Traveling faster than light poses all sorts of physics problems, so most hard SF has had to handwave FTL. But if light were faster, then ships could be commensurately faster without violating causality, developing infinite mass, etc.

If we postulate an unknown mechanism that changed “c”, leaving all other independent, fundamental constants unchanged, clearly there would be effects at galactic scale. But what about human scale? Would life on Earth be significantly changed? What would break if the universal speed limit were revised upward?

  • $\begingroup$ I did check physics stack exchange first... no one asked this there. I thought it might be too hypothetical for them. $\endgroup$
    – SRM
    Commented Nov 12, 2019 at 13:44
  • $\begingroup$ Increased relative to what other speed? Increasing it relative to a mass or an energy would not make sense. $\endgroup$
    – user38304
    Commented Nov 12, 2019 at 14:52
  • $\begingroup$ Don't think of it as the speed of light. Think of it as the tick rate of causality or the timestep of the simulation. Everything is built off it. $\endgroup$ Commented Nov 12, 2019 at 17:05
  • $\begingroup$ Why do you need your characters travelling faster than light if you can make them live longer which is more realistic? $\endgroup$
    – Anixx
    Commented Nov 13, 2019 at 7:18
  • $\begingroup$ @anixx Social/cultural drift, psychological challenges... science fiction is full of stories contemplating the problems of the longevity approach. Yes, it is more realistic, but it doesn’t tell stories of “today’a people in space” (a la Star Wars) well, do authors continue to look for physics loopholes. $\endgroup$
    – SRM
    Commented Nov 13, 2019 at 13:10

4 Answers 4


$$c_0 = \frac{1}{\sqrt{\varepsilon_0 \mu_0}}$$ where $\varepsilon_0$ is the electric permittivity of the vacuum, $\mu_0$ is the magnetic permeability of the vacuum, and $c_0$ is the speed of light in a vacuum.

This means that you cannot change the speed of light in a vaccum unless you change the strength of electromagnetic forces. Changing the strength of electromagnetic forces by necessity changes all of chemistry. No chemical reaction will work as in our world.

Changing chemistry means that there are no humans in that world; there may be some sort of intelligent beings, but for sure no humans.

Moreover, changing the strength of electromagnetic forces will dramatically change the properties of common materials such as steel or water.

Rather dire consequences...

  • $\begingroup$ +1. (Probably worth specifying that "c0" means "the speed of light in a vacuum") $\endgroup$
    – Qami
    Commented Nov 12, 2019 at 14:49
  • $\begingroup$ @Qami: Done. Good observation. $\endgroup$
    – AlexP
    Commented Nov 12, 2019 at 15:04
  • $\begingroup$ Succinct and to the point. Thanks. $\endgroup$
    – SRM
    Commented Nov 12, 2019 at 18:48

It would grotesquely affect chemistry. Just off the top of my head it would be tough to give details. But, the fine structure constant would change. Rydberg's constant would change. This would change the energy levels of atoms and molecules. Some molecules that are stable would become unstable, some that are not stable now would become so. Certain chemical reactions that currently produce energy would suddenly require energy to be put in, and certain others the other way around.

A 50% replacement of the water in mammals with heavy water is fatal. That replacement represents a change in the mass of a water molecule by about a ratio of 20:22. This is because most organisms are finely adjusted to the rate that various chemical reactions take place and the energy they release or require to proceed. If you increased the speed of light by even 10 percent, most mammals, birds, reptiles, and fish would die in minutes. It would not take much more than that to kill off insects and plants.

Also, it would affect nuclear physics. I'd have to dig further to get details on that. But, for example, it would certainly affect nuclear processes in the sun. I'd have to do a lot of digging to figure out if it made them faster or slower. But many of them are fairly sensitive to small changes in such things. It might well either extinguish the sun or cause it to suddenly increase energy output by a huge factor.

In the country of Gabon in the region known as Oklo, there are the remains of naturally occurring nuclear reactors. One of the very interesting investigations of this formation is tests of the constancy of various constants of nature. This abstract shows the results of one such study. Basically, various nuclear reactions are dependent on such things as $\alpha^4$ or even higher powers. This means that small variations in $\alpha$ will produce huge variations in nuclear reactions. So the maximum change in $\alpha$ since the Oklo reactor operated is about 1 part in 10 million. That is over 2 billion years. Even very tiny changes would have caused this reactor not to operate as it was observed to. It is possible that a change in the speed of light would cause existing deposits of Uranium to become critical masses. You could even have them explode. I don't know, off hand, how much Uranium there is in deposits near the surface, but there are certainly many thousands of tons. One such deposit is in McArthur River, Saskatchewan.. They produce 18 million pounds of "yellow cake" per year. It's quite possible that, if the speed of light was changing, this entire site would explode. Since 100 kg of Uranium produces kilotons, such a thing would probably blast a sizable chunk out of the planet.

