Re: Length definition in SR - can anyone explain


"David" <dseppala@xxxxxxxxxxxxx> wrote in message news:g6rc42logom5j8a5evesc16dttoae03rhp@xxxxxxxxxx
On Wed, 19 Apr 2006 16:09:26 GMT, "Dirk Van de moortel"
<dirkvandemoortel@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:


"David" <dseppala@xxxxxxxxxxxxx> wrote in message news:v8lc42dpp731snaggm8eg2me89b5ne4q77@xxxxxxxxxx
I'm trying to understand fundamental concepts of SR like time and
length. In my posting about clocks I got a varied opinions of what
constitutes a clock. This posting is a question about lengths. Can
anyone explain why this is or isn't allowed in SR?

I'm in an inertial frame. Since the speed of light is constant I
decide to make my standard unit of length equal to one light-second
(the distance light travels in one second as measured by synchronized
clocks in my inertial frame).

Now let there be an inertial frame moving along the x axis with
velocity V = 0.866c relative to my frame.
At time t0 I fire two lasers for a one second burst. The length of
each pulse is one light-second. The lasers are aligned along the
x-axis. One points in the positive direction, the other points in the
negative direction.

The moving observer measures the length of one pulse to be much less
than the length of the other pulse even though in my frame they are
identical.

What makes you decide that?

I thought this was a simple well known result of SR so I didn't
include any explanation of how I came to this conclusion.

Try to explain your reasoning by defining some relevant
events and calculating the lenghts of the pulses in the
moving frame.

But if you want some details here's how I came to this conclusion.

[snip]

Start by drawing a little spacetime diagram.
Hint: in your equations, use the variable v in stead of the
numeric value 0.866c.

Try to explain your reasoning by defining some relevant
events and calculating the lenghts of the pulses in the
moving frame. Start by drawing a little spacetime diagram.
Hint: in your equations, use the variable v in stead of the
numeric value 0.866c.

Dirk Vdm


.

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