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Linked but not a duplicate of this question.

An example of the problem

During the last Star-Wars movie, I was dumbfounded by the sheer stupidity of admiral Holdo's move, during this particularly visual scene:

Admiral Holdo turning her ship in a superluminal bullet.

While most people I know don't seem to realize how incredibly reckless this was, I'd like to offer one of my favorite quotes ever to summarize the situation:

Gunnery Chief: This, recruits, is a 20-kilo ferrous slug. Feel the weight. Every five seconds, the main gun of an Everest-class dreadnought accelerates one to 1.3 percent of light speed. It impacts with the force of a 38-kiloton bomb. That is three times the yield of the city buster dropped on Hiroshima back on Earth. That means Sir Isaac Newton is the deadliest son-of-a-bitch in space. Now! Serviceman Burnside! What is Newton's First Law?

First Recruit: Sir! An object in motion stays in motion, sir!

Gunnery Chief: No credit for partial answers, maggot!

First Recruit: Sir! Unless acted on by an outside force, sir!

Gunnery Chief: Damn straight! I dare to assume you ignorant jackasses know that space is empty. Once you fire this husk of metal, it keeps going till it hits something. That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years. If you pull the trigger on this, you're ruining someone's day, somewhere and sometime. That is why you check your damn targets! That is why you wait for the computer to give you a damn firing solution! That is why, Serviceman Chung, we do not "eyeball it!" This is a weapon of mass destruction. You are not a cowboy shooting from the hip!

Second Recruit: Sir, yes sir!

(Emphasis mine - quote from an anonymous human military on the Citadel in Mass Effect 2)

So, for those who've been sleeping in the back of the classroom during their physics class, Holdo accelerating a ship at a speed faster than light and ramming it into another ship is the equivalent of a cosmic shotgun. (Hence the pretty little light streaks scattering in a cone from the impact point and instantly destroying Star Destroyers).

A single bolt of 1g from this ship now has a bare minimum kinetic energy of $E = 0.5 \times m \times C^2 = 0.5 \times 0.001 \times 9 \times 10^{16} = 4.5 \times 10^{13} J$. 1

(A kiloton is $4,184 \times 10^{12} J$, so the absolute minimum energy on impact, if physics didn't break at that point, as explained in comment is around 10 kilotons. Hiroshima's Little Boy was estimated between 12 and 15 kilotons).

To be honest, I'm not even sure this law holds at speeds higher than light, and I don't have the theoretical knowledge to even make an educated guess.

Given that some several-years travel at the speed of light could be done in mere hours in the SW universe, I'd posit that those numbers are way higher, and each bolt (not even talking about chunks of the ship) are now delayed orbital strikes aiming god knows where.

At this point, you might as well charge the Rebellion for crime against the universe. No wonder why the Yuuzhan-Vongs paid us a visit.

Problem

The other question explains perfectly the problem. As soon as your universe includes FTL travel, an idiot somewhere is gonna make this mistake (on purpose or not) and a lot of people are gonna pay for it.

Following the example above, I struggle to create any universe with FTL travel, because each war would mean I'd have to wipe half of the celestial map.

A few propositions to protect a planet or a system from this kind of incident includes: giving the person using FTL the means to avoid said incident, trusting them to understand the risks, pre-emptive strike, or (my favorite) clouds of space dust.

Now, the two first answers are made irrelevant by idiocy. Holdo knew the risks and had the computers telling her not to do it. By hubris, despair or idiocy, someone in a space battle will end up pushing the red button. (You don't even have to sacrifice a pilot. Guided FTL rockets are the end-game)

Pre-emptive strike seems a bit radical. While they're targeted and shouldn't cause collateral damage, you can't just destroy every planet where FTL "might" happen.

The cloud of dust is useful to protect a single system or planet, but there is no way to effectively shield the universe, unless you want to fill all empty space with space dust.

Is there any way to devise a universe with FTL travel without realistically condemning half of said universe to utter destruction by FTL strikes? (Not asking how to shield a sole planet / a sole civilisation from a dumb accident)

  • Note that any reactionary counter measure suggests you know that a danger is coming your way. The problem of FTL is that the danger travels faster than the information. You'll "see" the explosion way after the upper deck of the destroyer tore through your planet. And the one behind. And the one behind the one behind. (But maybe quantum entanglement can help. I've read somewhere about research being done on the topic to communicate faster than light, but I don't understand the principle behind it).

  • To clarify what I'm asking for, I'm looking for references of universes with non-destructive faster than light space-travel, effective countermeasures covering the universe, or anything that allows you to write a story including both FTL and idiots without dramatic consequences.

1 Fixed thanks to elPolloLoco's answer. Don't do maths absent-mindedly during lunch break without double checking the data.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch Feb 7 at 10:38
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    $\begingroup$ “To be honest, I'm not even sure this law holds at speeds higher than light”—it does not: any massful particle requires infinite energy to get at the speed of light, and there isn't more than infinite energy available for accelerating it further. $\endgroup$ – Jan Hudec Feb 7 at 22:51
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    $\begingroup$ There are a number of problems with this question, though the leading two answers offer reasonable outs for a Worldbuilder. However, the biggest problem is a substantial misapprehension of scale, that any worldbuilder should correct before proceeding. You see Space is big. Really, really, really big. So big that the numbers that you site are nigh-trivial in comparison. Consider a supernova, they are many orders of magnitude larger, more energetic and more dangerous than any gun made by humans. But how much of your world, is gone because of supernovas? Add 0.01% onto that. $\endgroup$ – RBarryYoung Feb 8 at 2:46
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    $\begingroup$ To elaborate on what @JanHudec said, special relativity has it at $E=m \frac{1}{\sqrt{1-v^2/c^2}} c^2 - mc^2$, so the energy goes up asymptotically as you approach the speed of light. $\endgroup$ – AJMansfield Feb 8 at 4:48
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    $\begingroup$ You have much bigger problem anyway. Whatever energy your ships have must have been provided by the engines. If you have engines capable of providing that sort of energy, there are many more creative ways of releasing it in a very destructive manner. $\endgroup$ – Jan Hudec Feb 8 at 6:52

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But in most stories, FTL doesn't actually follow Newton's 1st law; everybody knows that once your hyperdrive fails, you're dead in the water. Otherwise, the Enterprise could hit warp 9.9 and easily coast across the galaxy.

