The reciprocal effect of length contraction and time dilation, which
appears by logical necessity to emerge from the kinematic part of the
special theory of relativity, has been variously explained as

1. true but not really true
2. real
3. not real
4. apparent
5. the result of the relativity of simultaneity
6. determined by measurement
7. a perspective effect
8. mathematical.

Unless placed in quotation marks, authors' assessments are summarized.

1. Effects are true but not really true:

Eddington [1928, pp33-34]:

"The shortening of the moving rod is true , but it is not really true."

2. Effects are real:

Arzelies [1966, pp120-121]:

The Lorentz Contraction is a Real Phenomenon. ...
Several authors have stated that the Lorentz contraction only seems to
occur, and is not real. This idea is false. So far as relativistic
theory is concerned, this contraction is just as real as any other
phenomenon. Admittedly ... it is not absolute, but depends upon the
system employed for the measurement; it seems that we might call it an
apparent contraction which varies with the system. This is merely
playing with the words, however. We must not confuse the reality of a
phenomenon with the independence of this phenomenon of a change of
system. ... The difficulty arises because we have become accustomed to
the geometrical concept of a rigid body with a definite shape, whatever
the measuring system. This idea must be abandoned. ... We must use the
term "real" for every phenomenon which can be measured ... The Lorentz
Contraction is an Objective Phenomenon. ...
We often encounter the following remark: The length of a ruler depends
upon its motion with respect to the observer. ... From this, it is
concluded once again that the contraction is only apparent, a subjective
phenomenon. ... such remarks ought to be forbidden.

Krane [1983, pp23-25]:

It must be pointed out that time dilation is a real effect that applies
not only to clocks based on light beams but to time itself. All clocks
will run more slowly as observed from the moving frame of reference. ...
The length measured by the moving observer is shorter. It must be
emphasized that this is a real effect.

Matveyev [1966, p305]:

The dimensions of bodies suffer contraction in the direction of motion
.... A body is, therefore, "flattened" in the direction of motion. This
effect is a real effect ...

Møller [1972, p44]:

Contraction is a real effect observable in principle by experiment. It
expresses, however, not so much a quality of the moving stick itself as
rather a reciprocal relation between measuring-sticks in motion relative
to each other. ... According to relativistic conception, the notion of
the length of a stick has an unambiguous meaning only in relation to a
given inertial frame. ... This means that the concept of length has lost
its absolute meaning.

Pauli [1981, pp12-13]:

We have seen that this contraction is connected with the relativity of
simultaneity, and for this reason the argument has been put forward that
it is only an "apparent" contraction, in other words, that it is only
simulated by our space-time measurements. If a state is called real only
if it can be determined in the same way in all Galilean reference
systems, then the Lorentz contraction is indeed only apparent, since an
observer at rest in K' will see the rod without contraction. But we do
not consider such a point of view as appropriate, and in any case the
Lorentz contraction is in principle observable. ... It therefore follows
that the Lorentz contraction is not a property of a single rod taken by
itself, but a reciprocal relation between two such rods moving
relatively to each other, and this relation is in principle observable.

Schwinger [1986, p52]:

Each will observe the other clock to be running more slowly. This is an
objective fact. It is not a property of clocks but of time itself.

Tolman [1987, pp23-24]:

Entirely real but symmetrical.

3. Relativistic effects are not physically real:

Taylor & Wheeler [1992, p76]:

Does something about a clock really change when it moves, resulting in
the observed change in the tick rate? Absolutely not! Here is why:
Whether a clock is at rest or in motion ... is controlled by the
observer. You want the clock to be at rest? Move along with it. ... How
can your change of motion affect the inner mechanism of a distant clock?
It cannot and it does not.

4. Relativistic effects are apparent:

Aharoni [1985, p21]:

The moving rod appears shorter. The moving clock appears to go slow.

Cullwick [1959, pp65, 68]:

[A] rod which is at rest in S' ... appears to the observer O to be
contracted ... Similarly, a rod at rest in S will appear in S' to be
contracted....

Eddington [1920, p23-24]

"It is the reciprocity of these appearances-that each party should think
the other has contracted-that is so difficult to realise. Here is a
paradox beyond even the imagination of Dean Swift. Gulliver regarded the
Lilliputians as a race of dwarfs; and the Lilliputians regarded Gulliver
as a giant. That is natural. If the Lilliputians had appeared dwarfs to
Gulliver, and Gulliver had appeared a dwarf to the Lilliputians-but no!
that is too absurd for fiction, and is an idea only to be found in the
sober pages of science.....It is not only in space but in time that
these strange variations occur. If we observed the aviator carefully we
should infer that he was unusually slow in his movements; and events in
the conveyance moving with him would be similarly retarded-as though
time had forgotten to go on. His cigar lasts twice as long as one of ours."

In the preface to modern editions of this book written in 1986 by
Hermann Bondi, Bondi identifies an "erratum" by Eddington
"There is only one point in the whole book to which (i) can be applied:
the Fitzgerald contraction cannot be seen. Eddington was in no way alone
in the error that it could be seen, indeed it now seems astonishing that
it was well over fifty years after Einstein's paper that Terrel,
stimulated by Weisskopf, showed that the Fitzgerald effect could not be
seen."

