# Science-based FTL drive

So I'm writing a story about a crew of scientists that explore the universe searching for a hidden knowledge in embed in the fabric of the space itself. More like to the Destiny's crew from Stargate Universe. Like their ship, I want mine to have some sort of FTL capability but not through hyperspace.

It's very important that the ship does not go out of the space, like when you use a hyperspace drive, because the crew need to be constantly gathering data from the ship equipment, and do not warp the space around it like the Alcubierre Drive, because the will render the data useless, since space was warped.

So, there is some way to achieve FTL travel like the one I'm proposing and being scientific correct? And yes, if possible I want to not suffer from time dilation due the speed.

• You're asking about a scientifically-correct FTL drive. If we knew how to do that with science, we'd've done it already. – iAdjunct Jul 8 '16 at 17:34
• @iAdjunct I don't think that's technically true. He's not asking for blueprints, he's asking for a concept. Most great works of engineering have approximate concepts written up on them before we figure out the practical details. Take the space elevator, for example, or the atomic bomb before the Manhattan Project finished. – Ranger Jul 8 '16 at 17:45
• Note that if you can simply approach the speed of light, then time will slow for the crew, so from the crews' standpoint, they are effectively going the speed of light or even faster than the speed of light. The downside, of course, is when they get home, many many years will have passed. – Jiminion Jul 8 '16 at 18:29
• I would just drop the "scientifically correct" concept and go for the more classic Science Fiction concept of "well, science hasn't proven we CAN'T do it". Scientists discover "ghost matter" which exists and can be observed but seems to have no mass. They figure out how to shift regular matter into ghost matter via NEW FOLGERS CRYSTALS, which allows ships to travel literally any speed while observing and not crashing into the regular matter universe. Slap a little scientific jargon around it and there you go. This is basically how all sci-fi authors do it. – JamieB Jul 8 '16 at 19:20
• "do not warp the space around it like the Alcubierre Drive, because the will render the data useless, since space was warped" Why would it render the data useless, as opposed to just requiring observers inside the bubble to compensate for the warping effects of the bubble when looking at their observations? We can compensate for all sorts of optical distortions with computers, I don't think there would be any great difficulty in creating a program that would allow an observer in the bubble to reconstruct the image they would be seeing if the bubble itself weren't there. – Hypnosifl Jul 8 '16 at 19:40

TL;DR: "Scientifically correct" (according to current established science) and "faster-than-light travel" cannot be used in the same context without some form of negation. What you are asking for is not possible within the boundaries of science as we know it.

Here's why:

Our best model for this type of effects, insofar as I know, is special and general relativity. Special relativity postulates that colinear velocities are added according to the formula $$s = \cfrac{v+u}{1 + \cfrac{vu}{c^2}}$$ for an initial velocity $v$ and a total acceleration $u$ (over some period of time) yielding a final velocity $s$.

For small values of $v$ and $u$, this behaves like we are used to, because for such values, the fraction $\frac{vu}{c^2}$ is very small, so the term $1 + \frac{vu}{c^2}$ is very close to 1 giving $s \approx v+u$. Of course, in some situations, even with everyday velocities this approximation might not be good enough.

However, look what happens if we set $v = 0.90c$ and $u = 0.10c$ (meaning that in an intertial reference frame, our initial velocity is 0.90 times the speed of light, and we increase our velocity by 0.10 times the speed of light). Intuitively, the velocity would come out as $(0.90 + 0.10)c = c$, but it turns out that this is not the case at all. Rather, using units of $c$ for simplicity:

$$s = \cfrac{0.90 + 0.10}{1 + \cfrac{0.90 \times 0.10}{1^2}} \approx 0.9174$$

See what happens? In an inertial reference frame, our velocity only rose from $0.900c$ to about $0.917c$, an increase of 1.9%, even though we tried to raise the velocity by 11% ($0.10c$ out of $0.90c$).

This effect becomes even more pronounced as your initial velocity approaches $c$ ($v \to c$). For example, look what happens if we are moving at $0.99c$ and increase our velocity by $0.10c$ (yes, I really mean that): $$s = \cfrac{0.99 + 0.10}{1 + \cfrac{0.99 \times 0.10}{1^2}} \approx 0.9918$$ for a 0.18% increase for the same effort that got us 1.9% starting at 90% of $c$.

