I am writing a hard science fiction story and I want to avoid violating known physics, while still enabling some of the classic mainstays of science fiction under the auspice of sufficiently advanced engineering. To that end, I have developed a method of interstellar travel that I believe avoids violating either relativity or causality. It is not faster than light travel.

It is, in fact, exactly as fast as light.

Question: The Fast as Light (FaL) system in my setting functions as follows. Does it violate causality or relativity, as described?

  1. The cornerstone of the FaL system is that nothing travels faster than light (masssless energy), and that nothing travels as fast as light except for light itself. There is no way, for instance, to move a physical spacecraft faster than light.

  2. Expanding on point 1, if massless energy always moves through a vacuum at light speed, then the quickest information can be transmitted between solar systems is by beaming energy between them in patterns.

  3. Mass can be converted into energy, and energy into mass.

  4. A sufficiently advanced civilization in my setting has a machine that translates mass into a vast and self-sustaining pattern of massless energy, structured in such a way that it collapses back into matter after a specified amount of time has passed. The pattern self-checks as much as it can, and contains enough redundancies that the interstellar medium doesn't damage it enough to destabilize it early (usually).

  5. This civilization can load any cargo that fits into their machine, activate it, and broadcast the cargo out at the speed of light - as a pattern of light. The pattern of energy then collapses back into matter in its original form once it reaches its pre-set destination.

  6. The practical outcome of point 5 is that this civilization can, at the cost of immense energy expenditure at the launch mechanism, send spacecraft or resources across interstellar space at light speed, though any actual crew or machinery would be shut down for the duration of the trip itself.

EDIT: As AlexP said in the comments, my saying "massless" was colloquial for "without a rest mass".

  • $\begingroup$ What Radovan said, plus a remark: that civilization could probably be dealing with tachyons. In sci-fi, tachyons are particles that do travel faster than light. In modern physics, though... I'll quote the wiki: > The possibility of particles moving faster than light was first proposed by O. M. P. Bilaniuk, V. K. Deshpande, and E. C. G. Sudarshan in 1962, although the term they used for it was "meta-particle". In the 1967 paper that coined the term, Gerald Feinberg proposed that tachyonic particle $\endgroup$ Commented Mar 20, 2018 at 14:17
  • $\begingroup$ A minor query on a cornerstone of this concept: Doesn't "massless energy" include things like sound-waves, which propagate at sub-light speeds? So, while the only thing that can travel at light-speed is massless energy, not all massless energy can travel at light-speed. Of course, this might be treated as an earlier / lower-power / short-distance /cheaper version of the same technology... $\endgroup$ Commented Mar 20, 2018 at 16:50
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    $\begingroup$ See also worldbuilding.stackexchange.com/questions/46873/… applies to any superluminous travel method. $\endgroup$
    – JDługosz
    Commented Mar 21, 2018 at 15:32
  • $\begingroup$ Point 4 is a problem. Could yould say ultrarelativistic rather than lightspeed (like neutrinos) to get around it — what’s a few nanoseconds delay matter? $\endgroup$
    – JDługosz
    Commented Mar 21, 2018 at 15:35
  • $\begingroup$ Actually, using the last concept, 'scaling down a universe' instead of a human, into wavepackets exclude time, as in being a perceived 'reality'. Time is needed for any outcome to exist. So doing it that way you might be presumed to reach outside the limits (laws properties etc) imposed on this universe. Would make it a enjoyable book actually :) $\endgroup$
    – Yoron
    Commented Mar 21, 2018 at 19:46

8 Answers 8


There is a problem with relativity. An object traveling at the speed of light does not experience time. Your suggestion is that the object will revert to matter as soon as a certain elapsed time. From the reference frame of the light-speed object, all events are simultaneous, and the elapsed time between any two events is exactly zero.

You almost have a solution. Instead of having a time based system for reverting to matter, you'll need to replace that with a system based on location. For example, the energy can turn into matter where it hits a large mass, or two beams can turn into matter where they intersect.

