On a mythical world almost identical to our own, deep space telescopes notice as an object appears out near the orbit of Jupiter. In the instance after its appearance, the light which bounced off it during its FTL entry into the solar system finally catches up with it, allowing the telescopes to see what general direction the ship came from.

Scientists determine that the ship is decelerating at such a pace and trajectory that it will pass close to the world while staying just outside of lunar orbit distance.

Over the course of the next hour, the ship does just that.

Then with every telescope in the world pointed at it, the ship re-accelerates to FTL, effectively vanishing. Never to be seen again.

Question : What can the scientists of this world learn from the event? More specifically, what does absolute proof that FTL is possible do to current scientific understanding? Would there be any immediate advancements or resulting technologies which might quickly become visible to a layman, non-physicist?

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    $\begingroup$ At c the wavelength is shifted to infinity, at ftl... hold on I am looking for a pen and the next day envelope. $\endgroup$
    – user6760
    Nov 26, 2019 at 12:03
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    $\begingroup$ The whacky thing about FTL is, the ship appears to be moving the other way. You see it HERE and then you see it THERE. Light from Jupiter gets here in about 43 minutes (give or take the Earth's orbit.) Then light from when the ship was at Saturn's orbit gets here. Then light from Neptune's orbit. And so on. $\endgroup$
    – puppetsock
    Nov 26, 2019 at 14:47
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    $\begingroup$ So it would look like the object which we first saw at jupiter has split in two with part of it heading away from earth and the other half approaching. Interesting! Thanks @puppetsock. I will use that. $\endgroup$ Nov 26, 2019 at 14:53
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    $\begingroup$ The first thing we can learn is FTL is achievable what, for our current knowledge, its not. $\endgroup$
    – jean
    Nov 26, 2019 at 20:06
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    $\begingroup$ Re. seeing it "split in two": this gets even more interesting as we think it all the way through. For one, we're talking about two copies of the ship, not "halves". But, about the copy we see fly backward: Imagine having a good enough telescope, knowing where to look, and looking at where it came from. We see its homeworld as it was however-many years before the ship left. Eventually we'll see the ship being built, launched, and start accelerating toward us. All the while we've still been watching it flying backward(!) The two copies approach each other, until they meet... and just vanish. $\endgroup$
    – dgould
    Nov 27, 2019 at 7:30

11 Answers 11



Let's try to understand what we actually see.

  1. "Deep space telescopes notice as an object appears out near the orbit of Jupiter":

    That's either one biggg object, or at least a veeery luminous object. The chances of any reasonably sized spacecraft being detected near the orbit of Jupiter are between zero and infinitesimally small.

    Anyway, and wherever the object is first detected, nobody will have any reason to suspect that the object just materialized in normal space. Everybody will more naturally assume that it had always been in normal space. It's not as if we have a complete catalog of stuff moving about in the outer solar system and beyond.

  2. "The ship is decelerating at a pace and trajectory that it will pass close to the world while staying just outside of lunar orbit distance":

    So we notice that an extra-terrestrial object is maneuvering and we now know for certain that extraterrestrial intelligent life exists; or at least that it existed thousands of years ago when the ship was launched in some star system far, far away.

    Depending on how exactly the ship is maneuvering we may learn quite a bit about their technology. Do they use some sort of reaction engine? We will see the spectrum of the exhaust, and we can measure its chemical composition and temperature. Do they use some sort of reactionless drive? That would be shocking, and will give a very strong impulse to conduct research in fundamental physics.

    A secondary effect of equal importance is that we will know now for certain that our defense systems are pitiful. Expect a huge investment in defense by all the great powers.

  3. "With every telescope in the world pointed at it, the ship re-accelerates to FTL, effectively vanishing":

    What we see in reality is that the ship accelerates and then vanishes. Nobody will ever suspect that it went FTL. What the relevant military agencies will deduce is that stealth in space is possible after all. This will downgrade the impression of our defense systems from being pitiful to being a very bad joke. Expect even larger investment in fundamental research in physics and in advanced weapons platforms.

