Re: Errors being made by SR experts.

On Aug 6, 7:33 pm, "Jeckyl" <no...@xxxxxxxxxxx> wrote:
"PD" <TheDraperFam...@xxxxxxxxx> wrote in message

news:1186408230.239837.222760@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

You don't measure light going past someone else's frame. That would be
the point. Any procedure to do so makes an implicit assumption about
how velocities combine, as I've told you in the past. That assumption
used is the one that is incorrect.

I must disagree (or at least clarify) on that point.

You can indeed calculate, the speed within our frame S that thing A moves
toward or away from thing B (which are both moving in our frame of reference
S), just like you'd would calculate speed of A or B in the frame of
reference. You'd find the difference in positions of A and B at two
different times and divide by the time.

I understand that, and we may have a semantic difference.
Let's look at what that does.
You measure the positions of A and B, say xA1 and xB1, at time t1, and
the positions xA2 and xB2 at time t2.
The prescription above says that the relative speed between A and B is
[(xA2-xB2) - (xA1 - xB1)] / (t2 - t1)
= [(xA2 - xA1) - (xB2 - xB1)] / (t2 - t1)
= (xA2 - xA1)/(t2 - t1) - (xB2 - xB1)/(t2 - t1)
= vA - vB

And we know this prescription for finding a relative velocity is
either *wrong* or it is special to *this particular frame* and does
NOT correspond to the relative velocity that any frame would measure.
One that DOES correspond and is therefore more "right" is
(vA - vB)/(1 - vA*vB/c^2). It's a simple matter to translate this back
into xA1, xA2, xB1, xB2, t1 and t2 data to see what the more durable
prescription for the measurement would be.

Now perhaps I'm being too strong in saying that this isn't a
measurement. What is certainly true is that SR does NOT say that the
value calculated in the naive manner, when one of the objects is
light, will always be c. It doesn't even say that the quantity
calculated in the naive manner above for ANY two objects would be the
same as measured in any other frame. Just the opposite, in fact. So
while it may be a measurement of sorts, it's not a particularly useful
measurement, and it certainly is not a measurement of any quantity
that SR suggests should be invariant. The better combination of xA1,
xA2, xB1, xB2, t1 and t2 that I suggest would be, however, and this is
certainly consistent with SR's principles and derivations.


You don't *need* to combine velocities to do that calculation .. but you can
easily see that simply vector addition of velocities gives you the velocity
that A moves past B in our frame S (often called closing velocity or
velocity of separation)

However, that is NOT the same as calculating how fast thing B sees thing A
go past it. For that you would need to either look at the rulers and clocks
that B would use and work out the speed B would calculate .. of you'd need
to put yourself in B's frame of reference and measure the speed of A from
there.

The point is, that is a different thing to calculating the speed of
separation/closure of A and B in our own frame.

NOTE: That the same is true in both LET and SR .. so if it is somehow and
error in SR, it is also and error in LET.


.

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