Jackson [1975, p520]:

The time as seen in the rest system is dilated.

Joos [1958, pp243-244]:

The interval appears to the moving observer to be lengthened. A body
which appears to be spherical to an observer at rest will appear to a
moving observer to be an oblate spheroid.

McCrea [1954, pp15-16]:

The apparent length is reduced. Time intervals appear to be lengthened;
clocks appear to go slow.

Nunn [1923, pp43-44]:

A moving rod would appear to be shortened. An interval is always less
than measured by the other observer.

Whitrow [1980, p255]:

Instead of assuming that there are real, i.e. structural, changes in
length and duration owing to motion, Einstein's theory involves only
apparent changes, and these are independent of the microscopic
constitution and hidden mechanisms controlling the structure of matter.
[Unlike]... real changes, these apparent phenomena are reciprocal.

5. Relativistic effects are the result of the relativity of simultaneity:

Bohm [1965, p59]:

When measuring lengths and intervals, observers are not referring to the
same events.

French [1968, p97],
Rosser [1967, p37],
Stephenson & Kilmister [1987, pp38-39]:

Measurements of lengths involve simultaneity and yield different
numerical values.

6. Relativistic effects are determined by measurements:

Schwartz [1972, p113]:

Each observer determines distances to be foreshortened.

7. Relativistic effects are comparable to perspective effects: Rindler
[1991, pp25-29]:

Moving lengths are reduced, a kind of perspective effect. But of course
nothing has happened to the rod itself. Nevertheless, contraction is no
illusion, it is real. Moving clocks go slow, a 'velocity-perspective'
effect. Nothing at all happens to the clock itself. Like contraction,
this effect is real.

8. Relativistic effects are mathematical:

Eddington [1924, pp16-18]:

The connection between lengths and intervals are problems of pure
mathematics. A travelling clock gives a low reading.

Minkowski [1908, p81]:

[The] contraction is not to be looked upon as a consequence of
resistances in the ether, or anything of that kind, but simply as a gift
from above, - as an accompanying circumstance of the circumstance of motion.

Rogers [1960, p496]:

Thus we have devised a new geometry, with our clocks and scales
conspiring, by their changes, to present us with a universally constant
speed of light.

References:

Aharoni, J., The Special Theory of Relativity, (1965), Dover, 1985.

Arzelies, H., Relativistic Kinematics, Pergamon, Oxford, 1966.

Bohm, D., The Special Theory of Relativity, W.A. Benjamin, New York, 1965.

Cullwick, E.G., Electromagnetism and Relativity, 2nd ed., Longmans,
London, 1959.

Eddington, A.S. Space, Time & Gravitation (1920) CUP, Cambridge Science
Classics, 1999

Eddington, A.S. The Mathematical Theory of Relativity, 2nd ed., CUP 1924.

Eddington, A. S., The Nature of the Physical World , 1928, CUP /
MacMillan (NY).

French, A.P., Special Relativity, Chapman & Hall, London, 1968.

Jackson J.D., Classical Electrodynamics, 2nd ed., John Wiley, New York,
1975.

Joos, G., Theoretical Physics, (1934), 3rd ed., Blackie, London, 1958.

Krane, K.S., Modern Physics, J. Wiley, New York, 1983.

McCrea, W.H., Relativity Physics, 4th ed., Methuen, London, 1954.

Matveyev, A., Principles of Electrodynamics, Reinhold, New York, 1966.

Minkowski, H., "Space and Time" (1908), in H.A. Lorentz et al., The
Principle of Relativity, Dover, 1952,75-91.

Møller, C., The Theory of Relativity, 2nd ed., OUP 1972.

Nunn, T.P., Relativity and Gravitation, University of London Press, 1923.

Pauli, W., Theory of Relativity (1921), Dover 1981.

Rindler, W., Introduction to Special Relativity, 2nd ed., Clarendon,
Oxford, 1991.

Rogers, E.M., Physics for the Inquiring Mind, Princeton U. P. 1960.

Rosser, W.G.V., Introductory Relativity, Butterworths, London, 1967.

Schwartz, M., Principles of Electrodynamics, McGraw Hill, New York, 1972.

Schwinger, J., Einstein's Legacy, Scientific American Library, New York,
1986.

Stephenson, G., & Kilmister, C.W., Special Relativity for Physicists
(1958), Dover, 1987.

Taylor, E.F., & Wheeler, J.A., Spacetime Physics: Introduction to
Special Relativity, 2nd ed., W.H. Freeman, New York, 1992.

Tolman, R.C., Relativity Thermodynamics and Cosmology (1934), Dover, 1987.

Whitrow, G.J., The Natural Philosophy of Time, 2nd Ed. OUP 1980.



No wonder the relativists in this newsgroup just give facetious replies
and never bother to answer questions. Even the "expert" relativists who
have written textbooks don't have a clue what's going on.

Dave wrote:

The reciprocal effect of length contraction and time dilation, which
appears by logical necessity to emerge from the kinematic part of the
special theory of relativity, has been variously explained as

1. true but not really true
2. real
3. not real
4. apparent
5. the result of the relativity of simultaneity
6. determined by measurement
7. a perspective effect
8. mathematical.