The point is that at present we are absolutely confident that faster than light travel is impossible and even nonsensical. We would be ready to accept that an advanced artificial intelligence is piloting the ship, that the aliens have some sort of stasis technology, or that they can build functional generation ships; all those possibilities are consistent with what we think we know for certain. Faster than light travel is so far out of our understanding of physics that no serious scientist will suggest it, and those who will suggest it will be labelled cranks and exiled to the remote corners of the Internet discussion forums.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Nov 28, 2019 at 12:30
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    $\begingroup$ You could mention the thought process that governs scientific minds. Occam's Razor, for example, would seek explanations in the plausible spectrum instead of going into "It's FTL". That would explain point #3 better. Also, they might as well just blame it on their detection devices. After all, it is too easy to evade a camera lens from millions of Km away. - - - - - One thing that would raise a ton of questions would be non-inertial movement, but that's beyond the scope of the question. Upvoted. $\endgroup$ Nov 28, 2019 at 13:02
  • $\begingroup$ @Mindwin How would you go about proving that it is non-inertial? It is impossible to prove a negative, especially from a few light minutes away. $\endgroup$
    – Aron
    Nov 29, 2019 at 8:27
  • $\begingroup$ One possible thing that could cause us to suspect FTL. If at the same time as the appearance of the extra-solar object, LIGO registers a huge gravitational wave event coming from the direction of Jupiter. $\endgroup$
    – Aron
    Nov 29, 2019 at 8:31
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    $\begingroup$ "This will downgrade the impression of our defense systems from being pitiful to being a very bad joke." -- I love this line so much. $\endgroup$ Nov 29, 2019 at 10:22

Obviously exciting things:

  • FTL is possible without the need for chunks of exotic matter than weigh more than the visible universe.
  • Interstellar travel appears to be possible and practical.
  • There exist intelligent beings capable of building such a device.

Possibly interesting things:

  • It probably isn't made of antimatter, or we'd see lots of gamma rays shooting out of it due to interplanetary dust and gas hitting it. Alternatively, it has a magical deflector shield. I'm not sure if we could tell if the latter existed or not, unless we actually spotted it hitting something, which we almost certainly will not.
  • If we get a good look at the object, we might be able to get a handle on the most minimal obvious requirements for FTL travel. Does it seem to have a huge shielding bumper? Massive heatsinks? Does it have any sort of design to minimise its cross section? Does it appear to be made of something that behaves like regular matter does? We might even be able to tell what it is made of, looking at the spectrum of light being reflected off it.
  • It almost certainly cannot use reaction engines to brake into and accelerate out of our system. If it does, there will be a really exciting amount of gamma rays, and we'll learn that whilst FTL appears to be possible, reactionless drives are not. If there is any exhaust, we'll be able to get a grip on what sort of rockets it uses.
  • If it does use reactionless drives, we might be able to see some sort of warping or frame-dragging distorting the view of the stars behind the object, if it uses some kind of warp-motor. If it does, we know that warp drives are possible, and may or may not be the same mechanism for FTL travel. If it does not, it suggests that our understanding of physics is even more lacking than we initially thought, because there's a non-warping yet still reactionless mechanism for travel through space.
  • If we can see the object accelerating smoothly up to the speed of light, that probably tells us that our understanding of relativity is deeply inadequate. What we see might be able to inform new theories.

But beyond that, I think things stay pretty murky.

Whilst FTL implies that timetravel is possible, it doesn't guarantee it (see also various chronology protection conjectures). I'm not at all sure whether analysis of the object and its trajectory can tell you whether it was travelling back in time or not. If it was travelling back in time, the people who made it might have been us, in the future, so the question of whether there is intelligent life elsewhere in the universe right now remains unanswered.

What can the scientists of this world learn from the event?

