Re: The true crackpots


Dirk Van de moortel wrote:
> "PD" <TheDraperFamily@xxxxxxxxx> wrote in message news:1128959594.681846.15000@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > Dirk Van de moortel wrote:
> > > "PD" <TheDraperFamily@xxxxxxxxx> wrote in message news:1128956256.543167.38340@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > > >
> > > > surrealistic-dream@xxxxxxxxxxx wrote:
> > >
> > > [snip]
> > >
> > > > subject?
> > > > >
> > > > > Proper length is an invariant. It can be regarded as an intrinsic
> > > > > property of a material body.
> > > >
> > > > I find this to be counter-productive and contributory to the confusion
> > > > that Luttgens and Seto and others suffer.
> > >
> > > Paul, if you had been on this forum since a bit more than a few
> > > months, you would have known that proper length has almost
> > > always been explained to both Luttgens and Seto, as physical
> > > properties - see below.
> > > That is why they suddenly are all over the place - again ;-)
> >
> > Which is precisely why I find it a poor term to use, the same way that
> > Newton's use of the term "inherent force" when describing inertia has
> > so thoroughly befuddled TomGee.
> >
> > The invariant interval, which *mathematically* has the same definition
> > as so-called "proper length", is indeed an inherent physical property
> > between two events. It is, however, not a length per se, because it
> > involves both space and time quantities. It may be a semantic quibble,
> > but the term "proper length" does not carry that semantic distinction,
> > which I think is crucial to the understanding of SR.
>
> Perhaps this is semantic indeed. But I have never seen "proper length"
> defined the way you define it.
> We have proper length of a *rod*, and we have spacetime interval
> *between two events*. The latter is clearly more general than the
> former. If the events are spacelike separated, both endpoints of a
> physical rod can be present at each event,

In my mind, an event is something that happens at a single location in
space and in time. Thus a rod cannot have both ends present in the same
event. The thing that happens at one end of a rod to mark its location
(say, its discharging a spark to a nearby measuring stick) is a single
event, but the thing that happens at the other end of the rod to mark
its location is another event. These two events may have the same time
mark.


> and if timelike separated,
> a single object (clock) can be present at both events. I think these
> are very useful -and physical- concepts.
>
> >
> > >
> > > >
> > > > "Proper length" is more neutrally (and therefore more suggestively)
> > > > called "spacetime interval", where it is explicitly recognized as the
> > > > invariant quantity that involves *both* space and time coordinates of
> > > > two events.
> > >
> > > On this forum proper length of a rod has always been defined
> > > as the length of a rod as measured in an inertial restframe in
> > > which it happens to be at rest, and where of course the
> > > distances of both sides of the rod are measured simultaneously.
> > > In this sense the spacetime interval of the two measuring events
> > > consists of a spatial component only, just like "proper time"
> > > between two events is essentially defined as the spacetime
> > > interval as measured in a frame in which the two events have
> > > zero spatial separation and a temporal component only.
> > > This has a nice and clear symmetry i.m.o.
> >
> > One *extremely* subtle but crucial correction:
> > "In this sense the spacetime interval of the two measuring events
> > consists of a *nonzero* spatial component only..."
>
> Of course, otherwise I would have been talking about
> two identical events, zero time separation and zero spatial
> separation, forming a zero-rod/zero-clock, being subject
> to both "time dilation" and "length contraction" in the
> "traditional" sense: dx = dt = dx' = dt' = 0.

Not quite what I meant. Two spacelike-separated events have a frame
where the time difference is 0. The interval in this case and in this
particular frame is I^2 = 0^2 - l^2. To me, this has extra meaning than
calling this proper length because "length" connotes a term that
involves no time components, zero or otherwise. Perhaps I should have
modified your sentence more substantially to read, "In this sense the
spacetime interval consists of a zero-magnitude temporal component and
a nonzero spatial component...." This to me has different qualitative
meaning (though of course the same numerical meaning) than "In this
sense the spacetime interval consists of a spatial component only..."

>
> > The problem is the subtle but tempting misconception that a zero
> > temporal component means no temporal component whatsoever. This
> > misconception doesn't happen to those who are comfortable with
> > spacetime, but it happens all the time to those who are not, and so the
> > extra care is warranted.
>
> Yes, the more care, the better.
> But the correction wasn't necessary in this case.
>
> >
> > >
> > > > It so happens that in some cases, it is possible to *find*
> > > > a frame of reference where the difference between the time coordinates
> > > > between those two events is zero. However, there is nothing physically
> > > > special about that frame and nothing special about the value of the
> > > > spacetime interval in that frame.
> > > >
> > > > The heuristic problem is that "length" is a term that (at least to the
> > > > beginner) means space coordinates *only*. The term "proper length" then
> > > > implies that there is a quantity that involves spatial coordinates
> > > > *only* that has some fundamental physical reality. That is not the
> > > > case. The thing that has fundamental physical reality involves *both*
> > > > space and time coordinates, regardless whether in one frame out of an
> > > > infinitude of choices, the time difference happens to be zero.
> > >
> > > There might be nothing physically special about that *frame*,
> > > but when bringing that same physical rod in *any* inertial frame
> > > whatsoever, the same value will always be found, and therefore
> > > that value clearly depends on the rod itself (and of course on the
> > > way we measure things), so we can argue that we have the right
> > > to call it a "physical property of the rod".
> >
> > Invariant interval is indeed a physical property of the rod (or, more
> > accurately, the two events that mark the measurement of the two ends of
> > the rod), but I decline for pedagogical reasons to call it a length
> > (which I've already described). The point being made to Seto and to
> > Luttgens is not to get too far ahead of oneself.
>
> Paul, they don't understand what an event is. They don't
> understand what coordinates are. They have been ahead
> of themselves before they were even born.

Then that is where we should begin.

>
> > First one has to
> > realize why length (spatial coordinates *only*) is not a physical
> > property of an object, then one has to realize why interval (with both
> > space and time coordinates) is the appropriate physical property of an
> > object, then finally one can loop back and find the connection or
> > special case where interval can be obtained via the procedure used to
> > definne length.
>
> Well, it seems that we disagree on what has the right to be called
> "physical property of an object". I can live with that.
>
> Apparently that seems to be just about the only thing the crackpots
> seem to be interested in anyway. "Is it physical??? - Ha, one SRist
> says yes, the other says no - you see, I always told you, all SRists
> are full of ***".
> Let's confuse them some more, shall we? ;-)
>
> Cheers
> Dirk Vdm

.

Quantcast