Chapter 1
God throws dice
If Nature were not beautiful, it would
not be worth knowing,
and if nature were not worth knowing,
life would not be worth living.
Henri Poincaré
We say that we should not “judge a book by its cover”. This is clear with regards to physicist Richard Feynman (1918-1988). Imagine a marihuana smoking bongo player known for his mad sense of humour and practical jokes. Feynman was of the opinion that the highest level of understanding you can achieve is through laughter and compassion, which probably explain why he was highly appreciated by his colleagues and students.
Feynman was an eccentric as well as highly intelligent and in 1965 he received the Nobel Prize for his work in quantum physics. Once he opened one of his lectures with the following: “Do not takes this lecture too seriously...just lean back and enjoy. I am going to tell you how nature works and if you are willing to admit she behaves in this way you will see that she is both delightful and magical. Do not ask yourself, why is that so? If you do so you will enter a labyrinth from which nobody has ever returned. Nobody knows why nature is the way she is”.
I will endeavour to follow Feynman’s footsteps by making this journey through the gates of knowledge to the building blocks of our reality a comprehendible, enjoyable and interesting journey. Science has shown us an image of nature that is quite weird and this is particularly true when we enter the sub atomic realm.
Your body and everything that exist around you is made up of trillions of tiny building blocks that we call particles. Just as you need different Lego’s to build certain things you also need a wide range of particles in order to build everything we can see around us. Particles have been given a host of names such as neutrons; protons, neutrinos, photons, quarks, bosons, gluons etc just to mention a few. This is the language in Quantum Physics and it aims to describe how matter and energy is operating on the microscopic level.
Quantum physics is often perceived as strange and incomprehensible, which is understandable. Niels Bohr (1885-1962), one of the great pioneers within the field used to say that if you aren't confused by quantum physics then you haven't really understood it. Even so, quantum physics is not something a group of clever physicist made up in order to make the world sound particularly obscure. The quantum theory has actually been met with a lot of resistance over the years because of its complexities and paradoxes.
The theory's mathematics is simple enough to be taught to undergraduates, but the physical implications of that mathematics give rise to deep philosophical questions that remain unresolved. Despite all the strangeness surrounding quantum physics, the theory offers the most precise and successful calculations in the history of science. It has provided us with remarkable insight into how the world around us is constructed and it underlies much of our modern lives. Products such as TV, radio, mobile phones and computers would not be possible without our understanding of quantum physics. In addition it has given us increased knowledge of biology and chemistry and it also may also have something to tell us about where the universe came from.
Let us therefore take a closer look at how we have come to this knowledge and to try and gain some insight into the paradoxical and sometimes bizarre consequence of what the quantum theory is telling us about reality.
The remarkable history of the Atom
Everything you can see around you is made up of tiny particles that we call atoms. Tiny is not a good description because the small size is almost incomprehensible. Atoms are smaller than one millionth of the thickness of a human hair. In order to construct a snowflake you need around 10 billion atoms. One curious thing is that even if we today need advanced technology to observe an atom they were actually well known already around 400 BC.
The name atom was invented by the Greek philosopher Democritus and his teacher Leucippus and atom means unbreakable! Democritus asked himself the following question: “What if I break this stick in two and then I break the following two parts into four and so on, can I then continue this process infinitely?” he concluded that the answer had to be “no”. Nature had to have “a smallest component”.
According to Democritus the atoms had different properties such as size, form and mass. Based on these properties the atoms could build everything. Depending on how the atoms were put together we would get different perceptions of tastes, colours etc. How wine would taste depended therefore on the organisation and dynamic of the atoms in the wine. Nature is hence structured like a machine and the atoms are the keys to understand the machine. Everything in nature from a flash of lightning, a flower to human consciousness is a consequence of vibrations, speed and collisions between atoms.
However, it is no small task to propose a theory that aim to explain everything. Democritus theories were met with a lot of criticism particularly from two of the most prominent thinkers in western history, Plato and Aristotle. In contrast to Democritus, they claimed that nature was continuous and that matter could be deconstructed infinitely. In addition they did not believe that atoms could explain all phenomena in nature and that his theory was too materialistic.
Democritus’ theory met such powerful resistance that it was forgotten about for more than 2000 years when it again would find some support. The support came from no other than the prominent Italian Galileo Galilei (1564-1642), the founder of modern science, astronomer, physicist, astrologer and philosopher. However Galileis’ support did not help the theory of the atom much and it was not until 17th century that when the Croatian philosopher and scientist Rudjer Joseph Boscovich (1711-1787) proposed that Democritus was actually wrong, that his theory was again met with interest.
Boscovich did not join Plato and Aristotle in thinking that the atoms were fundamental and infinite; he actually thought that the atoms consisted of even smaller parts. He proposed that that you could deconstruct the atoms until you got to the fundamental building blocks of matter. As opposed to atoms, these building blocks were so small that they actually had no size at all; they were more akin to geometrical points that could not be extended.
The next step in the development of the theory of atoms went hand in hand with the development of turning chemistry into a science. For centuries alchemist had been making persistent attempts to create gold from other elements and made many discoveries on how various substances impact each other. In 1803 the British chemist John Dalton proposed a theory that could explain chemical processes. His theory suggested that that various elements such as gold, phosphor, hydrogen, and oxygen consisted of atoms, just like Democritus believed. Dalton furthermore proposed that certain elements such as gold and iron were pure because the atoms they consisted of were the same, that is, each atom within each element had the same mass. That could explain why these elements were different to each other as well as why everything that was not pure, singular elements such as animals and rocks etc had to be constructed by atoms from different elements.