Huge amounts. There's decades worth of research to be done, I'm sure. As to the specifics, I can't say, because a) I am not an omnidisciplinary scientist and b) you left the nature of the object and its propulsion unspecified ;-)

More specifically, what does absolute proof that FTL is possible do to current scientific understanding?

It wouldn't totally upend it. It would upset various bits of theoretical physics, that's for sure, but the rest of reality will keep on working pretty much as it always did. If we see the object violating relativity, that'll be interesting, but in most circumstances we already know that general and special relativity hold true (see also: how GPS satellites were designed).

Would there be any immediate advancements or resulting technologies which might quickly become visible to a layman, non-physicist?

Who knows? It might produce a bunch of amazing things immediately, or it might take decades for actual end-products to appear. How long is a piece of string?

  • $\begingroup$ To be clear: is this the kind of really exciting gamma rays that we should be deeply concerned about, or only the kind we should be excited about? $\endgroup$
    – Cadence
    Nov 26, 2019 at 11:39
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    $\begingroup$ @Cadence probably interesting exciting rather than all your electrons just fell off exciting, given that the object is in translunar space, but it would depend on a bunch of factors like the nature of the exhaust plume and the trajectory of the ship and how massy it is and how fast it is accelerating, etc etc. An antimatter-fuelled photon rocket capable of promptly boosting someone up to relativistic speeds is not something you want pointed at you when it lights up, for example. $\endgroup$ Nov 26, 2019 at 12:00
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    $\begingroup$ @StarfishPrime loved the wit on "really exiting". $\endgroup$
    – Gustavo
    Nov 26, 2019 at 14:01
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    $\begingroup$ The Vulcan science directorate has determined that FTL travel is impossible. $\endgroup$ Nov 26, 2019 at 23:03
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    $\begingroup$ in most circumstances we already know that general and special relativity hold true It'll be like when Einstein showed that Newton wasn't 100% correct about gravity because of relativity - which is why Mercury's orbit seemed wrong. Newton's laws still hold true for the most part - we don't have to worry about relativity, and Newton certainly wasn't thinking about relativistic events at the time. $\endgroup$
    – Baldrickk
    Nov 27, 2019 at 10:44

The first thing that scientists will learn, is that there is a severely blueshifted object near Jupiter. This will be extremely bright for the same reason that a sonic boom is loud. This "luminal boom" will not be noticed by most people on Earth, though.

The blueshift will be far outside of the visible range, well past X-rays, and into Gamma rays. The broad-spectrum gamma ray observatories -- satellites that are tasked with detecting the Gamma Ray Bursts associated with hypernovas, and the double-spikes of gamma radiation associated with nuclear weapons testing -- will be the first to detect the blueshifted light from our FTL ship as it decelerates through the transluminal barrier.

In minutes, the directional gamma ray observatories will be retasked to observe the extremely strange signal... Extremely strange, because it's the only gamma ray source that isn't just a huge, short lived spike of high energy photons at some arbitrary wavelength, but is a long lasting source that is changing its wavelength very, very, very quickly.

If they're lucky, they'll notice the streak of very high energy gamma radiation traveling away from the point where the FTL ship crossed the transluminal barrier, but this signal will likely be completely lost in the huge amount of gamma -- then X-ray photons that the FTL ship is reflecting.

As the ship slows down, we'll be able to better resolve it -- both because it's closer, and because our abilities to focus photons improves as the frequency goes down. It won't be in visible wavelengths until it's at least .25c, though.

Our scientists will know that something is very strange. Some will suggest FTL. Most will say that FTL shouldn't be possible. Unless we notice (and understand) the stream of gamma radiation that's traveling away from the point where the ship crossed the transluminal barrier, though, there will be no proof that the ship ever travelled faster than light, since simply travelling near the speed of light, then decelerating as it nears Jupiter will have the same effect.

Even if instruments do record that telltale FTL echo, it isn't conclusive proof, as the same signal could be produced from a high energy communications pulse being reflected by interplanetary dust, perhaps letting their homeworld know that they've arrived.

