Re: HOCUS POCUS
- From: "harry" <harald.vanlintelButNotThis@xxxxxxx>
- Date: Tue, 12 Sep 2006 18:35:07 +0200
"PD" <TheDraperFamily@xxxxxxxxx> wrote in message
news:1158069037.261469.192290@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
mluttgens@xxxxxxxxxx wrote:
HOCUS POCUS
Two electrons E1 and E2 are ejected along a straight line with
opposite velocities v1 and v2 from a device stationary in S,
at t=0 according to S clock.
Assuming that v1 = -0.6 c and v2 = 0.8 c, what is the relative
velocity between E1 and E2 ?
After a time interval t measured on his clock, S will conclude
that the distance separating E1 from E2 is (0.6 + 0.8) ct = 1.4 ct,
hence that E2 is moving away from E1 at V = 1.4 c, or that E1 is
moving away from E2 at V = 1.4 c, meaning that the relative velocity
between E1 and E2 exceeds c.
Yes, and there is nothing wrong with that.
Ein Zwei Ein Stein HOCUS POCUS
E1 = S',
V = (.8 c-(-0.6c))/(1+0.8*0.6) = 1.40/1.48 c = ~.9459 c
E2 moves at 1.4/1.48 c relative to S'
E2 moves at 1.4/1.48 c relative to E1
ABRACADABRA
420000 = ~283784
By assimilating E1 to a frame S' moving away at -0.6 c from a frame S,
Srists claim that the electron E2, which had a velocity V = 1.4 c
wrt E1 measured in S, has only a velocity V' = ~0.9459 c measured in
S'.
Yes.
But the electrons don't bother about which name they are given, nor
does their relative velocity V depend on their velocity wrt the device
by which they have been emitted. Such device -the frame S according
to SRists- should be ignored after their emission, it belongs to
history.
Not at all. You seem to think that relative velocity between two
objects should be a frame-independent quantity. It's not. I don't know
why you think it should be.
Well put! - thus nothing to add, except for a precision below:
For instance, E1 could have been emitted at -0.5 c, and E2 at 0.9c,
hence their relative velocity V remains 1.4 c.
But, according to SRists, V' is then
(0.9 c-(-0.5c))/(1+0.9*0.5) = 1.40/1.45 c, which is different from
1.40/1.48 c!
This new V' is relative to *another* inertial frame. You correctly point out
that the choice of inertial frame matters for all relative speeds - even the
speed between two objects is not invariant. This point is even unclear to
some "SRists", possibly because the speed between E1 and E2 is the same for
the rest frames of E1 and E2.
Harald
Marcel Luttgens
.
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