It is a windy day in London when I meet physicist on the sidelines of a conference 鈥 which turns out to be a problem. We had agreed to chat in a tent, but the canvas is flapping so loudly that we can barely hear each other. In the end, we retreat to a nearby church hall.
But then again, maybe this ad-hoc venue is strangely fitting. As a choir sings tunefully about heaven a few doors down, Pasterski tells me that her work is all about the celestial sphere 鈥 and has something in common with the Christian concept of the Holy Trinity, the idea that God is three persons in one.
鈥淒uality is almost like the Trinity,鈥 Pasterski tells me, referring to one of the principles at the core of her work. 鈥淚鈥檓 not religious, but聽I was raised Catholic.聽It鈥檚 the sense that there can be different manifestations of something equivalent.鈥
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It isn鈥檛 the introduction I expected to Pasterski鈥檚 research, a subtle and complex idea known as celestial holography, which seeks to show how two separate descriptions in physics can be equivalent. But given that she is often called 鈥渢he next Einstein鈥 in the press in reference to her fierce intellect, I am excited to try to wrap my mind around her work, regardless of the venue.
It helps to start with the two key pillars of physics. On the one hand, there is general relativity, Albert Einstein鈥檚 theory of gravity, the key ingredient of which is a smoothly curving fabric of space-time. On the other, quantum theory wonderfully explains the world of particles and envisages reality as not smooth at all, but instead coming in chunks with discrete amounts of energy. Suffice it to say that both are fundamentally incompatible.
The dearest wish of many a physicist would be to find a way to unify the two, and so produce a theory of quantum gravity. That is what Juan Maldacena was attempting back in 1997 when he proposed an exciting kind of duality.
Imagine a three-dimensional cylinder of space-time with a two-dimensional surface. Maldacena constructed a model like this where, inside the volume of the cylinder, you have a description of space, time, particles, gravity 鈥 the lot 鈥 but the surface is purely quantum. But here was the crucial point: he showed that the two descriptions of the model 鈥 the surface and the internal volume 鈥 were equivalent. They were a duality, two inter-translatable descriptions of the same thing. Put another way, it suggested that the universe was like a hologram: that the reality we live in could merely be a projection of a simpler, lower-dimensional foundation.
All very exciting. The only trouble was that Maldacena鈥檚 idea 鈥 which came to be known as the AdS/CFT correspondence 鈥 worked for only a specific type of curved space-time. And that isn鈥檛 the same as the 鈥渇lat鈥 kind we really live in. Inevitably, physicists have wondered if the trick could be made to work for the real universe. It has been devilishly hard to pull off, but that is what celestial holography promises.
Meeting Pasterski, it is obvious she has a formidable brain. She speaks so quickly, it鈥檚 as if time is running out. And as she speaks, she gesticulates, energy levels sky-high as sharp gel nails scratch the air between us. My impression is that she processes information faster than the average person. I begin by asking how she feels about being labelled the next Einstein? 鈥淚t鈥檚 bullshit,鈥 she says, smiling. 鈥淚 don鈥檛 think I deserve it, first of all. The positive I try to turn it into now is that hype gets you a platform.鈥
This energy appears to have driven her since she was a child. At a young age, she told her parents she wanted a flying broomstick; in response, they got her flying lessons. At 12, she began to build her own plane from a kit, and by 16, she could fly it. She dreamed of working for Blue Origin, Virgin Galactic or SpaceX, viewing the founders of those companies as heroes. As a young adult, her version of rebellion was to then turn against this idea 鈥 鈥測ou鈥檙e basically just helping them build their thing鈥 鈥 and decide to major not in aerospace engineering, but in physics at the Massachusetts Institute of Technology instead.
I wonder if these tech founders are still her heroes? She will always appreciate Jeff Bezos, she says. He encouraged her as a child, penning a 鈥渘ice letter鈥. But when it comes to Elon Musk, she is more circumspect, going back and forth. 鈥淎 lot more people have seen [his] flaws, and yet he has done a lot of cool stuff,鈥 she says. But the fact that 鈥渟omeone like him can do all that cool shit鈥, she says, raises the question, 鈥淲hy can鈥檛 we?鈥
Pasterski began to lay some of the mathematical groundwork for celestial holography during her PhD with at Harvard University in around 2018, but she now works at the Perimeter Institute in Ontario, Canada. Pasterski鈥檚 hypothesis starts with particle collisions. She imagines the debris spraying off in all directions and that each piece of quantum shrapnel can be marked as a point on a sphere infinitely far away, along with information such as energy. Pasterski thinks of this sphere as being essentially like the night sky. 鈥淵ou see the stars, you think of them as being infinitely far away, like a sphere around you,鈥 she says. 鈥淭hat 2D sphere is the celestial sphere.鈥
Instead of wrestling with the mess in the middle, celestial holography reconstructs the entire universe from the pattern written around the edge. You can think of it like a new, improved version of Maldacena鈥檚 idea, except with a sphere that, this time, corresponds to the real space we live in. At least, almost. 鈥淚 also cheat,鈥 admits Pasterski. We know that real space is expanding ever-faster, but hers isn鈥檛.
