Who is in Motion: Two People, Constant Rate

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Discussion Overview

The discussion revolves around the concept of motion in a relativistic context, specifically whether it is possible to determine which of two observers is in motion towards the other when they are moving at a constant rate towards each other. The scope includes theoretical implications of relativity and the nature of motion as perceived by different observers.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant suggests that the question of determining which person is in motion is meaningless in the context of relativity, as all motion is relative to an inertial reference frame.
  • Another participant reiterates the idea that observers are in relative motion, emphasizing that the laws of physics are consistent across different inertial frames.
  • Several analogies are presented to illustrate the relativity of motion, with one participant comparing the situation to the relative sizes of two individuals, questioning the equivalence of perspectives in motion.
  • A participant argues that while analogies can help illustrate concepts, they cannot capture all aspects of the main system, particularly regarding the relevance of velocities in determining motion.

Areas of Agreement / Disagreement

Participants express differing views on the nature of motion and the applicability of analogies, indicating that multiple competing perspectives remain without consensus on the question posed.

Contextual Notes

The discussion highlights the dependence on reference frames in relativity and the challenges of using analogies to convey complex concepts of motion.

grounded
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Imagine space contains nothing but two people, and the distance between them is reducing at some constant rate.

Do we have the ability to tell which person is in motion towards the other, or if they are both in motion at half speed?
 
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The question is meaningless in relativity, all motion is defined relative to your choice of inertial reference frame, and the laws of physics work the same way in all inertial frames so there's no physical basis for preferring any of them.
 
grounded said:
Do we have the ability to tell which person is in motion towards the other, or if they are both in motion at half speed?
To add to the previous response: We therefore say things like "the observers are moving relative to each other at such-and-such speed" or that "they are in relative motion".
 
This is like asking-> james looks bigger paul. paul looks smaller than james.
But in reality is it that James is bigger than paul or paul is smaller than james?
 
thecritic said:
This is like asking-> james looks bigger paul. paul looks smaller than james.
But in reality is it that James is bigger than paul or paul is smaller than james?
It's not really like that, since James bigger than Paul and Paul smaller than James are equivalent, whereas relativity says that James can have a greater velocity than Paul in one frame, but Paul can have a greater velocity than James in another frame.
 
JesseM, no Analogy can be equivalent in all respect to the main system.
I was just analoging motion. I was just trying to show -- who was in real motion was as irrelevant as who was actually smaller or bigger.
My analogy doesn't (and i didn't claim it does) take velocities into account.
 

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