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Recurringly in Sci-Fi spaceships manage to make it to lightspeed and above. Is there a scientifically plausible explanation of such capacity? How does it impact the spaceship's design and its passengers lives?

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    $\begingroup$ I disagree that this is off-topic - how do colonists get to a distant world? A spaceship on a sufficiently long journey could be a person's whole world for their whole life. $\endgroup$ – Monty Wild Sep 21 '14 at 9:35
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    $\begingroup$ Even if the answer is "no", the details provided will help shape a story that is not quite realistic but allows faster than light travel that is comfortably believable. Definitely on topic. $\endgroup$ – trichoplax Sep 21 '14 at 17:19
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I was thinking about this topic a lot and I still didn't find any plausible FTL mechanism. Here is quick overview of important features of the most "standard" mechanisms used in science-fiction. None of them is scientifically sound, but some of them are plausible in principle.

Deformations of spacetime

This category of FTL transport mechanisms uses known fact, than general relativity theory on its own allows very broad class of spacetime deformations. Almost for any shape of spacetime, one can find distribution of mass that is capable of creating it. This allows solutions as the famous Alcubierre drive or traversable wormholes. There are, however, three very serious problems.

Problem of exotic matter

It is true, that the solutions of Einstein equations almost always exist and some of them allow FTL travelling, but almost always, this requires the so-called exotic matter. It is exotic in several aspects: it either has negative rest mass, or it has tachyonic properties. This means lot of troubles: in some cases, such matter could be spontaneously created from vacuum, producing energy in the process. By other words its existence would render our vacuum unstable. In other cases, this problem is not present, but such matter is likely to allow direct superluminar travelling and therefore necessarily time travelling. (In optimal case, we would like wormholes to grant us shortcuts to stars without breaking the causality, which is always troubling.) In all cases, such matter is unlike anything we have encountered so far and there is no good reason to think it will be discovered in the future.

Someone says quantum effects, like the Casimir effect, can take role of exotic mass. This seems very unlikely to me, since they are tiny compared to the rest masses already present in the system. For construction of warp-drive or a wormhole, we need effects comparable or bigger than the rest masses involved. If I would like to introduce exotic mass into my universe, I would probably postulate that new particles of these properties were discovered.

Problem of density

Another big problem is, that the spacetime is deformed only at very, very high densities. Even at densities of nuclei, around 1017 kg/m3, there has to be object of size of a neutron star to do anything at all. Probably much higher densities would be necessary for FTL construction. This means that some form of reasonably stable ultra-dense matter would probably have to be introduced to allow this form of FTL.

Problem of too big mass

To achieve wormholes or warp-bubbles on scale of meters or kilometers, masses comparable to the mass of sun would be probably needed.

Warp drive

  • There are no indications that required exotic high-density mass should exist.
  • Automatically allows time travelling in the universe.
  • Probably requires existence of some stable high-density exotic matter, like magmatter proposed in the Orion's Arm. It is unlikely we could build it only based on atoms. (This includes that it probably cannot be based on some field that is just "switched on", like it is in Star Trek. Required energy for such field would by far surpass any energy spaceship made out of atoms can carry, even in form of antimatter.)
  • Warp bubbles big enough to enclose spaceships require tremendous energies at very high densities. Civilization capable of their construction would probably find easier to "rewrite" themselves into objects of high densities and use smaller bubbles.

Wormholes

  • There are no indications that required exotic high-density mass should exist.
  • Wormholes would probably have mass comparable to planets or even stars.
  • Do not automatically allow time travelling. It can be prevented by Visser effect, how they called it in Orion's Arm. But the discussion of this effect is broader, including some scientific articles on the topic. Nicely explained on web of the Anderson Institute

    It is thought that it may not be possible to convert a wormhole into a time machine; some analyses using the semi-classical approach to incorporating quantum effects into general relativity indicate that a feedback loop of virtual particles would circulate through the wormhole with ever-increasing intensity, destroying it before any information could be passed through it, in keeping with the chronology protection conjecture.