And that actually makes sense in the warp-style of FTL: FTL travel is only possible within the field generated by a functioning FTL drive. In fact, within the reference frame of that field, matter still isn't moving faster than light! Once the drive is destroyed, the field collapses and that matter resumes its non-FTL velocity.

But regardless, there are many other FTL concepts that operate completely differently. See Hyperspace, Jump drives, Jumpgates, and more in Wikipedia's scifi section.

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    $\begingroup$ Yes! With this type of setup, what you have is a ship approaching an object at whatever it's "impulse speed" is, but wrapped in a bubble/field/whatever that changes the space around it. When any other object (another ship, planet, etc) enters that bubble/field and collides with the ship, the speed of the collision is the same as it would have been if neither of them were in the bubble at all, nothing more than "ramming speed". Could still cause massive local destruction of the two colliding objects, but nothing like a relativistic shotgun $\endgroup$ – Dalila Feb 6 at 16:59
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    $\begingroup$ A lot of those other FTL concepts operate similarly. You travel FTL in a different layer of subspace (or w/e), not in real space. Or through a temporary, artificial wormhole (or equivalent). All real space travel is sublight (and usually quite far below light speed). So the weapon potential is limited to dropping a package behind enemy lines, not ramming. $\endgroup$ – jaxad0127 Feb 7 at 1:40
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    $\begingroup$ As a matter of fact, warp bubbles solve nothing, only change threat profile. What happens when someone flies through a planet or star? I can tell you because star trek styled warp drive is not prohibited by general relativity and some researchers looked into that: grossly simplifying, matter scooped up by bubble is converted into radiation and released at destination. We are talking supernova type event. $\endgroup$ – M i ech Feb 8 at 13:42
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    $\begingroup$ "matter scooped up by bubble is converted into radiation and released at destination" Converting 100% of the mass of a single atom into light would release such a massive amount of energy... Every speck of dust you fly through would turn into an ultra death laser. I'd like to read that research, if you have a link. $\endgroup$ – BoomChuck Feb 8 at 23:05
  • $\begingroup$ Explained succinctly here in Futurama. $\endgroup$ – eyeballfrog Feb 10 at 1:10
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1. Space is mostly empty

If you pull the trigger on this, you're ruining someone's day, somewhere and sometime

Well, not necessarily. There's so much empty space between anything interesting in the universe, that it's very likely that those "shots from a cosmic shotgun" will just keep traveling through the cosmic void, never even approaching anything worth considering.

Additionally (as @DanDryant pointed out), objects traveling faster than the speed of light can escape any gravity well, and their trajectories wouldn't even change much when passing through one. This means that superluminal objects are less of a risk compared to "slow" moving objects - the first need a direct hit on their first and single pass through a system, while the latter can get caught in the gravity well - going around and around, increasing their odds of hitting the massive object in its center or some other objects caught in that well.

If you are specifically worried about space battles, where missing the tiny enemy fighter ships can mean bombarding the planet you are defending (or its sun!), just have these battles typically occurring far away from planets. From a worldbuilder's perspective, you can achieve this by two different strategies, used independently or combined:

2. Defensive technologies or circumstances

Intelligent races will just have to invent and deploy defensive technologies to survive the "superluminal idiots" of the universe.

Just as the Chinese Empire built The Great Wall of China to surround their entire territory. Any FTL capable civilization will need some sort of defense:

  • Planetary shields that block, disintegrate or teleport any unauthorized/unexpected approaching objects.
  • Scattering artificial "cosmic dust" / interceptor drones / etc. around important systems / sites / stations - anything that can absorb or dampen FTL projectiles.
  • Planetary or system-wide Warp Inhibitors, slowing down anything (or anything unauthorized) traveling towards it.
  • Strategic positions (inside nebulae, next to black holes etc. etc.) making any FTL dangerous - these can help even non star faring races to survive.

3. Limitations or variants on real-space FTL

As the worldbuilder, you can tweak physics or the nature of FTL so it is more difficult to weaponize, at least against settled planets:

  • No FTL next to gravity wells - this was used widely in Larry Niven's Known Space universe. FTL only works on relatively "flat" space - forcing space battles to commence at civilized speeds or far away from solar systems.
  • FTL is warp based rather than simply traveling very fast in real-space (that's the gist of Boomchuck's answer). This provides ships the ability to travel astronomical distances in reasonable time without gaining an insane amount of kinetic energy. It also means that debris of an FTL collision won't be traveling at superluminal speeds (though they can still be dangerously fast...). A typical trope of this approach demands an intact "warp bubble" around the FTL ship - which is dispersed if it interacts with another bubble (and or gravity wells etc.).
  • FTL is teleportation based. Your interstellar civilizations use stargates or hyper-jumps to skip from A to B without traversing the distance between them.
  • FTL is done in Hyper space rather than real space - a ship slips to another dimension, where it cannot interact with anything in this dimension, and where the physics are different to allow fast traveling (or distances are different there - same thing really). Ships may each have their own exclusive variant dimension, or they may interact with other hyperspace travelers (possibly only using a compatible technology / frequency etc.).

etc. etc. etc.

Finally, keep in mind the principle sometimes called the Kzinti Lesson:

"A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive."

If you have a functional interstellar drive, it will be very difficult to completely prevent its weaponization.

Atomic Rockets have an entry about propulsion systems in their exotic weapons page - it's really worth reading.

