Einstein actually won his Nobel Prize earlier in the 20th century for the photoelectric effect, but perhaps his most famous innovation was recognizing that mass and energy are essentially the same thing. We talk about the hybrid entity, mass-energy, because mass and energy are interconvertible. The most ubiquitous example of mass-energy conversion occurs in the thing that keeps us alive every second, sunlight. Deep within the sun, a three-step fusion process is converting a tiny piece of the mass of every atomic nucleus into radiant energy that travels out through the sun, eventually comes to Earth, sustaining all life on Earth. This three-step fusion process eventually turns four protons into a helium nucleus, which is two protons and two neutrons. If we do the book keeping, it turns out that the atomic nucleus that results, the helium nucleus is slightly lighter than the four particles that went into it. The tiny amount of extra binding energy released is radiant energy. In the sun, 0.7% of the mass of all the subatomic particles each reaction is being converted into radiant energy. If you want to think about this in a colloquial sense, it's as if mass is frozen energy. And under suitable conditions, typically extreme pressure and temperature, that energy can be liberated. Fusion is one example where that energy is liberated everyday in the sun and in all stars in the universe. The equation that codifies this relationship is Einstein's seminal e =mc squared. Let's unpack this equation and see what it means. It relates energy to mass with the multiplier of c squared. We know that c, the speed of light is an incredibly large number, so its square is a phenomenally large number. And in the equation, that means that a tiny mass has an enormous equivalent energy. So fusion and any process that liberate mass energy is a phenomenally efficient source of energy. For example, the ink in the period of the end of the sentence in a book that you might be reading, if converted by e equal mc squared into pure energy would power a family home for a year. Now remember in the sun and in a fusion process, only 0.7% or slightly less than 100th of the mass energy is liberated. There are situations where all the energy can be liberated. These are the situations where particle and an anti-particle annihilate and create pure radiant energy, the complete conversion of mass into energy. But even at an efficiency of slightly less than 1%, fusion is an extraordinarily efficient energy source. To see how, imagine the energy liberated when we eat a small hamburger patty, 100 grams. The chemical energy in that small patty is a few hundred Joules. That energy might sustain your activity for a few hours. If for example, we could somehow liberate the mass energy entirely of the material in that 100 grams, we would generate 100 billion times more energy than the chemical energy you get from eating it. That’s the phenomenal efficiency of fusion or mass energy. And you can see why people put a lot of effort into trying to generate fusion energy as a routine and reliable energy source. It turns out that it's so difficult, the fusion has only been harnessed for tiny fractions of a second and not really in a sustainable way. But it is indeed the holy grail of energy research to be able to get energy sources that are billion of times more efficient than chemical energy which is what we rely on now. In an aside on Einstein, he was aware that unlocking the power of the nucleus through E = mc squared could possibly have a down side. And indeed later in his life, he watched in dismay as the United States and then the Soviet Union developed the atomic bomb. Einstein and other concerned scientist wrote to President Harry Truman, urging him not to continue development of the bomb. But it was a horrible and amidst use of mass energy for destructive purposes. But the genie was out of the bottle, nuclear weapons are here to stay just as nuclear energy is not yet here to stay, an unfortunate sideline and important piece of physics. It would be unfair to pin nuclear weapons to Einstein's door. The physics of unlocking the power of the atom is just that physics, how human choose to use that power is up to them. Earlier in the 20th century, Einstein made a profound realization that mass and energy were equivalent. They're related by his iconic equation, E equals mc squared. c is a huge number, and this means that a tiny amount of mass is equivalent to a phenomenal amount of energy. The sun, and all stars like it in the universe, are liberating just under a percent of their mass into radiant energy and this sustains all life on Earth. Mass energy is the most efficient energy source imaginable, billions of times more efficient than energy from rearranging electrons in chemical bonds.