See Inside A Matter of Time

Atoms of Space and Time [Preview]

We perceive space and time to be continuous, but if the amazing theory of loop quantum gravity is correct, they actually come in discrete pieces

In Brief


  • To understand the structure of space on the very smallest size scale, we must turn to a quantum theory of gravity. Einstein's general theory of relativity reveals that gravity is caused by the warping of space and time.
  • By carefully combining the fundamental principles of quantum mechanics and general relativity, physicists are led to the theory of loop quantum gravity. In this theory, the allowed quantum states of space are represented by diagrams of lines and nodes called spin networks. Quantum spacetime corresponds to similar diagrams called spin foams, in which spin networks evolve over time.
  • Loop quantum gravity predicts that space comes in discrete lumps, the smallest of which is about a cubic Planck length, or 1099 cubic centimeter. Time proceeds in discrete ticks of about a Planck time, or 1043 second.


A LITTLE MORE THAN 100 YEARS AGO MOST PEOPLE—AND MOST SCIENTISTS—thought of matter as continuous. Although since ancient times some philosophers and scientists had speculated that if matter were broken up into small enough bits, it might turn out to be made up of verytiny atoms, few thought the existence of atoms could ever be proved. Today we have imaged individual atoms and have studied the particles that compose them. The granularity of matter is old news.

In recent decades physicists and mathematicians have asked if space is also made of discrete pieces. Is it continuous, as we learn in school, or is it more like a piece of cloth, woven out of individual fibers? If we could probe to size scales that were small enough, would we see “atoms” of space, irreducible pieces of volume that cannot be broken into anything smaller? And what about time: Does nature change continuously, or does the world evolve in a series of very tiny steps, acting more like a digital computer?

The past 25 years have seen great progress on these questions. A theory with the strange name of “loop quantum gravity” predicts that space and time are indeed made of discrete pieces. The picture revealed by calculations carried out within the framework of this theory is both simple and beautiful. The theory has deepened our understanding of puzzling phenomena having to do with black holes and the big bang. Best of all, it is possible that current experiments might be able to detect signals of the atomic structure of spacetime—if this structure actually exists—in the near future.

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