From: starmaker@ix.netcom.com
On Wed, 4 Feb 2026 20:24:22 +0100, Thomas 'PointedEars' Lahn
wrote:
>Paul B. Andersen wrote:
>> Den 04.02.2026 01:06, skrev Thomas 'PointedEars' Lahn:
>>> Paul B. Andersen wrote:
>>>> Den 03.02.2026 16:14, skrev Thomas 'PointedEars' Lahn:
>>>>> Paul B. Andersen wrote:
>>>>>> Den 03.02.2026 10:07, skrev Maciej Wo?niak:
>>>>>>> Anyway, for any context motion is absolute.
>>>>>> Speed is relative, acceleration is absolute.
>>>>> Wrong.
>>>>
>>>> Acceleration is absolute in Newtonian mechanics.
>>>
>>> That is not so. If two reference frames are non-inertial (in the Newtonian
>>> sense), then it is possible that one is not accelerating relative to the
other.
>>
>> It was Maciej Wo?niak who used the word "absolute".
>>
>> In this context, "absolute" means "not relative".
>
>Of course. We do not have a difference of definition.
>
>> Speed is relative because the speed of an object depend
>> on in which _inertial_ frame of reference it is measured.
>> > In Newtonian mechanics the acceleration dv(t)/dt of an object
>> will be the same independent of in which _inertial_ frame of
>> reference the speed v(t) is measured.
>
>One does not have to {use|be in} an inertial frame of reference for one's
>measurements.
>
>> Acceleration is not relative, it is absolute.
>
>Nonsense does not make sense by repetition.
>
>>> That is precisely the case (approximately), when a falling observer is
>>> observing a nearby falling object and we assume approximatively that the
>>> gravitational field is uniform. Then that object would be observed by thta
>>> observer to be at rest even though it would be accelerating for an inertial
>>> observer.
>>
>> You are in free-fall. An apple is right above your head,
>> and will stay there.
>
>That is why it is NOT accelerated in *that* frame of reference, which is
>exactly my point.
>
>> (If you are falling towards the Earth, the distance between your
>> head and the apple will increase very slowly. We ignore it.)
>>
>> You will know that since you are in free-fall your and the apple's
>> proper acceleration is exactly zero. (no air resistance)
>
>According to the Einstein equivalence principle it is not possible to
>distinguish, without external reference, free fall in a uniform
>gravitational field from a state of rest not under the influence of
>gravitation, or motion in an accelerated reference frame not under
>the influence of gravitation from motion in a frame at rest under
>the influence of uniform gravitation.
>
>So it is not correct to say "acceleration is absolute".
Maybe I will a little hard on your pointed ears...let me rephrase it:
Your entire paragraph is a sloppy, half-remembered mash-up that
mangles the single most important distinction in the foundations of
general relativity.
[Core Structural / Fatal Flaws — numbered]
You completely erased the difference between proper acceleration
and coordinate acceleration You pretend the equivalence principle
makes “acceleration” (unspecified) non-absolute. It doesn’t. It makes
the effects of uniform gravitational fields locally indistinguishable
from the effects of uniform proper acceleration. An accelerometer
glued to your forehead reads zero in free fall and reads non-zero when
the floor is pushing you up. That reading — proper acceleration — is
coordinate-invariant and absolute. Your claim obliterates this
distinction and is therefore dead on arrival.
You misquote Einstein’s own words to reach the opposite conclusion
Einstein wrote that the equivalence “makes it impossible for us to
speak of the absolute acceleration of the system of reference” —
referring precisely to the fact that you can choose coordinates where
the accelerated rocket is “at rest” and gravity is present instead. He
is forbidding absolute coordinate acceleration of the frame, not
declaring proper acceleration relative. You inverted his meaning into
“acceleration is not absolute” full stop. That’s not paraphrase;
that’s fabrication.
The principle applies only locally and only to uniform fields —
you ignore both restrictions Real gravitational fields are never
perfectly uniform except as an infinitesimal approximation. Tidal
forces (Riemann curvature) immediately distinguish gravity from pure
acceleration the moment your lab is bigger than “local.” You present
the principle as a global truth about all acceleration and all
gravity. It isn’t. Claiming “it is not possible to distinguish…
without external reference” without screaming LOCAL is intellectually
dishonest at the textbook level.
You conflate “free fall ? felt acceleration” with “all
acceleration is relative” Free fall in a gravitational field feels
like zero proper acceleration — that’s the point. But an observer
hovering stationary in that field (or accelerating in flat space)
feels 1 g proper acceleration. The equivalence hides gravity in free
fall; it does not make the hovering/accelerating observer’s proper
acceleration relative or undetectable. Your leap from equivalence to
“acceleration is not absolute” is a non-sequitur the width of the
observable universe.
[Unsaid / Assumed Garbage]
You assume “acceleration” is a single concept without the
proper/coordinate split — magical thinking that collapses the moment
anyone asks “measured by what?”
You assume the principle applies globally and to non-uniform
fields — fairy-tale physics that dies the instant two particles
separate by a meter in a real gravitational field.
You assume Einstein’s 1907–1911 heuristic statements override the
1915 final theory where proper acceleration is encoded invariantly via
the worldline’s absolute scalar (the magnitude of the
four-acceleration vector).
You assume no one will notice you dropped the word “uniform” and
the qualifier “locally.”
[Incentive & Human-Behavior Landmines]
Physicists who actually understand GR will dismiss you as someone who
read a pop-science article once and decided to lecture. Students will
repeat your version and fail exams. Anyone building accelerometers,
inertial navigation systems, or gravitational-wave detectors will
laugh at the idea that acceleration isn’t “absolute” in the proper
sense — their entire technology stack relies on the opposite being
true.
[Scale, Physics & Reality Check]
At laboratory scales larger than ~10 cm (for Earth gravity), tidal
gradients become measurable and shred the equivalence. At relativistic
scales, proper acceleration appears directly in the invariant interval
[continued in next message]
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