Scientists have discovered a new physical paradox

Researchers at Peter the Great St. Petersburg Polytechnic University (SPbPU) theoretically discovered and explained a new physical effect: the amplitude of mechanical vibrations can grow without external influence. The scientific group offered its explanation on how to eliminate the Fermi-Pasta-Ulam-Tsingou paradox.

The SPbPU scientists explained it using a simple example: to balance a swing, you must keep pushing it. Oscillatory resonance is generally believed to be impossible to achieve without constant external influence.

However, the scientific group from the Higher School of Theoretical Mechanics, Institute of Applied Mathematics and Mechanics SPbPU discovered a new physical phenomenon of ‘ballistic resonance’, where mechanical oscillations can only be excited due to the internal thermal resources of the system.

Experimental work by researchers around the world demonstrated that heat propagates at abnormally high speeds at nano and micro levels in ultrapure crystalline materials. This phenomenon is called ballistic thermal conductivity.

The scientific group, supervised by the corresponding member of the Russian Academy of Sciences Anton Krivtsov, derived the equations that describe this phenomenon and made significant progress in the general understanding of thermal processes at the micro level. In the study published in Physical Review E The researchers considered the behavior of the system in the initial periodic distribution of temperature in the crystalline material.

The phenomenon discovered describes that the thermal equilibrium process leads to mechanical vibrations with an amplitude that increases with time. The effect is called a ballistic resonance.

“In recent years, our scientific group has been investigating the mechanisms of heat propagation at the micro and nano levels. We found that at these levels, heat does not propagate in the way we expected: for example, heat can flow from cold to hot. This behavior of nanosystems leads to new physical effects, such as ballistic resonance, “said associate professor at the SPbPU Higher School of Theoretical Mechanics Vitaly Kuzkin.

According to him, in the future the researchers plan to analyze how this can be used in such promising materials as, for example, graphene.

These discoveries also provide an opportunity to resolve the Fermi-Pasta-Ulam-Tsingou paradox. In 1953, a scientific group led by Enrico Fermi carried out a computer experiment that later became famous. Scientists considered the simplest model of oscillations of a chain of particles connected by springs. They assumed that the mechanical movement would gradually fade, turning into chaotic thermal oscillations. Still, the result was unexpected: The oscillations in the chain first almost declined, but then revived and reached almost the initial level. The system reached its initial state and the cycle was repeated. The causes of mechanical oscillations of thermal vibrations in the system under consideration have been the subject of scientific investigation and dispute for decades.

The amplitude of the mechanical vibrations caused by the ballistic resonance does not increase infinitely, but reaches its maximum; after that it begins to gradually decrease to zero. Eventually, the mechanical oscillations fade away completely and the temperature balances across the glass. This process is called thermalization. For physicists, this experiment is vital because a chain of particles connected by springs is a good model of crystalline material.

Researchers from the Higher School of Theoretical Mechanics demonstrated that the transition from mechanical energy to heat is irreversible if we consider the process at finite temperature.

“In general, it is not taken into account that in real materials, there is a thermal movement, along with a mechanical movement, and the energy of the thermal movement is several orders of magnitude higher. We recreate these conditions in a computer experiment and demonstrate that It is thermal movement that dampens the mechanical wave and prevents the reactivation of oscillations, “explained Anton Krivtsov, director of the SPbPU Higher School of Theoretical Mechanics, a corresponding member of the Russian Academy of Sciences.

According to experts, the theoretical approach proposed by SPbPU scientists demonstrates a new approach to how we understand heat and temperature. It may be essential in the development of nanoelectronic devices in the future.

Provided by Peter the Great Saint-Petersburg Polytechnic University