Inertial Frame of Reference vs Spacetime

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Discussion Overview

The discussion revolves around the relationship between inertial frames of reference and spacetime, exploring whether they are equivalent concepts. Participants examine the definitions and implications of these terms within the context of physics, touching on theoretical and conceptual aspects.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that inertial frames of reference are not the same as spacetime, using analogies such as latitude and longitude lines on the Earth's surface to illustrate their point.
  • Others propose that reference frames are simply coordinate systems, with inertial reference frames being a specific class that simplifies calculations in spacetime for non-accelerating observers.
  • A participant questions if inertia is equivalent to spacetime when the term "reference frame" is omitted, to which others clarify that inertia refers to an object's tendency to maintain its motion in the absence of external forces.
  • There is a discussion about the bending of starlight around galaxies, with some questioning if this phenomenon could be considered a change in inertia, while others argue it does not qualify as such.

Areas of Agreement / Disagreement

Participants generally disagree on the equivalence of inertial frames of reference and spacetime, with multiple competing views presented regarding their definitions and implications. The discussion remains unresolved on several points, particularly regarding the nature of inertia and its relationship to spacetime.

Contextual Notes

Some claims rely on philosophical interpretations of spacetime, and there are unresolved aspects regarding the definitions and implications of inertia and reference frames. The discussion also touches on the complexity of quantum gravity, which is noted as beyond the current understanding of some participants.

Pjpic
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Is the inertial frame of reference the same thing as space time?
 
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No. Reference frames are to spacetime as latitude and longitude line are to the surface of the earth: The surface of the Earth is there and we can move across it as we please; the latitude and longitude lines are just conventions for attaching numeric labels to points on that surface.
 
Nugatory said:
No. Reference frames are to spacetime as latitude and longitude line are to the surface of the earth: The surface of the Earth is there and we can move across it as we please; the latitude and longitude lines are just conventions for attaching numeric labels to points on that surface.
If the term "reference frame" is dropped, is inertia the same as spacetime?
 
Pjpic said:
If the term "reference frame" is dropped, is inertia the same as spacetime?

No. Inertia is something completely different - it refers to an object's tendency to keep moving in a straight line if you apply no external forces to it.
 
Take a simple Euclidean plane. You can draw many coordinate systems on it - for example a Cartesian system with the origin at some point, a Cartesian system with the origin at some other point and rotated with respect to the first, or circular polars with the origin at yet a third point, and many more.

A reference frame is simply a coordinate system. Any one of the ones I listed above is a valid reference frame for a Euclidean plane (and there are many more that I did not list). An inertial reference frame is a particular class of coordinate systems for space time, as all the possible Cartesian systems on the plane are a class of coordinate systems. Inertial reference frames happen to be a natural way to label events in spacetime for observers who are not accelerating, and they happen to make maths with the coordinates relatively easy as well. That's all they are.

To put it another way, imagine a piece of paper. You can turn it into a piece of graph paper by printing a grid of some kind on it. Choosing a reference frame is like choosing which grid to print. Choosing an inertial reference frame is like choosing which Cartesian grid to print. Actually building a network of rods with clocks at each junction (as Einstein spoke about) is like printing the grid on the paper (you don't have to do this to use the coordinates). The paper is spacetime.

What spacetime "actually is" is a matter of philosophy at this point (it's probably not actually a piece of paper). I understand that a proper theory of quantum gravity might shed some light, but that's beyond my level of understanding.
 
Nugatory said:
No. Inertia is something completely different - it refers to an object's tendency to keep moving in a straight line if you apply no external forces to it.

When star light bends around galaxy would it not be considered a change in inertia because the light is following the natural curve of space?
 
Pjpic said:
When star light bends around galaxy would it not be considered a change in inertia because the light is following the natural curve of space?
No it would not be considered a change in inertia. A change in inertia would be if it were NOT following the natural "curve" of space time.
 
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