When you realize that relativity is ultimately about observation,
the 4,5,7, and 8 are more or less equivalent (with some qualification).
Although 2 and 3 sound contradictory, it depends on your definition
of "real" (believe it or not, "real" is not a well-defined term,
scientifically). Some people would consider a "real" length
contraction to be something involving some sort of physical
compression, understandable in terms of the bulk modulus, and
by that definition, the length contraction is
not "real". On the other hand if "real" means that I could
devise an experiment to measure the length contraction of
something that is moving in my frame, the it is *real*, but
now "real" is entirely consistent
with 4,5,7, and 8. I believe this distinction is what Eddington
was alluding to in comment 1.

As for 6, I don't believe that there have been direct measurements
of length contraction (although I may be wrong), but is an implicit
part of such things as relativistic heavy ion interaction
calculations. It's also used more precisely when doing calculations
involving lasers interacting with relativistic beams, but in that
case it's so intimately wrapped up with Maxwell's equations that
it's impossible to disentangle it. For example, if you try to
calculate something like Compton scattering from a relativistic
beam using some sort of energy density model (rather than
tranforming the field), you'll find that unless you include
length contraction, you'll never reconcile it with reality.

-E

Unless placed in quotation marks, authors' assessments are summarized.

1. Effects are true but not really true:

Eddington [1928, pp33-34]:

"The shortening of the moving rod is true , but it is not really true."

2. Effects are real:

Arzelies [1966, pp120-121]:

The Lorentz Contraction is a Real Phenomenon. ...
Several authors have stated that the Lorentz contraction only seems to
occur, and is not real. This idea is false. So far as relativistic
theory is concerned, this contraction is just as real as any other
phenomenon. Admittedly ... it is not absolute, but depends upon the
system employed for the measurement; it seems that we might call it an
apparent contraction which varies with the system. This is merely
playing with the words, however. We must not confuse the reality of a
phenomenon with the independence of this phenomenon of a change of
system. ... The difficulty arises because we have become accustomed to
the geometrical concept of a rigid body with a definite shape, whatever
the measuring system. This idea must be abandoned. ... We must use the
term "real" for every phenomenon which can be measured ... The Lorentz
Contraction is an Objective Phenomenon. ...
We often encounter the following remark: The length of a ruler depends
upon its motion with respect to the observer. ... From this, it is
concluded once again that the contraction is only apparent, a subjective
phenomenon. ... such remarks ought to be forbidden.

Krane [1983, pp23-25]:

It must be pointed out that time dilation is a real effect that applies
not only to clocks based on light beams but to time itself. All clocks
will run more slowly as observed from the moving frame of reference. ...
The length measured by the moving observer is shorter. It must be
emphasized that this is a real effect.

Matveyev [1966, p305]:

The dimensions of bodies suffer contraction in the direction of motion
... A body is, therefore, "flattened" in the direction of motion. This
effect is a real effect ...

Møller [1972, p44]:

Contraction is a real effect observable in principle by experiment. It
expresses, however, not so much a quality of the moving stick itself as
rather a reciprocal relation between measuring-sticks in motion relative
to each other. ... According to relativistic conception, the notion of
the length of a stick has an unambiguous meaning only in relation to a
given inertial frame. ... This means that the concept of length has lost
its absolute meaning.

Pauli [1981, pp12-13]:

We have seen that this contraction is connected with the relativity of
simultaneity, and for this reason the argument has been put forward that
it is only an "apparent" contraction, in other words, that it is only
simulated by our space-time measurements. If a state is called real only
if it can be determined in the same way in all Galilean reference
systems, then the Lorentz contraction is indeed only apparent, since an
observer at rest in K' will see the rod without contraction. But we do
not consider such a point of view as appropriate, and in any case the
Lorentz contraction is in principle observable. ... It therefore follows
that the Lorentz contraction is not a property of a single rod taken by
itself, but a reciprocal relation between two such rods moving
relatively to each other, and this relation is in principle observable.

Schwinger [1986, p52]:

Each will observe the other clock to be running more slowly. This is an
objective fact. It is not a property of clocks but of time itself.

Tolman [1987, pp23-24]:

Entirely real but symmetrical.

3. Relativistic effects are not physically real:

Taylor & Wheeler [1992, p76]:

Does something about a clock really change when it moves, resulting in
the observed change in the tick rate? Absolutely not! Here is why:
Whether a clock is at rest or in motion ... is controlled by the
observer. You want the clock to be at rest? Move along with it. ... How
can your change of motion affect the inner mechanism of a distant clock?
It cannot and it does not.

4. Relativistic effects are apparent:

Aharoni [1985, p21]:

The moving rod appears shorter. The moving clock appears to go slow.

Cullwick [1959, pp65, 68]:

[A] rod which is at rest in S' ... appears to the observer O to be
contracted ... Similarly, a rod at rest in S will appear in S' to be
contracted....

