JC's advice on the precision of phraseology is the most important
thing dealing with speeds/velocities.

To say, for example, "nothing that has mass can travel at or faster
than the speed of a photon moving through 'empty vacuum'", is one
thing; it is quite another to say "no speed, however derived,
whatsoever, may rival or surpass the speed of light."

Three examples, two faster than light speed derivatives, and one
about the contradiction of photon/light itself:

1. "inflation" (Guth), which most physicists seem to allow in their
thinking as being possible correct (not wrong anywhere theoretically),
DID expand FTL;

2. The DISTANCE (possible distances in the plural form) between
far-flung objects, can expand FTL, if the objects are far enough apart.

3. As the Nobel Laureate Robert B. Laughlin of Stanford said,
the notion that "nothing that has mass" can travel at the speed of
light is nonsense, is a contradiction: light (or as Laughlin puts
it [pp.125-6, A DIFFERENT UNIVERSE]: "real light", as opposed
to idealized Newtonian light), contrary to popular beliefs, even
among working physicists, DOES HAVE MASS,,, or that is,
OUGHT TO HAVE MASS, according to the theory of relativity.
"Real light," even when stone cold, has energy, which should
generate mass, which, in turn, should gravity,,,, But that's not
happening, Laughlin speculates, there is something wrong with
our understanding of light, or with Einstein's little beautiful
equation,
or both.

According to Einstein's relativity theory, then (PER Laughlin),
photon/light/energy should not be traveling around 300,000,000
meter/s (186,000 miles/s), if that's the upper asymptote for
non-mass object (which shouldn't be called "object" because
"object" connots some entity that possess definable and thus
measurable features, like mass, weight, shape, etc. --- that we
lack the tools to measure something or some entity at the photon
level or beyond is NOT the same thing as saying they don't have
mass, shape, texture, weight, so on and so forth, as our current
physical theories keep saying,,,,

Good advice.

Lao


cirejcon wrote:
Steven wrote:
I'm sure this question has been asked many times before, but here
goes...

If two objects are moving in opposite directions, and both are moving
at .9c relative to a stationary object, how is it that their speeds
relative to each other do not surpass the speed of light?

In cases like this, you have to be very precise about the term
"relative velocity". You might mean how fast the distance
between the two objects is changing in your frame of reference.
This is referred to as the "closing velocity", and in this case
it would be 1.8c, as you would expect. The other definition is
how fast one object is moving *in the frame* of the the other.
This will always be less than c. If this seems counterintuitive,
just remind yourself that you don't often travel near the speed
of light, so intuition is not of much use.

-jc