“A stitch in time saves energy “. Albert Einstein equation E=MC^2 proves that objects having mass have equivalent amount of energy and vice versa. At the same time, other scientists also raced to show that mass and energy are interchangeable. Before 1820, many scientists and chemists believed that energy exists in different forms and all of them were unique or different. But, Humphry Davy and Michael Faraday disproved this belief by their experiments. They found that energy in any form gets transformed into another energy and they are interconnected.
Humphry Davy and Michael Faraday
H.Davy and M.Faraday experimented on Mass-Energy Equivalence. The Mass-Energy Equivalence proves that when 1 Kg of gold heats to 10°C from its normal temperature, its mass increases. Here, heat energy converts to mass. When the metal gets cool, the metal loses the mass it gained. So, this proves mass and energy are the same. Though, people did not realize this and started arguing how mass and energy could be the same during conversion.
Speed of Light
To answer why the mass of an object is multiplied by the light speed to determine how much energy it has inside, whenever you convert part of an object or any other piece of matter to pure energy, the resulting energy is by definition moving at the speed of light. In practice, this is not true. When objects move at the speed of light, its mass increases proportionally with the energy required to move it. This is the reason why nothing can travel faster than light or equally as light. This gave rise to the theory of special relativity by Einstein over a period.
According to special relativity, distance and time are linked to objects moving at a consistent speed in a straight line. The equation E=MC^2 is based on special relativity, it explains energy and mass are interchangeable. It means that they are different form of the same thing. If the mass of an object is converted into energy it proves how much energy the object had earlier. This could be imagined as how powerful an nuclear explosion would be after its mass is converted into an explosion. Also the equation demonstrates how mass increases with speed, that puts a speed limit on how fast things can move.
Coming back to special relativity, time moves relative to an observer. This means that time travels more slowly when someone is moving than when someone is standing still. The Einstein theory imagines to a car moving at a point in a straight line equally between two trees. If a thunder-light hit both trees at the same time, due to the motion of the car, the person inside the car would see the thunder-light hit one tree before the other tree. But the person outside would see simultaneous strikes.
Einstein decided that concurrence is relative, that being said the events that are simultaneous for one observer may not be the same for another. This led Einstein to the conclusion that time flows differently according to the state of motion and to the conclusion that distance is also relative.