As the ship starts to depart our system, it will be severely redshifted, quickly becoming undetectable in the visible spectrum as the photons elongate out to radio frequencies. If it happened to have a clock on the rear, we would notice the clock slowing. Even though the redshift will happen at the same rate that we'd expect a ship traveling at near the speed of light will happen, the brightness will drop faster than we'd expect. This will be another hotly debated but still inconclusive detail in academia.

In all, several people will be convinced that FTL is possible -- a minority of physicists will deeply consider the implications but, unless they can replicate it, the only papers in journals will be theoretical discussions that conclude that more research is necessary.

However, seeing the object accelerate from (presumably) .999g to a relative standstill in the course of an hour, then back up to speed again over the next hour will generate a LOT of buzz. Physics will be turned over. I can't even begin to imagine the technology it would take to shove that much energy into a spacecraft in such a short amount of time without it turning into a stream of highly ionized and definitely-not-spacecraft-shaped dust. Cats-and-dogs-living-together scales of mass hysteria.

  • $\begingroup$ +1 for including blueshift and redshift. $\endgroup$
    – toolforger
    Nov 29, 2019 at 11:38

FTL can be achieved!

This is no pipe dream and providing resources to the investigation of Faster than light travel is now humanity's top priority.

  • All observatories pointing over it means a full spectre of radiation was recorded. From gravitational sensors, microwaves, even the shift on the lunar regolith can hold clues.
  • Fermi Paradox got an answer, we are not alone.
  • Militaries the world over want lasers and weapons able to shoot down such spaceship.
  • Chaos! Religions worldwide claiming an act of his/her/their gods. Churches burning down.

Average Joe will see a ton of Star Fleet paraphernalia and kids will fight over Kirk vs Picard.

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    $\begingroup$ Fermi paradox not answered. If FTL provides time travel (and I don't think we can say either way based on what we see here) the ship could be from our own future. $\endgroup$ Nov 26, 2019 at 11:16
  • $\begingroup$ Said AN answer, not The answer. $\endgroup$
    – Gustavo
    Nov 26, 2019 at 11:21
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    $\begingroup$ Picard, obviously. $\endgroup$ Nov 27, 2019 at 4:03
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    $\begingroup$ Fermi Paradox resolved: there were going to have been lots of alien civilisations, but future us will have gone back in time and broke them all. $\endgroup$
    – Ben
    Nov 27, 2019 at 4:45
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    $\begingroup$ @StarfishPrime Surely it could be from own future only if relativity is correct, and we've just observed strong evidence that it is not, in fact, correct? $\endgroup$ Nov 28, 2019 at 10:44

The most obvious thing that will be discovered from the passage of the FTL spacecraft from Jupiter orbit to close to the Earth will be that the spacecraft travelled that distance at an average velocity of around half-lightspeed. This indicates an acceleration of the order of approximately 9,000 gee. Similarly, it will accelerate away from the Earth at the same acceleration.

This is sufficiently remarkable in and of itself. The power required to accelerate a spacecraft is incredibly huge.

However, the most important aspect of the vehicle's FTL travel is the fact something that is made of bradyonic matter can pass through the lightspeed barrier. Going from, as it has been observed, above lightspeed to below it, and then transition from below lightspeed to above it.

This will imply that two of theories which allow for certain kinds of FTL travel may be possible. These will be the initial theories scientists will investigate in an attempt to explain the phenomenon. Extended special relativity which assumes the existence of tachyons, and the Alcubierre metric which might permit a kind of warp drive. The observations made of the FTL vessel may be able to either distinguish between these two theories or, possibly, discount both of them or give indications of an otherwise novel physical basis for faster-than-light technology.

