7. Mysteries of Light - Part 2
This is the seventh article in the series From Particles to Angels. If you are interested in this article you should read the previous articles in the series in order, beginning with the first (On Happiness).
In Mysteries of Light - Part 1 we talked about two of the paradoxes of light: the particle-wave duality, and the constancy of it's speed. Before looking at the modern interpretation of the particle-wave duality, I'd like to talk a little about the role and status of light in modern physics, at least as far as my understanding goes.
Aside from the 3 particles that comprise matter, physics also posits just 4 fundamental forces. These are gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. Everything that happens in the world is some combination of these 4 forces acting on the 3 kinds of matter. Gravity you know, and electricity and magnetism you know. The strong nuclear force operates in the nucleus of atoms. It is what binds quarks together to form protons and neutrons, and what binds protons and neutrons together to form the nucleus of atoms. The weak nuclear force is more obscure. It is involved in nuclear radiation. Every now and again, an up quark in a nucleus will change into a down quark, changing a proton into a neutron, or a down quark will change into an up quark, changing a neutron into a proton. The weak nuclear force mediates this rare change. But the forces we are most familiar with are gravity and electromagnetism.
You know that gravity is what keeps us all glued to the surface of the earth, and what stops all the oceans and atmosphere floating off into space. It keeps the moon in orbit around us, which tugs a little on the oceans, forming the tides. It holds all the planets in orbit around the sun, and all the stars in orbit around the centre of the galaxy. Galaxies too may orbit around larger clusters. If we want to overcome the force of gravity and leave the Earth, we need to build tall rockets filled with explosive liquid oxygen, and ignite it.
When you pick up your coffee mug, it is solid because of the electromagnetic force, and so is the floor beneath your feet. The crushing weight of miles of rock in the crust of the Earth under the force of gravity is resisted by the repulsive forces between the tiny atoms in the rock, that prevent the whole Earth from collapsing to a single point. It takes the weight of a collapsing star to overcome this resistive force. So most of what you know as physics and chemistry, biology and engineering, is the operation of the electromagnetic force. When paper burns, water boils or iron rusts, when a plant grows, your heart beats or your nerves transmit a signal, it is electromagnetism in action.
If we take the strength of the strong force as our unit of measure, that is, if we say the strong force has a relative strength of 1, then the electromagnetic force has a relative strength of 1/1000. That is, the electromagnetic force is one-thousandth the strength of the strong nuclear force. The weak nuclear force has a relative strength of 1/10,000,000,000,000,000. That is, ten-quintillionths of the strength of the strong force, or ten-trillionths of the strength of the electromagnetic force. The gravitational force has a strength of 1/100,000,000,000,000,000,000,000,000,000,000,000,000,000. So it is only 1/10,000,000,000,000,000,000,000,000 the strength of the electromagnetic force. We don't realise the incredible strength of the electromagnetic force because it comes in two opposite kinds: positive and negative, and most of the time they are present in equal amounts, and so they mostly cancel each other out. It only takes a small mismatch between positive and negative to power an electric train or a city, or cause a lighting storm. A lightning storm shows nature's strong inclination to restore the balance between the electromagnetic force, and so does the zap you get sometimes when you touch a metal door handle after walking across a carpet. The implacable force of gravity is because it only comes in a single variety. All the little tiny bits of gravity are permitted to add up. But consider a small piece metal sitting on the ground, held there by the combined might of the gravity of every atom in planet Earth. Now imagine picking up that bit of metal with a tiny magnet held in your hand. Have you ever tried to push the North pole of two magnets together?
Each of the fundamental forces is thought to be mediated by a special kind of particle, although there is some uncertainty around the strong force and gravity. The particle that transmits the strong force is called the "gluon" because it glues the nucleus together. The particle mediating gravity, if it exists, is called the "graviton". The weak nuclear force is mediated by two kinds of particles called "Z bosons" and "W bosons". If a proton turns into a neutron, or a neutron turns into a proton, a W boson is emitted. The electromagnetic force is mediated by photons. Pause dramatically.
It is important to remember that the parts of the electromagnetic spectrum are not a collection of different phenomena, but just a lot of different names for the same phenomena. To emphasise this fact I will be committing a slight misuse in terminology from now on, and will be referring to the entire electromagnetic spectrum and all photons as "light".
The electromagnetic force works by means of exchanging photons between charged particles. What this means, if you haven't guessed already, is that all the solid objects in the world are solid by virtue of being bound together by light. Mount Everest does not sink to the centre of the planet under the force of gravity because light is supporting it. It is light that stops you driving through a concrete wall. The phenomena of electronics and much of physics and chemistry, is the operation of light. Before we look at some more of the implications of this, let's return to the subject of the particle-wave duality and its modern form.
Reference is sometimes made to Einstein's exasperated statement that "God doesn't play dice with the world" as if he is denying the reality of the role of chance in the universe or the legitimacy of probability in statistical mechanics. It is then described how successful probabilistic models have been in describing and predicting physical events in the quantum world, with the implication that Einstein was some foolish old fuddy-duddy incapable of accepting the radical new physics.
"One could predict the approximate number of times that the result would be A or B, but one could not predict the specific result of an individual measurement. Quantum mechanics therefore introduces an unavoidable element of unpredictability or randomness into sicence. Einstein objectd to this very strongly, despite the important role he had played in the development of these ideas. Einstein was awarded the Nobel prize for his contribution to quantum theory. Nevertheless, Einstein never accepted that the universe was governed by chance; his feelings were summed up in his famous statement 'God does not play dice.' Most other scientists, however, were willing to accept quantum mechanics because it agreed perfectly with experiment. Indeed, it has been an outstandingly successful theory and underlies nearly all of modern science and technology."
