Craig Hogan: Well, it's one way of looking at it. Quantum mechanics, which is the standard matter of all of energy, that's true. States can have a particle or wave character, there's this complimentary principle, which is a property of matter and energy. We've never figured out how that could work for space and time itself. So spacetime is classical, it's just made out of points and lines. There's no Heizenberg-type uncertainty. That's the wave part captured in that description.
That's right. Physicists don't know how this works. The things we know about physics that are verifiable so far haven't shed any light on this. It's all in theory of space. The standard physics, which doesn't explain the quantum nature of spacetime is fine for the experiments what we have. So the physics at CERN, for example, doesn't care about this. What we've done is design an experiment that will care about it.
Can you explain how the experiment works?
"Command over nature on that incredibly small, fine scale is going to be really important"
The basic idea is something that's quite old. There's a machine invented by Robert Michelson in the 19th century called the Michelson interferometer. You shine light between mirrors that are far apart. We use lasers of course, but by using wave nature of the light, not of the spacetime but of the light, you can measure the relative positions of the mirrors very accurately. As the mirrors move through time, you can monitor their positions with the light. If there's this quantum property, quantum property of spacetime, we can see that in the light that comes out of the interferometers. We have fast electronics to read the light signals out of it. We use it as a very sensitive pair of microphones to listen to the jitteriness or uncertainty of spacetime.
It means space and time aren't infinitely divisible, that they aren't a continuum. You can't divide the line into smaller and smaller points forever. You get to the point where, like atoms, it's invisible. Except that invisible unit isn't even a unit of spacetime, it's a fundamental element of something that looks like spacetime when you have a lot of them together. It'd suggest that, when you dig down to some microscopic level, it's a quantum system.
Would that finding suggest the universe has more dimensions or that it can be explained with just two, or what?
"The fact that [the universe] feels three dimensional, in a way isn't a necessary property of it"
Well, there may be more dimensions, but the amount of information needed to describe it is consistent of having just two, which is like saying the third dimension of space, the fact that it feels three dimensional, in a way isn't a necessary property of it. It doesn't have as much information as an infinitely divisible, truly three-dimensional space does. It has as much information as a two-dimensional space, which is actually pixelated.This is something you're testing right now—this is not just theoretical talk.
In a way, that's right. You can think of it as a bandwidth limit to reality. Reality is unfolding, but it's not downloaded as an infinite bandwidth. There's a finite amount of total data. And the data rate we're sensitive to is 10^43 hertz, which is the Planck Scale. So far, that's such an enormous rate that for practical purposes it's almost infinite. We designed this experiment to try to see signs of the limitations.How far along is the experiment?We built [the Holometer] and it's been running. It runs pretty well, the first science results just came out. Not at Planck sensitivity level so we haven't seen the granularity of space time, but we have been able to do the world's best limits on the vibrations of spacetime, which are gravitational waves. At high frequencies, where we're collecting data at megahertz frequencies. We didn't find any, what that means is we can say some things aren't there, some exotic sources of vibrations like black hole binaries and cosmic strings. We didn't expect them, but we can say for sure they're not there.
Yeah, I think so. Is that clear? If we don't find anything, it's not clear what would happen. It's quite possible we don't find any new effect, and what we find is consistent with continuous spacetime.
In a way that's two different questions. Yes, there is a hole, there is this gap—a big gap people have been aware of for 100 years. Einstein's theory of relativity is in one hand and quantum mechanics is in the other hand. There were really interesting debates in the1920s and 1930s because those two theories at some level are incompatible. They live in different worlds. Modern physics has found ways to combine them, but they aren't really compatible. That's the opportunity. There is that gap to address.As far as what led me into it, there are some theory results that are fascinating that grew out of black hole physics in last 30-40 years. There is evidence in physics of black holes that there is holographic bound information. Black holes are 3D objects, but theory suggests very strongly that they have a 2D amount of information even though they are in a 3D space. All the information on a black hole fits in a two-dimensional surface of Planck resolution. That is kind of crazy, and it's just kind of hanging out there, people don't know how to reconcile it. There's a lot of theory on that, but no experiments. That was the motivation—is there any experiment that can shed light on this?