And of course, in the real world, these are both absurdly high values for $u$, reminiscent of instantaneous acceleration. Instead, we should be working with $u \to 0$ (because in the real world, the time over which we measure acceleration goes to 0), but since that's difficult to show in a single equation, I'll settle for $u = 10^{-12}c \approx 0.3~\text{mm/s}$ which isn't a totally unrealistic change of velocity over a short period of time given something resembling a real-world device trying to propel itself. Now look what we get if we start out at $0.90c$:

$$s = \cfrac{0.90 + 10^{-12}}{1 + \cfrac{0.90 \times 10^{-12}}{1^2}} = 0.900~000~000~000~189~999...$$

Our velocity increase, which we tried to make $\frac{10^{-12}}{0.90} \approx 1 \times 10^{-12}$, became $\frac{0.90000000000019 - 0.90}{0.90} \approx 2 \times 10^{-13}$. We only got 1/5 of the increase that we spent the effort for, and at 90% of $c$, we are still a good long ways away from $c$. It only gets worse from there.

Eventually, this means that the energy cost of increasing your velocity grows in an exponential fashion. If you work the math all the way, effectively solving $$\lim \limits_{v \to c, u \to 0} \cfrac{v+u}{1 + \cfrac{vu}{c^2}}$$ you end up with an energy requirement that grows toward infinity as you get closer and closer to the speed of light. Because instantaneous velocity changes are not possible (because of inertia, for one thing), you can't simply "jump past" the difficult part of the acceleration curve.

Because your spacecraft will, at every instant, have an instantaneous velocity (along some vector) and an instantaneous acceleration ($\vec{a} = \vec{\Delta v} / \Delta t$ for some $\Delta t \to 0$), you will thus only ever be able to (with humungous energy expenditure) approach the speed of light, but you will never be able to reach the speed of light. As your velocity increases, the marginal utility of any given acceleration (within the local frame of reference) decreases; you get less and less (inertial reference frame) acceleration out of any given amount of effort. Because inertial reference frames are what we are generally concerned with when going places, this means you work exponentially harder but get exponentially less utility for your efforts.

If you want a single formula that explains why faster-than-light travel is impossible in the real world as we currently understand it, the mass-energy equivalence $E=mc^2$ (as suggested by AndreiROM) isn't what you are looking for (in fact, it might even to a limited extent be your friend, if you can figure out how to do the mass-energy conversion); rather, the one you want is the relativistic colinear velocity addition formula and an understanding of how it behaves as $v \to c$.

• I've already marks you as my response, but can you explain to me what will happens if my ship would not have that moment of acceleration. I mean, $t_1 \to v=0$ $t_2 \to v=c$ ? – Lucas Leite Jul 8 '16 at 22:23
• @LucasLeite Unless you are redefining physics anyway, instantaneous velocity changes are not possible, due to inertia. ($a=\frac{\Delta v}{\Delta t}$ where $\Delta t \rightarrow 0,~\Delta t > 0$.) And if you are redefining physics at that level, then it's no longer science-based anything, but rather magic, and you can do whatever you want. Also, as I showed in the answer, you cannot reach $v=c$ (at least not with anything that has a mass) -- the energy expenditure grows to infinity if you try. If you have truly giganormous amounts of energy, you can get $v \rightarrow c$, but never $v=c$. – a CVn Jul 8 '16 at 22:29
• @JDługosz well, what would be an answer to the spirit of a question that asks how to do something which cannot be done? That seems to be the situation here. – David Z Jul 9 '16 at 7:11
• @JDługosz OP is asking for "some way to achieve FTL travel" which is "scientifically correct". I think my answer quite well shows how the two goals are incompatible given our current understanding of our universe. Sometimes, what one wants is something that simply cannot be done within the constraints established in the question, and in such a case, assuming that the explanation itself stays within the criteria established in the question, "that's not possible and here's why" is a perfectly correct and valid answer to the question as asked. – a CVn Jul 9 '16 at 8:53
• If the OP is happy with something handwavy that allows FTL travel without time dilation or breaking causality, and it isn't a central part of the story being told, then that's where story-teller fiat comes into play; no need to ask us. Note that realistically speaking, not even Star Trek-style warp drive (which very specifically avoids physically moving the craft at local FTL velocities) solves the causality issues; they just ignore them for the purpose of telling a story. – a CVn Jul 9 '16 at 8:53

There are many classic ideas on how an FTL drive might work. As other answers have noted, currently there are no actual science-based FTL drive ideas available; FTL travel appears to be impossible.

One of the classic ideas is quantum jumps, where the whole ship teleports a short distance instantaneously, and then repeats the jump at a high rate. For whatever reason, the teleportation can't work at a great distance, but a ship can send itself a short distance. The ship can jump from being motionless in one place to being motionless in another, nearby place; this avoids the huge energy cost of accelerating up to a significant fraction of the speed of light. The seeming motion can be called "pseudovelocity". (I know I've read stories featuring an FTL drive that works this way but I can't think of the names of the stories.)

