Discussion:
Apparent rotation
Luigi Fortunati
2022-12-29 07:29:13 UTC
Are the rotations (and accelerations in general) all of the same type
(that is, are they all real) or are there real ones and also apparent
ones?

It seems to me that they are not all the same.

In my simulation
https://www.geogebra.org/m/asbcp8sh
there is a circle (a merry-go-round) and there is an annulus all around
it.

In the reference of the merry-go-round (click on the appropriate box)
it seems to rotate the circular crown, in the reference of the circular
crown it seems to rotate the merry-go-round.

It seems to me that (on Earth) the reference system where the rotation
is real is the one where the man has to cling to the pole (so as not to
fall) and where the plumb bob assumes an inclined position.

Instead, the reference system where the rotation is apparent is the one
where the man stands calmly and in balance (without having to hold on
to the pole to avoid falling) and where the plumb bob maintains its
vertical position.

Is that it?
Richard Livingston
2022-12-30 16:45:53 UTC
Post by Luigi Fortunati
Are the rotations (and accelerations in general) all of the same type
(that is, are they all real) or are there real ones and also apparent
ones?
Luigi,

I believe you are understanding it correctly. In a rotating reference
frame there are two types of accelerations: true accelerations and false
or coordinate accelerations. The true accelerations are the same as would
be calculated in an inertial reference frame. The false or coordinate
accelerations are the result of the coordinate points following a curved
path in the inertial reference frame. Coriolis forces are in the category
of a coordinate acceleration.

You can find the math on the web, but the general idea is that in a
rotating reference frame when taking derivatives of the coordinate
positions of an object you have to take into account not only how the
object is moving wrt the reference frame, but also how the reference
frame coordinates are moving wrt an inertial frame, i.e. rotations. The
accelerations of the coordinate positions have to be subtracted from the
calculated accelerations (i.e. second derivative of the coordinate
positions) in order to get the "real" forces on the object.

Rich L.
Luigi Fortunati
2022-12-31 12:08:53 UTC
Post by Richard Livingston
Luigi,
I believe you are understanding it correctly. In a rotating reference
frame there are two types of accelerations: true accelerations and false
or coordinate accelerations.
Richard, and what are these accelerations you speak of?

In the rotating frame of my simulation
https://www.geogebra.org/m/asbcp8sh
the only visible accelerations are those of the *external* annulus.

Inside the rotating frame there is nothing that accelerates: the pole A
stands still and the man A holding on to the pole also stands still.

Everyone stands still!

In the rotating frame there are no rotations and there are no
accelerations, neither centripetal nor centrifugal.

There are only forces: there is the force of the post on the man and
that of the man on the post.

Both exert their force but do not move and do not accelerate.

In the rotating frame this is the situation: the *forces* are there,
the *accelerations* are not.
Richard Livingston
2023-01-01 08:45:52 UTC
Post by Luigi Fortunati
Post by Richard Livingston
Luigi,
I believe you are understanding it correctly. In a rotating reference
frame there are two types of accelerations: true accelerations and false
or coordinate accelerations.
Richard, and what are these accelerations you speak of?
In the rotating frame of my simulation
https://www.geogebra.org/m/asbcp8sh
the only visible accelerations are those of the *external* annulus.
Inside the rotating frame there is nothing that accelerates: the pole A
stands still and the man A holding on to the pole also stands still.
Everyone stands still!
In the rotating frame there are no rotations and there are no
accelerations, neither centripetal nor centrifugal.
There are only forces: there is the force of the post on the man and
that of the man on the post.
Both exert their force but do not move and do not accelerate.
In the rotating frame this is the situation: the *forces* are there,
the *accelerations* are not.
Luigi,

Newton's Laws are for an inertial reference frame. All accelerations are wrt
such a frame. A rotating reference frame is not inertial. In calculating real
accelerations using a rotating frame you have to compensate for the
motion of the rotating frame wrt an inertial reference frame. It isn't
complicated.

