only travel faster than light. So, interstellar travel in this fictional universe would be achieved by the ship's drive converting normal particles into tachyons.

However, I notice that in many other works of sci fi, the ship and its crew also get converted into tachyonic particles. Instead, what I am envisioning is more of the ship emitting a stream of tachyons that it rides along.

Edit: How would a tachyonic FTL system actually work?

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– L.Dutch
Commented May 7 at 16:40

Not even a little bit plausible. But neither is Star Trek's warp drive, or any other free-form FTL travel in any other setting, so there's not a problem. Just saying that there is faster than light travel, ignoring the consequences, and possibly adding a vaguely science-sounding but nonsensical justification, is completely fine.

Making the equations work out so that you can get $$\Delta v > c$$ isn't especially hard in mathematical physics. The problem is that...

• you need to have things that almost certainly do not exist - and then you need to have a way to create or turn yourself into those things!

• these invented, probably nonexistent things tend to blow up the laws of physics in ways that you don't want for your story

• the raw fact of being able to get from point A to point B before a c-propagating wave can do so, no matter how you get there, unravels causality, conservation laws, and logic itself.

If for some reason you want to have plausible FTL instead of just having FTL and not worrying about it, you can get around the third bullet point by having some sort of pre-built physical path along which anything, including your slower-than-light space ship, can travel, that happens to be much shorter than most other paths. This path must take at least as long to build as it would take a pulse of light to get from one end to the other without the path. (Which rules out Star Trek's warp drive, the wormhole drive from Event Horizon, and so on.)

The first two bullet points are more intractable, but at least they can be packed into a black box crafted by the Ancient Civilization That Went Before Us and left alone.

• FTL is absolutely supported by known physics. Quantum teleportation of information is already a thing and the Casimir Effect can theoretically achieve macroscopic FTL according to Alcubierre's equations. It's just the practical implementation of such technologies that are still lacking. Commented May 7 at 14:30
• @Nosajimiki Quantum teleportation of information only involves conveying information apparently faster than light, not actual mass-energy. Plus, the Casimir effect is virtually impossible to implement because the energy densities required by the Alcubierre drive require that the conducting plates generating the effect be picometers apart, at which point quantum effects take over and the effect stops working anyway. Commented May 7 at 14:31
• @Nosajimiki I disagree with controlgroup's reply: your criticism substantively addresses my answer, I just disagree with you. I was aware of both of those common beliefs before I wrote this answer. If you would like to search / ask about those claims on Physics SE I suspect you will get useful answers.
– g s
Commented May 7 at 15:19
• My only real objections to your answer are "Not even a little bit plausible." and "no matter how you get there, unravels causality, conservation laws, and logic itself". Causality and Conservation arguments are based on models that already have known exceptions, and the existence of those exceptions make it logically consistent. I'm not saying we have the tech today, but the physics exist to make the tech theoretically plausible. Commented May 7 at 16:11
• @JuimyTheHyena I believe so, yes.
– g s
Commented May 26 at 16:32

Nope. And here's a bit of why...

From any point in time and space there is a Schwarzschild double cone, extending into the past and future at the speed of light. Everything within the cone exists within time and space as we know it. Everything in the past cone can influence us and be seen by us. Everything in the future cone we can influence, and can see us. All the particles we know about within these two cones are called Tardions, which means they experience time as we do. Only photons in vacuum, having no rest mass can approach the cone surface. Anything else with a rest mass would take infinite energy to get there.

Tachyons are a theoretical construct. They would be particles on the other side of the cone surface. They would always travel faster than light. Our laws of cause and effect would not apply to them, and their laws would not apply to us. Nevertheless, we can make relativistic laws for particles like this. They would have a similar inability to approach the Schwarzschild double cone from the other side.

Physics advances by posing 'what if' questions like this. But, unlike asking 'What would happen if I could move at the speed of light?' this one currently gets us nowhere. Tachyons would have one length and three time axes. It is not clear how we could see them or they could see us without infinite energies. We don't see any FTL effects that might be explained by tachyons. We don't say 'There is no such thing as Tachyons' because we don't know, but we don't see any, and we don't really know how to look.

