In the last two centuries, Science has answered many questions about nature and the laws that govern it. We’ve been able to research galaxies and the atoms that make up matter. We’ve built machines that can compute and solve problems that no human can work out. We’ve solved age old problems in Mathematics and created theories that gave maths new problems. This article, however, is not about these achievements. It’s about those problems in science that have left scientists scratching their heads, wondering if they’ll ever yell, “Eureka!”
Turbulence isn’t a new word. You’re probably more familiar with it as the word that describes the sudden bumpiness during a flight. However, turbulence in fluid mechanics is a whole different ball game. Flight turbulence, more technically referred to as ‘clear-air turbulence’, is caused due to the meeting of two bodies of air moving at different speeds. Physicists, however, have a much tougher time explaining this phenomena of turbulence in fluids. Mathematicians have nightmares about it.
Fluid turbulence is all around us. The jet from a tap that’s turned on completely breaks into chaotic patches of fluid, different from the unified stream of jet we get when we only half open it. This is one of the classical examples of turbulence and is used to explain it to school students and graduates alike. Turbulence is ubiquitous in nature, occurring in various geophysical and oceanic flows. It’s also of importance to engineers as it occurs frequently in flow over turbine blades, aerofoils and other bodies. Turbulence is characterised by random fluctuations in variables like velocity and pressure.
While there have been plenty of experiments and empirical data regarding turbulence, we’re still far from a convincing theory about what precisely triggers turbulence in a fluid, how it is controlled, and what exactly lends it order through chaos. Making the problem even more difficult to tackle is the fact that the equations governing the motion of a fluid − namely the Navier-Stokes equations − are notoriously hard to analyse. Scientists resort to high performance computing techniques, along with experiments and theoretical simplifications to study the phenomena, but a complete theory of turbulence is still lacking, making fluid turbulence one of the most important unsolved problems in physics today. In fact, Nobel laureate, Richard Feynman, declared it as “the most important unsolved problem in classical physics.” Quantum physicist Werner Heisenberg, when asked what he would ask God if given the opportunity, remarked: “I’ll ask him two questions. Why relativity? And why turbulence? I really believe he will have an answer to the first.”
A simulation snapshot showing turbulence in a jet
We got a chance to speak to Padma Vibushan recipient, Prof. Roddam Narasimha and this is what he had to say: “To this day we are unable to predict the simplest turbulent flows starting from the first principles of mechanics like Newton’s laws, without ever appealing to experimental data about the flow itself. For example, it’s not currently possible to predict the pressure loss in a pipe in which the flow is turbulent, but it’s known thanks to clever use of data at some level obtained in experiments. The basic issue is that the turbulent flow problems we are interested in are almost always highly non-linear, and the mathematics for handling such highly non-linear problems does not seem to exist. There has been a widespread view among many physicists that whenever a new problem arises in their subject, it appears that somehow, like magic, the mathematics required for it is found to have already been invented. The problem of turbulence provides a counter-example to this rule. The laws governing the problem are well known and, for simple incompressible fluids under normal conditions, are embodied in the avier-Stokes equations. But the solutions remain unknown. It is clear therefore that current mathematics is singularly ineffective in solving the problem of turbulence. As Richard Feynman put it, turbulence remains the greatest unsolved problem in classical Physics.”
Such is the importance of turbulence studies that a whole new generation of computing techniques have spawned in its wake. Solving, or even coming close to a theory of turbulence will allow science to make better weather predictions, design power efficient automobiles and aircraft and have a better understanding of various natural phenomena.
The Origin of Life
We’ve always been obsessed with studying about the possibility of life on other planets, but another question that has baffled scientists even more is how exactly did life as we know it originate on Earth? Though answering this question will have little practical application, the path to the answer could lead to several interesting discoveries in fields varying from microbiology to astrophysics.