During the time that the ship isn't jumping, it's at rest in the normal universe, and cameras or other instruments could take measurements. But the data might resemble a time-lapse series.

How does the teleportation work? Handwave handwave quantum physics something something. But it's a classic old tradition: the FTL will work just how it needs to work for your story.

P.S. It may also be possible to use a space warping drive, and then explain that the data is computer-processed to remove the effect of the warping. But I think the quantum jumping would work well for your story.

• Congrats on being the first one to answer the OP as asked, with a (fictional trope) that meets his requirements. take note people the earlier answers are bad, wrong, irrelevant, or just plain not answering. Spend your upvotes here and why is anyone upvoting the others? – JDługosz Jul 9 '16 at 2:59
• Welcome to Worldbuilding, sheveha. I have high hopes we'll be seeing more of you. – JDługosz Jul 9 '16 at 3:01
• @Steveha The quantum micro-jumping pseudovelocity FTL drive appears in the novels of Poul Anderson. Sufficiently scientific for a fictionally plausible FTL drive. I posted a comment to another answer with a few more details. Anderson was good at using enough science to make his SF plausible. – a4android Jul 9 '16 at 8:33
• @JDlugosz This is the first answer to the OP's question. While it's nice to see some special relativity purity in action in other answers, it's not quite helpful. I upvoted this answer and hope others will too. – a4android Jul 9 '16 at 8:37
• Anderson was also one of the first to give Special Relativity a solid place as a plot element, in Tau Zero. – JDługosz Jul 9 '16 at 9:01

I am aware of only two scientifically postulated methods of FTL drive. The first has been mentioned on this board quite a bit and I thought was originally posed as an answer here, the Alcubierre Drive. The second is Quantum Tunneling.

## Alcubierre Drive

...a spacecraft could achieve apparent faster-than-light travel if a configurable energy-density field lower than that of vacuum (that is, negative mass) could be created.

Rather than exceeding the speed of light within a local reference frame, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel. Objects cannot accelerate to the speed of light within normal spacetime; instead, the Alcubierre drive shifts space around an object so that the object would arrive at its destination faster than light would in normal space.

Essentially, the object does NOT move faster than the speed of light locally. In its frame of reference it is moving slower than light. The Alcubierre Drive compresses space in front of the ship and stretches space behind the ship - essentially moving space.

2-D representation of Alcubierre space stretching:

## Quantum Tunneling

Any object can perform quantum tunneling, but the math of the problem shows that even for very light mass objects, the event is unlikely. That we see tunneling is because of the large number of objects observed.

As the mass of the object or width of the potential barrier increases, the likelihood drops. When you get to single macroscale objects performing tunneling events across interstellar distances, the average amount of time for that event to happen is a number with many, many zeroes.

Quantum tunnelling or tunneling (see spelling differences) refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the Sun...

and

Tunnelling is often explained using the Heisenberg uncertainty principle and the wave–particle duality of matter. Pure quantum mechanical concepts are central to the phenomenon, so quantum tunnelling is one of the novel implications of quantum mechanics.

and

It is possible for spin zero particles to travel faster than the speed of light when tunnelling.3 This apparently violates the principle of causality, since there will be a frame of reference in which it arrives before it has left. However, careful analysis of the transmission of the wave packet shows that there is actually no violation of relativity theory.

Now consider the barrier as the gravity hill sloping upwards away from your star. If used as an SF drive, the Quantum Tunnel event would cause your ship to disappear from this gravity well and spontaneously appear in another deeper gravity well somewhere else.

Quantum Tunneling of an Electron:

During Quantum Tunneling a particle appears on the other side of the potential barrier instantly (e.g. faster than light). For most particles, a delay at the barrier makes the total velocity of the even slower than light, even though the particle will appear spontaneously on the other side of the barrier. However, for spin zero particles calculations show that the entire event happens FTL.

There are three big drawbacks to using this in SF though.

### It's improbable

The first is that the probability of the event is inversely related to the mass of the object. For particles of the mass of a proton it becomes extremely unlikely to happen. To observe this happening to a specific macroscopic object (say a person or a space ship) you'd have to watch it for an extremely long time (a number with a huge number of zeroes in it).

### Only spin zero particles go FTL

The other problem is the key words "spin zero particle" and not particles like electrons and protons. So it is scientifically conceivable that this could be used to transmit some things at FTL speeds.

### We can't make it happen

Like other things quantum mechanical, there's simply a probability that the event happens. The only way we know how to manipulate the event is by changing the particulars of the particle or the barrier. We can't otherwise influence the likelihood of it happening.

