# Does Relativistic Mass Change at the Speed of Light?

• jishitha
In summary, relativistic mass is an SR concept that is not particularly useful in GR, and pretty much of a dead end.
jishitha
Hi.. What will be the nature of relativistic mass when a body moves with speed of light..?

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...

H2What is relative mass and velocity?

Relative mass and velocity are two concepts used to describe the mass and velocity of an object relative to another object. This means that the measurements of an object's mass and velocity will vary depending on the frame of reference used.

H2How is relative mass calculated?

Relative mass can be calculated by dividing the mass of an object by the mass of the reference object. This calculation takes into account the ratio of the two objects' masses and allows for an accurate comparison between them.

H2What is the relationship between relative mass and velocity?

The relationship between relative mass and velocity is that they are both affected by the frame of reference used. As the velocity of an object changes, so does its relative mass. This means that an object can have different relative masses depending on its velocity in relation to the reference object.

H2How does relative mass affect an object's motion?

Relative mass affects an object's motion by changing its inertia. As an object's velocity increases, its relative mass also increases, making it more difficult to change its motion or direction. This is known as the principle of inertia.

H2Why is it important to understand relative mass and velocity in science?

Understanding relative mass and velocity is important in science because it allows for accurate measurements and comparisons between objects. It also helps us understand how objects move and interact with one another in different frames of reference. This is crucial for many scientific fields, such as physics and astronomy.

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