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Credit: Tel Aviv University

A new study led by Tel Aviv University researchers demonstrates unprecedented sensitivity to an exciting dark matter candidate. As part of the new NASDUCK (Noble and Alkali Spin Detectors for Ultralight Coherent dark-matter) collaboration, the researchers developed unique innovative quantum technology that enables receiving more accurate information on invisible theoretical particles suspected of being dark matter with ultralight masses. The study was published in the prestigious Advanced Science journal.

The study was led by Prof. Tomer Volansky, research student Itay Bloch from the Raymond & Beverly Sackler School of Physics & Astronomy in the Raymond & Beverly Sackler Faculty of Exact Sciences at Tel Aviv University, Gil Ronen from the Racah Institute of Physics at the Hebrew University, and Dr. Or Katz, formerly of the Weizmann Institute of Science (now from Duke University).

Dark Matter is one of the great mysteries of physics. It composes most of the matter in theuniverse, and it is known to interact through gravity; however, we still know very little of its nature and composition. Over the years, many different theoretical particles have been proposed as good candidates to serve as dark matter, including the so-called axion-like particles.

Prof. Tomer Volansky explains: The interesting thing about axion-like particles is that they can be significantly lighter than any of the matter particles seen around us, and still explain the existence of dark matter, which for years was expected to be significantly heavier. One of the main ways of searching for dark matter is by building a large experiment with lots of mass, waiting until dark matter collides with it or is absorbed in this mass, and then measuring the minute energetic imprint it leaves in its wake. However, if the mass of the dark matter is too small, the energy carried by it is so insignificant that neither the collision nor the absorption effect can be measured. Therefore, we need to be more creative and use other properties of dark matter.

In order to discover these particles, the researchers have designed and built a unique detector in which compressed, polarized xenon gas is used to find tiny magnetic fields. Surprisingly, it turns out that axion-like particles which play the role of dark matter, affect the polarized xenon particles as if it is placed in a weak anomalous magnetic field which can be measured. The innovative technique used for the first time by the researchers, enabled them to explore a new range of dark matter masses, improving previous techniques by as much as three orders of magnitude.

PhD student Itay Bloch adds: This is quite a complex operation, since these particles, if they exist, are invisible. Nevertheless, we have succeeded with this study in constraining the possible properties of axion-like particles, by the very fact that we have not measured them. Several attempts have been made to measure such particles by turning them into particles of light and vice versa. However, the innovation in our study is the measurement through atomic nuclei without relying on an interaction with light, and the ability to search for axion-like particles in masses that were hitherto inaccessible.

The study is based on especially complex mathematical methods taken from particle theory and quantum mechanics and employs advanced statistical and numerical models in order to compare the empirical results with the theory.

Prof. Volansky concludes: After five months of sustained effort, we have presented a new method that expands what we thought was possible with magnetometers; therefore, this is a small but significant step towards finding dark matter. There are many more candidates for dark matter, each with its own quantum properties. However, axion-like particles are among the most interesting options, and if we ever find them, that would be a huge step forward in our understanding of the universe. This experiment was the first of the NASDUCK collaboration, showing the promise that lies in our detectors. I have no doubt that this is just the beginning.

New constraints on axion-like dark matter using a Floquet quantum detector

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For the first time, new quantum technology demonstrates capabilities that may enable detection of ultralight dark matter - EurekAlert

One of the greatest ideas of all time is reductionism, the notion that every system, no matter how complex, can be understood in terms of the behavior of its basic constituents. Reductionism has its roots in ancient Greece, when Leucippus and Democritus, in about 400 BC, proposed that everything is composed of atoms, which in Greek means that which cannot be cut. So, atoms came to signify the smallest constituents of matter, even though what we understand by smallest has drastically changed in time.

The focus is on the bottom layer of the material chain: matter is made of molecules; molecules are made of atoms; atoms are made of electrons, protons, and neutrons; protons and neutrons are made of up and down quarks, and so on to presumably other possible levels of smallness unknown to us at present. At the biological level, organisms are composed of organs; organs of cells; cells of organic macromolecules; macromolecules of many atoms, etc.