  • Would have to be transported in place by sub-light speed.

Tachyons, 'Mass Effect' or simply jump-drive

Jump-drive is used for example in Battlestar Galactica series without explanation of the mechanism. Similar jump drive is used in Mass Effect, where they offer basically this mechanism: Theory of relativity does not forbid existence of tachyons - particles that always move faster than speed of light. They can travel into past, thus breaking the causality, but this is not problem in principle.

Jump drive works so that the space-ship changes local Higgs field, which leads change of rest mass of particles of the ship. Eventually, all particles gain imaginary rest mass and become tachyons. Then they move faster than light.

  • Requires existence of tachyons. There are currently no indications they should exist.
  • Requires viability of change of the rest masses into imaginary values by changes of the Higgs field. (Probably not expected to be really possible, but plausible.)
  • Automatically allows time-machines.
  • Requires incredible amount of energy.
  • It is very improbable that the ship and crew would survive tachyonic state or transition to it. As the rest masses change, binding energy of every atom drop or wildly change and atoms become much bigger. Same things happen to the atomic nuclei.

Subspace/Hyperspace

Subspace/Hyperspace idea operates with assumption that it is possible to leave our spacetime into some other space, in which the speed of light would not be the limit. The biggest problem is how the transition into hyperspace should occur. We know how the matter behaves up to very high energies (hundreds of GeV) and we have not seen anything even remotely similar. From this, we can judge that if transition into hyperspace is possible, it would probably occur only using some very high energy mechanism. This brings similar problems as in the spacetime manipulation mechanisms.

  • There are no indications that the hyperspace should exist.
  • Does not necessarilly allow time travelling. Structure of the hyperspace could dictate what is really "now". This violates principle of equivalence, but only if the hyperspace is concerned. (And it is therefore not in contradiction with experiments done so far.)
  • If the transition to hyperspace is possible, it probably requires so high energies we haven't noticed it so far. This might be prohibitively difficult to construct.

Communication via Quantum Entanglement

Here, it is simple. Although it is often used in science fiction, quantum entanglement is shown not to be able to transmit data faster than light. Wikipedia:

It is not possible, however, to use this effect to transmit classical information at faster-than-light speeds

References

There is a very extensive and scientifically serious overview on the web of Project Rho. (Although some of the listed FTL drives are not very plausible - and the author admits it.)

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    $\begingroup$ As I recall, the BSG FTLs were stated off-screen by producers to utilise wormholes. $\endgroup$ – Lightness Races in Orbit Oct 8 '14 at 11:32
  • $\begingroup$ I enjoyed reading your well thought out answer. One thing no one ever mentions with regard to FLT is the issue of navigation. For one thing, how the hell would you stop? It would seem to require discharging an infinite amount of energy, just as accelerating to FLT would appear to require gaining an infinite amount of energy - or at least an unbounded amount of energy. $\endgroup$ – Epsilon Nov 2 '14 at 20:44
  • $\begingroup$ "but such matter is likely to allow superluminar travelling" — I thought that's exactly the point of a faster-than-light space travel system. $\endgroup$ – celtschk Nov 2 '14 at 21:04
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    $\begingroup$ Thank you, I will reformulate it. I wanted to point out that if you allow superluminar travelling in arbitrary direction, it automatically means time travelling. (Wormholes avoid this problem, since you rather change the distances in space than travelling superluminarly.) $\endgroup$ – Irigi Nov 2 '14 at 21:54
  • $\begingroup$ The thing about the warp drive field is the energy density required is very high, but it contains negative energy components. It may well be possible to drop the total energy to something manageable. Arithmetic on more complex fields has already dropped it from half the mass energy of the galaxy to a couple of Jupiter masses. $\endgroup$ – Joshua Aug 28 '15 at 17:34
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The Alcubierre drive is one of the proposed FTL drive systems that is the closest to being feasible. In effect, it is very similar to the Star Trek warp drive, though an Alcubierre ship would probably look nothing like a Star Trek ship. It works by distorting spacetime in front and behind the ship, compressing it in front and expanding it behind, while the ship moves in a bubble of undistorted space-time.