Many Chemists realised that Dalton was on to something; however they were not prepared to accept his ideas. A century later when the world of science had taken an interest in developing models that could explain how steam behaved in steam engines, the theory of atoms came into the limelight again. The property of steam was explained as being based on atoms and molecules (which is a collection of atoms). Pressure caused the atoms to hit the walls in a container and the heat was a result of the speed and random movement of the atoms. The hotter the steam the faster the atoms would move. In spite of the fact that the theory of atoms could explain in detail what was observed, this was still not enough to convince the sceptics.
The first indication that Democritus’ theory of atoms as nature’s building blocks was actually correct came in 1897 when the British physicist Joseph John Thomson (1856-1940) discovered an even smaller particle, the electron. Another strange consequence of what we know and understand of atoms is that you and I and everything around us consist of 99,99999999 % empty space. This is because the nucleus of the atom is tiny compared to the size of the actual atom. If we blew up the atom’s nucleus to the size of a hangar ship the whole of the atom would be the size of planet earth! So if everything around us mainly consists of empty space why do we experience them as solid? The answer is that our experience of solidity is caused by the electrons circling around the nucleus billion of times per second. The enormous speed is what gives the atom volume and why we then experience matter as solid.
Even though the discovery of the electron gave credibility to the theory of atoms, the debate was not over yet until Albert Einstein (1879-1955) introduced the phenomenon known as Browns movement in 1905. The phenomenon was discovered by the biologist Robert Brown (1773-1858) in 1827. Brown studied tiny seeds of pollen under water and discovered that the seeds moved very erratic, just like a drunken person trying to walk straight. When trying to explain why this was so he could not come up with an explanation, He tried with particles of dust in order to rule out that the pollen movement in anyway could be caused by the pollen being alive; however the same thing happened again. Einstein on the other hand could explain what was going on. The seeds erratic movements was caused by the seeds colliding with the evens smaller and almost invisible water molecules and these collisions would be completely random and of different strength, hence a very erratic pattern of movement would emerge, just as Brown observed.
The grand finale in the history of atoms came in 1910 when the New Zealander Ernest Rutherford (1871-1937) created his famous model of the atom. It showed an image of a tiny nucleus of an atom consisting of neutrons and protons with one or more electrons circling around it. At first this model was interpreted as a miniature solar system, however there was a crucial difference. It was not the force of gravity that kept the electrons in orbit around the nucleus but the electromagnetic force.
A big unsolved question within physics at that time was how the electron stayed in a stable orbit around the nucleus. The equations that described electromagnetism state that accelerating electrical charges emit electromagnetic waves. The electrons are accelerating because they whiz around the nucleus, constantly changing direction. So, the electrons must emit electromagnetic waves (light) and thereby lose energy. Then, like a satellite losing energy through atmospheric friction, they should spiral into the nucleus within a hundredth of a second. Since this did not happen, something about our understanding of the atomic forces had to be wrong.
In 1913 Niels Bohr came up with a radical solution to the problem. Bohr suggested that the electron could only move in specific orbits around the nucleus. This means that the electron does not move gradually in the way the moon or a tennis ball does, an electron moves in leaps, hence the expression quantum leaps (which actually are, in contrasts to how we use it on our daily speech, the smallest leap we know of). Nature is therefore not continuous as we experience it. It moves incrementally in unfathomably small steps. A little bit like the way the picture on your television screens is not smooth and continuous but made up of thousands of pixels.
Electrons also play a big part in explaining why we have different atoms. It turns out that electrons cannot stay in the same quantum condition, which in a plain language means that two electrons are in a sense anti "social”. For example, particles have a quantum property we call “spin”. Particles can either spin up or down and when they have different spins their quantum properties are different. As the electrons are circling around the nucleus in specific orbits, this means that only two electrons can be in the same orbit, one with an upward spin and one with a downward spin. This reluctance to share the same quantum condition is known as the Pauli Exclusion Principle, named by the Austrian physicist Wolfgang Ernst Pauli (1900-1958) who discovered it.
We can use the exclusion principle to understand how and why atoms are structured. When nature is creating different atoms, they first place an electron in the inner track, which becomes the hydrogen atom which is the lightest element. The next is helium, which has got two electrons in the inner track: the next element is helium which has got two electrons in the inner track, one upward spinning and one downward spinning. The third is lithium with three electrons spinning around the nucleus and because two of the electrons are occupying the inner track, the third has to find itself another track further out and so on.
As of today we have found 116 different types of atoms or base elements, of which 93 exist naturally here on earth. The rest of we have crated in particle accelerators. This means that the next time you marvel at the beauty and diversity on this planet, remember it is all because of the Pauli Exclusion Principle. It causes atoms to have unique properties and qualities due to the varying numbers of electrons and their orbits
What is light?
The basis of our understanding of quantum physics is also connected to our attempts at understanding what light is. Isaac Newton (1643-1727) was the first one out to study this phenomenon methodologically. By using a prism made out of glass Newton managed to prove that light could split into all the colours of the rainbow and when he was using two prisms and put the two rainbows together the light became white. This experiment proved that light is a combination of different colours and Newton concluded that light had to consist of particles that behaved like tiny balls.
Even though Newton is one of the greatest scientists in history, the answer he came up with turned out to be not so simple. The Dutch physicist and mathematician Christiaan Huygens (1629-1695) claimed that light most likely consisted of waves. Huygens was of the opinion that the only way to explain the speed of light was that it had to consist of waves that moved through some kind of invisible space called ether (see chapter 3). The disagreement between Newton and Huygens was not clarified while the gentlemen were still alive.