Rich L.
Tom Roberts
2022-12-31 12:08:23 UTC
[...]
Any rotating system is real, for any sensible meaning of "real".

If the man and plumb bob are rotating with the merry-go-round, the man
must hold on and the plumb bob hangs inclined.

If the man and the plumb bob are not rotating, the man need not hold on,
and the plumb bob hangs vertically.

In no case does it matter what coordinates or reference is used, what
matters is whether the objects themselves are rotating.

This OUGHT to be obvious.

Tom Roberts
Luigi Fortunati
2023-01-01 04:47:16 UTC
Post by Tom Roberts
Any rotating system is real, for any sensible meaning of "real".
If the man and plumb bob are rotating with the merry-go-round, the man
must hold on and the plumb bob hangs inclined.
If the man and the plumb bob are not rotating, the man need not hold on,
and the plumb bob hangs vertically.
In no case does it matter what coordinates or reference is used, what
matters is whether the objects themselves are rotating.
This OUGHT to be obvious.
Tom Roberts
I absolutely agree with you: this IS obvious.

And it leads to this consequence:
(1) The rotation is absolute and does not depend on the reference
system.
(2) In my simulation
https://www.geogebra.org/m/asbcp8sh
the rotation of the circular crown (observed from the reference of the
circle) is apparent and not real.

Luigi Fortunati

[[Mod. note --
(We're assuming Newtonian mechanics throughout.)

I'm going to call the non-rotating reference frame "C" (for "circle"),
and the rotating reference frame (the one you called the "circular crown"
"R" (for "ring"). And let's say that the rotation axis is vertical, so
that your animation shows a view from above, looking down on a horizontal
plane containing the circle and the ring.

Your point (1) is correct. That is, in Newtonian mechanics, both C and
R can (consistently) figure out that C is non-rotating and R is rotating.

For example, both observers can notice that the plumb bob in C hangs
straight down, and if we attach a billiard table (ruled with x-y grid
lines to define an x-y coordinate system) to C, both observers can observe
that billiard balls move in straight lines with respect to C's coordinate
system.

Likewise, both observers can notice that the plumb bob in C hangs to
one side, and if we attach a billiard table (again ruled with x-y grid
lines to define an x-y coordinate system) to R, both observers can observe
that billiard balls do NOT move in straight lines with respect to R's
coordinate system.

Your point (2) is a bit trickier, because it depends on just what you
mean by the words "apparent" and "real". An observer in R will measure
C to be rotating "backwards". This relative rotation is real. But the
observer in R also knows (or should know, via the plumb bobs and billiard
tables discussed above) that she's rotating, and that C is not rotating,
and all the dynamics are consistent with that.
-- jt]]
2023-01-01 16:01:17 UTC
Post by Luigi Fortunati
Post by Tom Roberts
Any rotating system is real, for any sensible meaning of "real".
If the man and plumb bob are rotating with the merry-go-round, the man
must hold on and the plumb bob hangs inclined.
If the man and the plumb bob are not rotating, the man need not hold on,
and the plumb bob hangs vertically.
In no case does it matter what coordinates or reference is used, what
matters is whether the objects themselves are rotating.
This OUGHT to be obvious.
Tom Roberts
I absolutely agree with you: this IS obvious.
(1) The rotation is absolute and does not depend on the reference
system.
(2) In my simulation
https://www.geogebra.org/m/asbcp8sh
the rotation of the circular crown (observed from the reference of the
circle) is apparent and not real.
Luigi Fortunati
[[Mod. note --
(We're assuming Newtonian mechanics throughout.)
I'm going to call the non-rotating reference frame "C" (for "circle"),
and the rotating reference frame (the one you called the "circular crown"
"R" (for "ring"). And let's say that the rotation axis is vertical, so
that your animation shows a view from above, looking down on a horizontal
plane containing the circle and the ring.
Your point (1) is correct. That is, in Newtonian mechanics, both C and
R can (consistently) figure out that C is non-rotating and R is rotating.
That is the well known fact that rotation, like other accelerations,
appears to be absolute. Somewhat puzzling is the question "relative to
what?" One can use it to make an argument for absolute space in the
sense of Newton. Or, following Mach, argue that it only appears to be
absolute and is actually relative to the distant galaxies or whatever,
in other words the behaviour would be the same if the rest of the
Universe were rotating around a bucket of water---water would still pile
up on the sides. In general relativity there is an effect known as
frame-dragging, or the Lense-Thirring effect, which has been observed.
However, as far as I know, there is still some genuine debate on this
issue within the general-relativity community (i.e. whether Mach's
principle explains why acceleration appears to be absolute).