If you are playing roulette in a casino, you can place your bet on one of the numbers. The courier spins the wheel in one direction, and tosses in the ball in the other direction. The wheel has 37 or 38 numbered pockets and eventually the ball will fall into one of these pockets. If the wheel has 38 pockets then you know that the laws of probability state that there is a 1 in 38 chance of the ball landing on the number you have placed your money on. The laws of probability and chance are a generally accepted part of reality, even for Albert Einstein. However, when we seek to explain the physics of how the ball interacts with the roulette wheel we are not content to account for its behaviour as being the result merely of chance. We will consider things like the mass and velocity of the ball, and the shape of the wheel. The motion of the ball is the result of deterministic physical laws, but a knowledge of those laws does not tell us which number the ball will land on because of the difficulty in accounting for and measuring all the variables. We could in theory determine it if we had a precise knowledge of the motion of the wheel and the momentum of the ball, but in most practical cases we do not have that knowledge. It is the same with choosing a card at random from a deck of cards, tossing a coin in the air, throwing a dice, or considering the collisions of molecules in a gas. Probability theory gives us a means of making a prediction about the outcome of the event, without a knowledge of all the underlying physical factors. We know the roulette wheel has 28 pockets and that the ball must land in one of them. A probability of 1/38 tells us that we're unlikely to land on a particular number the first time, but if we try 38 times we are likely to land on it at least once. Our world consists of an element of unpredictable chance, but underneath it is guided by predictable, deterministic laws. That is all Einstein was saying. His opponents were saying something strange. They were saying: "No, it's just chance, all on its own. There are no underlying deterministic laws leading to the predicted outcome". They were suggesting that chance and probability can serve as physical causes of events, acting all on their own.
We do not need to seek for any underlying physical causes, since after all: "They had measured [this, that and the other] and could write down a simple law relating [all and sundry]. [The] alleged [deterministic causes were], by contrast, ... invisible, intangible, and imperceptible. What was the point of explaining a straightforward law, derived directly from experiment, in terms of hypothetical entities that could not be seen and might never be seen?"
Einstein originally made his comment in a letter to Max Born in 1926.
"Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the 'old one.' I, at any rate, am convinced that He does not throw dice."
He repeated the statement at later times.
"As I have said so many times, God doesn't play dice with the world."
Louis de Broglie made the following comment on the difference between statistical prediction and physical explanation.
"Such is, in its main lines, the present state of the Wave mechanics interpretation by the double-solution theory, and its thermodynamical extension. I think that when this interpretation is further elaborated, extended, and eventually modified in some of its aspects, it will lead to a better understanding of the true coexistence of waves and particles about which actual Quantum mechanics only gives statistical information, often correct, but in my opinion incomplete."
Quantum Physics is becoming an increasingly impenetrable black box for most of us, and "probability as a sufficient physical cause" is justified with claims that the counter intuitive nature of the quantum world must be accepted. If the usual interpretation of chance events is used, it implies a whole other as yet undiscovered layer of physical causality of which the current probabilistic methods of quantum mechanics show us only the result. If we know that there is a three in four chance of something happening, but we don't know why or how, we can say that it happens three out of four times "because" there's a three in four chance of it happening. Then we can appeal to Occam's Razor to defend the sufficiency of "a straightforward law, derived directly from experiment" claiming that this is a simpler explanation than a whole other undiscovered, hidden layer of physical causation. Under normal circumstances we would suggest in such a case that nothing has been explained at all.
"A nice way of visualizing the wave/particle duality is the so-called sum over histories introduced by the American scientist Richard Feynman. In this approach the particle is not supposed to have a single history or path in space-time, as it would in a classical, nonquantum theory. Instead it is supposed to go from A to B by every possible path.... The probability of going from A to B is found by adding up the waves for all the paths."
Imagine you are planning a trip to Europe. You check the prices for all the different airlines and the different routes they take. Should you have a stop-over in Bangkok, or Morocco, or should you fly straight through? What hotels should you stay in? Will you join a tour group when you arrive, or hire a car and set out on your own? Your "plan" consists of all the "possibilities" of what you could do. But eventually you must make a decision and choose only a single one of all these possibilities. The sum of your various plans is the wave function. The collapse of that wave function is the single actual route you take. Richard Feynman uses the example of a photon bouncing off a mirror as part of ordinary reflection as an illustration.
"So the theory of quantum electrodynamics gave the right answer--the middle of the mirror is the important part for reflection--but this correct result came out at the expense of believing that light reflects all over the mirror, and having to add a bunch of little arrows [probability vectors] together whose sole purpose was to cancel out."
That the "light reflects all over the mirror" in some real sense is shown by the fact that we can exploit this feature in a device called a diffraction grating (see: QED the strange theory of light and matter by Richard P. Feynman, p.46). You may have heard of Schrödinger's "half dead cat". A cat is hidden in a box and whether it is alive or dead is determined by some quantum event. So that in the wavefunction view, the cat exists in both states (alive and dead) simultaneously. Some have chosen to go so far as to suggest a model where all possibilities exist as real parallel universes. Hugh Everett III (1930–1982) proposed the many-worlds interpretation of quantum physics.
"The simplification of the measurement problem achieved by the many-worlds approach, combined with the fact that the mathematical details of the theory turn out to be very elegant, has led the physicist Paul Davies to describe it as 'cheap on assumptions, but expensive on universes!'"
The philosopher David Lewis (1941 - 2001) pursued a similar idea in his book: "On the Plurality of Worlds", a defence of what is referred to as "modal realism". Erwin Schrödinger, originator of the wavefunction, makes the following comment.
"Nearly every result (the quantum theorist) pronounces is about the probability of this or that or that ... happening - with usually a great many alternatives. The idea that they be not alternatives but all really happen simultaneously seems lunatic to him, just impossible."