It was not until the start of 18th century that Huygens theory was proved when the Englishman Thomas Young (1773-1829) did some experiments with light. He split a beam of light through two thin openings in a plate in front of a light sensitive screen. If light consisted of particles you would expect two lines on the screen, however the pattern which came up in Young’s experiment showed something very different. He observed numerous lines and how could that be possible if there was only two openings? The explanation was that he had come across something we call interference. The interference pattern is caused by waves overlapping each other and when two waves meets they reinforce each other. When two waves met they created light lines on the screen compared to the dark lines which was created when wave crest meets a wave trough. Hence the experiment proved that light consisted of waves and not particles as Newton believed.
In 1865 we had breakthrough in how we perceive the phenomenon of light. The English physicist James Clerk Maxwell came up with an equation which managed to unite electricity and magnetism as a single force. He also proved that electric and magnetic fields move through space as waves with a constant speed of 300 000 km per second which is equivalent to circling the earth seven times in one second! Based on these discoveries Maxwell understood that light also was a form of electromagnetic waves. His equations were soon verified by experiments and Huygens theory that light consisted of waves was broadly accepted.
The further progress in our understanding of the nature of light was connected to the development of the industrial revolution. As mentioned earlier, scientist at the time was very interested in understanding how the steam engine was working. Heat, energy and pressure were therefore under intense scrutiny. This research was eventually grouped together into a science of its own called thermodynamics. At the closing of the 18th century, the ”hottest” subject in thermodynamics at the time was heat radiation. The big question at the time was why a heated piece of metal would change colour as it grew hotter. This looked like a simple phenomena to explain since we at the time had several theories that could describe both heat and radiation.
However, when the theories were applied, they gave nonsensical answers. One of the conclusions was that a theoretical object called a black body would emit an infinite amount of energy. The reason why the infinites crept into the equations was that there was no theoretical lower limit to how short the wavelength of the radiation could be (lower wavelength, higher energies and visa versa). Since experiments did not support this, something in our understanding had to be wrong. It was the German physicist Max Planck (1858-1947) that came up with a solution. He suggested that there was a lower limit for how small you could divide things. This implied that time, space, matter and energy had to be made up of discrete parts.
The notion that energy only could move in extremely small but incremental steps turned out to be one of the most radical ideas in the history of science. By using this assumption in his equations however, Planck was able to solve the riddle of how energy and temperature was connected. The reason why we perceive the transition between various states of heated metal as continuous is because the steps taken are so incredibly small. The steps taken in this process is the smallest unit in the universe. Nothing is smaller than this Plancklength which is about 1,6 x 10-35 m. Similarly we can use the Planck formulae to define the shortest moment possible. The plancktime is unbelievably short, approximately 10-43 seconds. In order to get a feel for how quick this moment passes, imagine a thousandth of a second which is written as 0,001 seconds. This duration of time is so short that for all practical purposes, we do not use it. In comparison, the planck length is only 0,00000000000000000000000000000000000000000001 of a second. This time unit represents the first moment of creation after the big bang. Before this, nothing existed since no shorter time interval is possible.
It is a peculiar feature of the universe that there is such a thing as the smallest component. As Plato and Aristotle thought more than two thousand years ago it is easy to think it should be possible to divide something for ever. When time, space energy or matter is divided down to the plancklength, we are still holding on to something. And that something must in turn be made up of something. For example, even though the plancklength is incredibly short, it still has a length. Thus it is easy to think that it should be possible to remove a tiny tiny piece away from that length. However, this is not the way nature works and this is what Max Planck was able to show us. Planck never thought about the philosophical implications of his theory and he thought that someone else would come up with an answer as to why his theory seemed to be correct.
It took another five years after Planck’s discovery before we understood more of the connection between atoms and temperature. It was discovered that by shining light on a piece of metal, electrons would be emitted through a process called the photoelectric effect. The energy of the light transferred energy to the electrons so that they could be released from the atoms on the surface of the metal. So far, nothing strange so what’s the point? The crucial point is that the electromagnetic equations by Maxwell stated that the more intense the light that was shone on the metal, the more energy the electrons should absorb and thus get travel further out from the metal. It would be the same principle as if you were kicking a ball. The harder you hit it, the further the ball travels. Experiments showed however that this was not the case with electrons. It did not matter how bright and energetic the light source was. This was a baffling result. It is as if a football is not affected at all by how hard you hit it, it travels the same distance. What experiments did show was that more electrons were released from the metal when the energy of the light was increased. Based on the understanding of light as waves, this did not make any sense.
It was Einstein that yet again paved way for a completely new way of thinking. He used Planck’s formula and suggested that light was not a wave but was made up of small bundles of energy named photons. A photon is an individual particle just like an electron. According to Einstein it was individual photons that hit the electrons in the metal and not a light wave, which was the ruling interpretation on the nature of light at the time. By using this interpretation instead, Einstein was able to explain the baffling result of the photoelectrical effect. When the energy level of the light source was increased, the number of photons increased but the individual energy level of each photon remained the same. Hence it was not so strange why more electrons were kicked out of the metal when you increased the energy. If you wanted to increase the energy of the electrons that were knocked out of the metal, you had to change the frequency of the light. For example by changing the light from red to blue, experiments verified that the energy of the knocked out electrons increased. It was therefore the colour of the light (wavelength/frequency) that defined the energy level of each photon.
In order to truly understand this result and compare it to our classical perception of the world it is comparable to kicking a ball and watch it travel a certain distance. If you kick it harder it does not go further, the increase in energy only leads to you kick more footballs at once. If you want to kick a ball further than your first shot you need to change “player” and in quantum physics this translates into changing the wavelength/frequency of the light. Einstein’s explanation was experimentally confirmed and he ended up receiving the Nobel price in physics for this work and not his famous relativity theory.
