From: bert on
On Feb 1, 1:14 pm, Sam Wormley <sworml...(a)gmail.com> wrote:
> On 2/1/10 8:55 AM, kens...(a)erinet.com wrote:
>
> > Is Length Contraction in SR physical??
>
>    Length contraction, time dilation and relativistic mass are
>    observer dependent!

Sam If a race car going at 10,000mph was measured it would be shorter
(very tiny difference) than at rest. An electron going at 99.999999 of
c gets very much heavier,and again must be foreshortened in the
direction the accelerators field is taking it. Einstein Mach,and I
have known this to be reality for 63 years. TreBert
From: PD on
On Feb 2, 10:39 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 2 Feb, 15:52, PD <thedraperfam...(a)gmail.com> wrote:
>
>
>
> > On Feb 2, 9:40 am, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > On 1 Feb, 20:52, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > On Feb 1, 2:49 pm, kenseto <kens...(a)erinet.com> wrote:
>
> > > > > So does that mean that length contraction is not physical?
>
> > > > No, it does not mean that.
> > > > Why do you think that "observer dependent" means "not physical"?
> > > > Lots of physical properties are observer dependent.
>
> > > Because for anyone who believes in material reality, physical
> > > properties should not be "observer dependent",
>
> > Then I would ask the following questions of you.
> > Is velocity a physical property? It is *clearly* observer-dependent.
>
> Velocity is a physical property that does not vary with observer-
> perspective, yes. The measurement of velocity is observer-dependent,
> yes.

Ah. Then I'll ask you what your physical velocity is right now. Along
the way of answering that question, please also explain how you would
go about determining what that velocity value is.

>
> The discrepancy between those statements is accounted for by the fact
> that I believe in *material reality*, where the measurement of
> variables does not necessarily reflect the value of the variable
> itself.

OK, so here's an interesting statement by you. There is the claim
about the existence of properties that will not necessarily be
revealed by measurement. One then immediately might ask what values
those properties have, and how would that be determined outside of
measurement? And if all we have is a set of observations
(measurements), then how do we glean the reality of those nether-
properties at all?

Finally, a basic notion: When we say we know how the universe works,
what we mean is that when we *see* circumstances A, B, and C prevail,
then we can expect to *see* an outcome X in quantity Q. It is our
ability to make this prediction reliably that gives us confidence that
we know what is going on -- otherwise we can't be sure. But since the
prediction has entirely to do with *observed* circumstances and
*observed* outcomes, what other connection do we have with the
operation of nature than by what is observed?

>
> > Having answered that, is kinetic energy a physical property, given
> > that (at least at low speeds) the kinetic energy is proportional to
> > the square of the velocity, and so is also clearly observer-dependent.
> > Having answered that, is the law of conservation of energy, which
> > includes kinetic energy as one of the contributions to the conserved
> > sum, a law about physical properties? If not, then what is it a law
> > about?
>
> I'm afraid you'll have to explain what you mean.

The law of conservation of energy says that the sum of measured energy
contributions in a closed system (and one of those contributions is
measured kinetic energy) will remain constant regardless of
interactions inside the system (including exchanges among different
types of energy). This statement is true regardless of choice of
reference frame, though the value of the system sum will be a
different constant depending on the reference frame. It is a
remarkable physical statement about a remarkable physical property,
and it is considered one of the core lynchpins of our understanding of
nature.

Now, you just made a statement that the *measured* velocity is not
necessarily the *physical* velocity. Since the kinetic energy is
proportional to the *measured* velocity squared, then it too must not
be related to a *physical* quantity. Therefore the law that makes a
statement about this *measured* and other *measured* contributions to
a system sum must not be dealing with physical properties, right?
Therefore, what is this law about that makes a remarkable statement
about *measured* properties but not physical properties?

