Re: Difference in speeds in special relativity

[Dirk Van de moortel]

Steven D'Aprano <st...@pearwood.info> wrote:
> Hi guys, I'm not sure if this is the right forum for this, but I
> thought I'd ask a serious question related to special relativity.

Should be in sci.physics.relativity.
I have set copy and followup to there.
If you reply, it will only show up there.

>
> Consider an observer O in an inertial frame of reference. A particle
> X is travelling towards the observer at v metres per second, while a
> second particle Y travels away from the observer at u m/s:
>
>
> X----> v O Y--------> u
>
>
> All three of X, O and Y are in a straight line, as seen by the
> observer.
>
> My question is this: how does the observer O calculate the difference
> in speeds between X and Y? I think that the difference is just (u-v),
> the same as in classical mechanics. Am I correct?

Yes, by definition of "difference". It is called the "closing speed
between X and Y as seen by O". It is the (time) rate at which
the distance, as measured by O (!), between X and Y changes.
It's not a good name. Depending on the magnitudes of u and v it
could be an "opening speed". It's also called the "relative speed
between X and Y in O's frame".

From the point of view of X (not light) the speed of Y (possibly
light) would be
the absolute value of
( u - v ) / ( 1 - u v/c^2),
and vice versa.

>
> Does this still apply if Y is a photon, and hence u = c?

Yes.

>
> How about if X or Y are travelling in the opposite direction? If X
> and Y are travelling away from each other, this would imply the
> greatest possible difference is 2c (two photons travelling towards
> each other would appear to O to be approaching at 2c, or two photons
> travelling away from each other would appear to be separating at 2c).
> Correct?

Yes. But in this case from the point of view of X (not light)
the speed of Y (possibly light) would be
( u + v ) / ( 1 + u v/c^2),
and vice versa. Here you can't have 2c because light is
never considered to be an observer.

Dirk Vdm

[Steven D'Aprano]

On Thu, 03 Jul 2014 08:43:47 +0200, Dirk Van de moortel wrote:

> Steven D'Aprano <st...@pearwood.info> wrote:
>> Hi guys, I'm not sure if this is the right forum for this, but I
>> thought I'd ask a serious question related to special relativity.
>
> Should be in sci.physics.relativity.
> I have set copy and followup to there. If you reply, it will only show
> up there.
>
>
>> Consider an observer O in an inertial frame of reference. A particle X
>> is travelling towards the observer at v metres per second, while a
>> second particle Y travels away from the observer at u m/s:
>>
>>
>> X----> v O Y--------> u
>>
>>
>> All three of X, O and Y are in a straight line, as seen by the
>> observer.
>>
>> My question is this: how does the observer O calculate the difference
>> in speeds between X and Y? I think that the difference is just (u-v),
>> the same as in classical mechanics. Am I correct?
>
> Yes, by definition of "difference". It is called the "closing speed
> between X and Y as seen by O". It is the (time) rate at which the
> distance, as measured by O (!), between X and Y changes. It's not a good
> name. Depending on the magnitudes of u and v it could be an "opening
> speed". It's also called the "relative speed between X and Y in O's
> frame".

Thanks very much! This is very useful.



Regards,




Steve

[Lord Androcles]

"Dirk Van de moortel" wrote in message
news:lp2u31$enh$1...@speranza.aioe.org...

========================================================
Einstein POSTULATED...

Here we are again, getting nowhere because of a disagreement over the
meaning of words. The following is the sense I am using the word, and
this is the sense explicitly called out in the books I've read about
relativity and its postulates -- shithead Bodkin

Einstein postulated in § 7. Theory of Doppler's Principle and of Aberration:
"It follows from these results that to an observer approaching a source of
light with the velocity c, this source of light must appear of infinite
intensity."

So observer X does travel at c, getting somewhere over the meaning of words.
(1+2)/(1+1*2/1^2) =3/3 =1
Bwahahahahaha!

Of course if X approaches a source of light at 0.9994 c, the source of light
approaches (closes on) X at gamma*(x-vt) divided by t* gamma = v * gamma^2
= 834c and the imbecile Dork Van de faggot will squawk about v = dx/dt being
different to v = x/t and different events.
ork's bluff:
news:lojehl$i1k$1...@speranza.aioe.org...

> As far as I can tell, that's 13 years of swapping (proper)
> length of an object in motion with (coordinate) length of
> that object at rest.
> His using t=0 denotes that an object is moving in the
> (x,t) system, in each case with proper length 1, but at
> rest in the (ksi,tau) system (with coordinate length gamma).

