# Tag Info

## New answers tagged mass-energy

1

One shoul think as c as a kind of space-time conversion constat, massless energy travel at this speed. Light and gravity are kinds of massless energy. The idea of E=mc^2 is that mass converts to energy like this.

2

The speed of light in a vacuum is invariant: it is the same no matter what point you pick as "stationary". So if I'm on a train, and you're on the ground, and we both measure $c$, we'll get exactly the same number. The speed of light does not depend on the wavelength. Gamma rays travel at the same speed $c$ as radio waves. The frequency $f$ and wavelength ...

6

The speed of light is there for much more than to look cool, and in fact there are a number of derivations of mass-energy equivalence that shows why $c$ is present; I will say that one basic reason is that the units of mass and energy are different, so we require at least some sort of constant factor to make the units work. I'll also say that we often use ...

0

Energy conservation may be better stated as, The total energy, i.e. energy in the form of mass + all other forms of energy is conserved for an isolated system. This would mean that annihilation is simply an example of interconversion of energy from 'mass-energy' to 'light(electromagnetic) energy'.

0

Photons aren't pure energy - they are a particle like all other particles. Admittedly photons are massless but then so are gluons, and indeed above the electroweak phase transition temperature so are all particles. So pair production from photons and annihilation into photons is just a scattering process like any other particle interaction. However if is ...

0

Conservation of energy refers to systems looked from the same reference frame, it does not make sense to require that energy of the same system to be the same in different reference frames. As a consequence of time translational symmetry, energy conservation is usually true unless we drive the system externally which may break this symmetry. Similarly, ...

8

The annihilation produces gamma photons, whose total energy sums up to the total energy $E_0=\sqrt{p^2 \,c^2 + m^2\,c^4}$ formerly contained in the matter / antimatter kinetic energy (the $p\,c$ term) and that "frozen" in rest mass (the $m\,c^2$ term). So energy is conserved. As for gravity, the Einstein field equations "can't tell the difference" between ...

5

According to general relativity the source for spacetime curvature/gravity is the stress-energy tensor. Mass contributes rest energy $mc^2$ but it is not the only form of energy that influences gravity. Therefore it's not exactly true to say that mass causes gravity, but energy does. The photons produced during annihilation will carry energy and so they will ...

6

The idea that gravity is sourced just by mass only hold in the nonrelativistic limit. In a fully relativistic treatment, which is necessary to understand any process involving particle annihilation, you must use the full stress energy tensor. This tensor includes mass, but it also includes, for instance, the energy of any photons created in the annihilation. ...

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