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    $\begingroup$ It also helps that if you're accelerating things to significant fractions of the speed of light, you're pretty much guaranteed to exceed escape velocity for the system, regardless of the direction of your velocity vector. The only thing you need to avoid is a direct hit, which is incredibly unlikely thanks to the Space is Really Big situation. And, even if you're fighting around a planet, any velocity you add is presumably going to be in addition to your existing orbital velocity, so you're unlikely to hit the planet unless you deliberately try to. $\endgroup$ – Dan Bryant Feb 6 at 20:26
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    $\begingroup$ How empty is the space? Well the density gives it away: ~10^-29 g/cm3. Density of lowest density element is hydrogen with ~10^-4 g/cm3. So basically your chances of hitting something when fired in a random direction in open space is 10^-25. Of course if you are firing towards a cruiser orbiting a planet, that is a different story. $\endgroup$ – Cem Kalyoncu Feb 6 at 21:46
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    $\begingroup$ The Kzinty Lession reminds me of Maxim #24 from Schlock Mercenary: "Any sufficiently advanced technology is indistinguishable from a big gun." $\endgroup$ – Mason Wheeler Feb 6 at 22:03
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    $\begingroup$ Luckily for us, ancient colonies on Earth deployed a planetary shield that's still functional today. We now call it "the atmosphere". $\endgroup$ – Vaelus Feb 7 at 2:17
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    $\begingroup$ +1 for mentioning Atomic Rockets. Anyone asking a question about space should be force to read Atomic Rockets first. $\endgroup$ – Tom Feb 7 at 8:53
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First of all, this is wrong: "A single bolt of 1g from this ship now has a bare minimum kinetic energy:

$$E = m \times C^2 = 0.001 \times 9 \times 10^6 = 9 \times 10^3 J$$

Kinetic energy is calculated as $0.5mv^2$

$v$ being $c = (3 \times 10^8) v^2 = 9 \times 10^{16}$

So you get

$$E = (10^{-3}) \times 9 \times (10^{16}) \times 0.5 = 4.5 \times (10^{13}) J$$

But it doesn't really matter does it?

Now here is the thing: This formula is not suited for speed close to $c$. That is because close to the speed of light, the mass of the object changes.[Edit: the mass of the object does not change, the formula is still useless at speeds close to c, reasons to be found in comments]

Maybe you once heard that it takes an infinite amount of energy to bring an object with mass to the speed of light? Using this formula, it wouldn't.

Now let's get to your problem. Obviously having all kind of people flying WMDs is kind of problematic and wouldn't do the galaxy any good. FTL by just flying really fast is already a violation of physics, so just leave it out.

You need your characters to be able to move between starsystems or even galaxies in little time? Give them some kind of warp- engine or wormholes or some other magic travelling aid that moves them from A to B without accelerating them and so without putting insane amounts of energy into them.

Edit: See the Ender series for example. I think FTL is introduced in "speaker of the dead" or "Ender in exile".

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    $\begingroup$ I looked for the value quickly and the final result seemed me weak. Thanks for pointing out the mistakes $\endgroup$ – Nyakouai Feb 6 at 13:04
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    $\begingroup$ "Now here is the thing: This formula is not suited for speed close to c. That is because close to the speed of light, the mass of the object changes. Maybe you once heard that it takes an infinite amount of energy to bring an object with mass to the speed of light? Using this formula, it wouldn't." I remember reading your mass increases when nearing C, but I never learned why. So assuming your bolt of 1g now has an exponentially increased mass and is travelling faster than C, as far as I understand it, the kinetic energy is way off the charts tby breaking laws of physics). $\endgroup$ – Nyakouai Feb 6 at 13:24
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    $\begingroup$ Me neither. It is being discussed on physics stockexchange: physics.stackexchange.com/questions/1686/… There is no faster than c traveling. To accelerate a body with mass to exactly c, the amount of energy needed is already infinite $\endgroup$ – elPolloLoco Feb 6 at 13:47
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    $\begingroup$ In the Ender series, FTL is introduced in "Children of the Mind". It's an ability that Jane "learns", IIRC $\endgroup$ – Brian Feb 6 at 17:27
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    $\begingroup$ FWIW the formula for kinetic energy, which applies even at speeds close to that of light, is $mc^2(1/\sqrt{1 - v^2/c^2} - 1)$. In this formula, $m$ (the mass) is an intrinsic property of the object and does not change depending on its speed. $\endgroup$ – David Z Feb 6 at 21:10
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Is there any way to devise a universe with FTL travel without realistically condemning half of said universe to utter destruction by FTL strikes?

My personal favorite answer to this is E.E. "Doc" Smith's FTL approach from the Lensman series: Inertialess drives.

Basically, you turn the inertialess drive on and immediately assume the velocity at which all forces acting on your ship cancel each other out. If you have a reasonably-powerful engine pushing in one direction, this will mean moving at an extremely high multiple of the speed of light, because that's what it will take for the drag from interstellar gases to match the power of your engine. But, if you run into a planet, you stop instantly with no impact effect, because the resistive force of the planet is enough to cancel out your engine's thrust.

This also makes most projectile or energy weapons useless when used from a lone attacker, as the impact of the attacker's guns or the light pressure of their lasers will simply push the defender away with no damage inflicted. The only effective means of space combat is for large fleets to form up as cylinders, cones, or spheres around the enemy and crush them with simultaneous weapons fire from all directions.

When the inertialess drive is turned off, you will immediately resume your original vector.

Although this eliminates the problems of superluminal impacts and accidental planetary devastation, it still allows for deliberate planet-killer attacks, such as finding another planet moving on the opposite vector, taking it inertialess, and inserting it into the target world's orbit. The kinetic energy of a 20kg slug at 0.13c is peanuts compared to a head-on collision between two planets at typical orbital velocities. And, if that's not destructive enough, in the later books of the series, they start finding antimatter planets to do this with because, sometimes, there's no such thing as too much overkill.