Eddington [1920, p23-24]

"It is the reciprocity of these appearances-that each party should think
the other has contracted-that is so difficult to realise. Here is a
paradox beyond even the imagination of Dean Swift. Gulliver regarded the
Lilliputians as a race of dwarfs; and the Lilliputians regarded Gulliver
as a giant. That is natural. If the Lilliputians had appeared dwarfs to
Gulliver, and Gulliver had appeared a dwarf to the Lilliputians-but no!
that is too absurd for fiction, and is an idea only to be found in the
sober pages of science.....It is not only in space but in time that
these strange variations occur. If we observed the aviator carefully we
should infer that he was unusually slow in his movements; and events in
the conveyance moving with him would be similarly retarded-as though
time had forgotten to go on. His cigar lasts twice as long as one of ours."

In the preface to modern editions of this book written in 1986 by
Hermann Bondi, Bondi identifies an "erratum" by Eddington
"There is only one point in the whole book to which (i) can be applied:
the Fitzgerald contraction cannot be seen. Eddington was in no way alone
in the error that it could be seen, indeed it now seems astonishing that
it was well over fifty years after Einstein's paper that Terrel,
stimulated by Weisskopf, showed that the Fitzgerald effect could not be
seen."

Jackson [1975, p520]:

The time as seen in the rest system is dilated.

Joos [1958, pp243-244]:

The interval appears to the moving observer to be lengthened. A body
which appears to be spherical to an observer at rest will appear to a
moving observer to be an oblate spheroid.

McCrea [1954, pp15-16]:

The apparent length is reduced. Time intervals appear to be lengthened;
clocks appear to go slow.

Nunn [1923, pp43-44]:

A moving rod would appear to be shortened. An interval is always less
than measured by the other observer.

Whitrow [1980, p255]:

Instead of assuming that there are real, i.e. structural, changes in
length and duration owing to motion, Einstein's theory involves only
apparent changes, and these are independent of the microscopic
constitution and hidden mechanisms controlling the structure of matter.
[Unlike]... real changes, these apparent phenomena are reciprocal.

5. Relativistic effects are the result of the relativity of simultaneity:

Bohm [1965, p59]:

When measuring lengths and intervals, observers are not referring to the
same events.

French [1968, p97],
Rosser [1967, p37],
Stephenson & Kilmister [1987, pp38-39]:

Measurements of lengths involve simultaneity and yield different
numerical values.

6. Relativistic effects are determined by measurements:

Schwartz [1972, p113]:

Each observer determines distances to be foreshortened.

7. Relativistic effects are comparable to perspective effects: Rindler
[1991, pp25-29]:

Moving lengths are reduced, a kind of perspective effect. But of course
nothing has happened to the rod itself. Nevertheless, contraction is no
illusion, it is real. Moving clocks go slow, a 'velocity-perspective'
effect. Nothing at all happens to the clock itself. Like contraction,
this effect is real.

8. Relativistic effects are mathematical:

Eddington [1924, pp16-18]:

The connection between lengths and intervals are problems of pure
mathematics. A travelling clock gives a low reading.

Minkowski [1908, p81]:

[The] contraction is not to be looked upon as a consequence of
resistances in the ether, or anything of that kind, but simply as a gift
from above, - as an accompanying circumstance of the circumstance of
motion.

Rogers [1960, p496]:

Thus we have devised a new geometry, with our clocks and scales
conspiring, by their changes, to present us with a universally constant
speed of light.

References:

Aharoni, J., The Special Theory of Relativity, (1965), Dover, 1985.

Arzelies, H., Relativistic Kinematics, Pergamon, Oxford, 1966.

Bohm, D., The Special Theory of Relativity, W.A. Benjamin, New York, 1965.

Cullwick, E.G., Electromagnetism and Relativity, 2nd ed., Longmans,
London, 1959.

Eddington, A.S. Space, Time & Gravitation (1920) CUP, Cambridge Science
Classics, 1999

Eddington, A.S. The Mathematical Theory of Relativity, 2nd ed., CUP 1924.

Eddington, A. S., The Nature of the Physical World , 1928, CUP /
MacMillan (NY).

French, A.P., Special Relativity, Chapman & Hall, London, 1968.

Jackson J.D., Classical Electrodynamics, 2nd ed., John Wiley, New York,
1975.

Joos, G., Theoretical Physics, (1934), 3rd ed., Blackie, London, 1958.

Krane, K.S., Modern Physics, J. Wiley, New York, 1983.

McCrea, W.H., Relativity Physics, 4th ed., Methuen, London, 1954.

Matveyev, A., Principles of Electrodynamics, Reinhold, New York, 1966.

Minkowski, H., "Space and Time" (1908), in H.A. Lorentz et al., The
Principle of Relativity, Dover, 1952,75-91.

Møller, C., The Theory of Relativity, 2nd ed., OUP 1972.

Nunn, T.P., Relativity and Gravitation, University of London Press, 1923.

Pauli, W., Theory of Relativity (1921), Dover 1981.

Rindler, W., Introduction to Special Relativity, 2nd ed., Clarendon,
Oxford, 1991.

Rogers, E.M., Physics for the Inquiring Mind, Princeton U. P. 1960.

Rosser, W.G.V., Introductory Relativity, Butterworths, London, 1967.

Schwartz, M., Principles of Electrodynamics, McGraw Hill, New York, 1972.

Schwinger, J., Einstein's Legacy, Scientific American Library, New York,
1986.

Stephenson, G., & Kilmister, C.W., Special Relativity for Physicists
(1958), Dover, 1987.