The reason that we can see this happening in a sample set is because there are so many particles and to us we don't care which one it happens to. The other is the very light mass of these particles.

### Other Stuff

I've often toyed with the idea of inventing an FTL drive using similar principals but adjusting the particulars to meet the needs of my SF Universe. I haven't developed a consistent set of rules for it yet though.

If you wish to use this as the basis of your drive, you'll want to consider the answers to these questions and what they mean to your Universe, even if you don't explain the particulars to your audience:

• How do we account for the very large mass of the object we want tunneled?
• How do we account for causing the event to happen when we want it to?
• How do we account for causing a specific object to tunnel rather than just any old object?

... Actually, Nimtz and coworkers observed that the measured tunneling time is spent at the barrier front, whereas inside the barrier zero time is spent. This result was observed in several tunneling barriers and in various fields. Zero time tunneling was already calculated by several theoreticians

• About the "spin zero" part: 1) Quantum tunneling is not restricted to spin zero particles, in fact your figure depicts an electron, with spin 1/2. 2) Photons have spin 1, not spin 0. 3) The only spin zero particle that we know, the Higgs boson, is not massless. And 4) Quantum tunneling should be described using QFT, which incorporates SR, and therefore doesn't allow FTL communication. – Bosoneando Jul 9 '16 at 22:16
• I never stated that Quantum Tunneling was restricted to spin zero, I stated that the FTL aspects were restricted to spin zero particles. The graphic was presented as a convenient visual depiction of tunneling. Another reference for zero-time tunneling is provided in the answer. – Jim2B Jul 10 '16 at 3:16
• Quantum tunneling cannot allow for transmission of any information-carrying signals faster than light in the current understanding of quantum field theory, see Eberhard and Ross. – Hypnosifl Jul 10 '16 at 18:35
• I think it is not correct to say that tunneling happens "instantly" and ftl. Any measurement of the particle being in some position is instant. The evolution of the wave function takes time to cross the barrier. In some sense the tunneling is not even "instant" because the superposition of positions evolves from being all on one side to all on the other side over a period of time. Instant is an artifact of trying to describe quantum processes in English. IAC, it is by no means a way to communicate faster than light. – JDługosz Jul 18 '16 at 6:34
• Looking at your link, it seems that only Günter Nimtz believes that quantum tunneling is faster than light. – JDługosz Jul 18 '16 at 6:49

I think I got something workable. But you have to invent a type of exotic matter that I'll call "negative mass" for now, not to be confused with antimatter.

In our universe you have to overcome the following problems,

1. the faster you go, the more mass you have, meaning you need even more energy to move. That means you keep pumping in energy and never quite get to c. And
2. speed necessarily means slower time for you in relation to someone not moving.

That's the way mass energy equivalence and special relativity work. So you'd have to change something about those. To counter 1) maybe you have some way of creating "negative mass" the faster you go, that way you don't get more and more massive. For bonus points you could put another theoretical speed limit, maybe you asymptotically approach instantaneous arrival in your universe, some kind of "infinite acceleration." You wouldn't have to necessarily explain why but it would serve to eliminate the going-faster-than-light = going-back-in-time problem.

To counter the time dilation effects you really need to alter something pretty fundamental to the way we understand acceleration and relative velocities, just understanding special relativity, as it currently is, is really hard. Again, I would just balance it out and say that the "negative mass" simultaneously creates a kind of time acceleration, increasing your progress through time. Or you could even have a whole other time engine just to accelerate your progress through time if you want and leave the negative mass out of it.

Thinking about the universe like this makes my head hurt because I don't know what ramifications this type of meddling would have. For example, if you accelerated up to c with your negative mass generator on, what would the lorentz transformations make you look like? If c isn't a speed limit anymore, maybe you would look normal. Another example would be what would the bouncing photon look like on a train going at c? It's still traveling farther for a stationary person. The problem is people going different speeds don't agree on what they see. Fundamentally I don't know how to imagine a universe where c isn't a speed limit but where "infinite acceleration" might be.

I'm sure real physicists are furrowing their eyebrows and shaking their heads at me now.

