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FQXI ARTICLE

February 6, 2023

Melting Spacetime

To understand how spacetime might emerge from string theory, in the early cosmos, we need to heat up the equations, and thaw the space and time dimensions.

FQXi Awardees: Joanna Karczmarek

April 30, 2012

JOANNA KARCZMAREK

University of British Columbia

Technically, our perceived reality is a gigantic series of approximations: The tables, chairs, people, and cell phones that we interact with every day are actually made up of tiny particles—as all good schoolchildren learn. From the motion and characteristics of those particles emerge the properties that we see and feel, including color and temperature. Though we don’t see those particles, because they are so much smaller than the phenomena our bodies are built to sense, they govern our day-to-day existence.

Now, what if spacetime is emergent too? That’s the question that Joanna Karczmarek, a string theorist at the University of British Columbia, Vancouver, is attempting to answer. As a string theorist, Karczmarek is familiar with imagining invisible constituents of reality. String theorists posit that at a fundamental level, matter is made up of unthinkably tiny vibrating threads of energy that underlie subatomic particles, such as quarks and electrons. Most string theorists, however, assume that such strings dance across a pre-existing and fundamental stage set by spacetime. Karczmarek is pushing things a step further, by suggesting that spacetime itself is not fundamental, but made of more basic constituents.

High-Risk, High-Payoff

Having carried out early research in atomic, molecular and optical physics, Karczmarek shifted into string theory because she "was more excited by areas where less was known"—and looking for the building blocks from which spacetime arises certainly fits that criteria. The project, funded by a $40,000 FQXi grant, is "high risk but high payoff," Karczmarek says.

Spacetime itself would literally

disappear because the theory

got too hot.

disappear because the theory

got too hot.

- Joanna Karczmarek

That may change though. Nathan Sieberg, a string theorist at the Institute for Advanced Study (IAS) in Princeton, New Jersey, has found good reasons for his stringy colleagues to believe that at least space—if not space

Brane Power

The first inklings that physicists may need to look for something more basic than strings and space came for Karczmarek in 2002 when she read some research about branes. Originally added to the string framework in the 1990s, branes are objects that can sprawl across a number of dimensions, and thus have more versatility than one-dimensional strings. For instance, a D0-brane would look like a particle, a D1-brane would look like a string, while a D25-brane would look like a giant membrane spread in 25 different dimensions.

The mathematics needed for describing strings together with branes is much more complicated than when describing strings on their own. In particular, in the late 1990s, Seiberg and string-supremo Ed Witten, also at IAS, discovered that to mathematically describe higher dimension D-branes as a combination of lower dimensional D-branes you have to use what is known as non-commutative multiplication (arXiv:hep-th/9908142v3). Normal multiplication is commutative, that is, it doesn’t matter which order you multiply a series of numbers, you will always get the same answer. If you want to find the area of a rectangular room, for instance, you could either multiply the length by the breadth, or you could multiply its breadth by its length—both would give you the same result. "But in noncommutative geometry x times y is not the same as y times x," says Karczmarek. "It is a very different space and a lot of really weird things happens."

Since D-branes are intimately connected to spatial geometry in string theory, the discovery that they used non-commutative mathematics led Karczmarek and other string theorists to speculate that space may behave in a non-commutative manner too. In turn, Karczmarek argues, that suggests that there is something more fundamental—an underlying algebraic structure that is non-commutative—from which space inherits these strange properties.

If trying to uncover the structure that underpins space was not hard enough, Karczmarek has also been thinking about whether time too may be emergent. Seiberg agrees that it is likely that if one aspect of spacetime is emergent, then both are, given that time and space get mixed together in ways we don’t fully understand yet in black holes and at the big bang. However, he is quick to point out that, if true, this opens up a whole new realm of challenges. Having an emergent theory of time confuses even our most basic assumptions when constructing models and theories.

HOT TIME

Melting dimensions in string theory models could help explain

how space and time emerge in reality.

Credit: Siarhei Hashnikau

Karczmarek’s work is still in the exciting early exploratory stages where the project could develop in many unpredictable directions. To try and simplify the problem, she is looking at a basic object—a sphere—and trying to calculate how the time dimension is affected as the sphere changes phase. "We think that when you heat it up the sphere might melt," says Karczmarek. But it’s not only the sphere that would be destroyed in her model, she adds: "It’s the spacetime itself that would literally disappear because the theory got too hot."

Melting spacetime may sound like an imaginative step too far! But Karczmarek hopes that her model will lead to a theory that could make predictions about the early universe, when spacetime would have emerged as the hot dense cosmos cooled, leaving observable signs that could be spotted today. The hope is that—eventually—others may be able to take her work and use it to identify signatures for what to look for in the cosmic microwave background radiation—the afterglow of the big bang.

Scarily, the work may also force us to rethink what time actually is: "If one could construct a theory where the entire spacetime including the time were emergent, then you would discover that time is an illusion and have a more fundamental understanding of why it is there," says Karczmarek. "But that’s the holy grail of the field, and I wouldn’t be surprised it if takes fifty years to make any progress on it."

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AMRIT SRECKO SORLI wrote on March 10, 2020

BB cosmology is a fairy tale.

BB cosmology is a fairy tale.

GEORGINA WOODWARD wrote on July 3, 2016

Hi Durga,

You have written "With such a high level of confusion in physics, we are bound by a theory called relativity. " You seem to be implying that confusion binds us to relativity. I don't think that is so but rather it can not be discarded because it is useful, fits experimental evidence and only suffers from the lack of general realization that there is an inherent category error in the theory.

What do you mean by " We have to come out from the paradigm of Einstein ideas...

Hi Durga,

You have written "With such a high level of confusion in physics, we are bound by a theory called relativity. " You seem to be implying that confusion binds us to relativity. I don't think that is so but rather it can not be discarded because it is useful, fits experimental evidence and only suffers from the lack of general realization that there is an inherent category error in the theory.

What do you mean by " We have to come out from the paradigm of Einstein ideas...

DURGA DAS DATTA. wrote on July 3, 2016

With such a high level of confusion in physics, we are bound by a theory called relativity. Why we can not think of space as absolute empty but filled with an exotic fluid called ether or dark energy or even geavitoewthertons in certain pockets where universe exist. The total landscape may be infinite with patches of absolute nothing and patches of something. Scientists are getting signal from absolute void and how do we ignore that. Balloon inside balloon theory what we call parallel...

With such a high level of confusion in physics, we are bound by a theory called relativity. Why we can not think of space as absolute empty but filled with an exotic fluid called ether or dark energy or even geavitoewthertons in certain pockets where universe exist. The total landscape may be infinite with patches of absolute nothing and patches of something. Scientists are getting signal from absolute void and how do we ignore that. Balloon inside balloon theory what we call parallel...

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