“The most incomprehensible thing about the universe is that it is comprehensible.”
Albert Einstein
The story of Isaac Newton and the falling apple is one of the best-known and endearing stories in all the history of science. After observing an apple fall from the tree at Woolsthorpe Manor (his mother’s farm), he developed the mathematics answering the question, “Why do apples fall?” He published the answer (the theory of gravity) in his monumental work, Principia Mathematica in 1687. His equations were used by NASA to navigate Neil Armstrong, Michael Collins, and Buzz Aldrin safely to the moon and back in July 1969. Newton did not realize that the bigger question was, “How the apple get up in the tree in the first place?” Answering that question is the objective of this essay and is indebted to Johnjoe McFadden & Jim Al-Khalili's wonderful 2014 book, Life On The Edge, The Coming Age of Quantum Biology.
The Massachusetts Institute of Technology (MIT) was founded in 1861 (during the Lincoln presidency) in Cambridge, Massachusetts and is one of the world’s premier citadels of science and technology. As of 2014 there were nine Nobel Prize winners among its one-thousand professors. Its alumni include astronauts (one-third of NASA’s space flights involved MIT graduates) and a host of prominent scientists and technology leaders, including Richard Feynman, a 1939 graduate and 1965 Nobel Prize winner in Physics. According to Wikipedia, “As of 2015, 84 Nobel laureates, 52 National Medal of Science recipients, 45 Rhodes Scholars, 38 MacArthur Fellows, 34 astronauts, and 2 Fields Medalists have been affiliated with MIT.” Not surprisingly MIT attracts more than its share of the brightest young people allowing it to be very selective with a selection rate of only 7.9% in 2014.
In spite of MIT’s abundance of super intelligent professors and students, its most illustrious resident is not a human at all, but an apple tree albeit not an ordinary one. Growing (since 1977) in the President’s Garden is a cutting from another apple tree kept in England’s Royal Botanic Gardens, a direct descendant of the actual tree that Isaac Newton allegedly sat under when he observed his famous apple fall from the tree. Newton’s apple tree growing in Cambridge, Massachusetts is poetic given all the years Newton spent at Cambridge, England as a student and professor. In October 2006 MIT’s hero-tree produced its first apple.
MIT has a long history of pushing science and technology to its outer limits. For example, MIT scientists have been working on developing a quantum computer starting with Richard Feynman’s 1981 proposal to build a computer that takes advantage of Quantum Mechanics (QM). Peter Shor's 1994 quantum factoring algorithm constituted another step forward, but there is a major gulf between theoretical quantum theory and the engineering required to build a quantum computer. Technical problems have defeated the best efforts of some of the world’s brightest scientists as of 2007.
Therefore, in April 2007 when an article appeared in the New York Times suggesting that “plants” were quantum computers, the MIT quantum group understandingly exploded into laughter. Seth Lloyd said it was “quantum hanky-panky” and “Oh, my God, that’s the most crackpot thing I’ve ever heard.” A the same time they were laughing their ‘quantum socks’ off, a photon of light was traveling at 186,000 miles per second toward the famous apple tree setting off a series of steps in the process of putting an apple in the tree. To appreciate the source of the laughter and continue the trip from Cambridge, England to Cambridge, Massachusetts it is necessary to examine QM in some detail. Since it takes a photon eight minutes and twenty seconds (on average) to travel from the sun to the surface of the earth and the MIT apple tree, there is ample time
Albert Einstein observed “If you can't explain it simply, you don't understand it well enough” and it is in that spirit that the following explanation is attempted. The underlying dynamics of QM (also called Quantum Theory or Quantum Electrodynamics) is sufficiently confusing even before invoking Groucho Marx’s famous adage, “Who are you going to believe, me or your own lying eyes?” First, it is important to understand the difference between waves and particles. Sound and ocean surface perturbations (water waves) are two examples of waves. When waves move they diffuse (spread out). Particles in contrast move in a stable and consistent trajectory. Throw a ball and its path is easily seen at a reasonable speed. Balls, bullets, and atoms are three examples of particles. The following graphic (the famous sine wave) will be familiar from high school mathematics and is useful to see what is meant by ‘peaks’ and ‘troughs.’
