Relativistic Spring-Mass Oscillator: A Paradox?

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Discussion Overview

The discussion revolves around the relativistic behavior of a spring-mass oscillator on a train moving at relativistic speeds. Participants explore the implications of special relativity (SR) on the mass, period, and spring constant of the oscillator, questioning the validity of classical mechanics in this context.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that both the mass and the period of the oscillator appear to increase by a factor of γ to a stationary observer, leading to confusion regarding the relationship between mass and period.
  • There is a discussion about the relativistic force transformation rules and how they may affect the spring constant when changing frames.
  • One participant emphasizes that the period is proportional to the square root of the mass and inversely proportional to the square root of the spring constant, questioning how the spring constant transforms under relativistic conditions.
  • Some participants suggest that the formula for the period of a spring is non-relativistic and may not hold true in relativistic contexts.
  • Concerns are raised about the invariance of the spring constant and Hooke's law, with some participants suggesting that these may not be invariant under relativistic transformations.
  • One participant argues that the oscillations can be slow in the moving frame, implying that Hooke's law should apply accurately in that frame, but questions remain about reconciling this with observations from a stationary observer.
  • Another participant points out that the derivation of frequency dependence on mass is based on non-relativistic laws, suggesting that such relationships should not be expected to hold in relativistic scenarios.
  • There is a contention regarding the interpretation of mass increase in relativistic terms, with one participant stating that this is a matter of terminology rather than a direct consequence of the postulates of SR.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the relativistic treatment of the spring-mass oscillator, particularly concerning the invariance of the spring constant and the applicability of Hooke's law. The discussion remains unresolved, with no consensus on how to reconcile the classical mechanics with relativistic effects.

Contextual Notes

Limitations include assumptions about the invariance of the spring constant and the applicability of classical laws in relativistic contexts, which remain under debate. The discussion also highlights the need for a relativistic version of Hooke's law, as the classical formulation is not considered relativistically invariant.

Alfred Cann
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Consider a spring-mass oscillator on a train moving at relativistic speed.
According to SR, to a stationary observer, both the mass and the period will appear to have increased by a factor of γ.
But the period is supposed to be proportional to the square root of the mass. Something is wrong.
Don't talk about longitudinal and transverse masses; I can orient the oscillator any way I want.
Don't talk about the mass oscillating at relativistic speed; I can keep the oscillation slow.
 
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Have you looked at the relativistic force transformation rules?
 
Alfred Cann said:
the period is supposed to be proportional to the square root of the mass.

It's proportional to the square root of the mass, and inversely proportional to the square root of the spring constant. So how does the spring constant transform when you change frames?

Alfred Cann said:
I can orient the oscillator any way I want.

So which way do you want to orient it?
 
Alfred Cann said:
Something is wrong.
Yes, the formula for the period of a spring is non-relativistic.
 
I had assumed the spring constant is invariant; you guys are implying that it changes.
 
Alfred Cann said:
I had assumed the spring constant is invariant; you guys are implying that it changes.
I suspect that not just the spring constant, but also Hookes law is not invariant.
 
Dale said:
Yes, the formula for the period of a spring is non-relativistic.

This shouldn't be an issue. As the OP said, the oscillations in the frame of the moving train can be slow, so Hooke's law should apply in the frame of the train to whatever accuracy you want. The question is how to reconcile this with what the stationary observer sees as the train speeds by. I think the right answer, as others have implied, is that the spring constant as perceived by the stationary observer is different from that perceived by the observer on the train. @Alfred Cann, imagine the train speeding by multiple times (or multiple trains) with different weights hanging from the spring. What would he see?
 
phyzguy said:
This shouldn't be an issue.
How could it not be? The derivation by which the OP expects the frequency to depend on the square root of the mass is based on Hooke’s law and Newton’s laws. If those are non relativistic then the relationship should not be expected to hold relativistically.
 
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Alfred Cann said:
I had assumed the spring constant is invariant

You should not assume. You should try to calculate in order to either demonstrate that it's correct or realize that it's not correct. In order to do that calculation you will first need to find a relativistic version of Hooke's law, since the ordinary version is written in terms of 3-vectors, not 4-vectors, and is therefore not relativistically invariant, as @Dale has pointed out.
 
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Alfred Cann said:
According to SR, to a stationary observer, both the mass and the period will appear to have increased by a factor of γ.

The period will increase, that's a consequence of the postulates. The same cannot be said for the mass. Saying that the mass increases is a choice about the meaning of a word, it's not a consequence of the postulates.
 

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