Imagine a completely empty universe. Would there still be inertia? If
one argues that there wouldn't be, because there is nothing acceleration
could be relative to, would that change if one introduced one or more
other bodies of arbitrarily small mass? If one then observes the
expected inertia, how can that be due to arbitrarily small masses? One
might argue that that would lead to a small amount of inertia and adding
more and more mass in the form of other bodies would increase inertia.

There is a huge amount of literature on Mach's principle. The
moderator's note mentions that Newtonian mechanics is assumed
throughout. If we drop that assumption, what happens? In other words,
what is the current thinking on whether Mach's principle explains the
origin of inertia?
Stefan Ram
2023-01-02 05:56:59 UTC
Post by Phillip Helbig (undress to reply)
In general relativity there is an effect known as
frame-dragging, or the Lense-Thirring effect, which has been observed.
This probably cannot be used to explain inertia because it
has different properties.

On december 25, I submitted a small snippet about the
Lense-Thirring effect to this newsgroup. I have still not
seen it appear in this newsgroup. Here it is again:

|A body falling towards a stream of matter is indeed pulled in
|the direction of its motion, but a body moving away from the
|stream is accelerated in a direction opposite to the motion
|of the stream! And a body at rest feels no influence from the
|motion of the stream at all.
"Gravity from the Ground up" (2003) - Bernard Schutz (1946/).
Post by Phillip Helbig (undress to reply)
Imagine a completely empty universe. Would there still be inertia?
I'm not trained in this area, but, following remarks by other
more trained individuals I read in the Usenet, one can say:

Even a space time with no mass and no electromagnetic field
energy has a metric.

This metric determines the possible geodesics of light. And
the geodesics of light are not rotating.

So, there always is a metric, and this defines the meaning
of "non-rotating" and "rotating".

these laws tell us that there is a conserved quantity for
every symmetry. In an empty universe, there would be perfect
homogeneity in three directions (axes). So the momenta in all
three directions would be conserved. Isn't this conservation
of momentum what one calls inertia?
Richard Livingston
2023-01-02 08:37:34 UTC
On Sunday, January 1, 2023 at 10:01:21 AM UTC-6, Phillip Helbig (undress to reply) wrote:
...
Imagine a completely empty universe. Would there still be inertia? If
one argues that there wouldn't be, because there is nothing acceleration
could be relative to, would that change if one introduced one or more
other bodies of arbitrarily small mass? If one then observes the
expected inertia, how can that be due to arbitrarily small masses? One
might argue that that would lead to a small amount of inertia and adding
more and more mass in the form of other bodies would increase inertia.
...
Actually, combining simple ideas from QM and SR give us momentum:
-Consider a mass $m$ at rest. Per SR this mass represents an energy $mc^2$.
-Per QM this energy is represented by a frequency $\omega = mc^2 /\hbar$.
Therefore the QM wave function is something like:
$\Psi = \Psi_0 e^{i mc^2 t /\hbar}$
-Now consider an observer moving at $-\beta$ wrt this mass. They will
transform the time to $t => \gamma t' - \gamma \beta x'$ (using units of
seconds for time and space).
-This gives the wave function in the observers frame as:
$\Psi = \Psi_0 e^{i ((\gammamc^2 /\hbar) t' - (\gamma mc^2 /\hbar) x')}$
-Note that in QM the wave number, $\gamma \beta mc^2$, is the momentum
of the mass.
-This means that if that mass had originally been in the observers frame, he
would have had to impart an energy $(\gamma -1) mc^2$ to the mass with
an accompanying momentum imparted of $\gamma \beta mc^2$. This is
what we mean by inertia, that it takes energy and momentum to make an
object move.

