Special relativity:Reality or measurment error(?

[ghwellsjr]

I have another question.In SR there is a notion called rest mass.but we know sth which is at rest in one frame of refrence,is moving in another.So How can we define rest mass when there is nothing called rest?

In SR, you must select just one frame at a time. Anything at rest in that frame will exhibit its rest mass.

 

[jester1989]

Hey! read 1 of the TS original questions. super interesting. So if we have a 2m car, a 1m hole and an stationary observer. now the 2m car is accelerated to approx c. then the observer measures the car nw, it is less than 1m, will it fall into the hole? (since the hole is not moving wrt to observer, even if he measures it again, it will still b 1m right?)

hmm from the car driver pt of view, he will always measure his car at 2m right? but he will see the hole contract? if so...then he (even more) won't fall into the hole from his pt of view.

 

[John232]

The car would contract he just wouldn't be able to measure it with a contracted ruler, so he would measure his car to still be 2m but it could still be smaller than the hole.

Same problem of two observers both measuring each other to contract doesn't fit into one single reality. I would think that since the car does the acceleration it is the one that actually contracts, but then acceleration doesn't cause spacetime dialation...

Earth has never contracted due to an object traveling close to the speed of light either.

 

[JesseM]

I did a Google search on the phrase "relativistic velocity composition formula" and found only 14 hits but I couldn't get a clear definition of the formula.

Usually terms for equations don't actually have the word "formula" at the end of them, if you search for "relativistic velocity composition" you get 1,990 hits and if you search for the more common synonym "relativistic velocity addition" you get 10,300 hits.

 

[jester1989]

why would it still be smaller than the hole? if the stationary observer measures it again(when it is traveling at near speed now) but this time it is less than 1m, shouldn't it fall into the hole?

sry if i seem slow. this concept is quite non intuitive.=x

 

[John232]

The saying goes that you can't measure your own length contraction because your ruler would contract by the same amount. So he would measure himself as being the same size but he would still be contracted to a smaller size.

But, he would only be contracted in the direction of motion so if he was traveling head on into the hole then he still wouldn't fit because his ship would still be just as wide.