Harvard physicist Andrew Strominger is among the pioneers of celestial holography, a new branch of physics he calls “unflinching” in its rootedness in reality.
The physics subfield of celestial holography is so new that Harvard physicist Andrew Strominger can clearly remember how it got its name.
A couple of years ago, he and a small group of colleagues and students were developing what he calls “a different kind of enterprise” to understand gravity and cosmic phenomena. Their research aimed to describe four-dimensional space using a two-dimensional theory on the edge of space – ultimately a much simpler framework than string theory and other mathematically complex approaches.
The subfield they were excitedly developing needed a name, and two contenders were in the running:
Flat holography, because it uses a 2D (flat) theory on the edge of space.
Celestial holography, because the theory posits this edge of space as a celestial sphere where light rays end up.
Each potential name had its supporters and detractors. Strominger preferred flat holography, because it describes the scientific approach pretty succinctly.
“Then I asked my wife, who is not a physicist,” recalls Strominger, referring to Harvard genetics professor Stirling Chuchman. “She said it should be celestial holography. She said ‘flat holography sounds like something you wouldn’t want to work on, but celestial holography sounds like something interesting people would want to work on.’ So I switched my vote.”
It turns out she was right on both counts – the name stuck, and the still-burgeoning field is growing quickly with a research community that now numbers in the hundreds (as opposed to the small team investigating it just a few years ago). What excites Strominger is the breadth of new ideas and perspectives these researchers are bringing to celestial holography – and the new directions celestial holography promises for their respective fields.
“It’s a very encouraging thing that we have connected with people studying twistor theory, soft theorems, string theory,” Strominger says, sitting beside a blackboard at Perimeter Institute in Waterloo, Canada, where he is teaching at a new celestial holography summer school.
“Celestial holography is not bigger than everything else, but it’s hooking things together in a new way.”
The field is still so novel that Strominger and colleagues – Nima Arkani-Hamed of the Institute for Advanced Study, Monica Pate of New York University, and Ana-Maria Raclariu of King’s College London and Perimeter Institute – published a kind of introductory handbook to the field in 2021. The following year, Sabrina Pasterski, a former PhD student of Strominger’s at Harvard, founded the Celestial Holography Initiative at Perimeter Institute, described on its webpage as “a concerted effort to tackle the problem of uniting our understanding of spacetime with quantum theory by encoding our universe as a hologram.”
To explain the basics of celestial holography, Strominger chalks-up a simple drawing on the blackboard. On the right is a circle inhabited by smaller circles – a representation of the celestial sphere that serves as the holographic plate; on the left, a rectangle representing the holographic image created by the plate.
The drawing depicts a model that, instead of dealing with the complicated events of our universe, uses a simpler 2D theory on the celestial sphere to describe those phenomena:
Unlike the established theories rooted in Anti-de Sitter (AdS) space, which is negatively curved and well-suited to string theory (but not reflective of our basically flat universe), celestial holography seeks to apply the holographic principle to flat space.
Of course, the notion that our universe is a hologram cast inward from a celestial sphere is not really simple, in the typical definition of the word. It defies our intuition and requires incredibly precise equations to accurately comprehend – but so do many of the phenomena explained by physics, he argues, which doesn’t make them any any less real.
Reality is on Strominger’s mind a lot these days. Celestial holography is, in part, Strominger’s effort to bring his research back to reality.
Although Strominger is one of the leading figures in string theory – he shared the prestigious Breakthrough Prize in 2017 for his “transformative advances in quantum field theory, string theory, and quantum gravity” – he sees celestial holography as a promising, data-driven direction “without all the stringy baggage.”
That baggage, he says, is a symptom of string theory’s “top-down” approach, which creates elegant mathematical models and then attempts to apply them to real phenomena. Celestial holography, on the other hand, is a “bottom-up” approach based on real phenomena and our best mathematical understanding of them.
“We don’t want to assume anything we don’t know to be true,” explains Strominger. “We want to know how much of this principle we can reconstruct in flat space from the bottom up. It is unflinching in demanding a connection to the real world. ”
To that end, celestial holography researchers have predicted new observable effects of spin-memory that can be tested experimentally by LIGO, the gravitational wave observatory. While it may be some years before LIGO can test it, the development of a testable prediction is something Strominger says is a promising sign that celestial holography is unencumbered by stringy baggage.
“We’re not assuming in our mathematical investigations anything that isn’t a tested law of physics,” says Strominger.
“In string theory, which I’ve fruitfully spent most of my career working on, one studies the many profound lessons of strings on how they manage to reconcile quantum mechanics and gravity, and tries to apply them to the real world. I’ve had a lot of lessons, and I’m ready to apply them. I’m ready to get out of school and be in the real world.”