Taylor, E.F., & Wheeler, J.A., Spacetime Physics: Introduction to
Special Relativity, 2nd ed., W.H. Freeman, New York, 1992.

Tolman, R.C., Relativity Thermodynamics and Cosmology (1934), Dover, 1987.

Whitrow, G.J., The Natural Philosophy of Time, 2nd Ed. OUP 1980.



No wonder the relativists in this newsgroup just give facetious replies
and never bother to answer questions. Even the "expert" relativists who
have written textbooks don't have a clue what's going on.

It is another posibility...
To replace "time dilation" with "signals' time interval (duration) dilation"

See:
www.freephysics.org

On 12/3/2003 10:52 AM, Dave wrote:
The reciprocal effect of length contraction and time dilation, which
appears by logical necessity to emerge from the kinematic part of the
special theory of relativity, has been variously explained as

1. true but not really true
2. real
3. not real
4. apparent
5. the result of the relativity of simultaneity
6. determined by measurement
7. a perspective effect
8. mathematical.

6,7,8 are indistinguishable. 1,2,3,4 depend intrinsically on he
definitions of non-physics words like "real". 5 is a completely
different type of statement. So it is not surprising there is a
divergence of opinion.

I believe you'll find that all authors agree on a few basic ideas:
A. The invariant interval between a given pair of events is
invariant (i.e. the same for all observers).
B. How a given observer measures the spatial and temporal
differences between those events depends on the coordinates
used by the observer.
C. The coordinate differences between the pair of events is
determined by projecting the invariant interval between them
onto the coordinate axes.

Depending on the definitions of words in your 1-8 above, these encompass
them all.

As I keep repeating around he BEWARE OF UNACKNOWLEDGED PUNS! Your 1-8
are full of them.


Tom Roberts

Dasve wrote:
The reciprocal effect of length contraction and time dilation, which
appears by logical necessity to emerge from the kinematic part of the
special theory of relativity, has been variously explained as

1. true but not really true
2. real
3. not real
4. apparent
5. the result of the relativity of simultaneity
6. determined by measurement
7. a perspective effect
8. mathematical.

Like many things in life different interpretations are often consistent with
the facts. The fact is lengths measure differently in different inertial
frames. That is an experimental fact - beyond question (by which I mean
that no experiment is inconsistent with it). Each of the above is just
different ways of looking at it - (also your expression of the different
views leaves a lot to be desired - true but not really true - is obvious
rubbish - unless context is given. The fact you choose not to do such I
believe speaks volumes).

Dave wrote:
No wonder the relativists in this newsgroup just give facetious replies
and never bother to answer questions.

You must be reading different posters than me. Bilge, Steve Carlip and Tom
Roberts do not fall into that category. Note to refute my assertion you
would need to show that in most cases their relies are facetious - not give
one example where you believes the reply was facetious.

Dave wrote:
'Even the "expert" relativists who have written textbooks don't have a clue
what's going on.'

And your distortion of the rules of logic, which in this case means not
realizing that different interpretations are equally valid as long ass they
are consistent with the facts, brands you as a shallow thinker.

Thanks
Bill

EjP wrote:
When you realize that relativity is ultimately about observation,
the 4,5,7, and 8 are more or less equivalent (with some qualification).
Although 2 and 3 sound contradictory, it depends on your definition
of "real" (believe it or not, "real" is not a well-defined term,
scientifically). Some people would consider a "real" length
contraction to be something involving some sort of physical
compression, understandable in terms of the bulk modulus, and
by that definition, the length contraction is
not "real". On the other hand if "real" means that I could
devise an experiment to measure the length contraction of
something that is moving in my frame, the it is *real*, but
now "real" is entirely consistent
with 4,5,7, and 8. I believe this distinction is what Eddington
was alluding to in comment 1.

For those that doubt length contraction simply analyze a current carrying
wire. If length contraction occurs then magnetic forces appear - exactly
as found from experiment - if length contrition does not occur then magnetic
forces do not appear - in disagreement with experiment.

Thanks
Bill

Bill Hobba wrote:

EjP wrote:
When you realize that relativity is ultimately about observation,
the 4,5,7, and 8 are more or less equivalent (with some qualification).
Although 2 and 3 sound contradictory, it depends on your definition
of "real" (believe it or not, "real" is not a well-defined term,
scientifically). Some people would consider a "real" length
contraction to be something involving some sort of physical
compression, understandable in terms of the bulk modulus, and
by that definition, the length contraction is
not "real". On the other hand if "real" means that I could
devise an experiment to measure the length contraction of
something that is moving in my frame, the it is *real*, but
now "real" is entirely consistent
with 4,5,7, and 8. I believe this distinction is what Eddington
was alluding to in comment 1.

For those that doubt length contraction simply analyze a current carrying
wire. If length contraction occurs then magnetic forces appear - exactly
as found from experiment - if length contrition does not occur then magnetic
forces do not appear - in disagreement with experiment.