I don't think we need to invoke the mass of the universe to explain inertia,
unless it is that mass that generates the Minkowski space-time geometry.

Rich L.
2023-01-02 19:02:26 UTC
Post by Richard Livingston
....
Imagine a completely empty universe. Would there still be inertia? If
one argues that there wouldn't be, because there is nothing acceleration
could be relative to, would that change if one introduced one or more
other bodies of arbitrarily small mass? If one then observes the
expected inertia, how can that be due to arbitrarily small masses? One
might argue that that would lead to a small amount of inertia and adding
more and more mass in the form of other bodies would increase inertia.
...
We can imagine the limits h-->0 and c-->. Would there still be inertia
in such cases?
Post by Richard Livingston
I don't think we need to invoke the mass of the universe to explain
inertia, unless it is that mass that generates the Minkowski
space-time geometry.
I think that that is the motivation of most people who invoke Mach's
principle.
Luigi Fortunati
2023-01-02 08:37:34 UTC
Phillip Helbigundress to reply domenica 01/01/2023 alle ore 01:01:17 ha
Post by Phillip Helbig (undress to reply)
That is the well known fact that rotation, like other accelerations,
appears to be absolute. Somewhat puzzling is the question "relative to
what?"
The question is not puzzling, it is poorly posed.

The *real* rotation is not the observed one and, therefore, does not
depend on the reference.

The real rotation of bodies is an *internal* matter which concerns the
atoms of which the body is composed.

If the atoms of the body are not in tension, if no atoms push or pull
adjacent ones, and if no atoms are pushed or pulled by them, then the
body is not rotating.

If, on the other hand, all atoms are in radial tension, if each atom
pushes and pulls adjacent ones in a radial direction and is pushed and
pulled by them, then the body is in rotation.

To establish whether a body is rotating or not, we must not rely on
external references but only on internal radial forces.
Luigi Fortunati
2023-01-04 17:12:48 UTC
In my simulation
https://www.geogebra.org/m/jvsxwjrb
there's a lighter sliding frictionlessly across the dashboard of a
cornering car.

But I'm not going to talk about the lighter here, I'm just going to talk
about the car that curves along a semicircle from time 4 to time 25,
rotating 180Â°.

There is a single force F (the friction between the tires and the
ground) which simultaneously acts on the car (in one direction) and on
the ground (in the opposite direction).

This force F, applied to the small mass m of the machine, generates a
good centripetal acceleration a=F/m.

The same force F, applied to the ground (and therefore to all the
immense mass M of the Earth), generates a centripetal acceleration a=F/M
equal (for all practical effects) to zero.

Consequently, in the inertial frame, the observer on the ground sees
this rotation and the driver of the car feels its force (because the
rotation is real).

And, instead, in the accelerated reference, the driver of the machine
sees the rotation of the ground but the observer on the ground does not
perceive any force (because the rotation of the ground is apparent).