• Tachyons, a hypothetical FTL Particle, would necessarily have either imaginary mass or imaginary energy based upon current understandings of relativity. Possibly similar? – MozerShmozer Jul 8 '16 at 18:56
• Maybe, but I really don't know enough about the math to understand what mass as an imaginary number would mean. – fet Jul 8 '16 at 19:39
• Honestly, no one does. – MozerShmozer Jul 8 '16 at 19:47
• No, no, no,… negative mass has nothing to do with anything here. The Minkowski spacetime is space itself and different kinds of things in space don't change that. – JDługosz Jul 9 '16 at 2:56
• Where to begin? For tachyons (as noted by @MozerShmozer) see this Answer for more depth. Coincedently, negative mass was discussed less than a fortnight ago in this post. – JDługosz Jul 9 '16 at 6:48

First of all all movement suffers from time dilation what so ever and the amount is mathematically defined, so by hard physics (as we understand it) you'll always get this if you're physically moving from point A to point B. Even as citizens of Earth we see this dilation compared to a completely stationary particle (not rotating, orbiting, or expanding with the universe) but the difference is fairly small.

Also as we understand velocity currently you can't actually travel faster than light, so if you want to stick to known science and have to go faster than light can we're kind of forced to abandon normal means of getting from point A to point B.

However we are already making making minor steps towards actual teleportation which negates the actual travel from point A to point B, and the teleported material would never enter 'hyperspace' or anything like it. I think it's the only thing that will work for your idea.

Alternately you could use the classic idea of wormholes to bridge two spots in space, but it should be noted that we have no experimental evidence for them whatsoever. Basically they're fiction, not science, like magic or the Force. But they sound sciency.

• “actual teleportation” does not exceed the speed of light. – JDługosz Jul 9 '16 at 2:52
• @JDługosz Actually, interestingly enough, it does, as it's transferring information, not physical matter, but the information appears as a photon on the other end. There's no actual movement, no actual velocity, so the speed of light need not apply. – Ranger Jul 9 '16 at 4:10
• That is completely wrong. The quantum state (entanglement, spin, energy, and superpositions of all that with no classical measurment that corresponds to the complete state) cannot be moved to the destination photon at faster than light. Think speed of causality: information is what matters so "actual velocity" is not what is important. – JDługosz Jul 9 '16 at 6:36
• You might be thinking of how measurement of entangled particles are corelated, and sometimes popularly described as instantanious but that's really not right; it's weider than that actually. But that is only one step of what it takes to teleport. You also must move the prepared target through space (which can be done in advance) and convey some normal information in the classic way. The receiver can't do its quantum step until the measurements from the sender are received. – JDługosz Jul 9 '16 at 6:41
• JDługosz is correct, it's been proven (by Eberhard and Ross) that without modifying the basic equations of quantum field theory it should be impossible to use quantum entanglement to transmit any signals faster than light. – Hypnosifl Jul 10 '16 at 18:34

There are several reasonably science-based forms of FTL travel in SF and usually written by SF writers with a science background. Here are some of the better examples. If I've missed anyone who should be on the list, please let me know.

Poul Anderson's quantum mechanical micro-jumping pseudovelocity drive. See his Flandry of Terra series, the Polesotechnic League series, and the Psychotechnic League series of novels. For more information about Poul Anderson's science fiction go here

Gregory Benford's tachyon jump. It assumes bradyons (slower-than-light particles) are virtual tachyons. The jump works by converting the vessel's bradyon matter into tachyons which move an infinite velocity from point A to point B where it reverts to its former bradyon matter state. Reaction mass is expelled during a jump, presumably to shift the vehicle's position. See The Stars in Shroud (1977). Note: this is also the revised edition of Deeper Than The Darkness (1970).

Benford, who as a working physicist was a co-author of antitelephone paper that came down against tachyons, no longer uses tachyons for FTL travel in his SF. For example, in Foundation's Fear: The Second Foundation Trilogy (1997) wormholes are the mechanism for FTL travel around the Asimovian Galactic Empire.

Stanley Schmidt's Rao-Chang tachyon drive. FTL ships 'shift' from the bradyon state to their equivalent tachyon state. Schmidt published an article "How To Move the Earth" in Analog (May, 1976) which has an explanation for this fictional tachyon drive. See The Sins of the Fathers (1976) and Lifeboat Earth (1978).

Robert L Forward uses wormholes for FTL travel in Timemaster (1992). Somewhat eccentrically the wormholes are created by friendly space-dwelling organisms which lay them, more or less, like eggs.

Stephen Baxter has wormholes supported by exotic matter structures for FTL travel in his Xeelee Sequence novels. See here for more details.

These are nothing more than science-based forms of FTL travel. None of the authors necessarily believe this is how FTL travel will be accomplished, if it can ever be accomplished, the role of the science is to make their versions of FTL travel appear to be scientifically plausible. This is what the OP wanted for his story. Here are some examples.

• He specifically wants something that lets them watch the view alomg the route. – JDługosz Jul 9 '16 at 9:22

Your main requirement is: the crew need to be constantly gathering data from the ship sensors. So you don't want to just jump to the destination or move in a different realm.

