# If the speed of light were slower, could we travel to the stars? **URL:** \ **Category:** Factual Questions\ **Created:** [May 25, 2021, 12:25am UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843 "2021-05-25T00:25:51Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![EdwardFound](https://avatars.discourse-cdn.com/v4/letter/e/4af34b/32.png) [@EdwardFound](https://boards.straightdope.com/u/EdwardFound)\ **Post date:** [May 25, 2021, 12:25am UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/1 "2021-05-25T00:25:51Z")
If I could just get my spaceship up to relativistic speed, I could travel to a star - or across the galaxy - in a comfortable ship-board time. Unfortunately, we don’t have (maybe never will have) a technology to accelerate spacecraft to anywhere near relativistic speed. But what if the speed of light (the Einstein constant) were much, much lower: something comparable to the speeds of our current spaceprobes. Could I then boost my ship up to a now-tractable relativistic speed and survive a trip to Alpha Centauri? I’m suspect the answer to this would be “no” - that mass increase would thwart me from achieving the needed time dilation. But I’m not up on the math, so maybe that’s wrong ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [May 25, 2021, 1:41am UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/2 "2021-05-25T01:41:44Z")
> [@EdwardFound](#): > > I’m suspect the answer to this would be “no” - that mass increase would thwart me from achieving the needed time dilation. But I’m not up on the math, so maybe that’s wrong There’s no such thing a relativistic mass increase. As has been discussed several times on this board in recent threads. However, the answer is still no. It would still take more and more energy to approach the speed of light, so a ship would still have difficulty getting near that speed. What it would mean is that the space probes we’ve sent out would have been going much slower than they actually were. ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [May 25, 2021, 2:30am UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/3 "2021-05-25T02:30:32Z")
There is no real answer to this because a universe with such a different set of universal constants - and a different c would mean different vacuum permittivity and permeability - is not likely one that would support life as we know it, much less a human civilization much like the one we have. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [May 25, 2021, 3:46am UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/4 "2021-05-25T03:46:42Z")
You might argue that the problem remains the same. In many ways the speed of causality (aka light) is 1. It is just the values of things like the fine structure constant that give us a way of expressing the resultant mundane things in other units. In order to get a useful time dilation you need to get to the same fraction of light speed, no matter what that speed is. This is usefully expressed as the gamma value. Sadly that same gamma is what controls the amount of energy you need to put into the system, so in your new universe nothing actually changes. Lots of handwaving above, as noted above, once you change the rules, it isn’t clear you can use other rules to work out what happens. But if you just play around with relativity, it doesn’t much care about that. c always equals 1, and you can’t change that. You can’t reason about anything if it isn’t. You are positing something other than relativity then. Time dilation may not even be part of that universe’s physics at all. ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [May 25, 2021, 12:46pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/5 "2021-05-25T12:46:39Z")
Mr. Tompkins covered the ramifications of a slower speed of light quite thoroughly. > **[Mr Tompkins](https://en.wikipedia.org/wiki/Mr_Tompkins)** > > Pages for logged out editors learn more > > Mr Tompkins is the title character in a series of four popular science books by the physicist George Gamow. The books are structured as a series of dreams in which Mr Tompkins enters alternative worlds where the physical constants have radically different values from those they have in the real world. Gamow aims to use these alterations to explain modern scientific theories. > Mr Tompkins' adventures begin when he chooses to spend the afternoon of ... ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [May 25, 2021, 4:23pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/6 "2021-05-25T16:23:07Z")
[Refractive index](https://en.wikipedia.org/wiki/Refractive_index) > In optics, the **refractive index** (also known as **refraction index** or **index of refraction** ) of a material is a dimensionless number that describes how fast light travels through the material. It is defined as > **n = c/v** So if the vacuum were made of water or diamond, the speed of light would be slower. I doubt you be able to go faster than c in such a medium. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [May 25, 2021, 4:40pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/7 "2021-05-25T16:40:13Z")
> [@Francis\_Vaughan](#): > > This is usefully expressed as the gamma value. Sadly that same gamma is what controls the amount of energy you need to put into the system, so in your new universe nothing actually changes. Exactly. But we could also say that it is actually even worse, since the difference between the ship’s clock and the earth’s is going to be that much greater on a round trip. I know this is not a concern of the OP, I’m just saying, the answer to the OP is: All the current problems remain, and the only difference is that one of the main problems that was not included in discussion is made much worse. ---
**Author:** ![Jasmine](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jasmine/32/2964_2.png) [@Jasmine](https://boards.straightdope.com/u/Jasmine)\ **Post date:** [May 25, 2021, 4:50pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/8 "2021-05-25T16:50:47Z")
It seems to me that the opposite would be true. If the speed of light was 100 times FASTER, and we could attain just 25% of the speed of light, we’d be going significantly faster than what the speed of light actually is now. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [May 25, 2021, 5:04pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/9 "2021-05-25T17:04:26Z")
