shuba a écrit :


Right. This is the normal definition, as it *is* the velocity of
an object in a frame. To find that velocity, we use the
hyperbolic trigonometry of special relativity.

define: A = arctanh(u/c) B = arctanh(v/c)

V_total/c = tanh(A + B)
= (tanh(A) + tanh(B))/(1 + tanh(A)tanh(B))
= (u/c + v/c)/(1 + uv/c^2)

For u = .9c and v = .9c we have,

V_total/c = (.9 + .9)/(1+.81) ---- V_total = .9945c

Good.

Also, you can use directly Lorentz's transformations.

http://hachel.chez.tiscali.fr/general2.gif



--Tim Shuba---

R.H.

"Steven" wrote in message
ups.com...
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?


Light is being transmittted by a stationary ether at a max. speed of c. For
a third party observer O he can see that A is moving at .9c away from him
and B is moving away from him at .9c and therefore O can conclude that A and
B are moving away from each other at 1.8c. However from A"s (or B's) point
of view, he will recieve light from B via the ether at a max. speed of c.
Therefore A can only see that B is moving away from him at a speed of .9945c

Ken Seto

On 21 Jun 2005 16:20:18 -0700, "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
1.8c? Have you taken any physics courses?

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

Aristotle wrote:
On 21 Jun 2005 16:20:18 -0700, "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
1.8c? Have you taken any physics courses?

I have taken them and taught them. I also regularly deal with
things going near the speed of light.

If two objects are moving directly toward each other, each moving
with a velocity .9c as measured in my frame of reference, then the
distance between them, also as measured in my frame of reference,
is decreasing at 1.8c

This follows quite simply from the definition of velocity. If you
get a different answer, it's because blindly plugged something
into an equation that wasn't applicable.


-jc

"cirejcon" wrote in message
oups.com...


Aristotle wrote:
On 21 Jun 2005 16:20:18 -0700, "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
1.8c? Have you taken any physics courses?

I have taken them and taught them. I also regularly deal with
things going near the speed of light.

If two objects are moving directly toward each other, each moving
with a velocity .9c as measured in my frame of reference, then the
distance between them, also as measured in my frame of reference,
is decreasing at 1.8c

I suggest you stop teaching and take a refresher course.


This follows quite simply from the definition of velocity.


The definition of velocity and the lorentz transformations show otherwise -
http://math.ucr.edu/home/baez/physic.../velocity.html

If you
get a different answer, it's because blindly plugged something
into an equation that wasn't applicable.

As I said above a refresher course is obviously in order.

Bill



-jc

wrote in message
oups.com...
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.

If he said that then I think that Noble prize may have been misplaced - but
then again I strongly suspect you are taking what he said out of context.

"Real light," even when stone cold, has energy, which should
generate mass, which, in turn, should gravity,,,,

If light has mass is purely a definitional thing depending on if you accept
the definition of realistic mass applies to light. Most physics I have read
do not accept such a definition as reasonable physically. Since the source
of gravity is the stress energy tensor then yes light creates gravity - but
that does not mean it has mass.

Bill

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

here we go, a moron talkin to another moron

"Bill Hobba" wrote in message
...

"cirejcon" wrote in message
oups.com...


Aristotle wrote:
On 21 Jun 2005 16:20:18 -0700, "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
1.8c? Have you taken any physics courses?

I have taken them and taught them. I also regularly deal with
things going near the speed of light.

If two objects are moving directly toward each other, each moving
with a velocity .9c as measured in my frame of reference, then the
distance between them, also as measured in my frame of reference,
is decreasing at 1.8c

I suggest you stop teaching and take a refresher course.


This follows quite simply from the definition of velocity.


The definition of velocity and the lorentz transformations show
otherwise -
http://math.ucr.edu/home/baez/physic.../velocity.html

If you
get a different answer, it's because blindly plugged something
into an equation that wasn't applicable.

As I said above a refresher course is obviously in order.

I have an apology to make. JC does not nee to take a refresher course - I
need to take a cause in comprehension. He is perfectly correct when he says
'If two objects are moving directly toward each other, each moving with a
velocity .9c as measured in my frame of reference, then the distance between
them, also as measured in my frame of reference, is decreasing at 1.8c'

With sincerest apologies to JC.
Bill


Bill



-jc

On Tue, 21 Jun 2005 21:49:18 +0000 (UTC), "Martin Hogbin"
wrote:


"Steven" wrote in message ups.com...
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?


Because space and time are not like you think they are.

Martin Hogbin

That is a non-responsive answer!

As measured by this Third Observer, their relative speed is 1.8c. As
measured by each other, their relative speed is 1.8c/1.81 which is
approximately 0.9945c. However notice that there is this Third
Observer. Relative to Him, neither object moves faster than c, nor
does any object relative to another object move faster than c. As far
as the Theory of Relativity is concerned, the Third Observer is
irrelevant. The theory is concerned with what two observers measure
with respect to each other.

Note well that the three different observers get two different
answers.

HTH,

the softrat
Sometimes I get so tired of the taste of my own toes.

--
It's like pushing a car uphill with a rope.