As explained earlier by steveha, you can use some short-range jumping tecnique to give a “pseudovelocity”. I recall a series of connected short stories by F. M. Busby expanded into a novel where this was explained as arising suppressing the normal rate of "appearances".

"The trick is that space and time are quantized. If you don't know what that means, wait and ask me later. Mainly, the universe doesn't exist continuously. It pulsates—appears and disappears at a rate much too high to measure. So when you move you do it by vanishing at one point and reappearing at the next—normally.

"Ordinarily, in moving we hit every point along the way. Skip Drive suppresses our appearances at most of those points. We beat lightspeed because it's the ins and outs that use up time and energy, not the motion itself."

An interesting side effect they later figured out is that this also allows drift between timelines! The travelers may arrive to a world with different history than when they left.

The “make small jumps” idea can be applied to adapt any FTL mechanism to your need to have a pseudo-path in normal space and see out. For example, use wormholes: but only a billion miles at a time. You find a wormhole in the quantum foam that goes to the desired destination, but as with electron probability clouds and such, the probability of finding one decreases with distance. So they find one with an endpoint about a billion miles away in the right direction, amplify it to ship size, open the ends and traverse. You can find any number of reasons why the jump distance is limited.

Space warp in the manner of Alcubierre Drive (but without the insane energy requirements)? The very point that the ship can't see out at all makes it difficult to navigate. So it makes a series of small jumps and takes pictues at each jump, making it seem like normal motion to those watching the screens.

But this is a survey ship or something, with the purpose of gathering readings along a survey route, right? So even if the technology were not limited, the ship would be made that way. A passenger liner goes into hyperspace, flys around in this other realm for a few days having nothing to do with out universe and what's to be found in the space between here and there, and returns to normal space there. But the survey ship wants specifically wants to see what lies along the route in normal space, so it goes into hyperspace for a minute, drops out, and records what it sees, then repeats.

Of course, any means of FTL can become time travel, so you need to be careful in explaining it or designing it so as not to be a general purpose time machine as well.

But, you can turn the relationship around. Consider making a normal journey at relativistic speeds, taking (say) 5 years to cover the distance in the reference frame of the destination planet. Then, use a time machine to go back 5 years! Now the ship arrived instantly, right?

But instead of jumping back in time when you get there, do it incrementally. The ship will appear in a short length of voyage from A to B during time t₁ through t₂ (say, one day), then jump back in time a day, and be seen in the outside world to travel from B to C, also, in the time interval from t₁+ε to t₂+ε, and so on, all along the path of its journey. Potentially you could make each leg as short as the length of the ship.

• Many of the novels and short stories of the science fiction writer Poul Anderson featured micro-jumping FTL travel. "I assumed-- FTL is possible. - not by simple aceleration -- a device for handling discontinuous psi functions permits a spaceship to assume a pseudo-velocity (not a true speed in the mechanical sense) limited only by the frequency of the engine's oscillators." Edited quote from Author's Notes , "Virgin Planet' (1959). A quantum mechanics based FTL drive with a lot of licence. Anderson had a BS in physics and knew how to deftly add science to his SF. – a4android Jul 9 '16 at 8:12
• "Of course, any means of FTL can become time travel" Assume there exists a mechanism that prevents closed timelike curves & other causality violation like chronology protection conjecture and it doesn't any more. While "FTL can become time travel" not every FTL trip is time travel and if those that are are forbidden, then only future-directed FTL would be possible. Stross' Eschaton novels has the transhuman future intervening to stop FTL time travel to protect its own future existence. Perhaps someone should show Tolman's paradox can be refuted on WB SE. – a4android Jul 9 '16 at 8:21
• I think making the universe safe for ftl is another question. But it might be too broad/opinion flagged if just asked like that. It's also an idea I've thought about as an essay. Singularity Sky certainly did it in a most explicit way! – JDługosz Jul 9 '16 at 8:28
• I understand the problem. Asking for science-based FTL implicitly assumes that objections like causality violation or how to cross the lightspeed barrier must be either held in check or fixed somehow instead of plain ignored as not relevant. I am also planning an essay on this topic. Stross is a rare example of a writer who took the causality issue head on. – a4android Jul 10 '16 at 9:47

Travel is defined as the movement of an object from Point A to Point B via the space inbetween. Travelling faster than light speed is not possible, the only way an object can appear to move faster than light is to warp space-time, typically with a wormhole, though again this is not travelling, since you do not occupy the space inbetween.