Scientists believe that the key to understanding the origin of life could be in finding out how two of life’s characteristic features – reproduction and genetic transfer − could have begun as processes involving molecules that have the ability to replicate. This led to the formation of the popular ‘primordial soup’ theory, according to which the early Earth somehow had a mixture, a sort of broth of molecules, that were energised by solar energy and electrical storms. Over a long period of time, these molecules would have reacted to form the more complex organic structures that make up life. This theory was buoyed by the famous Miller-Urey experiment, wherein the duo succeeded in creating amino acid by passing electrical discharges through a blend of simpler elements such as methane, ammonia, water and hydrogen. The discovery of DNA and RNA, however, doused the initial excitement as it seemed impossible that the complex and elegant structure of DNA could have risen from a primitive soup of chemicals.
A school of thought exists that believes that the early world was an RNA world rather than a DNA one. RNA was found to have the ability to speed up reactions and remain unchanged and to replicate and store genetic material. But in order for RNA to be hailed as the original replicator of life instead of DNA, scientists would have to find evidence of elements that could form nucleotides – the building blocks of the RNA molecule. The glitch is that nucleotides are extremely difficult to produce, even in a lab’s controlled environment. Again, a primordial soup seems incapable of creating these molecules. This conundrum led to another school of thought that believed that the essential organic molecules present in primitive life have an extra-terrestrial origin and were shipped to Earth from outer space via meteors, leading to the development of the ‘panspermia’ theory. Another possible explanation could be the ‘Iron-Sulphur world’ theory that proposes that life on earth has a deep sea origin, spawning from chemical reactions that occur in highly pressurised hot water found near hydrothermal vents with a volcanic nature.
It’s quite remarkable that even after a good 200-odd years post industrialisation we’re still not close to finding out how our Earth gave birth to life. There’s been massive interest in this problem from research groups in interdisciplinary sciences, resulting in several initiatives to carry out directed research in solving one of science’s biggest mysteries.
A trip down memory lane will take us back to high school chemistry/physics class where we all learnt (mostly by rote) that proteins are extremely important molecules and are the building blocks of life. Protein molecules are made up of amino acid sequences that influence their structure and, in turn, determine the specific action of the protein. How a protein folds and assumes a specific functional shape is a long-standing problem in science. It’s even been declared by Science magazine as one of the biggest unsolved problems in science. The problem in essence includes three parts: 1. How exactly does a protein evolve into its final native structure? 2. Can we come up with a computational algorithm to predict the structure of a protein given its amino acid sequence? 3. Given the large number of possible conformations how exactly does a protein fold so quickly? Over the past few decades, significant progress has been made on all three fronts, however, scientists are still to completely decode the driving mechanisms and underlying principles governing protein folding.
During the process of folding, a large number of forces and interactions are at play, driving a protein to reach a state of lowest free energy possible which lends it stability. Due to the sheer complexity of the structure and the large number of force fields involved, it’s difficult to understand the precise physics of a folding process beyond small proteins. The problem of structure prediction has been attacked by a combination of physics and powerful computers. While they’ve achieved success with small and relatively simple proteins, scientists still struggle with accurately predicting the folded shape of more complex multidomain proteins from its amino acid sequence, known as a ‘priori’.
To understand the process, imagine being at a crossroad with a 1,000 different roads going to the same destination and you choosing the path that took the least amount of time to get there. A similar, but larger scale problem is the kinetic mechanism of a protein folding to a specific state amidst many possible structures. It has been figured out that random thermal motions play a major role in the rapid nature of folding and that the protein ‘zips’ through conformations locally, avoiding unfavourable structures, the physical route is an open question − solving which could also lead to faster algorithms for protein structure prediction.
The protein folding problem is arguably the hottest topic today in biochemical and biophysics research. The physics and computing algorithms developed for protein folding have led to the development of new man-made polymeric materials. Besides contributing to the growth of scientific computing, the problem has resulted in a better understanding of diseases like Type II Diabetes, Alzheimer’s, Parkinson’s and Huntington’s – diseases in which protein misfolding plays a part. A better physical understanding of protein folding could not only lead to several breakthroughs in materials and biological sciences but could also revolutionise medicine.
