Clean up after Einstein

A new generation of physicists hopes to succeed where Einstein failed

Towards the end of his life, Einstein tried to create a theory of everything that describes all the forces in the universe.
He failed, and at least part of the explanation for this is that two of the fundamental forces of the universe, the weak force and the strong force, were still not well known at that time. Now physicists are trying again, using information about particles and new fields.

Leslie Rosenberg's attempt to decipher the universe looks like a makeshift hot water heater covered with some electrical wires and a plug
In a large, underground refrigerator. But this "water boiler", located in the laboratory adjacent to Rosenberg's office at the University of Washington, is actually an empty magnetized chamber immersed in supercooling and containing a sensitive detector whose function is to listen to the microwave "beep" of passing particles called axions. These particles are invisible, and as of now they are still only a hypothesis.

Rosenberg has been chasing these particles since his postdoc at the University of Chicago during the early 90s. During that time, he conducted experiment after experiment, each one more accurate than the last, but he always brought up clay in his hand and was unable to discover any sign of the axion - a particle that, if discovered, would save a scientific idea not through its star, Albert Einstein's greatest scientific idea.

Physicists call this idea the "Unified Field Theory," but it's better known by the piquant name "The Theory of Everything." This Torah strives to reach a single formal formulation that would encompass the behavior of all the fundamental forces of physics. Einstein began the search for this formulation 90 years ago. The fact that the two fundamental forces that shape the behavior of the universe - gravity and the electromagnetic force - behave differently from each other, bothered the great physicist. He wanted to show that all types of matter and energy are governed by the same basic logic.

Location: University of Washington, Seattle

The project: The ADMX detector located inside an empty magnetized chamber with supercooling listens to the "beep" of the microwaves that create particles called axions. These supposed particles may be able to explain dark matter, matter that should have a much greater mass than visible matter.
Crafters: Sierra Fox, Nick Posey, James Sloan, Clifford Plesche, Richard Ottens, Josh Povik, Karkyra Stockton; Hannah Le Tourneau, Leslie Rosenberg, Xavier Frost, Anna Malagon, Kiva Ramondo, Jacob Herr. Credit: Timothy Archibald

But encompassing the entire universe within a single formula was too high a hurdle, even for Einstein. "I want to know how God made this world," he wrote to a German physics student in 1920 in an oft-quoted letter. "I am not interested in this or that phenomenon, the spectrum of this or that element. I want to know what God was thinking. Everything else is details."

But as the Heidi proverb says: "Man plans and God laughs." Einstein chased God's thoughts for 30 years without results, going from one dead end to another. When he died in 1955 he left behind, scribbled on his blackboard, a collection of unsolved unified field equations.

The task of unifying the physical forces passed into the hands of subsequent generations of physicists. These broke down the problem into innumerable small parts: what began as a lofty vision of a single genius morphed into slow ants' work carried out by different groups of physicists, each trying to understand just a small piece of the great cosmic aggregate. Rosenberg, for example, is not decisive in his opinion about finding an all-encompassing theory. It is focused on one specific and troubling problem: the Axion. The assumed properties of this particle could eliminate the need to change the form of Einstein's gravity equations. "We will wait to see what the data will say," says Rosenberg. "I'm not interested in peeking into the mind of God."

Despite the narrow focus of their research, Rosenberg and his partners do not forget the big prize. They are involved in a large-scale effort to correct flaws in the impressive theoretical structure created by Einstein and to build a more complete model of particle physics from the bottom-up rather than the other way around. They aim to push science forward by discovering the way nature actually behaves rather than how scientists think it should behave (an approach Rosenberg dismisses as arrogant). Other researchers are planning experiments that will shed light on a mysterious aspect of physics called dark energy, or experiments that will discover two-dimensional quantum units that may be the building blocks of what appears to be a three-dimensional universe. The numerical data from these experiments may be just what today's physicists need to succeed where Einstein failed.

"We can actually test some of these crazy ideas about the evolution of the universe," says physicist Joshua Freeman of the University of Chicago. And without these experiments, he believes, there is almost no chance that physicists will arrive at a theory of everything.

The dark side of the universe

When you look at Rosenberg's Axion Dark Matter experiment (ADMX), you discover the power of the "few holding the many" approach. It is possible that this modest experiment, which is looking for only one particle, will also be able to disprove certain doubts about the theory of general relativity and solve a major cosmic puzzle.

