loop quantum gravity -- 12/11/19

Today's selection -- from Seven Brief Lessons on Physics by Carlo Rovelli. Loop quantum gravity seeks to reconcile general relativity and quantum mechanics:

"Loop quantum gravity is an endeavor to combine gen­eral relativity and quantum mechanics. It is a cautious attempt because it uses only hypotheses already contained within these theories, suitably rewritten to make them compatible. But its consequences are radical: a further profound modification of the way we look at the struc­ture of reality.

"The idea is simple. General relativity has taught us that space is not an inert box but rather something dynamic: a kind of immense, mobile snail shell in which we are contained -- one that can be compressed and twisted. Quantum mechanics, on the other hand, has taught us that every field of this kind is 'made of quanta' and has a fine, granular structure. It immedi­ately follows that physical space is also 'made of quanta.'

"The central result of loop quantum gravity is indeed that space is not continuous, that it is not infinitely di­visible but made up of grains, or 'atoms of space.' These are extremely minute: a billion billion times smaller than the smallest atomic nuclei. The theory describes these 'atoms of space' in mathematical form and pro­vides equations that determine their evolution. They are called 'loops,' or rings, because they are linked to one another, forming a network of relations that weaves the texture of space, like the rings of a finely woven, im­mense chain mail.

Artist's illustration of loop quantum gravity.

"Where are these quanta of space? Nowhere. They are not in space because they are themselves the space. Space is created by the linking of these individual quanta of gravity. Once again, the world seems to be less about objects than about interactive relationships.

"But it's the second consequence of the theory that is the most extreme. Just as the idea of a continuous space that contains things disappears, so the idea of an elementary and primal 'time' flowing regardless of things also vanishes. The equations describing grains of space and matter no longer contain the variable 'time.' This doesn't mean that everything is stationary and un­changing. On the contrary, it means that change is ubiquitous -- but elementary processes cannot be or­dered in a common succession of 'instants.' At the min­ute scale of the grains of space, the dance of nature does not take place to the rhythm of the baton of a single orchestral conductor, at a single tempo: every process dances independently with its neighbors, to its own rhythm. The passage of time is internal to the world, is born in the world itself in the relationship between quantum events that comprise the world and are them­selves the source of time.

"The world described by the theory is thus further distanced from the one with which we are familiar. There is no longer space that 'contains' the world, and there is no longer time 'in which' events occur. There are only elementary processes wherein quanta of space and matter continually interact with one another. The illusion of space and time that continues around us is a blurred vision of this swarming of elementary processes, just as a calm, clear Alpine lake consists in reality of a rapid dance of myriads of minuscule water molecules.

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Carlo Rovelli


Seven Brief Lessons On Physics


Riverhead Books


Copyright 2014 by Riverhead Books


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