Caption: Shown is the only physics formula everyone knows: E=mc**2, where E is energy, m is mass and the vacuum light speed c = 2.99792458*10**8 m/s (exact by definition) ≅ 3*10**8 m/s =3*10**5 km/s ≅ 1 ft/ns.
E=mc**2 is, of course, a basic result of special relativity (1905) and is obtained in the physicsy derivation of special relativity from the special relativity postulates. A physicsy derivation is one where you start with basic axioms, but introduce extra ones as you go along as seems reasonable to physical intuition (i.e., educated guessing) or by clairvoyance (i.e., you believe you know where you have to arrive). Pure mathematics it isn't.
In fact, the derivation of E=mc**2 is remarkably simple though well beyond the scope of this figure.
But what does E=mc**2 mean?
Actually, E=mc**2 means two things:
Such changes in mass are NOT noticed in everyday life NOR were they observed experimentally prior to the advent of special relativity in 1905 (when people started looking for them) because ΔE/c**2 is usually so small compared to a system's rest mass (see the Meaning 2 of E=mc**2 below). Nowadays, they have been super well verified at least by indirect means.
In modern formulation, the laws of conservation of mass and conservation of energy are fundamentally the same thing and are considered as separate laws only as emergent laws in cases where changes in rest mass are below notice (see the Meaning 2 of E=mc**2 below and Wikipedia: Relativistic mechanics: Rest mass and relativistic mass). However, it many applications scientific and technological, those changes are below notice.
Actually, yours truly believes that a measurement of mass of an object is the only direct measurement of energy and it is only a measurement of total energy. All other energy measurements are indirect measurements since you measure some other quantities and calculate energy from a formula. For example, kinetic energy KE=(1/2)mv**2 (the energy of motion in the classical limit) is measured by measuring m and v and using the just given formula.
Since kinetic energy changes with reference frame, you may wonder is gravity a reference-frame dependent quantity. In the classical limit, NO because in the classical limit we simply do NOT consider changes in mass given by the semi classical physics formula KE/c**2: they are deemed negligible.
When NOT in the classical limit, general relativity (1915) tells you how to calculate gravity effects in general and yours truly thinks the issue of gravity as reference-frame dependent quantity becomes a bit unmeaningful.????
There is such a thing as rest mass energy (usually just called just rest mass for simplicity) since massive particles have intrinsic mass NOT due to any other energy form.
This is the energy calculated from E=mc**2 for an object of mass m when the object is at rest in an inertial frame: hence the name rest mass.
Since any energy form can be converted into other all other energy forms, rest mass can be converted into all other energy forms. But note conversions are NOT always easy to do in practice.
Some further points:
Exotic dark matter particles (if they actually exist) also have rest mass and are massive particles, but they are NOT baryonic matter.
To emphasize, massive particles can be observed when at rest.
Note that objects including composite particles (e.g., atoms and molecules) that are at rest on some observed scale have contributions to their mass from many kinds of microscopic scale energy. Are those contributions part of the rest mass of the objects? Yes or no depending on what you mean with the meaning usually determined by context.
Note that massless particles always move at vacuum light speed c = 2.99792458*10**8 m/s (exact by definition) ≅ 3*10**8 m/s =3*10**5 km/s ≅ 1 ft/ns relative to any local inertial frame, and so are NEVER at rest in a local inertial frame. But could they still have rest mass in some sense? NO. If they had rest mass, then the formula
But E=mc**2 means that massless particles do have mass since they have energy (finite energy). To be super precise, rest-massless particles should be called rest-massless particles, but that's NOT the convention.
But it can be done in principle. For example, just rapidly collide 0.5 kg of matter with 0.5 kg of antimatter and both will annihilate to form gamma rays (and maybe other stuff ???) which will give explosive heating with the above calculated amount of 25 megatons TNT. But there is NO practical way to accumulate any macroscopic amounts of antimatter.
Pair creation is actually inverse annihilation since you create massive particles, NOT destroy them.
At the microscopic level, the mutual annihilation of matter and antimatter goes on all the time at a very low level including wherever you are. For example, positrons (the antiparticles of electrons) are products of certain radioactive decay processes (specifically some beta decay processes) that occur at a low level just about everywhere. But the rate of positron production is very low usually. The produced positrons run into electrons pretty quickly and mutually annihilate to create gamma rays. The rate of heating from this natural and common process is relatively low.
Ever since the advent of special relativity in 1905, people have been mesmerized by how much energy is available in principle in rest mass. But the practical ways of getting a significant fraction of it from matter are limited to nuclear reactors and nuclear bombs (see the figure below: local link / general link: explosion_1954_bikini.html).
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