Can Earth be considered an inertial reference frame?

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Discussion Overview

The discussion revolves around whether Earth can be considered an inertial reference frame in the context of special relativity. Participants explore the implications of Earth's motion, including its orbit around the Sun and its rotation, and how these factors relate to the definitions of inertial frames in both Newtonian mechanics and general relativity.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants suggest that while Earth is not a strict inertial frame due to its orbital motion, it can be treated as an approximate inertial frame for local experiments.
  • Others argue that local inertial frames exist in free-fall, and a frame at rest on the surface of the Earth is accelerating upwards, thus not inertial.
  • A participant notes that Newtonian mechanics and general relativity have different definitions of inertial frames, complicating the classification of Earth's frame.
  • Several contributions reference historical attempts, such as the Michelson-Morley experiment, to measure effects related to Earth's motion through the ether, highlighting the complexities involved in defining inertial frames.
  • It is mentioned that in relativity, there are no global inertial frames in the presence of gravity, only local ones that can be defined under certain conditions.
  • Some participants emphasize that the Earth can only be considered an inertial frame for specific local conditions, consistent with classical mechanics.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether Earth can be classified as an inertial reference frame. Multiple competing views are presented regarding the definitions and applicability of inertial frames in different contexts.

Contextual Notes

Limitations include the dependence on the definitions of inertial frames from both Newtonian mechanics and general relativity, as well as the unresolved nature of how to classify Earth's motion in relation to these definitions.

cooper607
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hi guys, i have a basic question on special relativity.. if the inertial reference frame denotes the frames that holds up the Newtonian 1st law, then can Earth be an inertial frame?
i mean it changes direction of velocity as it moves in the orbit around sun, so its not in constant velocity ??
please clear my concept..
thanks
 
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You can imagine an inertial reference frame on Earth with Earth's velocity being tangent to it's curved path.
but this frame would only be applicable at one point in spacetime.

Don't take my word for it though, I've only just recently started looking into physics :).
 
If you fix your reference frame to Earth then strictly it isn't an inertial one but you can take it to be an approximate one.
 
cooper607 said:
hi guys, i have a basic question on special relativity.. if the inertial reference frame denotes the frames that holds up the Newtonian 1st law, then can Earth be an inertial frame?
i mean it changes direction of velocity as it moves in the orbit around sun, so its not in constant velocity ??
please clear my concept..
thanks
And for those of us riding on the surface of the Earth not near one of its poles, our direction and magnitude of velocity go through a cycle of change daily. In fact it was this characteristic that Michelson and Morley were hoping to capitalize on to measure a daily fluctuation in the ether wind, but since they couldn't, they concluded that the Earth must be dragging the ether along with it. Of course, other explanations prevailed.
 
cooper607 said:
i mean it changes direction of velocity as it moves in the orbit around sun, so its not in constant velocity ??

Newtonian mechanics and GR have different definitions of an inertial frame. This frame is noninertial according to the Newtonian definition, but inertial according to the relativistic one.

http://www.lightandmatter.com/html_books/genrel/ch01/ch01.html#Section1.5

You asked about the frame of the orbiting earth, i.e., a frame tied to the Earth's center of mass. A frame tied to Los Angeles is noninertial according to both definitions, because of the Earth's rotation about its own axis.
 
Last edited by a moderator:
thanks to all of you...
 
cooper607 said:
hi guys, i have a basic question on special relativity.. if the inertial reference frame denotes the frames that holds up the Newtonian 1st law, then can Earth be an inertial frame?
i mean it changes direction of velocity as it moves in the orbit around sun, so its not in constant velocity ??
please clear my concept..
thanks
In relativity in the presence of gravity there are no global inertial frames, only local ones. In other words, you can make a reference frame where Newton's laws hold to within any arbitrary experimental precision by making your frame sufficiently small in both space and time that tidal effects cannot be measured.

One other thing to note is that local inertial frames in relativity are in free-fall. A frame at rest on the surface of the Earth is accelerating upwards.
 
cooper607 said:
hi guys, i have a basic question on special relativity.. if the inertial reference frame denotes the frames that holds up the Newtonian 1st law, then can Earth be an inertial frame?
i mean it changes direction of velocity as it moves in the orbit around sun, so its not in constant velocity ??
please clear my concept..
thanks
As others already mentioned, the Earth can only be approximately an inertial frame for certain "local" experiments. That is also the case in classical (Newtonian) mechanics, so it's nothing "new".

Very clearly, concerning special relativity:
"this theory asserts only the equivalence of all Galilean (unaccelerated) coordinate systems".
- https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity

See also:
- SR uses the reference systems of Newtonian mechanics. http://www.fourmilab.ch/etexts/einstein/specrel/www/
- Definition of inertial frames for classical mechanics and SR in: Fundamental University Physics (Mechanics), Alonso&Finn
(question: which textbooks do you use?)
 
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ghwellsjr said:
And for those of us riding on the surface of the Earth not near one of its poles, our direction and magnitude of velocity go through a cycle of change daily. In fact it was this characteristic that Michelson and Morley were hoping to capitalize on to measure a daily fluctuation in the ether wind, but since they couldn't, they concluded that the Earth must be dragging the ether along with it. Of course, other explanations prevailed.
To be precise, they were hoping to at least capitalize on the seasonal velocity differences of the Earth (from orbital motion) - which is exactly what the OP mentioned. See:
https://en.wikisource.org/wiki/The_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether
https://en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether

Note that I agree with your description of inertial reference frames in the other thread:
https://www.physicsforums.com/showpost.php?p=4228938&postcount=62
 
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