Unpacking a mystery of physics: Why processes in nature operate only in one direction

Why do processes in nature only work in one direction? For example, why can’t we heat up a cup of coffee in the fridge or prevent a drop of ink from spreading spontaneously in water?

It’s a question that’s puzzled many generations of physicists – and it stems from an incompatibility in the laws of physics, specifically between those that dictate the behaviour of macroscopic versus microscopic systems. Macroscopic systems can be seen with the naked eye; they consist of an extremely large number of atoms and molecules. Microscopic systems represent a different world: small enough that the behaviour of each individual atom or molecule can be described, but is not visible to our eyes.

Physicists can easily explain why the processes of macroscopic systems can’t reverse themselves spontaneously. It comes down to the second law of thermodynamics, which centres on the nature of the energy of a macroscopic system like a glass of water. This law provides a criterion that predicts the direction of spontaneous processes through the concept of entropy, a measure of order in matter. Liquids are less ordered than crystals, and gases are even less ordered. Hotter or more dispersed matter is higher in entropy. Simply put, entropy always increases; systems become more disordered as they progress spontaneously – and they cannot regress unless we supply energy.

A different set of physical laws exists when looking at the individual atoms and molecules that comprise a microscopic system. But these laws don’t explain what direction the processes in this system must take.

The matter and the processes are the same – but when they are studied from the macroscopic viewpoint the result may contradict that of the microscopic viewpoint. This is of course a problem.

In our new paper we argue that there’s a solution to this conundrum. The key is to distinguish between two types of reversibility: time-reversibility and thermodynamic reversibility. A smooth transition of the two types would pave the way to a unified theory that can describe all states of matter and all processes based on a single set of principles. This is what scientists are eagerly looking for.

Equilibrium and gradients

Consider a pendulum. It swings back and forth indefinitely in the absence of friction. If this motion is recorded and played backwards, there’s no difference; it would still look entirely natural. That’s a time-reversible process – the pendulum’s motion is symmetric with respect to time reversal.

But the heat that is dissipated from a cup of hot coffee never flows back. The heat inevitably flows from the hot coffee into the cooler air and the heat flow stops when the coffee and surrounding air have the same temperature. This final state is called equilibrium. Since it does not reverse like the pendulum the process is time-irreversible. A recording of it played backwards looks unnatural. This forward direction of processes in nature that stops at equilibrium is famously known as the arrow of time.

Then there’s thermodynamic reversibility. Heat dissipation is an example: it is driven by a heat gradient, going from warmer to cooler. In fact, all spontaneous processes are driven by some type of gradient – a temperature, concentration, or pressure difference. These processes proceed “downhill” along the gradient, from the higher to lower temperature, higher to lower concentration, or higher to lower pressure. This gradient provides the driving force of the process. Any process in the universe that is driven by some gradient is thermodynamically irreversible.

Gradients govern the course of events in small and large systems. The earth receives energy radiated from the hot surface of the sun and dissipates energy at a much lower temperature into the cold background of the universe. The processes of life (for plants, animals and humans, among other organisms) are also driven by gradients – their source of energy ultimately comes from the sun in the form of tiny light packets called photons.

All living organisms dissipate energy in the form of colder photons, which is eventually released into outer space.

Molecular memory

Time-reversibility doesn’t have anything to do with an entropy gradient. It’s about memory. A process is time-reversible if all the molecules can “remember” where they were and how fast they moved at every instance of time, so that every molecule’s motion can be reversed and the initial state restored. This can be simulated by modern computers if a system isn’t too large. As computer technology advances, increasingly larger and more complex systems can be described at the level of their individual atoms and molecules.

So, the apparent incompatibility between microscopic and macroscopic systems has nothing to do with the size of the system. It has to do with the type of process and whether that process wipes out the molecules’ “memory”.