One of my old favorites, although I'd doubt its scientific accuracy is the Jump Drive from C.J. Cherryh's fiction work.

https://en.wikipedia.org/wiki/Jump_(Alliance%E2%80%93Union_universe)

In the Alliance-Union universe faster-than-light (FTL) ships have two major drive systems, slower-than-light (STL) thrusters and FTL jump engines. The jump engines comprise vanes that are attached to the outside of the ship. When the vanes are pulsed, they generate gravity waves which create a field, or "bubble", around the ship that pulls it (and anything else in the field) along the interface between realspace (Einsteinian space) and hyperspace (jumpspace).

Jump takes place between two massive objects, called jump-points, which are generally stars, brown dwarves, or "rogue planets" sufficiently massive to make "pockmarks" in hyperspace. Prior to jumping, the ship's navigators calculate an outbound vector, targeting the destination jump-point with direction and speed. The ship accelerates along this vector with a long STL burn until it is clear of the current jump-point's gravity well and the requisite velocity is reached. The jump engines are then engaged and the ship enters the interface between realspace and hyperspace.

In this quasi-state of being half-in and half-out of hyperspace the ship is drawn along the interface to the nearest gravity well on the outbound vector, the destination jump-point. Here it re-enters realspace, travelling at the same speed and direction it entered the interface. Back in normal space the ship dumps velocity by cycling its vanes to graze the interface before the STL thrusters take over. It is possible to pass through several jump-points without slowing down, but this is risky as it can cause the ship's velocity to become uncontrollable.

Long story short, it's sort of a gravity winch. If you point yourself in the right direction, and speed up to near FTL, the drive can grab hold of a gravity well and drag you along with it, like a stone skipping across the surface of a pond.

The books explain the drive systems a little differently; it's not that the ship enters hyperspace, it's more akin to dropping a weight into a whitewater stream just below a mostly-calm surface while kayaking. You don't get pulled underwater, but along the surface.

• Of course a gravitational wave (gravity waves are something different) does alter (or rather, is an alteration of) the spacetime fabric. BTW, "speed up to near FTL" doesn't really make sense; you're always at rest in your own frame of reference, and near the speed of light in some other. – celtschk Jul 18 '16 at 7:12
• Would you prefer I described it as "approach 0.99 C?" – Kamilion Apr 10 '17 at 6:39
• You can even write "approach light speed" as "approach" does not mean "reach". You better say relative to what, though (I guess relative to the jump gate). For example you could say "approach the jump gate at almost light speed" as here the jump gate gives a frame of reference, and "almost light speed" then gives the relative(!) velocity. Of course "approach the jump gate approaching light speed" would be possible, too, but would not sound as good. – celtschk Apr 10 '17 at 7:11

We can take a page from the classics that first tried to get around Einstein's speed limit: Inertialess Drive.

E. E. "Doc" Smith has one of the better known examples of this type of drive in his Lensman series. This drive gets around the the "mass increases to infinite" issue by proposing a method that stores or hides the mass of the ship from the universe. With zero mass, even the smallest thrust could, theoretically, push a ship an infinite distance at infinite speed. However, any interstellar material would instantly stop the ship. So the speed of the ship ends up being whatever speed balances out the thrust with the drag of the interstellar medium. One issue is that you retain the velocity that you had when you engaged the inertialess drive. So, ships would have to thrust very hard at their destination to alter their velocity to the velocity of the object they wish to approach.

If you can hand wave the mass negation, you have your "scientific" FTL drive.

The fun part is that it comes with a bunch of issues that the characters absolutely must deal with.

• A massless object will travel at lightspeed, not infinite speed. – JDługosz May 6 '17 at 4:11
• Not only does it travel at lightspeed, it is required to travel at lightspeed and does not require acceleration. – Draco18s no longer trusts SE May 6 '17 at 16:16
• @JDługosz, what if you were to push whatever can hide the mass further? If it is given negative mass, could it fall away from lightspeed in the other direction? – ShadoCat May 8 '17 at 22:12
• No, negative mass still follows the rules of Minkowski spacetime it’s travelling through. – JDługosz May 8 '17 at 23:34

I have already thought of this idea for my story, and will gladly share it with you now. This solution DOES fit your criteria even though it requires that the multiverse theory be true. The ship stays in the universe, does not suffer time dilation, and does not disrupt the space around it at all. Many of the answers above mine have one common limiting factor: the laws of physics. Nothing can go faster than light. However, my FTL drive does something special that involves different laws of physics.