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    $\begingroup$ Ahem. ITKM There is no "overkill." There is only "open fire" and "reload." $\endgroup$ – Martin Bonner Feb 7 at 13:17
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    $\begingroup$ @MartinBonner More applicable is Rule 24: Any sufficiently advanced technology is indistinguishable from a big gun. $\endgroup$ – Ray Feb 7 at 13:55
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    $\begingroup$ and not just antimatter planets (that they create.) The ultimate overkill is they find planets from an alternate universe where the light speed barrier is reversed (ie. all "normal" matter is superluminal.) They drag a couple into normal space, and drop them inert (ie not under inertialess drive) aimed at a planet (and its sun) so they get by a planetary-mass superluminal projectile each. (hilariously, it;s noted that even this wasn't enough, as the big bad guys would have been able to duplicate the weapon within weeks, and then that would have been all she wrote.) $\endgroup$ – Wenlocke Feb 8 at 16:55
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    $\begingroup$ This was going to be my answer. The fact that an inertialess object just stops when any outside force is exacted upon it is beautiful. Though, as you point out there are so many other dangerous shenanigans you can get up to with Inertialess drive. $\endgroup$ – ShadoCat Feb 8 at 18:59
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    $\begingroup$ There's No Kill Like Overkill $\endgroup$ – Bob Jarvis Feb 9 at 3:18
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Kinetic Energy at High Velocities

If you design a universe that permits faster than light travel, you will need to invent laws for how kinetic energy works when traveling faster than light.

For centuries, we used the Newtonian formula for kinetic energy: $KE = \frac{m v^2}{2}$. And that worked well. It isn't wrong; it's just an approximation that only holds for velocities well below the speed of light.

But for high velocities, we need to use Einstein's version instead: $KE = \frac{m c^2}{\sqrt{1 - v^2/c^2}} - m c^2$. (m is the rest mass in that formula, not the relative mass.)

We could also use this for low velocities if we wanted to; it's approximately equal to $\frac{m v^2}{2} + \frac{3m}{8}\frac{v^4}{c^2} + \frac{6m}{16}\frac{v^6}{c^4} + ...$, so all terms except $\frac{m v^2}{2}$ will be very close to zero. But for that same reason, we don't need to bother with any term except the first when $v << c$, so it'd be overkill. We instead use the simplest approximation that works well at the sort of speeds we're looking at.


Now let's look at what those equations mean in practice. Suppose we have a 1 kg mass and see how much energy these predict at various speeds.

Low speed: 10 m/s

If it's moving at 10 m/s, then using the Newtonian approximation, we get a kinetic energy of $50 J$.

If we use the Einsteinian version, we get $50.00000000028 J$. Both versions are exceedingly accurate, and so we might as well use the simpler one.

Relativistic speed: 0.99c

The Newtonian approximation gives us $3.64 \times 10^{16} J$.

The Einsteinian one gives us $2.58 \times 10^{17} J$.

The Newtonian version is eight times too small; at these velocities, we can no longer use that approximation.

FTL speed: 2c

The Newtonian approximation gives $1.8 \times 10^{17} J$, but we don't care because we've long since passed the velocities where it's remotely accurate.

The Einsteinian approximation gives us $-1.038\;i \times 10^{17} J$. Since our kinetic energy is now both negative and imaginary, it's safe to say that this approximation is no longer accurate (or even meaningful) at faster than light velocities. (It's not even defined at exactly light speed; there's a division by 0.)


In our universe, the Einsteinian equation may not be an approximation; it may be completely accurate at all velocities for all we know. But if FTL is possible in your universe, than it must be an approximation that is only valid at low speeds.

I can't tell you what equation you should use for calculating kinetic energy at FTL speeds; I can just say that the one you're using will not give accurate results in our universe, and the best approximation known to modern science for our universe doesn't even permit FTL, so that won't be right, either. Decide how you want kinetic energy to work at FTL speeds, and then be consistent.

Energy Scale

You don't actually have to make up a new equation if you don't want to, though. You can safely assume that Newtonian physics hold at any velocity in your universe without creating the sort of problems that you're envisioning.

$10^{13} J$ may sound like a lot, but that's by puny Earthling standards. We've tested nukes that are 20,000 times larger than that, and we're all still here.

Let's consider something with a bit more energy than a mere hydrogen bomb. Suppose that Newtonian physics holds at high speed, and that we've got a ship the size of the Titanic moving at light speed. It would have a kinetic energy of $2.337 \times 10^{24} J$. That's a decent amount of energy, but it's 100 times less than the Sun outputs every second, and 100 million times less than what we'd need to blow up the Earth. (Which in turn means that the scene you mention was not the most energetic event we've seen in Star Wars.)

To put things into serious perspective, a supernova can output $10^{44} J$. That's $10^{20}$ times more than our ship's kinetic energy. Which is to say, our ship colliding with something outputs at least 100,000,000,000,000,000,000 times less energy than something that happens in the universe on a fairly regular basis.

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  • $\begingroup$ You need to correct Einstein's version of the KE equation above. Where it should read "mv^2" it reads "mc^2". It's a simple typo, we all make them. The good thing is they're easily fixed. Otherwise it's a great answer. Plus one for putting numbers into FTL physics. $\endgroup$ – a4android Feb 7 at 3:20
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    $\begingroup$ @a4android $mc^2$ is actually correct. $m$ is the rest mass, so it should more properly be displayed $m_0$, but I left it as $m$ so as to more easily compare it to the Newtonian version. $m_0/\sqrt{1-v^2/c^2}$ is the relative mass. $m_0/\sqrt{1-v^2/c^2} \times c^2$ is the total mass energy, and $m_0 c^2$ is the rest mass energy, which we subtract from the total to leave just the kinetic energy. Thus, $m_0 c^2/\sqrt{1-v^2/c^2} - m_0 c^2$. ($mv^2/2$ emerges in the Taylor expansion of that formula.) (source. chapter 15) $\endgroup$ – Ray Feb 7 at 3:44
  • $\begingroup$ Your clarification is much appreciated. May I suggest editing your answer, but specifying that the mass is the rest mass. That would improve it and not confuse dumbos like yours truly. $\endgroup$ – a4android Feb 9 at 8:06
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Drag

Space is big and very very empty... but it isn't totally empty.