• Tardon? Did you just make that up? Commented May 7 at 12:49
• No. It is a spelling mistake for 'Tardion'. Apple spell checker is a tad assertive even when it isn't sure. Apparently 'Bradyion' is a preferred term on the net these days, but I had never heard of that. Commented May 7 at 12:56
• Tardon, then Tardion, then Bradyion? This is going from mildly curious to bad to worse. Stop making words up! Commented May 7 at 13:01
• @stackoverblown, check out the first paragraph: en.wikipedia.org/wiki/Massive_particle Somebody made up those words (just like how somebody made up "tachyon,") but it probably wasn't Richard Kirk. Commented May 9 at 2:02
• @SolomonSlow Only tachyon is in use. The others were made up by one or two guys so suddenly that qualify as words in common usage? I think I will add the word Rubbishion onto Wiki so that now it is a word too. Commented May 9 at 13:25

Since this is SCIENCE-FICTION question, lets play a bit fast and lose here:

Your ship travels at FTL speeds by basically abusing Quantum Entanglement: It finds a spot somewhere else in the universe that is perfectly entangled with it, and then "flips" on quantum level, turning that empty spot into a copy of a ship, and the ship into a copy of empty space. Not every spot is good for flipping. You can neither flip into totally empty intergalactic space, or into a gravity well safely, and some regions are just stubbornly chaotic and unflippable at all.

Yes, this unfortunately means that everyone onboard dies and is replaced by a perfectly identical copy somewhere else, but civilizations had accepted that and pretty much nobody complains.

Why tachyons though? Its because you need to scan the universe to find the right spot to "flip" to, calculate its coordinates, and maintain an information link to do the flipping in a safe and intelligent manner. For that your scanner needs to use tachyon radiation which travels significantly faster than light on its own to "scout out", and return with information, sort of like a tachyon radar/sonar.

Sure, you can "flip" without a tachyon scan, simply by approximating the right coordinates. Good luck. Your chances of erasing the ship out of existence are about 35%. You might also end up in a completely wrong part of the galaxy, inside a black hole, or "telefrag" into a planet or an asteroid. You are also very likely to accidentally entangle with another ship, telefrag into them and annihilate both. Long story short, flipping without an active tachyon scan is both extremely stupid and extremely illegal.

So ship astrogators with actual brains smartly decide to spend credits on clouds of tachyons stored in antigrav bottles, which then they load into the tachyon scanner. Enormously expensive, since such tachyons have to be captured in extremely large particle colliders down in gravity wells, and are wasted each time a ship flips, but then again, it is better to be a little bit shorter on cash than a little bit smeared across spacetime.

• This is a fantastic answer in my opinion. Another thought is that a tachyon beam could be used to actually entangle the particles of the ship with the particles at the destination: entanglement doesn’t happen on its own, so you use a tachyon beam to “synchronize” the spacetime sat the destination and origin. Commented May 6 at 12:40
• @controlgroup, to be honest, I came up with the entanglement-tachyon combo because it would a useful plot device, not because it makes scientific sense. This way, FTL is difficult and expensive but not extremely so, and OP can always use excuses why some places cannot be FTLed to, or put the ship in exciting danger. Commented May 7 at 9:23
• Then I suppose you’ve stumbled upon something that makes slightly more scientific sense than you originally intended. My compliments to you! Commented May 7 at 12:34

"Does any of this seem plausible from a theoretical standpoint?" No, it doesn't.

Oh, and tachyons do not actually exist, therefore you cannot be "familiar with how they work". They don't work, because they don't exist. (And the mathematics describing their putative existence would be rather hairy, anyway.)

But then, Star Trek: STD featured¹ interstellar travel by means of magic mushrooms and piteous enslaved tardigrades, therefore if you are aiming for such a level of verisimilitude your main problem is formulating sufficiently bamboozling if meaningless technobabble, and not in the least any compatibility with known physics.

¹) I say featured because I just couldn't bear any more and stopped watching at some point in early season 2. I understand that the series somehow managed to shuffle to a total of 5 seasons.

• I didn't make it to the end of season 1. I barely made it through the first episode, when a crewman on a starship that can travel at many times the speed of light said that there were no enemies about because he had scanned everything within a thousand km! Commented May 5 at 2:01
• I'd claim that the right amount of magic mushrooms will lift you any amount of distance to any imaginable place in just under an hour. Even when this place is outside our universe. Commented May 6 at 13:54

# Magic

Let us ignore all the initial problems with tachyons being theoretical and the like. There is a very simple problem to solve. You want the spaceship and it's occupants to 'ride along' and go FTL. My question would be how? A normal particle can only go at a maximum speed. Riding along tachyons would be no different than riding along with light. Sure you might be able to accelerate, but never to FTL.

The alcubierre drive might have a solution at face value. It needs to use tachyons within itself. Unfortunately it is merely a way to have everything in the right place, like a pipe moving gasoline. The pipe is needed, but it doesn't provide the actual thrust.

Tachyons can have weird properties we do not know about. But that is the problem. Going FTL with tachyons is tantamount to magic.

Especially because you've chosen a theoretical construct that is highly doubted can exist in current science, it is actually quite moot what you write about it. You ignore people saying tachyons can't exist already, and anything else is as fiction in a scientific paper as it is in your book.

My take? You run with it. You're banking on some suspension of disbelief already. What is a bit more for FTL?

