Am Thu, 08 May 2008 11:21:08 -0700 schrieb liketofindoutwhy
in
:

I once heard that everything is relative, except the speed of light,
which is absolute...

is that true really? if living things cannot see, and therefore not be

The speed of light in a vacuum is absolute and is defined as
299'792'458 m/s exactly.

ref
http://physics.nist.gov/cgi-bin/cuu/Value?c|search_for=universal_in!

The fact that the speed of light in a vacuum should be self-evident since
one of Maxwell's equations is

epsilon_0 * mu_0 * c^2 = 1

Since epsilon_0 and mu_0 only depend on the nature of space and not the
observer, it follows that c should also depend only on the nature of
space and not the observer.

able to see light, then maybe we will think everything is relative, as
we don't know light exists.

we often hear that when the speed of an object increases (close to the
speed of light), then its mass increases, and its length decreases... is

Actually in modern Relativity, we don't say that the mass increases since
that makes for some logic problems. Instead it's the momentum that
increases with speed.

As for length, that only decreases for cotemporal events (cf Lorentz
Transformation).

it "absolute speed" here or "relative speed"? is it true that only an

Relative speed, since you can only talk about speed with respect to
something else.

observer which sees that object moving close to the speed of light will
measure that increase of mass and decrease of length, but let's say
there is an ant on that moving object, the ant won't measure the object
having increased mass and decreased length.

Relativistic effects become more pronounced as speeds approach the speed
of light in a vacuum. To see a minimum of 10% Relativistic effect you
will need to go at least 17%c.


(once i heard that an object cannot move faster than the speed of light,

Right since going faster would involve providing it more kinetic energy
than the total energy in the universe.

[...]
sorry can things be discussed as if it is explained to a 10-year old?
Thanks very much for answering to such a simplistic question.

No problem. I appreciate honest questions so I can give honest answers.