If the earth really rotated at the angular speed of 180Â° in 21 seconds
(as the driver of the car sees it do), the man on the ground and all of
us could not stay happy.
wugi
2023-01-09 12:15:37 UTC
Post by Phillip Helbig (undress to reply)
Post by Luigi Fortunati
Post by Tom Roberts
Any rotating system is real, for any sensible meaning of "real".
If the man and plumb bob are rotating with the merry-go-round, the man
must hold on and the plumb bob hangs inclined.
If the man and the plumb bob are not rotating, the man need not hold on,
and the plumb bob hangs vertically.
In no case does it matter what coordinates or reference is used, what
matters is whether the objects themselves are rotating.
This OUGHT to be obvious.
Tom Roberts
I absolutely agree with you: this IS obvious.
(1) The rotation is absolute and does not depend on the reference
system.
(2) In my simulation
https://www.geogebra.org/m/asbcp8sh
the rotation of the circular crown (observed from the reference of the
circle) is apparent and not real.
Luigi Fortunati
[[Mod. note --
(We're assuming Newtonian mechanics throughout.)
I'm going to call the non-rotating reference frame "C" (for "circle"),
and the rotating reference frame (the one you called the "circular crown"
"R" (for "ring"). And let's say that the rotation axis is vertical, so
that your animation shows a view from above, looking down on a horizontal
plane containing the circle and the ring.
Your point (1) is correct. That is, in Newtonian mechanics, both C and
R can (consistently) figure out that C is non-rotating and R is rotating.
That is the well known fact that rotation, like other accelerations,
appears to be absolute. Somewhat puzzling is the question "relative to
what?" One can use it to make an argument for absolute space in the
sense of Newton. Or, following Mach, argue that it only appears to be
absolute and is actually relative to the distant galaxies or whatever,
in other words the behaviour would be the same if the rest of the
Universe were rotating around a bucket of water---water would still pile
up on the sides. In general relativity there is an effect known as
frame-dragging, or the Lense-Thirring effect, which has been observed.
However, as far as I know, there is still some genuine debate on this
issue within the general-relativity community (i.e. whether Mach's
principle explains why acceleration appears to be absolute).
Imagine a completely empty universe. Would there still be inertia? If
one argues that there wouldn't be, because there is nothing acceleration
could be relative to, would that change if one introduced one or more
other bodies of arbitrarily small mass? If one then observes the
expected inertia, how can that be due to arbitrarily small masses? One
might argue that that would lead to a small amount of inertia and adding
more and more mass in the form of other bodies would increase inertia.
There is a huge amount of literature on Mach's principle. The
moderator's note mentions that Newtonian mechanics is assumed
throughout. If we drop that assumption, what happens? In other words,
what is the current thinking on whether Mach's principle explains the
origin of inertia?
I think that, as Richard Livings said elsewhere, acceleration in general
and rotation in particular are 'absolute' in the sense of non-inertial.
Any inertial system will 'detect' acceleration and rotation, and their
'inertia'.

As to why this is so, and whether Mach's principle and far away universe
parts should be called for, I doubt it. At least in the first degree. I
think the first actor is the behaviour of light, or EM radiation, ie,
the EM field, and 'local' light speed. Once you've these, you can start
playing with light clocks.

The photons 'ticktocking' in a light clock can be assigned mass (and
inertia). An inertially moving light clock is actually 'carrying' mass
at infra-luminal velocities... where's the difference with matter as a
carrier of mass at infraluminal velocities!? ;)

So, acceleration and rotation are 'absolute' WRT the EM field.

Now then, how does the EM field 'decide' about its local behaviour and
metric? Does Mach and the far away universe possibly intervene here 'in
the second degree'? That, I wouldn't know...
--
guido wugi
Luigi Fortunati
2023-01-11 04:37:15 UTC
Post by wugi
I think that, as Richard Livings said elsewhere, acceleration in general
and rotation in particular are 'absolute' in the sense of non-inertial.
Any inertial system will 'detect' acceleration and rotation, and their
'inertia'.
As to why this is so, and whether Mach's principle and far away universe
parts should be called for, I doubt it. At least in the first degree. I
think the first actor is the behaviour of light, or EM radiation, ie,
the EM field, and 'local' light speed. Once you've these, you can start
playing with light clocks.
The photons 'ticktocking' in a light clock can be assigned mass (and
inertia). An inertially moving light clock is actually 'carrying' mass
at infra-luminal velocities... where's the difference with matter as a
carrier of mass at infraluminal velocities!? ;)
So, acceleration and rotation are 'absolute' WRT the EM field.
Now then, how does the EM field 'decide' about its local behaviour and
metric? Does Mach and the far away universe possibly intervene here 'in
the second degree'? That, I wouldn't know...
Why bother the distant universe if rotation (like any other
acceleration) are "absolute"?