Thanks
Bill

This is not a good example. If you do the math you'll find that length
contraction cancels out of the equation, leaving the force as due to
only the relative velocity and proximity of the respective lines of
charge. Thus your example is actually a counter-argument to the
'reality' of length contraction, i.e. length contraction is unnecessary
and has nothing to do with the measured force. As an analogy let's
suppose that instead of length contraction we substitute line density
increase via spontaneous creation of extra electrons. Now it should be
obvious that this will provide exactly the same outcome, since the
relativistic length contraction hypothesis provides just this same line
density increase. Now let's suppose that instead of either of these we
postulate that the lines of charge simply 'appear' closer to each other
from their respective points of view when they are in relative motion.
Again we get the same result. In short, any ad hoc presumption that will
increase the force as a function of relative velocity will work equally
as well as any other that does the same, each will provide correctly for
the observed force.

Thus I trust you'll see that the development of this force is no proof
or even the slightest indication that length contraction is real.

Now look at
http://www.cswnet.com/~rper
and you'll find a mathematical treatment of this force that makes no
assumptions at all about the 'cause' of the force, it simply quantifies
it directly as a function of relative velocity (in a sense this reduces
to the actual cause), and thus by virtue of Ockham's Razor, it is a much
superior approach, eliminating as it does all of the excess baggage that
you brought along above.

Richard Perry

"Dave" wrote in message
...

The reciprocal effect of length contraction and time dilation, which
appears by logical necessity to emerge from the kinematic part of the
special theory of relativity, has been variously explained as

1. true but not really true
2. real
3. not real
4. apparent
5. the result of the relativity of simultaneity
6. determined by measurement
7. a perspective effect
8. mathematical.

All these divergence of opinions can be resolved as follows:
1. Lorentz contraction is not a physical contraction but rather
the lengthening of the light path length of a moving rod.
2. A moving clock runs slow is not the dilation of time but rather it is
a clock second contains a larger amount of absolute time than
before it is moving.

Ken Seto


Unless placed in quotation marks, authors' assessments are summarized.

1. Effects are true but not really true:

Eddington [1928, pp33-34]:

"The shortening of the moving rod is true , but it is not really true."

2. Effects are real:

Arzelies [1966, pp120-121]:

The Lorentz Contraction is a Real Phenomenon. ...
Several authors have stated that the Lorentz contraction only seems to
occur, and is not real. This idea is false. So far as relativistic
theory is concerned, this contraction is just as real as any other
phenomenon. Admittedly ... it is not absolute, but depends upon the
system employed for the measurement; it seems that we might call it an
apparent contraction which varies with the system. This is merely
playing with the words, however. We must not confuse the reality of a
phenomenon with the independence of this phenomenon of a change of
system. ... The difficulty arises because we have become accustomed to
the geometrical concept of a rigid body with a definite shape, whatever
the measuring system. This idea must be abandoned. ... We must use the
term "real" for every phenomenon which can be measured ... The Lorentz
Contraction is an Objective Phenomenon. ...
We often encounter the following remark: The length of a ruler depends
upon its motion with respect to the observer. ... From this, it is
concluded once again that the contraction is only apparent, a subjective
phenomenon. ... such remarks ought to be forbidden.

Krane [1983, pp23-25]:

It must be pointed out that time dilation is a real effect that applies
not only to clocks based on light beams but to time itself. All clocks
will run more slowly as observed from the moving frame of reference. ...
The length measured by the moving observer is shorter. It must be
emphasized that this is a real effect.

Matveyev [1966, p305]:

The dimensions of bodies suffer contraction in the direction of motion
... A body is, therefore, "flattened" in the direction of motion. This
effect is a real effect ...

Møller [1972, p44]:

Contraction is a real effect observable in principle by experiment. It
expresses, however, not so much a quality of the moving stick itself as
rather a reciprocal relation between measuring-sticks in motion relative
to each other. ... According to relativistic conception, the notion of
the length of a stick has an unambiguous meaning only in relation to a
given inertial frame. ... This means that the concept of length has lost
its absolute meaning.

Pauli [1981, pp12-13]:

We have seen that this contraction is connected with the relativity of
simultaneity, and for this reason the argument has been put forward that
it is only an "apparent" contraction, in other words, that it is only
simulated by our space-time measurements. If a state is called real only
if it can be determined in the same way in all Galilean reference
systems, then the Lorentz contraction is indeed only apparent, since an
observer at rest in K' will see the rod without contraction. But we do
not consider such a point of view as appropriate, and in any case the
Lorentz contraction is in principle observable. ... It therefore follows
that the Lorentz contraction is not a property of a single rod taken by
itself, but a reciprocal relation between two such rods moving
relatively to each other, and this relation is in principle observable.

Schwinger [1986, p52]:

Each will observe the other clock to be running more slowly. This is an
objective fact. It is not a property of clocks but of time itself.

Tolman [1987, pp23-24]:

Entirely real but symmetrical.

3. Relativistic effects are not physically real:

Taylor & Wheeler [1992, p76]:

Does something about a clock really change when it moves, resulting in
the observed change in the tick rate? Absolutely not! Here is why:
Whether a clock is at rest or in motion ... is controlled by the
observer. You want the clock to be at rest? Move along with it. ... How
can your change of motion affect the inner mechanism of a distant clock?
It cannot and it does not.

4. Relativistic effects are apparent:

Aharoni [1985, p21]:

The moving rod appears shorter. The moving clock appears to go slow.

