SPACE-TIME may not be fundamental. Instead, according to the holographic principle, it emerges from something deeper, like a 3D hologram emerges from a flat surface. The principle says that space-time, and by extension gravity, arises from quantum entanglement.
With that in mind, (pictured above), a physicist at Stanford University in California, is trying to create space-time from scratch. Her approach simulates a 2D holographic boundary around a universe, which, according to the holographic principle, is enough to encode all the information that describes the universe within. This 鈥渉olographic duality鈥� says that space-time and the lower-dimensional boundary that it emerges from are equivalent.
In essence, Schleier-Smith鈥檚 methodology involves tabletop experiments that have the potential to reveal how the holographic principle contributes to phenomena all the way down to those on the smallest scales, where space-time would emerge.
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Lyndie Chiou: What is your experimental set-up?
Monika Schleier-Smith: The tools I work with are laser-cooled atoms. We have isolated atoms in a vacuum chamber and we use lasers to bring them to very low temperatures 鈥� millionths of a degree above absolute zero. We pin them where we want them and it鈥檚 essentially a starting point for having a very well-controlled model of a quantum system.
How can you tell the particles are entangled?
We鈥檝e been studying this idea of holographic duality [by trapping] atoms between two mirrors that form an optical resonator. The cool thing about this optical resonator is it lets any atom talk to any other atom. Photons can travel between these atoms and act like messengers that convey quantum information between them. The light generates correlations, or entanglements.
Once we have prepared atoms and let them interact and entangle, we send in light again to take a picture. So it鈥檚 literally light just scattering off the atoms. We can see not only where the atoms are, but also which state the atoms are in. The atoms have an internal spin, they can point up or down, and we can look at the spin correlations between different sites of our array of clouds of atoms. So basically, we take a bunch of pictures and analyse the correlations.
How would quantum gravity emerge from this?
I credit my students, who were very clever in thinking through how you should analyse the data. They realised that one thing you can do is measure the correlations between different pairs of sites in our array of atom clouds and ask which pairs are the most strongly correlated. You can draw a line that connects any of the pairs of clouds that are the most correlated.
What pops out is a tree graph. And thanks to a wonderful collaboration with a theorist, the late Steven Gubser from Princeton, I knew that this tree graph is a representation of curved space-time. The geometry that emerges is something that looks like a space-time with negative curvature. [It is the same kind of space-time involved in the theory of] holographic duality, which doesn鈥檛 quite look like gravity in our universe, but it鈥檚 fascinating. This was an important first step for us experimentalists to wrap our heads around precisely what you should measure to see gravity emerge from quantum mechanics.
Does everyone agree your experiment is a simulation of space-time?
First of all, there isn鈥檛 a consensus about whether holographic duality describes our universe. That is absolutely an open question.
There鈥檚 a second question of whether these experiments are still a fruitful area of research. Some people are pessimistic, but I have the more optimistic view for two reasons. One is that it may ultimately teach us something deep and fundamental about space-time and gravity in our universe. Secondly, maybe it will help us understand quantum mechanics better, specifically entangled quantum systems.
What other physics are you using your experiment to study?
There are fascinating predictions about what happens to information that falls into a black hole. Not long ago, it seemed like information gets lost when you throw a book into a black hole, which isn鈥檛 allowed under the laws of quantum mechanics.
The resolution [of this so-called black hole information paradox] is that information isn鈥檛 completely lost, it鈥檚 鈥渟crambled鈥�. It becomes very difficult to recover because information that was initially locally stored in one quantum bit, or qubit, becomes delocalised and hidden in complex entanglements among many qubits.
There鈥檚 a theoretical prediction that says if you have the holographic dual of a black hole [the 2D version from which the 3D black hole can arise], it should scramble information extremely quickly. The rate at which that information is scrambled is a fundamental limit for how fast that can happen in any quantum system. This is known as 鈥渇ast scrambling鈥� and it鈥檚 neat because [this limit] came from thinking about gravity.
Lyndie Chiou is a freelance writer based in California