• You can't exceed the speed of light via light-pressure drives for the same reason you can't sail with the wind and expect to go faster than the wind. As you approach the velocity of your propelling particles the amount of push they can provide drops to zero. If you were somehow being pushed by FTL particles then something different could definitely happen. The main problem would seem to be that going that fast messes with your position in time as well as in space... Exactly how we're not sure. We've never done it. At least, nobody remembers ever having done it... Commented May 7 at 15:55

Tachyons are imaginary, but that doesn't mean they don't exist. The way they're theorized, they're like quarks, but with a different set of quantum statistics. The creation of the Bose-Einstein condensate demonstrated that we can change the behavior of matter by combining and otherwise altering those statistics.

Let's say you can flip one of those statistics, converting an entire spaceship into tachyons. One of the theorized qualities of tachyons is that, just like baryonic matter, the more energy you put into them, the closer they get to the speed of light.

So, let's say you convert a particle at rest to a tachyon. Since it had no motion, it would translate to a tachyon with infinite speed. Think "infinite improbability drive", where it passes through every part of the universe at once. You can bet that you'd never see them again.

baryon energy = velocity/C
tachyon energy = C/velocity

So an object moving at half the speed of light would translate to something moving twice the speed of light.

So, here's your challenge: getting your ship to wind up where you want it to. Our fastest probe goes 450,000 mph. Convert that to tachyons, and it would be moving 1450x the speed of light. At that speed, you'd have to time your transition within about 2 seconds to hit a target the size of the orbit of Mars.

The unavoidable problem is that tachyons and baryons can't interact. For all we know, tachyons may very well be a form of dark matter.

You couldn't emit tachyons as a thrusting force unless the conversion of baryonic matter to tachyonic matter was an energy-generating event. You couldn't emit tachyons forward and ride on its gravitational wake unless you were throwing more mass into tachyon-ness than you currently had.

Even if either of those strategies were to provide significant acceleration, you still have the issue of baryonic matter having that whole "speed of light" limitation.

• Question: half the speed of light relative to what? How do you measure what speed you are going compared to the universe? Otherwise a +1 from me Commented May 5 at 10:11
• @demigan, The speed of light is an absolute -- everything else is relative to it. When measuring relativistic speeds, it's always measured as a percentage of C. I don't understand your question. Commented May 5 at 17:37
• c is a constant, not an absolute velocity. Light has the same speed in all frames, this is not the case for things moving at fractions of c. What's moving at 0.5c in one frame can be anywhere from 0 to 0.999... to any arbitrary number of 9's of c in another frame. Commented May 6 at 1:43
• Ok, how do you determine how fast you are going relative to the speed of light? Because no matter how fast you go, the light will overtake you with the speed of light. So you “standing still” or “going 99% of the speed of light”, that light would still careen past you at the speed of light. Commented May 6 at 10:49
• @RobertRapplean I tend to agree that people get overly attached to the math as though the math is the universe we're trying to describe. Relativity gave us math that worked without reference to a stationary frame, so therefore most physicists concluded that such a frame does not exist. That's something of a leap really. Commented May 21 at 20:04

The basic problem here is that you are asking about something that is forbidden by physics and then asking how would physics deal with it.

In science nothing is absolutely written in stone and any good scientist should accept that all scientific theories are open to revision in the light of new evidence, however unlikely that might be.

But in the highly unlikely event that relativity is wrong and that faster than light travel is actually possible, there is then no point is poking about in the blackened shards and ashes of modern physics asking how it works. Because modern physics would be wrong and would have to be rewritten from the ground up.

So you are on your own and will have to invent a new physics or more likely come up with some vague handwaving that is enough to suspend disbelief but doesn't really provide much of a deeper explanation.

If you have tachyons there is one obvious way this is possible: Alcubierre drive.

The current theoretical problem with the Alcubierre drive is that to keep contracting space in front and expand it behind, you need particles that get there to do that in time. You can’t travel faster than the particles that contract the space in front of you, and those particles still go slower than light.

But if these tachyons go faster than light, they could be used to contract space in front while expanding it behind fast enough to do FTL.

• The other, more pressing theoretical problems with the A-drive also include the necessity for an enormous amount of mass-energy (700 kgs worth in an extremely tight and well-defined space) and also negative energy in some cases (not known to exist, might be satisfied by tachyons). Commented May 6 at 12:38

If you want a sort-of-coherent description of tachyon behavior, consider the following: a tachyon is a specially-engineered microscale black hole with an Alcubierre-like warp field built into it.

You can produce these MBH tachyons, better known as geons - gravitational wave packets that are held together by their own mass-energy - by using a quantum resonant flux capacitor to generate a small, concentrated gravitational field of high enough density that it holds itself together (already fairly hard). Then, you tweak the grav field around the geon to give it a small warp drive of its own creation; as it accelerates through space, it gains energy, thus empowering the warp drive further and accelerating it faster. It would have exponential speed as it rockets across space, eventually exceeding light speed (which it can do because the warp drive only affects its coordinate velocity, not its local light speed-limited velocity).

