Followup to my lower bound light speed question. I was told on the Sandbox to split the lower and upper bound for light speed in different Questions. Yes they are similar but it is possible that different formulas bind the light speed to a low bound than to an upper bound.

I am investigating the possibility of modifying the speed of light in my world. Assuming that the speed of light in a vacuum can (and in my world, is) modified, what upper bound exists that would permit humanity to exist?

I want to know how high light speed could get while still allowing for humans to live.

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    $\begingroup$ Given L.Dutch's answer and my comment on your lower-bound question, this question will probably go the way of the dodo. $\endgroup$
    – JBH
    May 22, 2019 at 17:46
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    $\begingroup$ I would remove this question, indeed. It's basically a duplicate of your other one $\endgroup$
    – L.Dutch
    May 22, 2019 at 18:19
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    $\begingroup$ There's no reason to think humans (or even the universe as we know it) would even exist if light speed wasn't exactly what it is. $\endgroup$ May 22, 2019 at 18:26
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    $\begingroup$ The question looks like a duplicate as a consequence of the answer. Question-wise it makes sense to separate the two cases. $\endgroup$
    – L.Dutch
    May 22, 2019 at 19:50
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    $\begingroup$ See my answer at the sister question about a lower bound. A different $c$ means different $\varepsilon_0$ or a different $\mu_0$ or both, which means that all electromagnetic phenomena have different strength, which means that all chemistry is different. Different chemistry entails no humans. Life might be possible, intelligent life might be possible, but there will be no humans. $\endgroup$
    – AlexP
    May 22, 2019 at 20:14

2 Answers 2


The fine-structure constant is the one controlling most of the properties allowing life as we know to exist.

It can be expressed as $\alpha=\cfrac{ k_e\cdot e^2}{\hbar\cdot c}$, where

  • $k_e$ is the Coulomb constant
  • $e$ is the elementary charge
  • $c$ is the speed of light in vacuum
  • $\hbar$ is Planck constant

As you see, if you change $c$ you change $\alpha$, and that would make life impossible.

The anthropic principle is a controversial argument of why the fine-structure constant has the value it does: stable matter, and therefore life and intelligent beings, could not exist if its value were much different. For instance, were α to change by 4%, stellar fusion would not produce carbon, so that carbon-based life would be impossible. If α were greater than 0.1, stellar fusion would be impossible, and no place in the universe would be warm enough for life as we know it.


What would be the lower bound for light speed which still allows Human live?

Exactly what it is: $c$

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    $\begingroup$ Seriously y’all, don’t mess with the fundamental constants. $\endgroup$
    – Joe Bloggs
    May 22, 2019 at 20:58
  • $\begingroup$ @JoeBloggs ...without reading the manual first. $\endgroup$
    – Ray
    May 23, 2019 at 7:20
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    $\begingroup$ Useful link for next time: MathJax basic tutorial and quick reference and, for those times when you know a symbol but not the LaTeX for it, Detexify2 (that's also mentioned in one of the answers to the earlier-linked Meta question). $\endgroup$
    – user
    May 23, 2019 at 14:03
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    $\begingroup$ I don't think your conclusion in this answer or in the related answer follow the setup you've provided. You are given "c is one parameter that influences α" and "if α changes even by a small amount, life is impossible" and then conclude that "if c changes by a small amount, life is impossible". There are other constants in your function that the OP could modify in parallel or opposition to "c" and still have the same "α". You should elaborate on why this strategy would not work to make life possible with a different "c". $\endgroup$ May 23, 2019 at 20:47
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    $\begingroup$ @DavidCoffron, OP is asking about changing, c, not about changing c and other constants. $\endgroup$
    – L.Dutch
    May 24, 2019 at 2:35

I don't think anyone here has sufficient expertise to answer your question definitively -- and I have certainly not noticed anything in the professional literature which answers your question. But I think I can approach it with an analogy:

Some scientists have looked at the possibilities of a universe without the weak force (the weak force is responsible for radioactivity and has a major impact on how stars produce energy. The conclusion is that such a universe could evolve stars and planets and thus life.

Here's a link to one such paper: Universes without the Weak Force: Astrophysical Processes with Stable Neutrons and here's another: Nuclear Processes in Other Universes: Varying the Strength of the Weak Force.

My strong guess is that if a life-bearing universe without the weak force is possible, one with a somewhat larger speed of light ought to be, also. But going much deeps requires detailed analysis.


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