It seems as though fundamental particles are able to make plans about what they are going to do, and to compare all of these plans simultaneously to determine what they will actually do. The parallel worlds hypothesis takes a very literal interpretation of this process. In our example of planning a trip to Europe, the planning takes place in our head while the actual event takes place in the real world. But presumably particles don't have heads in which to do their planning, and how would they go about gathering all the information they need about their environment? What seems to be required is some new formulation of the notion of a physical "possibility", a formulation that is not quite what we think of as an actual physical event, but also not quite what we think of as a mental process in someone's head. That is, that the alternatives exist not as "real events" in parallel universes, but as "real possibilities" in this one. We might segue here into speculation on the place of consciousness in nature, but let's leave it there for now. The philosophy that possibilities have some kind of (non-actual) existence is called "possibilism". It stands in contrast to the philosophy called "actualism", that asserts that the only existing things are actual.
"What makes actualism so philosophically interesting, is that there is no obviously correct way to account for the truth of claims like ‘It is possible that there are Aliens’ without appealing to possible but nonactual objects."
("Actualism" by Christopher Menzel, Stanford Encyclopedia of Philosophy)
The preference we have met with several times already to believe that something does not exist until it has been definitely discovered finds its ultimate expression in the collapsing wavefunction (as represented in the thought experiment known as "Schrödinger’s Cat"). The wavefunction is a mathematical description of probabilities, but it is able to give rise to concrete realities (that is: collapse) by means of the creative power of the act of an observer looking at it.
Modal Realism may serve to illustrate the pitfalls of tailoring reality to the interests of logic, rather than the other way around.
Once upon a time matter was just matter. It was real substance, actual stuff. Today, matter is technically defined as particles possessing mass. But not all particles possess mass. One notable example is the photon, which has zero mass. That is, there is no matter in a photon. But if there is no matter, what is there? What we mean by something having mass, is that it is something that obeys certain behavioural rules, primarily it demonstrates inertia.
At the same time as we have particles with no mass, we have the modern conception of "space-time" describing a void which nevertheless has characteristics and out of which matter can spontaneously emerge.
Einstein's famous e = mc2 equation says that mass and energy are interchangeable. But what do we mean by energy? This concept has gone through some changes over the centuries. Back in 1667 the alchemist Johann Joachim Becher suggested that flammable objects contained a fire-like substance he called "phlogiston", and that the act of burning something was the act of releasing this fire-like substance previously locked in the object. It was assumed that phlogiston had mass, and therefore that objects would weigh less after burning because the phlogiston had been lost. Some careful experiments showed that objects did not weigh less after burning, as long as you didn't let the smoke escape, and the phlogiston theory was abandoned. It was replaced for a time by the more sophisticated "Caloric" theory. Caloric was a weightless, "subtle fluid" that was lost as bodies cooled. The problem Caloric theory encountered was that physical processes do not necessarily conserve heat, but do conserve energy. So instead of heat as a kind of stuff, a substance, we needed the more general notion of energy that could take the form of heat, but could also take other forms such as momentum or sound. Caloric theory is remembered in our word "calorie", the name of the unit for the measure of energy.
Energy then came to be thought of not as a substance, but as an ability to do work. Energy was an amount of "horse-power" or "kilowatts". It wasn't a noun, but an adjective or a verb depending upon whether it was potential or actual. Slowly moving particles of little mass had low energy. fast moving particles had a lot of energy. The drivers of all this activity was the 4 fundamental forces. If I have a coffee mug sitting on the edge of a table it has potential energy on account of its distance from the floor, because if I bump it with my elbow off the edge of the table, the force of gravity will accelerate it to the floor where it may shatter. It might make a loud noise as it hits the floor, as some of the energy of its motion (its kinetic energy) is converted into sound, while the rest is inherited by (the kinetic energy of) its fragments as they fly out in all directions.
Putting aside the effects of gravity, most of what we refer to as energy boils down to the activity of photons. If you are playing pool or billiards, you shoot the queue ball at another ball, and the energy you imparted to the queue ball with the stick, the queue ball then transfers to the ball it hits. If the queue ball hits the other ball head on, the queue ball may transfer all of its kinetic energy to the other ball, so that the queue ball stops moving altogether, while the other ball flies off almost at the same speed the queue ball originally had, minus some energy lost in the "clack!" sound the balls made when they collided, and perhaps a tiny amount of heat. The transmission of momentum (the tendency to move and keep moving) from one object to another, is effected by photons, even though photons, having no mass, have no momentum of their own.
If we blow up a building with C4, bits of building will fly in every direction, having kinetic energy provided by photons. From the explosion will radiate light (photons) and heat (kinetic energy transmitted by photons) and sound (kinetic energy imparted to the air ... by photons). The "release of energy" from an object, is customarily the release of photons, rather like the old phlogiston theory, except that photons (like caloric) don't have mass. I present that in my role as patron saint of abandoned notions. If you want to put energy into something, by heating or accelerating it, pump some photons into it. So when we talk about the conversion of matter into energy and energy into matter, we are usually talking about converting matter into photons and photons into matter, respectively. Accelerate something enough, and it gains mass. In this sense, matter is made out of light, and dissolves back into light. If energy is more primary than matter, then light is the fundamental stuff out of which the universe is made.
But just how do photons communicate momentum from one object to another when they have none of their own? That is, how does the electromagnetic force actually work? Consider this additional problem. You are probably aware that a proton and an electron will attract one another because they have opposite charge (a proton has a positive electric charge, while an electron has a negative charge), while two electrons will repel each other because they have like charge; similar to the way the north poles of two magnets will repel each other, while the south pole of one magnet and the north pole of another will be attracted to each other. Now imagine that a lone photon sets out into space from an electron, the effect of that photon on any other particle that it encounters on it's journey will depend upon what kind of particle it meets. For instance, if it meets a proton it must say to it: "Dear proton, I come to you from an electron, so I need you to move in the direction from which I came." If the photon reaches another electron instead, it must say to it: "Dear electron, I come to you from another electron, so I need you to head off in the direction opposite to that from which I came." But as far as we know, the only characteristics that photons have is wavelength, frequency and magnitude of energy, all of which are dependent on each other, as well as a constant velocity and some direction of motion. There's not supposed to be any identifiable difference between a photon that leaves an electron, and a photon that leaves a proton. Having no mass it has no momentum of its own. So how is the photon able to communicate all this information to the other particles it meets? Physicists talk now about photons transferring "information" between particles, but no particular mechanism is suggested.
