Question

If you observe people moving by you at very high speed, you will say that their time runs slowly, their lengths are contracted in the direction of motion, and their masses are increased from their rest masses. What will they say about you?

Answer 1

Answer: They would observe same things about you all due to relative motion.

Explanation: An observer traveling at a very high speed close to the speed of light {c} would observe time to be slower, lenght to be shorter and mass to be reduced by a factor of √{1 - v²/c²}. That is square root of {1 - v²/c²}. Where v is the moving observer velocity which is close to the speed of light say 0.8c. And c is the speed of light 3*EXP(8)m/s².

For example if the stationary observer measured the length to be L¹ then the person moving at a very high speed will measure a shorter lenght as,

L = L¹ *{√{1 - v²/c²}}.

Same thing apply to mass m and time t.