Such a ship would require exotic matter (matter with negative mass), or perhaps some manipulation of the Higgs-field (that could become possible following the discovery of the Higgs Boson), and would require a particular design in order to operate. Depending on the multiple of lightspeed such a ship could achieve, it may still take quite some time to make a journey to another star, necessitating efficient self-sustaining life support or some sort of hibernation technology, or both.

The other mechanism for FTL travel that may be scientifically feasible is the traversable wormhole, but this is not so much travelling at greater than lightspeed as bypassing most of the space between here and there. Such technology would require that the journey be made once in real space in order to carry the mouth of the wormhole to the desired destination. Traversable wormholes may eventually not even require a ship, but if they are possible, I would imagine that at least initially, it would be desirable to have the mouths of the wormhole located a safe distance away from planets. A wormhole ship could be little more than a sealed can shot into the mouth of the wormhole and caught on the other side; the passengers would not need to be aboard it very long at all, perhaps only an hour or so. It may not even need to have more life support than a few emergency supplies.

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  • $\begingroup$ Fun fact: According to Alcubierre any FTL vehicle is potentially a time machine. But if you try to actually use it as a time machine, bad things could happen. (source: wikipedia and ccrg.rit.edu/files/FasterThanLight.pdf ) $\endgroup$ – user31389 Sep 23 '14 at 21:07
  • $\begingroup$ I’ve only taken a basic special relativity undergrad course, so it’s extremely easy that I have something wrong here, but the formula for contraction of time when traveling at relativistic speeds has a limit of zero as you approach c. That is, when traveling at c, all “travel” takes 0 time and is instantaneous, from your own perspective. The math gets wonky with tachyons, since, IIRC, the formula would have those experience negative time, whatever that means. But anyway it may imply that from the perspective of those on board, no time at all passes while traveling FTL. $\endgroup$ – KRyan Sep 24 '14 at 14:01
  • $\begingroup$ @KRyan, I'm not even sure there would be any time dilation, as from the ship's POV, it is not moving. Any light that passes the ship from ahead or behind (inside the warp bubble) would do so with no red-shift or blue-shift. $\endgroup$ – Monty Wild Sep 24 '14 at 16:34
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    $\begingroup$ Upvoted. This is more or less the answer I was going to give. I was going to say, under completely “hard” science, (i.e. the unimaginative and narrow-minded type that basically operates under the absurd assumption that no new scientific discoveries beyond what we have right now will ever be made. Ever.) no. Under semi-hard but still at least flexible science, the Alcubierre drive, as Monty mentioned. $\endgroup$ – Josh Zmijewski Nov 4 '14 at 3:10
  • $\begingroup$ while you could possibly get an Alcubierre drive to work, the radiation inside the bubble would ensure you'd not survive the experience (and most likely neither would anything else inside the bubble). $\endgroup$ – jwenting Nov 4 '14 at 13:00
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I will take "scientifically sound" to be "we are sure of the principles involved, and need technologies with better precision, more energy, or need to experiment with the engineering. In practice the solution might be very hard, and centuries away from being a normal technology." I would count nuclear fusion power in this category. Or perhaps a Dyson Sphere.

In which case the answer to the question in the title is "No."

The best examples worked through in accordance with theories of relativity, are scientifically plausible (by untested extrapolations of the theory) only if you allow for unknown exotic materials, or access to stupendous amounts of energy that would be game-changing beyond FTL travel in their own right.

The topic of FTL communication and travel is driven by science fiction first and foremost, because it gives access to a whole cosmos of settings and venues for adventure and story telling.

