A recent experiment by scientists from Brazil, the United Kingdom, and Germany achieved a spontaneous flow of heat from a cold object to a hot object, which is in complete violation of the classical formulation of the second law of thermodynamics. The thermodynamic time arrow defined by the second law of thermodynamics also corresponds to The ground was reversed. Does humanity create a time machine to return to the future?
Fig. Entropy and disorder
In the classic thermodynamic representation, the second law of thermodynamics states that the entropy of isolated systems will not decrease, and that all spontaneous reactions are the direction of entropy increase. Macroscopically, entropy is defined by balancing the heat exchange between the system and the environment; microscopically, entropy is a measure of the degree of system chaos. The whole content of classical thermodynamics is based on empirical observations and limited mathematics. These empirical observations have never been overturned, that is, since the mid-19th century, the zeroth, first, second, and third laws of thermodynamics have been many The iron law in the field of science. The physicist LÃdia del Rio once wrote: "The law of thermodynamics is like a witch. Many people think she is a bit strange and wants to find something in her, but no one dares to have a direct conflict with her."
The time arrow is a concept proposed by Sir Arthur Stanley Eddington in 1927 to describe the asymmetry of time, the direction of time. On the microscopic level, almost all physical processes are time-symmetrical, that is, all theoretical representations are still correct under the condition of time reversal. But on the macroscopic level, the direction of time is intuitive and obvious. For example, the spread of ink in water, heat does not spontaneously flow from cold objects to hot objects, etc. These time-related phenomena will never automatically reverse. . The second law of thermodynamics states that the spontaneous reaction direction is the direction in which the entropy increases, so the direction in which the entropy increases is also the direction of the macroscopic time arrow.
Fig.2 Thermodynamic time arrow
Is thermodynamics still valid at the microscopic or quantum scale? In the 1980s, the British physicist Kolsoff began to explore this problem. In recent years, with the development of quantum-scale theory and experiments, quantum thermodynamics has gradually become a hot area. Hundreds of theoretical and experimental scientists have done corresponding research, such as temperature The concept and picture based on the statistical behavior of a large number of particles are still applicable in the quantum scale, the new definition of free energy on the quantum scale, the unusual physical processes beyond the traditional thermodynamic content, and so on. At present, there have been several experiments with quantum scales that seem to violate the laws of thermodynamics, such as the enhancement of particle spin fluctuations by the phonon reflection of Schaetz et al., the diamond quantum heat engine of Kosloff et al.; in theory, interpretation by Freitas et al. Quantum refrigerators can never reach absolute zero due to the spontaneous generation of photons when they approach absolute zero, and the system originally entangled by quantum as described by Bera et al. makes the cold source cooler and the heat source hotter.
In an experiment by Brazilian physicists Micadei and Peterson and British physicist Serra, German physicist Lutz et al., a 13C-labeled CHCl3 dilute acetone solution was placed in a superconducting magnet and made 13C with 1/2 spin. Nuclear and 1H Nuclei Form a 2-Qubit Quantum System. As the experiment progressed, the degree of correlation between the 13C core and the 1H core gradually weakened, the number of states of the quantum bits gradually decreased, and the entropy associated with the degree of system chaos also became smaller; at the same time, the cold qubit became colder and hotter. The qubits get hotter. The spontaneous process performed by the experiment reverses the direction of the thermodynamic time arrow. Is the second law of thermodynamics broken?
Fig.3 The Entropy Variations of Classical Systems and Quantum Systems
Maxwell had a famous ideological experiment 150 years ago. His hypothesis is that if there is a monster that can move molecules with fast speeds to one side according to the speed of the molecules to get higher temperatures, and put slower molecules on the other side to get lower temperatures, Then you can extract useful energy based on the difference in temperature. The problem is that this monster does not work on those molecules in addition to the operating switches. The behavior of the monster violates the second law of thermodynamics. In fact, physicists have come to realize that monsters must pay the thermodynamic price to deal with information about the speed of molecular motion. It must store, delete, and rewrite that information in its head. These processes consume energy and increase total entropy. Information was once considered non-material, but Maxwell's demon showed that information can have physical and objective effects.
Fig.4 Maxwell's Demon
The experiments of Micadei and Serra et al. essentially reversed thermodynamic time arrows in their isolated quantum systems. Serra said: This reversal is a trade between quantum associations and entropy. The weakening of quantum associations has caused the flow of heat from cold qubits to hot qubits. Experimental results show that the time arrow is not an absolute concept, it is strongly It is relative, depending on the choice of initial conditions. Prior to the report by Micadei et al., French scientists Przadka et al.'s 2012 gravitational capillary wave experiment and ETH Zurich scientist Hofmann et al.'s 2016 GaAs/AlGaAs quantum dot theoretical and experimental studies have also achieved time reversal.
The experiments and theories of the quantum thermodynamic time arrow did not allow us to be closer to the time machine of manufacture, but it is possible to advance our thinking about the origin of the universe. One of the puzzles that has long puzzled people in cosmology is why the universe originated from a state with very low entropy, which allowed the entropy to increase over time in the entire development history of the universe. Combining entropy with quantum entanglement, Serra hopes that their experiments will also promote the discussion of the cosmic time arrow.
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