The more radical view of reductionism claims that all behaviors, from elementary particles to the human brain, spring from bits of matter with interactions described by a few fundamental physical laws. The corollary is that if we uncover these laws at the most basic level, we will be able to extrapolate to higher and higher levels of organizational complexity.

Of course, most reductionists know, or should know, that this kind of statement is more faith-based than scientific. In practice, this extrapolation is impossible: studying how quarks and electrons behave wont help us understand how a uranium nucleus behaves, much less genetic reproduction or how the brain works. Hard-core reductionists would stake their position as a matter of principle, a statement of what they believe is the final goal of fundamental science namely, the discovery of the symmetries and laws that dictate (I would say describe to the best of our ability) the behavior of matter at the subatomic level. But to believe that something is possible in principle is quite useless in the practice of science. The expression fundamental science is loaded and should be used with care.

There is no question that we should celebrate the triumphs of reductionism during the first 400 years of science. Many of the technological innovations of the past four centuries derive from it, as does our ever-deepening understanding of how nature works. In particular, our digital revolution is a byproduct of quantum mechanics, the branch of physics that studies atoms and subatomic particles. The problem is not so much with how efficient reductionism is at describing the behavior of the basic constituents of matter. The problems arise as we try to go bottom-up, from the lowest level of material organization to higher ones.

We know how to describe with great precision the behavior of the simplest chemical element: the hydrogen atom, with its single proton and electron. However, even here, trouble lurks as we attempt to include subtle corrections, for example adding that the electron orbits the proton with relativistic speeds (i.e., close to the speed of light) or that its intrinsic rotation (or spin) gives rise to a magnetic force that interacts with a similar magnetic force of the proton. Physicists take these effects into account using perturbation theory, an approximation scheme that adds small changes to the allowed energies of the atom.

Physicists can also describe the next atom of the periodic table, helium, with considerable success due to its high degree of symmetry. But life gets complicated very quickly as we go up in complexity. More drastic and less efficient approximation schemes are required to make progress. And these dont include the interactions between protons and neutrons in the nucleus (which calls for a different force, the strong nuclear force), much less the fact that protons and neutrons are made of quarks and gluons, the particles responsible for the strong interactions.

Physics is the art of approximation. We dress down complex systems to their bare essentials and model them in as simple terms as possible without compromising the goal of understanding the complicated system we started from. This process works well until the complexity is such that a new set of laws and approaches is necessary.

At the next level of complexity are the molecules, assemblies of atoms. In a very rough way, all chemical reactions are attempts to minimize electric charge disparities. How many molecules can exist?

Lets jump to biochemistry for an illustration. Proteins are chains of amino acids. Since there are 20 different amino acids and a typical protein has some 200 of them, the number of possible proteins is around 20200. Increasing the length of the protein and hence the possible choices of amino acids leads to a combinatorial explosion. Physicist Walter Elsasser coined the term immense to describe numbers larger than 10100, a googol (that is, a one followed by 100 zeroes). The number of possible proteins is certainly immense. We see only a small subset realized in living creatures.

The number 10100 is not arbitrary. Elsasser showed that a list containing 10100 molecules would require a computer memory containing more than all the matter in the universe. Worse, to analyze the contents of the list, we would need longer than the age of the Universe, 13.8 billion years. There is an immense number of new molecules with unknown properties to be explored. The same goes for the number of genetic combinations, cell types, and mental states.

It is thus impossible to predict the behavior of complex biomolecules from a bottom-up approach based on fundamental physical laws. Quarks do not explain the behavior of neurons. The passage from one level of material organization to the next is not continuous. New laws are required for different layers of material organization, as described in the fast-growing field of complex systems theory. There are many texts on the subject, including this somewhat technical book. The exciting aspect of this new field is that it calls for new ways of thinking about natural systems, which are by nature more holistic such as network theory, nonlinear dynamics, chaos theory and fractals, and information theory. Climate science is another clear example.

