Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|cite|improve this question
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 '14 at 0:47
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 '14 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 for GR and QM having a "sentient being" is ok but for SR its Not. – Peter Mar 10 '14 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 '14 at 10:50

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.