In the case of heat, or of energy more generally, the same amount of energy that is used to synthesise a sugar molecule is set free when the molecule fuels a process in our body and decays back to its initial constituent molecules. This is the thermodynamic view; it neglects the aspect of time.

If it takes five minutes to synthesise the molecule it does not mean that the molecule also decays after exactly five minutes. We can’t predict the exact time that a molecule will decay because the process of decay is governed by a certain probability per unit time. And, importantly, probabilistic processes are never time-reversible because they contain no memory for the state in earlier times. A complete description of a probabilistic process requires one to take account of both the energetic and the timing aspects.

In this example both the synthesis of the sugar molecules and their decay are thermodynamically irreversible processes because a lot of energy must be added to reverse them. But this is completely different from time reversibility where memory is involved. So in this case, thermodynamic reversibility and time reversibility do not have the same origin.

This is the essence of the problem at hand. It is generally assumed that thermodynamic irreversibility and time irreversibility have the same probabilistic origin, which is often the truth but not always. Our paper argues that these two types of reversibility must be separated.The Conversation

Tjaart Krüger, Associate Professor in Biophysics, University of Pretoria and Emil Roduner, Professor, University of Stuttgart

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Prof Tjaart Krüger and Prof Emil Roduner

February 28, 2022

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Researchers
  • Prof Tjaart Krüger
    Professor Tjaart Krüger has been doing research at the University of Pretoria (UP) for nine years. Prof Krüger has established and leads the biophysics research team which is based in the Department of Physics. He completed his undergraduate studies at North-West University (then Potchefstroom University).

    Biophysics is a field of research that uses the rigorous laws and principles of physics to describe the complexity of biological systems. Prof Krüger says he became fascinated with biophysics a few years before embarking on his PhD, but because there was limited activity in this field in South Africa, he looked for PhD opportunities abroad, with hope of returning to South Africa to help establish the field in the country.

    Prof Krüger says going to Amsterdam to do his PhD was a life-changing experience. It was not only a career shift to switch from an MSc in Space Physics to a PhD in Biophysics, but he found the vibrant scientific environment stimulating.

    During his postdoctoral period, UP’s Department of Physics showed an interest in the field and offered Prof Krüger a position. He accepted the offer because, among other things, the position promised a reduced teaching load that would enable him to focus on establishing the new research field. UP also had a relatively large number of physics students compared to many other South African universities.

    “I considered human capital development to be an important element in establishing this new research field, as this would have been difficult without a growing critical mass,” he says, adding that despite a lot having happened over the past nine years, support at various levels is disappointing.

    In terms of how his field of research contributes to the betterment of the world, Prof Krüger says: “Without the advances in physics over the past two centuries, the world would have looked entirely different, because most technological advances are based on new discoveries in physics. It still applies to the world we live in today: the complex problems of today demand multidisciplinary efforts, and because physics is a fundamental, enabling science, it plays a critical role in addressing these issues.”

    Biophysics is multidisciplinary by definition, and aims to solve complex problems in biology and medicine, he adds.

    “Over the past few decades, biophysics has not only contributed to great advances in solving important and fundamental questions in biology, but has also shown to be a notable source of innovation. The following extract from the educational website of the international Biophysical Society states it well: ‘Biophysics is a wellspring of innovation for [any] high-tech economy... Society is facing physical and biological problems of global proportions. How will we continue to get sufficient energy; how can we feed the world’s population; how do we remediate global warming; how do we preserve biological diversity; how do we secure clean and plentiful water? These are crises that require scientific insight and innovation. Biophysics provides that insight and technologies for meeting these challenges.’

    “Biophysics underpins very large sections of the global bio-economy; therefore, a strong, diverse biophysics research and commercial sector is vital for the success of the African economy,” Prof Krüger says.

    Interestingly, he explains, many of the key-enabling technologies that the European Union is seeking to realise find examples in nature. In other words: “humankind is desperately seeking technological solutions that already exist in nature – biophysics is key to identifying and implementing these design principles in nature”.

