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

#### Gear300

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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#### kista

Re: Mass

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.

Staff Emeritus
Re: Mass

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.

#### haushofer

Re: Mass

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?

#### clem

Re: Mass

Just old-fashioned.

#### Gear300

Re: Mass

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)?

#### Meir Achuz

Homework Helper
Gold Member
Re: Mass

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

#### Dale

Mentor
Re: Mass

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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