A Quantum Theory of Gravity
We all know that an apple supposedly fell on Newton’s head and led to the discovery of gravity. Suffice to say, the world wasn’t the same anymore. Then came along Albert Einstein with his celebrated theory of general relativity. He re-imagined gravity as the distortion of space-time, the fabric that makes up the universe. Imagine a heavy ball on a stretched bedsheet and another smaller ball a little further away from it. The big ball would press down on the sheet depressing it, causing the smaller ball to be drawn towards it. Einstein’s theory of gravity worked wonders and even explained the bending of light. However, when it came to sub-atomic particles governed by the law of quantum mechanics, the theory threw up all sorts of weird results. Developing a theory of gravity that can unify quantum mechanics and relativity, two of 20th century’s most successful theories, has become the biggest research topic ever since.
This problem has birthed some new and exciting areas in physics and mathematics. The area that has gained the most traction is ‘String theory’. String theory replaces the notion of particles with tiny vibrating strings that can assume various shapes. Each string can vibrate in a particular mode lending it a specific mass and spin. String theory is incredibly complicated and mathematically deals with ten dimensions in space-time − six more than what humans can physically perceive. The theory has been successful in explaining many of the oddities Of Gravity’s marriage with quantum mechanics and has been hailed as one of the top contenders for a ‘theory of everything’.
Another theory for formulating quantum gravity is ‘loop quantum gravity’ (LQG). LQG is relatively less ambitious, in the sense that it primarily aims to be a consistent theory of gravity without trying to achieve any grand unification of sorts. LQG imagines space time to be comprised of fine fabric stitched from tiny loops, hence the name. Unlike string theory, LQG doesn’t involve added dimensions.
While both the theories have their own upsides and flaws, the theory of quantum of gravity remains an unresolved question because neither of the two has been proved experimentally. Validating any of the above mentioned theories is a major challenge in experimental physics.
The theory of quantum gravity might not have any noticeable effect in our daily lives, but it will be a true testament of how far we’ve progressed in science and allow us to make leaps in fascinating futuristic areas such as black hole physics, time travel and wormholes.
In an interview, distinguished number theorist, Terence Tao referred to prime numbers as the atomic elements of number theory, which of course is a spot-on characterisation. Prime numbers have no factors besides 1 and themselves and as such are the simplest construction elements of numbers. Prime numbers are also extremely erratic and follow no set pattern. A large number (product of two primes) is used to encrypt the millions of secure transactions processed online. The prime factorisation of this number currently takes practically forever. However, if we were somehow able to tame the seemingly random nature of primes and better understand how they work, we could allegedly come close to actually breaking the internet. Solving the ‘Riemann Hypothesis’ could bring us ten steps closer to understanding primes and could have major implications on banking, commerce and security.
As mentioned, primes aren’t exactly known for being well behaved. What Bernhard Riemann did in 1859 was find out the number of primes that are less than a given number. Riemann’s solution is related to a function called the ‘Riemann Zeta’ function and the related distribution of points on the integer number line for which the function becomes 0. The hypothesis is concerned with a special set of such points called ‘non trivial zeros’, which are supposed to lie on a critical line. This hypothesis has been verified for more than a billion such zeros and could unlock the mystery surrounding the distribution of prime numbers.
Anyone in the mathematics circle will be aware that the Riemann Hypothesis is one of the biggest unanswered problems there is. Not only will the solution have a huge impact on science and society, it will also guarantee the solver a million dollar prize. The Riemann hypothesis has been listed by the Clay Mathematics Institute (a private philanthropic foundation dedicated to mathematics) as one of the seven Millennium prize problems. The internet is rife with attempted proofs of the hypothesis, all of which have been rejected as valid proofs by the mathematics community. Rest assured, the internet is safe, for now.
The Survival Mechanisms of the Tardigrades
Tardigrades are a class of microorganisms found abundantly in nature in regions spanning most climatic zones and altitudes across our seven continents. But these are not your common microorganisms, they’re creatures with such extreme survival skills that all those survival reality TV shows seem dull in comparison. Let’s start with a fun fact – tardigrades are the first animals that could survive the dangerous vacuum of space. A bunch of these creatures orbited in outer space on the outside of a FOTONM3 rocket. They were bombarded with all sorts of cosmic radiations and plenty of them made the trip back home to tell us their story!