In his prescient 1972 essay More is Different, Nobel laureate physicist Philip Anderson argued for this layering of physical laws, which are irreducible: We cannot deduce laws from a higher layer by starting at a lower level of complexity. The reductionist program meets a brick wall, where progress needs to be carved at each specific level of complexity. There are theories of things and not a theory of everything.

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Why reductionism fails at higher levels of complexity - Big Think

The present has a special status for us humans our past seems to no longer exists, and our future is yet to come into existence. But according to how physicists and philosophers interpret Einsteins Theory of Relativity, the present isnt at all special. The past and the future are just as real as the present - they all coexist and you could, theoretically, travel to them. But, argues Dean Buonomano, this interpretation of Einsteins theory might have more to do with the way our brains evolved to think of time in a similar way to space, than with the nature of time.

The human brain is an astonishingly powerful information processing device. It transforms the blooming buzzing confusion of raw data that impinges on our sensory organs into a compelling model of the external world. It endows us with language, rationality, and symbolic reasoning, and most mysteriously, it bestows us with consciousness (more precisely it bestows itself with consciousness). But, on the other hand, the brain is also a rather feeble and buggy information processing device. When it comes to mental numerical calculations the most complex device in the known universe is embarrassingly inept. The brain has a hodge-podge of cognitive biases that often lead to irrational decisions. And when it comes to understanding the nature of the universe, we should remember that the human brain was optimized to survive and reproduce in an environment we outgrew long ago, not decipher the laws of nature.

Is Einstein still right?Read moreTo date, the most powerful tool we have devised to overcome the brains limitations is called mathematics. Once in a while an outlier such as Einstein or Schrdinger conjures up equations that allow us to describe and predict the external world, independently of whether the human mind is capable of intuitively understanding those equations. We can plug those equations into a computer, which can then pump out predictions about what will occur when, whether or not we (or the computer) understand those equations.

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Much as chess is beyond the grasp of Schrdingers cat, an intuitive understanding of quantum mechanics is probably beyond the grasp of the human brain.

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Mathematics, however, is mostly agnostic to the interpretation of the equations of modern physics. This is particularly clear in the case of Schrdingers equation, which helped master the quantum world of particles that underlies much of our digital technology. No one can really claim to intuitively understand what a wavefunction actually is, or what it means for two photons two be entangled. Much as chess is beyond the grasp of Schrdingers cat, an intuitive understanding of quantum mechanics is probably beyond the grasp of the human brain.

The equations that comprise the laws of modern physics have proven accurate beyond any reasonable expectation, but when we interpret the equations of relativity and quantum mechanics, we often forget to take into account the inherent limitations, constraints, and biases, of the organ doing the interpreting. This point is particularly relevant in the context of what the laws of physics tell us in regard to the nature of time.

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Under eternalism time-travel is a theoretical possibility, as my past and future selves are in some sense physically real. In contrast, under presentism the notion of time travel is impossible by definition.

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While there is no universally accepted view as to the nature of time, the two main views are referred to as eternalism and presentism. In its simplest form eternalism maintains that the past, present, and future all stand on equal footing in an objective physical sense. The past, present, and future all coexist within what is called the block universe. Under presentism, my local present moment is fundamentally and objectively different from the past and future, because the past no longer exists and the future is yet to exist. Importantly presentism is local, and distinct from the empirically disproven Newtonian notion of absolute time, in which clocks moving at different speeds will remain synchronized. While some have argued that the distinction between eternalism and presentism is a false dichotomy, the fundamental difference between them can be easily captured in the context of time travel. Under eternalism time-travel is a theoretical possibility, as my past and future selves are in some sense physically real. In contrast, under presentism the notion of time travel is impossible by definition, one cannot travel to moments that dont exist.

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It is important to note that relativity does not predict that we live in an eternalist universe, rather it allows for an eternalist universe.

