UChicago astrophysicist Craig Hogan tests the digital nature of space.
This past August, headlines started popping up announcing that the universe might be a hologram, conjuring thoughts of Princess Leia’s message to Obi-Wan Kenobi or Tupac Shakur’s posthumous 2012 Coachella performance or even CNN’s roundly ridiculed 2008 foray into holograms, “beaming” a correspondent from Chicago to New York City. Of course, those weren’t real holograms. CNN used a green screen; Coachella used a high-powered projector and an old magician’s trick called Pepper’s Ghost; Leia’s technology is from a galaxy far, far away.
A real hologram is a two-dimensional surface that contains three-dimensional information. When light reflects off the hologram, the angle from which you view the surface determines whether you will perceive the information as 2-D or 3-D. When Craig Hogan, UChicago astrophysicist and director of Fermilab’s Center for Particle Astrophysics, describes the universe as “holographic,” he is referring to the way space-time is theorized to contain information, with 2-D sheets coding what we perceive as 3-D reality. Hogan’s Holometer experiment at Fermilab, which sparked the media buzz, probes the holographic, information-storing nature of the universe.
The notion of reality as encoded information is difficult to grasp. The Holometer website explains it with an analogy: a painted wall. If you zoom in on a blue wall, you will see droplets of blue paint. Zoom in further, and you will see blue molecules that make up blue paint. But zoom in even further, and the individual electrons, neutrons, and protons have no intrinsic color. They don’t appear blue. By working together, they provide the information that ultimately reads as blue. This is an example of emergence—when a property becomes apparent only on a macroscale. Likewise, when we look at the world around us, we see three dimensions. Zoom in far enough, and we may find that space-time is emergent, made of two-dimensional sheets encoding a three-dimensional environment.
Digital photographs also exemplify this concept of emergence. Tiny pixels carry the information that, when viewed at a distance, creates the picture you see. The universe’s “film” would be those 2-D sheets. The universe’s “pixels” would be the information coded on those 2-D sheets: the bits of a digital universe.
The Holometer experiment, whose ultimate goal is to better understand the relationship between matter, energy, and space-time, has to do with these pixels. When looked at very closely, holograms have “fuzziness” directly proportional to pixel size. The smaller the pixel, the clearer the 3-D image. But some fuzziness is always there. Hogan is looking for that fuzziness in space-time, which he calls “holographic noise.”
The universe’s suspected pixel level is the Planck scale, the smallest possible units of length, mass, time, charge, and temperature, based on universal constants like the speed of light and gravity. The smallest known particle is orders of magnitude larger than a Planck length (~1.6 x 10−35 m). It’s hard to comprehend and impossible to directly observe this scale: there are more Planck lengths across a grain of sand than grains of sand required to span the observable universe. “We’ve never seen any sign of direct Planck scale physics,” says Hogan. “So that’s what the experiment is trying to do,” to produce experimentally measurable data on the smallest scale.
The Holometer, which was devised more than four years ago and began producing data last summer, is run by a team of 21 scientists and students from UChicago, Fermilab, MIT, and the University of Michigan, led by UChicago astronomy and astrophysics professor Stephan Meyer and Fermilab physicist Aaron Chou. The experiment is “conceptually simple,” Hogan says. “Simplicity is necessary because we’re trying to isolate this fundamental effect.”
The team measures light using a Michelson interferometer, a device consisting of a laser, three mirrors, and a detector. A high-frequency laser is shot at a beam splitter (a half-silvered, one-way mirror), and the resulting twin beams travel down 40-meter-long perpendicular arms with mirrors at each end. (The distance the light must travel amplifies the predicted effect.) When the light reflects back and recombines at the splitter, the team measures the beam’s intensity. If it’s dimmer upon return, the twin beams’ wavelengths no longer line up perfectly; one beam’s round-trip took longer than the other. The beam splitter moved.
The experiment actually uses two adjacent interferometers. The team measures a combined signal that over time gradually reduces other sources of noise, yet can indicate whether both beam splitters are “wandering” in exactly the same, random way, moved by the “jittering” of space itself. This correlated—or coherent—jitter is holographic noise. By measuring jittering space-time, which should bounce around randomly about one Planck length every Planck time, the experiment provides access to Planck scale physics.
So why does this motion—this jitter—amount to the space-time fuzziness Hogan seeks? It’s a behavior displayed by other systems experimentally known to consist of information bits. Matter and energy are quantum—or digital—systems. They are composed of discrete packets of information, building blocks that can be quantified. Particles are the bits of matter, photons of light. The smallest packets of energy are called quanta, defined by the father of quantum mechanics—Max Planck, whose research led to the Planck scale.
A key element of a quantum system is its ability to behave as both waves and particles, which gives rise to the uncertainty principle that states you can’t know exactly where a particle is and how it’s moving at the same time. This uncertainty creates a jitter, and matter and energy both exhibit this property.
A century ago, Einstein showed that “space-time is dynamical,” says Hogan. Unlike Newton’s conception of “stuff moving around in this empty container,” space-time “carries energy and information just like matter does. It’s an active player.” But Einstein’s space-time, which still governs our concept of it today, is smooth and continuous—an analog system. Hogan’s experiment could change that. “We think that space-time is a quantum system together with matter and energy,” says Hogan. And a measurable jitter would support that claim.
If the team does detect a space-time jitter, the next step is to “measure it better” using more light detectors or longer arms, but “first we have to prove it’s real. People will not believe this,” says Hogan. Viewing space-time as a quantum, digital system would change how we understand the universe on a fundamental level—how matter, space, and energy interact; how the universe stores information; and how that information translates to what we deem reality.
What Hogan and the Holometer experiment are not trying to do is prove that reality is a projection or Matrix-style simulation, as some recent articles seem to imply. This experiment is about illumination, not illusion—how two-dimensional sheets of information create our multidimensional universe. We may all be holograms, but the wall is still blue and we are still real.