# Can we convert energy to mass?

Dale
Mentor

What kind of mass would we say a photon has. Aren't there like 3 different types of masses.
A photon has "relativistic mass", but not "invariant mass". Most physicists don't use relativistic mass any more, but it is still found in a lot of textbooks and many historical papers, so it is good to know what it is.

ok , I was just reading Dalespams post and he said the mass would be unchanged but we could transform energy into matter.
I was actually refering to the invariant mass of the system. This can be much higher than the rest masses of the individual particles that make up the system, actually that is the whole purpose of a particle collider. As an example, a 1 MeV photon moving to the right it has 0 invariant mass, and a 1 MeV photon moving to the left also has 0 invariant mass, but a system of a 1 MeV photon moving to the right and a 1 MeV photon moving to the left has an invariant mass of 2 MeV/c². The invariant mass is given by m²c² = E²/c² - p². For a system with no overall momentum this reduces to the familiar E = mc².

Yes, exactly.

And heat has mass. Energy has mass, it does not need to be "transformed into" mass.
hey hey....heat has mass??

if energy has mass ...can we measure it practically without imparting to any mass??

hey hey....heat has mass??
It was one of Einsteins favourite illustrations to state that an object when hot is more massive than the same object when cold.

Drakkith
Staff Emeritus

if energy has mass ...can we measure it practically without imparting to any mass??
No, any measurement requires an interaction of forces, which will alter the energy of the systems involved in some way. But you can calculate it.

It was one of Einsteins favourite illustrations to state that an object when hot is more massive than the same object when cold.
but hot water is lighter than cold water....!!!

but hot water is lighter than cold water....!!!
Are you confusing mass with density?

Mass and energy are just metrics applied to systems; unlike Transformers they are not actual objects. E=mc^2 says that the energy in a system is equal to the mass of a system times the speed of light in a vacuum. This implies that adding energy to a system increases its mass; its 'rest mass' is the mass it has when it has no energy. Sometimes rest mass is created by particle accelerators or positron emission; because of the law of conversation of mass, it is assumed that the rest mass created is equal to the mass of the energy that is consumed by its creation.

The nuclear reactions inside stars cause them to emit cosmic rays including protons. Those protons (hydrogen nuclei) can be used as fuel for nuclear fusion. The fused nuclei (or just the protons emitted by stars) can drift into a forming star, becoming fuel for that star's nuclear reactions.

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Drakkith
Staff Emeritus