# How long would it take to reach the speed of \[light\]? **URL:** \ **Category:** Factual Questions\ **Created:** [June 14, 2013, 7:52pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018 "2013-06-14T19:52:39Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [June 14, 2013, 7:52pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/1 "2013-06-14T19:52:39Z")
And not crushed by excessive G’s. ---
**Author:** ![Prof.Pepperwinkle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/prof.pepperwinkle/32/13627_2.png) [@Prof.Pepperwinkle](https://boards.straightdope.com/u/Prof.Pepperwinkle)\ **Post date:** [June 14, 2013, 7:54pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/2 "2013-06-14T19:54:58Z")
Uh, the thread title says Speed of Sound, the OP says Speed of Light? Which is it? Or both? ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [June 14, 2013, 7:55pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/3 "2013-06-14T19:55:49Z")
Light, I edited and it didnt come out on the board. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 14, 2013, 8:12pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/4 "2013-06-14T20:12:47Z")
> [@HoneyBadgerDC](#): > > And not crushed by excessive G’s. You’ll never reach the speed of light. But if we pretend the universe is Newtonian, and we accelerate at a comfortable 1 G, then it takes 300,000,000 m/s / 9.8 m/s^2 = 30,600,000 s, or just under a year. ---
**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:** [June 14, 2013, 8:32pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/5 "2013-06-14T20:32:22Z")
> [@Dr.Strangelove](#): > > You’ll never reach the speed of light. But if we pretend the universe is Newtonian, and we accelerate at a comfortable 1 G, then it takes 300,000,000 m/s / 9.8 m/s^2 = 30,600,000 s, or just under a year. Sounds easy, doesn’t it? - Until you consider that in order to achieve this, you must set off with all of the fuel you will need for the whole year - and the weight of this fuel means you need even more fuel to accelerate your vessel, etc. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [June 14, 2013, 8:41pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/6 "2013-06-14T20:41:30Z")
G’s are from acceleration, not speed. If you accelerate at 1G you’ll be fine for the entire year. If you accelerate at 50Gs you’ll be awfully uncomfortable but for a shorter time. G forces are entirely dependent on how fast you wish to achieve light speed (which you can’t reach, but let’s just say .99 C). ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [June 14, 2013, 8:51pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/7 "2013-06-14T20:51:28Z")
You can take a lot of g’s before you suffer actual physical crush injuries. There’s a much lower limit before you have trouble getting blood to continue flowing through the parts of your body that are on the side of your body that you are accelerating toward. The limit varies depending on orientation and length of time. For fighter pilots, who are in a sitting position, they regularly tangle with 9 g’s for short periods, but that takes [special equipment](http://en.wikipedia.org/wiki/G_suit) and [constant physical exertion.](http://navyflightmanuals.tpub.com/P-868/P-8680094.htm) by making yourself shorter in the direction of acceleration, you can tolerate more g’s. If you’re laying down, you can tolerate quite a bit of back-to-front acceleration (i.e. spine toward belly button) without any special measures. If we assume 20 g’s is tolerable, then building on **Dr. Strangelove** ’s math, it would take 1/20th of a year, or about 18 days. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [June 14, 2013, 11:17pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/8 "2013-06-14T23:17:55Z")
I know it could never be reached I was just curious. ---
**Author:** ![bonitahi](https://avatars.discourse-cdn.com/v4/letter/b/7993a0/32.png) [@bonitahi](https://boards.straightdope.com/u/bonitahi)\ **Post date:** [June 15, 2013, 12:04am UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/9 "2013-06-15T00:04:47Z")
"How long would it take to reach the speed of sound? " Walking or riding? If you were walking it would take quite a long time, 10-11 months or more. But if you were is a car driving, say 55 mph, it could take as little as less than a month, considering no traffic lights or stop signs. That’s assuming you did not stop for gas. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 15, 2013, 12:15am UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/10 "2013-06-15T00:15:43Z")
> [@Mangetout](#): > > Sounds easy, doesn’t it? - Until you consider that in order to achieve this, you must set off with all of the fuel you will need for the whole year - and the weight of this fuel means you need even more fuel to accelerate your vessel, etc. You could power the vessel externally with a (big) laser. Sure, it would require a non-trivial fraction of the Sun’s output as an energy source. And there’s no way to slow down. But other than that… ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [June 15, 2013, 1:12am UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/11 "2013-06-15T01:12:57Z")
> [@Machine\_Elf](#): > > … > by making yourself shorter in the direction of acceleration, you can tolerate more g’s. If you’re laying down, you can tolerate quite a bit of back-to-front acceleration (i.e. spine toward belly button) without any special measures. If we assume 20 g’s is tolerable, then building on **Dr. Strangelove** ’s math, it would take 1/20th of a year, or about 18 days. Plus you get shorter the closer you are to the speed of light. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 15, 2013, 1:29am UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/12 "2013-06-15T01:29:39Z")
> [@Machine\_Elf](#): > > If you’re laying down, you can tolerate quite a bit of back-to-front acceleration (i.e. spine toward belly button) without any special measures. If we assume 20 g’s is tolerable, then building on **Dr. Strangelove** ’s math, it would take 1/20th of a year, or about 18 days. It doesn’t seem like this helps you much. A person can handle 20 Gs for short durations, but not 18 months. I doubt that anyone could even take 2 Gs for 6 months. 1.5 might be OK for fit individuals, which gives you 8 months. You could do a lot better if you put a person on a heart-lung machine, and in a liquid-filled tank. They might want to be in an induced coma, though, since no real movement would be possible, and the journey would be pretty boring. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 15, 2013, 1:34am UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/13 "2013-06-15T01:34:34Z")