Typo:
... moving in the
(x,t) system, in each case with shorter coordinate length 1,
but at rest in the (ksi,tau) system (with longer proper length
gamma).

He wouldn't have noticed anyway.

Dirk Vdm

===========================================
Dork has finally got it.
The proper stationary coordinate length (x-0) = 1 at rest in the stationary
system K is shorter than the proper moving coordinate length (x'-0') = gamma
(= 28 at v = 0.9994c) properly at rest in the proper moving coordinate
system k, and gets shorter the faster the proper longer coordinate length
gamma moves while at rest in its proper moving coordinate frame k the same
way I am properly at rest in a proper moving aircraft pulled along by a
proper propeller.
Well done, Dork, it only took you 13 years for it to sink in, you proper
dead brained maggot with one working neuron.
Notice Dork is claiming the length x-0 gets shorter than x-0 as chsi
increase with v.

For those that are sane and can manage without Dork's proper verbal
diarrhoea, the moving chsi = 28*dx when v = 0.9994c and t = 0 (but not dt)
and is called length expansion.

-- Lord Androcles, Zeroth Earl of Medway

[Jack...@hotmail.com]

On Thu, 3 Jul 2014 08:43:47 +0200, "Dirk Van de moortel"
<dirkvand...@hotspam.not> wrote:

>Steven D'Aprano <st...@pearwood.info> wrote:
>> Hi guys, I'm not sure if this is the right forum for this, but I
>> thought I'd ask a serious question related to special relativity.
>
>Should be in sci.physics.relativity.
>I have set copy and followup to there.
>If you reply, it will only show up there.
>
>>
>> Consider an observer O in an inertial frame of reference. A particle
>> X is travelling towards the observer at v metres per second, while a
>> second particle Y travels away from the observer at u m/s:
>>
>>
>> X----> v O Y--------> u
>>
>>
>> All three of X, O and Y are in a straight line, as seen by the
>> observer.

Look again. There is no known way for the observer at O to determine
velocity of X or Y. In cosmology. It is done by red shift, but that's
not available here. He might judge that one or the other is
approaching or going away, by the change in the angular size with
time. This is a one-dimensional problem, and you are treating it as if
there was an observer Z in a 2nd dimension that could make these
measurements.
It makes more sense if you change O to P, to designate some point, but
not an observer, who otherwise would require supernatural powers.

John Polasek

[pcard...@volcanomail.com]

On Monday, July 21, 2014 11:42:02 AM UTC-7, Jack...@hotmail.com wrote:
> On Thu, 3 Jul 2014 08:43:47 +0200, "Dirk Van de moortel" <dirkvand...@hotspam.not> wrote: >Steven D'Aprano <st...@pearwood.info> wrote: >> Hi guys, I'm not sure if this is the right forum for this, but I >> thought I'd ask a serious question related to special relativity. > >Should be in sci.physics.relativity. >I have set copy and followup to there. >If you reply, it will only show up there. > >> >> Consider an observer O in an inertial frame of reference. A particle >> X is travelling towards the observer at v metres per second, while a >> second particle Y travels away from the observer at u m/s: >> >> >> X----> v O Y--------> u >> >> >> All three of X, O and Y are in a straight line, as seen by the >> observer. Look again. There is no known way for the observer at O to determine velocity of X or Y.

How ridiculous. The are many ways to measure velocity.

[Jack...@hotmail.com]

On Mon, 21 Jul 2014 12:21:41 -0700 (PDT), pcard...@volcanomail.com
wrote:

>On Monday, July 21, 2014 11:42:02 AM UTC-7, Jack...@hotmail.com wrote:
>> On Thu, 3 Jul 2014 08:43:47 +0200, "Dirk Van de moortel" <dirkvand...@hotspam.not> wrote: >Steven D'Aprano <st...@pearwood.info> wrote: >> Hi guys, I'm not sure if this is the right forum for this, but I >> thought I'd ask a serious question related to special relativity. > >Should be in sci.physics.relativity. >I have set copy and followup to there. >If you reply, it will only show up there. > >> >> Consider an observer O in an inertial frame of reference. A particle >> X is travelling towards the observer at v metres per second, while a >> second particle Y travels away from the observer at u m/s: >> >> >> X----> v O Y--------> u >> >> >> All three of X, O and Y are in a straight line, as seen by the >> observer. Look again. There is no known way for the observer at O to determine velocity of X or Y.
>
>How ridiculous. The are many ways to measure velocity.

