RECENT ARTICLES

Why do we remember the past and not the future? Untangling the connections between cause and effect, choice, and entropy.

Philosophers, physicists and neuroscientists discuss how our sense of time’s flow might arise through our interactions with external stimuli—despite suggestions from Einstein's relativity that our perception of the passage of time is an illusion.

A devilish new framework of thermodynamics that focuses on how we observe information could help illuminate our understanding of probability and rewrite quantum theory.

An unusual approach to unifying the laws of physics could solve Hawking's black-hole information paradox—and its predicted gravitational "memory effect" could be picked up by LIGO.

Objective reality, and the laws of physics themselves, emerge from our observations, according to a new framework that turns what we think of as fundamental on its head.

FQXI ARTICLE

January 19, 2020

Wrinkles in Spacetime

Searching for defects in the fabric of the cosmos could help physicists home in on the correct theory of quantum gravity.

FQXi Awardees: Sabine Hossenfelder

March 31, 2016

Sabine Hossenfelder

Frankfurt Institute for Advanced Studies

Her characteristic sharpness and no-nonsense attitude may well give Hossenfelder just the right qualities for discerning which research avenue is best for physicists to follow, when faced with many competing options. In particular, Hossenfelder is searching for a good theory of quantum gravity—a framework that would bring together Einstein’s theory of gravity, general relativity, which describes how cosmic bodies move, and quantum theory, which governs the behaviour of particles on the smallest scales. "Some people work on problems that I don’t think are problems at all," she says. "But the question of how to find a consistent theory that combines gravity with quantum field theory is one that everyone agrees is a problem—and one that has to have a solution."

A working theory of quantum gravity is needed to explain events in the very early universe and within the core of black holes, which are both instances where strong gravity is confined in a small region. So far, describing what happens in these regimes has defied physicists. "We know the theories we have right now are inconsistent—when you combine them the answer is nonsense," she says. "It is clearly not how nature works, there has to be a better answer."

Shooting Down Theories

The trouble is that there are many contenders for that better answer. Hossenfelder—who has worked on black holes, particle physics beyond the standard model, cosmology, quantum foundations and, most recently, condensed matter physics—is hoping she can shoot some of them down, leaving a smaller range of possibilities to pursue further. The key to unlocking these mysteries of quantum gravity could involve investigating whether spacetime is continuous or discrete, when you zoom in to look at it closely. This is a question that she will spend the next two years investigating thanks to an FQXi grant of $126,000.

People like to talk

about ’atoms of

spacetime.’

about ’atoms of

spacetime.’

- Sabine Hossenfelder

Hossenfelder is working on identifying experiments that may be able to probe whether spacetime is indeed discrete, allowing physicists to rule out rival theories that have it pinned as continuous. She’s particularly focussing in on potential imperfections in the discreteness. "If spacetime is not fundamentally continuous, then the smoothness we use in general relativity must have defects in it due to quantum effects in its discrete structure," she says. It is a bit like the defects you find within the lattice of a crystal such as a diamond—the underlying structure does not repeat perfectly. And if a particle travelling through spacetime encounters one of these defects then its energy and momentum will be altered. It is these changes that Hossenfelder is hoping to find experimental evidence for. (See also, "Journeying Through the Quantum Froth.")

Defect Detectors

Particles traveling from distant quasars could reveal whether spacetime is

continuous or discrete.

Credit: ESO/M. Kornmesser

According to Frans Klinkhamer, a theoretical physicist from the Karlsruhe Institute of Technology in Germany, Hossenfelder is taking a "healthy approach" to the problem. He also adds a word of caution, however. "How much progress can be made remains to be seen," he says. But if successful, the pay off would be huge. Measurements that reveal the size and spacing of the defects could tell us much about the underlying theory of quantum gravity, says Klinkhamer. "It would show how classical spacetime emerges from quantum theory."

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!

function ValidatePostText_main () {
form = document.addPostForm_main;
var recaptcha = $("#g-recaptcha-response").val();
if (recaptcha === "") {
event.preventDefault();
alert("The reCaptcha Box below must be checked before you submit the form");
}
else if (form.postText_main.value == '') {
alert ("The post contains no text");
return false;
}
else {
return true;
}
}

**Your name:**
(optional)

Recent Comments

read all article comments

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 (10

^{100}) and subscript (A_{2}) 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

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

Attachments
[hide]

You may optionally attach up to two documents to your post. To add an attachment, use the following feature to browse your computer and select the file to attach. The maximum file size for attachments is 1MB.

Once you're done adding file attachments, click the "Submit New Post" button to add your post.

You may optionally attach up to two documents to your post. To add an attachment, use the following feature to browse your computer and select the file to attach. The maximum file size for attachments is 1MB.

Once you're done adding file attachments, click the "Submit New Post" button to add your post.

WAYNE wrote on May 8, 2018

It is good that FQXi is supporting work to find detectable signatures of these 3D+t defects. Any better ways to observe and usually exclude hypotheses is beneficial, especially if they are inconsistent or allow infinities or, conversely, singularities. These are the bane of physical theories anyway, and since the community demands experimental disproof, well so be it.

I'd also accept that a comprehensive theory must meet necessary conditions, such as consistency across all physical...

It is good that FQXi is supporting work to find detectable signatures of these 3D+t defects. Any better ways to observe and usually exclude hypotheses is beneficial, especially if they are inconsistent or allow infinities or, conversely, singularities. These are the bane of physical theories anyway, and since the community demands experimental disproof, well so be it.

I'd also accept that a comprehensive theory must meet necessary conditions, such as consistency across all physical...

QUANTUM ANTIGRAVITY wrote on April 17, 2017

EXPERIMENTAL quantum Anti-gravity — https://quantumantigravity.wordpress.com

I have made a theoretical as well as an empirical scientific discovery

of quantum gravity and quantum antigravity.

Present day quantum gravity theories suffer from

too many mathematical space dimensions, and from

too few conclusive experimental results.

My hypothesis is simple, clear,

and subject to easy empirical verification :...

EXPERIMENTAL quantum Anti-gravity — https://quantumantigravity.wordpress.com

I have made a theoretical as well as an empirical scientific discovery

of quantum gravity and quantum antigravity.

Present day quantum gravity theories suffer from

too many mathematical space dimensions, and from

too few conclusive experimental results.

My hypothesis is simple, clear,

and subject to easy empirical verification :...

STEVE DUFOURNY wrote on July 6, 2016

Hello,theproblem isthat gravitons are bosons.And that we have a problem of equivalence.This weakest quantum force cannot be bosonic.That is not really dterministic universally speaking.You think really that if an infinite entropy above our simple human understanding ,above our physicality,has created an universewith intrisic laws.I am doubting that the special relativity ,heat and thermo and standard model are the only one Tools.it is not really logic philosophically speaking, nor...

Hello,theproblem isthat gravitons are bosons.And that we have a problem of equivalence.This weakest quantum force cannot be bosonic.That is not really dterministic universally speaking.You think really that if an infinite entropy above our simple human understanding ,above our physicality,has created an universewith intrisic laws.I am doubting that the special relativity ,heat and thermo and standard model are the only one Tools.it is not really logic philosophically speaking, nor...

read all article comments