So far into the history of quantum physics it was finally determined that all matter was made up of tiny blocks called atoms. Understanding the true nature of light was a little bit trickier. As we saw, in the 17th century Newton and Huygens had opposite views with regards to if light was made up of particles or waves. Maxwell solved the twist in the 18th century and proved that light was made up of waves until Einstein in the beginning of the 19th century proved in the photoelectric effect that light was made up of particles. Physics was then confronted by to opposing explanations; either light was made up of particles or waves. The big question at the time was therefore, who was right? One intuitive answer would be to compare light to water. Water has wave characteristics and at the same time, a wave of water is made up of billions of water molecules. Why can’t we use this analogy as well on the nature of light? Let’s return to Thomas Young’s experiment where he was sending light through two slits. The result from Young’s experiment could only be explained if light was travelling as a wave. If waves consist of particles as Einstein proved, it should be possible to send single photons and not a whole bunch of them.
Experiments were set up in order to facilitate this. The result was mind boggling to say the least. It turned out that even though we are sending one photon at a time, the same interference patterns emerges. How is that possible? Interference patterns come about because of overlapping waves. How can it arise if we send individual photons, one at a time? This result is one of the most difficult ones to understand and it’s also quite tricky to understand why it is so strange so let’s go through it nice and slow. Imagine that you are holding a “laser gun” which can fire single photons instead of bullets. Our target is two thin slits placed close to each other. Behind the slits we have put up a screen where each photon leaves an imprint on impact. After we have fired a few rounds, a pattern will slowly emerge as the photons hit the screen behind the slit. It’s the same principle as if you were firing a machine gun at a metal plate with two small openings in the middle and a screen behind the metal plate. After you have fired a few rounds, some of the bullets will have passed through either of the two openings and two narrow patterns will emerge behind the metal plate. If we fire individual photons you would expect to see two narrow bands behind the two slits but this is not what happens. Instead an interference patterns slowly emerges consisting of multiple bands. This is exceedingly strange. We have fired individual photons but the result indicates that we have sent waves. How is this possible? If we think of a water wave, it is not possible to form a wave with individual water molecules. The wave pattern is only possible with a large collection of water molecules. How can individual photons then behave as if they are a wave? Despite this paradoxical result, the nature of light as having both wave and particle characteristics has been verified experimentally numerous of times.
As in every good story you do of course save the good things for last. In 1923 we reached a mind blowing new insight into the nature and structure of reality. A young Frenchman by the name Prince Louis de Broglie (1892-1987) suggested that the wave/particle question not only applied to light but also to matter. He argued that Einstein’s famous equation E=mc² proved that matter was equal to energy. Since light is energy and consists of electromagnetic waves, shouldn’t also matter have wave characteristics? Let’s pause for a moment and reflect on what this means. De Broglie’s suggestion is that all matter, what you, me and everything around us is not made up of tiny particles like billiard balls but of waves. De Broglie sent his paper to Einstein who later replied; it looks insane but you are probably right”. Since then the wave characteristics of matter have been verified experimentally. Instead of firing individual photons, electrons are fired and the same interference appears when there are two open slits present.
Waves of Probability
In 1925 the Austrian physicist Erwin Schrödinger (1887-1961) came up with an equation that later gave him the Nobel price in physics. The equation has become one of the most fundamental equations within physics and chemistry and is known today as the Schrödinger equation. It describes all the properties of an electron or any other sub atomic particle. By using the wave characteristics of a particle as a starting point, the Schrödinger equation s is able to describe how particles move and behave over time.
Because of all the paradoxes within the quantum realm, the Schrödinger equation has been referred to as a mathematical beast. The solution to the equation is not predetermined like ordinary equations but it gives accurate results if it is used on a large number of occurrences (f. ex to determine where the photons in Thomas Young’s experiment will hit the screen). Even though the equation is able to describe the quantum process perfectly, Schrödinger was unsure how he was supposed to interpret it and understand the philosophical implications of it. One of the underlying assumptions is that the equations must be operating in six dimensions. Since ordinary reality consists of four (one in time and three in space), the true nature of the Schrödinger equations is not easy to grapple with. It is perhaps fitting that the equations sometimes is referred to as a mathematical beast due to its highly abstract and complex nature.
In 1926 an interpretation of the Schrödinger equations was raised that shows that reality has no difficulty in surpassing science fiction in weirdness. The German physicist Max Born (1882-1970) concluded that the wave characteristics of sub atomic particles are best described as a wave of probability. This concept was so strange and so radical that no one else in history had even been close to think along those lines. A probability wave is meaningless if we try to picture it as something physical. It only makes sense if we view it through the eyes of a mathematician. Probability says something about our knowledge of a given situation. For example even though we do not know all the factors that affects the outcome when we throw a dice (speed of the hand, friction of the material that the dice lands on, material the dice is made of etc) the results can still be quantified and given a precise value. If we do this a number of times, the outcome of each throw will move closer and closer to the assigned probability values. A probability wave can therefore best be described as a tendency for something. It represents a kind of mix between possibility and reality. Places where the probability wave has a crest, is where we are likely to find the electron but it is not for certain. Likewise there is a low probability of finding the electron in one of the troughs. The only certainty that we can assign to sub atomic particles is that if we carry out identical experiments, we will not find the electron in the same location every time.