You probably don't want to play with the speed of light.


The trouble with 'just' tweaking one of the fundamental constants like this is that a lot of the deep interactions in physics are linked together. Let us try, say, doubling $c$. Since Maxwell's Equations of electromagnetism define $c$ in terms of the electric and magnetic constants:

$${\displaystyle c={\frac {1}{\sqrt {\varepsilon _{0}\mu _{0}}}}}$$

we must also decrease one or both of these by a combined factor of 4. Decreasing the magnetic constant is really dangerous, as it affects the Fine Structure Constant:

$${\displaystyle \mu _{0}={\frac {2\alpha }{e^{2}}}{\frac {h}{c}}}$$

and now we are required to change either $e$ the charge on the electron or $h$ the Plank constant. Either way we're really falling down a rabbit hole as changing the strength of the electrostatic interaction (via $e$) will probably require recalculating all of chemistry, and $h$ affects the energy of all photons and everything to do with heat transfer through systems, as well as affecting lots of quantum interactions which may or may not mess with subatomic physics depending on which unification theory you prefer.

Probably safer to just modify $\varepsilon_0$, which is actually similar to filling the universe with a magical conductive ether. You're still messing with the practical realisation of the electrostatic interaction, which means all your electron shells will be more tightly bound, the enthalpy of chemical reactions will be affected, and anything that depends on precise interplays of electrostatic attractions to hold their shape (like, for instance, proteins) would stop being correctly assembled. If the change happened over too short a span for evolution to adapt, end of all life.

In other news, the famous $E = mc^2$ means that the relativistic energy of everything just went up by a factor of four. This has implications for high-energy astronomy, as you increase things like the Schwarzschild radius of black holes:

$${\displaystyle r_{s}={\frac {2GM}{c^{2}}}} $$

With the effect that lots of accretion disks that were previously inactive would become activated as more material was suddenly 'within range', leading to a massive increase in high-energy radiation, although this would take millions or billions of years to affect life on Earth, although once it did so it would probably do so very negatively. The high-energy charged particles streaming out of the sun in the solar wind, however, would reach Earth in just a few minutes and would become significantly more effective at blowing away parts of the atmosphere.


Let's ignore the fact that life is impossible if you change any of the basic constants -- because that means there's no story.

Anything that depends on light speed for timing would be profoundly affected. Radar comes to mind instantly -- triple the speed of light, suddenly aircraft 50 km away look like 16-17 km. Except that radar (and anything else that runs on radio) would quit working instantly, because the antennas, tuned to the wavelength of the emission, would suddenly be far too short. Efficiency would drop sharply, and the change in impedance might result in destruction of transmitter final amplifier stages. There goes broadcast TV and radio, ham, cell phones, remote control for drones, and so forth.

I think it's fair to say that a sudden large-factor change in the speed of light (in either direction) would be the end of modern civilization -- we'd be back to 1950s power distribution (except that old equipment is long gone) so the power grid would fail because the equipment that controls it has failed.

Guns would still work fine, though, and they'd probably get a lot of exercise over the short period just after the fall.

  • $\begingroup$ Hey Zeiss! I gave a -1 because you're missing/ignoring one of the most important effects of changing the speed of light--namely, the catastrophic effect it would have on the chemistry and basic cohesion of matter (see Stephen or AlexP's answers). $\endgroup$
    – Qami
    Commented Nov 12, 2019 at 14:53
  • 2
    $\begingroup$ @Qami Sure, if every answer isn't the same, the different is just plain wrong. Their answers boil down to "there is no story, because there is no universe after the change*. This isn't a hard-science tag, it's science-based -- and science fiction has always depended on not having to know all the implications of things. Ever actually checked whether Heinlein's Nazis could have gotten to the Moon with alcohol-based rockets? Never mind winged SSTO spaceplanes, even with staged rockets it'd require something the size of Sea Dragon. But it's still a good story. $\endgroup$
    – Zeiss Ikon
    Commented Nov 12, 2019 at 14:57
  • $\begingroup$ Fair point! For the record, I was not trying to be snooty or arrogant by commenting what I thought was wrong with your answer. I just understand that it's considered polite to comment when you make a downvote... $\endgroup$
    – Qami
    Commented Nov 12, 2019 at 15:02

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