# 1. Materials Needed

An atom called "Hyperium" or colloquially known as "FTL Fluid" is used. Negative ions of Hyperium are put into a pistol-shaped engine. A layor of positive ions surrounds the ship, easily created by making the outer layer of the ship from Hyperium metal (Fe2HyC4). For a tie-fighter sized ship, the engine only needs to be pistol sized; for a star destroyer, it would have to be as big as two or three tie-fighters. So, you need: a FTL engine, a bullet sized amount of negative Hyperium ions that is relative to the size of your engine, and an outer coating of positive Hyperium ions.

# 2. How does it do it?

When the engine is started up, it electrifies and shoots a blast of Hyperium. This unstable element splits apart into two, but both parts are connected to each other and exists in the same space. One part zooms off in this universe, touches the field of positive ions, and gets stuck. The other half zooms off into a different universe that is extremely small compared to ours and empty as well. Why does it go there? Because of its electrons configuration and instability and whatever are aligned so that the electron blast charges them into the other universe. Read this next part carefully. Once that part of the Hyperium blast enters the other universe, it is still going the fast speed BUT it is in a smaller universe at the bigger universe speed, so is going relatively uber overly faster than light speed. But it is connected to the other Hyperium, where the ship is. And that Hyperium is connected to the other ions. And the ship is connected to that. So the ship blasts off at the relatively uber overly faster than light speed; both universe reflect each other for the benefit of the ship drivers. But wait, what if you want them to only be able to go slightly over the speed of light?

No problem. Many different society, planets, or peoples have different methods for making this engine. Some are better at it than others, meaning that YOU the writer/designer get to decide how fast their engine is. Were they excellent at making it and can go across galaxies in the blink of an eye? Or are they not very good, and can only travel between planets in that time?

After your journey is done, simply sever the link between the Hyperium and the ship via another electric blast. The ions will become temporarily not-attracted, flinging the Hyperium blast into oblivion and instantaneously stopping the ship. Also, to the crew, there doesn't appear to be a sudden OOF from stopping and/or going; [insert excuse with science here that relates to trans-universe thing].

# 3. Pros

This ship requires Hyperium to exist. Otherwise it cannot. That means you can choose if, in your universe, the element is very rare and only elites have it, or if it is common and everyone has a FTL drive.

As aforementioned, the speed of the ship can also be decided to fit your needs as the designer.

The trans-universe thing causes NO time dilation because the other universe doesn't have that physical law. The trans-universe thing causes NO cross-universe travel for the ship; only for the Hyperium. That's actually a lie, because the ship also becomes partially embedded in that universe, still able to detect things. The only parts that are effected are the laws of physics limitations, not the ship's ability to collide with objects.

The engine could have a cool down or not; as the designer of your universe, you decided how efficient their engines are. For plot purposes, any different limitations can be added or changed. Perhaps your FTL drives needs [some other substance] to function?

# 4. Cons

If you're going really fast, you might miss your destination. But you can slow down gradually, maybe.

This engine is kind of cheaty because it often answers questions with, "How? Because the other universe has different laws physics. How does that effect things? Because it's science!". If I knew some complex math and physics, I could probably actually say, "Well, A happens because B times C equals the square root of A!". However, if you don't mind that, than I humbly think this is an answer to your question.

In addition to the problem of a lack of ideas on FTL drives meeting your requirements there's an even greater problem:

Special Relativity. Now, there's no question that it's not the final answer as to how the universe works but it's been tested enough that it's obvious that it's basically correct, although there might be edge cases it doesn't handle well. (Think of how Einstein displaced Newton—edge cases only, for almost everything we do we use Newton's answers.)

Einstein prohibits accelerating through the speed of light, although there is no prohibition on a system to transform your ship into tachyons for the duration of your voyage. However, this enters into the realm of paradoxes and I do not believe that a solid object could thus be transformed. The transformation field can only propagate at c—but the part of the ship that's already transformed must take off at >c. This means your ship is dismantled at the subatomic level by the transformation.

This basically says that any FTL transport system must somehow leave the universe in order to work. (I see nothing precluding a tachyon-beam communicator however, other than the issue of paradoxes.)

You could possibly use some kind of temporal warping around the ship. This would allow the data to filter in, albeit at a much accelerated rate. While this would leave the ship trapped in the future, with temporal warping systems you could either throw the ship and crew back in time or throw the universe around you forward in time.

• Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! – Secespitus May 6 '17 at 9:45
• Hi Cameron Brownsey, and welcome to Worldbuilding and Stack Exchange! Please note that the OP specifically asked for ways that were "scientifically correct". Your answer would be much improved by editing to add a discussion on how temporal warping could be combined with the known sciences. (I very much doubt myself that "temporal warping" is even possible, but even an explanation that allows the reader to maintain suspension of disbelief might be acceptable.) – a CVn May 6 '17 at 10:24