The beginning of the riddle dates back to the 30s, when astronomers began to realize that the universe is full of an invisible component whose presence is felt only by the gravitational pull it exerts on the visible stars. The discovery became even stranger during the 20s when new models of the Big Bang showed that this invisible (or "dark") matter, whatever it is, cannot be made up of ordinary atoms. This realization left only two disturbing possibilities: either on large scales gravity does not behave as predicted by Einstein's theory of general relativity, or the universe is full of particles of an unknown type that our telescopes are unable to see.

Location: American Fermi National Accelerator Laboratory (Fermilab), Batavia, Illinois

The project: The holometer experiment will look for tiny changes in a laser beam that is sent through two paths perpendicular to each other. Such changes would indicate that space and time are made up of fundamental quantum units - a theory known as the holographic principle.
Cast: Sam Waldman, Oykwang Kwon, Robert Lanza, Aaron Chu, Craig Hogan, Ray Tomlin, Stephen Meyer, Brittany Kamai, Lee McCuller, Jonathan Richardson, Chris Stoughton, Rainier Weiss, Richard Gustafson. Credit: Sandy Nicholson

The overwhelming majority of physicists reject the first hypothesis for at least two reasons: first, because it seems to be an ad hoc hypothesis, and second, because it is difficult to reconcile it with observations of the movements of galaxies. The mainstream of physicists therefore believes that the second option is the correct one, which has led to dozens of attempts to remove the lot from those invisible dark particles. This is where ADMX comes into play.

The predicted properties of axions match nicely with the properties expected from dark matter particles, so if axions are discovered by Rosenberg and his team, we can get a much more complete picture of the formation and evolution of galaxies. Such a discovery would also allow us to dispense with the need to make some ugly changes to Einstein's gravity equations. But most of all, the existence of axions will force us to rewrite the standard model of particle physics. This model is a comprehensive – but clearly incomplete – theory of fundamental forces and fields. The discovery of axions will confirm the claim that the Standard Model needs to be extended, an extension that has long been disputed, thus bringing physicists one step closer to a true theory of everything.

Until recently, axions were considered an unlikely bet in the hunt for dark matter. Most of Rosenberg's colleagues concentrated their efforts on particles of a different type called WIMPs (literally: "weak"), which are considered more theoretically attractive. "I've always been an oddball," Rosenberg cheerfully admits. But although many wimp detectors became more and more sensitive, they failed to detect anything. The watershed came last year, when an ultra-sensitive detector called "Large Underground Xenon" (LUX), located under the hills of South Dakota, went into action. So far, he too has brought up pottery in his hand.

Now this is a crucial moment for Rosenberg: will he succeed in proving that cations are the solution to the puzzle and by the way provide support for the theory of general relativity - Einstein's idea that gravity arises from the curvature of time-space. The principle behind the ADMX is delightful in its simplicity. If indeed the dark matter is composed of particles, there must be an incessant wind of these particles blowing through the earth and everything on it (including yourselves). And if these particles are axions, every now and then a part of them is expected to disintegrate. But while axions themselves are invisible, in their rare decay process they are supposed to turn into weak microwaves that will produce a faint but detectable signal. The idea may sound simple, but from a practical point of view it is very complicated to carry it out.

"We have a container the size of an oil barrel," says Rosenberg, "cooled to a temperature of 100 millikelvin," meaning a tenth of a degree above absolute zero. The low temperature ensures that the detector itself will produce almost no noise in the form of microwaves. In the next step, the container is magnetized to cause axions to disintegrate. And finally, a small pencil-like detector eavesdrops on microwaves that aren't supposed to be there. And as if these challenges weren't enough - no one knows exactly what kind of microwaves we should listen to; The frequency of the signal depends on the mass of the axon, which is of course unknown.

loops of strings

The Project: String theorists attempt to unify all known forces in nature into a single theoretical framework by describing both particles and forces as being generated by oscillations in loops of strings. Some versions of the theory provide predictions about phenomena that occurred at the beginning of the universe. These phenomena may have left their mark on the radiation that reaches us from the farthest reaches of the universe.

