The debate on whether or not we can have “free energy” has been going on for a while: is there a way to produce energy that comes in the form of photons or some other form of energy?
In the 1960s, Albert Einstein gave an informal explanation of this question to physicist Eugene Wigner. He defined energy as “the capacity of any system to produce an amount of energy equivalent to the energy that it would need to produce that amount of energy itself”.
Image copyright Thinkstock Image caption The amount of light emitted by a star or sun depends on how much its atoms collided
A physicist who was familiar with the Einstein equations could say that, for each electron, there was a certain energy level. He could also say that, if this energy were produced the same way that the electrons that form the atoms do, then that energy would be the same as the total amount of energy the system needed to produce.
Wigner, on the other hand, looked at the quantum world of the electron and the atom, and realised that he could calculate what would happen to the energy in a star or sun if you dropped a star into its orbit at a particular energy.
He realised that you could say that there is an energy level at which you would not need to give more energy to the star because it would just give back more energy – like what you get with a solar power plant.
Image copyright SPL Image caption Photons behave in much the same way as atoms – but they have different masses
His approach to the question of whether it was physically possible was to look at the quantum state of a photon, looking at all the energy levels associated with it.
The photons that come from a star are not of the same energy. Even if they have the same mass, and the same spin, and they have the same frequency, they are different photons.
In light, for example, if an atom has a certain energy and you drop it into its orbit, that atom will go faster on average than if you hadn’t dropped it in.
In Einstein’s words “if in a black hole, you drop a particle into its orbit, you will have, say, an energy of one with respect to any of the quantum states associated with it.
“But if you drop it into a gas in which it has only very small temperatures, and at other times its temperature has been dropped to extremely low positions, you get an energy of only
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