To achieve FTL, simply flood the region of space around your ship with geons traveling in the right direction, and their collective warp fields add up, carrying you with them.

Frame Challenge

I suggest that you look at some writers who have used FTL in their stories. I'd start with Ursula LeGuin, Arthur C Clark, Isaac Asimov, even E. E Smith. They all use magic or hand waving. They also make their stories sufficiently gripping that the reader is too interested in what is going to happen, so they don't stop and ask the question "how is this supposed to work?". Learn how to do that, and don't worry about a scientific justification. Also read the Silmarillion: Tolkien starts with a Flat Earth, and bends it into a round one at the end of the 2nd Age. How is that supposed to work? If your readers start to ask that question, you have already lost.

We don't know whether tachyons exist at all. Tachyons are (as far as my limited understanding of physics goes) a hypothetical particle.

Here's how that works: Einstein says particles (with a rest mass, so this doesn't go for photons and neutrinos) traveling the speed of light have infinite mass and therefore infinite energy. That means it's impossible for matter to be accelerated to the speed of light. Only to close to the speed of light, and even that takes a ridiculous amount of energy.

However, according to Einstein's equations, particles that go faster than the speed of light have finite mass again. We have no idea whether such particles exist (probably not), but his equations say they'd have finite mass and energy. However, they can never slow down to below the speed of light, just like we can never accelerate past it.

So "turning particles into tachyons" doesn't mean anything. You're just saying you've got a magical way to make them suddenly go faster than the speed of light without accelerating through the speed of light.

But even if the equations allow for this without violating preservation of energy by magically skipping past the infinite energy zone, you are blatantly violating preservation of momentum.

If you want FTL, you'd better forget about hard physics, or resort to wormholes or fancier ways to warp spacetime.

## What is a Tachyon anyway?

To an outside observer, no object can move faster than the speed of light... but the important part here is "to an outside observer". Tachyon theory states that particles move faster than light all the time, but spacetime prohibits us from perceiving this. Any time a photon is going down a gravity well, it moves at a speed faster than light, but that same gravity slows down any return signal in equal measure. So, even if a particle were to fall into a black hole at superluminal speeds, as someone standing outside of the black hole, we could never observe this action.

The problem with tachyons is not getting things moving faster than light, it's that whole slowdown effect you get trying to then put that particle back into normal space. If you had a miniature black hole, then you could accelerate a photon to superluminal speeds, but then it would just slow back down when it tries to climb back up the other side giving you a zero sum benefit.

The only conceivable way to make a tachyon would be if you could bend spacetime at will. So, if you were to fire a photon down an artificial gravity well, and then remove the gravity well out from under the particle, then it might conserve its momentum, leaving the gravity well as a superluminal particle.

Normally you can't impart the energy to exceed 1c because no two wave or partials can achieve a relive motion in excess of 1c when cohabitating the same area of spacetime, but in this case, the increased speed is not coming from a mass-energy conversion, but from spacetime itself.

## Why this is not a practical propulsion system

If you have the technology to make an unbound tachyon, then you don't need a reaction mass to achieve superluminal speeds. It would take way less energy to just use your ability to manipulate spacetime to create an Alcubierre style warp bubble around your ship so that you can ride the self made spacetime gradient itself.

There is a 2nd universe, parallel, folded away into the bubble-centers of https://en.wikipedia.org/wiki/Galaxy_filament, a anti-verse, where the speed of light is different and does not apply. Space curvature pushes it away, so you can not enter that universe, from curved gravitational influenced space. You can have that far away from galaxies - and in small Lagrange points of really massive object constellations. Between two singularities, there is a ever bopping (https://en.wikipedia.org/wiki/Gravitational_wave_background) around needle-head of space that can be a door.

Once your magic machinery folds space around you, you slip through the door- to be "reconstructed" from anti-verse matter. Which behaves very differently - allowing no longer for biology to function. You are machine while in anti-verse, traveling, searching a fast, cyclically expanding and contracting foam-bath for the target constellation. And in that target constellation for a little pinhole.

You are violating causality, but at the same time, your ship lives through the same anti-verse again and again, as if it had never been there. Finally it slips through the door, the machinery reconstructs your bodies and the ship and heals the tear in the universe, the singularity ripped, that is in the heart of your ship. And of course - in the anti-verse, your copy has to die. Less it would repeat its spawn attempt, creating copies of you. There have been whole wars between civilizations of runaway copies traveling tachyonic.

There are bubbles of space times whos la grange points are mined in the anti-verse and vice versa.