The photoelectric effect is used in solar cells and a photomultiplier (which is used to detect photons). I am sure there are other applications too but trying to find them (even with the whole internet at my disposal) is a rather hard thing to do.
Some of the applications of the theory of energy quanta are; in Geiger counters where the ability of photons to ionise gas molecules is used to detect radiation (and since the theory of energy quanta is such a large part of quantum physics I think I will throw in a few things that quantum physics does for us), quantum mechanics is vital in the understanding of Chemistry (as all Chemistry can be explained using physics), chemistry is all about understanding the energetic properties of bonding, and this is explained through quantum mechanics, the use of transistors, lasers and diodes all rely on quantum effects (that covers just about all of electronics) and greater understanding of quantum effects leads to innovation in electronics, providing faster and more powerful computing (that’s the main use really). Though it is probably debatable as to whether or not the understanding of quantum effects has led to a great change in modern electronics, we may have ended up where we were without fully understanding it, but then that is just me speculating.
"On the Motion of Small Particles Suspended in a Stationary Liquid, as Required by the Molecular Kinetic Theory of Heat", this paper provided an explanation to Brownian motion, which is the movement of a particle or molecule due to bombardment by many smaller particles or molecules (at least that is my understanding, and I don’t really get it at all, so I could be wrong there), Einstein’s paper also provided empirical evidence for the existence of the atom, which had just been a ‘nice way to think about particles’ when trying to explain scientific problems before his explanation.
Many people use this theory to explain fluctuations in the stock market, they can do this because the theory behind Brownian motion is very close to being an equation which describes random motion (though naturally a perfect equation would be impossible), and fluctuations in stock market prices are very random. This being said, the very famous mathematician Benoît B. Mandelbrot argued that it couldn’t explain stock market fluctuations, for Brownian motion to explain the stock market several assumptions about the variation in stock price would have to be made; the change in price of shares would have to be independent of previous changes in the stock price, that the factors affecting change in stock prices remain the same and that prices changes follow the proportions of a normal distribution. Unfortunately in reality Mandelbrot states that “life is more complex” and that these three assumptions are not met in the real world, so Brownian motion cannot therefore be used to predict or explain fluctuations in stock price (Mandelbrot 2004 [4]).
“On the Electrodynamics of Moving Bodies”. This is where things really get interesting, the theories proposed in this paper later become known as Einstein’s “special theory of relativity” and this paper explains how the speed of light is a constant for all frames of reference, and how special dimensions have to warp in order to obey this law. Einstein cannot claim all the glory for this work though, as much of it built upon the works of other famous scientists, just in his own paper he mentions the names of Isaac Newton, James Clerk Maxwell, Heinrich Hertz, Christian Doppler and Hendrik Lorentz, but naturally his work would have been influenced by other scientist’s thoughts and questions too.
Einstein begins his paper with a brief scientific fact that shows that relative motion can affect the properties of a system, then he jumps straight into the two theories he wants to put forward, the first being that “the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good”, and the second being “that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body” (Einstein 1905 [5]), which I believe means that whatever the relative movement of a system, compared to that which it is measuring, the laws of electrodynamics and optics will stay the same, so no matter how fast you are moving relative to a system within which you are measuring the speed of light (for instance) the speed of light will be the same from your measurements as those taken from the much slower system, so more generally the speed of light is constant no matter how fast you are going it will appear the same speed. This is a pretty big idea to throw to the floor, but unfortunately the laws of physics support this hypothesis; no matter how hard it is for you or me to quite comprehend (physics would be boring if all of the theories involved actually followed logic).
Einstein’s postulates on this supported the evidence collected from the Michelson-Morley experiment - in 1887 - into the ether that acted as a medium for electromagnetic waves. Their experimental data showed that there was not background ether, since light travelling into the ‘ether wind’ was not slowed down like it should have been if there was ether. Einstein states that “the unsuccessful attempts to discover any motion of the earth relatively to the "light medium," suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.” ( Einstein 1905 [6]).
