A Question of Unification: from atoms to galaxies

Rakshak Adhikari

Staff Writer

Every Monday, Wednesday and Friday physics students at Troy take two very peculiar physics classes: General Relativity in the morning and Quantum Mechanics in the afternoon.

General Relativity, Einstein’s magnum opus, deals mostly with large scale structures in the universe. The deviation of planetary orbits from Newtonian laws, the slowing down of time near massive objects, the existence of certain exotic objects called black holes and the recently discovered gravitational waves are all consequences of general relativity.

Quantum Mechanics, on the other hand, was developed by a group of physicists over many years and deals primarily with the microscopic world. It tells us that light can act like particles and matter can behave like waves and also gives extremely accurate predictions of events in the microscopic world. The revolutionary ideas of quantum computing and superconductivity stem from quantum mechanics, as does the fact that the world is inherently non-deterministic.

While these theories have survived countless tests in their respective fields of application, there are two extreme scenarios where they cross paths, and the result is nothing short of a catastrophe. The two instances are called singularities where physical quantities like temperature, density, etc. become infinitely large and hence “unphysical.”

The “Big-Bang Theory,” the most widely accepted cosmological model today, posits that about 14 billion years ago the universe existed in a very hot and infinitely dense state which is one of the singularities where the aforementioned theories fail. The second singularity occurs when we try to peek into black holes — remnants of dead stars where the mass is packed so densely that not even light can escape its immense gravitational attraction.

While there have been numerous attempts to reconcile quantum mechanics with general relativity to build a grand “theory of everything,” none of them have been successful so far. The proposed theories either have gaps in them, do not make any testable predictions, or only work in higher dimensions than have been observed so far.

The two major contenders for uniting the seemingly unresolvable theories are String Theory and Loop Quantum Gravity.

The more popular of the two, String Theory, postulates that the world is made up of tiny strings, the vibration of which emerges as the different physical properties of perceived particles. However, the strings are far too small to be directly observed (about millionth of a billionth of a billionth of a billionth of a centimeter) and the different properties of elementary particles require the strings to vibrate in many more dimensions than the ones we have observed.

A newer proposal for unification comes from Loop Quantum Gravity — a theory that proposes that space and time are by themselves discrete and not continuous.

One of the interesting predictions of this theory is that the speed of light could be frequency-dependent, which contradicts the postulate of Einstein’s special theory of relativity that states that the speed of light is a universal constant.

While both the theories are promising, neither of them is complete or verified. All we need is some testable process that will determine which – if either – of the two theories will triumph.

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