Heat flows from hot to cold objects. When a hot and a cold body are in thermal contact, they exchange heat energy until they reach thermal equilibrium, with the hot body cooling down and the cold body warming up. This is a natural phenomenon we experience all the time.
It is explained by the second law of thermodynamics, which states that the total entropy of an isolated system always tends to increase over time until it reaches a maximum. Entropy is a quantitative measure of the disorder in a system. Isolated systems evolve spontaneously toward increasingly disordered states and lack of differentiation.
An experiment conducted by researchers at the Brazilian Center for Research in Physics (CBPF) and the Federal University of the ABC (UFABC), as well as collaborators at other institutions in Brazil and elsewhere, has shown that quantum correlations affect the way entropy is distributed among parts in thermal contact, reversing the direction of the so-called "thermodynamic arrow of time".
In other words, heat can flow spontaneously from a cold object to a hot object without the need to invest energy in the process, as is required by a domestic fridge. An article describing the experiment with theoretical considerations has just been published in Nature Communications.
The first author of the article, Kaonan Micadei, completed his PhD under the supervision of Professor Roberto Serra and is now doing postdoctoral research in Germany. Serra, also one of the authors of the article, was supported by São Paulo Research Foundation - FAPESP via Brazil's National Institute of Science and Technology in Quantum Information. FAPESP also awarded two research grants linked to the project to another coauthor, Gabriel Teixeira Landi, a professor at the University of São Paulo's Physics Institute (IF-USP).
"Correlations can be said to represent information shared among different systems. In the macroscopic world described by classical physics, the addition of energy from outside can reverse the flow of heat in a system so that it flows from cold to hot. This is what happens in an ordinary refrigerator, for example," Serra told.
"It's possible to say that in our nanoscopic experiment, the quantum correlations produced an analogous effect to that of added energy. The direction of flow was reversed without violating the second law of thermodynamics. On the contrary, if we take into account elements of information theory in describing the transfer of heat, we find a generalized form of the second law and demonstrate the role of quantum correlations in the process."
The experiment was performed with a sample of chloroform molecules (a hydrogen atom, a carbon atom and three chlorine atoms) marked with a carbon-13 isotope. The sample was diluted in solution and studied using a nuclear magnetic resonance spectrometer, similar to the MRI scanners used in hospitals but with a much stronger magnetic field.
"We investigated temperature changes in the spins of the nuclei of the hydrogen and carbon atoms. The chlorine atoms had no material role in the experiment. We used radio frequency pulses to place the spin of each nucleus at a different temperature, one cooler, another warmer. The temperature differences were small, on the order of tens of billionths of 1 Kelvin, but we now have techniques that enable us to manipulate and measure quantum systems with extreme precision. In this case, we measured the radio frequency fluctuations produced by the atomic nuclei," Serra said.
The researchers explored two situations: in one, the hydrogen and carbon nuclei began the process uncorrelated, and in the other, they were initially quantum-correlated.
"In the first case, with the nuclei uncorrelated, we observed heat flowing in the usual direction, from hot to cold, until both nuclei were at the same temperature. In the second, with the nuclei initially correlated, we observed heat flowing in the opposite direction, from cold to hot. The effect lasted a few thousandths of a second, until the initial correlation was consumed," Serra explained.
The most noteworthy aspect of this result is that it suggests a process of quantum refrigeration in which the addition of external energy (as is done in refrigerators and air conditioners to cool a specific environment) can be replaced by correlations, i.e., an exchange of information between objects.
Maxwell's demon
The idea that information can be used to reverse the direction of heat flow - in other words, to bring about a local decrease in entropy - arose in classical physics in the mid-nineteenth century, long before information theory was invented.
It was a thought experiment proposed in 1867 by James Clerk Maxwell (1831-1879), who, among other things, created the famous classical electromagnetism equations. In this thought experiment, which sparked a heated controversy at the time, the great Scottish physicist said that if there were a being capable of knowing the speed of each molecule of a gas and of manipulating all the molecules at the microscopic scale, this being could separate them into two recipients, placing faster-than-average molecules in one to create a hot compartment and slower-than-average molecules in the other to create a cold compartment. In this manner, a gas initially in thermal equilibrium owing to a mixture of faster and slower molecules would evolve to a differentiated state with less entropy.
Maxwell intended the thought experiment to prove that the second law of thermodynamics was merely statistical.
"The being he proposed, which was capable of intervening in the material world at the molecular or atomic scale, became known as 'Maxwell's demon'. It was a fiction invented by Maxwell to present his point of view. However, we're now actually able to operate at the atomic or even smaller scales, so that usual expectations are modified," Serra said.
The experiment conducted by Serra and collaborators and described in the article just published is a demonstration of this. It did not reproduce Maxwell's thought experiment, of course, but it produced an analogous result.
"When we talk about information, we're not referring to something intangible. Information requires a physical substrate, a memory. If you want to erase 1 bit of memory from a flash drive, you have to expend 10,000 times a minimum amount of energy consisting of the Boltzmann constant times the absolute temperature. This minimum of energy necessary to erase information is known as Landauer's principle. This explains why erasing information generates heat. Notebook batteries are consumed by heat more than anything else," Serra said.
