Object's mass does not increase as it approaches the speed of light?

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Discussion Overview

The discussion revolves around the concept of mass in the context of special and general relativity, particularly whether an object's mass increases as it approaches the speed of light. Participants explore the definitions of invariant mass and relativistic mass, and how these concepts relate to energy and gravity.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that mass is invariant, suggesting that an object's mass does not increase as it approaches the speed of light.
  • Others argue that while rest mass is invariant, the mass of an object appears larger when it is moving relative to an observer, indicating a distinction between rest mass and relativistic mass.
  • A participant notes that in modern physics, the term "relativistic mass" has fallen out of favor, with an emphasis on energy increase instead.
  • There is a question raised about the advantages of distinguishing between rest mass and relativistic mass, with one participant labeling the latter as old-fashioned.
  • Another participant inquires whether gravity should be considered a consequence of total energy or rest mass, linking this to concepts of time dilation and length contraction.
  • In response, a participant mentions that in general relativity, the space-time tensor is related to the energy-momentum tensor, indicating a more complex relationship.
  • Further clarification is provided that in general relativity, gravity is sourced not just from mass or energy, but from the entire stress-energy tensor, which includes various factors such as momentum and pressure.

Areas of Agreement / Disagreement

Participants express differing views on the relevance and validity of the concept of relativistic mass, with some considering it outdated while others see value in the distinction. The discussion regarding the source of gravity remains unresolved, with multiple perspectives presented.

Contextual Notes

There are unresolved assumptions regarding the definitions of mass and energy in different contexts, as well as the implications of these definitions for understanding gravity in general relativity.

Gear300
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I read in a book that mass is invariant. Does this mean that an object's mass does not increase as it approaches the speed of light?
 
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Gear300 said:
I read in a book that mass is invariant. Does this mean that an object's mass does not increase as it approaches the speed of light?

If an object starts moving relative to you then its mass would be larger compared to when it was at rest relative to you. What is invariant is rest mass i,e the mass of any object when it is at rest relative to you.
 


Gear300 said:
I read in a book that mass is invariant. Does this mean that an object's mass does not increase as it approaches the speed of light?

In most modern treatments, that's the case. What increases is the object's energy. In the past this was called "relativistic mass", but since we already have a perfectly good word to describe it, this terminology has fallen out of favor.
 


Is there any advantage of considering mass as not being a scalar, but making the distinction between rest massa and relativistic mass? Or is the notion of "relativistic mass" just old-fashioned?
 


Just old-fashioned.
 


So then when we consider gravity, do we consider it as a consequence of the total energy or the rest mass (I had viewed time dilation and length contraction as a result of the curvature of space-time due to the heavier mass of an object approaching light-speed)?
 


In GR the space-time (Riemann) tensor is related to the energy-momentum tensor.
 


To second what Meir said, in GR the source of gravity is not just mass or energy, but the entire stress-energy tensor. There are terms including energy, momentum, pressure, stress, etc. So it is not correct to either say that the invariant mass or the relativistic mass is the source of gravity.
 

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