Update: A few folks more knowledgable folks on the topic, including Cliff Harvey and Sabine Hossenfelder have informed me that this post is “completely wrong.” In particular the experiment was intended to test one wild extrapolation of the holographic principle that is not well founded. Point taken. For a not completely wrong (and quite cogent) take on these results, check out Sabine’s post on her blog.

Every now and then there’s a proposal that the universe might be a hologram. Not a hologram in the sense that we’re living in a virtual universe, but rather that the universe obeys the holographic principle, where the physical properties of an enclosed volume can be determined entirely by the behavior of its enclosing surface. At Fermilab an experiment known as the holometer was devised to look for evidence of this holographic effect, and the first results are in.

The idea behind the holometer is that if the holographic principle is correct, then (so the holometer team argues) there should be quantum fluctuations within space itself due to its dual nature. This would produce a background of “holographic noise” that could in principle be detected. After a year of observations, the holometer has found no evidence of holographic noise. So, it looks like the universe is not a hologram, at least not the easily detectable kind.

This result isn’t unexpected given what we know about quantum physics. The experiment has been somewhat controversial because even if they did detect noise it would not rule out other sources of noise. The team is looking at other ways to increase the sensitivity of their experiment, so we can expect to see this story pop up again in the future.

Paper: Aaron S. Chou, et al. Search for Space-Time Correlations from the Planck Scale with the Fermilab Holometer.  arXiv:1512.01216 [gr-qc] (2015)

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Modern cosmology is dominated by two fundamental theories: general relativity, which describes the structure of space and time as manifold that interacts with mass/energy (aka gravity), and quantum theory, which describes the fundamental interactions of protons, electrons, light, etc. (aka quanta). Both models are strongly supported by experimental and observational evidence. The problem is that each theory makes fundamental assumptions about the way the universe works, and they contradict each other at a basic level. This isn’t a problem if you are interested in things on a large scale, such as planets and galaxies (general relativity), or things on a small scale such as nuclear fusion (quantum theory). The contradiction arises when you want to understand objects that are both very dense and interact at high energies, such as black hole interiors, the big bang, etc. So one of the challenges of modern cosmology is to develop a unified theory of quantum gravity, which would combine the predictions of general relativity and quantum theory in a consistent way.

There are lots of approaches to quantum gravity, including string theory and loop quantum gravity, that try to unify these two models, but one of the big challenges is that many of their predictions are difficult if not impossible to verify. But new observations of distant quasars has put some observational constraints on the type of unified model the universe might allow.

The research focuses on a property common to most unified theory approaches, known as quantum foam. The idea behind quantum foam is that at a fundamental level the quantum aspect of things dominates. This means that on a small enough scale, the precise nature of space and time itself breaks down into a nebulous flurry of quantum fluctuations or quantum foam. In this approach the structure of space and time we see around us is a macroscopic approximation arising out of this foam, just as a table appears solid when in fact it is a dynamic interaction of atoms and molecules. The scale at which the foamy nature of spacetime becomes evident is known as the Planck scale, which is about 10 billionths of the width of a proton. That’s far too small for us to probe directly.

But it turns out that this quantum foam (assuming it exists) should interact very slightly with light. Basically, a photon traveling through spacetime has a tiny chance of interacting with the quantum foam in such a way that its wavelength and direction could be changed. The chances of such an interaction is so small as to almost be zero, but over a billion light year journey it would have a measurable effect. Depending on the quantum foam modal, distant light could appear blurred at certain wavelengths so that our view of distant quasars would become too blurry to be observed.

Based upon observations of distant quasars, the team found no evidence of any quantum foam blurring. Given the constraints of their observations, this means that spacetime is completely smooth down to a scale of at least a thousandth of the width of a proton. This is actually precise enough to eliminate some quantum foam models. In particular, it eliminates one popular model known as the holographic model. As the authors point out, while the holographic model is a popular model relying upon the holographic principle, this research doesn’t invalidate the holographic principle itself.

So to the limits of observation, there is no evidence for a quantum foam. Whether it exists but has more subtle effects is something that will require further research.

Paper: E. S. Perlman et al. New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations. ApJ 805 10 doi:10.1088/0004-637X/805/1/10 (2015)

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Tis the season for claims that the universe is a hologram again. Just to be clear, there’s no observational evidence that the universe is a hologram, and the latest research driving the sensational headlines doesn’t claim that there is. But there is some interesting theoretical work regarding the holographic principle that is worth discussing.

The holographic principle argues that the information contained within a region of space can be determined by the information at the surface that contains it. For example, imagine a road 10 miles long that is “contained” by a start line and a finish line. Suppose the speed limit on this road is 60 mph, and I want to determine if a car has been speeding. One way I could do this is to watch a car the whole length of the road, measuring its speed the whole time. But another way is to simply measure when a car crosses the start line and finish line. At a speed of 60 mph, a car travels a mile a minute, so if the time between start and finish is less than 10 minutes, I know the car was speeding. Mathematically, the space can be represented as a hologram of the surface that contains it. Unfortunately the term hologram invokes images of virtual reality and the idea that we’re living in the Matrix, which couldn’t be farther from the truth.