Since electrons and all other sub atomic particles is the basic building blocks of matter and therefore our physical reality, the fundamental structure of the universe is made up of probability! Let’s pause for a moment and reflect on this. Everything around us like the chair you are sitting on or the air you breathe is made up of probability waves. If you are having difficulties comprehending this you can perhaps find comfort in the fact that you are not alone. Einstein who was one of the founding fathers of quantum physics never accepted the idea that the universe was governed by probability. His stance was summed up in the famous quote “god does not play dice”. Bohr who was Einstein’s intellectual sparring partner replied to Einstein that he could not dictate god what he was able to do or not.
If everything around us is made up of small areas of probability, what does it imply for our reality? How is it that we feel solid? The answer to the first question we will delve deeper into as we go along. The answer to the second questions goes something like this: Even though all the sub atomic particles in the universe are described as probability waves, you do not have to worry about the particles in your body. The reason why the atoms in your hand do not suddenly vanish and appear somewhere else is simply because it is highly improbable.
De Broglie’s equation showed that probability waves only have an effect on very short distances. As soon as we move outside the area where the electron has a high probability of being located, the probability of finding it somewhere else quickly drops towards zero. In addition, since you hand is made up of an enormous amount of electrons, the probability of all of them moving outside their probability fields is highly unlikely. Even though I am willing to bet my life on that something like this will never happened to you, it is quiet fascinating that it cannot be ruled out either.
The Uncertainty Principle
Shortly after Max Borne’s explanation of the electron as a probability wave the German physicist and later Nobel Prize winner Werner Heisenberg (1901-1976) launched his famous uncertainty principle. In contrast with the Schrödinger equation which only looked at the wave aspect of particles, the uncertainty principle could also take into account the particle nature of sub atomic particles.
Let’s delve a little bit deeper into what this entails. If we want to locate the planet Mars, all we need to do is search for it in the night sky. Thereafter we can follow its transition across the night sky as it moves. If we try to the same with electrons we quickly realize that it is impossible. The physicist and philosopher Henry Margenau (1901-1997) has said that studying electrons is a little bit like watching fireflies in the night sky. You can see a blink here and there but you have absolutely no idea where the firefly is located between these two observations. It is nearly impossible to specify the direction where the fly will be moving next. In order to do that we would have to be able to pinpoint the location and speed at a given point and follow it from there. That could be accomplished if we had a powerful light source that could light up the area where the firefly was located. If we tried to pull that stunt on electrons however it would not work. The reason is that the light would interfere with the electron so that its original path would be lost. By using light with a short wavelength (high energy) we find location of the electron but we interfere with the original direction. If we instead use light with a longer wavelength so as to not interfere with the path the electron is travelling, we can find the direction the electron is travelling but not its location. We are then left with a «catch 22» situation. The more precise we want to know the direction the electron is travelling, the less we know about the electrons position and visa versa.
Heisenberg conclusion was that we are never able to say with certainty where a sub atomic particle is or how it’s moving. It had not nothing to do with the fact that we did not have accurate equipment to measure these variables or that we used the wrong method. It was a fundamental property of nature and it applies to not only matter but to energy, time and even space itself.
On a more practical level, the uncertainty principle explains amongst other things why stars are shining or why the ground is solid despite being made up of mostly emptiness. Let’s look at the sun first. The sun generates heat by fusing hydrogen atoms into the slightly heavier helium atoms. This process has a by-product that we experience as sunlight. What is remarkable is that this process at first glance should not be possible. Just like to magnets are repulsive if you place the equal poles opposite each other, protons are not to keen on coming to close to each other.
This resistance is incredibly strong and in order to overcome it, protons need to collide with tremendous force. Calculations show that in order for protons to obtain the necessary energy to fuse, the temperature in the sun would have to be close to 10 billion degrees. Since the temperature in the suns core is a meagre 15 million degrees it seems like we have a problem if the theory is to be correct. This is where the uncertainty principle comes to the rescue.
When a radioactive material decays it emits an alpha particle. Before the emission, the alpha particle is “trapped” inside the nucleus. What’s strange is that it should be impossible for the alpha particle to escape its subatomic prison. The energy required is many times greater than what the alpha particle possesses. The question then is how does the alpha particle free itself? This is where the uncertainty principle comes into play. It states that the more defined the position of a particle is, the more uncertain its energy levels and visa versa. When the alpha particle is trapped inside the nucleus which is unfathomably small, the space around the alpha particle restricts its possible movement. Therefore the energy level of the particle becomes more uncertain and that’s how it’s able to escape its subatomic Alcatraz. This kind of escape is in some sense comparable to us walking through a solid wall, not probable but still possible. For individual alpha particles the possibility turns out into a certainty. This kind of Houdini like ability is known as tunnelling and it is a direct consequence from the workings of the uncertainty principle.
In a similar way the protons in the sun have a small, yet finite probability to come sufficiently close to fuse. It is this probability that enables the protons to fuse despite the ridiculously "low temperature" of fifteen million degrees Celsius inside the sun’s core. The uncertainty principle also explains why the ground underneath us feels solid. It is correct as stated earlier that the electron travels at unbelievable speeds around the nucleus, thus making the atom appear solid. However, the uncertainty principle also plays a major role. First and foremost is the fact that the uncertainty principle does not allow the electrons to orbit to close to the nucleus, thus creating volume. The reason why the electron has to travel a certain distance from the nucleus is that if it were able to be close to the nucleus, we would be able to pinpoint the location with a high degree of certainty. This would mean that the energy level would be uncertain and thus electrons would be able to escape from the atoms. The uncertainty principle therefore causes electrons to circle around the nucleus at a distance, creating a lot of space inside the atom and thus causing things around to have volume.