Although photons communicate the effect of charge, they have no charge of their own, and therefore are neither attracted nor repelled by charged particles. And given that photons, electrons and other fundamental particles are all considered as having no physical size, that is, they are treated as point particles, how do photons manage to find and connect with other particles? This is usually accounted for by their being "smeared out" in space.
"Heisenberg's uncertainty principle implies that particles behave in some respects like waves: they do not have a definite position but are 'smeared out' with a certain probability distribution."
That is how a dish microwave antenna that is a mesh (that is, full of large holes) is able to detect a signal. If we think back to Feynman's sum over histories approach, or Schrödinger’s wave function, individual photons seem to know a lot about their environment, and are able to interact with it in a coordinated way. If you Google the words "two slit experiment" you will find an interesting example.
When we must reformulate our understandings we must decide whether to retain the term used for the old formulation and merely adjust its meaning, or whether to do away with the old term altogether and allocate a new term for the new meaning, such as replacing phlogiston with photon. When the ancients spoke of the elements: earth, water, air and fire, they were not using the words as we use them. When water freezes we call it ice, but although it is a solid, we consider that ice is still composed of the chemical compound we call water. When water evaporates we call it water vapour, but although it is like a gas, we still consider it to be molecules of water in the air.
"In the first place, we see that what we just now called water, by condensation, I suppose, becomes stone and earth; and this same element, when melted and dispersed, passes into vapour and air. Air, again, when inflamed, becomes fire; and again fire, when condensed and extinguished, passes once more into the form of air; and once more, air, when collected and condensed, produces cloud and mist; and from these, when still more compressed, comes flowing water, and from water comes earth and stones once more; and thus generation appears to be transmitted from one to the other in a circle. "
So when the ancients spoke of "earth, water, air and fire"; they were thinking of what we would call: "solid, liquid, gas and radiant energy", and they recognised that each could transform into the others. So we could just as well call the photon by its ancient name: "fire".
"In the next place we have to consider that there are divers kinds of fire. There are, for example, first, flame; and secondly, those emanations of flame which do not burn but only give light to the eyes; thirdly, the remains of fire, which are seen in red-hot embers after the flame has been extinguished."
I hope I've managed to convey some of the magic and mystery of the thing we call "light", and maybe even some excited anticipation about what layers of reality nature might reveal to us next. "What was the point of explaining a straightforward law, derived directly from experiment, in terms of hypothetical entities?" I leave that for you dear reader. Perhaps we can merely call it "curiosity". We complete our discussion of matter in the article Atom.
Any comments welcome.
In Mysteries of Light - Part 1 we talked about two of the paradoxes of light: the particle-wave duality, and the constancy of it's speed. Before looking at the modern interpretation of the particle-wave duality, I'd like to talk a little about the role and status of light in modern physics, at least as far as my understanding goes.
Fundamental Particles and Forces
In a way, the story of the development of physics and chemistry and science in general has been about explaining more and more with less and less. Since ancient times, human beings have imagined that the great diversity of things that exist; trees, animals, rocks and clouds; were composed of a few simple elements. The ancients suggested these might be earth, water, air, fire; and the aether. This conception has evolved quite a bit over the centuries. You are probably aware that physics now says that all the matter we see is composed of atoms, and these atoms are composed of protons, neutrons and electrons. Protons and neutrons form the nucleus of the atoms, while electrons orbit around the nucleus. With the development of quantum physics, although the electron is still a fundamental particle; protons and neutrons are thought to be composed of smaller particles called "quarks". The proton is composed of two "up" quarks and one "down" quark, while the neutron is composed of two down quarks and one up quark. So every physical object in the world is composed of just these 3 elements: electrons, up quarks and down quarks. All the chemical elements from hydrogen to plutonium are a configuration of these 3 elements, and every compound from wood and flesh, to chocolate, is some configuration of these chemical elements. Quantum physics posits a whole zoo of other particles, but these others are exotic and fleeting and don't compose ordinary matter. So we can ignore these others.Aside from the 3 particles that comprise matter, physics also posits just 4 fundamental forces. These are gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. Everything that happens in the world is some combination of these 4 forces acting on the 3 kinds of matter. Gravity you know, and electricity and magnetism you know. The strong nuclear force operates in the nucleus of atoms. It is what binds quarks together to form protons and neutrons, and what binds protons and neutrons together to form the nucleus of atoms. The weak nuclear force is more obscure. It is involved in nuclear radiation. Every now and again, an up quark in a nucleus will change into a down quark, changing a proton into a neutron, or a down quark will change into an up quark, changing a neutron into a proton. The weak nuclear force mediates this rare change. But the forces we are most familiar with are gravity and electromagnetism.
You know that gravity is what keeps us all glued to the surface of the earth, and what stops all the oceans and atmosphere floating off into space. It keeps the moon in orbit around us, which tugs a little on the oceans, forming the tides. It holds all the planets in orbit around the sun, and all the stars in orbit around the centre of the galaxy. Galaxies too may orbit around larger clusters. If we want to overcome the force of gravity and leave the Earth, we need to build tall rockets filled with explosive liquid oxygen, and ignite it.