    Prof Krüger is currently conducting research into the primary processes of photosynthesis.

    “The light-harvesting protein complexes of photosynthetic organisms are amazing molecular machines,” he explains. “They use quantum mechanics to optimise their functions, a property that has captivated physicists for many decades; they feature as delicate, smart nano-switches to maintain a fine balance between their light-harvesting and photoprotective functions, and exhibit an amazing variety. They offer various technological applications such as nano-sensing, molecular switches and light-activated molecular processes. These systems are also a great source of inspiration for finding green, sustainable energy solutions. Also, much of our food security depends on photosynthesis. Certain types of cyanobacteria, for instance, are viable sources of future food.”

    He says he has recruited a few new postgraduate students over the past 18 months and has embarked on several new research projects.

    “The focus of most of these research projects is still in line with the research I am doing, but there are many new elements and a broadening of the research scope. One good example is our development of new research methods. Over the past few years, my research group has built the first single molecule spectroscopy experimental facility in Africa, and we are continuing to broaden its scope of capabilities, making it a unique piece of equipment.”

    A specific highlight is the success of his project on time reversibility with Prof Emil Roduner; the project involves the notion of “molecular memory”. The outcome of their research was published in the journal Physics Reports earlier this year and Prof Krüger contributed an article about the research to The Conversation Africa. In May 2022, the two UP academics participated in a Cassyni seminar based on the content of their research.

    As far as interdepartmental and interdisciplinary research goes, Prof Krüger embarked on a new project with one of his postdoctoral fellows and Prof Don Cowan, Director of UP’s Centre for Microbial Ecology and Genomics. They investigated the photosynthetic properties of hypoliths, organisms that live (and thrive) underneath stones in the Namib Desert. Their research findings drew the attention of Physics Today, the flagship publication of the American Institute of Physics. It is the most influential and closely followed physics magazine in the world.

    In 2021, Prof Krüger joined the editorial board of The Journal of Physical Chemistry Letters; he was also invited to deliver keynote lectures at international conferences and to review grant applications for international grant agencies.

    He singles out a particularly inspiring incident at a conference in Berlin in 2014. He was talking to Prof WE Moerner of Stanford University, with whom he was collaborating at the time, and they were joined by Stefan Hell, a director at the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany. “Little did I know that a few weeks later, both would receive the Nobel Prize in Chemistry,” Prof Krüger says. “They are great scientists and still inspire me.”

    Prof Krüger says his grandfather is another great role model. “He was one of the key people in my life who showed me the beauty of nature through the eyes of a physicist. Even today, at 95, he is doing research in physics and is still enthusiastic and supportive of my research.”

    Prof Krüger hopes to witness the steady growth of biophysics research and education in South Africa to a level that would enable the establishment of strong biophysics institutes in the country in order to address important biophysics research questions and to serve as springboards for spin-off companies.

    Prof Krüger’s advice to school learners or undergraduates who are interested in his field is to nurture an inquisitive mind and feed their curiosity with excellent resources like the many stimulating educational websites, podcasts and video channels that are at our disposal. “Find people who are passionate about their research and learn from them,” he adds. “Learn to see every part of the world through the eyes of a physicist. See mathematics as a useful tool for physicists to describe the world. When your circumstances are challenging, do not blame them, your history or other people – rise above that and embrace every opportunity that comes your way.”

    He also encourages aspiring scientists to adopt the following qualities: “Perseverance (no matter the obstacles you encounter); creativity (which requires knowing cutting-edge problems, reading widely and setting aside sufficient time for reflective thinking); good interpersonal skills (because it is unlikely that you will succeed in doing impactful research on your own); good time management skills (to juggle all the responsibilities you have to face each day); and a healthy work-life balance (no matter how stimulating or demanding your work is).”

    His free time is devoted to his wife, three daughters and two sons, who he enjoys spending quality time with. He also loves music – he plays three musical instruments and composes his own music.
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