We don’t really have any data on long-term human tolerance of any acceleration other than 0 or 1g. For all we know, maybe a human can take 2g for months on end. Or maybe they’d have a higher chance of heart attack and stroke during that time, but if it didn’t happen, they’d be fine. Or maybe they’d age faster. Or maybe they’d actually come out of it healthier. Or maybe, or maybe. We just don’t know. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 15, 2013, 1:46am UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/14 "2013-06-15T01:46:40Z")
> [@Chronos](#): > > We don’t really have any data on long-term human tolerance of any acceleration other than 0 or 1g. For all we know, maybe a human can take 2g for months on end. Or maybe they’d have a higher chance of heart attack and stroke during that time, but if it didn’t happen, they’d be fine. Or maybe they’d age faster. Or maybe they’d actually come out of it healthier. Or maybe, or maybe. We just don’t know. I wouldn’t say we know nothing. It’s just a plain physical fact that only a quite fit individual can even walk around in 2 G. And we know for various reasons that people don’t do all that well laying in one position in a bed–bedsores and other problems can be very serious. And there are people [doing](http://en.wikipedia.org/wiki/Hypergravity) [research](http://science.nasa.gov/science-news/science-at-nasa/2003/07feb_stronggravity/). It’s a little counterintuitive that hypergravity research is far easier than hypogravity, and yet we have far more data on the latter (the former requires a cheap centrifuge; the latter requires a space station). I guess the answer is that there’s little practical application right now. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [June 15, 2013, 4:34am UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/15 "2013-06-15T04:34:40Z")
> [@HoneyBadgerDC](#): > > Light, I edited and it didnt come out on the board. Fixed. You can only edit the body of the post after posting, not the thread title. Colibri General Questions Moderator ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 15, 2013, 2:29pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/16 "2013-06-15T14:29:04Z")
> [@](#): > > It’s a little counterintuitive that hypergravity research is far easier than hypogravity, and yet we have far more data on the latter (the former requires a cheap centrifuge; the latter requires a space station). Well, we have data on the specific case of zero gravity (which causes a number of problems). Would we get the same problems with a half of a g, or a third, or a sixth? Maybe, maybe not, or anything in between. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [June 15, 2013, 2:38pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/17 "2013-06-15T14:38:15Z")
Yes Chronos, that’s my biggest beef with NASA. That’s actually an experiment they could have done by now. Putting people in zero G for 40 years give or take and going “yep, still very bad for you” is getting old. The pessimist in me think’s NASA doesn’t want to find out that Mar’s or the Moon’s level of gravity is still very bad for you long term. That would put a real damper on those colony dreams. ---
**Author:** ![Jake](https://avatars.discourse-cdn.com/v4/letter/j/f475e1/32.png) [@Jake](https://boards.straightdope.com/u/Jake)\ **Post date:** [June 15, 2013, 2:48pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/18 "2013-06-15T14:48:15Z")
> [@Leo\_Bloom](#): > > Plus you get shorter the closer you are to the speed of light. Shorter relative to what? ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [June 15, 2013, 4:03pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/19 "2013-06-15T16:03:35Z")
> [@billfish678](#): > > Yes Chronos, that’s my biggest beef with NASA. That’s actually an experiment they could have done by now. Putting people in zero G for 40 years give or take and going “yep, still very bad for you” is getting old. > > The pessimist in me think’s NASA doesn’t want to find out that Mar’s or the Moon’s level of gravity is still very bad for you long term. That would put a real damper on those colony dreams. Other than constructing a rotating space habitat which would be signfiicantly more complex than any orbital structure previously erected (e.g. Mir and the ISS), how would they go about doing this? **Chronos** is correct that we have essentially no data on the long term effects of fractional acceleration on mammals, and even minimial data on accelerations of greater than 1 G (on mice in a centrafuge in laboratory conditions) but we can make some rough interpolations from the effects of zero G and terrestrial acceleration. An acceleration of at least 1/3 G is probably sufficient to alievate many of the symptoms seen during the first few months of spaceflight, but the longer term and chronic effects on cariovascular and osteoskeletal health are complete unknowns, and a degradation of skeletal musculature can be assumed just by lack of use. However, compared to the problems of hygeine (e.g. bacteria not settling as readily on surfaces) and radiation, this is easier to mitigate via exercise and diet supplementation. However, I think that long term habitation off the Earth is going to require either recreating Earth-like conditions as much as possible in massive orbiting habitats, or modifying the human form to mitigate all of the deficiencies that would occur in low or zero gravity conditions. As for the question of the o.p., consider that as you approach _c_, you become effectively more massive (to the external observer) and thus the thrust is less effective. Realistically, there is no way to carry enough propellant to achieve even a small fraction of _c_ with any conventional propulsion system, even using nuclear fusion, as the mass ratio is simply prohibitive. The only way to achieve this with a reaction engine would be the direct conversion of matter (which is a bound and limited form of energy) into photons, or at least, very low mass particles, provided that they can be focused to go opposite the direction of travel. Stranger ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [June 15, 2013, 4:08pm UTC](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018/20 "2013-06-15T16:08:15Z")
> [@Mangetout](#): > > Sounds easy, doesn’t it? - Until you consider that in order to achieve this, you must set off with all of the fuel you will need for the whole year - and the weight of this fuel means you need even more fuel to accelerate your vessel, etc. I wonder if you could send fuel pods out ahead of time? Robotic cargo tugs, no payload, just fuel tanks. Shoot a dozen of those out ahead of the main vessel. You’d be going pretty quick as you approached the later modules, might need some fancy space-wrangling technology. [Next page](https://boards.straightdope.com/t/how-long-would-it-take-to-reach-the-speed-of-light/661018.md?page=2)