Over the past few years, Pasterski has been developing ways to translate between the boundary of the sphere and real space. Initially, the focus was on modelling how single particles behave, but she and Justin Kulp at Stony Brook University in New York have recently taken the big step of showing .
She has also been exploring the concept of entanglement, the mysterious quantum process in which two particles can be connected so that if you measure one, you can immediately infer something about its partner 鈥 even if they are very far apart. In AdS/CFT, physicists have shown that entanglement between quantum particles at the boundary can directly produce the fuller dimensional world we experience. In celestial holography, things are more complicated. 鈥淭hat鈥檚 really well understood in AdS/CFT,鈥 says Pasterski. 鈥淚t鈥檚 harder to generalise to flat space.鈥
One reason it鈥檚 so tricky is that it is hard to keep the boundary part of celestial holography free of gravity 鈥 it isn鈥檛 a neat wall of a box or a cylinder like it is in Maldacena鈥檚 theory, it is a sphere infinitely far away. This makes the mathematics messier. But Pasterski and her colleagues have made a start in working it out, in particular, the connection between the entanglement present on the boundary of the sphere and the geometry of flat space-time. In the models they studied, the flat-space region was associated with reduced entanglement in the boundary compared with what you get when you use AdS/CFT.
Pasterski refers to the list of constraints, rules and equations she has developed as a 鈥済rammar鈥 for celestial holography. While work on the dictionary translating the four-dimensional physics into a two-dimensional boundary description is progressing rapidly, there are still things missing.
It is heady stuff and I am keen to know if any of this is testable. Potential theories of quantum gravity are notoriously difficult to subject to experiment, sometimes requiring particle colliders so powerful they simply cannot be built. However, Pasterski has thought of other possible ways to probe celestial holography.
Although the hypothesis typically deals with particle collisions, it is more general than that. 鈥淚t could be that the collision is black holes,鈥 says Pasterski. When black holes collide, they give rise to ripples in space-time called gravitational waves. 女生小视频s believe that when these waves pass through regions of the cosmos, they leave behind small but permanent distortions in space-time.
Around a decade ago, Pasterski and her colleagues developed an extension to this idea known as the . This proposes that the distorted space-time also carries an imprint of the gravitational wave鈥檚 angular momentum or rotation. And this, in turn, can be directly translated to one of Pasterski鈥檚 main hypotheses for how things work on the boundary, which potentially gives a link between something in our 4D universe and celestial holography. Excitingly, space-based detectors such as the gravitational wave observatory LISA, expected to be launched within a decade, could find evidence of the spin memory effect.
Another possibility is the Large Hadron Collider (LHC) at the CERN particle physics laboratory near Geneva, Switzerland. The trick here is to treat the LHC鈥檚 detectors, such as CMS, as a holographic sphere. 鈥淭he kind of fun thing about our framework is that we can talk about [a boundary at] infinity being literally the CMS detector or it can be like LISA,鈥 she says. 鈥淏ut you have to adjust because the geometry of the detector isn鈥檛 exactly like a sphere.鈥 This means physicists can pore over collider data to try to discern the patterns her celestial grammar proposes. For example, it suggests there should be a geometric pattern in how energy is distributed during particle collisions.
One thing I have always wondered about physicists who work on holography is whether they really believe it. The maths may work, but how seriously should we take the idea? 鈥淚 study the mathematics that underlies these different theories that we have,鈥 says Pasterski. 鈥淪o, I don鈥檛 take it very literally that I live in a hologram 鈥 but I think that it actually is quite powerful to ask that question.鈥
Indeed, if there are two equal universes, which one is the real one, the most fundamental? If it is the celestial boundary, does that mean gravity and the 3D space we so clearly experience are just an illusion? Pasterski isn鈥檛 too worried about which is the real deal. 鈥淭he way that we set up [the celestial holography research] programme, I think it鈥檚 very much still the one that we鈥檙e in,鈥 she says. 鈥淏ut I think in a world where we鈥檇 be able to fully understand this dual description, then they would be equally good鈥 they would be equivalent.鈥
When we arrived at the church earlier, I wasn鈥檛 sure if Pasterski鈥檚 cerebral intensity was going to make her an intimidating interviewee. She is certainly on another level to me, and as we leave the church hall, she keeps chatting with the same speed and energy, despite having flown in from Canada that morning. It was a relief, however, to also find her warm, relaxed and open.
Earlier on, I had become flustered trying to book a taxi for us on my phone. My signal was poor, nothing was happening, and at one point I looked at her pleadingly, wondering if she could dream up a technological solution with her powerful brain. But she just smiled and offered to book it on her phone instead. So, is she really the next Einstein? Time will tell. But I certainly warmed to her.