Waves have peaks (highest point) and troughs (lowest point). Since waves spread out in what is called diffraction, they can interfere with each other. If they collide ‘in phase’ that is ‘peak to peak’ their strength (amplitude) is doubled in what is called constructive interference. If they collide ‘out of phase’ or ‘peak to trough’ they cancel each other out and in what is called destructive interference.
In classical physics (Newtonian) light was always thought of as being a wave. It is now known that light can behave both as a wave and also as a particle, and particles can also behave both as particles and as waves in what is called the wave–particle duality. This wave-particle duality lies at the heart of QM and can be demonstrated by one of the most famous experiments in science, the two-slit experiment. The apparatus required for the two-slit experiment is quite simple, consisting of two screens The first screen acts as a barrier and the second as a detector. The first screen has two vertical slits cut out and spaced some distance apart of a size large enough to permit waves or particles to pass through them when ‘fired’ at the screen with a device that can be thought as a gun. The second screen has a surface that can record or detect any wave or particle that manages to pass through the slits and strike the screen. As will be seen, sometime the researcher will open just one slit and at other times open both slits. The experiment is conducted in stages in order to contrast waves from particles. In the first stage a series of bullets (particles) are fired toward the barrier with one slit open. As expected a vertical pattern of bullet holes will appear behind slit 1. After opening the second slit and firing another volley, a second vertical pattern will appear right next to the first one. In other words the bullets behave just like particles in accordance with classical physics as would be expected.
In stage two a monochromatic light (light with just one wavelength) is emitted toward the screen. This is in contrast to the light from a normal light bulb that is composed of many differing wavelengths (making it difficult to observe the interference pattern). The second screen is coated with a chemical that illuminates when impacted by light. When the monochromatic light is emitted with only one slit open, a pattern of brightness appears just behind the slit. But with both slits open an interference pattern is observed consisting of bright bands where constructive interference occurred and dark bands where destructive interference occurred. Again, this is just what is expected.
In the third stage electrons (particles of tiny bits of matter) are fired at the screen with one slit open resulting in a pattern just like the one the bullets made in stage one with only one slip open. But when the second slit is also opened, a interference pattern develops just like what happened with the monochromatic light with two slits open. In other words the electron particles are behaving like waves in an example of wave-particle duality. Some early investigators thought maybe the electrons being fired in a volley were ‘bouncing’ off each other causing the interference pattern. To eliminate this as a possibility the electrons were fired, one at a time with both slits open. Surely this would eliminate the interference pattern. After all, how could a single electron interfere with itself? After repeating the test by firing one electron at a time repeatedly with two slits open, the same interference pattern was produced meaning a single particle was transforming into a wave passing through both slits at the same time and interfering with itself creating an interference pattern exactly like the monochromatic light in the earlier test. Richard Feynman said that the two-slit test “has in it the heart of quantum mechanics.” Physicists refer to the electron’s ability to be in two places at the same time as superposition and is just one part of the weirdness of QM.
Physicists, however, are very clever people and they were not going to surrender to QM weirdness easily. They set up another detector near slit 1 positioned so it could ‘see’ and record the electron if it enters slit 1 and repeated the experiment. The result only demonstrated another aspect of QM weirdness, the measurement effect. With the detector “watching” slit 1 the electrons reverted to behaving just the way particles are expected to behave! Yes, the mere act of observing the experiment changes the outcome. Feynman once famously described the universe in one sentence, “Everything is made of atoms and they are always wiggling.” The detector that was used in an attempt to see if the electron passed through slit 1 is, of course, made of atoms and their wiggling causes what physicist call decoherence.
Johnjoe McFadden & Jim Al-Khalili aptly described decoherence as follows: “This complex interaction causes the delicate quantum coherence to leak away very quickly and be lost in the incoherence noise of its surroundings. This process is called decoherence. . . but decoherence does not need a measuring device to come into effect. It's taking place all the time inside every single classical object as its quantum constituents–the atoms and molecules–undergo thermal vibrations and get buffeted around by all the surrounding atoms and molecules, so that their wave-like coherence is lost. In this way we can think of decoherence as the means by which all the material surrounding any given atom, say–what is referred to as its environment–is constantly measuring that atom and forcing it to behave like a classical particle. In fact, decoherence is one of the fastest and most efficient processes in the whole of physics.” Please note that coherence means that some physical system is exhibiting the wave–particle duality of superposition.
