Different values for the inertia of a moving object?

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Discussion Overview

The discussion revolves around the concept of inertia in the context of relativity, specifically addressing the claim that relativity necessitates different values for the inertia of a moving object based on its direction of motion. Participants explore the implications of this claim on the principles of physics and the nature of physical laws across different frames of reference.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that relativity maintains isotropy in space when transforming to a frame where an object is at rest, challenging the idea that different inertial values contradict physical laws.
  • Others argue that the notion of transverse and longitudinal relativistic mass stems from the use of classical mechanics (F=ma) and suggest that using 4-vectors simplifies the understanding of force and momentum without needing to differentiate between mass in different directions.
  • A participant questions the origin of the initial claim about inertia, linking it to a specific source that critiques relativity.
  • There is acknowledgment that the concept of transverse and longitudinal relativistic mass is not entirely dismissed, as it appears in older literature, but participants suggest that simpler definitions should be preferred to avoid unnecessary complexity.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the claim regarding inertia in relativity, with some supporting the idea that it contradicts isotropy while others defend the consistency of physical laws across directions. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants highlight the dependence on definitions and the evolution of concepts in physics, indicating that certain definitions may lead to complications that could be avoided with alternative approaches.

csmcmillion
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I would appreciate terse responses to the following statement:

"Relativity requires different values for the inertia of a moving object: in its direction of motion, and perpendicular to that direction. This contradicts the logical principle that the laws of physics are the same in all directions."

Thanks.
 
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Relativity says that when you transform into a frame in which you are standing still, space has the same properties in all directions. Which is true, and necessary for isotropy.

A lot of people assume that if you have all the laws of physics, the Newtonian laws apply without any modification. This is incorrect and a source of confusion.

Another way of saying this is that the statement is not a statement about the laws of physics, but about human choices, coordinate choices. If it were about the laws of physics, there would be some sort of experiment that tells you which frame is moving, and which isn't. But the point of relativity is that there isn't.
 
To add to Pervect, the idea of transverse and longitudinal relativistic mass derives from the choice to use F=ma, where 'a' is spatial acceleration. To my mind, this is as silly as trying to come up with compensations to mechanics such that you can use normal velocity addition in SR rather than relativistic velocity addition.

If, instead, you use 4-vectors, you have:

p = m U (U being 4-velocity)

and

F = dp/d tau (F now being 4 force)

with no need for any transverse and longitudinal relativistic mass.
 
Last edited:
csmcmillion said:
"Relativity requires different values for the inertia of a moving object: in its direction of motion, and perpendicular to that direction. This contradicts the logical principle that the laws of physics are the same in all directions."

Are you quoting this from Conservapedia's "Counterexamples to Relativity" page?
 
jtbell said:
Are you quoting this from Conservapedia's "Counterexamples to Relativity" page?

Yes - that's where I found this. Supposedly, CP's founder has a BSEE from Princeton... (scratching head).
 
csmcmillion said:
Yes - that's where I found this. Supposedly, CP's founder has a BSEE from Princeton... (scratching head).

Well the idea of transverse and longitudinal relativistic mass is not clearly wrong. In fact, it is fairly common in old books. I would describe the evolution away from it as: if some definitions force such complexity, and other definitions that are at least as simple do not, let's avoid the definitions with adverse side effects.
 

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