The answer from Copenhagen
The fact that we discovered that sub atomic particles could be described as either a microscopic object or as a probability wave made it difficult to interpret the results in a way that would make sense. In 1927 Niels Bohr and Werner Heisenberg met up in Copenhagen in order to find an explanation to this apparent paradox. Their interpretation became known as the Copenhagen interpretation. Bohr argued that the only way you could explain the uncertainty principle was to assume that nothing exists prior to being observed. According to Bohr, there was no such thing as an objective reality without an observer. Let’s take a closer look at what he meant by this.
Previously we saw that the Thomas Young’s double slit experiment confirmed that light had to be made up of waves. A twist to this experiment illustrates something remarkable. This time we cover one of the slits with duck tape and make sure we fire only one photon at a time. Slowly a pattern emerges behind the screen in the same manner as bullets would do. The result of this experiment shows in contrast to the results obtained from Young’s double slit experiment that light is made up of particles. If we return to the set up in the double slit experiment and leave both slits open, an interference pattern emerges and this confirms that light is made up of waves. The odd thing about this result is how a photon “knows” whether or not the other slit is open so it can adjust its behaviour so to speak. No one knows the answer to this question. All we know is that sub atomic particles display different characteristics depending on the set up of the experiment.
Bohr explained this paradox by saying both explanations is right. When we are describing sub atomic particles we have to use two opposing explanations because at the sub atomic level, everything is both a wave and a particle at the same time until an observer comes along and decide which aspect shall be observed in an experiment. By deciding if one or two slits shall be open during the experiment, the observer determines the outcome of the experiment. This result is in itself is baffling but the quantum realm is much stranger than this.
Let’s continue with the two-slit experiment. We leave both slits open and we place a monitor at each slit in order to observe the wave characteristics as it passes through both slits. What’s remarkable is that when we do this, the interference pattern disappears and the photon turns into a particle and pass through just one of the slits. In quantum terminology we say that the wave function collapses. So even though we have two open slits and thus would expect to see an interference patterns on the screen, we only see two narrow bands when we try to sneak a peek at the wave function.
What is true of photons also hold for all other sub-atomic particles as well. Sub-atomic particles, it seems, act like waves as long as they are permitted to act like waves, spread out through space with no definite position. But the moment someone asks where the particles are by determining which slit they actually went through or making them hit a screen-they abruptly become particles. This is result is what Richard Feynman described as the "central mystery" of quantum physics.
However, more recent two-slit experiments suggest that the quantum world is even more baffling. When we talk about observation in experiments, we presume that it entails a measurement of some sort and therefore requires direct physical intervention with the phenomenon at hand. The classic double slit experiments discussed so far do in fact involve intrusive measurement whereby we physically interact with a photon, thus disturbing it and causing the wave function to collapse. However, there are other ways in which we can do the double slit experiment without actually interfering with the photon.
The results from these experiments show that even if you do not physically interact with the photon but nevertheless are able to infer which slit it went through due to a specialised experimental set up, the interference patterns disappears. Stranger still, if you add a polarisation filter that scrambles the information just before the photon hits the detector, then the interference pattern reappears. This means that a collapsed wave function can be put back together again in what is now referred to as a quantum eraser experiment.
What these experiments show is that it is not anything that we physically do to the photon that determines which way it will behave. Rather what determines the outcome of experiments is whether or not we are able to know which path the photon takes to the detector. The crucial part is that we have done nothing to the photon except our ability to know which path the photon took
The plot thickens further when we consider a variation of this experiment called Delayed Choice Experiment. Common sense tells us that influencing the past is impossible however when dealing with the quantum world, nothing seems impossible. John Archibald Wheeler (1911-2008), Nobel Price winning physicist, a former student of Bohr and a mentor for Richard Feynman proposed a variation of double slit experiment. In this experiment which is referred to as Delayed Choice Experiment, the method of detection can be changed after the photon passes the double slit, so as to delay the choice of whether to detect the photons as particles or waves. When the photon passes through the slits, it can't "know" whether the experiment is set up to detect particles or waves. The photon will therefore enter both slits and appear as an interference pattern. However, if we change the mode of detection and take a picture of the photon just after it has passed both slits but before it hits the detector screen in the back, it becomes a particle again.
To illustrate how strange this result is a bit further, consider a cosmic version of this experiment that has yet to be carried out but nonetheless is in full accordance with Einstein's theory of relativity. Imagine a distant star, billions of light years away from earth. On its way towards the earth, the gravitational field around a massive object like galaxy will bend light so that a single photon can take either of two paths around the galaxy and still reach earth. Bending around the left side is the experimental equivalent of going through the left slit of a barrier; bending around the right side is the equivalent of going through the right slit. It is then possible to set up an experiment billions of years later so that an astronomer can either detect the photon coming from both sides of the galaxy and hence seeing an interference pattern or he can choose to determine to see the photons coming exclusively from either the left hand side or the right hand side, and therefore observing the photon acting like a particle. Hence, the experiment is all about choosing to know which side of the galaxy the photon passed by. What is remarkable is that we in this experiment have delayed this choice until a time long after the particles "have passed by one side of the galaxy, or the other or both sides of the galaxy at the same time. It seems paradoxically to say the least that our later choice of whether to obtain this information determines which side of the galaxy the light passed billions of years ago. In effect, the observation you make affects the nature of the past. Physicists call this strange phenomenon "quantum post-selection".