Those who do the work: Andrey Linda, Renata Kalosh, Ahmed Elmakhiri, Leonard Susskind, Schmitt Cashro, Patrick Hayden, Lampros Lampro. Credit: Timothy Archibald

The only way around the problem is to scan the microwave field frequency by frequency; In many ways, then, any ADMX experiment is very similar to what radio amateurs do when switching between channels. Rosenberg's face lights up when I offer this comparison: "I have always been intrigued by the electronics of radios. As a child I played with them, sending radio waves to the moon and waiting to hear the return signal. But the detectors we use today to look for signals in the experiment are so sensitive that if we put them in a cell phone, we would have full reception on Mars!" Rosenberg is also very proud that, unlike Einstein's endless search for the unified field theory, his experiment will have unequivocal results.

"By 2018 we will cover the entire frequency range relevant to Axion," says Rosenberg. "Then we will know for sure if he exists, or not." In other words: either we will have a big new clue in which direction to go in order to develop the theory of everything, or we will know that we have to delete another idea from the list of possible ideas.

The energy of empty space

While Rosenberg is patiently unraveling the problem of dark matter, other researchers are trying to reach a complete picture of physics by searching for the other invisible component of our universe: dark energy. The action of dark energy is the opposite of that of dark matter: while dark matter exerts a gravitational attraction, dark energy creates a repulsive force. Because of its action against gravity, dark energy has immediate implications for our interpretation of the equations of general relativity. And more than that: it is impossible to explain dark energy with the help of the current model of particle physics. So, the ability to explain dark energy is a crucial test for any theory that claims the crown of the theory of everything.

Just such a test is currently being conducted by Freeman of the University of Chicago. Freeman attached a custom-built camera to the Blanco Telescope, a four-meter aperture telescope located on Cerro Tulolo in Chile, a peak that rises more than two kilometers above sea level. The basic idea is to collect a huge amount of images of distant galaxies. Each image in the camera contains 570 million pixels, a huge amount of data, and the camera will take 400 such images every night, 105 nights every year, for five years. Not surprisingly, the project is called the "Dark Energy Survey" and by the time it ends in February 2018, 300 million galaxies and about 4,000 supernovae will be scanned. (For comparison, the most advanced automatic search for supernovae to date, conducted by the University of California at Berkeley from 1998 to 2000, found a total of 96 supernovae.)

Like Rosenberg, Freeman also began his professional career as a theoretical physicist but was drawn to the observational side because of the idea of ​​designing actual tests. But now he must face the difficulties of the task he has taken upon himself. "It's hard to collect the data, it's hard to process the data," he says.

Freeman and his team divide the results from the observations into four parts, with each part allowing the processing of a different aspect of the behavior of dark energy. One part of the results focuses on exploding stars of a type called Type Ia supernovae, which serve as milestones in space. The brightness of these supernovae makes it possible to determine the distance to them and their color indicates the speed at which they are moving away from us. Combining the information that comes from several such milestones makes it possible to assess how the expansion of the universe has changed over time. The three other parts of the analysis examine different patterns in how galaxies form clusters. Gravity causes things to attract each other, and dark energy causes them to move away from each other. Mapping changes in galaxy clusters over cosmic periods of time can therefore reveal the strength of dark energy's influence.

As strange as they are, you can think of dark matter and dark energy as decoration on top of a reality that even Einstein would have recognized. But is it possible that the description of reality itself needs adjustments in order for us to move forward?
The simplest models of dark energy describe it as an unchanging property of empty space itself, and which prevails throughout it. It turns out that the standard theories of particle physics can explain the existence of such energy; But the power they contract for this energy is 10 times greater to the power of 120 than the observed one. (This prediction is sometimes considered the worst prediction in the history of physics.) The ability to explain the true, much smaller value of dark energy is one of the most important tests facing any future theory of everything. Also, astronomers still don't know at this point if the size of dark energy is indeed constant. If Freeman and his team find that the strength of this energy varies with time, the theory of everything will be required to explain this finding as well.

But before we get to that matter, there is a fundamental matter of it that we must settle. "Our assumption is that dark energy is driving the expansion of the universe, but there is no certainty that this is indeed the case. It may well be that on the largest scales, general relativity is simply not the right theory," says Freeman. There may be a way to modify general relativity slightly to account for the observed effects of dark energy, a possibility that Freeman plans to investigate more closely. In any case, there must be a theory that will expand Einstein's, and the dark energy survey project will certainly contribute to the development of this theory.