What the researchers observed was that the information present in the quantum correlations can be used to perform work, in this case the transfer of heat from a colder to a hotter object, without consuming external energy.
"We can quantify the correlation of two systems by means of bits. Connections between quantum mechanics and information theory are creating what is known as quantum information science. From the practical standpoint, the effect we studied could one day be used to cool part of a quantum computer's processor," Serra said.
###
About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at http://www.FAPESP.br/en and visit FAPESP news agency at http://www.agencia.FAPESP.br/en to keep updated with the latest scientific breakthroughs FAPESP helps achieve through its many programs, awards and research centers. You may also subscribe to FAPESP news agency at http://agencia.FAPESP.br/subscribe.
Throughout the universe, it’s natural for energy to flow from one place to another. And unless people interfere, thermal energy — or heat — naturally flows in one direction only: from hot toward cold.
Heat moves naturally by any of three means. The processes are known as conduction, convection and radiation. Sometimes more than one may occur at the same time.
First, a little background. All matter is made from atoms — either single ones or those bonded in groups known as molecules. These atoms and molecules are always in motion. If they have the same mass, hot atoms and molecules move, on average, faster than cold ones. Even if atoms are locked in a solid, they still vibrate back and forth around some average position.
Thank you for signing up!
There was a problem signing you up.
In a liquid, atoms and molecules are free to flow from place to place. Within a gas, they are even more free to move and will completely spread out within the volume in which they are trapped.
Some of the most easily understood examples of heat flow occur in your kitchen.
Conduction
Put a pan on a stovetop and turn on the heat. The metal sitting over the burner will be the first part of the pan to get hot. Atoms in the pan’s bottom will start to vibrate faster as they warm. They also vibrate farther back and forth from their average position. As they bump into their neighbors, they share with that neighbor some of their energy. (Think of this as a very tiny version of a cue ball slamming into other balls during a game of billiards. The target balls, previously sitting still, gain some of the cue ball’s energy and move.)
As a result of collisions with their warmer neighbors, atoms start moving faster. In other words, they are now warming. These atoms, in turn, transfer some of their increased energy to neighbors even farther from the original source of heat. This conduction of heat through a solid metal is how the handle of a pan gets hot even though it may be nowhere near the source of heat.
Convection
Convection occurs when a material is free to move, such as a liquid or a gas. Again, consider a pan on the stove. Put water in the pan, then turn on the heat. As the pan gets hot, some of that heat transfers to the molecules of water sitting on the bottom of the pan via conduction. That speeds up the motion of those water molecules — they are warming.
Lava lamps illustrate heat transfer via convection: Waxy blobs get warmed at the base and expand. This makes them less dense, so they rise to the top. There, they give off their heat, cool and then sink to complete the circulation.Bernardojbp/iStockphoto
As the water warms, it now begins to expand. That makes it less dense. It rises above denser water, carrying away heat from the bottom of the pan. Cooler water flows down to take its place next to the hot bottom of the pan. As this water warms, it expands and rises, ferrying its newly-gained energy with it. In short order, a circular flow of rising warm water and falling cooler water sets up. This circular pattern of heat transfer is known as convection.
It’s also what largely warms food in an oven. Air that’s warmed by a heating element or gas flames at the top or bottom of the oven carries that heat to the central zone where the food sits.
Air that’s warmed at Earth’s surface expands and rises just like the water in the pan on the stove. Large birds such as frigate birds (and human flyers riding engineless gliders) often ride these thermals — rising blobs of air — to gain altitude without using any energy of their own. In the ocean, convection caused by heating and cooling helps to drive ocean currents. These currents move water around the globe.
Radiation
The third type of energy transfer is in some ways the most unusual. It can move through materials — or in the absence of them. This is radiation.
Radiation, such as the electromagnetic energy spewing from the sun (seen here at two ultraviolet wavelengths) is the only type of energy transfer that works across empty space.NASA
Consider visible light, a form of radiation. It passes through some types of glass and plastic. X-rays, another form of radiation, readily pass through flesh but are largely blocked by bone. Radio waves pass through the walls of your home to reach the antenna on your stereo. Infrared radiation, or heat, passes through the air from fireplaces and light bulbs. But unlike conduction and convection, radiation doesn’t require a material to transfer its energy. Light, X-rays, infrared waves and radio waves all travel to Earth from the far reaches of the universe. Those forms of radiation will pass through plenty of empty space along the way.
X-rays, visible light, infrared radiation, radio waves are all different forms of electromagnetic radiation. Each type of radiation falls into a particular band of wavelengths. Those types differ in the amount of energy they have. In general, the longer the wavelength, the lower the frequency of a particular type of radiation and the less energy it will carry.
To complicate things, it’s important to note that more than one form of heat transfer may occur at the same time. A stove’s burner not only heats a pan but also the nearby air and makes it less dense. That carries warmth upward via convection. But the burner also radiates heat as infrared waves, making things nearby warm up. And if you’re using a cast-iron skillet to cook a tasty meal, be sure to grab the handle with a potholder: It’s gonna be hot, thanks to conduction!