imagine a very long string (as long as the distance from Earth to the Sun)
that is continously unwound by traction at a certain speed in a middle of an
empty space (no gravity). the string is supposed to be thin, unbreakable and
inextensible. I decide to stop the unreeling at one end at time t.
Does the other end continue to go on by the time the information from the
blocking end arrives (that is 8 minutes later at t+8mn)
or does it block instantaneously (at time t) because of the fact that the
string is not extensible ?

The same way, two persons walking in opposite direction are pulling the same
very long string. When the string is hard strained at time t, does one stop
before the other (the information of a strained string has to propagate at
maximum light speed from the middle to the extremities) or do they stop at
the same time ?

Jean DAVID
http://www.geocities.com/jeandavid5.geo

"Jean DAVID" wrote in message ...
imagine a very long string (as long as the distance from Earth to the Sun)
that is continously unwound by traction at a certain speed in a middle of an
empty space (no gravity). the string is supposed to be thin, unbreakable and
inextensible. I decide to stop the unreeling at one end at time t.
Does the other end continue to go on by the time the information from the
blocking end arrives (that is 8 minutes later at t+8mn)
or does it block instantaneously (at time t) because of the fact that the
string is not extensible ?

The picture is not clear to me.
Perhaps the answer to your next question helps.


The same way, two persons walking in opposite direction are pulling the same
very long string. When the string is hard strained at time t, does one stop
before the other (the information of a strained string has to propagate at
maximum light speed from the middle to the extremities) or do they stop at
the same time ?

According to someone according to whom the persons walk
in opposite direction with the same speed, the two persons
stop at the same time.
According to each walking person, the other person stops
later than himself.

Dirk Vdm

Jean DAVID wrote:
imagine a very long string (as long as the distance from Earth to the Sun)
that is continously unwound by traction at a certain speed in a middle of an
empty space (no gravity). the string is supposed to be thin, unbreakable and
inextensible. I decide to stop the unreeling at one end at time t.
Does the other end continue to go on by the time the information from the
blocking end arrives (that is 8 minutes later at t+8mn)
or does it block instantaneously (at time t) because of the fact that the
string is not extensible ?

If you truly had an inextensible string, the speed of sound in that
string (the speed of propagation of disturbances) would be
infinite. Such materials are physically impossible according
to relativity.

There are many such thought experiments you can do, all
arriving at the same conclusion: you can violate relativity if
you can create perfectly rigid materials. "Perfectly rigid rods"
are another common theme.

- Randy

I don't think that such string (inextensible) is physically impossible
because relavity laws must be observed. We can make shorter strings (or
rods) that can resist a certain tension of pulling. Those objects don't have
to be perfectly rigid. I have supposed that the speed of unreeling of the
string is small enough so not to create a big tension to break the string.
The only difficulty I admit is the length of such string that we can product
for the experience.

You also said that speed of propagation in such a material is infinite, I
really doubt so.

"Randy Poe" a écrit dans le message de news:
...

Jean DAVID wrote:
imagine a very long string (as long as the distance from Earth to the
Sun)
that is continously unwound by traction at a certain speed in a middle of
an
empty space (no gravity). the string is supposed to be thin, unbreakable
and
inextensible. I decide to stop the unreeling at one end at time t.
Does the other end continue to go on by the time the information from the
blocking end arrives (that is 8 minutes later at t+8mn)
or does it block instantaneously (at time t) because of the fact that the
string is not extensible ?

If you truly had an inextensible string, the speed of sound in that
string (the speed of propagation of disturbances) would be
infinite. Such materials are physically impossible according
to relativity.

There are many such thought experiments you can do, all
arriving at the same conclusion: you can violate relativity if
you can create perfectly rigid materials. "Perfectly rigid rods"
are another common theme.

- Randy

"Jean DAVID" wrote in message ...

[ text moved to bottom - please do not top-post ]

"Randy Poe" a écrit dans le message de news:
...

Jean DAVID wrote:
imagine a very long string (as long as the distance from Earth to the
Sun)
that is continously unwound by traction at a certain speed in a middle of
an
empty space (no gravity). the string is supposed to be thin, unbreakable
and
inextensible. I decide to stop the unreeling at one end at time t.
Does the other end continue to go on by the time the information from the
blocking end arrives (that is 8 minutes later at t+8mn)
or does it block instantaneously (at time t) because of the fact that the
string is not extensible ?

If you truly had an inextensible string, the speed of sound in that
string (the speed of propagation of disturbances) would be
infinite. Such materials are physically impossible according
to relativity.

There are many such thought experiments you can do, all
arriving at the same conclusion: you can violate relativity if
you can create perfectly rigid materials. "Perfectly rigid rods"
are another common theme.

- Randy


I don't think that such string (inextensible) is physically impossible
because relavity laws must be observed.

It's the other way around.
The laws of relativity are made in such a way that *they* obey
what we think we know about nature.


