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From: Hendrik van Hees <hees@itp.uni-frankfurt.de>
Newsgroups: sci.physics.research
Subject: Re: Equivalence principle
Date: 10 Jun 2024 12:10:37 GMT
Organization: Goethe University Frankfurt (ITP)
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Or to put it simpler. In a local inertial reference frame, realized by a 
point-like non-rotating body in free fall, you observe (e.g., by using 
pointlike test particles) only the "true gravitational forces", i.e., 
the tidal forces.

If you sit on the surface of a planet, you are not in free fall, because 
there are (electromagnetic) forces keeping you there.

That's why the accelerometer of your smart phone at rest on Earth shows 
an acceleration of 9.81 m/s^2, because it measures accelerations 
relative to a local inertial frame of reference! See, e.g.,

https://physlab.org/wp-content/uploads/2016/03/primer_smartphones.pdf

On 10/06/2024 13:46, Mikko wrote:
> On 2024-06-08 17:40:15 +0000, Luigi Fortunati said:
> 
>> In the video https://www.youtube.com/watch?v=R3LjJeeae68 at minute 6:56
>> it states that there is no measurement that can be made to distinguish
>> whether you’re being accelerated or whether you are sitting still on the
>> surface of a planet.
>>
>> So, I ask: what stops us from measuring the presence (or absence) of
>> tidal forces? If tidal forces are there, then we are stationary on the
>> surface of a planet, if they are not there, we are experiencing a
>> non-gravitational acceleration.
> 
> Consider a situation where you are not sitting on a surface of a planet
> but acclerated by a real non-gravitational interaction; and this happens
> near a planet or a star: you can measure a tidal force (if your instruments
> are big and sensitive enough).
> 

-- 
Hendrik van Hees
Goethe University (Institute for Theoretical Physics)
D-60438 Frankfurt am Main
http://itp.uni-frankfurt.de/~hees/