It is the fragility of coherence was the source of the laughter of the MIT group working on building a quantum computer which had to be maintained in a state of superposition in order to make quantum calculations. The advantage of a quantum computer over the familiar digital computer is it can be in three states called qubits (on, off, and both) whereas the digital computer has only two states (on and off). The MIT group at the time (October 2006) was combating decoherence by lowering the temperature in their quantum computer to extremely low temperatures to combat the heat caused by atoms colliding with each other in what is called thermodynamics. This enabled them to maintain the coherence necessary to perform quantum computing. The idea that coherence could be maintained in the leaves of the apple tree with its high temperature was beyond laughable, but as they soon learned, was true. The idea of an apple tree maintaining coherence gives a deeper appreciation to Leslie Orgel’s famous quip, “Evolution is cleverer than you are.” Before leaving QM behind and moving on to the apple tree, the last two bits of its weirdness must be mentioned, Entanglement and Tunneling.
Entanglement is perhaps the strangest and most difficult to accept. A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. They have various intrinsic properties including spin. When a pair of particles is generated in a way that their total spin is known to be zero, and if one particle is found to have a clockwise spin, then the spin of the other particle, measured on the same axis will always be counterclockwise. If the spin of one of the quarks is changed (for example by being measured), its partner quark will automatically and instantly change its spin to be opposite. Entanglement occurs because each particle of an entangled pair “knows” what measurement has been performed on the other, and with what outcome, even though there are not any known means for such information to be communicated between the particles which at the time of measurement may be separated by arbitrarily large distances. Einstein called entanglement “spooky action at a distance” and could never accept QM in spite of playing a large role in its development. As McFadden & Al-Khalili observed, “Einstein was skeptical because entanglement appeared to violate his theory of relativity, which stated that no influence or signal can ever travel through space faster than the speed of light. Distant particles should not, according to Einstein, possess instantaneous spooky connections. In this, Einstein was wrong: we now know empirically that quantum particles really can have instantaneous long-range links. But, just in case you are wondering, quantum entanglement can’t be invoked to validate telepathy.” To date, none of the world’s greatest minds have been unable to unravel this paradox.
Tunneling is the ability of a particle to break through some barrier in a fashion that would not be permitted by the laws of classical physics. A tennis player would not expect his ball to pass through a concrete wall while practicing his serve hitting the ball repeatedly against the wall. It must be mentioned that it is possible (but extremely unlikely) that the ball would pass through the wall just as easily as if there existed a large hole in it. If (the mother of all big ifs) all the electrons in the atoms in the tennis ball and all the electrons in the atoms of the wall were to align in a precise manner, the ball would pass right through the wall! The probability of this happening is so low that the tennis player would have to continue hitting the ball against the wall continuously for over five billion years. The sun is consuming hydrogen at the rate of 620 million metric tons each second, and at this rate will exhaust its hydrogen fuel supply sometime in the next five billion to seven billion years causing it to collapse into its core. At this time the sun will expand into a red giant as its atoms of helium undergo fusion and swallow the earth in a great fireball thus putting an end to the persistent tennis player.
Without quantum tunneling, the principal player in the story of the apple, the photon (the basic component of light) would not be available to make its journey to the earth. The hydrogen atom consists of a single positively charged proton and a single negatively charged electron. The sun’s energy is produced in three steps whereby four hydrogen atoms are squeezed together in a process called nuclear fusion into two helium atoms. The mass of the resulting two helium atoms is less than the mass of the original four hydrogen atoms with the difference released as energy in accordance with Einstein’s famous E=mc2 where e=energy, m=mass, and c2=the speed of light squared. According to the Law of Attraction, “opposites attract” and “likes repel.” An extreme force is necessary to force a pair of hydrogen atoms close enough to fuse together. Scientists have calculated that the gravitational force and heat found in the core of the sun is not sufficient for hydrogen fusion to occur without the assistance of quantum tunneling.
