I was recently looking at this question, which seems to argue that FTL travel within the existing laws of physics can risk resulting in the destruction of half the universe. (Specifically, bad things occur when anything moving FTL hits anything else).

So, say you've handwaved this issue to allow for less dangerous FTL travel, by entering a separate hyperspace dimension where the rules are different, or whatever else. But say this mode of FTL is only possible under certain circumstances, or perhaps your handwavium drive is broken.

Have you noticed how in space opera when the "hyperdrive" breaks, somehow the heroes still manage to get from here to there in a reasonable amount of time? "Slower" maybe, but they still usually make it to "the nearest inhabited star system." Basically, if we don't want to risk wiping out large portions of the universe, how feasible is this?

In a situation where you cannot use your normal "safe" mode of FTL, how fast can you travel without risking an unacceptable "universe destroying" potential? The speed of light? Slightly slower? And following these guidelines, how long will it take you to go anywhere? How long to get to Pluto from Earth, or to Alpha Centauri? Would the valiant Captain be dead before he ever arrived at the nearest hyperdrive repair facility?

  • 5
    $\begingroup$ Well considering that distances from stars is usually measured in light-years and if you are going at around the speed of light, the maths is not very difficult. Alpha Centauri is 4.37 light-years from the Sun, therefore Captain Picard travelling at light-speed should make it in around 4.37 years. $\endgroup$ Apr 24, 2019 at 23:14
  • 1
    $\begingroup$ I'm having a hard time wrapping my head around your question. You have FTL space travel and you don't define the speeds. You now can't use the FTL systems but don't define the speeds you can achieve. And you have some unspecified danger of destroying half the universe, but don't tell us what conditions will make that happen. You do give us enough details to calculate sample distances though. When ST:TNG discovered that warp drives caused damage to (very tiny bits) of the universe, it was that only high warp did it. They saved warp 7-10 (or there abouts) for emergencies and only... $\endgroup$
    – Cyn
    Apr 24, 2019 at 23:47
  • 1
    $\begingroup$ went up to warp 4 or so under normal circumstances (at least for the rest of the episode). Warp is still FTL and they could still get places fast, just not as fast. So what exactly are you asking here? $\endgroup$
    – Cyn
    Apr 24, 2019 at 23:49
  • 1
    $\begingroup$ @Cyn This question is related to the linked question's claim that moving at FTL speeds can put the universe at risk. So that part is discussed in the other post; I took it at face value. If their claims are wrong or I'm misunderstanding them, by all means let me know. I'm not talking about star trek specifically $\endgroup$
    – MarielS
    Apr 24, 2019 at 23:54
  • 1
    $\begingroup$ I think this question needs some additional work, because it's completely not clear what you are asking. Your hypothetical spaceship had some FTL drive that is broken. Presumably, your ship has some sublight secondary drive for maneuvering and planetary approach. It can try getting somewhere on this secondary drive - it will take exactly as long as it takes for this particular drive, and will depend on the fuel the ship has availabile - half of the fuel is spent on accelerating, another half on braking... $\endgroup$
    – Cumehtar
    Apr 25, 2019 at 0:57

2 Answers 2


Universe-Destroying Potential Isn't the Problem

The problem with the linked question is the required energy, and it's the problem here as well. If you accelerate something to fractional-C, it takes more and more energy the closer you get to lightspeed. This energy will subsequently be released if that object hits something (which is the source of the excellent Mass Effect line quoted in the question).

But getting the energy is the problem. It's why in most hard-science space stories, ships don't tend to move too quickly, on an interstellar scale.

So, in softer sci-fi, the solution is hyperspace/warp fields/alcubierre drives. Bend the rules, or find somewhere where they don't apply, and travel very fast for much less energy.

All this is to say that if your FTL drive breaks, your ship won't have a means of accelerating to anything near lightspeed. It won't have dozens of times its own mass in reaction mass, nor a power plant capable of expelling that mass at the velocities needed to get your ship up to those speeds. If the drive breaks, and you can't fix it, you're hooped.


A potential solution is, a la the old Star Wars Expanded Universe, that the FTL drive isn't binary. It can break a bit, but keep working. You can't go a million times the speed of light... but you can go a hundred times the speed of light.

This still can result in a long journey - if your drive's high gear breaks halfway to Alpha Centauri, resulting in the ability to travel only a hundred times the speed of light, it'll take a week to get to one end or the other, rather than the 2 minutes it'd take travelling at a megalight.


So that aside, you needn't worry about universe-destroying potential, because unless your ship is independently capable of generating enough power to destroy the universe, it won't do so by doing whatever its best sublight speed is.

  • $\begingroup$ Good answer, thanks. Just to be a bit more complete, however, let's say somehow your ship does have enough superfuel (unobtainium) to accelerate itself to exactly lightspeed when the hyperdrive goes out? Or, if this is impossible due to infinite energy needs, as close to lightspeed as can feasibly imagined. Is the "universe-destroying potential" going to be an issue at any speed equal to or less than the speed of light, or does it only become a serious concerns when FTL speeds enter into the equation? (I'm bad at math and science stuff, so I hope you forgive me if this is stupid). $\endgroup$
    – MarielS
    Apr 25, 2019 at 2:38
  • $\begingroup$ It occurs to me that perhaps I'm thinking about this wrong. Is it FTL speeds specifically that are prevented given our understanding of the laws of science, or is it all related to required energy and thus almost equally as impossible to actually reach the speed of light as it is to cross the famous "FTL barrier"? $\endgroup$
    – MarielS
    Apr 25, 2019 at 2:41
  • $\begingroup$ It is literally impossible to reach the speed of light. Under the laws of physics to the extent we have explored them, accelerating any mass to the speed of light requires infinite energy. (Massless particles, on the other hand - like photons - must travel at the speed of light.) $\endgroup$
    – jdunlop
    Apr 25, 2019 at 17:55
  • $\begingroup$ @MarielS - conversely, depending on your frame of reference, it may not be impossible to exceed the speed of light, without transitioning through a point of travelling at or close-to the speed of light. (This violates causality, so it's still probably impossible, but less impossible than an object with mass moving at the speed of light.) $\endgroup$
    – jdunlop
    Apr 25, 2019 at 17:56
  • $\begingroup$ But the "Universe destroying potential" in the linked question is wrong in the first place (since it's based on a truly stupid moment in the Star Wars canon) - if you have the energy to accelerate a mass to universe-imperiling speed, having a reactor failure would also imperil the universe, because it's all energy. You can't make something travel fast in Newtonian space without possessing the energy it would release on impact. $\endgroup$
    – jdunlop
    Apr 25, 2019 at 17:59

Emergency fallback FTL travel.

portal minecraft


You hate to have to resort to it. The maps are crap. There is beaucoup weird stuff in there. Sometimes you feel like it gets on you and then weeks later, you smell it again. Those horrible portals. A distant second best to friendly streak-starred hyperspace, to be sure. But if you gotta get there fast the portals might be your only option. Tie back your hair, fill your pockets with pennies and jump in.

  • $\begingroup$ Minecraft is a good call but warhammer 40k would also be a good inspiration. $\endgroup$
    – Eric
    Apr 25, 2019 at 15:08

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .