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Ask a Caltech Expert: Physicists Explain Quantum Gravity

Quantum gravity is one of the biggest mysteries in physics today. Quantum gravity is a set of theories that aim to combine the concepts of quantum physics and gravity. Theorist Kathryn Zurek and experimentalist Rana Adhikari are working with others to create a small experiment that could potentially detect signatures of quantum gravity.

As part of Conversations on the Quantum World, a webinar series hosted by the Caltech Science Exchange, Professor of Theoretical Physics Kathryn Zurek and Professor of Physics Rana Adhikari talk about one of the biggest mysteries in physics today: quantum gravity.

Quantum gravity refers to a set of theories attempting to unify the microscopic world of quantum physics with the macroscopic world of gravity and space itself. Zurek, a theorist, and Adhikari, an experimentalist, have teamed up with others to design a new tabletop-size experiment with the potential to detect signatures of quantum gravity.

In conversation with Caltech science writer Whitney Clavin, the scientists explain that at the microscopic, or quantum, level, matter, and energy are made up of discrete components; in other words, quantized. Many scientists believe that gravity is also quantized: if you magnify space itself enough, you should see discrete components. In this webinar, Zurek and Adhikari discuss why measuring quantum gravity is so difficult and how they plan to go about searching for its elusive signatures.

Highlights from the conversation are below.

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The questions and answers below have been edited for clarity and length.

Can you start out by orienting us to the world of quantum physics?

Kathryn Zurek: Sometimes I think about the quantum world as a pointillism painting. When you look at the painting from a distance, of course, it just looks like an ordinary painting. But as you start to zoom in to it on smaller and smaller scales, you start to notice that there’s more structure there. And, in fact, rather than it being a continuous object, you start to notice that it’s actually made up of individual points. And as you zoom in further and further, you can see the individual points, the quanta, that make up that painting. And that’s what we do in particle physics. We’re zooming in on smaller and smaller structures, smaller and smaller scales.

What is quantum gravity?

KZ: What we’ve been doing over the last hundred years is to zoom closer and closer in to smaller and smaller structures. And we’ve learned a lot about the fundamental forces of nature by doing that: electromagnetism, the strong and the weak forces, etc. We understand those forces in the language of quantum mechanics. The one really big piece that doesn’t fit into that puzzle is gravity. That’s actually not surprising based on what we know about gravity. We expect that we’re going to have to keep zooming in to smaller and smaller scales to be able to start to see the quantum effects of gravity.

Rana Adhikari: You can think about how things move in a swimming pool. Macroscopically, we would look at a body of water and there’s waves on top of it. But if you really want to know how sticky the water is or how smooth it feels, you have to zoom in and find out what’s in the water. And that comes from the quantum mechanics of the particles. But fish don’t really care about that. They just swim around, and they feel things like viscosity and temperature, and they don’t really need to know about quantum mechanics. And planets are like that in space. They don’t need to know about quantum mechanics. They just feel the gravity and do what they’re supposed to do.


When you take simple microscopic laws and put things together, and you have, really, billions and trillions of these things, they have properties that maybe you didn’t expect at first. I have a hunch that gravity comes in the same way.

Why do researchers want to unify quantum physics with gravity?

RA: I just want to know what’s going on. It would be very strange, if everything in the world is quantum, how it could possibly be that we have spacetime or gravity and it’s not quantum? It would be mind-blowing that I could do things like make gravitational perturbations here with my hand and then somehow that gets communicated to Kathryn across campus through gravity, but that is not somehow a quantum information channel. That would be the first thing in the universe that is not like that. And so I feel it’s got to be quantum, and I want to know how that works. It’s going to be amazing if we ever figure out how quantum gravity works. And maybe we’ll never use it for something practical, but that is what they told Faraday about electromagnetism.

KZ: A hundred years ago or so, we had this beautiful unified picture of how all the classical forces worked. And then we had quantum mechanics and quantum field theory, which now explain all the forces of nature except gravity. And yet we know that these things have to work together, they have to fit together. And so if you’re a physicist, you’re always trying to solve the puzzle: How do these things fit together? How do they work together? How do I make a unified picture to understand all of the forces of nature together? And there are very deep, good reasons why we expect that there should be quantum effects in gravity. We also have an understanding of why it is that they’re so hard to see.

How do you propose to find evidence of quantum gravity?

KZ: We are looking for ripples in the fabric of spacetime. You can think about gravity and spacetime as this stretchy sheet. And classical gravity is when you put a mass down on it, and it causes a sheet to bend. But with quantum gravity, in general, we expect that there’s going to be ripples in that fabric. And, in fact, we already see this with the ordinary forces, with electromagnetism, that there are actually fluctuations in empty space. Empty space, the vacuum, is not so boring.

We want to look for the fluctuations in spacetime due to the quantum nature of gravity. Now, if those effects just occurred at extremely small-length scales, we would never be able to see them. What I’ve been thinking about for the last several years is whether there’s a real possibility that those fluctuations in the fabric of spacetime are actually larger than you might naively expect. The fabric of spacetime is like a pond, a very smooth pond of water. We’re looking for drops on it. Those little drops create a wave pattern on the water. And we’re looking for the interference between the waves.

RA: Kathryn gave a good visual description; I’m giving the audio equivalent. People detected the cosmic microwave background a long time ago, and it is like a hiss. But that comes from far out in space. This hiss is a little bit different. This is more like a hiss that is inherent to spacetime itself. It’s analogous to the electromagnetic fluctuations that Kathryn was mentioning. If you look into empty space, the electric field has fluctuations, it has noise. And if you could measure that, it would tell you something about the electromagnetic field in space (which is cool, and people have done it). What we’d like to do is measure something like that—but the gravitational fluctuations in space when there are no sources for it, when it’s not coming from outer space or stars or anything like that.

What will the experiment look like?

RA: This experiment is exactly the same shape and setup as LIGO but on the scale of several meters. The laser comes in on one side, the light goes two different directions, and then it comes back, and we measure how long it took the light to go this way and that way. But this is the next step: to prepare quantum states that are really unusual and to use those to dig deep, deep into what you can possibly measure on the earth. I think no one’s ever attempted such a precise measurement of distance. If it works, it’ll be the most precise distance measurement ever done.

Where will the experiment live?

RA: Caltech is building a new center for quantum precision measurement here on the Caltech campus. And you might ask, why are we doing it here? You could do it any place in the world. We have a unique opportunity here. We have Kathryn here who knows about how this spacetime is supposed to work. We have people in my group, who know about measuring small displacement. And our goal is to go to the big leagues in terms of quantum measurement. And for that, we need people working on the theory of quantum information and measurement, and also other people who do measurements like us. The basement of this new building will have the new Kip Thorne laboratories, where we’ll be doing all of our stuff and where Kathryn will come visit us in the labs.


Here are some of the other topics addressed in the video linked above:

  • The day-to-day work of a theorist versus an experimentalist and how the two work together
  • How future quantum computers can aid in studies of quantum gravity
  • The holographic principle (how three-dimensional objects can be described by what’s taking place on a two-dimensional surface)
  • The role of the uncertainty principle in quantum gravity
  • Measuring entanglement
  • Spacetime emerging out of quantum processes

Source: SciTechDaily