Problem about Einstein's postulate

[silver]

Question:
evaluate its validity:
"einstein said that nothing can go faster than c but if I'm in a rocket going at 1/2c and I shine a laser out of the noes of my rocket, then the laser beam is going 1+1/2c, so einstein is wrong."

Please help...it is so hard to understand...

 

[sridhar_n]

..

einstein said that nothing can go faster than c

You have answered ur question... "c" is the ultimate speed. Though its value is finite it has got infinity like properties: c+c=c, c+1 = c etc.Special Relativity also says that C is constant with respect to all reference frames. Which means whether u are moving, stationary, accelerating, C is always constant.

Sridhar

 

[HallsofIvy]

If you are going to "evaluate" relativity, then you need to work in terms of relativity.

The formula you are using: "if I'm in a rocket going at 1/2c and I shine a laser out of the noes of my rocket, then the laser beam is going 1+1/2c," or more generally, "if my speed relative to point O is u and the speed of some other object relative to me is v, then the speed of that object relative to point O is u+v" is classical mechanics, not relativity.

The relativity formula is "(u+v)/(1+ uv/c²)".

In the particular example where v= c this reduces to
(u+c)/(1+ u/c)= c(u+c)/(c+u)= c. Light travels at "c" relative to ANY reference point.

 

[silver]

thank you very much...
but my teacher hasn't talked too much about the relativity yet, she just wanted us to use the words to explain what happen...
so i just used "the speed of light appeared the same to everyone,everywhere" this postulate to explain it...
but the main thing is, i don't understand how this postulate made up...can you explain how it was made up,or suggest where could find some brief information about it...

 

[arcnets]

silver,
I think there's two different things being mixed up here.

I think the basic postulate (backed directly by experiment) is:
"Vacuum lightspeed is the same for any observer, regardless of his state of motion".

So the answer to your original question is easy:
"The laser beam is going at c. For any observer."

The other postulate is "Nothing can move faster than c".
I think this is much more sophisticated than the other one. If you make a statement containing the word "nothing", you have to check very carefully on all aspects of physics - and define very precisely what you consider a "thing". You might (or not) arrive at this statement if you carefully draw the implications of the other one.

I think the postulate "Nothing can move faster than c" sounds spectacular, but is not a good starting point when teaching relativity. The other one, IMO, is.

 

[arcnets]

Ambitwistor: Agree. Nice post IMO.