Can you describe the Holometer? It's in a trailer, right?
"You can't make fundamental advances in technology and engineering without making fundamental advances in physics"
I read that somewhere, it's not in a trailer. The control room is in a trailer, which is not uncommon at Fermilab. The experiment itself, part of it is in a tunnel, part of it is sticking out into a prairie. The Holometer looks like, well, somebody said the internet is a series of tubes. Well, the Holometer is a series of tubes with mirrors in it. There's a series of 40 meter, 6-inch diameter vacuum tubes, an empty space, except for laser beams, shining down 40 meter tubes between very precise mirrors also in the vacuum.Each of the devices is not a mirror but a piece of glass with a mirror on it. The light has to go through the glass a centimeter thick, and it only absorbs a part per million of the light. It's basically looking for an invisible piece of matter. Everything that doesn't reflect all the way goes through the glass. Anyway, there's that and then there's a bunch of computers of course to keep everything aligned. It's more or less just bolted to the ground, it's anchored in concrete pylons. We have to have electronics to keep it locked down so the mirrors don't wander all over the place. We are looking for a precision smaller than an atom so we can detect the fast vibrations which are a billion times smaller than that.
What are potential places where noise that would ruin the experiment can come in?
"If we're out there and find the bandwidth limit to reality, it's important to how we live"
There's a whole series of them. A lot of it has to do with frequency dependence. This signal we're after is in high frequencies. It's megahertz experiments so it's like AM radio. So it's between 1 and 10 million times per second oscillation. Most of the noise in the environment is at low frequencies, it's at kilohertz and below. We certainly see that, it's a very sensitive microphone. But like the gravitational wavelengths, we can immediately detect vibrations of the ground from ocean wave surf—the Atlantic and Pacific coasts are thousands of miles away. But that's a very low frequency. At high frequencies, almost nothing shakes. The only thing we can detect that isn't on signal is there's a few radio stations we can pick out—there's shielding from that. And the mirrors themselves, they're sitting there at room temperature, and an unavoidable property of anything is the molecules move around.
Well there aren't any other experiments. It's a unique experiment. On the theory side, there's a lot of stuff built around string theory or things like that. They're addressing things that happen at very small scales, they're worried about the consistency of the theory on a microscopic scale. There isn't much about how it works for macroscopic objects like mirrors. There's no theoretical consensus about what we should see. The theorists I talk to say that they're interested in the result of the experiment, but few of them say they know what we should see.
If the universe is a hologram, will you be the first to know?
"Gosh, we could be like Galileo or Newton, you know, that would be great"
I guess so. But if we don't see a result, that doesn't mean the universe is not a hologram. It just says that it's not a hologram in this way. We could have finite information, but we're looking for the most obvious way it could be blurry. If we do see a result we're looking for, I think it'll be a big surprise to everyone, including us.When do you expect results?
I hope this summer or fall. I think you'll hear about it pretty quickly; we'll be out there talking about it.If we are living in a hologram, how do you think people will respond? Does it matter?
There's many many examples of fundamental physics breakthroughs transforming society. You can make a list better than I can. If you know how space and time works, that's valuable, it's really useful. In the past, that very esoteric theory of Einstein, the theory of relativity, everyone has it in their cell phone with GPS. So similarly if we're out there and find the bandwidth limit to reality, it's important to how we live. It's going to matter, at some point.The more fundamental a finding is, the longer it takes to translate into technology. The transistor was invented 60 years ago, so it takes a while, but now it has really mattered a lot.As a scientist is it weird working on something that may not translate for 60 years?
We take the long view. Gosh, we could be like Galileo or Newton, you know, that would be great. It's not that unusual, sometimes it's call curiosity-driven research.But more than just curiosity, you can't make fundamental advances in technology and engineering without making fundamental advances in physics. Part of building the civilization of the future is figuring out how the world works. It's this iterative thing, because we couldn't have built this experiment as cheaply as we did 10 years ago because we're relying on fast electronics, photonics technology. You can now buy catalogs on the internet to build it. If we happened to detect something, it means there's a machine that can detect it, so there's another machine that can use it. Command over nature on that incredibly small, fine scale is going to be really important.