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February 6, 2023

Much Ado About Nothing
Does the vacuum regenerate itself to fill the gaps as spacetime is pulled apart? Could a growing vacuum explain dark energy?
by Bob Swarup
FQXi Awardees: Ted Jacobson
May 8, 2009
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University of Maryland
Ted Jacobson is a man who worries about nothing. More precisely, he worries about the vacuum, its place in quantum cosmology and how it might grow as the universe expands.

Underlying this is the suspicion that though space and time are assumed to be continuous in standard physics, this is only an approximation. At a more basic level, the fundamental fabric of our universe may be a patchwork of discrete units, just as what appears to be continuous matter is actually composed of discrete atoms. This, however, presents a problem. According to Jacobson, as the universe expands, the patches it’s made from would be spread apart. The only way to explain why the mirage of a spacetime continuum doesn’t vanish as this happens, is if new patches of spacetime are created to fill the gaps, he argues.

"My hunch is that this ability of the system itself to grow new parts is a fundamental aspect of the nature of spacetime and the vacuum that we have so far missed in physics," explains Jacobson, a physicist at the University of Maryland, College Park.

Armed with a $82,127 grant from the Foundational Questions Institute, Jacobson is now working on that hunch to address two basic questions: How can we modify the laws of physics to allow for this growth, and could we possibly see signs of this underlying discreteness in weird phenomena such as dark energy?

This ability of the
system to grow new
parts is a fundamental
aspect of the nature
of the vacuum.
- Ted Jacobson
Asking "what if?" has always been second nature to Jacobson. As a child, he often annoyed his unscientifically-minded family with his persistent curiosity. He can trace his confidence in the pursuit of the unknown to a teacher in the third grade of elementary school, who assured him that questions were the most important things and that he should never stop asking them. The lesson was only reinforced later in high school and channeled into a future career when he took a physics class. He fell in love with its penetrating insights into how the world works.

Even then, the building blocks of spacetime was a topic uppermost in his mind as he learned more about quantum mechanics and the notion that certain physical properties are packaged into discrete bundles. "I learned near the end of the year about the quantization of angular momentum and energy, and it struck me as very strange that these could be quantized, when others entering into the formulae—such as velocity, mass and distance—were not," remembers Jacobson. "It seemed clear that space and time themselves should be quantized and I tried to show that the quantization of space and time could explain why nothing can go faster than light. It probably didn’t make much sense, but it intrigued me."

A PhD at the University of Texas Austin with Cecile DeWitt-Morette, an expert in the mathematics behind the method of path-integral quantization, and a spell as a Fulbright Scholar at the Technion in Haifa, Israel with Larry Schulman, only strengthened his belief that quantization of spacetime was a fundamental question.

Infinite Wiggles

Jacobson’s research over the years has focused on several topics, including the problem of quantization of General Relativity, the thermodynamics of black holes, and the testing of relativity. "I have explored many different research areas, but the discreteness of spacetime is one theme that has always remained," says Jacobson. "It is this theme that underlies my current work on the growth of the vacuum."

As the universe expands, new vacuum wiggles could be created to fill the gaps.
The word "vacuum" often connotes nothingness," an absence of everything. However, physicists do not believe that "empty space" is truly empty. In the case of an electromagnetic wave such as light, for example, a completely dark room does not mean that there are no electric and magnetic fields, as one might naively assume. Rather, the fields are still there fluctuating and the darkness is due to the absence of any electromagnetic excitations above the absolute minimum allowed by the uncertainty principle of quantum mechanics.

"The fields are always there, wiggling around," explains Jacobson. "The vacuum corresponds to the lowest energy state of that wiggling."

These "wiggles" can be classified into their wavelengths, and standard theory holds that all wavelengths are present—but that’s where the trouble begins. It means that there should be an infinite amount of wiggling in any region of space, no matter how small—a fact that has led to theoretical difficulties. For instance, it predicts that interactions between particles are infinitely strong, and that the entropy of a black hole is infinite.

Quantum physicists have devised ways to dodge this problem and postpone its resolution to later developments in the theory; but Jacobson believes it remains a fundamental flaw. However there’s a way around it. If spacetime is discrete, rather than continuous, then there is a minimum wavelength that can exist in the vacuum.

Filling in the Gaps

While others before Jacobson have explored what a discrete spacetime might look like, his research is unique because he is focussing on the implications for the quantum vacuum and its growth over the lifetime of the universe. Jacobson reasons that the expansion of the universe must stretch wavelengths—so any minimum wavelength would get longer. This potentially creates a huge problem as the universe has expanded significantly over its lifetime; in just the early moments of the universe a sharp burst of accelerated expansion, known as “inflation” multiplied its size innumerably many times over. This would have exposed the effects of discreteness because we’d only see very large minimum wavelengths, unless somehow new short wavelengths were created as part of the expansion process, to "fill in" for the ones that have been stretched, says Jacobson.

Jacobson is working in
an area that merits deeper
study of the quality
that he is sure to provide.
- David Finkelstein
According to Jacobson, this would amount to the creation of new elements of the physical system, an idea that transcends the usual framework of physics in which the fundamental parts of a system are only rearranged as time goes on, not created. But the notion is a dangerous one. "It’s a challenge to modify the fundamentals of the vacuum without losing control of the theory," says Jacobson. "I am interested in looking at the problem in two different ways," he says, "one is to explore how to allow for such growth, and the other is to look for consequences that might show up in observable phenomena." Prime candidate phenomena would be the primordial cosmological fluctuations, and the ubiquitous but mysterious dark energy that may be causing the current accelerated expansion of the universe.

Galaxy Cluster Abell 85 is one of several used to track dark energy
Credit: NASA/CXC/SAO/A. Vikhlinin/SDSS
"Many physicists these days like to casually throw around the idea that space and time are discrete," says Brendan Foster at the Institute for Theoretical Physics in Utrecht, Holland. "Very few of them, though, have seriously examined whether their theories deliver acceptable results in the continuum limit, or even what the continuum limit means in the context of their theory. Ted’s work addresses this overlooked issue."

David Finkelstein at the Georgia Institute of Technology in Atlanta agrees. "The assumptions that you do not state are the ones that get you in the end," he says. "He is working in an area that merits deeper study of the quality that he is sure to provide."

Jacobson is hopeful. "The work is just at the beginning," he says. "I find that many colleagues are quite supportive and open minded to see others exploring nonstandard ideas, even if they don’t share the same intuitions or hunches about how the world might work."

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