It was in the year between 1904 and 1905 that Albert Einstein embarked upon three studies that would change the face of physics. The first was the study of heat. He studied Brownian Motion – the way in which particles in a colloid jiggle about. This explained how energy is stored in material as warmth. Ludwig Boltzmann had discovered the mechanical equivalent of heat.
The second study took him into the realm of photoelectrics. Max Planck had discovered that light consists of a kind of “atoms.” The term “atomiki” (indivisibles) had been coined by Democrit of Smyrna, who stated that only atoms and free space are real. The term was picked up by Dalton and JJ Thomson as the smallest, indivisible piece of material. So Planck, in his search for a new word, introduced the concept of a “quantum” for the smallest piece of energy.
Planck’s constant, when multiplied by the frequency of any particular color of light, will tell you how many watt-seconds are the least amount of light of that color that is possible.
The third study by Einstein went into the atom’s nucleus, in the search for an explanation of the riddle of radium. How can a piece of metal stay warm – seemingly forever? Where is the energy coming from?
Einstein’s second work – on the photoelectric effect – was the one that brought him the Nobel prize. It related to the puzzle that light could travel at a huge, but fixed and finite, speed through free space – and therefore must consist of nothing solid. Nevertheless, when it collides with something, it can cause that thing to move.
Such calculations are traditionally based upon studies of momentum – the product of mass and velocity. We know what the mass is – it is nothing at all because the light is nothing at all. We also know the speed of light. In a vacuum, it is known as c (about 300 million meters per second, or 186 thousand miles per second).
So when we use the MKS system (meters, kilograms and seconds), we multiply zero by three hundred million – and this gives us the momentum of light. It must surely be nil.
However, when light hits an atom of metal, it can throw out an electron from the metal. This is the photoelectric effect. Planck goes into some detail about the quantization of energy in his Nobel prize lecture:
So we can say that one quantum of light delivers one single electron.
The fact that the electron moves means that it has momentum. That is the product of the mass of the electron and its speed. So when we divide the momentum by the speed of light, we come up with the mass of a quantum of light. This is a sensation. Energy should be weightless, but here we find that it has mass.
Einstein then continued his studies of the movement of the electron by likening that movement to its behavior under the influence of a voltage. It turns out that for each frequency of light, there is an “electron-volt” rating that describes what would happen if that light struck an electron. The electron-volt value can be obtained from Planck’s result in Joules (Watt-seconds) simply by multiplying by a special constant.
His research showed that the electrons are held inside the atoms by means of known voltages, the work functions. When light sets them free, the energy of the light in electron-volts is divided up in two ways. The first part of the voltage if that which is needed to overcome the work function. The second part is the energy that is leftover in the liberated electron.
This work on the photoelectric effect was so important because it tied together many loose ends of physics. The spectral lines of light could now be defined as electron-voltages, and one could tell exactly where the light originated in the atoms. The atom was now looked upon as an electrical machine, and one could predict how metals would behave when used as photocells, for example.
So the whole world of photoelectric arrived – with sound-stripe on film and invisible-ray burglar alarms. At the same time, one could predict metals’ properties in an electroplating bath (although the photoelectric work-function was becoming supplemented with the electrochemical work-function).
Einstein received the 1920 Nobel prize for physics
But it was delayed for a year. He received the prize itself while onboard a ship was visiting Japan. His lecture to the Nordic Assembly of Naturalists was not his acceptance speech, therefore. Already he is thinking of relativity:
We have entered into a world where a matter has mass (weight under standard gravity), and so also has energy. But are the two kinds of mass the same?
When we have a lump of the material of mass M1 and place it at a distance D from the second piece of material of mass M2, we get an attraction between the two.
The law is simple. It is M1 times M2 divided by D and divided again by D.
That calculation defines the acceleration, or pull that one piece of material will exert on the other.
But what of light?
Max Planck had said that the smallest quantum of light is h times v. Here h is Planck’s constant and v is the frequency of that light.
Einstein now delivered his famous clichÃ© (that E=mc-squared). So the mass m is E divided by the speed of light and again by the speed of light.
From this, we discover that the smallest “mass” of light is h times v divided by c divided by c.
We have seen that c is an enormous number. When we divided h times v by c, we get a truly tiny quantity of mass. A further such division makes it ridiculously small. Nevertheless, the smallest quantum of light is not weightless – it is almost weightless.
If we want to take a photograph without a lens, we can make a pinhole camera. A bundle of light-rays will go through the pinhole to the film, and the sharpest detail on the resulting image will be only as sharp as the pinhole.
Now we make another camera, with a pinhole half as high and half as wide. We need to expose the film for four times the time, but it does indeed become twice as sharp.
Now we make another, with the pinhole four times smaller. At some point, we get a serious disappointment. The exposures are getting longer, but the images are not getting any sharper.
What is happening is that the quanta of light, as they pass through the tiny pinhole, are forced so close together that their masses interact. Rays of light are pulling rays of light. This is known as diffraction.
So the rule M1 times M2 divided by D-squared seems to hold for the mass of energy as well as for the mass of matter.
It was about this time that questions were being asked as to what would happen in one of the masses – say M1 – was due to matter, while the other – M2 – was due to energy.
Einstein’s answer was simple – try it.
As the mass of a quantum of light is so tiny, we need to counterbalance it with some enormous object if we are to see some visible effect. Einstein suggested the sun.
The sun is about a hundred-thousand times heavier than the earth. So it is extremely heavy. If Einstein’s prediction were to come true, a quantum of light of mass M2, skimming past the sun with its huge mass M1, should be pulled off course.
This would not be gravity, because gravity is something exerted by matter upon the matter.
This would not be diffraction, because diffraction is something exerted by energy upon energy.
This would be a half-way thing, neither one nor the other. It would be graviffraction. It would be the gravity of the sun causing the diffraction of the light.
One has to be careful, because a slight haze of gasses in the vicinity of the sun may act as a lens – causing refraction rather than diffraction.
When Einstein predicted the bending of light by the sun, in 1916, scientists waited three years for an eclipse.
Sure enough, as stars and planets on the opposite side of the solar system tried to drift behind the edge of the sun, the sun pulled the rays of light off course. The stars stayed visible due to the curved light path. The prediction had been confirmed.