Electromagnetism
Electricity and magnetism are two aspects of a single phenomenon called electromagnetism. James Clerk Maxwell (1831–1879) discovered that. Since then, we have discovered the thoroughgoing importance of the electromagnetic force. We learned above that the strong nuclear force is what forms the nucleus of atoms. The electromagnetic force is what binds electrons in orbit around the nucleus, completing the atom. It is also the force that binds atoms together to form molecules, solids and liquids. If you drive your car at high speed into a concrete wall, it is the electromagnetic force between that molecules in the concrete that stops you and your car abruptly. I read a nice quote not too long ago, but haven't been able to find it again. It was about a scientist (possibly Feynman, ... or someone else) who used to like to compare the relative strengths of gravity and the electromagnetic force by talking about someone falling down an elevator shaft. It requires the distance of tens of metres or more for the force of gravity to slowly accelerate them to their final velocity downwards. But it only takes a tiny fraction of an inch for the electomagnetic force to decelerate them down to zero velocity at the bottom.When you pick up your coffee mug, it is solid because of the electromagnetic force, and so is the floor beneath your feet. The crushing weight of miles of rock in the crust of the Earth under the force of gravity is resisted by the repulsive forces between the tiny atoms in the rock, that prevent the whole Earth from collapsing to a single point. It takes the weight of a collapsing star to overcome this resistive force. So most of what you know as physics and chemistry, biology and engineering, is the operation of the electromagnetic force. When paper burns, water boils or iron rusts, when a plant grows, your heart beats or your nerves transmit a signal, it is electromagnetism in action.
If we take the strength of the strong force as our unit of measure, that is, if we say the strong force has a relative strength of 1, then the electromagnetic force has a relative strength of 1/1000. That is, the electromagnetic force is one-thousandth the strength of the strong nuclear force. The weak nuclear force has a relative strength of 1/10,000,000,000,000,000. That is, ten-quintillionths of the strength of the strong force, or ten-trillionths of the strength of the electromagnetic force. The gravitational force has a strength of 1/100,000,000,000,000,000,000,000,000,000,000,000,000,000. So it is only 1/10,000,000,000,000,000,000,000,000 the strength of the electromagnetic force. We don't realise the incredible strength of the electromagnetic force because it comes in two opposite kinds: positive and negative, and most of the time they are present in equal amounts, and so they mostly cancel each other out. It only takes a small mismatch between positive and negative to power an electric train or a city, or cause a lighting storm. A lightning storm shows nature's strong inclination to restore the balance between the electromagnetic force, and so does the zap you get sometimes when you touch a metal door handle after walking across a carpet. The implacable force of gravity is because it only comes in a single variety. All the little tiny bits of gravity are permitted to add up. But consider a small piece metal sitting on the ground, held there by the combined might of the gravity of every atom in planet Earth. Now imagine picking up that bit of metal with a tiny magnet held in your hand. Have you ever tried to push the North pole of two magnets together?
Each of the fundamental forces is thought to be mediated by a special kind of particle, although there is some uncertainty around the strong force and gravity. The particle that transmits the strong force is called the "gluon" because it glues the nucleus together. The particle mediating gravity, if it exists, is called the "graviton". The weak nuclear force is mediated by two kinds of particles called "Z bosons" and "W bosons". If a proton turns into a neutron, or a neutron turns into a proton, a W boson is emitted. The electromagnetic force is mediated by photons. Pause dramatically.
Light
You may have heard of the electromagnetic spectrum. At one end of the spectrum are gamma rays and x-rays and ultraviolet. At the other end are radio waves, microwaves and infrared; and in between is visible light. But this entire spectrum refers to photons of certain frequencies and corresponding wavelengths (because remember they're also waves). What we call radio waves may have a wavelength from about 1 metre (m) up to many thousands of kilometres (km). Radio waves shade into microwaves having a wavelength from about 1 metre down to about 1 millimetre (mm). Then from 1 millimetre down to about 1 micrometre (1 μm = 1/1,000,000 m, one-millionth of a metre) is infrared light. At the micrometre scale is all of the spectrum of visible light. From the micrometre scale down to the nanometre (1 nm = 1/1,000,000,000 m, one-billionth of a metre) scale is ultraviolet light. X-rays and gamma rays are on the picometre (pm) scale (1 pm = 1/1,000,000,000,000, one-trillionth of a metre). In the visible sprectrum of light, red is at one end with a wavelength of around 700 nm, while blue is at the other end with a wavelength of around 400 nm. If you're wondering how real these so called "wave properties" of photons are, take a look at a few dish microwave antennas, the ones that are just a metal mesh; then ask yourself how a mesh is going to be suitable for catching tiny particles.It is important to remember that the parts of the electromagnetic spectrum are not a collection of different phenomena, but just a lot of different names for the same phenomena. To emphasise this fact I will be committing a slight misuse in terminology from now on, and will be referring to the entire electromagnetic spectrum and all photons as "light".
The electromagnetic force works by means of exchanging photons between charged particles. What this means, if you haven't guessed already, is that all the solid objects in the world are solid by virtue of being bound together by light. Mount Everest does not sink to the centre of the planet under the force of gravity because light is supporting it. It is light that stops you driving through a concrete wall. The phenomena of electronics and much of physics and chemistry, is the operation of light. Before we look at some more of the implications of this, let's return to the subject of the particle-wave duality and its modern form.
Playing Dice with the World
Since the discovery of the photon, the particle-wave duality problem has infected all of fundamental physics, because it was found that electrons and other particles, and even molecules exhibit wave characteristics together with their particle characteristics.Reference is sometimes made to Einstein's exasperated statement that "God doesn't play dice with the world" as if he is denying the reality of the role of chance in the universe or the legitimacy of probability in statistical mechanics. It is then described how successful probabilistic models have been in describing and predicting physical events in the quantum world, with the implication that Einstein was some foolish old fuddy-duddy incapable of accepting the radical new physics.
