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FQXI ARTICLE

January 18, 2018

The Cosmic Family Tree

Mapping the ancestral history of spacetime in an effort to unite quantum mechanics and general relativity.

FQXi Awardees: David Rideout

January 6, 2012

DAVID RIDEOUT

University of California, San Diego

When physicists try to calculate what happens in regimes where huge masses are confined to small regions, such as the points of infinite density— singularities—within black holes, their equations of gravity run into a snag. "Inside a black hole, the fabric of spacetime seems to end. The theory is calling for help!" says Rafael Sorkin, a physicist at Syracuse University, New York. Quantum theory is not immune either, he adds: "The bare electric charge of the electron is very badly infinite" in the quantum equations of electromagnetic force, though of course electrons do not carry infinite charge in reality. One possible way round these problems is to treat spacetime as a network of discrete chunks, just 10

Discreteness is not the whole story, though. If it were, says Rideout, "Spacetime would be like a pile of dust." To cast a complete description of the universe, physicists need something more to mathematically connect the points together, and Rideout thinks that "something" just might be causality. He hopes that combining causal structure with underlying discreteness—with the help of an FQXi grant for over $110,000—will fill in the remaining gaps and yield complete information about space and time all the way back to the Big Bang.

The Genealogy of Spacetime

That’s where Rideout and his cosmic family tree comes in. Family historians looking at an ancestral tree can work out which members could have influenced others: a mother affects her baby, for instance, but a baby could not have influenced his great-great-great-grandmother. In similar manner, Rideout and his colleagues are using "causal set theory" to deduce which spacetime points in the universe could have influenced each other.

It may lead to another paradigm shift

of the way we conceptualize the world

around us.

of the way we conceptualize the world

around us.

- David Rideout on quantum gravity and causal set theory.

Causal sets might also help unite the disparate views of the nature of time expressed in quantum theory and general relativity—which have thus far made it hard to gel the two into a unified theory of quantum gravity. In quantum mechanics, a universal clock drives the evolution of quantum systems. While developing relativity, on the other hand, Einstein showed that our idea of simultaneity—that two distant events can occur "at the same time"—is fundamentally flawed. Many gravitational theorists have therefore come to view the universe as a static block of spacetime, with no global "now," only a series of causally connected happenings. (See "The Crystallizing Universe" for more on the block universe.)

Rideout, however, believes that causal sets offer a view of time as a process that is occurring—as in quantum theory—with the future being formed as we live our lives and make decisions: "The growth dynamics of causal sets expresses the progression of time as a continual process of

The Universe? There’s an App For That

This growing tree, which currently has 10

Rideout is a contributor to a computing framework called Cactus, which he believes changes the paradigm for writing software: "Each scientist writes their own module, and then the framework takes care of making sure the modules work together." Though Cactus emerged more than a decade ago as a tool for numerical relativity, scientists now use it to study everything from quantum gravity to ocean coasts. "David’s been very involved as a co-developer" of Cactus’ open-source modules, says Gabrielle Allen, an associate professor of computer science at Louisiana State University, Baton Rouge.

THE BIGGEST TREE OF ALL

The causal relationships between different spacetime points, just a fraction

of a second after the Big Bang.

Credit: David Rideout, UCSD

When Rideout was his daughter’s age, he recalls, his interest in science was inspired by the prospect of far-flung exploration described in

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ANONYMOUS wrote on November 8, 2013

Baggott[Farewell to Reality: How Fairy-Tale Physics Has Betrayed The Search For Scientific Truth] and even more spot-on Unzicker-Jones[Bankrupting Physics: How Top Scientists Are Gambling Away Their Credibility] critiques shame physics’ shameless rock-star media-hype P.R. spin-doctoring veracity-abandoning touting sci-fi “show-biz” trending viral exacerbated by online social networks veritable diarrhea via proliferation of uncritical “pop-sci” science-writers where all is spectacle...

Baggott[Farewell to Reality: How Fairy-Tale Physics Has Betrayed The Search For Scientific Truth] and even more spot-on Unzicker-Jones[Bankrupting Physics: How Top Scientists Are Gambling Away Their Credibility] critiques shame physics’ shameless rock-star media-hype P.R. spin-doctoring veracity-abandoning touting sci-fi “show-biz” trending viral exacerbated by online social networks veritable diarrhea via proliferation of uncritical “pop-sci” science-writers where all is spectacle...

JONATHAN KERR wrote on August 17, 2012

Hello Eckard, our posts crossed. Thanks for your kind comments on my essay, I'll read yours. Best wishes, Jonathan

Hello Eckard, our posts crossed. Thanks for your kind comments on my essay, I'll read yours. Best wishes, Jonathan

JONATHAN KERR wrote on August 17, 2012

I should add that recently attempts have been made to escape from the block universe picture, which contradicts many other areas of our view of the world. These attempts include the 'crystallising block universe' and other ideas bringing in the wave function collapse of quantum theory, as a way of changing future to past. But they go on looking like fix-up attempts, not just because they tend to fail to address time dilation, but because in Minkowski spacetime, if you believe it to be completely...

I should add that recently attempts have been made to escape from the block universe picture, which contradicts many other areas of our view of the world. These attempts include the 'crystallising block universe' and other ideas bringing in the wave function collapse of quantum theory, as a way of changing future to past. But they go on looking like fix-up attempts, not just because they tend to fail to address time dilation, but because in Minkowski spacetime, if you believe it to be completely...

read all article comments