Theoretical physicists will have the proverbial field day in concocting a whole slew of new theories to explain FTL physics. These will endeavour to reconcile whatever observations and measurements were made of the FTL spacecraft during its flyby of planet Earth, with science as we know it, and to extend it beyond what we do know at present. The querent's post doesn't give a complete enough picture of the FTL spacecraft to determine what could be discovered from observing it. That will determine what its impact will be upon science so massively that it is nigh impossible to predict what the changes to science will be.

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    $\begingroup$ We can also conclude they have something to "cancel inertia" or a really tough material supports ridiculous acceleration. without crumbling $\endgroup$
    – jean
    Nov 26, 2019 at 20:19
  • $\begingroup$ @jean, 9000 g is less acceleration than an artillery shell experiences when fired. We've got electronics that can stand up to that sort of acceleration with no trouble. $\endgroup$
    – Mark
    Nov 26, 2019 at 23:18
  • $\begingroup$ @Mark Maybe for small solid metallic shells, but anything the size of a ship, with moving parts and hollow compartments...that's pretty impossible. $\endgroup$
    – jean
    Nov 27, 2019 at 15:41

Many existing scientific theories would need to be revisited. Any sort of FTL travel would be so fundamentally at odds with known physics; many of its theories (or even laws) would need to be revisited and rethought. There would be an enormous flurry of activity in the academic world due to this one discovery.

The ship's pattern of movement would open lines of investigation. In order for this ship to interact with the local environment, it would need, at least sometimes, to be traveling at a sublight speed. At one moment the ship would appear to be travelling very fast, but still at sublight speeds-- but then the ship would vanish and appear impossibly far away.

Hidden variables. Given this pattern of behavior, scientists would want search for any evidence of hidden variables. For example, it would be of interest to know whether the frequency of disappearances is periodic, and if it is, if it is periodic with respect to time or distance traveled. Inconsistencies in the periodicity would give rise to additional theoretical hidden variables; for example, perhaps they occur more frequently near other objects or fields of radiation, suggesting that the FTL drive relies in part on its immediate environment to operate.

Environmental side effects may indicate some clue as to how it operates. For example, perhaps the ship does not interact gravitationally with nearby bodies, or light reflected from its surface has unusual characteristics, or it releases a radio burst each time it disappears.

Does it "jump? Of considerable interest would be whether there are any environmental effects evident in the space between where it disappears and reappears. A complete absence of any environmental effects in these areas would suggest the FTL drive relies on some of of "jump" or "skip" effect, in contrast to continuous movement that is simply too fast to witness except by its trail.

The shape of the vessel may give several sorts of clues:

  • Is the vessel occupied by biological life?

  • How could they possibly survive under such extreme acceleration or in the presence of exotic physical effects?

  • Is there evidence of shielding, or are the engines houses in an outboard structure of some kind?

  • Does the vessel carry fuel, or collect it?

  • Is it driven by thrusters of some kind, or operate a sail?

Historical or mythological implications. Is it possible the vessel has been by the Earth before? Is it shaped like any known mythological create or archeological symbol?

  • $\begingroup$ Nice answer. However, the laws of physics won't need to be complete rewritten. There are several well-established physical theories that have potential superluminal effects. Special relativity itself, general relativity, string theory, and potentially some versions of quantum gravitation. Currently there are no experimentally verified superluminal results. Effectively there's nothing to be theoretically explained. If FTL effects were observed, the theories will follow. $\endgroup$
    – a4android
    Nov 27, 2019 at 12:16
  • $\begingroup$ All of them? Even Newton's first law? $\endgroup$ Nov 27, 2019 at 17:20
  • $\begingroup$ @user253751: Revisited, not corrected. "Yup, checked it, that one still holds." $\endgroup$ Nov 28, 2019 at 9:58

It's been said that you can have relativity, causality, or FTL. Pick any two.

We have numerous things that require that relativity be true(one example being the GPS system), thus causality cannot be true. This then implies that time travel is also possible.

The problem with a new scientific theory that implies FTL is that it must also explain all our current understanding of physics as well. This is known as the correspondence principle, and also applies to the thermodynamics problem of stealth spaceships.