"One could predict the approximate number of times that the result would be A or B, but one could not predict the specific result of an individual measurement. Quantum mechanics therefore introduces an unavoidable element of unpredictability or randomness into sicence. Einstein objectd to this very strongly, despite the important role he had played in the development of these ideas. Einstein was awarded the Nobel prize for his contribution to quantum theory. Nevertheless, Einstein never accepted that the universe was governed by chance; his feelings were summed up in his famous statement 'God does not play dice.' Most other scientists, however, were willing to accept quantum mechanics because it agreed perfectly with experiment. Indeed, it has been an outstandingly successful theory and underlies nearly all of modern science and technology."
(A Brief History of Time: from the Big Bang to Black Holes, by Stephen W. Hawking, p.60.)
If you are playing roulette in a casino, you can place your bet on one of the numbers. The courier spins the wheel in one direction, and tosses in the ball in the other direction. The wheel has 37 or 38 numbered pockets and eventually the ball will fall into one of these pockets. If the wheel has 38 pockets then you know that the laws of probability state that there is a 1 in 38 chance of the ball landing on the number you have placed your money on. The laws of probability and chance are a generally accepted part of reality, even for Albert Einstein. However, when we seek to explain the physics of how the ball interacts with the roulette wheel we are not content to account for its behaviour as being the result merely of chance. We will consider things like the mass and velocity of the ball, and the shape of the wheel. The motion of the ball is the result of deterministic physical laws, but a knowledge of those laws does not tell us which number the ball will land on because of the difficulty in accounting for and measuring all the variables. We could in theory determine it if we had a precise knowledge of the motion of the wheel and the momentum of the ball, but in most practical cases we do not have that knowledge. It is the same with choosing a card at random from a deck of cards, tossing a coin in the air, throwing a dice, or considering the collisions of molecules in a gas. Probability theory gives us a means of making a prediction about the outcome of the event, without a knowledge of all the underlying physical factors. We know the roulette wheel has 28 pockets and that the ball must land in one of them. A probability of 1/38 tells us that we're unlikely to land on a particular number the first time, but if we try 38 times we are likely to land on it at least once. Our world consists of an element of unpredictable chance, but underneath it is guided by predictable, deterministic laws. That is all Einstein was saying. His opponents were saying something strange. They were saying: "No, it's just chance, all on its own. There are no underlying deterministic laws leading to the predicted outcome". They were suggesting that chance and probability can serve as physical causes of events, acting all on their own.
We do not need to seek for any underlying physical causes, since after all: "They had measured [this, that and the other] and could write down a simple law relating [all and sundry]. [The] alleged [deterministic causes were], by contrast, ... invisible, intangible, and imperceptible. What was the point of explaining a straightforward law, derived directly from experiment, in terms of hypothetical entities that could not be seen and might never be seen?"
Einstein originally made his comment in a letter to Max Born in 1926.
"Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the 'old one.' I, at any rate, am convinced that He does not throw dice."
(Albert Einstein, letter to Max Born (4 December 1926).)
He repeated the statement at later times.
"As I have said so many times, God doesn't play dice with the world."
(Einstein and the Poet: In Search of the Cosmic Man, by William Hermanns, p.58.)
Louis de Broglie made the following comment on the difference between statistical prediction and physical explanation.
"Such is, in its main lines, the present state of the Wave mechanics interpretation by the double-solution theory, and its thermodynamical extension. I think that when this interpretation is further elaborated, extended, and eventually modified in some of its aspects, it will lead to a better understanding of the true coexistence of waves and particles about which actual Quantum mechanics only gives statistical information, often correct, but in my opinion incomplete."
(Interpretation of quantum mechanics by the double solution theory, by Louis de Broglie)
Quantum Physics is becoming an increasingly impenetrable black box for most of us, and "probability as a sufficient physical cause" is justified with claims that the counter intuitive nature of the quantum world must be accepted. If the usual interpretation of chance events is used, it implies a whole other as yet undiscovered layer of physical causality of which the current probabilistic methods of quantum mechanics show us only the result. If we know that there is a three in four chance of something happening, but we don't know why or how, we can say that it happens three out of four times "because" there's a three in four chance of it happening. Then we can appeal to Occam's Razor to defend the sufficiency of "a straightforward law, derived directly from experiment" claiming that this is a simpler explanation than a whole other undiscovered, hidden layer of physical causation. Under normal circumstances we would suggest in such a case that nothing has been explained at all.
The Wave Function
The modern form of the particle-wave duality in quantum physics is the "wave function" and its "collapse". The wave function is a three-dimensional model representing the probabilities of finding particles in given locations. The collapse of the wavefunction refers to the actual location of the actual particles. The wave functions are elegant and deterministic in themselves (in the sense that the precise probabilities are predetermined), and the predictions they make are, overall, very accurate. But what is actually going on? This is where things start to get cool."A nice way of visualizing the wave/particle duality is the so-called sum over histories introduced by the American scientist Richard Feynman. In this approach the particle is not supposed to have a single history or path in space-time, as it would in a classical, nonquantum theory. Instead it is supposed to go from A to B by every possible path.... The probability of going from A to B is found by adding up the waves for all the paths."
(A Brief History of Time: from the Big Bang to Black Holes, by Stephen W. Hawking, p.65.)
Imagine you are planning a trip to Europe. You check the prices for all the different airlines and the different routes they take. Should you have a stop-over in Bangkok, or Morocco, or should you fly straight through? What hotels should you stay in? Will you join a tour group when you arrive, or hire a car and set out on your own? Your "plan" consists of all the "possibilities" of what you could do. But eventually you must make a decision and choose only a single one of all these possibilities. The sum of your various plans is the wave function. The collapse of that wave function is the single actual route you take. Richard Feynman uses the example of a photon bouncing off a mirror as part of ordinary reflection as an illustration.
