Does Relativistic Mass Change at the Speed of Light?

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Discussion Overview

The discussion revolves around the nature of relativistic mass as a body approaches the speed of light, exploring theoretical implications and definitions within special and general relativity. It touches on concepts of mass, energy, and gravitational effects related to light and massive bodies.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions the nature of relativistic mass at the speed of light.
  • Another participant asserts that a massive body cannot reach the speed of light, and that mass-less bodies complicate the definition of relativistic mass.
  • A viewpoint suggests that relativistic mass is synonymous with energy, but cautions against using it to describe gravitational effects, indicating that it is not a useful concept in general relativity.
  • It is proposed that modern treatments of special relativity often avoid the term "relativistic mass," favoring other definitions of mass in general relativity, which include multiple competing definitions like Komar, ADM, and Bondi masses.

Areas of Agreement / Disagreement

Participants express differing views on the utility and definition of relativistic mass, with no consensus reached on its relevance or application in gravitational contexts.

Contextual Notes

The discussion highlights limitations in the definitions of mass and the applicability of relativistic mass in various contexts, particularly in relation to gravitational effects and modern theoretical frameworks.

jishitha
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Hi.. What will be the nature of relativistic mass when a body moves with speed of light..?
 
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You're in luck. There's a website devoted to your question:

http://www.relativisticmass.com
 
Last edited by a moderator:
A massive body cannot move at the speed of light, and a mass-less body does not allow a good definition of relativistic mass.
 
Relativisitc mass is just another name for energy, so a light wave has some. However its wrong to ascribe gravity as being a force due to relativistic mass (though if you're lucky you'll be within a factor of 2 of the correct answer. For instance, you'll get only half the expected deflection of light by using a quasi-Newtonian approach.)

"Relativistic mass" is an SR concept that is not particularly useful in GR, and pretty much of a dead end. The good news is it can be defined, the bad news is it's not particularly good for anything, including computing gravity, and the things that it can do can be done just as well if not better by other means.

So if you're asking about the relativistic mass of light in order to compute the gravitational effect of light, you're asking the wrong question.

Many modern SR treatments of SR don't even bother with relativistic mass anymore, though you'll find the occasionall odd enthusiast of the topic, and an occasional textbook reference (mostly in older textbooks).

"Relativistic mass" seems very appealing to the mass media and lay audience more than it's use in the professional literature for some reason.

Good relativistic treatments of mass by General Relativity are a fairly advanced subject in general relativity. In the current state of the art, there is not one, but several competing definitions of mass, each of which can be applied to their own class of problem.

The Komar mass, which applies to static space times, is one of the simplest. A few other examples are ADM and Bondi masses, and there are others, though these are probalby the 3 most common.
 
Thaaanks...
 

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