Matter is made up of atoms with a nucleus inside.

If we rotate the matter (ie the atoms) the nuclei that "float" inside
them "push" outwards and generate centrifugal force opposed by the
centripetal force of the molecular bonds.

The presence of these two opposing internal forces of matter is
confirmed by the internal tension of the rotating bodies.
Luigi Fortunati
2023-01-02 05:55:22 UTC
(1) The rotation is absolute and does not depend on the reference=20
system.
(2) In my simulation
https://www.geogebra.org/m/asbcp8sh
the rotation of the circular crown (observed from the reference of the=20
circle) is apparent and not real.
[[Mod. note --
(We're assuming Newtonian mechanics throughout.)
I'm going to call the non-rotating reference frame "C" (for "circle"),
and the rotating reference frame (the one you called the "circular crow=
n"
"R" (for "ring"). And let's say that the rotation axis is vertical, so
that your animation shows a view from above, looking down on a horizontal
plane containing the circle and the ring.
Your point (1) is correct. That is, in Newtonian mechanics, both C and
R can (consistently) figure out that C is non-rotating and R is rotating.
For example, both observers can notice that the plumb bob in C hangs
straight down, and if we attach a billiard table (ruled with x-y grid
lines to define an x-y coordinate system) to C, both observers can observe
that billiard balls move in straight lines with respect to C's coordinate
system.
Likewise, both observers can notice that the plumb bob in C hangs to
one side, and if we attach a billiard table (again ruled with x-y grid
lines to define an x-y coordinate system) to R, both observers can observe
that billiard balls do NOT move in straight lines with respect to R's
coordinate system.
Your point (2) is a bit trickier, because it depends on just what you
mean by the words "apparent" and "real".
For me, "real rotation" is that of the frame of reference where there
are centripetal and centrifugal forces, and "apparent rotation" is that
of the frame of reference where these forces are not there.

For me, the "real rotation" is that of the frame of reference where the
plumb bob remains tilted, and the "apparent rotation" is that of the
frame of reference where the plumb bob remains vertical.
An observer in R will measure C to be rotating "backwards".
This relative rotation is real.
If I told you that I "measured" the rotation of the Sun (in 24 hours)
around the Earth, you would answer me that I "observed" (and not
"measured") this rotation.

And if I told you that this rotation of the Sun is real, you would
answer that it is not true and that it is only of an apparent motion,
because it is not the Sun that has rotated (in 24 hours) around the
Earth but it is the Earth that (at the same time) has rotated on
itself.

And then, in the same way, I too tell you that the rotation of C,
observed (and not measured) by R, is an "apparent" motion, because it
is not C that rotates on itself but it is R that rotates around it.
Richard Livingston
2023-01-04 01:09:38 UTC
On Monday, January 2, 2023 at 1:02:30 PM UTC-6, Phillip Helbig (undress to reply) wrote:
...
We can imagine the limits h-->0 and c-->. Would there still be inertia
in such cases?
Taking the limit of c => infinity suggests that the inertia would
become infinite, not zero, provided hbar remains finite. I'm not
sure the wave function makes sense anymore if hbar goes to zero.
If hbar also goes to zero as c goes to infinity, then it would
depend on how fast each goes to the limits. One case would give
infinite inertia and the other would give zero, maybe. It isn't
clear since we don't know what energy and momentum really are.

