Ever wonder what theoretical physicists actually do? In honor of the 100th anniversary of Albert Einstein’s theory of general relativity, Brandeis University writer Leah Burrows sat down with some to find out.

Theoretical physics is a lot like sex, Nobelist Richard Feynman once quipped. “Sure, it may give some practical results, but that’s not why we do it.”

The prevailing stereotype outside—and inside—the sciences is that theoretical physicists have their gaze firmly fixed on their navels and play in a sandbox of their own creation.

It’s time to throw that stereotype out the window (and note how it falls to Earth with constant acceleration. Thanks, theoretical physics!)

Sure, theoretical physics can get weird, and some theories are pretty far out, but inquiry is always driven by a hunger to understand the universe fundamentally.

The physicists in Brandeis’ High Energy and Gravitational Theory Group research bizarre principles like holography, which postulates that all the information in the universe is stored on a two-dimensional surface, and we are mere projections of that information. And then there’s quantum entanglement, which even Einstein called “spooky.”

But at the core of the group’s research is a simple question: What is space?

Einstein described the way space is connected to time and how it interacts with mass. But he never theorized what space is, how it’s formed or what it’s made of.

“Since Einstein, our questions have gotten bigger and deeper,” says Matthew Headrick, assistant professor of physics. “We want to figure out the nature of space.”

### Two major theories

The answer lies somewhere between two pillars of modern theoretical physics—general relativity (GR, which describes gravity) and quantum field theory (QFT, which describes, among other things, particle physics). These fundamental theories describe two very different aspects of our universe and are written in different mathematical languages.

“Shockingly, in certain cases, theorists have discovered that these two very different theories are actually secretly the same,” Headrick says. “Between GR and QFT, there is some kind of one-to-one map. We know some of the shared points but we’re still in the dark about many others.”

This one-to-one map is holography and it represents GR, which lives in ordinary three-dimensional space, by a QFT living on a two-dimensional surface—just like a hologram.

Essentially, Headrick and other theoretical physicists are building a Rosetta Stone—a bilingual dictionary of sorts—using holography, general relativity, and quantum field theory. This translational tool will expose how GR and QFT are connected to each other and how to build a new language that obeys the properties of both GR and QFT.

Word by word, Headrick and his colleagues are testing and building a framework of conjectures.

“If you have confidence that a conjecture obeys the properties it needs to obey, you can enter it into the dictionary,” Headrick says. “Each small entry tells us something more about space.”

### Bubble wand

One conjecture Brandeis theorists pioneered has to do with the relationship between the geometry of space and the quantum information it contains.

Mathematically, certain areas in curved space contain minimal surfaces—a surface that minimizes its area. Dip a wand into soapy water and the soap film will stretch perfectly flat across the shape of the wand. This is a minimal surface.

“If the area of that soap film is expressed in fundamental units, it tells us about the quantum entanglement in the QFT,” says Headrick.

In other words, Headrick and his colleagues use the mathematical language of general relativity—geometry—to extrapolate a quantum property. That calculation, in turn, provides new information about how the two languages are interconnected.

That idea also provides a clue to the nature of space.

“It suggests that, fundamentally, the space that we live in and take for granted is stitched together out of quantum entanglement,” Headrick says.

The next step is to figure out why.

### Patience

The answers to these and other questions will, with any luck, give researchers the words and syntax to compose a theory of gravity in the language of quantum mechanics. It’s the Holy Grail of modern theoretical physics: a theory of quantum gravity.

But what does this have to do with reality? Richard Feynman may not have cared about the practical results of theoretical physics, but some do.

Be patient, Headrick says.

It took Einstein 10 years to develop general relativity, and it took physicists another 40 years to understand the black holes it predicted. Now, 100 years after the theory’s publication, relativity is ubiquitous in our daily lives. Without an understanding of it, for example, we wouldn’t have GPS.

But more important than the inventions a theory spurs, is the knowledge a theory advances, Headrick says.

“Einstein, Feynman, and others profoundly changed our understanding of nature,” he says.

And that’s why theoretical physicists do it.

*Source: Brandeis University*