Re: relativity? paradox?

Harry wrote: 
"Ulala" <zhulien..REMOVETHIS..@xxxxxxxxxxx> wrote in message
news:438EB310.2030906@xxxxxxxxxxxxxx

Bill Hobba wrote:

"Ulala" <zhulien..REMOVETHIS..@xxxxxxxxxxx> wrote in message
news:438d8000$1@xxxxxxxxxxxxxxxxxxxxxxxx


Hi,

I have been thinking after reading the Physics FAQ the following.

I guess there is so much to swallow and I can never know everything, so 

I

would like to toss in a few questions not specifically with the hope of 

a

straight answer, but moreso a reference to which theories to read up on
(not really interested in forumulas or proving the theory though).


If I take an object (eg: myself).  I am the observer and I have a mass
(assume whichever interpretation of mass you like).  I stand still (from
my observation) but I am moving at a high velocity because I am on Earth
which is moving at high speed around it's orbit.  The earth is in our
solar system which is also moving around.


First mistake - motion is not absolute. The earth is not 'moving' any 

more

than a car stationary at a stop sign 'moves'. All motion is relative to 

a

coordinate system and that coordinate system is arbitrary.



Our galaxy is moving around. Perhaps there are clusters of galaxies that
move around.  Relative to myself I am still, relative to Earth I am


still.

My mass in relation to Earth can be calculated, but is this my true mass
considering all the moving around that's going on?


By definition rest mass is the mass measured at rest relative to an 

inertial

coordinate system and it is invariant.  You can't really argue with a
definition and ask questions like is it the real mass - such questions


are

pretty pointless.



Now, given the unknown levels of components of motion (could it be
infinate?) it would suggest that it isn't possibly infinate because I
would be approaching the speed of light just by standing still


(observing

myself).  Given a finate known number of components of motion, perhaps a
true actual velocity could be calculated and therefore my true mass


also?

Another way of thinking about this could be if I had a train moving
forward at 100km/h on Earth, on it there was a smaller train travelling


at

100km/h (ie: it is 200km/h if Earth is the observer), on this smaller
train was another smaller train travelling at 100km/h, how many trains
upon would we need to travel at the speed of light?  Given that Earth is
an observer of the first train, but the Earth itself is in a


multi-leveled

state of motion.


Multi-leveled state of motion?????????????. Any object can make 

virtually

any motion you like by specifying the appropriate coordinate system 

which is

one reason relativity concerns itself with quantities that are the same
regardless of coordinate system - that is the importance of the


principle of

general covariance - nature should not depend on the coordinate system
chosen.


My thought was entirely that mass must only be relative but after
reading through the physics FAQ, even something like e=mc squared how
can that be?  given that if a mass is relative depending on the
coordinate system (I give the train as a simple to picture example),
each train is relative to eachother.  The topmost train would be closer
to the speed of light than the bottommost, of course two nearby trains
are not much difference.  If the topmost physically couldn't travel at
the speed of light because supposedly it is impossible from what I have
read, then couldn't similar principles be worked from backwards using
the universe ie: what is the total mass of the universe?  It couldn't be
infinate because e=mc squared would suggest that I am moving at the
speed of light from the outermost point of the universe.  If it is true
that I cannot move at the speed of light (relative to anything) then one
could calculate the physical mass of the entire universe as it would
therefore be finite.



Probably you did not do the exercises of Dirk, and I suspect that you didn't
even *look* at the references. Here again the one about how the additional
100 km/h is measured from earth:

 http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html


Thanks for that, yes I did read that and my thoughts are that *if* mass is constant, one can never know the actual mass of an object, but only the mass of an object 'relative' to another. (I read that mass increases as velocity increases) but I so far conclude we cannot know the 'actual' velocity, just the velocity in 'relation' to another. Is this correct?


Perhaps I am mis-interpretting Einstein's theory of relativity, but to
me I feel that two vehicles travelling >.5 lightspeed away from
eachother would therefore be travelling away from eachother faster than
lightspeed - rather than somehow magically being incapable in order to
satisfy the above rule.

Regards, Julian 


As measured from earth, they can each travel with <c in opposite directions,
which obviously makes them travel away from each other with <2c. But
measured from one of each, the other can of course only travel at <c. You
could already have understood that from the above link with which you can
actually calculate that for yourself!

Cheers,
Harald


.

Relevant Pages

  • Re: relativity? paradox?
    ... > (assume whichever interpretation of mass you like). ... > my observation) but I am moving at a high velocity because I am on Earth ... coordinate system and that coordinate system is arbitrary. ... > forward at 100km/h on Earth, on it there was a smaller train travelling at ...
    (sci.physics.relativity)
  • Re: relativity? paradox?
    ... I am the observer and I have a mass. ... I stand still but I am moving at a high velocity because I am on Earth which is moving at high speed around it's orbit. ... Another way of thinking about this could be if I had a train moving forward at 100km/h on Earth, on it there was a smaller train travelling at 100km/h, on this smaller train was another smaller train travelling at 100km/h, how many trains upon would we need to travel at the speed of light? ... Any object can make virtually any motion you like by specifying the appropriate coordinate system which is one reason relativity concerns itself with quantities that are the same regardless of coordinate system - that is the importance of the principle of general covariance - nature should not depend on the coordinate system chosen. ...
    (sci.physics.relativity)
  • Re: relativity? paradox?
    ... >(assume whichever interpretation of mass you like). ... >my observation) but I am moving at a high velocity because I am on Earth ... on it there was a smaller train travelling ...
    (sci.physics.relativity)
  • relativity? paradox?
    ... I am the observer and I have a mass. ... I stand still but I am moving at a high velocity because I am on Earth which is moving at high speed around it's orbit. ... Another way of thinking about this could be if I had a train moving forward at 100km/h on Earth, on it there was a smaller train travelling at 100km/h, on this smaller train was another smaller train travelling at 100km/h, how many trains upon would we need to travel at the speed of light? ...
    (sci.physics.relativity)
  • Re: relativity? paradox?
    ... > my observation) but I am moving at a high velocity because I am on Earth ... > forward at 100km/h on Earth, on it there was a smaller train travelling ...
    (sci.physics.relativity)