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    $\begingroup$ From the reference frame of the light-speed object, all events are simultaneous, and the elapsed time between any two events is exactly zero. You are trying to apply a version of Zeno's paradox to relativity. However, as anyone who has ever been hit by a projectile will be able to tell you, Zeno was full of fertilizants. If I point a flashlight at you, the individual photons might not experience time in a way that is meaningful to us, but they will still go from the lamp to you in an amount of time that makes some sense to us. $\endgroup$ Commented Mar 20, 2018 at 18:48
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    $\begingroup$ Say you stood next to me and shone a light on me. I can easily say it took 5 nanoseconds for the photons to get to me. My twin brother who was on a relativistic rocket and watching me carefully can say it only took 2 nanoseconds. Our other brother who was made of photons at the time can say it took 0 nanoseconds, his stopwatch wasn't doing anything, and he's really glad someone used a stopwatch made of matter to determine when to turn him back. $\endgroup$
    – Jetpack
    Commented Mar 20, 2018 at 20:03
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    $\begingroup$ Yes! The OP said that the energy collapses once it reaches a preset destination. Maybe that's done with a receiver. But my interpretation was that there's some sort of 'timer' encoded in the stream of light, and that simply doesn't work. The only thing I can think of to get out of using a receiver is to transmit 2 streams and have them interfere at some location and have that trigger the conversion to matter. Unfortunately this is no longer light speed. $\endgroup$
    – bendl
    Commented Mar 20, 2018 at 20:51
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    $\begingroup$ Doesn't exactly work like that. In the limit not only is time dilation infinite, but so is length contraction. The distance between the light and the brother is 0 "according to the photon". But that doesn't make any sense, regardless. Relativity explicitly forbids a light speed frame of reference. Talking about things from the perspective of a photon is nonsense. $\endgroup$ Commented Mar 21, 2018 at 2:38
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    $\begingroup$ @Jetpack I believe the 13 upvotes for your comment are for "say my other brother who was made of photons..." $\endgroup$
    – NL628
    Commented Mar 21, 2018 at 3:35

I see no causality arguments to be had, but there is an interesting philosophical one. If your system works on people, that proves that consciousness and the soul (if they exist at all), are successfully transferable in this way. That conundrum is one which philosophers have played with for centuries, so it would provide some fascinating background material you could add to your world.

Also, you could explore the question of whether or not this pattern of light waves is measurable. If it's measurable, you open the door for using this sort of technology for things like cloning, and all the complications which arise from that. If it's not measurable, you have a society that is likely constantly trying to understand why it's not measurable. Either case may add richness to your story, borrowing from works of past philosophers.

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    $\begingroup$ Comments are not for extended discussion; this philosophical conversation has been moved to chat. $\endgroup$ Commented Mar 22, 2018 at 2:53

I'm surprised no one has mentioned one of the major impediments to this form of travel. The key to your whole proposal is that you're converting the mass into pure energy and somehow encoding information in the structure of this energy to allow it to be converted back to mass when it arrives at its destination.

The problem you will face though is that only a fraction of the energy sent will arrive at your destination, meaning you won't have enough energy left to convert back to the mass that was sent (not to mention you may not have the information to reconstruct the mass either). The reason for this is that it is physically impossible to construct a collimated light beam, that is, a beam of zero disperson. This is a fundamental fact of physics, because light is a wave and exhibits diffraction, causing it to be dispersive.

This means that no matter how tight and straight you make your transmission beam of energy, it will disperse as it travels through space, making it so that only a tiny fraction actually hits your destination, with most of the energy missing your target. Your destination just won't receive enough energy to construct the matter that was transmitted.

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    $\begingroup$ Good point, though one that the superciv may be able to work around. The emitter itself is probably immense and uses extremely hard gamma rays, in order to have a small point at destination. It is also probably adding massive amounts of energy to boost the signal so enough arrives. The apparatus is probably mounted on at least a partial Dyson sphere, and the remaining backup energy would fry anything on the path in the next few parsecs, so avoid aiming at a planetary surface. It is also, obviously, an unstoppable interstellar weapon of mass destruction, namely a Dyson Beam. But hey, why not? $\endgroup$
    – Eth
    Commented Mar 21, 2018 at 18:03
  • $\begingroup$ @Eth That's true, if their destination was close, and their energy beam powerful enough, they could in principle have enough energy reach the destination that they can do what they want. I have debated doing the calculations necessary to send a reasonable mass to Proxima Centauri. I do have to wonder what the energy requirements would be. $\endgroup$
    – zephyr
    Commented Mar 21, 2018 at 18:06
  • $\begingroup$ Perhaps it needs the energy of a nearby solar wind or photosphere to reconstitute the matter at the destination. The solar gravity well might even be the trigger for the fractal signal packet to coalesce back into matter. Sort of like a supersaturated solution that needs a seed crystal to start coming together. $\endgroup$
    – KalleMP
    Commented Mar 21, 2018 at 21:59
  • $\begingroup$ You could add immense amount of redundancy and error correction algorithms such as Hamming codes to workaround this. Granted, it would take even more energy, but still... $\endgroup$
    – Pedro A
    Commented Mar 22, 2018 at 11:35


There is already an existing physical phenomenon travelling at the speed of light, namely the light itself, and it does violate neither causality nor relativity (in fact, its speed being an invariant is the basis of (special) theory of relativity).