There is a fine mist of dust and gas and atoms between the stars.

It's not much. The cold depths of interstellar space have a purer vacuum than almost anything you'll find in a lab on earth. But if you go fast enough it really matters.

In the solar system with the solar wind the density of atoms is 2x10^7 per cubic meter, mostly hydrogen or helium.

Outside the solar system it varies quit a bit by temperature and charge but if you take a steel ball and throw it into the cold dark night...

Lets see what happens

if you hit something, easy (scaled down to keep the photographer alive)

fire in the hole

But what if you miss...

The 20-kilo ferrous slug leaves the ships railgun at 1.3% of the speed of light.

The slug is pure iron.

The sphere has a radius of 8.464 centimeters.

cannon ball

It's going at 1.3% of the speed of light so lets wait about 1.3 light years away and see what it looks like when it arrives.

Hmm. It's a little bit late... not very late but a little bit...

In order to reach us the sphere has had to pass through 2.768×10^20 cubic centimeters of "empty" space

A Cold Neutral Medium (CNM) in interstellar space has about 20-50 atoms per cubic cm mostly hydrogen or helium. We'll assume 25 hydrogen atoms per cm^3 to make this easier.

So while it's traveled it's hit (2.768×10^20 *25) atoms of hydrogen... about 11.58 mg (milligrams) of hydrogen.

Assuming it doesn't hit a grain of sand long the way and turn itself into a cloud of atoms.

That's not a lot... but by the time the slug has traveled 2,245,230 light years it's struck about 20kg of hydrogen atoms. Notice that that's it's own weight in hydrogen.

That's a long way, almost the distance to Andromeda.

So by then it's going about half it's original speed and some of the iron has likely ablated away from those little impacts.

The universe doesn't fill up with high speed debris because every piece of debris experiences a tiny and subtle drag from hitting the fine mist of atoms between the stars.

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    $\begingroup$ You managed to make an absolutely coherent answer with a really good explanation as why the problem isn't one while missing the significant bit about the projectile being FTL and not the slug of the quote. (Though I'm curious to know if the answer differs at speed higher than C since laws of physics get messy/do not exist) $\endgroup$ – Nyakouai Feb 8 at 13:22
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    $\begingroup$ @Nyakouai projectiles can't be ftl. It's like saying a rock is going "slower than stopped". Other issues include the problem that anything that allows something to go FTL is automatically a time machine as well. Even in most scifi, you need a magi-teck FTL engine to keep going FTL. The debris from any impacts or explosions will always be limited to slower than light because they would require infinite energy to send them flying out from the explosion faster than that. Any object thrown out then experiences the same issue as the slug. $\endgroup$ – Murphy Feb 8 at 13:29
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    $\begingroup$ @Nyakouai If you edit the laws of physics to be Newtonian rather than Einsteinian with no speed limit at the speed of light, no relativity and no time travel... then the same logic applies. it still hits dust and a fine mist of atoms and gradually slows while it crosses the universe. $\endgroup$ – Murphy Feb 8 at 13:33
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tl;wr: Based on Star Wars canon: Gravity sucks you out of hyperspace.


At least in the written Star Wars universe it is canon, that while you may travel faster than light (no details known on how that works), ships can not fly through gravity wells. Meaning, before a ship travelling FTL hits a sun or a planet, it gets sucked out of hyperspace and has to travel STL. The same should go for any matter/particles/shipwreck parts which are propelled to FTL by any possible freak "accident".

Ships are usually not large enough to generate a gravitic field of their own, so basically after disabling all security measures etc. Admiral Holdo would have been able to execute the pictured manoeuvre, resulting in an at least close-to-lightspeed blast of shrapnels, shredding half of the fleet. And while that shrapnels may race through space for a very long time and distance, at least planets are rather safe from them, due to their solar systems gravity wells. Any debris would slow down to anything STL and at least partially burn in the atmosphere. Sure, there could still be some damage, but the total number of freak accidents and thus shrapnels is to be estimated rather small. The lone ship in space could get unlucky though, to be honest.


Thought on the side: It would be interesting to know, whether the Death Stars were massive enough to create their own gravity field.


The movies are - sadly - another topic. At least in Episode VII Disney wrecked the canon when the Millenium Falcon travelled through hyperspace right above the surface of Starkiller Base, below its energy shields. A jump actually impossible since the gravity should have sucked the Falcon out of hyperspace long before it got there. I can't cite numbers here, but I estimate that, when it comes to planets you have to be at least in a geostationary orbit or higher in order to be able to jump into hyperspace - meaning, you'd be about the same distance from the planet, when you get sucked out of hyperspace.

Suns are even another topic, since they are even more massive and have very large gravity wells. E.g. in either one of the Thrawn Trilogy books or one of the X-Wing-series books a New Republic spy with an Imperial background gets kind of pressed back into Imperial service, placing them on the command ship of either Thrawn or an Imperial Warlord. They manage to delete some navigational data, leading to the Star Destroyer being sucked out of hyperspace by a suns' gravity and stranding the ship effectively for several hours or even days, throwing off the imperial time plan and thus saving the day.