"So the theory of quantum electrodynamics gave the right answer--the middle of the mirror is the important part for reflection--but this correct result came out at the expense of believing that light reflects all over the mirror, and having to add a bunch of little arrows [probability vectors] together whose sole purpose was to cancel out."
(QED the strange theory of light and matter, by Richard P. Feynman, p.45.)
That the "light reflects all over the mirror" in some real sense is shown by the fact that we can exploit this feature in a device called a diffraction grating (see: QED the strange theory of light and matter by Richard P. Feynman, p.46). You may have heard of Schrödinger's "half dead cat". A cat is hidden in a box and whether it is alive or dead is determined by some quantum event. So that in the wavefunction view, the cat exists in both states (alive and dead) simultaneously. Some have chosen to go so far as to suggest a model where all possibilities exist as real parallel universes. Hugh Everett III (1930–1982) proposed the many-worlds interpretation of quantum physics.
"The simplification of the measurement problem achieved by the many-worlds approach, combined with the fact that the mathematical details of the theory turn out to be very elegant, has led the physicist Paul Davies to describe it as 'cheap on assumptions, but expensive on universes!'"
(Quantum Physics: Illusion Or Reality?, by Alastair I. M. Rae, p.79.)
The philosopher David Lewis (1941 - 2001) pursued a similar idea in his book: "On the Plurality of Worlds", a defence of what is referred to as "modal realism". Erwin Schrödinger, originator of the wavefunction, makes the following comment.
"Nearly every result (the quantum theorist) pronounces is about the probability of this or that or that ... happening - with usually a great many alternatives. The idea that they be not alternatives but all really happen simultaneously seems lunatic to him, just impossible."
(Erwin Schrödinger, from Schrödinger’s Philosophy of Quantum Mechanics, by Michel Bitbol, p.127.)
It seems as though fundamental particles are able to make plans about what they are going to do, and to compare all of these plans simultaneously to determine what they will actually do. The parallel worlds hypothesis takes a very literal interpretation of this process. In our example of planning a trip to Europe, the planning takes place in our head while the actual event takes place in the real world. But presumably particles don't have heads in which to do their planning, and how would they go about gathering all the information they need about their environment? What seems to be required is some new formulation of the notion of a physical "possibility", a formulation that is not quite what we think of as an actual physical event, but also not quite what we think of as a mental process in someone's head. That is, that the alternatives exist not as "real events" in parallel universes, but as "real possibilities" in this one. We might segue here into speculation on the place of consciousness in nature, but let's leave it there for now. The philosophy that possibilities have some kind of (non-actual) existence is called "possibilism". It stands in contrast to the philosophy called "actualism", that asserts that the only existing things are actual.
"What makes actualism so philosophically interesting, is that there is no obviously correct way to account for the truth of claims like ‘It is possible that there are Aliens’ without appealing to possible but nonactual objects."
("Actualism" by Christopher Menzel, Stanford Encyclopedia of Philosophy)
The preference we have met with several times already to believe that something does not exist until it has been definitely discovered finds its ultimate expression in the collapsing wavefunction (as represented in the thought experiment known as "Schrödinger’s Cat"). The wavefunction is a mathematical description of probabilities, but it is able to give rise to concrete realities (that is: collapse) by means of the creative power of the act of an observer looking at it.
Modal Realism
"Modal" philosophy concerns the nature of "possibility", "actuality" and "necessity". That is: what can be, what is, and what must be. The philosopher Saul Kripke (1940 - ) introduced the practice of considering possibility in terms of alternative "possible worlds". "Modal Realism" is a literal implementation of this practice that suggests that every possibility actually exists in a vast array of parallel universes. The suggestion is much the same as Hugh Everett's, but given a philosophical basis rather than one in theoretical physics. Its virtues and faults are also similar to those of Hugh Everett's theory. Its virtue is its simplicity, elegance and self consistency. Its flaw is that it seems wasteful of universes, and not a little flippant and trivial in their multiplication. It also implies that somewhere there exists the worst of all possible worlds. If you crossed the street today without being hit by a bus; in some other possible world there is another version of you who was not so lucky.Modal Realism may serve to illustrate the pitfalls of tailoring reality to the interests of logic, rather than the other way around.
Miscellaneous Oddities
In this final section of the article I just follow up on a few of the odd implications of the modern theory of light and electromagnetism (referred to as Quantum Electrodynamics (QED)).Once upon a time matter was just matter. It was real substance, actual stuff. Today, matter is technically defined as particles possessing mass. But not all particles possess mass. One notable example is the photon, which has zero mass. That is, there is no matter in a photon. But if there is no matter, what is there? What we mean by something having mass, is that it is something that obeys certain behavioural rules, primarily it demonstrates inertia.
At the same time as we have particles with no mass, we have the modern conception of "space-time" describing a void which nevertheless has characteristics and out of which matter can spontaneously emerge.
Einstein's famous e = mc2 equation says that mass and energy are interchangeable. But what do we mean by energy? This concept has gone through some changes over the centuries. Back in 1667 the alchemist Johann Joachim Becher suggested that flammable objects contained a fire-like substance he called "phlogiston", and that the act of burning something was the act of releasing this fire-like substance previously locked in the object. It was assumed that phlogiston had mass, and therefore that objects would weigh less after burning because the phlogiston had been lost. Some careful experiments showed that objects did not weigh less after burning, as long as you didn't let the smoke escape, and the phlogiston theory was abandoned. It was replaced for a time by the more sophisticated "Caloric" theory. Caloric was a weightless, "subtle fluid" that was lost as bodies cooled. The problem Caloric theory encountered was that physical processes do not necessarily conserve heat, but do conserve energy. So instead of heat as a kind of stuff, a substance, we needed the more general notion of energy that could take the form of heat, but could also take other forms such as momentum or sound. Caloric theory is remembered in our word "calorie", the name of the unit for the measure of energy.