Two other very bizarre outcomes of Einstein’s theories are that when a body is moving close to light speed it’s surrounding space-time undergoes ‘time dilation’ and ‘length contraction’. That is to say that when something is going very, very fast space-time warps around it, so that time becomes slower and lengths become shorter, these differences can only be seen by an external observer however, so a person going at such a speed to produce an effect wouldn’t be able to notice it themselves. As to finding actual reasons for this happening, I have yet to find any, but my own guess would be that the warping of space-time around objects close to the speed of light allows the very fabric of reality to control an objects speed so that it can never exceed the speed of light.
These two effects are also seen when the force of gravity is exerted on a body, this is covered by ‘general relativity’ (but I thought I would just add this on anyway because it’s cool), this leads to some very interesting theoretical physics surrounding black holes, which exert such a powerful force of attraction that light itself cannot escape it’s grasp. So now try to image the effects of time dilation and length contraction of light entering a black hole, that’s like both theories together, the effects would be huge, but also impossible for us to measure, as we couldn’t get close enough without being destroyed.
Applications of this today: anything where there is large changes in altitude between two objects which require time synchronicity have to account for time dilation between the two time measuring devices, almost all GPS rely on such synchronicity to accurately calculate the speed direction and position of a car/plane/person etc., it was also used in the recent test at CERN where they fired neutrino’s through the Alps, due to difference in heights of the detector compared with the source of the neutrinos, there was a significant amount of time dilation that they had to account for so that there results for the neutrino’s speed would be accurate enough. Other than those two I couldn’t find any other practical applications of this theory, but I am sure that the world of physics does have many uses for them, things to do with astronomy and such things.
"Does the Inertia of a Body Depend Upon Its Energy Content?”, this is the paper that led to the equation E=mc^2 which is one of the most famous, if not the most famous, equation in human history. This equation is based upon Einstein’s investigations into the “Maxwell-Hertz equations for empty space, together with the Maxwellian expression for the electromagnetic energy of space” (Einstein 1905 [7]). Einstein then does loads of crazy maths that I don’t really understand, but basically comes to the conclusion that “The mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by L/9 × 10^20” (Einstein 1905 [8]) (‘L’ is used to represent energy), I am unsure why it’s 10^20 and not 10^16 (which would be roughly the speed of light squared, but that’s the quote for you), this equation which he has written in words is just a rearrangement of E=mc^2 (it is, in his words, m=E/C^2).
This equation led to the ability for nuclear physicists to calculate the energy yield of nuclear reactions, and also allowed for more general theories of how increasing the energy of a system increases it’s mass (not through any mass being created, nor energy being created, but by the transfer of mass an energy into the respective other form).
A few examples of how changes in energy lead to changes in mass are:
A spring's mass increases whenever it is put into compression or tension. Its added mass arises from the added potential energy stored within it;
The worlds official standard mass for the kilogram, made of platinum/iridium, will undergo a change in mass of 1.5 picograms (1 picograms is equivalent to 1x10^-20 grams) if it’s temperature is raised 1 degree;
The Earth itself is more massive due to its daily rotation, than it would be with no rotation. This rotational energy {2.14 x 10^29 J (Infranetlab 2008 [9])} represents 2.38 billion metric tons of added mass.
He also published many other papers and wrote many books too, but there isn’t the time nor the words left to go into them, I will just add that as well as his love for physics, Einstein also dabbled in philosophy, as many theoretical physicist do, and wrote a few books on it, this rewarded him with an honourary degree in philosophy. Einstein was one of the leading figures in the World Government Movement and was offered the Presidency of Israel after WWII had ended, he declined it however. I would assume that this was for political reasons, he was a pacifist, and was against the use of atomic weapons, which the American’s used on Japan in 1945 [10].
Personally I find Einstein’s theories fascinating, and inciting of much thought. They challenge the understanding and extend the knowledge of the surrounding world.
