# Question about light speed. **URL:** \ **Category:** In My Humble Opinion\ **Created:** [April 28, 2014, 11:22pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224 "2014-04-28T23:22:39Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![msmith537](https://avatars.discourse-cdn.com/v4/letter/m/d9b06d/32.png) [@msmith537](https://boards.straightdope.com/u/msmith537)\ **Post date:** [April 30, 2014, 10:38am UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/21 "2014-04-30T10:38:09Z")
> [@Der\_Trihs](#): > > I on the other hand think you are creating a problem that doesn’t exist. The technology doesn’t have to be especially durable, it just needs to be repairable. Reparable with what? Where does the ship store 10,000 years worth of spare parts? Or the raw materials to fabricate spare parts if you prefer? > [@Der Trihs](#): > > Originally Posted by Mangetout View Post > Generation ships aren’t the answer, because that involves building a completely self-sustaining environment with machines and facilities that will keep functioning for tens/hundreds of thousands of years with no external support. > > We know that’s possible because we live on one that’s lasted for billions of years not just thousands. We just can’t steer it. We aren’t doing a great job of keeping it running. And it becomes a much bigger and more complex problem when you scale the problem down to something small enough to move to another star system. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 30, 2014, 12:24pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/22 "2014-04-30T12:24:57Z")
> [@Der\_Trihs](#): > > We know that’s possible because we live on one that’s lasted for _billions_ of years not just thousands. We just can’t steer it. It remains to be seen whether modern humans (at our level of technology or beyond) can live there for hundreds of thousands of years without rendering it uninhabitable. > [@](#): > > Actually the solution to that is simple: you bring along enough technology to repair or rebuild everything, including the repair & manufacturing machinery; you don’t try to haul along several thousand years worth of spare parts. And raw materials (no system of recycling will be perfect, especially not on the scale of millennia. And energy to run the process. If we have fusion power (or better) and replicator-level manufacturing, then a couple of problems are solved. But even if we solve all of the material problems, it’s still a fact that no human society or culture has lasted as long as the trip time of a generation ship - and the cultural constructs that have been explicitly hand-made, as this would need to be, are amongst the least durable. And of course there is the question of _why we would do such a thing_ - invest a significant portion of the effort and power resources of the whole world, to send a small number of humans off on a suicide mission (by which I mean all of the initial crew will die), their children, and grandchildren imprisoned and enslaved for tens of generations, to arrive somewhere that might (for some quite small reason) be completely uninhabitable, long after anyone who planned and invested in the venture is dead and forgotten. Why would we do that? Certainly not for anything like the reasons that people crossed oceans. What kind of humanity would we even be exporting? The nth-generation slave descendants of the initial crew are going to do what when they get there? ---
**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\ **Post date:** [April 30, 2014, 4:24pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/23 "2014-04-30T16:24:59Z")
> [@Der\_Trihs](#): > > We know that’s possible because we live on one that’s lasted for _billions_ of years not just thousands. We just can’t steer it. And if we somehow managed to steer it into interstellar space, it would cease working pretty damned fast. We’re awfully dependent on the proximity of that huge furnace we call the Sun. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [April 30, 2014, 4:33pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/24 "2014-04-30T16:33:38Z")
Anytime I go anywhere I try to find a shortcut. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [April 30, 2014, 4:58pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/25 "2014-04-30T16:58:38Z")
> [@msmith537](#): > > Reparable with what? Where does the ship store 10,000 years worth of spare parts? Or the raw materials to fabricate spare parts if you prefer? No, you’re missing the point. You don’t bring spare parts. You bring raw materials and machine tools. You can use machine tools to make other machine tools as well as other final products. In other words, when the generation ship finally arrives 100 or 1000 years later, you’re in the Ship of Theseus. Every single component has been replaced and recycled several times over, including the crew. Anything that you cannot build a replacement for is a point of failure that could doom the mission. This requires a level of technology we do not currently have, but I don’t think it’s impossible to acquire that technology. Perhaps more importantly, this is technology that is useful even outside of generation ships. This is the kind of tech that we’d need to reverse pollution, reverse global warming and make human civilization essentially zero impact to the environment. Whether we take it to space or not, the goal of perfectly recycling everything we need is already a valid goal. So once the tree huggers have perfected the self-contained, fusion-powered eco-house, all I’m proposing is that we up the recycling efficiency another 1% and strap engines to the back of it. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [April 30, 2014, 8:40pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/26 "2014-04-30T20:40:59Z")