What all of these different experimental designs indicate is that somehow the photon and all other sub-atomic particles for that matter either "knows" what the experimenter wants to measure before it reaches a detector or that it somehow are able to adjust its behavior in the past to fit in with the future experimental set up. John Wheeler expresses this deep quantum mystery in the following way:
"We used to think that the world exists 'out there' independent of us, we the observer safely hidden behind a one-foot thick slab of plate glass, not getting involved, only observing. However, we've concluded in the meantime that that isn't the way the world works. In fact we have to smash the glass and reach in."
What all of these experimental results indicate is that sub-atomic particles are adjusting their characteristics according to if they are being observed or not. Einstein was not happy about the implications of this and apparently asked Bohr one time if the moon was still there if when Bohr was not watching.
A central question to the weirdness of quantum physics is what constitutes an observation? Is it when the scientist learns of the result of the experiment or before? It is a little bit like the famous philosophical question about the falling tree. Is there a sound when a tree falls in the forest if there is no one there to hear it? The answer to this question is not clear. The Copenhagen interpretation was the best answer that Bohr and Heisenberg were able to come up with. It is not so much of an answer, rather it represents more of an agreement to which the majority of physicists today share.
The essence of the Copenhagen interpretation is that it is only the results from experiments that are important. Philosophical questions as to why sub atomic particles can display both particle and wave like characteristics is not scientific because it cannot be explained by physics. The uncertainty principle tells us that there is an inherent limit to what we can know about nature. It has nothing to do with limitations on our measuring equipment. It is a fundamental property of the universe. The only thing we can say with certainty is that an observation causes the wave function to collapse into a defined state that we can observe as a particle. According to the Copenhagen interpretation it is impossible to find out what exactly an observation does to a quantum system so that the wave function collapse. We just have to accept the experimental results without delving into philosophical debates.
Schrödingers cat
It was the paradoxical nature of quantum physics and the somewhat unsatisfactorily explanation that the Copenhagen interpretation presented that eventually lead to the famous question about Schrödingers cat. The famous physicist Stephen Hawking supposedly said that every time he heard the term Schrödinger’s cat he looked for his gun. To see why Hawking feels this way, we have to go back to year 1935. That year Erwin Schrödinger came up with a thought experiment that showed how bizarre the consequences of quantum physics were if you took the results from experiments and transferred them to our everyday macroscopic world. Schrödinger wanted to show that the Copenhagen interpretation had to be wrong when applied to everyday objects.
Schrödinger set up an imaginary experiment where his cat was placed inside a box. Connected to the box was a device that contained a cylinder of cyanide and some radioactive material. All radioactive materials are unstable. This means that the atom may decay any time and release energy in the form of radiation. It can happen within a minute, a day or it might take a million years. Because of the uncertainty principle it is impossible to accurately determine when. We can only say something about the probability of when it will happen.
The essential feature of Schrödinger’s thought experiment was that when the radioactive substance decayed the radiation that was unleashed would start a process where a hammer would break the cylinder containing cyanide and thus killing the cat. However, until we open the box and verify the results of this experiment, both actualities exist simultaneously in superposition. Therefore, until we look inside the box, the cat is both dead and alive at the same time. We know from experience that a cat can’t be both dead and alive at the same time. This raises the difficult question; if the Copenhagen interpretation is right, when do the bizarre quantum effects of superposition stop and turn into our everyday solid objects like tea cups and dogs? This is a pivotal question within quantum physics. The Schrödinger equation does not impose and limits so in theory the whole universe could be described as a probability wave.
Recent experiments have shown that it is possible to bring entire molecules up to a state of super position. Nowadays there are even serious talks about bringing viruses into a state of superposition by using advanced lasers. This shows that we are closing in on gap between quantum objects and the size of everyday objects. The philosophical implications of this are profound. If the whole universe can be described as a wave function and we know that the wave function is affected by observation, what does this mean? As we have seen so far, the Copenhagen interpretations tries to push this question under the carpet by saying its irrelevant but as we proceed, we will see later, it might have profound implications for how we exist in the world.
The EPR Paradox
As mentioned earlier, Einstein was not to happy about the implications of what quantum physics told us about the way nature worked. He recognized that the theory was unto something because it could predict the outcome of experiments but even so he felt that the theory still was incomplete. Einstein believed that a deeper explanation existed that would get rid of the apparent paradoxes. In the same year that Schrodinger set up his cat challenge, Einstein and two of his colleagues, Boris Podolsky and Nathan Rosen set up a thought experiment that would prove that quantum physics were incomplete. Their argument was that even if it was not possible to measure both position and velocity of particle at the same time, hidden variables existed that could explain everything if we only knew them. By setting up a thought experiment, they wanted to show that all particles had defined position and velocities at all times, thus proving that nature was not so messy as quantum physics suggested.
The starting point for the experiment was a phenomenon in physics where one particle is split into two smaller particles; f. ex when a particle called a pion splits up into two photons. When quantum occurrences like this happen we say that the particles are entangled. They are like a pair of identical twins which share the same characteristics. In the subatomic world, this translates into having the same velocity, mass etc but with opposite spin (all particles have spin that is either up or down). Since quantum physics through the Pauli Exclusion Principle forbids to particles to share the same quantum state (equal spin or orbit), it means that when one of the entangled particles have spin up, the other particle must have spin down. Because of this effect, when we measure the state of one particle we automatically know the state of the twin particle. What is strange is that according to the Copenhagen interpretation, prior to us measuring the quantum state of one of the particles, both the particles exist in super position with no predetermined characteristics. As soon as we make a measurement, the wave functions collapses and the two particles are assigned their respective quantum states. Einstein referred to this effect as “spooky action at a distance”. He argued that an object over “here” cannot be influenced by what you do to an object over “there”. He therefore designed a thought experiment to show that he was right.