With the rock coming right at him. How is he going to measure the
velocity?

[Tom Roberts]

Normally in gedankens we augment an observer with an array of assistants at rest
in her inertial frame, with clocks synchronized in the frame. This makes it
trivial to measure the velocities of moving objects relative to that frame. More
importantly, it avoids having to deal with signal or light propagation delays,
because an assistant is always located at any event of interest and can provide
its coordinate values.


Tom Roberts

[Jack...@hotmail.com]

It seems to me that neither the observer nor any of the assistants can
actually measure the velocity, a quantity which is not easy to
measure.
My favorite example of a valid measurement would be to measure the
speed of a magnetic slug going through a coil driving a calibrated
galvanometer. It would measure speed.
I have to expect that in the present case, the assistants would hand
in reports consisting of pairs of numbers, e.g., x1, t1, x2, t2 so
the quotients could be computed by the observer. But the quotients are
not velocity. It's really too contrived.
There is no meter, or combination of meters that would allow the
observer to measure the velocity toward or away from him.
I just want to stress that too many such easy statements are made,
especially in special relativity where the observer is said to see the
rapidly moving rod as contracted, when in fact it's not possible for
him to make that judgment from any distance

John Polasek

[al...@interia.pl]

W dniu czwartek, 3 lipca 2014 08:43:47 UTC+2 użytkownik Dirk Van de moortel

> > My question is this: how does the observer O calculate the difference
> > in speeds between X and Y? I think that the difference is just (u-v),
> > the same as in classical mechanics. Am I correct?
>
> Yes, by definition of "difference". It is called the "closing speed
> between X and Y as seen by O". It is the (time) rate at which
> the distance, as measured by O (!), between X and Y changes.
> It's not a good name. Depending on the magnitudes of u and v it
> could be an "opening speed". It's also called the "relative speed
> between X and Y in O's frame".

This is called a relative velocity of two bodies;
not a speed, because this is just |v-rel|.

The relations are frame independent, thus the reast of
your speech is a complete garbage.

> From the point of view of X (not light) the speed of Y (possibly
> light) would be the absolute value of
> ( u - v ) / ( 1 - u v/c^2),
> and vice versa.

And this sketch is defined in the SR - formally it's a relativistic velocity,
frame-observer dependent, ie. not a relative.

[Tom Roberts]

On 7/21/14, 7/21/14 - 7:28 PM, Jack...@hotmail.com wrote:
> On Mon, 21 Jul 2014 16:31:00 -0500, Tom Roberts
> <tjrobe...@sbcglobal.net> wrote:
>> Normally in gedankens we augment an observer with an array of assistants at rest
>> in her inertial frame, with clocks synchronized in the frame. This makes it
>> trivial to measure the velocities of moving objects relative to that frame. More
>> importantly, it avoids having to deal with signal or light propagation delays,
>> because an assistant is always located at any event of interest and can provide
>> its coordinate values.

> It seems to me that neither the observer nor any of the assistants can
> actually measure the velocity, a quantity which is not easy to
> measure.

No individual one can do so, but the COLLECTION of assistants can easily provide
the raw data needed to measure the velocity of any object relative to their
inertial frame.


Tom Roberts

[al...@interia.pl]

W dniu wtorek, 22 lipca 2014 17:23:27 UTC+2 użytkownik tjrob137 napisał:

>
> No individual one can do so, but the COLLECTION of assistants can easily
> provide the raw data needed to measure the velocity of any object
> relative to their inertial frame.

No, the measurement of a one-way speed is very hard problem.
Practically we always detect - measure the two-way speed/velocity only.

[hit...@yahoo.com]

On Wednesday, July 23, 2014 3:20:34 AM UTC-6, al...@interia.pl wrote:
>
> No, the measurement of a one-way speed is very hard problem.

Complete nonsense. Dr. Physics does it all the time. He uses two high-speed
detectors at each end of a 4 km evacuated tube, which detectors are connected
to synchronized clocks and data collectors. A short pulse from a laser is
sent down the tube and the time is recorded as it passes each detector.
Voila!

> Practically we always detect - measure the two-way speed/velocity only.

Dr. Physics understands that it is easier to measure the two-way speed,
but he must ask the unenlightened ones why they believe there would be any
difference from the one-way speed? Or if there were a difference, why two-
measurements would not detect it?

Dr. Physics