Construction of the Holometer, an experiment to test a hypothesis about the structure of spacetime on the microscopic scale. [Credit: Fermilab]

A hologram is a way to encode information about a three-dimensional object in two dimensions. Even though it’s flat, if you look at a hologram from different angles, you see different sides of the original object. However, the illusion isn’t complete: even a good hologram isn’t a precise replica of its three-dimensional subject.

So when we speak of the possibility of the Universe being a hologram, we don’t mean exactly the same thing. Instead, we mean that maybe one of the three dimensions of space (e.g. “depth”) is superfluous, that we can encode the same information in two dimensions instead. The statement of this idea is the *holographic principle*, which describes the mathematical correspondence between two different physical theories that still result in the same behavior. It was formulated for an imaginary test case in quantum gravity, so we don’t currently know if the holographic principle works for our real Universe.

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. [Read more...]

My latest article in The Daily Beast is too short to do more than scratch the (holographic?) surface of this very interesting topic. For more information, I recommend this blog post on the Holometer by Sabine Hossenfelder and the relevant sections of Sean Carroll’s book *From Eternity to Here*. However! The holographic principle is also one of the topics in my upcoming class, “Black Holes Into Darkness”, which begins next week. Consider that a hint, if you will.

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