# Just to make sure about special relativity effects...

## Main Question or Discussion Point

Hello Forum,
I have been studying special relativity and its unique phenomena (time dilation, length contraction, mass dilation, etc.)
I would like to make sure that my understanding of these interesting phenomena is correct. For example, the length of an object is not an absolute. What we ordinarily call the length is the proper length in our proper reference frame. Someone moving relative to us would judge that length to be different. Who's right? we all are. We are just so accustomed to measure length (distance interval) in a certain way and time interval the same way...

What about mass? Well, relativistic mass can increase with increasing speed. But does not mean that the object acquires more atoms and molecules because it really doesn't, correct?

thanks,
fog37

## Answers and Replies

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PeterDonis
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does not mean that the object acquires more atoms and molecules because it really doesn't, correct?
The number of atoms or molecules that an object contains is an invariant; it's the same in all reference frames.

Ibix
One can visualise length contraction as an effect similar to a slice through a sausage, which is circular if you slice through perpendicular to the sausage and elliptical if you slice through at an angle. Neither is the "correct" cross-section; they're just different. There is something special about the perpendicular slice, though, in that there's only one way to get that. You can get a 45° slice, for example, by holding the knife at 45° or at 135°.

Relativistic mass is a concept that's largely fallen out of fashion these days. People tend to use "mass" to mean "rest mass", which is invariant between frames. Doing it that way makes the answers to questions like yours rather more obvious - the mass doesn't change, so the number of atoms doesn't change.

Sounds like you are on the correct track...

You might find this article of interest regrading 'length contraction'....a misnomer....
https://en.wikipedia.org/wiki/Terrell_rotation

Here is an interesting tidbit from SR I picked up in these forums that had not occurred to me:

"Consider a rocket that’s initially at rest in some inertial coordinate system, and then accelerates gently to a speed where relativistic effects are noticeable. Viewed from the inertial coordinate system where the rocket started from rest, the rocket is now shorter than before by a factor of gamma. This means that the rear must have had a larger acceleration than the front!"

If interested, search 'special relativity' in these forums.....lots of Q&A available.