> [@Mangetout](#): > > True, but analogies about how we may be as ignorant as our ancestors contain an inherent contradiction, as well as plain factual inconsistencies. > > For example, I’ve heard it said that we will probably be able to travel faster than light in the future - and that our current view of its impossibility is like the prediction of 19th Century scientists, who asserted that people would suffocate if the speed of railway trains exceeded some trivial value. > But really, it’s nothing like that. They pulled that idea out of their asses, and it was subsequently overturned by experiment. c as a fundamental speed limit, on the other hand, is not only _not_ pulled out of asses, it’s also been very substantially supported by experiment, not to mention the implementation of many technologies. . . . I acknowledge the difficulty. This isn’t like an ancient Greek saying, “It is impossible to climb Mt. Olympus.” But…you may be taking the argument too far the other way. Your position depends, I think, too heavily on the rejection of the possibility of the future invention of really revolutionary technology – or even significant improvements on technology we have now. One interesting science-fictional idea, for instance, is that we might be able (in the future, using not-yet-discovered technology) feed an electric current into a solid, to reinforce the chemical bonds, thus making the substance vastly harder than it is in nature. Imagine such reinforced carbon: a “super diamond.” This one change, all by itself, would change the space age incredibly. We could build much more powerful rocket motors, and much safer spacecraft hulls. I agree it would be unfair of me to say, “…And such advances will occur.” But that isn’t what I say: I only say they might occur. And this, you cannot reasonably rebut. Best of all, this kind of idea doesn’t violate any laws that we think we understand today. It doesn’t fly in the face of relativity. There’s no concrete law that says this cannot happen. This, or a vast number of other unforeseen discoveries. > [@](#): > > The contradictory part is where the argument says (essentially): our ancestors were ignorant, but we know better, but actually, we don’t know better, because our predictions could be completely false. I’m saying, rather, we know much more than we did 4,000 years ago, and it is not unreasonable to suggest that we might know a lot more 4,000 years from now. Not that we will know, only that we might. How does one rebut that? “Our predictions could be completely false.” Do you deny that? Given that we have been making mutually contradictory predictions in this thread, at least some of our predictions must be completely false! > [@](#): > > For FTL to be possible, we have to discover that not only were we wrong about a huge swathe of science, but also, that all the things we did (and that worked) with that science, _worked for completely unknown reasons_. Agreed. > [@](#): > > For generation ships to be possible, we have to build stuff that lasts, and keeps working longer, than the whole of human civilisation. It’s a tall order - I’m not saying it’s absolutely impossible, just that I think most people who propose it enormously underestimate its difficulty. Actually, I agree with this, too. All I ask is that you not reject generation ships as being of the same order of impossibility as FTL. It’s like building a space elevator, or terraforming Mars, or building undersea cities. It isn’t like squaring the circle, or finding “the last digit of pi.” (Actually, I hold that FTL is between those categories. Some notions involving “negative energy” and wormholes have interesting implications.) ---
**Author:** ![ThisUsernameIsForbidden](https://avatars.discourse-cdn.com/v4/letter/t/b4bc9f/32.png) [@ThisUsernameIsForbidden](https://boards.straightdope.com/u/ThisUsernameIsForbidden)\ **Post date:** [April 30, 2014, 8:56pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/27 "2014-04-30T20:56:10Z")
> [@Voyager](#): > > Why do you think his watch would say anything different from what relativity says it would say? There is plenty of experimental evidence already. What would it say travelling at 0.5c for one year (our time)? ---
**Author:** ![MrKnowItAll](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrknowitall/32/87_2.png) [@MrKnowItAll](https://boards.straightdope.com/u/MrKnowItAll)\ **Post date:** [April 30, 2014, 9:36pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/28 "2014-04-30T21:36:21Z")
Of those options, I voted yes, but really I’d say “No, because we’ll find a way to travel the via wormholes before we can get anywhere by a vehicle.” ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [April 30, 2014, 10:54pm UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/29 "2014-04-30T22:54:20Z")
> [@ThisUsernameIsForbidden](#): > > What would it say travelling at 0.5c for one year (our time)? The compression ratio would be .866 [(that much math, I can do!)](http://en.wikipedia.org/wiki/Time_dilation) So the guy in the spacecraft experiences 316 days while we experience 365. ---
**Author:** ![ThisUsernameIsForbidden](https://avatars.discourse-cdn.com/v4/letter/t/b4bc9f/32.png) [@ThisUsernameIsForbidden](https://boards.straightdope.com/u/ThisUsernameIsForbidden)\ **Post date:** [May 1, 2014, 1:15am UTC](https://boards.straightdope.com/t/question-about-light-speed/687224/30 "2014-05-01T01:15:24Z")
> [@Trinopus](#): > > The compression ratio would be .866 [(that much math, I can do!)](http://en.wikipedia.org/wiki/Time_dilation) So the guy in the spacecraft experiences 316 days while we experience 365. That’s not going to be enough. We need to get much closer to c for it to make any big difference. Next problem: how long would it take your standard launch rocket (with an infinite amount of fuel) to reach 0.995 c? I’ll get to it. [Previous page](https://boards.straightdope.com/t/question-about-light-speed/687224.md?page=1)