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FQXI ARTICLE

March 18, 2018

Journeying Through the Quantum Froth

Are cosmic rays revealing the quantum nature of spacetime? Could *theories of (not) everything* help solve the puzzle of quantum gravity? The architect of *doubly special relativity* thinks so.

FQXi Awardees: Giovanni Amelino-Camelia

August 8, 2009

THE FERMI GAMMA RAY TELESCOPE REVEALS

BRIGHT EMISSIONS IN THE SKY

Is it also uncovering features of quantum spacetime?

Credit: NASA/DOE/Intl. LAT Team

Lucky for him, and, in many ways, lucky for us, because almost 30 years later Amelino-Camelia, at La Sapienza University in Rome, Italy, is dedicated to pursuing answers to foundational questions. Armed with a $65,000 grant from FQXi, he is currently making a run at redefining the way physicists attack perhaps the most vexing and elusive puzzle of all, the problem of quantum gravity. Many lament that experimental tests of quantum gravity lie way beyond reach. However, Amelino-Camelia believes that cosmic rays may already be revealing clues about the frothiness of the fabric of the universe, and he is using these hints to probe the quantum nature of spacetime—one step at a time.

Some believe we will

be smart enough to solve

everything in one shot.

Take my word for it, we

are not smart enough.

be smart enough to solve

everything in one shot.

Take my word for it, we

are not smart enough.

- Giovanni Amelino-Camelia

That doesn’t mean that Amelino-Camelia thinks that searches for a theory of everything should be abandoned. In fact, the majority of the simple models exploring portions of the quantum gravity problem that he works on were inspired by results obtained in the development of loop quantum gravity.

"Even assuming my concerns are correct—and only time will tell—research looking for a full solution of the quantum gravity problem would be very valuable because it provides guidance for the development of meaningful theories of

Doubly Special Relativity

Okay, we now have a decent understanding of what Amelino-Camelia is

The concept gels well with a theory that Amelino-Camelia introduced in 2000, which he dubs "

Instead of trying to fit this notion of a quantum spacetime straight into a larger quantum-gravity model, Amelino-Camelia argues that it will be fruitful to first fully examine its implications. Taking smaller bites out of the quantum gravity puzzle, in this way, gives physicists manageable chunks that they can chew on more easily.

Crucially, this opens up the possibility of examining each portion thoroughly and subjecting each portion to experimental scrutiny, explains Amelino-Camelia. Some aspects of the puzzle can be tested using today’s technology, while others will hopefully be testable in the coming years, as new technologies are developed.

Probing Spacetime

GIOVANNI AMELINO-CAMELIA

La Sapienza University

Cosmic rays have already helped to rule out some of the simplest quantum pictures of spacetime. Those models predicted that the maximum possible energy of cosmic rays, known as the GZK cutoff, would be significantly higher than currently thought. However, data recently gathered by the Pierre Auger cosmic-ray observatory does not support this. "In order to get to the level of probing more promising pictures, we still have some way to go," says Amelino-Camelia. "Although it is something we will realistically start doing within a few years."

In addition, some quantum-spacetime models predict a particular relationship between the speed of photons in gamma rays and their energy. There have already been tantalizing hints of these effects. In 2007, for instance, the MAGIC gamma-ray telescope collaboration based on La Palma in the Canary Islands announced that they had measured a 4-minute time difference between the arrival of high and low-energy gamma rays released at the same time in a flare from the Markarian 501 galaxy, some half a billion light years away (

It is much more appealing to

find partial answers that close

the door on a few open

issues, but open the door

on many more puzzling issues.

find partial answers that close

the door on a few open

issues, but open the door

on many more puzzling issues.

- Giovanni Amelino-Camelia

John Stachel, a physicist at Boston University, Massachusetts, admires this drive to produce a falsifiable theory. "Amelino-Camelia grew impatient with the exclusively theoretical nature of most work on quantum gravity," he says. "The smallness of most predicted effects threatened to make the field more an area of speculative scholasticism than a part of science."

But it’s not just about being able to scrutinize the theory using experiments and observations. Ultimately for Amelino-Camelia,

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LE VAN CUONG wrote on November 21, 2010

I think that Einstein's Special Relativity should correct an invariability of a light velocity, because if the measurement of space and time change from km and s to km' and s', then light velocity will also change from c-300,000 km/s to c'=300,000 km'/s'. We must confirm that c=300,000 km/s is different from c'=300,000 km'/s' because unit of measurement of the velocity: km/s is different from unit of measurement of the velocity: km'/s'. ( of which space is denoted by km', km' and time is denoted...

I think that Einstein's Special Relativity should correct an invariability of a light velocity, because if the measurement of space and time change from km and s to km' and s', then light velocity will also change from c-300,000 km/s to c'=300,000 km'/s'. We must confirm that c=300,000 km/s is different from c'=300,000 km'/s' because unit of measurement of the velocity: km/s is different from unit of measurement of the velocity: km'/s'. ( of which space is denoted by km', km' and time is denoted...

DOV HENIS wrote on April 23, 2010

Theory Of Everything Without Strings Attached.

Embarassingly Obvious And Simple.

See the signature links.

Life's Genesis Was Not Cells But First Gene's Self Reproduction.

Life Is Just Another Mass Format.

Since July 5 1997 I have developed and been proposing the following scenario of life's genesis:

* Life's genesis was not cell(s), but the self reproduction of yet uncelled ungenomed gene(s).

* There was NOT any "Pre-History Of Life" evolving in...

Theory Of Everything Without Strings Attached.

Embarassingly Obvious And Simple.

See the signature links.

Life's Genesis Was Not Cells But First Gene's Self Reproduction.

Life Is Just Another Mass Format.

Since July 5 1997 I have developed and been proposing the following scenario of life's genesis:

* Life's genesis was not cell(s), but the self reproduction of yet uncelled ungenomed gene(s).

* There was NOT any "Pre-History Of Life" evolving in...

WILLIAM H FOEHRINGER wrote on January 29, 2010

Is there data that indicates that the time delay is proportional to distance in a simple relationship? Is the time delay different when looking in different directions but the same approximate distance away? Teasing out these variables might be helpful. Bill Foehringer

Is there data that indicates that the time delay is proportional to distance in a simple relationship? Is the time delay different when looking in different directions but the same approximate distance away? Teasing out these variables might be helpful. Bill Foehringer

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