Rich L.
2023-01-11 08:32:14 UTC
Post by Luigi Fortunati
Post by wugi
I think that, as Richard Livings said elsewhere, acceleration in general
and rotation in particular are 'absolute' in the sense of non-inertial.
Any inertial system will 'detect' acceleration and rotation, and their
'inertia'.
As to why this is so, and whether Mach's principle and far away universe
parts should be called for, I doubt it.
Now then, how does the EM field 'decide' about its local behaviour and
metric? Does Mach and the far away universe possibly intervene here 'in
the second degree'? That, I wouldn't know...
Why bother the distant universe if rotation (like any other
acceleration) are "absolute"?
Matter is made up of atoms with a nucleus inside.
If we rotate the matter (ie the atoms) the nuclei that "float" inside
them "push" outwards and generate centrifugal force opposed by the
centripetal force of the molecular bonds.
The presence of these two opposing internal forces of matter is
confirmed by the internal tension of the rotating bodies.
Yes. No-one debates the fact that accelerations are absolute. The
question is WHY that is the case. Imagine an empty universe with one
object in it, say a merry-go-round. Should it be possible to tell if it
is rotating, as it would be under normal conditions? If so, with
respect to what is it rotating? There is nothing else in the Universe.

Some would claim that there would be no way to tell in such a case, i.e.
no inertia. Add a small amount of matter to the universe and there
would be a small amount of inertia. Add more and there would be more.
And so on. That would make sense if inertia is somehow caused by the
presence of other matter, which is the essence of Mach's Principle.
Certainly the Lense-Thirring effect indicates that the idea that
relative rotation has physical effects is not absurd.

As far as I know the extent to which, if any, Mach's Principle is real
is still an open question.

The alternative seems to be absolute space, which is usually associated
with Newton rather than Einstein.
Luigi Fortunati
2023-01-11 18:58:51 UTC
Phillip Helbigundress to reply mercoled=EC 11/01/2023 alle ore 09:32:14
Post by Phillip Helbig (undress to reply)
Post by Luigi Fortunati
Why bother the distant universe if rotation (like any other
acceleration) are "absolute"?
Matter is made up of atoms with a nucleus inside.
If we rotate the matter (ie the atoms) the nuclei that "float" inside
them "push" outwards and generate centrifugal force opposed by the
centripetal force of the molecular bonds.
The presence of these two opposing internal forces of matter is
confirmed by the internal tension of the rotating bodies.
Yes. No-one debates the fact that accelerations are absolute. The
question is WHY that is the case. Imagine an empty universe with one
object in it, say a merry-go-round. Should it be possible to tell if it
is rotating, as it would be under normal conditions? If so, with
respect to what is it rotating? There is nothing else in the Universe.
There is a contradiction in what you write.

First you say that accelerations are absolute and then you ask "with
respect to what is it rotating?".

If they are absolute, they cannot depend on the reference!

I say that the "real" rotations (those where centripetal and
centrifugal forces are manifested) are absolute and the "apparent"
rotations (those where neither centripetal nor centrifugal forces are
manifested) are relative.

In an empty universe there could be only real rotations, those where
the question "with respect to what is it rotating?" it has no reason to
exist,being absolute and not relative.
Post by Phillip Helbig (undress to reply)
Some would claim that there would be no way to tell in such a case, i.e.
no inertia.
I did not get this.

Do you think that in a completely empty universe there would be no
centripetal and centrifugal forces?

What does it mean: there would be no inertia?
Luigi Fortunati
2023-01-14 08:15:52 UTC
Phillip Helbigundress to reply giovedì 12/01/2023 alle ore 09:23:17 ha
But do such real rotations imply some sort of absolute space?
There is no absolute space and there is no absolute reference frame.

Space is not a reference frame, neither relative nor absolute.

The bodies are a reference frame, not the space.

Space doesn't stand still, it doesn't rotate, it doesn't accelerate.

They are the bodies that (in space) rotate, accelerate or remain inert
(inert, not still).

Every body is a reference frame.

The body without internal tensions is an inertial reference frame.