Also Grand Admiral Thrawn himself makes very good use of the few "Interdictor" ships he controls. Those are star destroyers equipped with powerful gravity well projectors, able to pull ships from hyperspace. Sure, you have to know where the enemy will pass through, but the man was brilliant and thus several times trapped unprepared New Republic convoys or whole fleets right before the guns of his fleet, the most prominent - and also last - example being the battle of Bilbringi, during which Thrawn is murdered by one of his alien bodyguards.

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    $\begingroup$ "At least in the written Star Wars universe it is canon" isn't that not canon any more? $\endgroup$ – VLAZ Feb 7 at 9:25
  • $\begingroup$ One movie contradicts it, the others don't really speak about it. $\endgroup$ – Erik Feb 7 at 10:51
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    $\begingroup$ When Disney acquired the Star Wars license, they declared the Expanded Universe materials non-canon and rebranded them as "Legends". In-universe they would be the equivalent of hearsay or "my friend's third cousin's aunt once heard". So the only "true" materials right now, as far as SW universe is concerned, are the movies. $\endgroup$ – VLAZ Feb 7 at 11:31
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    $\begingroup$ Sounds like politicians declaring climate change isn't real. $\endgroup$ – Erik Feb 7 at 11:45
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    $\begingroup$ Nothing really like that. $\endgroup$ – VLAZ Feb 7 at 11:45
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It's not "faster than light", it's "shorter than space"

If your FtL drive functions by moving your ship outside of normal space, and then back into normal space some distance away, then countermeasures are easy: any device that prevents reentry into normal space makes that space impregnable to FtL intrusion.

Schlock Mercenary

In the earliest days of the comic, FtL travel was done using Wormgates - enormous portals that ships could move between instantaneously. In this case, the energy cost of wormholes increased exponentially with the size of the wormhole, meaning that any wormhole large enough to transport a ship required its own infrastructure - hence the wormgates.

Then the protagonists invented a new FtL system called the teraport. It functions by creating a ridiculous number of tiny wormholes, and then pushing an object's individual molecules through the wormholes and reassembling them on the other side. (Hence the name 'tear-apart' 'teraport'). Because of the way the energy costs scaled, the teraport was extremely energy-efficient compared to a wormgate, and a device capable of teraporting a person could be held in your hand.

This immediately revolutionized warfare, because it turns out that being able to drop a bomb anywhere you want on your enemy's ship (or house) makes a battle very short. Of course, they can do the same...

The Teraport Wars came to an end with the invention of Teraport Area Denial, which projects a field which prevents the opening of the tiny wormholes that the teraport uses. (higher energy wormholes can punch through a TAD field, but the whole reason the teraport is usable is because of how low energy the individual wormholes are). And since the teraport process requires ripping the objects being moved into infinitesimal chunks and then reassembling them, interrupting the process can be rather messy for those involved.

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    $\begingroup$ +1 for quoting a fundamental principle from Gallifreyan transdimensional engineering. $\endgroup$ – A. I. Breveleri Feb 11 at 7:58
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One of the quirks of travelling at such speeds in "normal space" is that a stationary rocks are effectively travelling at that speed relative to you as you crash into them.

That's true of any particle or object between your point of departure and your point of arrival. What's important to note is that physics doesn't care whether I punch you in the face or you headbutt my fist, the outcome is the same.

In other words, in a universe with normal space ftl, it is essential to have some easily accessible form of resistance to simple objects at relativistic speeds. What this means is that the mass driver described in the question is a useless weapon in ship to ship combat as it wouldn't even reach the same level of damage as hitting a rock on your way to the battle.

Since ships tend to be aimed at planets and boosted up to such speeds, it's perhaps not unreasonable to shield your planets as well.

You don't need to worry about species that don't think of this, or don't have a solution to it, as they'll either never make it out past their own Oort cloud or blow up their own planet long before they much trouble the rest of us.

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    $\begingroup$ In Isaac Asimov's novel Nemesis anything travailing at > c has the sign of g flipped. So FTL objects repel rather than attract. $\endgroup$ – JGNI Feb 7 at 9:30
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    $\begingroup$ @JGNI, but gravity propagates at the speed of light so that wouldn't affect objects in the ship's path $\endgroup$ – Separatrix Feb 7 at 9:31
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Since no one answered it I'll go with the FTL answer from a special universe you might be acquainted with: our own.

The Alcubierre drive is derived from the equations of Relativity, and would allow the object to travel faster than light without all the problems that would normally entail. Below you can first find a publication with the benefits quoted in this answer, but I suggest you read the article as well. Below that is a video of PBS Space-time about the subject, I dont know how reliable they are but apparently the speaker does research in Quasars so the channel should have more reliability than the average site.

https://www.researchgate.net/publication/258317793_The_Status_of_the_Warp_Drive

Notable are the advantages in this paper:

Benefit 1: Removal of interstellar distance barrier, as no longer restricted to subluminal speed limitations. Get faster than light travel, as measured by distant observer outside of disturbed region. This will allow missions to the nearby stars and closer examination of astrophysical phenomena than is possible today.

Benefit 2: It is a conventional transport scheme, in that it requires no ‘tearing’ of space or non-trivial topologies (i.e. wormholes) and does not require the transmission of copies of objects across space as a means of getting to the destination (i.e. teleportation).Warp drive is a simple transport from origin to destination through space.

Benefit 3: No time dilation effects, as usually expected with other space propulsion schemes due to special relativity. This is because the vehicle could be moving at subluminal speeds so that clocks on board would remain synchronized with the origin and destination.

Benefit 4: No relativistic mass increase of vehicle, since ship is at the centre of warp bubble is at rest with respect to locally flat space.

Benefit 5: No requirement for rocket type propulsion to achieve near light speed, which usually restricts the maximum speed attainable due to special relativistic effects such as infinite thrust for infinite masses.

Benefit 6: Technological and economic benefits to mankind.

(1) (PDF) The Status of the Warp Drive. Available from: https://www.researchgate.net/publication/258317793_The_Status_of_the_Warp_Drive [accessed Jan 29 2019].

