Changes

'''Relativistic mass''' is a mass, e.g. <math>M_1</math> of a particle (or any object) that any observer is seeing in her/his reference system of coordinates. It is the same as [[invariant mass]] (denoted <math>m</math>) of the particle (or object) multiplied by <math>\gamma</math> factor that tells how much mass (or rather ''enery'') a moving particle has in relation to [[invariant mass]] (called sometimes the [[rest mass]] and then denoted by <math>m_o</math>).  

'''Relativistic mass''' is a mass, e.g. <math>M_1</math> of a particle (or any object) that any observer is seeing in her/his reference system of coordinates. It is the same as [[invariant mass]] (denoted <math>m</math>) of the particle (or object) multiplied by <math>\gamma</math> factor that tells how much mass (or rather ''enery'') a moving particle has in relation to [[invariant mass]] (called sometimes the [[rest mass]] and then denoted by <math>m_o</math>).  

It means that for different observers (moving with different  velocities) the '''relativistic mass''' of the same particle is in general different which would mess the calculations in physics a little bit, and that's why physicists decided to introduce the notion of ''[[invariant mass]]'' that for every observer is the same and simplify calculations in physics. Presently when a physicist says ''"[[mass]]"'' she/he always understands it as [[invariant mass]] though it is not that simple when non physicists get into a discussion since many of them assume that ''"mass is growing with its velocity"'' as it was understood still in Einstein's times. Of course the physics is the same but the terminology changed. Now the ''"mass that is growing with with its velocity"'' is called by physicists "energy" and '''relativistic mass''' or ''"inertial mass"'' by everybody else.

It means that for different observers (moving with different  velocities) the '''relativistic mass''' of the same particle is in general different which would mess the calculations in physics a little bit, and that's why physicists decided to introduce the notion of ''[[invariant mass]]'' that for every observer is the same and simplify calculations in physics. Presently when a physicist says ''"[[mass (science)|mass]]"'' she/he always understands it as [[invariant mass]] though it is not that simple when non physicists get into a discussion since many of them assume that ''"mass is growing with its velocity"'' as it was understood still in Einstein's times. Of course the physics is the same but the terminology changed. Now the ''"mass that is growing with with its velocity"'' is called by physicists "energy" and '''relativistic mass''' or ''"inertial mass"'' by everybody else.

The "[[mass]]" on the right side of Einstein's equation <math>E=mc^2</math> is invariant (or rest mass) for a physicist and generally '''relativistic mass''', or ''"inertial mass"'', for non physicists, which is rather non intuitive arrangement but luckily the Einstein's equation works for both kinds of mass (unless one wants spacially accurate terminology, which then may to involve also [[curvatures of spacetime]]).

The "[[mass (science)|mass]]" on the right side of Einstein's equation <math>E=mc^2</math> is invariant (or rest mass) for a physicist and generally '''relativistic mass''', or ''"inertial mass"'', for non physicists, which is rather non intuitive arrangement but luckily the Einstein's equation works for both kinds of mass (unless one wants spacially accurate terminology, which then may to involve also [[curvatures of spacetime]]).

Now we have a problem which ''"[[invariant mass]]"'' or '''relativistic mass''' is responsible for gravitation? It turns out it is '''relativistic mass''' and therefore it is the energy content of mass. And that's why for light the gravitational effects are the same as for other "masses". Photons (articles of light) don't have ''"[[invariant mass]]"'' (their [[invariant mass]] is equal zero, and that's why they are called "massless" particles) yet they have '''relativistic mass''' (energy) and therefore they may take part in gravitational phenomena (as they do). Energy of each photon is <math>E_\nu=h \nu</math>, where <math>h</math> is Planck constant, and <math>\nu</math> if frequency of light carried by this photon.

Now we have a problem which ''"[[invariant mass]]"'' or '''relativistic mass''' is responsible for gravitation? It turns out it is '''relativistic mass''' and therefore it is the energy content of mass. And that's why for light the gravitational effects are the same as for other "masses". Photons (articles of light) don't have ''"[[invariant mass]]"'' (their [[invariant mass]] is equal zero, and that's why they are called "massless" particles) yet they have '''relativistic mass''' (energy) and therefore they may take part in gravitational phenomena (as they do). Energy of each photon is <math>E_\nu=h \nu</math>, where <math>h</math> is Planck constant, and <math>\nu</math> if frequency of light carried by this photon.