Not only are these organisms surprisingly suited for life in space, they can also withstand temperatures ranging from just above absolute zero to way above the boiling point of water. Also, sample this: they can survive vacuum as well as the pressure at Mariana Trench – 11 kilometres deep in the Pacific Ocean.
Studies have attributed some of these amazing powers to the tardigrades’ ability to practice forms of cryptobiosis – a state in which metabolic activity is severely slowed down. This state, called desiccation, allows the creature to lose water and almost arrest all metabolism. When blessed with water, the tardigrade simply regains its rest state and moves on in life as if nothing happened. Sure, this ability can help it survive deserts and droughts, but how does it help this water bear survive in space or extreme temperatures?
In its desiccated form, the tardigrade performs some life-saving functions. A sugar molecule prohibits cellular expansion and anti-oxidants produced neutralise the threat posed by oxygen-reacting molecules present in outer space radiation. The anti-oxidants can help repair damaged DNA and this ability could explain the tardigrades’ knack for surviving extreme pressures. While all these functions present clues to the secret of the tardigrade’s superpowers, we understand very little about how the tardigrade functions on a molecular level. There’s little evidence about its evolutionary history as well. Does its flair for outer space survival point to an extraterrestrial origin?
Research about tardigrades can have exciting implications. If cryonics became feasible, think of the applications! Medicines and tablets could be stored at room temperature and perhaps, we could design a super-suit designed for extreme exploration. Astrobiologists will be able to stretch their parameters to search for life outside Earth. If a microorganism on Earth could test nature’s limit to such an extent, maybe there are such organisms on places like Jupiter’s moons, lying in an unactivated state, just waiting to be discovered.
Dark energy and Dark matter
The study of matter on Earth is an allencompassing field. It’s a pity that all matter known to us makes up just about 5% of the known universe! The rest of the universe is “dark” and its major constituents are aptly named ‘dark matter’ (~27%) and ‘dark energy’ (~68%).
Any list of unsolved problems in science is incomplete without the mention of the mystery of dark matter and dark energy. Dark energy is the proposed reason for the acceleration of the universe. In 1998, when two different research teams confirmed that the expansion of the universe was accelerating, it debunked the then popular notion of gravity slowing down the universe’s expansion. Theorists had, and still have, a headache explaining this, and in grand fashion named the possible explanation as dark energy. So far, no real contender has emerged for a correct explanation of the form dark energy takes. Current solutions state that dark energy could be a property of space and lend empty space energy, or it could be a sort of space-filling fluid that somehow leads to the acceleration of the universe, in the way that ‘normal’ energy cannot.
Dark matter is weird. It interacts with almost nothing, not even light, making its detection very difficult. Dark matter was discovered when researchers found oddities in the dynamics of certain galaxies. The known mass of the galaxy couldn’t explain the deviations and they concluded that some form of invisible matter with gravitational pull existed throughout the universe. Dark matter has never been directly detected, but scientists have however observed the effects of dark matter via gravitational lensing (bending of light due to gravitational interaction with invisible matter). Thank god for dark matter’s gravitational pull!
The constituent of dark matter is one of the biggest problems in particle physics and cosmology today. Scientists are of the opinion that dark matter is made up of certain exotic particles called ‘WIMP’ (or ‘Weakly Interacting Massive Particles’) that owe their existence to a theory called ‘Supersymmetry’. Scientists also postulate that dark matter could be made up of baryons. Baryonic dark matter could be ‘MACHOs’ (or ‘Massive Astrophysical Compact Halo Objects’) like small black holes.
While both theories – dark matter and dark energy − arise from our inability to explain some observed features of the universe, they’re in essence competing forces of the cosmos and both attract massive funding for large scale experiments. Dark energy repels and dark matter attracts. The dominance of either force will decide if the universe will expand forever or be drawn towards itself and ultimately implode in a rather interesting, Big Bangtype event, christened the “Big Crunch”. For now, however, we’re truly in the dark about both theories.