Search FQXi


Georgina Woodward: "On obtaining the singular, relative, measurement product it replaces the..." in The Present State of...

Steve Dufourny: "The paper of Wilczek of course is very relevant considering the idea about..." in The Noise of Gravitons

Georgina Woodward: "Material neuronal structure in which memory is encoded, physical records..." in Quantum Physics and the...

Steve Dufourny: "It is really how we consider the structure of the spacetime, and also how..." in The Noise of Gravitons

Aleksandr Maltsev: "Hi Georgina, Write a letter to" in Quantum Physics and the...

Georgina Woodward: "In quantum experiments using particles, there won't be swapping with a..." in The Present State of...

Aleksandr Maltsev: "I shortened the phrase Zeeya Merali  «Why does time flow….?    How..." in Time's Arrow, Black Holes...

Deserdi Chapas: "As reported on another forum, this first part of February 2023 has served..." in Alternative Models of...

click titles to read articles

The Entropic Price of Building the Perfect Clock: Q&A with Natalia Ares
Experiments investigating the thermodynamics of clocks can teach us about the origin of time's arrow.

Schrödinger’s A.I. Could Test the Foundations of Reality
Physicists lay out blueprints for running a 'Wigner's Friend' experiment using an artificial intelligence, built on a quantum computer, as an 'observer.'

Expanding the Mind (Literally): Q&A with Karim Jerbi and Jordan O'Byrne
Using a brain-computer interface to create a consciousness 'add-on' to help test Integrated Information Theory.

Quanthoven's Fifth
A quantum computer composes chart-topping music, programmed by physicists striving to understand consciousness.

The Math of Consciousness: Q&A with Kobi Kremnitzer
A meditating mathematician is developing a theory of conscious experience to help understand the boundary between the quantum and classical world.

February 5, 2023

The Cosmic Family Tree
Mapping the ancestral history of spacetime in an effort to unite quantum mechanics and general relativity.
by Kate Becker
FQXi Awardees: David Rideout
January 6, 2012
Bookmark and Share

University of California, San Diego
David Rideout, a physicist at the University of California, San Diego, is working on a family tree like no other—one spanning the genealogy of the universe. While the recent birth of Earth’s seven-billionth baby raised population alarms, the human family tree is only the tiniest sapling compared to Rideout’s effort, which has 10240 "leaves"—one for every point in spacetime—and grows with every passing moment. His hope is that this cosmic tree will help physicists bridge the gap between Einstein’s laws of gravity (which describe how huge masses warp spacetime) and the strange predictions of quantum theory (which governs the realm of the very small).

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 10-35 meters across, rather than a continuous fabric. In this case, spacetime would never shrink to infinitely small volumes.

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.
- David Rideout on quantum gravity and causal set theory.
The project goes like this: Start with a list of every point in space and time, all the way back to the Big Bang. Then add information about the relationship between every pair of points in the set. Luckily, there is only one thing you need to know about the relationship between those two points: Can A affect B? Thanks to Einstein’s theory of special relativity, we know that nothing can travel faster than the speed of light. This means that some pairs of points in spacetime will be forever disconnected. So, if the two points are suitably distant in time and space, the answer will be no, one could not have affected the other. If they are close enough, the answer is yes. A causal set is a roster of those "no" and "yes" answers for every pair of points in space and time.

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 becoming, like the unfolding of a flower or the growth of a tree," he says.

The Universe? There’s an App For That

This growing tree, which currently has 10240 leaves, might seem like an impossibly daunting subject for study, however. "The number of patterns of ancestry that are possible is enormous," says Sorkin, who first proposed using causal sets to probe quantum gravity, in 1987. That means that probing the mechanics of causal sets requires serious computing power. Fortunately, Rideout is distinguished by his facility with the sort of numerical computation that is done on high-performance computers.

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 causal relationships between different spacetime points, just a fraction
of a second after the Big Bang.

Credit: David Rideout, UCSD
Rideout’s computational approach also generates eye-popping images, such as the one shown on the right, which recently garnered third prize in the University of California, San Diego’s Art of Science competition. Rideout credits his artistically-inclined eight-year-old daughter Eleanor with selecting the image and its colors.

When Rideout was his daughter’s age, he recalls, his interest in science was inspired by the prospect of far-flung exploration described in Star Trek and other science fiction. Today, he hopes that his work with causal sets will prove that we do not have to travel through space to advance our understanding of the universe. "If we are on the right track, then the resulting theory of quantum gravity may have far-reaching consequences way beyond anything I may have imagined as a child," Rideout says. "It may lead to another paradigm shift of the way we conceptualize the world around us."

Comment on this Article

Please read the important Introduction that governs your participation in this community. Inappropriate language will not be tolerated and posts containing such language will be deleted. Otherwise, this is a free speech Forum and all are welcome!
  • Please enter the text of your post, then click the "Submit New Post" button below. You may also optionally add file attachments below before submitting your edits.

  • HTML tags are not permitted in posts, and will automatically be stripped out. Links to other web sites are permitted. For instructions on how to add links, please read the link help page.

  • You may use superscript (10100) and subscript (A2) using [sup]...[/sup] and [sub]...[/sub] tags.

  • You may use bold (important) and italics (emphasize) using [b]...[/b] and [i]...[/i] tags.

  • You may also include LateX equations into your post.

Insert LaTeX Equation [hide]

LaTeX equations may be displayed in FQXi Forum posts by including them within [equation]...[/equation] tags. You may type your equation directly into your post, or use the LaTeX Equation Preview feature below to see how your equation will render (this is recommended).

For more help on LaTeX, please see the LaTeX Project Home Page.

LaTeX Equation Preview

preview equation
clear equation
insert equation into post at cursor

Your name: (optional)

Recent Comments

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

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...

Jonathan Kerr (JK),

You did not comment on my comment on your essay:

"You concluded "block time and the accompanying picture must be false". You correctly realized that the block-time view is rooted in an observer-dependent perspective considering "the same event in the past for one observer but in the future for an other one"."

Didn't you read my essay ? I do not accept future spacetime as reality.

Eckard Blumschein

read all article comments

Please enter your e-mail address:
Note: Joining the FQXi mailing list does not give you a login account or constitute membership in the organization.