OK, how did the apple get up in the tree? The short answer is photosynthesis and the longer answer is, well, a bit longer. Photosynthesis and DNA are two of the most efficient processes found in nature and this efficiency is difficult to explain strictly in terms of classical physics. According to McFadden & Al-Khalili, every leaf on every plant or tree contains an incredible piece of machinery that creates “. . . about 16,000 tonnes of new organic matter in the form of trees, grass, seaweed, dandelions, giant redwood, and apples” every second. This photosynthesis process starts when a photon of light strikes the surface of a leaf activating chlorophyll and creating an exciton (“an electron that has been knocked out of its orbit in an atom, together with the hole it leaves behind.”) Excitons can be thought of as “a tiny battery with positive and negative poles capable of storing energy for later use.” The exciton must now travel through a maze of structures to reach a molecular manufacturing unit called a reaction center. To accomplish this feat it must solve the problem faced by all power plants: energy lost during transmission. For example, in 2003 the US lost up to 5% of the electricity generated during transmission. Just a small reduction in that loss would be worth billions of dollars.
Excitons are unstable and have to travel quite a distance, in molecular terms nanometer distances (measured in billions of a meter) from the excited chlorophyll molecules to the reaction center (the organic manufacturing plant). The energy has to be transferred from one antenna molecule to another within the chlorophyll forest to reach the reaction center. As McFadden & Al-Khalili wrote, “This can happen thanks to the tightly packed nature of the chlorophyll. Molecular neighbors of the one that has absorbed the photon can themselves become excited, effectively inheriting the energy of the initially excited electron, which is then transferred to their own magnesium atom’s electron. . . The problem, of course, is which route this energy transfer should take. If it heads in the wrong direction, randomly hopping from one molecule to the next in the chlorophyll forest, it will eventually lose its energy rather than delivering it to the reaction center. It doesn’t have very long to find its way to its destination before the exciton expires.”
The exciton must somehow travel through a forest-like maze of chlorophyll and reach the reaction center before its energy is lost. Its problem is very similar to the famous traveling salesman’s problem. A salesman needs to visit the capitals of all the lower 48 states and is allowed to start his trip from the capital city of his choice and continue on until passing through all 48 capitals. The idea is to plan the route so as to travel the least total distance. It took a supercomputer to find the shortest route of 10,628 miles. There is, of course, “many” other routes that could solve the problem and some of them might even be shorter than 10,628 miles (the shortest route found to date).
The problem facing the exciton is equally daunting and a leaf hardly has the space to house a supercomputer. At one time it was thought that plants utilized a statistical technique known as a random walk. In the author’s words, “This is sometimes referred to as a ‘drunken walk’ because it resembles the path taken by an intoxicated drinker exiting a bar, wandering this way and that until he eventually finds his way home. But random walks are not a very efficient means of getting anywhere: if the drunk’s home is far away, he may well wake up in the following morning in a bush on the other side of town. An object engaged in a random walk will tend to move away from its starting point by a distance proportionate to the square of the time taken. If in one minute a drunk has advanced by one meter, then after four minutes he will have advanced by two meters, and after nine minutes, only three meters. Given this sluggish progress, it is not surprising that animals and microbes seldom use a random walk to find food or prey, only resorting to the strategy if no other options are available. . . Drop an ant onto unfamiliar ground and as soon as it encounters a scent, it will abandon a random walk and follow its nose.”
Given the fact that photosynthesis is the second (only to DNA) most efficient biological system in nature, it is rather obvious that the exciton is not using the random walk. Almost all the energy absorbed by a chlorophyll molecule finds its way to the reaction center. How this energy can find its way to the reaction center so much better then drunkards, or ants searching for a meal, or even technologically advanced power plants is one of natures greatest puzzles. In an application of Arthur Conan Doyle’s observation, “Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.” Scientists now believe (with considerable experimental data) that only QM is capable of solving the search problem facing the exciton in finding the shortest path (in time) to the molecular manufacturing unit supplying the energy necessary to manufacture the apple in our story.