That is not to say that these unknown materials do not exist (note they would not be combinations of atoms, electricity or anything technology can handle today). We still do not know what dark matter or dark energy are, and the big bang theory is widely accepted to include a short period of Inflation, where space expanded orders of magnitude faster than the speed of light. If these, or similar things at the edge of known physics, turn out to be true, then we don't fully know the consequences, and that might mean we would have a way to send material faster than light between locations, at a practical cost. However, I think it is quite reasonable to forecast that any such new physics will not have practical use for transport.


Analogy: In the past, a weak understanding of chemical reactions prompted much searching for ways to transmute base materials to gold (even scientific luminaries, like Isaac Newton, spent time on this goal). Eventually that gave way to chemistry, one of the tenets of which was that transforming elements (i.e. altering the nature of elemental atoms) was considered impossible. Then came understanding of sub-atomic structure and radioactivity, followed by tools like linear accelerators that can and do transform elements between types. However, these tools and knowledge are not a practical way to make gold on demand, and never will be, although they have spawned useful technologies such as radioactive materials for medical imaging.

In the above if you take "making gold from other elements" -> "FTL Transport", and "unknown exotic matter" -> "sub-atomic theory", then this is an example of how wishful thinking can inspire research, but that the end scientific results are only vaguely related in the end. In my opinion, this applies to 99%+ of science and science fiction conjecture, pretty much for any technology that we don't understand well enough to be building it today.


The most likely long-distance starships are in my opinion most likely to be a generation ship - a self-sustaining colony taking many generations to arrive at its destination. Or perhaps an interstellar ark would be more practical, where most life forms are kept in or close to stasis in order to reduce resources needed by the ship to maintain them (and thus the size of the ship).

These types of long-term transport would be major projects and very exciting in their own right. However, they do lead to a very different kind of narrative. The stories associated with FTL can have a new adventure location each episode, or a galaxy-spanning grandeur. The stories associated with a travelling colony are much more inward-focussed.

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  • $\begingroup$ +1 for "this is an example of how wishful thinking can inspire research, but that the end scientific results are only vaguely related in the end" ´, that is easy to forget and it kind of matters. Both to those doing the speculating and to those dismissing the speculation. $\endgroup$ – Ville Niemi May 8 at 12:56
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There are a few slightly far fetched theories. Here is one:

One found here, that NASA is supposedly working (I'm skeptical), is that by creating a special propeller that expands space time behind it, and contracts it in front of it, you can have a ship travel just about as fast as you want. But this would require special exotic matter, and about the energy mass of the planet Jupiter. Basically, with Earth technology,that would be impossible. But, maybe, if you got a certain shaped ring around the propeller you could reduce the energy mass required to about 1,600 pounds.


I originally thought that FTL (faster than light travel) was impossible. Above is what I came up with before I did a quick google search.
But if you want one with some interesting possibilities you could look at the one from Xenocide by Orson Scott Card. In that story faster than light travel was accomplished by going into another dimension. If a mind could physically hold the pattern of something (the layout of cells and molecules), then that something could be moved to another dimension, and destroyed in this one. It could then be remade in this dimension in another place. This "other" dimension would be defined for the specific story.

This mind that could hold the pattern would have to be very large, many hundreds of computer's worth (as it is in Xenocide)

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  • $\begingroup$ NASA's article, you're pointing to, talks about works actually based on Alcubierre drive, mentioned by Monty Wild, above. It seems, that both your answers talks about the same. $\endgroup$ – trejder Sep 21 '14 at 11:53
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    $\begingroup$ @trejder Pretty much, mine was first, but his has more detail. I admit, mine is the lesser answer. (I upvoted his) $\endgroup$ – DonyorM Sep 21 '14 at 13:13
  • $\begingroup$ Great! Just, please, don't take my comment as compliance against your answer. I just observed the fact, that both answers generally rely on the same mechanism, shown in two different sources. $\endgroup$ – trejder Sep 21 '14 at 16:00

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