Regarding first detection and the telescope observations:

As someone before me pointed out, us detecting a sensible sized ship at Jupiter's distance is next to impossible without some heavy immediate clues. We indeed don't have that good knowledge of stuff flying around there. Luckily there are plausible clues!

There are automatic alarm mechanisms and equipment in space to detect a few types of short-lived interesting events. One of the best for this purpose are gamma flash detection satellites. They were first developed and deployed to enforce ban of atmospheric nuclear testing. They made detections, which turned out to be of astronomical origin, and so we found gamma bursts. Your ship exiting FTL speeds might emit such a signal.

Another more modern - and in that sense cooler - prospect are the gravitational wave detectors! Similarly, we could see a signal there that is either strange in shape, or strength and that pokes curiosity. With current detectors we can already manage some poor direction finding, but with some more detectors around the world, and improving sensitivity, it is already estimated that we will soon be able to point exactly where a gravitational wave came from

Additionally there is the point that if there is any fleeting chance of fitting FTL to our current understanding of physics, it involves excessive abuse of general relativity, and gravitational signatures would be more than expected of such exploits.


Let's take a step back, because it is much easier to imagine people less smart and knowledgeable:

You barely managed to get your stone axe mostly fixed to that piece of wood and just then see a bird fly by. What would you learn from that event? Almost nothing useful - that flight is possible for birds, and that's about it.

The same would happen here. Assuming ship gradually continuously accelerates from 0 to c (and beyond), then yeah, it might be the simplest explanation is that FTL is indeed possible, winning against competing "they have super-duper broadband cloak they turned on just when they reached almost c". But beyond making a huge mess in physics, best case nothing would really change for science.

For example, LHC results might be possibly useful to laymen in several decades. Or perhaps never. We shall see. Cosmology and particle physics are more about pushing boundaries of knowledge than anything useful - these are experimental math. And these are results we have managed to produce ourselves (so we knew what to search for, where is the boundary and so on), unlike an one-off event.

What could change is that we now know others have FTL and know how to get to our solar system. Maybe leading to a lot of money for science and defense. But this would be a political development, not a scientific one.


Assuming that coming out of FTL is a pretty energetic event, what you'd first detect is probably a massive(?) gamma ray burst, so that's the context in which the observations will happen. We don't expect GRBs to move, so by the time the source of the GRB is pinpointed in the sky and optical telescopes repoint themselves to the indicated coordinates, they'll find nothing.

Optical sky survey telescopes might pick out the object at some later time (depending on how big or luminous it is), but if it still moves much faster than the orbital velocity of more or less anything in our solar sytem (e.g. >1000km/s), it's unlikely it will be flagged up as a single moving object at first, in fact it might be discarded as a bad pixel.

It will take years if not decades for someone to piece the puzzle together, the strange GRB without any optical counterpart and the systematic "bad pixels" in subsequent survey photos, and chances are they won't publish it anywhere because it sounds so crazy.

p.s.: Of course the above scenario isn't what would definitely happen, like most sci-fi authors, I cheated. I started out with my own (not too flattering) view of human society, and worked my way backwards from there.

p.p.s: It is also feasible that such an event also generates gravitational waves, which combined with the GRB would point towards a black hole merge scenario, except it would very quickly become apparent that the waves don't match the pattern we know about the mergers. It could be this anomaly that sets someone off to check the sky survey photos.


One very important discovery would be: Not only are we alone in the universe, but an alien civilization definitely knows we exist.

Traveling "just outside the lunar orbit" almost certainly puts the ship within the sphere of influence of the planet. Given the great distance traveled, the incredible speeds, the fact that it decelerates and accelerates at the planet, and the tiny sphere of influence of the planet, the people of the planet will be able to be absolutely certain that the ship was sent to observe the planet.

This will cause a huge increase in SETI-like activity, as well as a huge increase in planetary defense spending, and spending on looking for exoplanets and general astronomical observation.


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