As a theoretical tool the holographic principle is useful because it is easier to do some calculations on a boundary than it is on the enclosed volume. One of the most popular uses of the principle is in string theory, through something known as the AdS/CFT correspondence, which uses the holographic principle to connect the strings of particle physics string theory with the geometry of general relativity. The AdS stands for anti-deSitter space, which is a non-flat model universe. Anti-deSitter space can’t be used as a model for the physical universe, because we know observationally that the universe is extremely flat.

It would be nice if there were a similar holographic correspondence for a flat universe model, but proving one has been difficult. Now a new paper has shown that the holographic principle can apply to flat space models, at least in some cases. The team looked at an aspect of quantum theory known as entanglement. If two objects such as electrons are entangled, they can be described in quantum theory as a single entity. Entanglement is one of the more strange aspects of quantum theory, and leads to some strange predictions about the universe, but has been experimentally validated. What the team found was that a calculation in standard quantum theory of a particular entanglement property dealing with entropy gave the same result when done using a holographic version. In other words, the standard and holographic versions are mathematically equivalent. They did these calculations in a model universe that’s flat, which demonstrates the holographic principle can work in flat space.

This is not a general proof, and it doesn’t show that the holographic principle does work for a flat universe like ours, only that it might when it comes to quantum systems. There’s a lot more work to do before anyone can say with certainty that there is a flat space version of AdS/CFT correspondence. And even then it won’t mean the universe is a hologram.

Paper: Arjun Bagchi, et al. Entanglement Entropy in Galilean Conformal Field Theories and Flat Holography, Phys. Rev. Lett. 114, 111602 (2015)

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There has been a flurry of news articles about a new experiment that could prove we live in a two-dimensional hologram. Needless to say, we do not live in a 2-D hologram, and even if successful this new experiment would prove nothing of the sort. Unfortunately the “universe is a hologram” headlines always make great link-bait, and it doesn’t help that the press release for this experiment uses a similar link-bait headline. That said, the experiment is is very real, and if it succeeds it could revolutionize our understanding of the cosmos, so it is worth talking about.

The experiment in question is known as the Holometer, being run at Fermilab, and its goal is to detect very small fluctuations in space itself.  In physics there are two main theoretical frameworks to describe the universe: general relativity and quantum mechanics. Both of these theories have been verified numerous times by experiment, and both are extraordinarily accurate descriptions of their respective regimes. The problem is that each of them paint the universe in very different ways.

In general relativity, objects are solid and continuous. Space and time can be warped by the presence of mass, and can in turn cause masses to deviate from their normal, linear paths. General relativity is a classical theory, using many of the same assumptions about physical objects that Newton did in the 1600s. Quantum mechanics, on the other hand, proposes that objects are not solid. Instead they possess a duality of particle-like and wave-like characteristics. Quantum objects are typically described within a space and time that is fixed and unaffected by things like mass.

For large objects like apples and planets, you don’t typically need to worry about their quantum nature, so the assumptions of general relativity are perfectly fine. For small objects like atoms, you don’t typically need to worry about gravity, so the assumptions of quantum mechanics are fine. The problem comes with things that are both massive and small, such as black holes and the earliest moments of the big bang. In those cases we aren’t sure where the assumptions break down, and trying to figure out the physics gets problematic at best.

It’s generally thought that at some point the quantum nature of space and time can’t be ignored. This presumes that general relativity must give way to a quantum description of space and time. Two main approaches to quantum gravity are string theory (which generalizes particle physics to include gravity) and loop quantum gravity (which strives to quantize general relativity directly). One idea that seems central to both of these approaches is known as the holographic principle, from which all the “universe is a hologram” statements arise.

The holographic principle states that the information contained within a region of space can be determined by the information at the surface that contains it. For example, suppose there is a road 10 miles long, and its is “contained” by a start line and a finish line. Suppose the speed limit on this road is 60 mph, and I want to determine if a car has been speeding. One way I could do this is to watch a car the whole length of the road, measuring its speed the whole time. But another way is to simply measure when a car crosses the start line and finish line. At a speed of 60 mph, a car travels a mile a minute, so if the time between start and finish is less than 10 minutes, I know the car was speeding. The holographic principle applies this idea to quantum gravity. Just as its much easier to measure the start and finish times than constantly measure the speed of the car, it is much easier to do physics on the surface “hologram” than it is to do physics in the whole volume.

If the holographic principle is correct, then (so the Holometer team argues) there should be quantum fluctuations within space itself due to its dual nature. This would produce a background of “holographic noise” that could in principle be detected. The Holometer team hope to detect this quantum noise over the next few years.

It should be noted that this experiment is somewhat controversial. Theoretical calculations don’t clearly support the existence of holographic noise, and observations of gamma ray bursts seem to disprove its existence at a level detectable by the Holometer experiment. This is really cutting edge science, so it’s difficult to predict what the outcome will be.

What we do know for sure is that if the project is successful there will be lots of headlines declaring that the universe is a hologram. They will be wrong. It would just be the first direct detection of the quantum nature of gravity, which we’ve long suspected but haven’t been able to prove.

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