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-------------------------- 2451 - TIME CRYSTALS - tell us about time?
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- Crystals are orderly states of matter in which the arrangements of atoms take on repeating patterns. In physics, they are called “spontaneously broken spatial symmetry.”
“Time crystals” are states of matter whose patterns repeat at set intervals of time rather than space. They are systems in which time symmetry is spontaneously broken.
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- The notion of time crystals was first proposed in 2012, and in 2017 scientists discovered the first new materials that fully fit this category. These and others that followed offer promise for the creation of clocks more accurate than ever before.
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- Crystals are nature's most orderly substances. Inside crystals atoms and molecules are arranged in regular, repeating structures, giving rise to solids that are stable and rigid.
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- People have found crystals fascinating and attractive since before the dawn of modern science, often prizing them as jewels. In the 19th century scientists' efforts to classify forms of crystals and understand their effect on light catalyzed important progress in mathematics and physics.
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- In the 20th century, study of the fundamental quantum mechanics of electrons in crystals led directly to modern semiconductor electronics and eventually to smart phones and the Internet.
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- The next step in our understanding of crystals is occurring today using Albert Einstein's relativity theory where space and time are intimately connected. It is natural to wonder whether any objects display properties in time that are analogous to the properties of ordinary crystals in space.
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- In exploring this question “time crystals.” were discovered. In common usage, “symmetry” very broadly indicates balance, harmony or even justice. In physics and mathematics, the meaning is more precise. We say that an object is symmetric or has symmetry if there are transformations that could change it but do not change it.
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- Consider a circle. When we rotate a circle around its center, through any angle, it remains visually the same, even though every point on it may have moved. A circle has perfect “rotational symmetry“.
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- A square has some symmetry but less than a circle because you must rotate a square through a full 90 degrees before it regains its initial appearance.
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- We say a law has symmetry if we can change the context in which the law is applied without changing the law itself. For example, the basic axiom of special relativity is that the same physical laws apply when we view the world from different platforms that move at constant velocities relative to one another. Relativity demands that physical laws display a kind of symmetry.
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- A different class of transformations is important for crystals, including time crystals. Whereas relativity says the same laws apply for observers on moving platforms, “spatial translation symmetry” says the same laws apply for observers on platforms in different places.
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- “Time translation symmetry” expresses a similar idea but for time instead of space. It says the same laws we operate under now also apply for observers in the past or in the future. In other words, the laws we discover at any time apply at every time.
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- Without space and time translation symmetry, experiments carried out in different places and at different times would not be reproducible. In their everyday work, scientists take those symmetries for granted. Indeed, science as we know it would be impossible without them.
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- But, it is important to emphasize that we can test space and time translation symmetry empirically. Specifically, we can observe behavior in distant astronomical objects. Such objects are situated, obviously, in different places, and thanks to the finite speed of light we can observe in the present how they behaved in the past. Astronomers have determined, in great detail and with high accuracy, that the same laws do in fact apply.
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- For all their aesthetic symmetry, it is actually the way crystals lack symmetry that is, for physicists, their defining characteristic. Consider a drastically idealized crystal. It will be one-dimensional, and its atomic nuclei will be located at regular intervals along a line, separated by the distance “d“.
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- If we translate this crystal to the right by a tiny distance, it will not look like the same object. Only after we translate through the specific distance “d” will we see the same crystal. Thus, our idealized crystal has a reduced degree of spatial translation symmetry, similarly to how a square has a reduced degree of rotation symmetry.
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- Physicists say that in a crystal the translation symmetry of the fundamental laws is “broken,” leading to a lesser translation symmetry. That remaining symmetry conveys the essence of our crystal. If we know that a crystal's symmetry involves translations through multiples of the distance “d“, then we know where to place its atoms relative to one another.
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- Crystalline patterns in two and three dimensions can be more complicated, and they come in many varieties. They can display partial rotational and partial translational symmetry.
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- We can extend the classification of possible crystalline patterns in three-dimensional space to crystalline patterns in four-dimensional spacetime. Whereas ordinary crystals are orderly arrangements of objects in space, spacetime crystals are orderly arrangements of events in spacetime.
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- For example, Earth repeats its orientation in space at daily intervals, and the Earth-Sun system repeats its configuration at yearly intervals. Inventors and scientists have, over many decades, developed systems that repeat their arrangements at increasingly accurate intervals for use as clocks.
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- Pendulum and spring clocks were superseded by clocks based on vibrating traditional crystals, and those were eventually superseded by clocks based on vibrating atoms. Atomic clocks have achieved extraordinary accuracy, but there are important reasons to improve them further using “time crystals“.
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- When a liquid or gas cools into a crystal, something fundamentally remarkable occurs: the emergent solution of the laws of physics, the crystal displays less symmetry than the laws themselves. As this reduction of symmetry is brought on just by a decrease in temperature, without any special outside intervention, we can say that in forming a crystal the material breaks spatial translation symmetry “spontaneously.”
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- An important feature of crystallization is a sharp change in the system's behavior or, in technical language, a sharp “phase transition“. Above a certain critical temperature we have a liquid; below it we have a crystal, objects with quite different properties.
