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The two tenants of Special Relativity are:

  1. The laws of physics are the same in all inertial frames of reference.

  2. The speed of light in free space has the same value $c$ in all inertial frames of reference.

So according to 1 above: if I am in a large closed box traveling a constant velocity ie an Inertial Frame of Reference and I set up an experiment to measure the speed of light then my result would be 300,000 kps. If I then set up a a gun that can fire a bullet a $1/2 c$ I would measure its velocity at 150000 kps.

But in this "closed" frame of reference where I am the only observer would the bullet gain mass,and Time dilate? In other words at a certain percentage of the speed of light start would the bullet start to behave Relativistically.

And if so doesn't that violate the first Proposition of SR?

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4  
If the bullet is moving at $1/2 c$ in your reference frame than you would certainly observe relativistic effects. How does that violate SR? That's exactly what should happen according to the first postulate. –  user27578 Mar 10 at 0:47
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You're not the only observer and you're not even required to be in the box in the first place. In SR, "observer" doesn't mean "sentient being". From Wikipedia "Observer (special relativity)": Speaking of an observer in special relativity is not specifically hypothesizing an individual person who is experiencing events, but rather it is a particular mathematical context which objects and events are to be evaluated from. The effects of special relativity occur whether or not there is a sentient being within the inertial reference frame to witness them. –  Alfred Centauri Mar 10 at 1:59
    
Well no wonder 'modern' physics is in such a state of confusion. If you want to read on in the wikipedia article that you use for a source of "Authority" "In general relativity the term "observer" refers more commonly to a person (or a machine) making passive local measurements, a usage much closer to the ordinary English meaning of the word. In quantum mechanics, "observation"is... "observer" with a measurement apparatus and observable with what can be measured.so for GR and QM having a "sentient being" is ok but for SR its Not. –  Peter Mar 10 at 10:24
    
The "laws of physics" meaning the classical laws of physics regarding Force, momentum. acceleration etc. SR is not a law its a theory. Its a theory about how different observers view events in different inertial frames of reference moving at different speeds relative to each other. –  Peter Mar 10 at 10:50

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