For some background information you can watch this: https://www.youtube.com/watch?v=94ed4v_T6YM

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Independently of all the in-universe specific scenarios described and calculated so far, I think in any setup where your technology allows you to accelerate matter close to c (or above) in normal space, you will have the rough equivalent to today's landmine and explosives disposal units.

Your rookie Gunner shot the Everest main gun by mistake? Call in the Sweepers ! They will calculate the round's position in normal space and, while you slap the Gunner, will move to intercept and dispose of the round diligently with means appropiate to the technological background of your Universe. For Mass Effect, that would actually include calculating the Gate it is going to be closest to for practical interception and schedule it for sometime in a few decades. For Niven's Known Space, you can be much more accurate and intercept inside Oort's cloud in hours or days.

Say you had a mayor battle with lots of potential debris and lost rounds. Sweepers of winning faction will calculate where the spherical front of debris and rounds is, based on used weapons known speed, the sectors of the sphere posing a risk to the elliptical plane of nearby populated stars, and setup clean up patrols spanning for years or decades. Would not be too different from the efforts to clean up the Zone Rouge in France post WWI. You do not need to clean up the whole expanding sphere, just the parts heading to known settelments, the rest can be integrated in navigational charts with big red signs "Everest rounds on the loose. Drop off to normal space at your own risk".

To deal with unreported accidents, the odd Evil Scientist, and forgotten civilization's debris from centuries ago, you will have to trust in local defense measures with integrated protocols to deal with incoming garbage at c speed as pointed in answers above.

Why is any of this never described in stories? Because everyone in-universe gives it for granted and nobody cares to worry or followup, same as we do not see the news opening with every landmine field or random bomb from 50 years ago neutralized... unless you have to evacuate half of Hamburg because of it. It would actually make for nice background story material.

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    $\begingroup$ Personally, my favorite answer. It's a fun and also new possibilities for plot hooks. May not go in depth about relativity problems, but really under-appreciated and deserve more recognition, in my opinion. $\endgroup$ – Nyakouai Feb 8 at 9:24
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Idk, man. Just use a normal Alcubierre drive or basically any Space-time warping FTL. Since these drives move the space around the ship instead of the ship itself, a ship with a failed drive will exit warp at the same (sublight) speed with which it entered, which may even be a state of rest.

And as far as the SW portion is concerned, I would like you to know that Shit-do ramming was completely lore-inaccurate, and a result of Disney, as usual, confusing hyperdrives with warp drives. Hyperdrives send a ship into a separate dimension, where they move much larger distances in realspace with every unit they move in hyperspace. Of course, objects which generate large mass shadows (think the gravity wells of planets, stars or black holes) generate "mass shadows" in hyperspace, which pull a ship back into normal spacetime of entered and may even destroy it (this is why ships cannot jump anywhere near a planet). The Supremacy, though large, will not generate a mass shadow even close to what is required, and even if it did, the Raddus would only collide at sublight speeds. Thus, that scene broke lore very badly.

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  • $\begingroup$ Maybe there's a brief moment of warp speed before the ship gets into the hyperspace dimension? That would explain the directional flash that comes right after or before a ship goes into or out of hyperspace. So perhaps Raddus was rapidly accelerating but never made it into hyperspace. $\endgroup$ – LukeN Mar 8 at 22:13
  • $\begingroup$ The distance was too great to not make it into Hyperspace. There was a scene (probably in Rogue One) where a Rebel Cruiser successfully jumps to hyperspace right in front of an ISD. $\endgroup$ – Budhaditya Ghosh Mar 9 at 1:19
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From a Traveller pen-and-paper RPG point of view:

FTL travel is more like "wrinkle in time" travel than "really really fast" travel. The jump drive steps you into jumpspace, you fly along at no net change in realspace velocity or heading, and a week later you pop out of jumpspace as if nothing happened, except you're now a few light-years away from where you started. Almost like a side-effect of something else happening.

That said, even Traveller struggles with the fraction-of-C-rock bomb effect.

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    $\begingroup$ You don't need FTL for that to be a problem. You just need a sufficiently powerful drive and enough time. $\endgroup$ – Keith Morrison Feb 6 at 23:37
  • $\begingroup$ +1 Too right. So the original question is not "really" about FTL either! $\endgroup$ – rje Feb 7 at 16:12
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Sir! An object in motion stays in motion... unless acted on by an outside force, sir!

You don't need space dust.
There are already known outside forces in space which cause drag. The solar wind from our sun is an example.
Whatever speed the ship pieces were at, they would not continue at that speed indefinitely.

From a practical "world building" view this isn't widely known/understood/appreciated, so I'd go with "the lack of a functioning hyperdrive" making this irrelevant, instead of the physics.stackexchange.com view.

To be blunt, some things are best ignored.
Like if you allow a FTL bullet and miss a ship but hit a planet (or maybe it deflects off the ship) the inhabitants who aren't hit still have to deal with the burst of x-rays, gamma rays, and scattered particles caused when the near light speed debris hits their atmosphere.

You can see the effects of a baseball traveling at 0.9c here (https://what-if.xkcd.com/1/) the physics of which appear to be legitimate and then extrapolate in your head what an item of larger mass would do.

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The FTL drive of the ship might have a, for now, undiscovered physical mechanism to propell the ship beyond c. However this mechanism works in detail, it might only be able to affect a confined system (like the spaceship). So even if your superluminar vessel "hits" another ship or even just an asteroid: it could transfer absurd amounts of engergy/momentum, but the striken object might not accelerate beyond c itself. So the excess of kinetic energy might just be instantly transformed into heat or radiation. At worst you may have a flaming ball of plasma shooting near the speed of light towards a random destination but this isn't really "unusual" in our universe?

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Consider that a black hole is already dragging in light, it has "FTL" effects.

