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From: Hendrik van Hees
Newsgroups: sci.physics.research
Subject: Re: Apparent rotation
Date: Thu, 12 Jan 2023 13:16:59 PST
Organization: Goethe University Frankfurt (ITP)
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From: hees@itp.uni-frankfurt.de
On 12/01/2023 09:23, Phillip Helbig (undress to reply) wrote:
> In article , Luigi Fortunati
> writes:
>
>> Phillip Helbigundress to reply mercoled=EC 11/01/2023 alle ore 09:32:14
>> ha scritto:
>>
>>>> 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?
>
>> 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.
>
>> 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.)
>
>> 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.
>
>>> 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".
>
>> 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/
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