by jinydu » Sat Jun 12, 2004 4:15 am
Let me try to make this clear: There is no universal reference frame! Therefore, you CANNOT say that length actually contracts and that it is only your measurements that are going wrong!
It may be helpful to think about it this way. Suppose you are in a car, travelling down a road. Your speedometer says that you are traveling at say, 20 m/s. "Common sense" suggests that the ground is stationary while you are moving at 20 m/s. Thus, we think that the ground provides an absolute reference frame. But is this necessarily true?
Suppose were not in a car, but instead a truck. Also, say that the road was perfectly smooth, so that it didn't have any bumps. Further suppose that this is no ordinary truck, but a VERY large truck. In fact, it is so huge that there is an entire society that lives inside the truck. People are born and raised inside this truck. From a very early age, they are taught that the truck is stationary and the Earth is moving backwards from under them.
Say, one day, this truck happens to pass by your home and you decide to talk with one of the people on the truck (call him T). You tell T that you are stationary while he is moving forward. T disagrees: He says that he is stationary while you are moving backward. Trying to prove him wrong, you tell him to drop a ball. You observe the ball moving in a parabola. Triumphantly, you tell T: "See, the ball moved in a parabola, therefore you are moving.". He replies "No, I saw the ball move in a straight line downwards. How about you try dropping a ball." This time, you claim that the ball moved in a straight line downwards while he claims that it moved in a parabola. According to the Special Theory of Relativity (SR), there is NO experiment that will prove either you or T wrong.
To see this more clearly, imagine two spaceships, X and Y. If X says that Y is moving in a particular direction with speed v, Y will say that X is moving in the opposite direction with speed v. How are we to decide who is correct? If you say X is right and Y is wrong, can you give a reason? If you say Y is right and X is wrong, can you give a reason? If you say that both are wrong and appeal to some special reference frame (Z), what should Z be? You may claim that Z should be space itself. But in order to observe space, you need an observer (call it O). Furthermore, either X or Y will claim that O is moving, and hence O will not correctly determine Z. Once again, you are caught in the same conundrum.
SR provides a way out of the problem: Abandon the notion of an absolute reference frame. That is, it is meaningless to ask whether a particular object is moving or stationary, because there is no universal "judge" to decide. Now, SR does show that our measurements of an object's mass and length (in the direction of relative motion) do depend on the object's relative velocity. But because there is no absolute reference frame, it is incorrect to say that travelling near the speed of light will make you see wrong length and wrong mass because:
1) There is no universal judge to say that you are travelling near the speed of light.
2) There is no universal judge to say that you are measuring the wrong length.
3) There is no universal judge to say that you are measuring the wrong mass.
In short, there is NO ONE correct value of an object's velocity, length or mass.
What SR does allow you to do is predict what particular reference frames will observe.
Furthermore, arsenic said that:
"Eientien has created spacial relative theory
and the theory is about light because human use light to see things "
This is also incorrect. SR does give light (or more precisely, electromagnetic radiation) a "special" status, in that all non-accelerating observers will observe light as moving at the same speed. However, electromagnetic radiation has this special status NOT because humans use it to see things, but rather because of Maxwell's Theory. Maxwell's Theory predicts that electromagnetic waves are measured to have the same speed by all non-accelerating observers. The reason for this is (I think) buried deep inside Maxwell's Theory. But it is this prediction that gives electromagnetic radiation its special status. Therefore, if a blind but non-deaf person will NOT come up with a theory (at least not an accurate theory) where sound is given the same status because sound does not have this same property of speed constancy. That is, if you run away from a sound source, will WILL measure the speed of sound to be lower than normal, while if you run away from a light source, you WILL NOT measure the speed of light to be lower than normal.
Please remember that "It is impossible to reach the speed of light." is NOT a postulate of SR (i.e. it is not assumed). Instead, it is proven from more basic assumptions.