The body with internal tensions is an accelerated reference frame.
Hendrik van Hees
2023-01-12 21:16:59 UTC
Post by Luigi Fortunati
Phillip Helbigundress to reply mercoled=EC 11/01/2023 alle ore 09:32:14
Post by Phillip Helbig (undress to reply)
Post by Luigi Fortunati
Why bother the distant universe if rotation (like any other
acceleration) are "absolute"?
Matter is made up of atoms with a nucleus inside.
If we rotate the matter (ie the atoms) the nuclei that "float" inside
them "push" outwards and generate centrifugal force opposed by the
centripetal force of the molecular bonds.
The presence of these two opposing internal forces of matter is
confirmed by the internal tension of the rotating bodies.
Yes. No-one debates the fact that accelerations are absolute. The
question is WHY that is the case. Imagine an empty universe with one
object in it, say a merry-go-round. Should it be possible to tell if it
is rotating, as it would be under normal conditions? If so, with
respect to what is it rotating? There is nothing else in the Universe.
There is a contradiction in what you write.
First you say that accelerations are absolute and then you ask "with
respect to what is it rotating?".
It is an empirical fact that they are absolute. But the very word
"rotation" implies that it is rotating with respect to something. But
what?
Post by Luigi Fortunati
If they are absolute, they cannot depend on the reference!
Another way of looking at it is that they provide an absolute reference,
absolute space, a Newtonian idea which some think Einstein did away
with.
A more modern interpretation of the Newtonian space-time framework is
that there is not an absolute space and time but that there exists a
class of inertial frames, in each of which Newton's 1st Law holds true.
Further the assumption is that any inertial observer describes space as
a 3D affine Euclidean manifold, and time is just an independent
parameter parametrizing a causal order.

Then special relativity has been discovered out of the necessity to also
make electromagnetism consistent with the special principle of
relativity and the observation that there's no preferred inertial frame
(something like an "ether rest frame"). The result is that instead of
the Galilei-Newtonian fiber-bundle structure one get's a 4D affine
Lorentzian manifold as the spacetime model with the Poincare group as
symmetry group. Since Newton's first postulate still holds there's still
the class of global inertial frames.

General relativity then can be understood as the idea that Poincare
symmetry is made a local symmetry, i.e., there exists only local
inertial frames, and rotations or other proper accelerations are always
relative to the local inertial frame.
Post by Luigi Fortunati
I say that the "real" rotations (those where centripetal and
centrifugal forces are manifested) are absolute and the "apparent"
rotations (those where neither centripetal nor centrifugal forces are
manifested) are relative.
I am sitting on a chair. If I can feel it pushing on me, then I am
really being accelerated, as opposed to someone thinking I am because of
some strange coordinates. (Ignoring for the moment that I also feel it
pushing on me at rest in a gravitational field.)
Post by Luigi Fortunati
In an empty universe there could be only real rotations, those where
the question "with respect to what is it rotating?" it has no reason to
exist, being absolute and not relative.
Right. But do such real rotations imply some sort of absolute space?
It's a hard question. Einstein spent years thinking about it.
Post by Luigi Fortunati
Post by Phillip Helbig (undress to reply)
Some would claim that there would be no way to tell in such a case, i.e.
no inertia.
I did not get this.
That is a claim some people make. If one thinks that what determines a
real acceleration is acceleration relative to some average of mass in
the Universe, then it makes sense for inertia to be proportional to such
mass.
I don't think that GR in any way has something to do with this "Machian
ideas", because it's a theory, which is strictly local, i.e.,
interactions are described by a local field theory, and thus
accelerations of (test) bodies relative to a local inertial frame are
due to interactions of the body with a field (e.g., the electromagnetic
field, acting on an electrically charged test particle). The
gravitational interaction is usually reinterpreted as "geometrized",
i.e., a free test particle moves on geodesics in curved spacetime, and
relative to a local inertial frame there's no force, and only "tidal
forces" on extended bodies are the "true gravitational forces".
Post by Luigi Fortunati
Do you think that in a completely empty universe there would be no
centripetal and centrifugal forces?
I don't know.
--
Hendrik van Hees
Goethe University (Institute for Theoretical Physics)
D-60438 Frankfurt am Main
http://itp.uni-frankfurt.de/~hees/