Is life a hologram?

Strange as they may be, it is still possible to think of dark matter and dark energy as nothing more than an ornament to the known universe: a glaze of additional particles or fields on top of a reality of the kind that Einstein would have recognized without any difficulty. But is it possible that the description of reality itself needs adjustments so that we can move towards a more comprehensive physical theory? Is it possible that space-time itself has new, as yet undiscovered properties that are not described by general relativity?

Craig Hogan, director of the Center for Particle Astrophysics at the American Fermi National Accelerator Laboratory (Fermilab), delves into this puzzle with an experiment he calls the Holometer. The experiment tries to check if space and time are built from elementary units: a universe composed by its very nature around the ticks of a clock and years of a ruler. According to this alternative point of view, our feeling that we live in a XNUMXD world is nothing more than an illusion: if we could look at space with sufficient magnification - more or less so that we would be looking at a piece of space that is ten billion billion times smaller than the size of an atom - we would see two pixels -Dimensional, which only look three-dimensional when viewed from a sufficiently distant perspective, similar to dots on a television screen.

Each of these elementary units will obey quantum laws. Thus, for example, there will be a certain degree of inherent uncertainty regarding the location of each such unit. On a large scale, space will appear continuous, just as Einstein thought, but at the fundamental level it will have a quantum structure. Such a pixelated universe would impose quantum mechanics on relativity and thus remove a serious obstacle to the creation of a unified theory of physics as a whole.

The idea of ​​two-dimensional reality appearing as if it were three-dimensional is known as the holographic principle, and this is where Hogan drew inspiration for the name of his experiment. "Holometer" is also a kind of play on words, reproducing the name of a device from the 16th century that was designed to accurately measure soils. The device built by Hogan, which is already collecting data at Fermilab, was also designed to test the area and make measurements with unprecedented precision. The device launches a laser beam that is split into two beams, each of which is sent to a different tunnel. The two rays hit the mirrors, are returned from them and then reunited. If the space has a quantum structure, the uncertainty in each of the pixels of the space should cause a jitter inside the device, a jitter that will cause a relative displacement between the waves of the two rays so that they are not coordinated. In theory, the holometer is able to measure indentations on the order of an atomometer: 10-18 meters!

But even that may not be small enough: some of Hogan's colleagues warn him that if space has an underlying quantum structure, it may be on an even smaller scale—so tiny that it would be impossible to detect experimentally. Hogan sees their skepticism as a challenge. In our conversation, he seems particularly amused by the way his experiment upsets Stanford University's Leonard Susskind, one of the main contributors to the idea of ​​the holographic universe. "Lenny has an opinion about how the holographic principle works, and it's not the idea tested in my experiment. He is quite convinced that nothing will be found. We met at a conference last year, and he stated that he would slit his own throat if we found the effect in our experiment," Hogan recalled.

It is likely that this debate will be decided one way or the other soon. After collecting data for an hour, the holometer is approaching Planck sensitivity, the scale at which Hogan thinks the bumpiness of space will begin to appear. Hogan predicts that a full answer will be received within a year - the time required to reach a much higher sensitivity than Planck's sensitivity, and then something will happen, only Hogan does not know for sure what: "Either we don't see something or we do see something. In any case, the space of possible ideas will be reduced. No one has a clue what to expect."

Einstein's dream, again

After hearing Hogan, I was also eager to get Susskind's perspective. Contrary to the stereotype of a thoughtful theoretical physicist in love with mathematical models, Susskind is quick to dive into a conversation about concepts that can be tested practically: "People complain that theoretical physicists spread ideas without responsibility because they don't need to test them experimentally. Nonsense. The importance of being able to test a theory is very clear to all of us," he says. But if there is a laboratory experiment to test the idea, the holometer is not the right experiment according to him.

Cosmic Crusaders
Location: Perimeter Institute for Theoretical Physics, Waterloo, Ontario. The project: theorists try to find ways to describe the entire universe within a single theoretical framework. One idea that has been proposed is that Einstein's time relativity could be replaced by Godel's relativity, which could lead to a new formulation of general relativity, one in which time and shape are meaningful while size is irrelevant.