Looks legit, but there are some subtleties with physics side of things and with the design itself.

@Jetpack's answer and @zepthyr's answer raise valid concerns: light-to-matter transmission and dispersion.

1) Lightbeam-to-matter at the designated location

I see two and a half ways how that could be acomplished.

You can take a look at rogue waves. Basically, the signal could consist of many waves of not-exactly-the-same frequency that once in a blue moon (at the destination) line up to produce local EM field of extreme density. High-energy physics are different from ones we're used to so there's actually a lot of leeway there.

Another way to trigger the transition at the specified location would be to place an obstacle there. For example, you could shoot some pretty fast neutrinos and let your lightbeam catch up with those. Sure, the whole system would be only as fast as your neutrino beam but 0.99c is pretty fast too (and there's a room for tradeoffs - you can travel faster by investing into better neutrino beam).

And, of course, the easiest way to go would be aimimg at some known obstacle at your destination system - they probably have some asteroids to spare. There, carefully formed wave package carves into the obstacle a simple reciever which bootstraps production of assembly line that uses asteroid matter and lightbeam information/energy to reconstruct the payload.

I think this makes the most sense because almost all your points of interest would have some matter flying around.

Transforming all payload into self matterifying lightbeam would have a greater overhead than focusing on constructing some simple reciever with some basic assembly capabilities and then bootstrapping into thing that can produce whatever you need from matter at arrival site and energy and specs via the lightbeam. If you can also harvest the energy locally then economically you beat any competition that pumps the energy at departure site.

2) Dispersion.

At the destination, your lightbeam should have energy density greater than some cutoff or it just won't have any effect. Unfortunately, lightbeams are actually cones with a finite angle. If you increase the height n times, cross-section area grows n^2 times and you would have to pump n^2 more energy into the whole affair. If your energy expenditure grows quadratically with length and you are doing interstellar travel then you are going to have a bad time.

You can work around that by using relays.

You can send relays the same way you want to transmit the payload. However, using n relays makes you spend n times less energy or so. That is, your expenditure only grows linearly with the trip length. Also, relays can correct the course in case of something you couldn't possibly predict from your departure site.

Implication would be that using an existing reciever is way cheaper than building your own, thus there would be, like, 1 reciever per system (actually per point of interest) and instead of jumping from arbitrary point A to arbitrary point B vessels would usually go from transport hub A to transport hub B - and self-matterifying part of your technology would be used to construct new hubs.


While not directly violating causality you do run afoul of the no-teleportation theorem.


In quantum information theory, the no-teleportation theorem states that an arbitrary quantum state cannot be converted into a sequence of classical bits (or even an infinite number of such bits); nor can such bits be used to reconstruct the original state, thus "teleporting" it by merely moving classical bits around. Put another way, it states that the unit of quantum information, the qubit, cannot be exactly, precisely converted into classical information bits. This should not be confused with quantum teleportation, which does allow a quantum state to be destroyed in one location, and an exact replica to be created at a different location.

In crude terms, the no-teleportation theorem stems from the Heisenberg uncertainty principle and the EPR paradox: although a qubit | ψ ⟩ can be imagined to be a specific direction on the Bloch sphere, that direction cannot be measured precisely, for the general case | ψ ⟩ ; for if it could, the results of that measurement would be describable with words, i.e. classical information.

The no-teleportation theorem is implied by the no-cloning theorem: if it were possible to convert a qubit into classical bits, then a qubit would be easy to copy (since classical bits are trivially copyable).

  • $\begingroup$ Can you elaborate on this please? It looks like an important point. $\endgroup$
    – Jetpack
    Commented Mar 21, 2018 at 13:55
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    $\begingroup$ No teleportation theorem says about the inability to transfer a quantum state. As far as the method does not try to transfer quantum states, but just the classical, measured information (say, positions and momenta of atoms as measured), everything is fine. Even so, if the transfer itself is not classical, but helped with pre-shared entangled system, you can transfer quantum state - see Quantum teleportation. $\endgroup$ Commented Mar 21, 2018 at 14:49

The science starts to fail at point 3 and 4 depending on your interpretation.

Point 3 says that mass converts to energy and vice versa, this of course is true. Combined with point 4 it however suggests that this can happen arbitrarily, this is where the physics fail, most notably the conservation of momentum. Bad practice, certainly for science-based, but here is the wiki quotation:

Because of momentum conservation laws, the creation of a pair of fermions (matter particles) out of a single photon cannot occur. However, matter creation is allowed by these laws when in the presence of another particle (another boson, or even a fermion) which can share the primary photon's momentum. Thus, matter can be created out of two photons.

