The two giants of Physics: Quantum Mechanics and General relativity
Quantum mechanics is perhaps one of those very few areas of research in Physics which even the adepts in the field have a hard time getting their heads around, and rightfully so. The bizarre laws of Physics on the smallest of scales completely defy common sense, so much so that the esteemed theoretical Physicist of the twentieth century, Albert Einstein, who, ironically, is regarded as one of the earliest contributors to Quantum Physics, always remained suspicious of it in spite of its success, maintaining that “God does not play dice.” The dependence on probability, an intrinsic part of Quantum mechanics, which for Einstein suggested a lack of awareness of the laws governing reality, was perhaps the one thing that caused him to develop resentment towards the field, and this resentment persisted until the day he died.
The first major step into the weird world of Quantum mechanics was taken by Max Planck in 1900 when he, in an attempt to explain the ultraviolet catastrophe which could not be explained using classical mechanics, proposed the idea that the energy of electromagnetic waves is quantized rather than continuous i.e. it could only take up certain discrete values. As bizarre as the idea might have been, it not only resolved the ultraviolet catastrophe but also laid the groundwork for future research in Quantum mechanics. Moving forward to 1905 and a young Albert Einstein publishes four scientific papers that will contribute substantially to the foundation of Modern Physics. One of these papers will go on to win him the Nobel Prize in 1921 and is an explanation of the photoelectric effect whereas another one introduces the world to special relativity, one of his most well-known ideas.
The photoelectric effect is perhaps one of the most easily observable phenomena in Physics: you illuminate the surface of a metal and, if the energy of the light is high enough, calculate the energy and the number of electrons emitted by measuring a small current and voltage across a circuit connected to two plates. The weirdness, though, begins when you change the intensity and the frequency of the light hitting the metal’s surface. Where classical mechanics predicted that the energy of the emitted electrons would increase with increasing intensity of light (as the energy of a wave is proportional to its intensity, and scientists had already established that light was a wave), physicists were shocked to observe that the energy of the ejected electrons depended on the frequency of the light being shone, and that there was a threshold frequency for any electrons to be emitted i.e. no electrons were emitted if the frequency was less than a certain minimum.
The explanation of this phenomenon required the brilliance of Albert Einstein, and he did that by positing that light, aside from being a wave, was made up of particles called photons. This particular nature of light is referred to as ‘wave-particle duality of light.’ Later scientists like Louis De Broglie proposed that particles like electrons also exhibit wave behavior, and the works of physicists such as Niels Bohr (who won the Nobel Prize in 1922), Werner Heisenberg (who introduced his famous uncertainty principle in 1927) and many others laid the foundations on which Quantum theory as we know it was built.
Where Quantum mechanics was developed over several years by numerous scientists working together, or independently, the one name that comes to mind whenever general relativity is mentioned is: you guessed it, Albert Einstein. Einstein’s 1905 paper on special relativity was a breakthrough in the physics of his time, and he spent the next ten years working to develop a more ‘general’ form of his theory. As the name implies, special relativity is only applicable to objects that have a special property: they have zero acceleration. According to Einstein, any observer moving at a constant velocity relative to a frame also moving at constant velocity sees events in the frame unfold slower in time than they do for anyone inside the frame(being stationary is just a special case of moving at a constant velocity of zero). The phenomenon, however, is only really observable at very high speeds.
As a very basic example, imagine you’re standing at the railway station when your friend somehow hops onto a train moving at a constant speed relative to you. Your friend then uses a torch to shine light onto a mirror on the train’s floor. You and your friend now measure the time that the light takes to make a round trip from the torch to the mirror and back to the torch. For your friend, light only moves in one direction: the vertical direction. For you, however, the light’s path has both a vertical component (equal to the distance that light has travelled for your friend) AND a horizontal component (by virtue of the horizontal movement of the train relative to you.) Hence, for you, the light follows a v-shaped path whose length is obviously longer than the distance that light has travelled for your friend.
Now, the obvious explanation for the above phenomenon would be that light, like any other object in the universe, has a speed that changes with the speed of the observer or the source. However, reliable experiments such as the Michelson-Morley experiment have proven that light has a constant speed of about 300 000 meters per second in vacuum for every observer regardless of their velocity or the velocity of the source of light. So, the only way out of this dilemma is to accept that time as we know it is not absolute but relative. It moves slower for you than for your friend! The absoluteness of time is merely an illusion. In the words of Einstein:
“The distinction between the past, present and future is only a stubbornly persistent illusion.”
Einstein published his theory of general relativity in 1915 and revolutionized the way people looked at gravity. He began with the simple principle that a frame accelerating upwards is, as far as the laws of Physics are concerned, identical to a frame that stands stationary in a gravitational field (provided that the magnitude of the acceleration is equal to the gravitational field strength) and came to the shocking realization that space and time were not independent but linked together in the form of a single entity called space-time. This space-time, which is spread throughout the universe, acts like a fabric: it is smooth, continuous and can get curved and deformed by the presence of energy and matter, much like the surface of a trampoline gets deformed when a mass is placed on it. The conclusion of all this is that the force we know as Gravity is nothing but distortions in the fabric of space-time caused by objects possessing mass! Take a moment to let all that sink in.
So, physicists have sorted almost everything out and there is very little to be known about the universe now, right?
Well, not exactly.
Over the years, physicists have managed to merge quantum mechanics with special relativity into a beautiful theoretical framework they call ‘Quantum field theory’. Doing the same with general relativity, however, proves to be much more difficult as quantum mechanics, which accurately describes the physics at the atomic and subatomic level, doesn’t really see eye to eye with general relativity, which describes the laws of physics at the largest scales in our universe. Scientists have been in search of a single theory that would arise from the combination of these two giants of Physics and would possess the ability to link together all physical aspects of the universe. Such a theory, if there is one, will therefore be called…..a theory of everything.