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One of the strongest arguments for eternalism was planted in 1908 by Herman Minkowskis geometric interpretation of Einsteins special theory of relativity. In it, time is represented as one axis in four-dimensional space, and movement of a clock along any of the three spatial dimensions will slow the rate at which it ticksMinkowski bound space and time into spacetime. But any geometric representation of time inevitably corrals the brain to think about time much like spacethinking of past and future moments in relation to now, as being as real as positions to the left and right of here. Indeed geometry, as formalized by Euclid over two thousand years ago was the study of static spatial relationships, and it was likely the first field of modern science because it had the luxury of ignoring time. Einsteins theory of general relativity further cemented the concept of spacetime into physics. But it is important to note that relativity does not predict that we live in an eternalist universe, rather it allows for an eternalist universe. Relativity makes no explicit testable predictions regarding eternalism versus presentism. Indeed, it is far from clear that there are any testable predictions that could prove or disprove eternalism or presentism (other than the emergence of a confirmed time traveler). And if advanced aliens ever came to Earth and assured us that we live in a presentist universe, I dont think anybody would claim that proves relativity is wrong (although presentism does set boundaries on the solutions to the equations of general relativity).

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Contrary to our everyday experiences, when interpreting the laws of physics, perhaps the architecture of the human brain imposes a bias towards eternalism.

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While the laws of physics do not assign any special significance to the present, they are ultimately agnostic as to whether the present may be fundamentally different from the past and future. Why then, despite our clear subjective experience that the present is special, is eternalism the favored view of time in physics and philosophy? Contrary to our everyday experiences, when interpreting the laws of physics, perhaps the architecture of the human brain imposes a bias towards eternalism. Thinking about time as a dimension in which all moments are equally real, better resonates with the brains architecture which readily accepts that all points in space are equally real.

The human brain is unique in its ability to conceptualize time along a mental timeline and engage in mental time travel. We can think about the past and simulate potential futures to degrees that evade the cognitive ability of other animals. It is mental time travel that allows us to engage in species-defining future-oriented activities, such as agriculture, science, and technology development. But how did humans come to acquire this ability? Evidence from linguistics, brain imaging, psychophysics, and brain lesion studies, suggest that the human brain may have come to grasp the concept of time by co-opting older evolutionary circuits already in place to represent and conceptualize space. A common example in the context of linguistics is that we use spatial metaphors for time (it was a long day; I look forward to seeing you). Imaging studies show a large overlap in brain areas associated with spatial and temporal cognition, and people with brain lesions that result in spatial hemineglect (generally characterized by an unawareness of left visual space), often exhibit deficits in mental time travel.

Our brains certainly did not evolve to understand the nature of time or the laws of the physics, but our brains did evolve to survive in a world governed by the laws of physics. Survival, of course, was not dependent on an intuitive grasp of physical laws on the quantum and cosmological scaleswhich is presumably why our intuitions epically fail on these scales. But questions pertaining to the reality of the past and future, fall squarely within the mesoscale relevant to survival. Thus, if one accepts that our subjective experiences evolved to enhance our chances of survival, our subjective experience about the passage of time and the fundamental differences between the present, past, and future, should be correlated to reality. A common counterexample to this point is our incorrect intuitions about the movement of the Earth. However, our incorrect perception that the Earth is static while the sun moves around us, pertains to the cosmological scale and is largely irrelevant to survival.

Empirical evidence from physics should always override our intuitions about the world. Yet in the case of the presentism versus eternalism debate there is actually no empirical evidence for eternalism. But there is some empirical evidence for presentism. Our brains are information processing devices designed to take measurements and make inferences about the physical world. Indeed, on the mesoscopic scale the brain does an impressive job at creating a representation of reality by measuring the physical properties of the world. It measures light, weight, temperature, movement, and time, in order to simulate the world well enough to survive in it. Our subjective experience of color or temperature, help us survive because they are correlated with reality.

I suspect that our subjective experiences regarding the nature of time also evolved because they capture some truth about the nature of the universe.

Perhaps one day objective evidence will emerge that we live in an eternalist universe, and we will understand why our subjective experiences are misleading. But until that day, we should accept our experience that the present is objectively different from the past and future as empirical evidence in favor of presentism.

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Einstein and why the block universe is a mistake - IAI