9 Answers

If you are familiar with physics, then you know that the definition of energy is equivalent to work, and work can be defined as a force applied over a distance. Work is a force x the distance over which it is applied. But Newton described a force as a mass x the acceleration that results from the force application. So, if you substitute Newton’s Second Law into the definition of work, you get that work = energy = mass x acceleration x distance. Using metric units, this can be written as:
E = kg x (m/sec^2) x m or E = kg x m^2/sec^2.
Note that m^2/sec^2 is just a velocity (m/sec) squared. This means that the units of energy can be expressed as a mass x a velocity squared. In the case of kinetic energy, we use a proportionality factor of 1/2 in order to make the standard units come out right.
In the case of nuclear physics, the proportionality constant is c^2. It is, of course, a constant, since c is a constant within all inertial reference frames. So, this is a statement of the equivalence of mass and energy. They are really two ways of looking at the same thing – mass/energy – that is inherent in all matter at rest within the reference frame that you are using. C^2 is just a dimensional conversion to change the units from what we call mass to what we call energy.
This is the result of applying the Lorentz Transformation within Special Relativity to momentum. It is a description of the often misunderstood notion that mass increases as you approach the speed of light. It does not. What increases is relativistic mass; rest mass remains constant as defined by this relation, and that rest mass is the same thing as energy, just in a different form.

Energy inherent to the structure of a mass is the Sum of all the micromass energy of the substance of light which make up the atomic structure of the material. Energy is defined per Quantron theory as a force which moves one period of oscillation. Hence any masses that is caused to moves experience a moving force. When a mass is in motion its is said to have Energy. We measure energy as the product of a mass and the velocity square that it has acquired during its motion. The larger the mass the slower it moves. Ths smaller the mass ,the faster it moves. Light being composed of a flux of micromasses which consists of the smallest masses in the Universe would be the fastest moving micromass in the Universe. Hence the Energy one basic micromass of light would equal to Ms x C^2 defined as Joules in Units of kg m^2/sec^2. Where C is the speed of light measured in meters per seconds. Ms is the mass singularity of light ,measured in Kilogrames. Note ;what is called photons is a misnomer its neither a particle mass or a wave in reality. It is just a convenience term to describe how much radiative Energy flux moving at a particular frequency is falling on a surface per second per unit area of surface. The motion of light micromass has a duality nature type of motion, in the sense that its motion is both inertial and noninertial as well.; Light like any other mass experiences also an osciliatory moving force during its motion . See Henri Poincare original publication in 1901 of the Equation E=MC^2. Its all very vely simple.

Actually, the real answer is: because it happens to be.
When we find “equivalencies”, we normally have a conversion factor (a.k.a. a “constant”).
In Newtonian gravity, using international units (kg, metres and Newtons) we write:
F = G * M * m / d^2
where G is called the “Gravitational constant”; its purpose is to make the units match up.
G = 6.674*10^11 N m^2 / kg^2
It’s purpose is to convert the kilograms of the mass and the metres of the distance, in units of force (Newtons).
Newton himself would have used “slugs” as unit of mass (he invented the name of the unit as a representation of the “sluggishness” caused by inertia), feet as distance and pounds as the unit of force.
Thus, his conversion factor would have had a different value and its units would have been “lb* ft^2 / slug^2”
When Einstein calculated the full series of the energy contained in a moving object, he found an infinite series that looks like:
E = m + (m/2)v^2 + (3m/8)v^4 + …
The second term (1/2)mv^2 is the one we call “kinetic energy” in Newtonian physics.
Then Einstein does strange things to turn the infinite series into a manageable equation using tensors (don’t ask me, it is beyond vector calculus) and ends up showing that if you use the proper units, then the conversion factor happens to be the speed of light.
For example, an intermediate result is that the total apparent energy (the sum of the infinite series), once differentiated, follows the Lorentz transformation (which had been determined separately earlier).
This is the result that is misinterpreted as: the mass increases as the speed approaches the speed of light. It is not the mass that increases, but the total energy content of the moving object.
Einstein figured out the energy content in some basic unit (he used the dyne) and, as long as he used a speed in cm/s, when he allowed the speed (v) to approach zero, the conversion factor happened to be the speed of light squared (around 9*10^20 cm^2 / s^2).
With modern units, we say
E = m c^2
where the energy is expressed in Joules (equivalent to 1 N m)
the mass is in kg and
c is the speed of light in metres per second (which then gets squared).
The conversion factor is left incomplete.
There are other conversion factors we use without thinking, because their value happens to be 1. For example:
F = m a
1 N = 1 kg * 1 m/s^2
We should really write
F = [1 N s^2 / (kg m)] m a
but we don’t.
or
F = [1 lb s^2 / slug ft] m a
for mass in “slugs” and acceleration in “feet per second per second”.
In the case of the “famous” equation, we should write:
E = [1 N s^2 / (kg m)] m c^2
which would give us an answer in N m (Newtonmetres) which happens to be the unit we call Joule.
Then, if we know the rate at which the matter is transformed into energy, we can find the rate of Joules per second (an output unit called Watt).
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It is simply because mass is energy. When mass is distorted, it can be converted into enormous amount of energy linked by the speed of light(squared). Einstein perceived that if space and time become distorted, then everything else is also distorted including all forms of mass and energy. E=MC2 or Viceversa. This lead to the discovery of nuclear fusion.

in physics, energy has a relationship to mass (for whatever reason–don’t ask me, its the laws of physics fault) in which energy is squared. the rest mass of an object therefore is equal to the mass (obviously) times the speed of light (because the object will be converted into electromagnetic energy) squared (because of the massenergy relationship).

because these are directly protional to each other
* eintien proved that energy and mass are intrrelated with each other
* acc. to him mass and energy are not disputed.
* acc to him energy and mass transfored with each other

i have a simple answer. one of the things einstien was thinking about was photons and how the **** they do the things they do, so, if energy was a function of the speed of said photon…. this just clicked with him it made alot of sense. to the average person all the rest a bunch of mathmatical papers trying to rationalizing the simple concept, makes little sense. none of said papers can prove anything but we all like to belive einstien was some kind of god.
why dose it make sense tho? well for 1; photons are suposedly tiny tiny packets of matter, 2; matter should be convertable to energy, 3; how would you calculate the energy of an elephant? you can’t, how would you calculate the energy of a photon? come up with a simple formula that seams to make sense and rationalize it with a mountain of mathmatics till your enemys faint or fall of (einstien probably played king of the hill as a small kid)

Because rest mass has energy.

For the same reason 2 + 2 = 4. IOW, there are some things for which there are reasons and there are some things that – unless you want to get into VERY complicated math – “just are”. Even Einstein had trouble with the math behind e=mc^2.