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

is that true really?

Pretty well. According to Einstein's theory of
relativity, which has been around for over a century,
is now fully accepted, and for which there is a vast
ammount of experimental evidence, all ineretial
observers will measure light to travel at the same speed.

if living things cannot see, and therefore not
be able to see light, then maybe we will think everything is relative,
as we don't know light exists.

I do not understand that.

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 it "absolute speed" here or "relative speed"?

It is relative speed that matters. There is no
absolute speed (except for light in a certain sense).

If I am in motion relative to you then you will measure
me to be contracted along the line of relative motion,
but you will measure me to be contracted. Weird, but
that how things seem to be.

is it true that only
an 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.

Correct.

(once i heard that an object cannot move faster than the speed of
light, as the mass will approach infinity and therefore no energy is
enough to make it move faster... so is that not true, since its mass
according to the ant is unchanged).

No object can travel at the speed of light, measured
relative to any other inertial object.


Martin Hogbin