Remember 3 ways heat transfer? In physics textbooks, except for thermal radiation, thermal conduction, heat convection, these two ways of heat transmission through the acoustic, can not occur in a vacuum. But in the eyes of quantum physicists, vacuum is not a real “empty”, but full of quantum ups and downs. A recent experiment published in the journal Nature has proved for the first time that quantum effects allow phonoones to transmit heat in a vacuum. Finally, a whole new way of heat transfer was found.
Quantum ups and downs allow the soundtosin to transmit heat in a vacuum. (Photo credit: Zhang Xiang/University of California, Berkeley)
We are taught from an early age not to touch the pot on the stove when cooking, and not to approach the flame, or it will be burned. Whether through direct contact or through the radiation of light, heat transfer will always make us suffer and impress.
In our middle school physics class, we learned three further ways of heat transfer: thermal conduction through direct contact to transmit heat, thermal convection through liquid or gaseous media, and thermal radiation transmitted by photons (carriers of electromagnetic radiation). In addition to thermal radiation, the first two heat transfer methods can not be carried out in a vacuum.
Now, scientists have discovered a whole new way of heat transfer. They used the incredible nature of quantum mechanics to transfer heat from one point in the vacuum to another without the help of photons.
Heat with quantum ups and downs?
Heat is a manifestation of the irregular motion of microscopic particles inside an object — when microscopic particles move faster, the object’s temperature is higher. On the cosmic scale, most of the heat of a star is transmitted in a vacuum by photons — that’s how the sun transmits heat to Earth from 150 million kilometers away. On Earth, however, heat is transmitted in contact with the collective excitation of the soundatosin (the collective excitation of atomic vibrations).
According to the previous view, if the soundises to transmit heat, then two objects must be in contact, or there must be at least air and other media between them. If the vacuum separates the two objects, it is impossible to transmit heat through the sound. The kettle is made according to this principle: the shell of the kettle and the inner bile between the vacuum, so that the water in the bottle can be kept warm for a long time. However, with the development of quantum mechanics, some scientists began to speculate that phonons might be able to spread heat in a vacuum. This hypothesis is based on the incredible fact that, from a quantum mechanics perspective, a vacuum of nothing is non-existent.
According to quantum mechanics, the universe is inherently fuzzy. For example, you can’t do your best to determine the momentum and position of a subatomic particle at a certain time. The consequence of this uncertainty is that the vacuum is never completely empty, but is filled with quantum ups and downs , the emergence and disappearance of so-called “virtual particles”.
Decades ago, scientists discovered that virtual particles do not exist only in theory. In fact, the forces they produce can be detected. The Kasimir effect, for example, refers to the tiny gravitational pull between two objects when they are placed in a vacuum at close range. For example, if you put two mirrors face to face in a vacuum, the force generated by the virtual photons will appear and disappear, and the forces they produce will bend the surface of the mirror.
The phenomenon has inspired physicists to think that if these brief quantum ups and downs produce real force, they may also have other effects — such as passing heat without heat radiation.
To understand how phonoids transmit heat through quantum ups and downs, let’s assume that there are two separate objects in the vacuum with different temperatures. The soundinas in a high-temperature object can transmit heat to virtual particles in a vacuum, which in turn transmit heat to a low-temperature object. If we think of both objects as a collection of vibrating atoms, then virtual particles are like a spring that transmits the vibration of one object to another.
John Pendry, a physicist at Imperial College of Technology who was not involved in the study, said the question of whether quantum ups and downs could really deliver heat from a sound in a vacuum and how efficient it would be if it could be transmitted, “these issues have been controversial over the last decade.” There are big differences in estimates by different theoretical physicists because the calculation process is very difficult. In general, he explains, previous studies predicted that this effect could only be observed when the distance between two objects was at a nanoscale. However, with such a short distance, the electrostatic action between the two objects or other nanoscale softenings can cause strong interference, so it is very difficult to observe the thermal transfer effect of the sound.
New mechanisms for heat transfer
In this latest paper, Zhang Xiang of the University of California, Berkeley, led the team in the experiment in order to spread the heat of the sounds at a scale of several hundred nanometers. They used two silicon nitride films, each about 100 nanometers thick. The vibration atoms in the membrane cause each membrane to oscillate back and forth at a certain frequency, so when the temperature changes, the way the membrane moves also changes. The membrane is light and thin, so when one piece of energy affects the motion of the other, researchers can easily observe the effect.
Experimental device diagram. (Photo credit: Zhang Xiang/University of California, Berkeley)
Zhang’s team realized that if the two films were the same size but at different temperatures, they would vibrate at different frequencies. As a result, the researchers specifically customized the different sizes of the two membranes so that they could vibrate at frequencies of 191,600 times per second at different starting temperatures (13.85 degrees C and 39.35 degrees C, respectively). When the two membranes resonate, the energy is quickly exchanged.
In addition, the researchers ensured that the two membranes were parallel to each other, with errors of no more than a few nanometers. They also ensure that the membrane is very smooth, the surface of the bump no more than 1.5 nanometers. In the experiment, two membranes were fixed to both sides of the vacuum chamber, and they heated one with a heater and cooled the other with a chiller. In order to detect the vibration frequency, i.e. the change in temperature, the surface of both membranes is covered with a gold reflector layer as thin as a cobweb and irradiated with a weak laser. After many experiments, the team confirmed that there was no thermal conduction on the contact surface of the membrane and the vacuum chamber, and that there was no thermal radiation between the two membranes with the help of electromagnetic waves.
“This experiment has very high control accuracy for temperature, distance and calibration,” Zhang said. We had an attempt to do this experiment in the summer and the result was an increase in room temperature in the lab. In addition, the measurement itself took a long time to eliminate noise, and each data point required four hours of measurement. ”
Eventually, the team found that when the distance between the two membranes was less than 600 nanometers, their temperature changed, and the change could not be explained by other theories. When the distance is less than 400 nanometers, the rate of heat exchange is sufficient for the membrane temperature to change significantly.
After the experiment was successful, the researchers calculated the maximum efficiency of the sonar to transmit energy: about 6.5 x 10-21 joules per second. At this rate, it takes 50 seconds to transfer the full energy of a visible photon. Although this may seem trivial, Zhang says this is still “a new mechanism for heat to pass between two objects”.
According to Zhang Xiang, in principle, stars can also heat their planets through this mechanism. However, given the distance between them, the scale of the effect may be “very small” and almost ignored.
In our lives, from smartphones to laptops, almost all electronic devices are getting smaller and smaller. The findings may help engineers deal with heat problems in nanoscale technology. “In hard drives, for example, the gap between the movement of the read and write head on the surface of the disk is only 3 nanometers, ” Zhang said. ” This should be taken into account when designing magnetic recording devices. ”
Zhang Xiang mentioned that quantum ups and downs are not just about virtual photons. In addition to virtual photons, there are many other kinds of virtual particles, such as virtual gravinom, which is the energy of a gravitational field. “Whether the quantum fluctuations of gravitational fields can trigger a cosmic-scale heat transfer mechanism is an interesting question, and it awaits us to explore,” Zhang said. “