Those doing the work: Daniel Carrasco Guariento, Gabriel Hertzeg, Flavio Mercati, Sean Grieve, Hamish Forbes; Niall O'Morcedha, Enrique Gomez, Andrea Napoletano, Julian Barbour, Lee Smolin. Credit: Sandy Nicholson.

A more promising approach, Susskind argues, is to look toward the edge of the visible universe and look for behavior that supports string theory. According to this theory, all particles and forces are actually different modes of oscillations in vibrating strings of energy, so there is basically a unified explanation for all of them. (These strings are different from the term "cosmic strings," which describes presumed defects in spacetime in the early universe.) The theory also has predictions about the physical conditions during the Big Bang. And what's more amazing: certain versions of this theory - the ones Susskind is working on - even have predictions about the conditions at an even earlier stage, before our universe was formed. Susskind believes that astronomers will be able to discover remnants of that primordial phase scorched by radiation coming from the far reaches of the universe.

But it is more likely, according to him, that the next steps towards the great unification of physics will not come from experiment or observation, but from intensive mathematical studies of black holes and of space and time. "Important things will happen in the next five, ten years," predicts Susskind. "I am not saying that we will have a complete theory of everything; We're not even close to that. But very deep insights await us regarding the connection between gravity and quantum mechanics."

And when the connection is discovered, Susskind expects - similar to most theoretical physicists today - that quantum mechanics will come out on top, so that gravity and general relativity will be forced into its framework. But since Einstein was the first to follow this path, it seems only fair to give the last word to one of his greatest proponents of the present time, physicist Lee Smolin of the Perimeter Institute for Theoretical Physics in Ontario.

Smolin is convinced that many of his colleagues who are immersed in quantum theories are simply thinking "small", literally, in their search for the final theory. "The only way to understand quantum mechanics is as a subsystem theory," says Smolin, "but general relativity is not a description of subsystems. It is a description of the world as a closed system." If we want to understand the entire universe, we must look at the world in relative terms, just as Einstein did.

This approach led Smolin to the sensational hypothesis that the laws of physics may change over time and that the universe has a memory of its own history - something he calls the principle of precedence. In this way, he hopes to move beyond the focused questions that quantum mechanics left unanswered (the strength of some field, or the mass of some particle) and treat them all as developmental aspects of a single universe functioning as a closed system. He even has an idea of ​​how to test his idea.

"If we succeed in developing a system that is large and complicated but still describable by a pure quantum state, we will force nature to produce new classification methods. One can imagine how this can be done with quantum devices," says Smolin. After we create the same system over and over again in the lab, nature may begin to develop a preference for a particular quantum state. "It will be difficult to distinguish between this situation and all the normal noise that is obtained in experiments. But it won't be impossible."

Smolin does not intend to sound mystical, but it seems that in a certain way he is not talking about the physical universe, but about Einstein's spirit. A century ago a single man discovered an innovative way of thinking about the universe. Sixty years ago this life was extinguished, as is the fate of all human life sooner or later. But Einstein's spirit still leaves a clear mark on today's researchers. They conduct new experiments in the service of an old ideal. This drive seems unstoppable: young researchers continue to reproduce his search for a deeper truth, a higher enlightenment.

About the writers

Corey S. Powell
He is a science reporter, editor and blogger living in Brooklyn, New York. He is a visiting scholar at New York University's Science, Health, and Environment Coverage Program. He can be followed at twitter.com/CoreysPowell.
for further reading

Search for Hidden Sector Photons with ADMX Detector. A. Wagner et al. in Physical Review Letters, Vol. 105, no. 17, Article No. 171801; October 19, 2010
Time Reborn: From the Crisis in Physics to the Future of the Universe. Lee Smolin. Houghton Mifflin Harcourt, 2013
Mass and Galaxy Distributions of Four Massive Galaxy Clusters from Dark Energy Survey Science Verification Data. P. Melchior et al. in Monthly Notices of the Royal Astronomical Society, Vol. 449, no. 3, pages 2219–2238; May 21, 2015
On the Generalized Theory of Gravitation. Albert Einstein; April 1950
Is the space digital? Michael Moyer. Scientific American Israel, June 2012
Atoms of space and time. Lee Smolin, Scientific American Israel, April-May 2004
The bad boy of physics, an interview with Leonard Susskind. Peter Byrne, Scientific American Israel, October-November 2011
Ion, website design and construction: Internet Technologies Unit, Ort Israel