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This might be a question purely of words and the meaning of them but isn't Newtonian physics more "relative" than Einstein's Special relativity? Newtonian physics predicts that laws of momentum & inertia behave the same at whatever speed you are traveling. Einstein said things change, and do more and more the closer you get to the speed of $c$. You become more and more massive as you approach $c$, therefore requiring more and more energy until you would require infinite energy to break the speed of light.

Why is Einstein's theory called the theory of relativity when it described a world where (for the first time) physics occurring in one spacecraft traveling fast is not analogous to the physics in another spacecraft standing still?

Apologies if this is a stupid question.

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Mike, I added the "History" tag to your question. The answer is really about history and what was known to the protagonists in your tale and at what time. As a principle, relativity was embraced every bit as fully by Newton and Galileao as it was by Einstein - it's just that Einstein had a few more experimental results he had to gather into relativistic thinking. –  WetSavannaAnimal aka Rod Vance Feb 25 at 1:23
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It could be argued that Galileo was empirically shown to be highly committed to relativity, having refused to forsake his commitment in the threat of persecution and torture, whereas Einstein was never put to that test! –  WetSavannaAnimal aka Rod Vance Feb 25 at 1:43

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This question is really about history and what was known to the protagonists in your tale and at what time. As a principle, relativity was embraced every bit as fully by Newton and Galileao as it was by Einstein - it's just that Einstein had a few more experimental results he had to gather into relativistic thinking.

As in dgh's answer the whole point of special relativity is that velocity is a relative concept insofar that the physical laws will seem the same to all inertial observers.

This concept was well appreciated by Galileo and Newton. See for example the quote of Galileo's character Salviati in the Galileo's Ship Thought Experiment of 1632. Saviati's narrative is clearly saying that there is no experiment whereby one could tell whether or not the ship were moving uniformly.

So, in spirit, Galileo's and Newton's physics were no less "relative" than Einstein's. The problem is that there is no unique way to make physical laws the same for all inertial observers. See the derivation of the Lorentz transformation under the heading From Group Postulates on the Lorentz Transformation Wikipedia Page. Here the Lorentz transformation is derived from very basic symmetry and homogeneity assumptions about the Universe. Take heed that the derivation only yields the form of the transformation, it does not yield the universal speed parameter $c$. So there are a whole family of relativities that fulfill these basic symmetry and homogeneity assumptions - roughly that physics is the same for all uniformly moving observers - and both Einstein's special relativity and Galilean relativity belong to this general family; the latter is simply the limit as $c\to\infty$.

Not to detract from Einstein's achievement, Einstein was responding to experimental results not known to Galileo and Newton - namely that speed of light seemed to be constant for all inertial observers - and showing that, against intuition at the time, this observation could still be consistent with the principles of inertial invariance of physics so clearly stated by Galileo's Salviati. It's simply that the reference frame transformation laws had to be different. To Galileo and Newton, the transformation laws implied by inertial invariance would have seemed to be unique; Einstein simply disproved this and showed instead that there was a more general transformation. That transforamtion challenged to notion of absolute time and simultaneity. This was the leap that revealed the non-uniqueness of Galilean relativity, which is indeed unique if one insists on upholding the notion of absolute time. Einstein's physics is "more relative" than that of Galileo and Newton, but only by dint of experimental results. These results forced the forsaking of a principle that Galileo and Newton had no experimental grounds to forsake - that of absolute simultaneity and time.

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Physics occurring in one spacecraft traveling fast (at a uniform speed) is the same as physics in another spacecraft "standing still". In fact, the whole point is that these words - traveling fast and standing still - are relative. All the things you are describing like relativistic mass are only apparent to observers in other reference frames. The person in the rocket ship would not notice anything is different but would, instead, attribute effects like time dilation and increasing inertia to the observer you say is "standing still".

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Also, for what it's worth, we do recognize the relativistic nature of Newtonian mechanics. It's called "Galilean relativity". –  user27578 Feb 25 at 0:48

Both Newton and Einstein say that all the laws of physics remain same in a frame at rest with respect to the observer and also in a frame moving with constant velocity. Now Maxwell showed that speed of EM wave in vacuum equals 'c' is a law of physics. But this law was inconsistent with the Galilean transformation, hence Lorentz transformation were introduced. Lorentz transformations conserve all the laws of physics when going from one reference frame to other (both non-accelerated). I think your problem is inconsistency of Special-relativity with human intuition. I also faced same doubts. This hypothesis helped me- Suppose human had the natural capability to travel near the speed of light, then we would have concluded all the effects of SR beforehand and may have derived Maxwell's laws working backwards. For reference on inertial-frames i suggest you read Mach's principle versus Newton's principle on what is an inertial frame.

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Now for an easy answer: Physics moving (at a constant speed) is physics standing still. From one person looking at another, they will generally disagree about how fast the other is moving and what time it is, but the underlying formulas of physics are unchanged.

The space and time part is a really strange idea, but just imagine what the hell this place would be like if all physics was different.

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How does this message provide an answer to the original question? –  Hunter Feb 25 at 7:32
    
The title question doesn't make much sense. I answered his last question, which gets at the heart of the subject. –  Sun God Feb 26 at 2:56

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