The area of theoretical physics is pretty cool. Mind experiments are used to come up with ideas of how things work at the most fundamental levels. Physicists look at the universe and try to find out how things are put together and how they interact. We all know about protons, neutrons and electrons which are the basic building blocks of matter, but theoretical physicists look at the building blocks of these – muons, gluons, quarks. They then pass off their ideas for the particle and experimental physicists to do the actual work. The CERN supercollider is the place where a lot of cutting-edge particle physics is taking place, helping us understand the makeup of the elementary particles. It is amazing that we can understand things we cannot see, but we know exist because of the effects they have on other things. I have highlighted uncertainty in the article in Red and my comments are in Blue.
An Experiment in Zurich Brings Us Nearer to a Black Hole’s Mysteries
By KENNETH CHANG
JULY 19, 2017
The equations that describe the universe at the smallest and largest scales — how the tiniest elementary particles dance, how the space-time of the cosmos bends — predicted a slight incongruity, a tiny unbalancing in the numbers of certain particles under certain circumstances.
But physicists have yet to observe this phenomenon, with the unwieldy name of mixed axial-gravitational anomaly, and confirm the prediction. The imbalance is negligible except when the warping of space-time is extreme — like next to a black hole or the moment after the Big Bang.
It turns out there was somewhere else to look, and it was much closer. An international team of scientists discovered this anomaly in a tabletop apparatus in Zurich examining the properties of a tiny metallic ribbon.
“There was no way to test this effect until now,” said Johannes Gooth, a scientist at IBM Research in Zurich who is the lead author of a paper published on Wednesday by the journal Nature.
The IBM experiment did not involve black holes, or even gravity. Instead, it took advantage of a class of exotic materials known as Weyl semimetals named for a German scientist, Hermann Weyl, whose equation first gave rise to the possibility of such materials. A solid Weyl semimetal crystal was first created a couple of years ago, enabling the IBM study.
The motion of electrons inside a ribbon of a semimetal is governed by essentially the same space-time-warping equations as the original mixed axial-gravitational anomaly.
The advance could have practical uses in electronics, similar to how the invention of the transistor led to computer chips. (I love to see practical applications from theoretical work. This may happen with this here and give us some wonderful advances to the technology we use every day.)
“This could be opening the door to something new,” said Bernd Gotsmann, an IBM physicist and a co-author of the Nature paper, who said the company was investigating how the anomaly could be exploited for generating electricity out of waste heat and for other uses.
The gravitational anomaly popped out from equations that describe how particles called pions moving at close to the speed of light could decay into gravitons, the fundamental particles that carry the force of gravity.
Usually, the laws of physics prohibit pions from falling apart in this way.
But under Einstein’s theory of general relativity, the curving of space-time can tip the balance to allow this decay to occur. (This is a mathematical presumption. Because the math allows for this does not necessarily mean that it will happen.)
A pion consists of two smaller pieces: a quark, a building block of protons and neutrons, and an antiquark, the antimatter equivalent of a quark.
Many elementary particles, including quarks and antiquarks, can be thought of as darts that are spinning as they fly through space. They can spin clockwise or they can spin counterclockwise.
Usually, in the decay of pions, the number of clockwise particles would exactly equal the number of counterclockwise particles.
But the anomaly resulting from the warping of space-time can flip a clockwise spin to counterclockwise, or vice versa, with more particles spinning in one direction than the other.
That then circumvents the prohibition, allowing pion-to-gravitons decay to occur.
But that is currently an impossible experiment. Physicists have yet to find a single graviton. (This does not necessarily mean that we will never find a graviton, but until we do, this is simply theoretical. However…)
“We would never be able to detect this,” said Karl Landsteiner, one of the authors of the Nature paper and a physicist at the Institute for Theoretical Physics in Spain. (This is about the most definitive statements I have ever read out of a theoretical physicist – and it goes to the point that we simply cannot know or test everything. The graviton, for example, if it exists, is theorized to have no mass and no charge. It is undetectable by any means we have. This should be a wake-up call for those who deny the existence of God. There are things that are unknowable. Period.)
The same equations became of interest to scientists working in solid state physics, studying the electronic properties of materials.
In this Weyl semimetal system explored in the experiment, a difference in temperature is analogous to the warping of space-time, and a magnetic field separates electrons into the opposite spins.
“You can now suddenly use all these concepts in a tabletop experiment,” Dr. Gooth said.
Dr. Landsteiner said the movement of electrons in a semimetal is very much like the behavior of matter at the event horizon of a black hole, the region where the gravitational pull is so strong that not even light can escape. (While the experiment is similar, the mathematics are not exact. This means that while we might get to a close approximation with this experiment, it will not be possible for us to replicate the results as if we were at the boundary of a black hole or being impacted by a gravitational wave.)
The anomaly leads to more electrons of one spin moving from the hot side to the cold side of the semimetal ribbon, generating an electric current, which the experiment measured.
“To see this analogy work out is quite exciting,” said Subir Sachdev, a theoretical physicist at Harvard who was not involved with the research. “It’s an important step. It’s a beautiful step.”
The semimetal results could, in turn, improve understanding of black holes, Dr. Landsteiner said.
The experiment is also a success for string theory, or at least an offshoot of the underlying mathematics. By imagining particles as strings vibrating in 10 or more dimensions, physicists have been trying to tie gravity into the Standard Model, the laws of physics that describe the other forces in the universe. But such attempts at a grand unified explanation of fundamental physics have been maligned because they do not produce testable predictions. (Humans understand things in four dimensions – length, height, width and time. These dimensions are related to one another – you cannot have one dimension without the others. Theoretically, we cannot have these other dimensions without the first four. Nor can we detect the other dimensions with things that are available in the first four. But exactly what are the other dimensions and why do they exist theoretically? String theorists believe there are things that exist in one dimension, having length, for example, but no width or height and not existing in time. Creationists believe that God exists beyond our four dimensions. Scientists are trying to create particles in the place that is God’s domain. I think that is kind of interesting.)
Here, Dr. Landsteiner said, some of the techniques that originated in string theory turned out to be useful for something different: to calculate the expected anomaly. “It puts string theory onto a firm basis as a tool for doing physics, real physics,” he said. “It seems incredible even to me that all this works, falls all together and can be converted into something so down to earth as an electric current.”