Einstein equation: Mass and Energy are the same

Einstein find out that mass and energy are the same. But what this really means?
Everybody has seen something with a mass (any object). But nobody can "see" energy. And actually, nobody can describe energy, as we can only experience the effects of it.
We describe many type of energy, or better of effects of energy. We know for instance that what we call "termal" energy is given by the mouvement of the molecules of the body considered (solid, liquid or gas), thus thermal energy is a sum of kinetic energies of the single parts.
And here you see already how statistic and probability are so important in our everyday life: those macroscopic effect, like the one that we call the "temperature" of a body, are the result of the sum of an immense number of small contributions on a microscopic scale.
But some energy is hidden: consider for instance the kernel of an atom build up with protons and neutrons: those protons and neutrons are less heavvy when measured while inside the atom then when we measure their weight when they are free (free means that they are not having any interaction with other particles). The reason is that some of the mass (energy=mass) is needed as energy in order to keep them together (we know that charged particles like the protons otherwise repel each others). And we know that to keep those particle together in the kernel we need even more energy, which will then be released if we split the kernel, as we do in an atomic reactor.
By the way, a similar effect can be observed on a bigger scale then the atomic one:
Consider the change of state of water: Water molecules free to move in the air (as vapour) have a certain temperature (thus cinetic energy). Now let us cool down this water vapour till it reaches the so called dew point: the cinetic energy is therefore lower then before and the molecule start to stick together, i.e. the vapour "condenses" into drops of liquid water, giving out some of their original cinetic energy to the surrounding gases, thus increasing the temperature around them.
This happens when we observe a storm: a "bubble" of air ascend because is warmer then the surrounding one, but in going up, the overall air pressure diminish, thus this "bubble" of air expand and cool down. But when it reaches the dew temperature, the water vapour in this bubble starts to condense, building drops of water and giving out energy which again increases the temperature of the whole bubble, which will be pushed up even more, building an upward stream of air which finally we see as a "cumulus" cloud.
Water is an interesting object for such kind of thinking on how physic works in our life. Think of when you would like to transform ice or snow into liquid water: you need to provide cinetic (termal) energy in order to "free" the water molecules from the ice. This energy will be "given back" when liquid water get frozen. Simple, isn't it?
I am aware that I am describing things in a very simplified way, but I hope that in this way I can show some of the mistery that we still have in our theory of physic.
I would like to write about another field of physics: electromagnetism.
This is a good example of how little we understand our universe. We have some very good equations from Maxwell, and with them we can precisely describe what happens with the electromagnetic fields. But nobody can describe what such fields are, as we know them only by their interactions (their effects).
Our civilisation is today based mainly on applications of the electromagnetic theory, just think of the radio waves, the whole electronics, computers, mobile phones and so on. We use this theory even if we do not understand everything. But it is a good analogy of the universe. Indeed we can take a neutral particle, for instance an atom, which is neutral. This simply means that the sum of the charges present inside the atom is zero. This means that we have the same number of electrons and protons. If you are away enough from this atom, you will not measure any electromagnetic field. Thus we can say that we are in a field-free space. But let send enough energy to this atom, for instance with a photon (electromagnetic wave), to have one electron to escape the atom. Suddently we have two opposite charges which produce a field in the space: the escaped electron and the atom left behind which now is an ion, i.e. has a total positive charge. We cannot "see" this field, but it is everywhere around those two charges. But we can detect the presence of the field using another "test" charge: moving this other charge from far away into the proximity of the original two charges will submit this test charge to some force given by the field present in the space, and in this way we can detect the presence of the field. Let now forget the test charge and move the two original one apart from each others. Of course we need to supply energy in order to do this because a negative and a positive charge attract themselves. We are also putting energy into this system. We do not "see" it, but is anyway there and we can get this energy back when we let the two charges come together again. This principle is applied in the everyday life in the device called "capacitor".

The important point now is to understand that the transformation of energy in its different forms need a space, i.e. our tri-dimensional space. This did not existed before the Big-Bang event: only at this initial time the tri-dimensional space started to be built, and it is still building up now, as the mouvement of the galaxies around us shows.
Work in progress .... sorry about that, be patient please.