>
> > > and if physical
> > > properties do indeed seem to change depending on how they are
> > > observed, then there should be some coherent explanation for it rather
> > > than just a statement that things are "observer dependent", which in
> > > most people's minds rather leaves the glaring question "why".
>
> > I think there may just be a clash regarding an unfounded expectation
> > that physical properties "should" be observer-independent. Why would
> > this be an expectation?
>
> Oh I know that is the source of the clash. The reason I expect
> observer-independence is because I believe in material reality, where
> variables exist independent of observation or measurement.

Oh the variables do exist. My question is why you think there is one,
"true" value associated with a physical variable that is independent
of reference frame?

And on what basis do you have the expectation that material reality
has variable values that exist independent of observation or
measurement?

Lest you think that where physics has gone wrong is in the last 100
years or so, I'll remind you that it was Galileo in the 1600's who
made the observation that properties of material reality depend on the
reference frame. He noted, for example, that the speed of an object
has no inherent value other than by choice of reference frame. This is
not modern physics. This is CLASSICAL physics, pre-Newton.

>
> > The reality is that physics is littered with physical properties,
> > about which there are many laws that seem to hold remarkably well.
> > Some of those properties are observer-dependent, and some are observer-
> > independent. There is no rationale required for "why" they are not all
> > one way or the other, any more than there has to be a reason cited for
> > "why" all animals are not mammals.
>
> There are no properties of the physical world that are "observer-
> dependent", although the measured value of those properties may depend
> on the circumstances of the observer.

This has not been thought true since the mid-1600s.

From: PD on
On Feb 2, 9:24 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 1 Feb, 19:14, PD <thedraperfam...(a)gmail.com> wrote:
>
>
>
> > On Feb 1, 1:03 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > I think others will contend that "what is measured" does not
> > > necessarily relate to a physical reality. After all, if I have an
> > > elastic meter ruler, and apply various amounts of tension to it, then
> > > the measurement of an object can be changed without any "physical"
> > > change in that object. On the other hand, if the object to be measured
> > > is also elastic, and I apply the same amount of tension to the object
> > > as well as to the meter ruler, then there are "physical" changes even
> > > though there is no change in the measurement.
>
> > This is why we have standards of measurement. A standard is defined so
> > that a replicable process conducted *locally* yields the same
> > measurement when measured again. Thus if you measure the half-life of
> > an isotope locally with this clock, and you measure the half-life of a
> > different sample of the same isotope over there with a clock that is
> > local to that sample, you should get the same number. Now, the local
> > criterion is important and it is built into the standards, because it
> > can be shown (and this is what Einstein showed) that standards applied
> > nonlocally with the expectation that they would yield common results
> > is an expectation that violates the known laws of physics. This
> > connection between the two statements is not obvious to the amateur
> > but is extremely important.
>
> I am aware of the point you make. What you're saying basically is that
> physicists are always careful to stretch the elastic object as much as
> the elastic ruler - and they only guarantee consistent results if this
> stretching is also done consistently, otherwise a conversion factor is
> required.

Um, no. That is not the point I make, nor basically what I'm saying at
all.

>
> But the question remains, what is the *physical* cause of the
> discrepancy when the object to be measured is moving differently from
> the measuring equipment. It should be a question taken seriously in
> physics.

And it is.
I will give you just a sketch, meant to inspire further investigation,
not to teach.
The physical cause is traced to the *meaning* of the physical property
length, which is the difference in the locations of the endpoints of
something, where those locations are measured *simultaneously*. The
importance of the simultaneity becomes apparent if you try to measure
the locations of the endpoints of, say, a moving car and attribute the
difference to the length. Then in turn, it becomes clear that length
is frame-dependent if simultaneity is frame-dependent -- the frame-
dependence of simultaneity is the physical cause of the frame-
dependence of length. So now the question becomes, what is the
physical reason for the frame-dependence of simultaneity? This in turn
is traced to the *meaning* of simultaneity, and how we would know that
two spatially separated events are simultaneous; this in turn hinges
on the synchronous arrival of equal-speed signals from the events at
the midpoint between the events. From here it is relatively simple to
show that the frame-independence of the speed of light *demands* that
for a given pair of events, that definition of simultaneity will be
satisfied in one reference frame and not in another reference frame.
That is, the frame-independence of the speed of light is the physical
cause of the frame-dependence of simultaneity, which in turn is the
physical cause of the frame-dependence of length. So then the question
is, what is the physical cause of the frame-independence of the speed
of light? The answer to this question has to do with the hyperbolic
structure of space-time and the fact that causal relationships are
limited to a region of spacetime called the past- and future-light
cones. This hyperbolic structure of space and time is thus the
physical cause of the frame-independence of the speed of light, which
is in turn the physical cause of the frame-dependence of simultaneity,
which in turn is the physical cause of the frame-dependence of length.

