@John Rennie offered a link where 18 answers were given to the reality
of length contraction. I read some, but it's anyway disconcerting that
113 years after Einstein's paper we are still arguing about its
meaning. And not only we, but authoritative books and so on...
Yet it appears to me that something is never (or very seldom) clearly
stated. Perhaps this explains why so many beginners (and not only)
get confused by dilation and contraction matters. I already examined
these points in other posts, in recent days too. But there is always
something which requires a better clarification.
The first point is about so-called "coordinate contraction". To me
this is an unfortunate definition. It isn't a matter of coordinates,
but of reference frames. The difference is very sharp, yet frequently
neglected. Coordinates are mathematics, frames are physics. In order
to observe (much better, measure) length contraction you need first of
all a frame of reference, which is a set of points stationary wrt one
another, but also a set of measuring instruments of various sorts.
You'll ever do without a set of synchronized clocks.
Then you must have the objects you want to measure distance in that
frame. Frequently the objects will be the extremes of a rod, or
something like that, but not necessarily. For instance, in the
well-known experiment about half-life of muons there is no rod. There
are two labs, one up on a mountain, the other at sea level, and
counters to measure muons flux. It is important therefore to be
prepared to identify the objects we are talking about.
But let's think of a rod, as it gives a more concrete vision of the
whole matter. A third thing is however necessary: how do you define
the length you are about to measure or to compute? If the rod stands
still in your frame, no problem: length is simply a comparison with
some standard. Grossly said, you see how many times your standard
metre fits between the rod's extremes, and that's all.
But the rod is moving: what is its length? A precise answer is needed,
and not always it's given. For instance octonion replied
You can't assume $t_B = t_A$.
He's right, but why? Simply because the understood definition of length
of a moving object is another. It's still a definition however.
In order to measure length of a moving rod you have to record
positions of its ends at the same time of your frame. Then you
will at ease measure the distance between these positions.
The point is that this procedure is not exactly simple. In order to
put it in operation you need
- to have a whole battery of synchronized clocks, ideally an infinite
number of them
- to program clocks to record - each one by itself - the passage times of the rod's ends
- to search in this collection of data two recordings which report the
same time, one for passage of left extreme, another for the right one
- to get the corresponding coordinates of those clocks and take the
And you're done: this difference is the required length of the moving
Once you know this, you may also do the calculation you tried, but
with $t'_B=t'_A$, and you know why :-)
I can't help remarking that I said nothing about a very delicate
point: how are clocks synchronized? This was a deliberate omission.
This time I preferred to focus attention on a different topic.