Cullwick [1959, pp65, 68]:

[A] rod which is at rest in S' ... appears to the observer O to be
contracted ... Similarly, a rod at rest in S will appear in S' to be
contracted....

Eddington [1920, p23-24]

"It is the reciprocity of these appearances-that each party should think
the other has contracted-that is so difficult to realise. Here is a
paradox beyond even the imagination of Dean Swift. Gulliver regarded the
Lilliputians as a race of dwarfs; and the Lilliputians regarded Gulliver
as a giant. That is natural. If the Lilliputians had appeared dwarfs to
Gulliver, and Gulliver had appeared a dwarf to the Lilliputians-but no!
that is too absurd for fiction, and is an idea only to be found in the
sober pages of science.....It is not only in space but in time that
these strange variations occur. If we observed the aviator carefully we
should infer that he was unusually slow in his movements; and events in
the conveyance moving with him would be similarly retarded-as though
time had forgotten to go on. His cigar lasts twice as long as one of
ours."

In the preface to modern editions of this book written in 1986 by
Hermann Bondi, Bondi identifies an "erratum" by Eddington
"There is only one point in the whole book to which (i) can be applied:
the Fitzgerald contraction cannot be seen. Eddington was in no way alone
in the error that it could be seen, indeed it now seems astonishing that
it was well over fifty years after Einstein's paper that Terrel,
stimulated by Weisskopf, showed that the Fitzgerald effect could not be
seen."

Jackson [1975, p520]:

The time as seen in the rest system is dilated.

Joos [1958, pp243-244]:

The interval appears to the moving observer to be lengthened. A body
which appears to be spherical to an observer at rest will appear to a
moving observer to be an oblate spheroid.

McCrea [1954, pp15-16]:

The apparent length is reduced. Time intervals appear to be lengthened;
clocks appear to go slow.

Nunn [1923, pp43-44]:

A moving rod would appear to be shortened. An interval is always less
than measured by the other observer.

Whitrow [1980, p255]:

Instead of assuming that there are real, i.e. structural, changes in
length and duration owing to motion, Einstein's theory involves only
apparent changes, and these are independent of the microscopic
constitution and hidden mechanisms controlling the structure of matter.
[Unlike]... real changes, these apparent phenomena are reciprocal.

5. Relativistic effects are the result of the relativity of simultaneity:

Bohm [1965, p59]:

When measuring lengths and intervals, observers are not referring to the
same events.

French [1968, p97],
Rosser [1967, p37],
Stephenson & Kilmister [1987, pp38-39]:

Measurements of lengths involve simultaneity and yield different
numerical values.

6. Relativistic effects are determined by measurements:

Schwartz [1972, p113]:

Each observer determines distances to be foreshortened.

7. Relativistic effects are comparable to perspective effects: Rindler
[1991, pp25-29]:

Moving lengths are reduced, a kind of perspective effect. But of course
nothing has happened to the rod itself. Nevertheless, contraction is no
illusion, it is real. Moving clocks go slow, a 'velocity-perspective'
effect. Nothing at all happens to the clock itself. Like contraction,
this effect is real.

8. Relativistic effects are mathematical:

Eddington [1924, pp16-18]:

The connection between lengths and intervals are problems of pure
mathematics. A travelling clock gives a low reading.

Minkowski [1908, p81]:

[The] contraction is not to be looked upon as a consequence of
resistances in the ether, or anything of that kind, but simply as a gift
from above, - as an accompanying circumstance of the circumstance of
motion.

Rogers [1960, p496]:

Thus we have devised a new geometry, with our clocks and scales
conspiring, by their changes, to present us with a universally constant
speed of light.

References:

Aharoni, J., The Special Theory of Relativity, (1965), Dover, 1985.

Arzelies, H., Relativistic Kinematics, Pergamon, Oxford, 1966.

Bohm, D., The Special Theory of Relativity, W.A. Benjamin, New York, 1965.

Cullwick, E.G., Electromagnetism and Relativity, 2nd ed., Longmans,
London, 1959.

Eddington, A.S. Space, Time & Gravitation (1920) CUP, Cambridge Science
Classics, 1999

Eddington, A.S. The Mathematical Theory of Relativity, 2nd ed., CUP 1924.

Eddington, A. S., The Nature of the Physical World , 1928, CUP /
MacMillan (NY).

French, A.P., Special Relativity, Chapman & Hall, London, 1968.

Jackson J.D., Classical Electrodynamics, 2nd ed., John Wiley, New York,
1975.

Joos, G., Theoretical Physics, (1934), 3rd ed., Blackie, London, 1958.

Krane, K.S., Modern Physics, J. Wiley, New York, 1983.

McCrea, W.H., Relativity Physics, 4th ed., Methuen, London, 1954.

Matveyev, A., Principles of Electrodynamics, Reinhold, New York, 1966.

Minkowski, H., "Space and Time" (1908), in H.A. Lorentz et al., The
Principle of Relativity, Dover, 1952,75-91.

Møller, C., The Theory of Relativity, 2nd ed., OUP 1972.

Nunn, T.P., Relativity and Gravitation, University of London Press, 1923.

Pauli, W., Theory of Relativity (1921), Dover 1981.

