# Galileo's relativistic postulate question

1. Mar 8, 2009

### bernhard.rothenstein

Consider the following statements:
1. If you move relative to me with velocity V I move relative to you with velocity -V.
Consider that I am equipped with a machine gun and with a laser gun at rest relative to me and you are equipped with identical machine gun and laser gun at rest relative to you. We are in relative motion with constant speed. If I measure the speed of the light emitted by my laser gun and the speed of a bullet fired by my machined I obtain c and u respectively. If you measure the speed of the light emitted by your laser gun and the speed of the bullet fired by your machine gun you will obtain c and u as well.
Would Galileo o.k. the two statements.
If yes I will continue.

2. Mar 8, 2009

### tiny-tim

No …

Galileo would have said that the light from a laser gun goes at c only as measured by an observer who says the gun is stationary

(except i'm not sure he wouldn't have said it goes infinitely fast )

3. Mar 8, 2009

### bernhard.rothenstein

Thanks.
I have mentioned that "my laser gun is at rest relative to me and that your laser gun is at rest relative to you. I measure the speed of the light signal emitted by my gun, you measure the speed of light emitted by my laser gun. As I see you aggree with the fact that we both obtain the same c. Can I consider that your answer is yes...?
I have in mind a modern Galileo being able to measure very short time intervals.

4. Mar 8, 2009

### tiny-tim

oh i see … then yes … the situation is symmetric anyway

5. Mar 8, 2009

### sganesh88

"Would Galileo o.k. the two statements."
Yes. So is your next question, "Why do I observe the speed of the light from YOUR laser gun also as c?" :)

6. Mar 8, 2009

### bernhard.rothenstein

Good question.
Consider that I could imagine a scenario in which I not obliged to measure the speed of the light signal emitted by your laser gun and you are not obliged to measure the speed of the light signal emitted by my laser gun. If I could derive, following that scenario, the formulas which account for relativistic effect then I could consider that they are the result of the first postulate. It seems to me that time dilation and length contraction could be derived so.