Much research and experimentation has taken place since the progeny of Newton’s apple tree produced its first apple in MIT’s President’s Garden. On January 9, 2014, Edward J. O’Reilly and Alexandra Olaya-Castro published an article titled, Non-classicality of the molecular vibrations assisting exciton energy transfer at room temperature in the journal in Nature Communications. In their abstract they state, “Our results therefore suggest that investigation of the non-classical properties of vibrational motions assisting excitation and charge transport, photoreception and chemical sensing processes could be a touchstone for revealing a role for non-trivial quantum phenomena in biology”. Of course, as we have learned, “Non-classicality” refers to QM.
When will science be able to state that photosynthesis utilizes QM to enable the exciton to find its path through the chlorophyll jungle and reach the reaction center before losing its energy at the level of a Scientific Theory? When will there be a Theory of Quantum Photosynthesis similar to the Theory of Evolution or the Theory of Relativity? To date, there exists considerable evidence indicating that is the case and classical physics does not appear to be up to the job, but that alone does not reach the standard demanded of any scientific theory which, after all, is the highest degree of truth possible in science. The hypothesis is the heart of any scientific theory.
When Pierre Laplace presented his definitive work on the properties of the solar system (Celestial Mechanics published in 1799) to Napoleon, he was asked if it was true that there was no mention of the solar system’s Creator (God) in his opus magus. Laplace’s famous answer , “I had no need of that hypothesis” provoked a challenge from Joseph Lagrange, another great French mathematician and physicists, who replied to Laplace saying, “Ah, it is a fine hypothesis; it explains many things.” Laplace replied, “This hypothesis, Sir, explains in fact everything, but does not permit to predict anything. As a scholar, I must provide you with works permitting predictions. This is the ultimate insult in science: it explains everything but predicts nothing.” And that is the gold standard of a hypothesis, it must make verifiable predictions and they must come to pass and be independently observed.
In contrast to “The Theory of Quantum Photosynthesis” the Theory of QM (or Quantum Theory) is a proven scientific theory by virtue of both the inductive and deductive methods of the Scientific Method. The two-slit test is an example of the inductive method where observable and replicable data is accumulated suggesting that coherence, decoherence, superposition, measurement effect, tunneling, and entanglement are all taking place. But this does not explain how it is happening. A hypothesis must provide an explanation for what is happening, and also make predictions. In the case of the QM the task was fulfilled by Richard Feynman, who used the deductive method of science to develop the mathematics that explained the experimental results. He was awarded the Nobel Prize winner in Physics in 1965 for his work. When asked, “What did you do to earn the Nobel,” he answered, “If I could explain it to you, they wouldn’t have given it to me.”
As has been suggested, Quantum computing is well-suited for solving search-problems because of its ability to take all possible paths at the same time. Finding the factors (especially for large numbers) is a major motivation for developing a quantum computer. According to phys.org/news/2014-11, Nike Dattani & Nathan Bryans broke the record (on 11/28/2014) for factoring the largest number (56,153) on a quantum device. The previous record was 143.
The role of QM in photosynthesis is only one story about this amazing fundamental feature of the universe found in Life On The Edge, The Coming Age of Quantum Biology. The story begins with the creation of the universe and continues to the story of nuclear fission taking place in the core of the sun and the birth of the photon that took just over eight minutes to travel 93 million miles to set off the process that put an apple in that famous MIT apple tree. Life On The Edge also tells the story of how enzymes, magnetoreception (animal navigation), and even consciousness depend on QM. Newton once said that “a midget standing on the shoulders of a giant, sees further.” The question that needs to be asked is, “Where will we find the 'midgets' ready to climb on the shoulders of the likes of Newton, Einstein, and Feynman?” It is estimated that 3% of the school children in the world are gifted. Our only hope is that each one receives a quality education, because our future on earth depends on them.
Sources:
1. Life On The Edge, The Coming Age of Quantum Biology (2014) by Johnjoe McFadden & Jim Al-Khalili
2. A Brief History of Time: From the Big Bang to Black Holes (1998) by Stephen Hawking
3. Genius: The Life and Science of Richard Feynman (1992) by James Gleick