To expand this in a way that would be compellingly easy to follow
would take more pages than I'm willing to invest. Fortunately, this is
done in numerous books designed to explain these things in a
compellingly easy-to-follow fashion. Would you like some
recommendations?

From: PD on
On Feb 2, 10:41 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 2 Feb, 15:59, JT <jonas.thornv...(a)hotmail.com> wrote:
>
>
>
> > On 2 Feb, 16:25, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > On Feb 2, 7:55 am, kenseto <kens...(a)erinet.com> wrote:
>
> > > > On Feb 1, 6:38 pm, artful <artful...(a)hotmail.com> wrote:
>
> > > > > On Feb 2, 1:55 am, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote:
>
> > > > > > Is Length Contraction in SR physical??
>
> > > > > Define 'physical'
>
> > > > Look up the dictionary.
>
> > > That's a bad idea. The definitions listed in a dictionary for words
> > > that are special in physics are usually NOT the meaning of those terms
> > > as used in physics.
>
> > > > > > According the most informed SRian, Tom Roberts, the answer to this
> > > > > > question is NO.
>
> > > > > I doubt that
>
> > > > You are not qualified.
>
> > > > > > His answer is that length contraction is the geometric
> > > > > > projection effect of the length of a moving meter stick onto the SR
> > > > > > observer's frame.
>
> > > > > No .. it can be MODELLED by geometry
>
> > > > So Modelled by geometry is not physical.
>
> > > > > > When a moving meter stick rejoins the stay at home
> > > > > > meter stick they will have the same physical length
>
> > > > > Irrelevant
>
> > > > So there was no physcial contraction.
>
> > > > > > Furthermore if
> > > > > > length contraction is physically real how come SR does not predict
> > > > > > length expansion?
>
> > > > > How come something can't be slower than at rest?
>
> > > > No object in the universe is in a state of absolute rest.
>
> > > > > > Is that becasue SR assumes that the observer is in a
> > > > > > state of absolute rest?
>
> > > > > No.  Just rest relative to himself .. everything else is either also
> > > > > at rest, or has a non-zero speed
>
> > > > ROTFLOL....at rest relative to himself is a oxymoron statement.
>
> > > > > > So what does it mean when SR says that a moving meter stick is
> > > > > > contracted?
>
> > > > > That is it measured as shorter (eg if you measure poles by whether
> > > > > they fit inside a barn, then the pole is shorter)
>
> > > > No measurement of physical length contraction due to relative motion
> > > > ever been made.
>
> > > > > > The answer:
>
> > > > > You wouldn't know
>
> > > > > > 1. An SR observer assumes that the light path length of his meter
> > > > > > stick is the same as the physical length of his meter stick.
>
> > > > > Nonsense
>
> > > > You are nonsense.
>
> > > > > > 2. Using this standard the light path length of a meter stick moving
> > > > > > wrt an SR observer is shorter than the light path length of the stay
> > > > > > at home meter stick. Why? Because light generated at the front end of
> > > > > > the stick will reach the rear end of the stick sooner for a moving
> > > > > > meter stick (c+v) according to the stay at home SR observer.
>
> > > > > Totally irrelevant
>
> > > > You are totally irrelevant.
>
> > > > > > 3. So according to the SR observer the light path length of a moving
> > > > > > meter stick is as follows:
> > > > > >      L' = L_o/gamma.
>
> > > > > Nonsense
>
> > > > You are nonsense.
>
> > > > > > 4. The above interpretation avoids all the paradoxes that arise due
> > > > > > to
> > > > > > the bogus interpretation that a moving meter stick is physically
> > > > > > contracted.
>
> > > > > There are no paradoxes
>
> > > > There are paradoxes if length contraction is physical. You are an
> > > > idiot.
>
> > > > Ken Seto
>
> > > > > > However, the above interpretation is incomplete. Why?
>
> > > > > Because it is nonsense and wrong
>
> > > > > > Because the
> > > > > > light path length of a meter stick moving wrt the observer may be
> > > > > > longer than the observer's meter stick. In that case the light path
> > > > > > length of such meter stick is calculated as follows:
> > > > > > L' = L_o(gamma)
> > > > > > This interpretation is included in a new theory of relativity called
> > > > > > Improved Relativity Theory (IRT). IRT includes SRT as subset.
>
> > > > > Which is nonsense
>
> > > > > > However,
> > > > > > the equations of IRT are valid in all environments, including
> > > > > > gravity.
>
> > > > > Nonsense is nonsense no matter where you use it
>
> > > > > > A complete description of IRT is available in the following link:http://www.modelmechanics.org/2008irt.dtg.pdf-Hidequotedtext-
>
> > > > > - Show quoted text -- Dölj citerad text -
>
> > > - Visa citerad text -
>
> > Yes he should be ashame i mean physical does not even have a
> > meaningfull definition in SR.
>
> > Shame on you Seto this is common knowledge, both length and time have
> > no meaningfull definition within SR, it is simply variable units they
> > are not universal appliance. You seem to think that a unit should be
> > applicable as a sort of comparisson value that have a equality sign,
> > no such things exist.
>
> The only thing that is "shameful" is that "physicists" should be using
> a theory for which there is no definition of "physical", and moreover
> there is no attempt being made to discern it's "physical" basis.

