These are the two principles of Special Relativity:

  1. The laws of physics are applied identically in all Inertial frames of reference.

  2. The speed of of light is the same in all frames of reference.

Did Einstein come up with the first one or did he just used what Galileo said about motion in 1600? Galileo was the first to state that velocity is relative. And since velocity is relative I don't see what else includes the above-mentioned first principle of Relativity. All Classical Physics up 1900 included that and so does modern Relativistic Mechanics.

  • 1
    $\begingroup$ 1. This question seems to be more about history than actual physics. 2. If you know that Galileo (and others) already thought about relative velocity (and indeed inertial frames feature in Newton's laws, too!), then what, exactly, is the question? $\endgroup$
    – ACuriousMind
    Dec 15, 2016 at 14:06
  • $\begingroup$ 1. is absolutely Galileo. Do you know about his allegory? I think in 200 years (IF humans haven't wiped themselves out), Galileo will be remembered as the relativity guy, and Einstein as the locality guy. But, as Maxis Jaisi rightly points out, it was Einstein who broadened it to all known physics, particularly electromagnetism, because it was widely assumed that Galileo was wrong for that in the 19th century (he didn't know about EM, of course)! $\endgroup$ Dec 15, 2016 at 14:14
  • $\begingroup$ @ACuriousMind It's not about History. I am just asking if the first principle alone brings something to Classical Physics that isn't already there $\endgroup$
    – Bill
    Dec 15, 2016 at 14:14
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    $\begingroup$ I think people overstate the case that Einstein's work was broadened the relativity postulate. Sure, you can compute the speed of light from Maxwell's equations and that computation is frame independent; but you can also compute the speed of gravity ways or sound or waves on a string from theory and those computations also are frame independent. The crucial difference in understanding is that for most waves the speed is relative a medium and the speed of light is not. That is, only the second postulate is new. The first postulate is required, of course but it is the same, just reiterated. $\endgroup$ Dec 15, 2016 at 18:39

2 Answers 2


You're correct that Galileo enunciated the first principle long before Einstein. His principle of relativity is illustrated in a rather picturesque manner in the Dialogue Concerning the Two Chief World Systems. The thought experiment involved is famously known as Galileo's Ship.

But it was Einstein who generalised the principle, postulating that the principle of relativity be true for any physical law, not just the laws of mechanics, but optics and electromagnetism as well. Here's the relevant snippet of Einstein's wonderful 1905 paper, "On the Electrodynamics of Moving Bodies" :

They suggest rather that, as has already been shown to the first order of small quantities, the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good. We will raise this conjecture (the purport of which will hereafter be called the “Principle of Relativity”) to the status of a postulate, and also introduce another postulate...

  • $\begingroup$ "Galileo enunciated the first law"? You mean the principle of inertia? What do you mean the "first law be true for any physical law"? Or by "first law" here, do you mean Einstein's first postulate? $\endgroup$
    – Geremia
    Dec 17, 2016 at 2:38
  • $\begingroup$ I've edited my answer. I hope it's less muddled now. $\endgroup$ Dec 17, 2016 at 7:33

Einstein's first postulate is novel because

  1. "laws of physics" is an idea that is no older than a few centuries (cf. Cornoldi's ch. on "physical laws"),


  1. what Einstein means by "laws of physics" is really the mathematical form of the equations; viz., he is speaking of covariance, how the mathematical form remains the same in different situations.

As described in

p. 1103 (my emphasis):

The two Einstein postulates state that the [mathematical form of the equations representing the] laws of physics and speed of light [$c$] do not change for relatively moving reference systems (being covariant and invariant, respectively).


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