Perhaps your black holes are your inter-galactic defenders. The shrapnel will be subject to getting chomped up by sentient and hungry black holes roaming within and without galaxies.

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For this answer I'm assuming you want a ship to be able to get to a destination FTL instead of just being able to move FTL.

If you just want a way to give FTL travel but none of the kinetic headaches why not just have a drive that folds space on itself and the ship transposes itself into the new location. It would drastically reduce the effectiveness of using FTL as a Military weapon. Defense against it would be a generator that creates a field that causes improper folding and sends the FTL somewhere else.

Making all/Most/Some planets equipped with these generators would prevent drunk Aliens from transposing whatever they want anywhere on the planet. Wouldn't be able to use it to rob a bank.

If that is still to powerful just make it so that any type of FTL needs an end anchor point to guide it and that using it without it would just scatter your energy across the universe because of [Insert Tchnobable]

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Holdo's maneuver doesn't make sense, even in the SW universe. (I didn't let it ruin the movie for me, but it's dumb.) You can't go FTL and stay in normal space; this is basic science that every geek knows. So you can't ram another ship using your FTL engines. You could maybe purposefully exit from FTL in the same (normal) space occupied by another ship. I would think that would do a lot of damage, cause reactor breaches, decompression, etc. Just not a spectacular ramming. So, the answer to your question is that your "problem" is a non-problem.

BTW, a spaceship firing a kinetic weapon with that much energy would have to SUPPLY that energy.

BTW, energy from kinetic rounds doesn't necessarily all get deposited in the target, but instead punches holes through it. The energy only gets deposited if the round encounters armor thick enough to absorb all that energy. So, the proper defense against sci-fi railguns is to build ships with fairly thin walls, many compartments, multiple redundancies, and explosion failsafes. Also, make ships very maneuverable. The counter to this defense is to have the ammo spread out into many pieces after launch -- a railgun shotgun. The counter to the railgun shotgun is...armor. ;-)

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Energy is conserved.

This is one of the laws of physics that was discovered by Newton. The rest of Newton's physics has been changed with both relativity and quantum mechanics, but this basic fact stands solid.

FTL travel must mean that physics as we know it is not complete, but the safest bet is that energy will still be conserved in the new physics. Of course, you as the author can just state it as a fact, if you want to.

What does this mean? It means you can't get more energy out of an FTL collision than you put in as starship fuel. This makes the universe safe.

The most energy-dense fuel we know is anti-matter. If put in contact with the same amount of ordinary matter, both will convert completely in energy. The maximum can be found using Einstein's famous $E=mc^2$.

You end up with energies that can be very bad news to a single planet, but not noticeable to a star.

However, simply storing the same amount of anti-matter and having an accident with that would be just as bad as having star ship shrapnel rain down.

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I have read a book (i don't remember which one, but it was from the age of Asimov's first books), in which this problem was neatly solved: FTL in that universe used some kind of "physics bubble" (compare it to the warp field in star trek), which extended a few light-seconds around the producing vessel. These bubbles were unable to interact, so any time two ships would be closer than that light second to each other, the FTL would automatically fail, dropping the ships into sub-luminar speeds untill they got far enough away.

Making FTL work in a way similar to this will indeed limit the dangers of such idiots, since super-luminar collisions are impossible by definition

(for reference, a light-second is roughly the distance between the earth and the moon)

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    $\begingroup$ Welcome to Worldbuilding, ThisIsMe! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$ – Gryphon Feb 8 at 15:00
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Robotech!

I am surprised that I have not seen this answer yet, but if anyone remembers the Americanized Anime show from the 1980s, Robotech, they had a unique and interesting take on FTL travel. Basically an FTL drive in the Robotech universe was a device which would create a sphere of energy around itself and essentially teleport anything inside that sphere to a location potentially many light years away. This is similar to the "space fold" concept. The ship is not achieving superluminal velocity at all, it is simply exchanging it's place with whatever is at a different location.

In the very first episode of the series, we see the potential to weaponize this type of FTL drive when the SDF-1 is forced to make an emergency space fold while still inside the Earth's atmosphere, not very far above an island in the South Pacific. The result is that the ship inadvertently teleports a significant chunk of the island with it into space. Clearly, this kind of thing could damage planets, cut enemy ships in half, etc, but the total potential for destruction is fairly limited by the size of the field which can be generated by the drive. In an interesting side note, the size of the FTL field that can be generated seems to be proportional to the size of the ship creating the field, which kind of makes sense (it's never spelled out in the series or completely explained) which may explain the incredibly large size of some of the ships that exist in the Robotech universe.

In any case, this basic concept: that the ship's engines (whatever allows it to move through space) are a totally separate system from the FTL "fold drive" which basically teleports the ship to a different location is a foundation which can be used to plan out a science fantasy/space opera universe. I have no idea how the creators of Robotech intended to deal with the admittedly unlikely event of two ships choosing to fold to the exact same location at the same time. I think they didn't think it through that far. I would suggest that either both folds fail, or there would be a huge explosion: whatever is more interesting for the story.

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Conservation of Energy

Simply posit the following for your FTL drive:

  • The vessel goes into an alternate space or alternate phase or whatever you want to call it, in which all FTL travel takes place;
  • In this space or phase, distances are somehow "shorter" than regular space, so that travel can be accomplished far more quickly, even by a ship travelling at subliminal speeds.
  • It takes a certain amount of energy to put a vessel into FTL space; the energy is linear to the mass of the vessel, and inversely proportional to the flatness of space.
  • A vessel can only exit FTL space at a point whose escape velocity in normal space is equal to the escape velocity of the entrance point.

All of this means that your thrusters only need to get you far enough out to where your FTL drive will work, and no more (so nobody will build them). There won't be a danger from ships traveling at relativistic speeds because there won't be any ships going this fast in normal space in the first place.

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