According to Einstein’s theory of relativity, nothing can travel faster than the speed of light. If two twin particles are located on opposite sides of the universe and we take a measurement on one of the particles, it should take billion of years for the information to travel to the other side of the universe to inform the other particle that a measurement was carried out on its entangled twin. However, quantum theory postulated that it would happen immediately thus seemingly violating Einstein’s cosmic speed limit set by the speed of light. Einstein, Podolsky and Rosen therefore believed that there had to be hidden variables at work that would be able to explain this apparent contradiction.
Despite being backed by Einstein’s enormously successful relativity theory, the EPR guys did not manage to impress the defenders of the quantum theory. Their answer was that there were no hidden variables. The Copenhagen principle clearly stated that there was a limit to our knowledge of a system thus making any suggestion about hidden variables irrelevant. Due to these two opposing viewpoints, the thought experiment became known as the EPR paradox.
For several years it looked as if this was one the paradox within quantum physics that one just had to accept. However, in 1964 the British physicist John Bell devised a thought experiment that would be able to solve the standoff. At the time there was no technology available that was advanced enough to conduct the experiment so the answer to the paradox had to wait almost twenty years before it was settled. Numerous experiments have now shown that Einstein, Podolsky and Rosen were wrong. An object (particle) over “here” is in fact influence by what you do to an object (particle) over “here”. Quantum physics have therefore shown us that there are connections between matter and energy that transcend our current understanding of space and time.
Infinite worlds
A third theory on what is causing the Schrödinger equation to collapse was proposed by the American Hugh Everett III in 1957. Everett did not use the idea that the wave of probability collapsed on its own accord nor that you needed an observer to define reality. Everett meant that the wave of probability never collapsed. In any quantum event that have more than one outcome (for example that an electron will hit one of many atoms) the universe will split. In one universe the electron will hit atom A, while in another universe the electron will hit atom B. With regards to Schrödinger’s cat this mean that in one universe the cat is dead while in another one it is alive. Absolutely all possible outcome of one single micro event will become real in one universe or another. This is known as the “Many world’s interpretation of quantum physics.
Since everything is made up of sub atomic particles, then it follows that we as conscious beings must also be on the probability wave ourselves (as probabilities). The Many Worlds theory therefore imply that every possibility exist as discrete reality, not only on the micro level of atoms but also on our level of perception. This means that there is in an infinite variety of universes that contains an exclusive "me" that is unable to experience any of my other selves residing on other exclusive probability waves.
The many worlds’ interpretation means that all thinkable outcomes must exist in one universe or another. If the theory gives us a correct description of reality, then it means that in a parallel universe there is another version of yourself writing this book that you are now reading. You may think this an obscure theory I have chosen to include just for fun but statistics show that in an opinion poll among physicists, 58 % of them believed that the many worlds interpretation of reality is the most plausible description of how nature works.
Consciousness causes collapse
In 1961 the Nobel Prize winner in physics, Eugene Wigner, came up with a very surprising interpretation of what makes the probability wave collapse. Wigner took the thought experiment with Schrödingers cat a bit further and introduced a friend in the experimental setup. In this experiment it is a friend of Wigner who performs the experiment after Wigner leaves the laboratory. Only when Wigner returns does he learn the result of the experiment from his friend, that is, whether the cat is alive or dead. A fundamental question is then raised: when was the state of the system determined? When Wigner learned the result of the experiment or was it determined at some previous point? If it was determined when Wigner learned the result, then the system must have been in a superposition of dead cat/sad friend and alive cat/ happy friend. Wigner used this thought experiment to support his view that human consciousness was necessary to the quantum mechanical measurement process. Wigner believed that the consciousness of his friend would have brought about the collapse whether or not Wigner (the second observer) made contact with the situation in the lab.
Critics of this interpretation say that this theory does not explain which things have sufficient consciousness to collapse the wave function. Another point is what was the wave function doing before consciousness appeared in the universe? Was the wave function waiting to jump for thousands of millions of years until an organism with sufficient consciousness were able to collapse it? It is also not clear whether measuring devices might also be considered conscious.
The key mystery that is still unsolved within quantum physics is how or why the probability wave collapse and turn into something as we perceive as solid. What makes this so difficult to figure out is that although quantum effects generally occur at small scales there is no apparent transition or cut-off due to size or scale, the Schrödinger equation does not in any way restrict even macroscopic objects like yourself to be in a superposition. The whole problem therefore is a bit like trying to solve a Gordian knot.
Many physicists believe that the probabilistic aspect of matter cancels out when there are a large number of particles around. The particles are in constant interaction and their environment serves to eliminate the super positions by a process called decoherence. Interaction between objects and their environments then does the job of 'observation' which Wigner accorded to conscious observers, Even though this sounds like a plausible explanation, it is important to be aware that decoherence does not generate actual wave function collapse. It only provides an explanation for the appearance of wave function collapse. Decoherence is therefore just an assumption.
In 2007 an Austrian research team, led by Markus Aspelmeyer and Anton Zellinger performed experiments that ruled out a broad class of hidden-variables theories. The results supported the notion that reality does not exist when we are not observing. It seems that science has finally managed to answer the age old philosophical question;” is there a sound if a tree falls in the forest and there is no one around to hear it?” What we can conclude is that the fundamental building blocks of reality exist as a potential when they are not being observed. What finally makes reality appear solid to our senses is still a mystery. If it is down to an infinite number of universes, decoherence, some still undiscovered physics or our consciousness, no one knows for sure. It looks like you as an observer of this information is the one to decide.