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- The transition occurs predictably and is accompanied by the emission of energy in the form of heat. The rigidity of crystals is another emergent property that distinguishes them from liquids and gases. From a microscopic perspective, rigidity arises because the organized pattern of atoms in a crystal persists over long distances and the crystal resists attempts to disrupt that pattern.
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- The three features of crystallization reduced symmetry, sharp phase transition and rigidity are deeply related. The basic principle underlying all three is that atoms “want” to form patterns with favorable energy. Different choices of pattern, different phases, can win out under different conditions of various pressures and temperatures.
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- A profound theorem proved by mathematician Emmy Noether in 1915 makes a connection between symmetry principles and conservation laws—it shows that for every form of symmetry, there is a corresponding quantity that is conserved.
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- Noether's theorem states that time is basically equivalent to the conservation of energy. Conversely, when a system breaks time, energy is not conserved, and it ceases to be a useful characteristic of that system. ( See Review 2052 to learn more about Emmy Noether’s mathematical theories.)
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- The usual explanation for why spontaneous symmetry breaking occurs is that it can be favorable energetically. If the lowest-energy state breaks spatial symmetry and the energy of the system is conserved, then the broken symmetry state, once entered, will persist.
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- But that energy-based explanation will not work for time breaking, because time breaking removes the applicable measure of energy. This apparent difficulty put the possibility of spontaneous time breaking, and the associated concept of time crystals, beyond the conceptual horizon of most physicists.
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- In the right atmosphere, if the temperature is hot enough, a diamond will burn into inglorious ash. Diamonds are not a stable state of carbon at ordinary temperatures and atmospheric pressure. They are created at much higher pressures and, once formed, will survive for a very long time at ordinary pressures.
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- But physicists calculate that if you wait long enough, your diamond will turn into graphite. Even less likely, but still possible, a quantum fluctuation can turn your diamond into a tiny black hole. It is also possible that the decay of a diamond's protons will slowly erode it. In practice, what we mean by a “state of matter” (such as diamond) is an organization of a substance that has a useful degree of stability against a significant range of external changes.
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- “New” time crystals arrived with the March 9, 2017, issue of Nature, which featured gorgeous (metaphorical) time crystals on the cover and announced “Time crystals: First observations of exotic new state of matter.”
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- The spin direction of the atoms (either with the ytterbium ions or the diamond defects) changes with regularity, and the atoms periodically come back into their original configurations.
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- Researchers have used lasers to flip the ions' spins and to correlate the spins into connected, “entangled” states. As a result, though, the ions' spins began to oscillate at only half the rate of the laser pulses.
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- Scientists have also used microwave pulses to flip the diamond defects' spins. They observed time crystals with twice and three times the pulse spacing. In all these experiments, the materials received external stimulation, lasers or microwave pulses, but they displayed a different period than that of their stimuli. In other words, they broke time symmetry spontaneously.
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- These experiments started a direction in materials physics that has grown into a minor industry.
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- These new time crystals display strong rigidity and stability in their pattern. This feature offers a way of dividing up time very accurately, which could be the key to advanced clocks. Modern atomic clocks are marvels of accuracy, but they lack the guaranteed long-term stability of time crystals.
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- More accurate, less cumbersome clocks based on these emerging states of matter could empower exquisite measurements of distances and times. Applications range from improved GPS to new ways of detecting underground caves and mineral deposits through their influence on gravity or even gravitational waves.
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- The steady-state-universe model was a principled attempt to maintain time in cosmology. In that model, popular in the mid-20th century, astronomers postulated that the state, or appearance, of the universe on large scales is independent of time, in other words, it upholds time symmetry.
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- Although the universe is always expanding, the steady-state model postulated that matter is continuously being created, allowing the average density of the cosmos to stay constant.
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- But the steady-state model did not survive the test of time. Instead astronomers have accumulated overwhelming evidence that the universe was a very different place 13.7 billion years ago, in the immediate aftermath of the Big Bang, even though the same physical laws applied.
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- In that sense, time is spontaneously broken by the universe as a whole. Some cosmologists have also suggested that ours is a cyclic universe or that the universe went through a phase of rapid oscillation. These speculations bring us close to the circle of ideas around time crystals.
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- The equations of general relativity, which embody our best present understanding of spacetime structure, are based on the concept that we can specify a definite distance between any two nearby points. This simple idea, though, is known to break down in at least two extreme conditions: when we extrapolate Big Bang cosmology to its initial moments and in the central interior of blackholes.
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- Elsewhere in physics, breakdown of the equations that describe behavior in a given state of matter is often a signal that the system will undergo a phase transition. Could it be that spacetime itself, under extreme conditions of high pressure, high temperature or rapid change, abandons time?
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- Ultimately the concept of time crystals offers a chance for progress theoretically in terms of understanding cosmology and blackholes and practically.
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- The novel forms of time crystals most likely to be revealed in the coming years should move us closer to more perfect clocks, and they may turn out to have other useful properties. Only time will tell.
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- October 18, 2019
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