Oh, there IS a definition of physical. It just isn't what you thought
it was. That doesn't mean it's absent. It means you're ignorant of it.
From: PD on
On Feb 2, 10:46 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 2 Feb, 16:09, "J. Clarke" <jclarke.use...(a)cox.net> wrote:
>
>
>
> > Ste wrote:
> > > On 1 Feb, 20:52, PD <thedraperfam...(a)gmail.com> wrote:
> > >> On Feb 1, 2:49 pm, kenseto <kens...(a)erinet.com> wrote:
>
> > >>> So does that mean that length contraction is not physical?
>
> > >> No, it does not mean that.
> > >> Why do you think that "observer dependent" means "not physical"?
> > >> Lots of physical properties are observer dependent.
>
> > > Because for anyone who believes in material reality, physical
> > > properties should not be "observer dependent", and if physical
> > > properties do indeed seem to change depending on how they are
> > > observed, then there should be some coherent explanation for it rather
> > > than just a statement that things are "observer dependent", which in
> > > most people's minds rather leaves the glaring question "why".
>
> > The "why" is because it is in the nature of spacetime to be that way.
> > Physics is decriptive.
>
> Not according to Paul Draper. Apparently it is necessary that a
> "scientific theory" be predictive (and therefore prescriptive), not
> just "explanatory" (i.e. descriptive).
>
> In any event, the question of "why" has perhaps been misinterpreted. I
> really meant "how". That is, what is happening in the physical system
> to cause a change in measurements which are "observer dependent".

Then let's start with the basics.
Let's suppose there is, as you claim, an observer-independent value of
the speed of your coffee cup right now.
For a different observer, that coffee cup will be measured to have a
value of velocity that is different than the observer-independent
value.
What is happening in the physical system of the coffee cup and that
observer to yield a different measured value of velocity?

Since this is so basic, we should be able to answer it, if it has any
answer at all.