# Where does the extra mass come from if an object approches speed of light? [duplicate]

According to scientists, if an object approaches the speed of light, a stage is reached where the object's speed is responsible for the increase in mass of the object. Beyond that stage, all the acceleration will increase the mass of the object. So where does this mass come from and how this mass created?

Uranium is dug up from the ground, some of its mass is converted to energy in a powerplant, then the energy is transferred all around along powerlines.

It happens to be so that heating an object increases its mass. So when mass of frying pans and boilers and other thing increases, mass of some radioactive matter decreases. Also the mass of a battery increases when it's charged.

Now is it correct to say that the extra mass that appears in a battery that is being charged comes from the fuel in a powerplat? Well I don't see anything wrong in that.

Finally to get to the subject of acceleration, I note that heating an object is about the same as increasing the speed of the object's molecules.

• Please don't post wild uninformed guesses as answers. – WillO Feb 24 '17 at 20:58
• @peterh: Are you unaware that the phenomenon the OP is asking about follows from the postulates of special relativity, which do not require the existence of molecules? – WillO Feb 24 '17 at 23:29
• @WillO I was. Sorry :-) – peterh Feb 24 '17 at 23:37
• @WillO ...well, if a particle is accelerated in an accelerator driven by a nuclear plant, he may be even right. At least, the masses of the particles will be increased by the same mass as with the mass of the fuel decreased. – peterh Feb 24 '17 at 23:38
• @peterh Moving bodies do not have extra mass. Hot bodies have extra mass. According to modern physicists it's like that. So when one radioactive atom decays in a pool of water there is the mass defect between the atom and its daughter particles, and then there is a mass increase of the water. We can't say that fast moving particles in a particle accelerator have some extra mass, because physicists would disagree! – stuffu Feb 25 '17 at 9:38