We can make shorter strings (or
rods) that can resist a certain tension of pulling. Those objects don't have
to be perfectly rigid. I have supposed that the speed of unreeling of the
string is small enough so not to create a big tension to break the string.
The only difficulty I admit is the length of such string that we can product
for the experience.

You also said that speed of propagation in such a material is infinite, I
really doubt so.

It would be if it were perfectly rigid.
Look at the speeds of sound in different materials.
Compare the speed with the rigidity.

Dirk Vdm

Jean DAVID wrote:
I don't think that such string (inextensible) is physically impossible
because relavity laws must be observed. We can make shorter strings (or
rods) that can resist a certain tension of pulling.

What does "resist a certain tension of pulling" mean?. If you pull one
end of a real rod or a real string, the immediate effect is that
only your end will stretch. The stretch reaches the other end
at the speed of sound, and takes a measurable amount of
time to reach the other end.

Those objects don't have to be perfectly rigid.

No, but they also aren't "inextensible". When you pull them,
they stretch. All real materials stretch when you pull them.

I have supposed that the speed of unreeling of the
string is small enough so not to create a big tension to break the string.
The only difficulty I admit is the length of such string that we can product
for the experience.

You also said that speed of propagation in such a material is infinite, I
really doubt so.

You said so by definition.

I pull one end of the string 1 mm closer to me. The entire string,
being inextensible, moves 1 mm instantaneously. The disturbance
has propagated through the entire string in 0 time.

What is the speed of propagation if the time of propagation is 0
but the length of propagation is something large?

- Randy

"Randy Poe" a écrit dans le message de news:
...

Jean DAVID wrote:
I don't think that such string (inextensible) is physically impossible
because relavity laws must be observed. We can make shorter strings (or
rods) that can resist a certain tension of pulling.

What does "resist a certain tension of pulling" mean?. If you pull one
end of a real rod or a real string, the immediate effect is that
only your end will stretch. The stretch reaches the other end
at the speed of sound, and takes a measurable amount of
time to reach the other end.

Those objects don't have to be perfectly rigid.

No, but they also aren't "inextensible". When you pull them,
they stretch. All real materials stretch when you pull them.

I have supposed that the speed of unreeling of the
string is small enough so not to create a big tension to break the
string.
The only difficulty I admit is the length of such string that we can
product
for the experience.

You also said that speed of propagation in such a material is infinite, I
really doubt so.

You said so by definition.

No, you misunderstood me.
I only ask if the blocking is instantanous at the other end . If so then the
speed of propagation is infinite and is in contradiction with relativity.
In the other case, the string will "stretch", as you said, as long as the
information travels through the wire.
But how long will it stretch ?
Will the string at the time of the arriving of the information be longer
than the length it had when I decide to block the unreeling at time t ?

You must be right. Perfect rigidity is impossible in relativity world.

"Jean DAVID" wrote in message
...
I don't think that such string (inextensible) is physically impossible
because relavity laws must be observed.

Inextensible is physically the same as rigid - ie if it was inextensible
then when it is pulled taught pulling one end will imply the other end would
move instantaneously - by the definition of inextensible. This is not
possible according to SR. It is the same reason rigid rods do not actually
exist - which is rather amusing because in formulating relativity you will
come across the concept of small rigid rods. It is not a contradiction
because where it is used the rod is so small that for all practical purposes
the time delay in moving one end before the other moves is negligible.

Thanks
Bill

We can make shorter strings (or rods) that can resist a certain tension
of pulling. Those objects don't have to be perfectly rigid. I have
supposed that the speed of unreeling of the string is small enough so not
to create a big tension to break the string. The only difficulty I admit
is the length of such string that we can product for the experience.

You also said that speed of propagation in such a material is infinite, I
really doubt so.

"Randy Poe" a écrit dans le message de news:
...

Jean DAVID wrote:
imagine a very long string (as long as the distance from Earth to the
Sun)
that is continously unwound by traction at a certain speed in a middle
of an
empty space (no gravity). the string is supposed to be thin, unbreakable
and
inextensible. I decide to stop the unreeling at one end at time t.
Does the other end continue to go on by the time the information from
the
blocking end arrives (that is 8 minutes later at t+8mn)
or does it block instantaneously (at time t) because of the fact that
the
string is not extensible ?

If you truly had an inextensible string, the speed of sound in that
string (the speed of propagation of disturbances) would be
infinite. Such materials are physically impossible according
to relativity.

There are many such thought experiments you can do, all
arriving at the same conclusion: you can violate relativity if
you can create perfectly rigid materials. "Perfectly rigid rods"
are another common theme.

- Randy

Talking of strings,

Joao Magueijo (in his VSL theory) suggests "fast tracks", along which
the speed of light is/can be much higher than normal.

Jean There would be a time lapse. No action is faster than 'c' Bert