You’re right, but the OP is talking about time dilation and how it changes travel time _for the ship’s occupants_. So, in our universe, c is 300,000 km/s, which means a trip to Alpha Centauri takes a minimum of 4.37 years, Earth time. But, on the ship, if you’re close enough to c, that same journey may take you just 2 years, or indeed two weeks. It’s just that if you make it a round trip, when you arrive back on earth you’ll find that no matter how little time passed according to your ship’s clock, on Earth over 8.74 years have passed (with just how far “over” depending on just how close you got to c). Meanwhile, if c is 30 gigameters per second, and the ship travels at 25% c, then the travel time for a round trip, according to Earth clocks, is 127 days. Meanwhile the ship time for 0.25c would be…121 days. Assuming I’ve calculated correctly. You have to get very close to c for time dilation to really ramp up. ---
**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:** [May 25, 2021, 6:14pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/10 "2021-05-25T18:14:53Z")
> [@RealityChuck](#): > > Mr. Tompkins covered the ramifications of a slower speed of light quite thoroughly. That phrase reminded me of a video game that was produced several years back: > **[A Slower Speed of Light](https://en.wikipedia.org/wiki/A_Slower_Speed_of_Light)** > > A Slower Speed of Light is a freeware video game developed by MIT Game Lab that demonstrates the effects of special relativity by gradually slowing down the speed of light to a walking pace. The game runs on the Unity engine using its open-source OpenRelativity toolkit. > In A Slower Speed of Light, the player controls the ghost of a young child who was killed in an unspecified accident. The child wants to "become one with light", but the speed of light is too fast for the child. This is solved th... The official trailer for the game: [![](https://img.youtube.com/vi/uu7jA8EHi_0/maxresdefault.jpg "A Slower Speed of Light Official Trailer — MIT Game Lab") ](https://www.youtube.com/watch?v=uu7jA8EHi_0) ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [May 25, 2021, 6:41pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/11 "2021-05-25T18:41:09Z")
> **[Goodreads](https://www.goodreads.com/book/show/1382560.Redshift_Rendezvous)** > > Discover and share books you love on Goodreads. Redshift Rendezvous by John Stith is set on a starship traveling in a space where c is much slower than ours. It is a murder mystery, but the fun part is dealing with the relativistic effects of everyday shipboard life. ---
**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:** [May 25, 2021, 8:07pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/12 "2021-05-25T20:07:37Z")
> [@Squink](#): > > So if the vacuum were made of water or diamond, the speed of light would be slower. > I doubt you be able to go faster than c in such a medium. In practice, the index of refraction of water or diamond isn’t all that high, and so the speed of light through those media is still hella fast by human standards, and so it would indeed be wholly impractical for a human-bearing vessel to exceed those speeds. But it’s still possible, and if you relax a few of those constraints, it’s actually commonplace: Subatomic particles, for instance, are routinely accelerated to speeds greater than the speed of light in water, which can produce an effect called Cherenkov radiation which is analogous to the wake of a boat or a sonic boom. And there are some exotic materials with a ludicrously high index of refraction, such that the speed of light through them is only a few tens of meters per second. (also, as a nitpick, you appear to be using c to mean the speed of light in the medium, but the convention is to use that symbol only for the speed in a vacuum, and some other variable to represent the speed in some medium) ---
**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:** [May 25, 2021, 9:23pm UTC](https://boards.straightdope.com/t/if-the-speed-of-light-were-slower-could-we-travel-to-the-stars/942843/13 "2021-05-25T21:23:04Z")
> [@EdwardFound](#): > > But what if the speed of light (the Einstein constant) were much, much lower: something comparable to the speeds of our current spaceprobes. Could I then boost my ship up to a now-tractable relativistic speed and survive a trip to Alpha Centauri? Aside from the other problems brought up, the effect of radically changing the value of _c_ would have would be wide-ranging, from electrodynamics and quantum field theory to general relativity and cosmology, likely rendering the universe uninhabitable (at least, for us). The problems with interstellar travel are really three distinct problems (aside from the issues of recycling resources, psychosocial dynamics, and so forth); propulsion, power generation, and thermodynamics, and they’re interlinked problems as illustrated below: The propulsion issue is limited by momentum exchange, e.g. to go anywhere you have to balance the change in momentum of the ship by an equal and opposite transfer of momentum in the opposite direction. We do this with chemical propellants, and to a limited degree with ionized plasma, but this requires carrying reaction mass with the ship which means that most of the momentum is used in accelerating propellant mass that will only later be used, and the of course enough mass to reverse the acceleration at the end of the trip. You get better efficiency the faster you eject mass, but this also requires more energy per unit impulse, which not only means you have to be able to generate power but results in more ‘waste heat’, i.e. energy that is not used for propulsion. We tend to think of energy as being an essentially unlimited resource since virtually all of our energy comes from the Sun (except that from terrestrial nuclear decay, e.g. fission and geothermal), but in interstellar space your entire energy supply has to be carried with you, which is yet more mass. And energy, once ‘used’, e.g. moved from an accessible ‘ordered’ state to an inaccessible ‘disordered’ (or ‘low temperature) state, cannot be renewed. Finally, the problem that few people other than actual spacecraft and satellite designers consider is thermodynamics, i.e. the rejection of ‘waste heat’, not only from the propulsion system as noted above (although while in operation it will be by far the largest source) but all operations aboard the ship including computing, recycling, cooling, and whatever else is needed to maintain functionality and habitability. All of the unrecoverable waste heat needs to be ultimately rejected into the cosmic background, which means any large spacecraft is going to be mostly outward-facing radiator surface, which is again more mass (and inert mass that isn’t even usefully for eventual momentum transfer). The need for radiating surface is why the Space Shuttle Orbiter Vehicle had its payload bay doors always open while in orbit and is actually the limiting factor in International Space Station operations. So, getting to Alpha Centauri or other interstellar destinations is not just a matter of getting there quickly but also being able to get there at all while maintaining some acceptable threshold of habitability and functionality. Stranger