With the detection of gravitational waves, we’re now able to observe black holes as they merge. We’re already able to determine the mass and rotation of the merging black holes, but gravitational waves might be able to settle the fierce debates over the conflict between black holes and quantum gravity. 

The signal of a classic black hole merger. Credit: LIGO

The LIGO signals we have so far show the classic properties of a black hole merger. Two orbiting black holes create a regular pattern of gravitational waves that gradually increase in frequency. Eventually the two masses merge, creating a chirp and “ringdown” as the newly formed black hole settles into a stable state. According to general relativity, once the new black hole settles down, it should no longer emit gravitational waves. That’s because a single black hole simply has the properties of mass and rotation (and theoretically charge), but nothing else. This is known as the “no-hair theorem.”

While relativity is a well-tested scientific theory, it runs into problems when you try to incorporate it into quantum theory. The foundational principles of quantum mechanics are very different from that of general relativity, so the two models don’t play well together. Since we have reasons to presume the ultimate theory of gravity is a quantum theory, there has been a lot of research on what such a theory would look like. When we try to develop a quantum version of black holes, weird paradoxes arise. One of them is known as the firewall paradox, where quantum fluctuations would create intense heat near the event horizon of a black hole, though this would seem to violate the equivalence principle, upon which relativity is based. Another is the information paradox, where knowledge of an object disappears when it crosses the event horizon, which violates a fundamental principle of quantum theory. Theorists have developed possible resolutions to these paradoxes, but there hasn’t been any way to test them. We can’t travel to a black hole to look at one up close.

Secondary echoes could be evidence of quantum effects. Credit: Jahed Abedi, et al.

But a new paper argues that LIGO might actually be able to test these ideas. While a classical black hole should be silent after the merger, quantum interactions near the event horizon could create small secondary chirps. These chirps should be regularly spaced, and their timing could put constraints on various quantum models. Interestingly, the team looked at data from the three black hole mergers that have been publicly announced, and found some evidence of these secondary signals. The statistics isn’t particularly strong, so it can’t be confirmed as a real effect, but that will change as we observe more black hole mergers. If these secondary chirps keep showing up, then we might be able to test the quantum behavior of black holes.

It’s an interesting result, and it demonstrates the power of gravitational astronomy.

Paper: Jahed Abedi, et al. Echoes from the Abyss: Evidence for Planck-scale structure at black hole horizons.  arXiv:1612.00266 [gr-qc] (2016)

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In physics there are two broad ways to look at the world. One is the classical realm of Newton and Einstein, where objects have definite form and interact in clearly determinate ways. The other is the quantum realm, where objects seem nebulous, with a strange mix of particle-like and wave-like behavior. The classical view gives us a wonderfully accurate description of everything from planets to baseballs. The quantum view is necessary to accurately describe the behavior of light and atoms. The classical world dominates on the scale of our daily lives, but nature seems to be rooted in quantum theory at its most basic level.

While both the classical and quantum approach are extremely accurate in their respective regimes, what happens in the intersection of the two regimes is still unclear. We don’t have a rigorous theory combining our classical and quantum models. We also don’t have certain key observational evidence, particularly in the nexus of quantum theory and gravity. But as quantum experiments increasingly study more massive objects and gravity experiments become increasingly sensitive, we’re approaching the point where “quantum gravity” experiments could be made. That’s the goal of a recently proposed experiment.

Since there isn’t yet a unified theory of quantum gravity, folks have instead focused on approximate approaches. One such approach is to add gravity to quantum theory a little bit at a time. This perturbative approach quantizes objects and their gravitational fields, and it works well for weak gravitational fields. One of the predictions of this approach is the existence of gravitons as the field quanta of gravity, much like photons are the field quanta of electromagnetism. However with stronger gravitational fields the approach becomes problematic. Basically, perturbative gravity builds upon itself in a way that is unphysical, so the model breaks down.

Another approach is known as the semi-classical method. Here gravity is treated as a field of space and time just as Einstein proposed, but the objects in spacetime are treated as quantum objects. The most famous prediction of this model is the Hawking radiation of black holes. The semi-classical model is not without its problems, particularly with strong gravitational fields, but for weak gravitational fields it reduces to the Schrödinger–Newton equation, which describes quantum objects interacting through classical Newtonian gravity.

What’s interesting is that these two approaches to gravity make fundamentally different predictions, even for weak gravitational fields. Either gravity is a classical background in which quantum objects interact, or both gravity and objects are quantized. This proposed experiment could determine which approach matches reality. Their idea is to take a charged disk of osmium with a mass of about a billionth of a gram and suspend it an electric field. This is small enough that its energy levels in the electric field would take on quantum behavior when cooled to temperatures a fraction of a Kelvin above absolute zero, but its also massive enough that its gravitational pull would affect the quantum behavior.

As the authors point out, the Schrödinger–Newton equation predicts energy levels that are different from perturbative approach. So such an experiment could determine whether semi-classical gravity is valid. It should be stressed that this is currently a proposal, not an actual experiment, but it could be a feasible way to bring gravity into the quantum realm.

Paper: André Großardt. Optomechanical test of the Schrödinger-Newton equation. arXiv:1510.01696 (2015).

This post originally appeared on Forbes.

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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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