String Theory & Quantum Gravity: Simplified

Physics is the study of matter and how it moves through spacetime. For many years, physicists have attempted to understand how the forces of the gravity, which regulates large bodies like the planets, and those of quantum mechanics, which regulates small particles like atoms, can be explained in a “unified theory of everything.” String theory attempted to make sense of these various forces in a mathematically coherent way.  Problems with string theory spawned an alternate model called the loop quantum gravity theory which I will cover as well in today’s post.

What Is String Theory?
In the 1960s, string theory proposed that the universe is composed of tiny filaments or “strings” and membranes that vibrate in different ways to create the objects we see around us. Strings may have open ends or may be closed into loops at various times as they vibrate. These strings attach at one or both ends to membranes to create the world of matter that we know. In the 1970s, string theory proposed that all matter is composed of fermions and bosons connected by a supersymmetry which corrects for some mathematical inconsistencies of the formulae.

Brian Green: The Universe on a String
In clear, nontechnical language, string theorist Brian Greene explains how our understanding of the universe has evolved from Einstein’s notions of gravity and space-time to superstring theory, where minuscule strands of energy vibrating …

Advantages of String Theory
String theory successfully unifies gravity and quantum mechanics mathematically, as well as a number of other widely accepted scientific principles. Though it is still in an early stage of experimentation, it offers a consistent and logical method of integrating our current understanding of the universe.

Problems of String Theory
One of the problems of string theory is that many of the particles suggested by the theory have not actually been observed by particle theorists. Another problem is that the theory does not work mathematically unless there are 10 additional dimensions beyond the 3 + 1 that we currently understand, that is, length, width, depth plus time. Some theorists propose that these extra dimensions are curled up into incredibly small sizes so that we are not able to perceive them.

What is Loop Quantum Gravity Theory?
In the 1980s, loop quantum gravity theory offered a different mathematical model for the universe and its motion. Also called loop gravity and quantum geometry, this idea proposes that space is composed of an extremely fine fabric “woven” of tiny loops that exist in excited gravitational fields called “spin networks.” Loop quantum gravity can be explained as new mathematical variables, and a model that these network, on a sort of graph with nodes and links, where each node and link consists of mathematical values. In this concept, spacetime does not exist fundamentally, but emerges from the association of nodes and links.

Advantages of LQG Theory
Loop quantum gravity theory does not need an observed supersymmetry as string theory does, nor does it require additional dimensions. It preserves the features of general relativity while employing quantization of space and time on the Planck scale in the tradition of quantum mechanics.

Problems of LQG Theory
While loop quantum gravity solves some problems of reconciling general relativity and quantum mechanics, it creates other problems in regard to the behavior of geometry at short distances. Space-time becomes a more variable entity of loops upon loops, instead of the fixed background “stage” previously thought. The theory cannot be directly tested because of this variability.

What’s Ahead
Many physicists continue to work on both of these theories to discover which can be used to consistently explain how the universe is put together and how it continues to function. Their work is important to the future of space exploration and other scientific fields alike.

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