So unless you can shoot one pattern of very high energy photons from one side and a second set from the other you can't do this.

Another physics law you are breaking is Heisenberg. You can either know the exact location of a particle or the exact speed. Not both simultaneously. To create matter in a specific configuration at an atomic level you need to know the exact location of the colliding photon. Since you know the velocity of the photon you would be in breach of the uncertainty principle of Heisenberg.

Two more points I would be highly doubtful about, but can't give you concrete evidence for, is whether you can possibly create a (stable) photon pair with a high enough energy to create neutrons and protons. Also at very high energy densities the non-linear regime of electro-magnetic waves will probably become annoying.

Lastely point two might not strictly speaking be true. Some loop holes in quantum entanglement theory might allow for FTL communication (see comment) but as far as I know there is no major consensus about the loop holes in quantum entanglement. To give a gist of it, quantum entanglement is instantaneously (experimentally proven B. Hensen, et. al., Nature). So if you take a pair of particles, quantum entangle them, move one particle to the other side of the universe as soon as the quantum entanglement is broken you will know it instantly on both sides of the universe. If you could do this with enough mass kept in a quantum entangled state you could send messages instantaneously. Of course you should send the mass there in the old fashioned way. Saw this method once already used in a science fiction novel very nicely.

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    $\begingroup$ Quantum entanglement doesn't permit you to send messages. You can prove after-the-fact by way of non-FTL communication that the entanglement breakage was transmitted instantaneously, but since anything you do to the entangled state (including seeing if it's still entangled) breaks the entanglement, you can't transmit information. So your first paragraph is irrelevant. $\endgroup$
    – Mark
    Commented Mar 20, 2018 at 23:20
  • $\begingroup$ @Mark, You have a valid point and indeed I think it is still in principle uncertain whether we can use QE for communication. In principle information is transferred FTL with QE, this however does not mean communication. In the sense of direct communication QE will not work but there might be some interesting loop holes that still allow for FTL communication. I thought one such loop hole is the reading of QE matter based on a time interval. If the sender breaks the QE before the reader reads it you could still send information. Still the possible loop holes in QE are still very much in debate. $\endgroup$
    – D.J. Klomp
    Commented Mar 21, 2018 at 9:40
  • $\begingroup$ @Mark, I shifted my first paragraph to the last with a clear indication that there is no consensus yet. $\endgroup$
    – D.J. Klomp
    Commented Mar 21, 2018 at 9:48

Under the assumption of no data loss during the transfer (due to any object in the transfer medium). Your system still has two problems.

  1. Energy. The amount of energy to convert mass to energy (twice) is huge, remember the formula E=mc2. This means that transfer a 10 tons of mass means 10^20 [J], which is insane.
  2. Information. You are not transfering energy, you're transfering information, data related to how mass should be built. A human body has 7*10^27 atoms, while a terabyte are only 10^12 bytes. So the data transfer assuming a single "wire" will need so much time that the system would be useless to transfer more than few molecules.
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    $\begingroup$ I did say that the civilization using this was "sufficiently advanced". $\endgroup$
    – TPK
    Commented Mar 20, 2018 at 13:39
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    $\begingroup$ That was a little blunt. Let me rephrase. This civilization can lift hydrogen from a star and fuse it on massive scales for energy, if it isn't capturing that energy with massive solar collectors. The broadcasting array for the pattern can be considered to be of arbitrary size and resolution. It is "sufficiently" advanced to solve the requisite engineering problems. $\endgroup$
    – TPK
    Commented Mar 20, 2018 at 13:48
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    $\begingroup$ The energy requirement should be a breeze if the civilization in question is able to build a Dyson Sphere - which they would probably do with the tech in the question. $\endgroup$ Commented Mar 20, 2018 at 14:09
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    $\begingroup$ @TPK There's a point where "sufficiently advanced" runs into hard physical limitations. Believe it or not, the limitation on brute-forcing a 256 bit key isn't time, it's energy. (Or number of atoms in the galaxy, depending on whose math you prefer). Your system is quickly going to run into the same problem. You won't find enough energy or matter in a solar system to run the required calculations, let alone actually do the work your system requires. $\endgroup$ Commented Mar 20, 2018 at 20:24
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    $\begingroup$ @Renan the energy requirement is not a problem anyway, as the transportation system is supposed to convert mass to energy, transport it and convert the energy back to exactly the same mass. The challenge is to handle that large amount of energy within the device. $\endgroup$
    – Holger
    Commented Mar 21, 2018 at 7:53

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