The Universe we inhabit seems to be four-dimensional: the three dimensions of height, length, and depth, along with time. Together those are known as spacetime, which is the background of our successful theories describing the forces governing matter and the cosmos itself. Other models, such as string theory, propose more dimensions, but those are coiled up too small to be seen.
But there’s another idea: maybe one of the ordinary three dimensions of space is unnecessary for a full understanding of the Universe, with the “depth” information encoded in the other dimensions. Because the concept is similar to a hologram, the idea is known as the holographic principle, and it’s a hot topic among those trying to understand the quantum nature of gravity.
The holographic principle isn’t a theory that describes our Universe yet: it’s more a fascinating conjecture that might let us solve some thorny problems in fundamental physics someday. However, a few researchers think we might be able to detect some discrepancies between the three dimensions of space we perceive and a lower-dimensional hologram in the structure of spacetime on the very microscopic level.
Their experiment is known as the Holometer, located at Fermilab in Illinois. The Holometer is starting operations to detect hypothetical “holographic noise”: vibrations in the structure of spacetime caused by the encoding of the third dimension in the other two.
However, not everyone who studies the topic agrees on whether holographic noise exists or not, and failure to detect anything won’t tell us if the Universe is a hologram or not. To see why this is, though, we need to look more closely at the holographic principle.
An ordinary hologram is typically constructed by using lasers to scan three-dimensional objects, resulting in a picture that looks different depending on the angle of viewing. The third dimension is encoded in the viewing surface, tricking our eyes into seeing depth that isn’t there. The illusion isn’t perfect: nobody but Homer Simpson would be fooled into thinking the hologram of a donut was actually a donut.
The holographic principle is a slightly different matter. The idea derives from an interesting mathematical correspondence between two theories: a five-dimensional toy universe filled with a variation of dark energy, the stuff that makes our real Universe accelerate) and a four-dimensional toy universe with no gravity.
While neither of those models describes the real world, they show in principle that a lower-dimensional hologram could contain all the information of a more complex cosmos. And more importantly, the correspondence is exact: unlike a normal hologram, you wouldn’t be able to peek around the edge of the cosmic hologram to notice that it’s actually flat.
The holographic principle may or may not work for our real Universe; it’s an area of active research. Based on studies of black holes, some physicists think the holographic principle will be part of any reasonable quantum theory of gravity. While this theory doesn’t exist in any complete form, it’s the hoped-for reconciliation of the two major branches of modern theoretical physics: general relativity, which describes gravity, and quantum field theory, which covers the other fundamental forces of nature.
In its purest form, the holographic principle doesn’t have any immediate testable consequences: we’d be hard-pressed to know we lived in a hologram, because the correspondence between the three-dimensional world we see and the two-dimensional hologram is nearly perfect. (I say “nearly” because it should have consequences for the behavior of black holes and other exotic phenomena, but that’s a story for another day.)
Fermilab physicist Craig Hogan proposed a refinement: maybe the encoding of the three-dimensional Universe in the hologram isn’t perfect, but instead is “pixelated” on a very small scale, leading to holographic noise. The Holometer is designed to test this idea using two laser interferometers: devices that measure the difference in the distance light travels along different paths. If Hogan’s holographic noise idea is correct, there should be random fluctuations in the interferometers not accounted for by any other explanation.
But that means the Holometer doesn’t actually test whether the cosmos is a hologram, only if there’s holographic noise. Because noise in the interferometers can come from a lot of different—yet very ordinary—sources, the Holometer team has to be damn sure they can account for every piece of it. Ironically, one of Hogan’s original motivations for his hypothesis was mysterious fluctuation in the GEO600 gravitational wave detector, which have since been cleared up through an improved method of reading out experimental data.
Several outcomes to the Holometer experiment are possible. The most interesting but probably least likely result would be the detection of holographic noise, unexplainable by more mundane sources. That would be a nice bit of evidence for the holographic principle, but without telling us much about the nature of the hologram we inhabit. The more likely (and disappointing) outcome is that any signal will be ambiguous or nonexistent, which tells us nothing about whether we live in a hologram or not. After all, while the holographic principle is necessary for holographic noise, the reverse is not true.
So I’m not holding out hope that the Holometer will reveal either way whether we live in a holographic Universe, but I respect the research team for testing the holographic noise theory. While some ideas in science may be true yet untestable (for example, we’re very sure there are galaxies too far away to see), we’re all about the evidence. And if not the Holometer, maybe another experiment in the future can tell us if we live in a hologram or not.