Rindler, W., Introduction to Special Relativity, 2nd ed., Clarendon,
Oxford, 1991.

Rogers, E.M., Physics for the Inquiring Mind, Princeton U. P. 1960.

Rosser, W.G.V., Introductory Relativity, Butterworths, London, 1967.

Schwartz, M., Principles of Electrodynamics, McGraw Hill, New York, 1972.

Schwinger, J., Einstein's Legacy, Scientific American Library, New York,
1986.

Stephenson, G., & Kilmister, C.W., Special Relativity for Physicists
(1958), Dover, 1987.

Taylor, E.F., & Wheeler, J.A., Spacetime Physics: Introduction to
Special Relativity, 2nd ed., W.H. Freeman, New York, 1992.

Tolman, R.C., Relativity Thermodynamics and Cosmology (1934), Dover, 1987.

Whitrow, G.J., The Natural Philosophy of Time, 2nd Ed. OUP 1980.



No wonder the relativists in this newsgroup just give facetious replies
and never bother to answer questions. Even the "expert" relativists who
have written textbooks don't have a clue what's going on.

Dave wrote in message ...
The reciprocal effect of length contraction and time dilation, which
appears by logical necessity to emerge from the kinematic part of the
special theory of relativity, has been variously explained as

1. true but not really true
2. real
3. not real
4. apparent
5. the result of the relativity of simultaneity
6. determined by measurement
7. a perspective effect
8. mathematical.
SNIP

No wonder the relativists in this newsgroup just give facetious replies
and never bother to answer questions. Even the "expert" relativists who
have written textbooks don't have a clue what's going on.

- According to SRT-Lorentz, it is a real effect although what is
measured is only apparent. That may sound incomprehensible or
confused, but is clearly explained by him and others, without any
ambiguity (needs however more than a few lines). The clock paradox and
inertia increase in particle physics underscore that these effects are
real and not some kind of optical illusion.
SRT is definitely about measurable effects (that is what SRT is
about).

Now I read it again, likely SRT-Einstein more or less agrees with
this.
To sharpen it a bit more, according to SRT-Lorentz what is measured is
generally not conform the reality of nature but affected by speed
through the ether (the reality of nature is assumed to be
observer-independent).

Harald

Richard wrote in message ...
Bill Hobba wrote:

EjP wrote:
When you realize that relativity is ultimately about observation,
the 4,5,7, and 8 are more or less equivalent (with some qualification).
Although 2 and 3 sound contradictory, it depends on your definition
of "real" (believe it or not, "real" is not a well-defined term,
scientifically). Some people would consider a "real" length
contraction to be something involving some sort of physical
compression, understandable in terms of the bulk modulus, and
by that definition, the length contraction is
not "real". On the other hand if "real" means that I could
devise an experiment to measure the length contraction of
something that is moving in my frame, the it is *real*, but
now "real" is entirely consistent
with 4,5,7, and 8. I believe this distinction is what Eddington
was alluding to in comment 1.

For those that doubt length contraction simply analyze a current carrying
wire. If length contraction occurs then magnetic forces appear - exactly
as found from experiment - if length contrition does not occur then magnetic
forces do not appear - in disagreement with experiment.

Thanks
Bill

This is not a good example.

Sure it is, (I've read your post)
See Purcells, "Electricity and Mangnetism" pg. 153
and read on.

If you do the math you'll find that length
contraction cancels out of the equation, leaving the force as due to
only the relative velocity and proximity of the respective lines of
charge.

IMO, this is a narrow intrepretation. Purcell has
done a very good job of relating Magnetism to
the Lorentz Transformation.

Thus your example is actually a counter-argument to the
'reality' of length contraction, i.e. length contraction is unnecessary
and has nothing to do with the measured force. As an analogy let's
suppose that instead of length contraction we substitute line density
increase via spontaneous creation of extra electrons.

Oh boy, in another thread Rich converted my
mirror into a light bulb, and now a wire is being
converted into a generator!

Now it should be
obvious that this will provide exactly the same outcome,

I think you screwed charge conservation in the process!

since the
relativistic length contraction hypothesis provides just this same line
density increase. Now let's suppose that instead of either of these we
postulate that the lines of charge simply 'appear' closer to each other
from their respective points of view when they are in relative motion.
Again we get the same result. In short, any ad hoc presumption that will
increase the force as a function of relative velocity will work equally
as well as any other that does the same, each will provide correctly for
the observed force.

Thus I trust you'll see that the development of this force is no proof
or even the slightest indication that length contraction is real.
Now look at
http://www.cswnet.com/~rper
and you'll find a mathematical treatment of this force that makes no
assumptions at all about the 'cause' of the force, it simply quantifies
it directly as a function of relative velocity (in a sense this reduces
to the actual cause), and thus by virtue of Ockham's Razor, it is a much
superior approach, eliminating as it does all of the excess baggage that
you brought along above.
Richard Perry

Is Rich pulling my leg, or is he serious?
A lot of people respect Purcell, and I agree
with Hobba, who is usually careful. Rich, I
think you are having a *bad hair day* or
your theories are very complex. Is there
some need to correct the theory of magnetism?

Yours Truly
Ken S. Tucker