There's a famous saying in science fiction: "Relativity, causality, FTL: pick two".

I choose causality and FTL. In a Newtonian universe, where there's a privileged reference frame, the speed of light isn't an absolute limit, and Einstein was wrong, what parts of physics would I need to re-work?

In particular,

  1. Do I need to re-formulate Maxwell's equations?
  2. Do nuclear reactions still work?
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    $\begingroup$ I don't know about the effects on the entire rest of physics, but I can answer #1 easily. The entire point of relativity was because Maxwell's equations seemed to need a privileged reference frame, and when we went to go looking for it, we didn't find one (we found a Lorentz transform instead). No relativity means Maxwell's equations can just work without modifications. $\endgroup$
    – Cort Ammon
    Commented Mar 21, 2017 at 1:54
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    $\begingroup$ It's not a famous saying in science-fiction. It might have currency among SF fans and writers with some knowledge of relativity & FTL travel. It originated with Jason Hinson on his Relativity and FTL website which discusses relativity and FTL travel (surprise!) at physicsguy.com/ftl. The correct form of words is: "Relativity, causality, FTL: pick any two." This has been a service to guard against historical error. And keeping quotations correct. $\endgroup$
    – a4android
    Commented Mar 21, 2017 at 5:18
  • $\begingroup$ @a4android, I guarantee that's not the origin: I've seen the phrase in sources that pre-date the World Wide Web. $\endgroup$
    – Mark
    Commented Mar 21, 2017 at 6:42
  • $\begingroup$ Isn't light instantaneous in Newtonian universe? And if it is, FTL is a void idea. $\endgroup$
    – Mołot
    Commented Mar 21, 2017 at 7:17
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1 Answer 1


Maxwell's equations, yes. Magnetism is closely tied to relativity. That's how the numbers work out anyway.

Two particles of equal charge will repel each other. But, if they move in parallel lines, there is an attractive magnetic force between them. Fun aside, when lightning strikes hollow objects, it shrivels them up because a very very strong current flows down the sides, causing an attractive force inwards.

The faster the two particles move in parallel, the stronger the magnetic force, while the electric force will remain constant. So, when do these two forces cancel? How fast do the particles have to travel in order to equal out the electric and magnetic forces? The answer, according to math, is C.

For a more clear picture, check out the two forces for electricity and magnetism:

$$F_{electric}=\frac{1}{4\pi\epsilon_0}\frac{q_1q_2}{r^2}, F_{magnetic}=\frac{\mu_0}{4\pi}\frac{qv}{r^2}$$

Nothing in any of those formulas is important except the constants. It turns out that


which is cool.

I don't know enough physics to explain exactly how they are related except for the above arguments, or how this affects things like permanent magnets. You'll no doubt have fun googling "why is the magnetic force is what it is" and turn up some cool stuff.

Anyways, from what I can tell, you might have to lose magnetism in your universe. That probably does it for nuclear reactions, too, considering it's called electromagnetic radiation.

Edit: As a disclaimer, I hope I haven't insulted your intelligence with this. You clearly knew they were related already or you wouldn't have mentioned those two specific points of interest. I hope this at least provides a decent starting point for showing how the magnetic force probably wouldn't exist in a universe without relativity, and, consequentially, light.

  • $\begingroup$ Out of curiosity, what is the precise situation in which the equation for magnetic force applies? It seems like the force is purely radial, which is odd. $\endgroup$
    – HDE 226868
    Commented Mar 21, 2017 at 2:45
  • $\begingroup$ It's the Biot Savart law for reference. Magnetic attraction of moving charges. It's been awhile since I've had to do problems with these so keep me honest. Regardless, mu naught shows up everywhere in magnetism. $\endgroup$ Commented Mar 21, 2017 at 3:45
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    $\begingroup$ See this answer. You will have no magnetic fields and no light waves. Nuclear reactions are not “electromagnetic radiation”. $\endgroup$
    – JDługosz
    Commented Mar 21, 2017 at 8:42
  • $\begingroup$ This is correct, nuclear reactions produce em radiation, which is light. So I guess reactions could happen, they just couldn't produce light $\endgroup$ Commented Mar 21, 2017 at 12:30
  • $\begingroup$ @BrianWoodbury Oh, so you assumed a point charge. I thought you were discussing a more general case. $\endgroup$
    – HDE 226868
    Commented Mar 21, 2017 at 13:16

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