Energy then came to be thought of not as a substance, but as an ability to do work. Energy was an amount of "horse-power" or "kilowatts". It wasn't a noun, but an adjective or a verb depending upon whether it was potential or actual. Slowly moving particles of little mass had low energy. fast moving particles had a lot of energy. The drivers of all this activity was the 4 fundamental forces. If I have a coffee mug sitting on the edge of a table it has potential energy on account of its distance from the floor, because if I bump it with my elbow off the edge of the table, the force of gravity will accelerate it to the floor where it may shatter. It might make a loud noise as it hits the floor, as some of the energy of its motion (its kinetic energy) is converted into sound, while the rest is inherited by (the kinetic energy of) its fragments as they fly out in all directions.
Putting aside the effects of gravity, most of what we refer to as energy boils down to the activity of photons. If you are playing pool or billiards, you shoot the queue ball at another ball, and the energy you imparted to the queue ball with the stick, the queue ball then transfers to the ball it hits. If the queue ball hits the other ball head on, the queue ball may transfer all of its kinetic energy to the other ball, so that the queue ball stops moving altogether, while the other ball flies off almost at the same speed the queue ball originally had, minus some energy lost in the "clack!" sound the balls made when they collided, and perhaps a tiny amount of heat. The transmission of momentum (the tendency to move and keep moving) from one object to another, is effected by photons, even though photons, having no mass, have no momentum of their own.
If we blow up a building with C4, bits of building will fly in every direction, having kinetic energy provided by photons. From the explosion will radiate light (photons) and heat (kinetic energy transmitted by photons) and sound (kinetic energy imparted to the air ... by photons). The "release of energy" from an object, is customarily the release of photons, rather like the old phlogiston theory, except that photons (like caloric) don't have mass. I present that in my role as patron saint of abandoned notions. If you want to put energy into something, by heating or accelerating it, pump some photons into it. So when we talk about the conversion of matter into energy and energy into matter, we are usually talking about converting matter into photons and photons into matter, respectively. Accelerate something enough, and it gains mass. In this sense, matter is made out of light, and dissolves back into light. If energy is more primary than matter, then light is the fundamental stuff out of which the universe is made.
But just how do photons communicate momentum from one object to another when they have none of their own? That is, how does the electromagnetic force actually work? Consider this additional problem. You are probably aware that a proton and an electron will attract one another because they have opposite charge (a proton has a positive electric charge, while an electron has a negative charge), while two electrons will repel each other because they have like charge; similar to the way the north poles of two magnets will repel each other, while the south pole of one magnet and the north pole of another will be attracted to each other. Now imagine that a lone photon sets out into space from an electron, the effect of that photon on any other particle that it encounters on it's journey will depend upon what kind of particle it meets. For instance, if it meets a proton it must say to it: "Dear proton, I come to you from an electron, so I need you to move in the direction from which I came." If the photon reaches another electron instead, it must say to it: "Dear electron, I come to you from another electron, so I need you to head off in the direction opposite to that from which I came." But as far as we know, the only characteristics that photons have is wavelength, frequency and magnitude of energy, all of which are dependent on each other, as well as a constant velocity and some direction of motion. There's not supposed to be any identifiable difference between a photon that leaves an electron, and a photon that leaves a proton. Having no mass it has no momentum of its own. So how is the photon able to communicate all this information to the other particles it meets? Physicists talk now about photons transferring "information" between particles, but no particular mechanism is suggested.
Although photons communicate the effect of charge, they have no charge of their own, and therefore are neither attracted nor repelled by charged particles. And given that photons, electrons and other fundamental particles are all considered as having no physical size, that is, they are treated as point particles, how do photons manage to find and connect with other particles? This is usually accounted for by their being "smeared out" in space.
"Heisenberg's uncertainty principle implies that particles behave in some respects like waves: they do not have a definite position but are 'smeared out' with a certain probability distribution."
(A Brief History of Time: From Big Bang to Black Holes, by Stephen Hawking, p.60-1.)
That is how a dish microwave antenna that is a mesh (that is, full of large holes) is able to detect a signal. If we think back to Feynman's sum over histories approach, or Schrödinger’s wave function, individual photons seem to know a lot about their environment, and are able to interact with it in a coordinated way. If you Google the words "two slit experiment" you will find an interesting example.
Why Ether?
In the end, whether the ether has been disproven or is yet to be established may be more a matter of semantics than physics because it depends upon how we define ether. Michelson and Morley had a particular conception of ether that they were testing, and they proved that conceptualisation of it to be false. But Einstein, Dirac, de Broglie, Laughlin and Wilczek, all believe in the existence of something they feel is deserving of the name "ether". The ether is a very ancient notion referring to the substance occupying the heavens. What that substance is and how it works is yet to be determined, but it is useful to have a word to refer to it. Imagine if we were not allowed to use the word "wood" to refer to blocks of wood, but were instead required to refer each time to the "volume of space-time in which wood-like properties and behaviours are manifest", out of the void. In the absence of another term, the term "space-time" or "space-time continuum" has come to be adopted as the de facto label for the stuff of space. But periodically we are reminded that space-time is only a set of coordinates in a void, out of which material particles and antiparticles routinely emerge, and disappear back into: "space endowed with physical qualities". With smeared out particles and three-dimensional wave-functions we are presented with a kind of intelligent void.When we must reformulate our understandings we must decide whether to retain the term used for the old formulation and merely adjust its meaning, or whether to do away with the old term altogether and allocate a new term for the new meaning, such as replacing phlogiston with photon. When the ancients spoke of the elements: earth, water, air and fire, they were not using the words as we use them. When water freezes we call it ice, but although it is a solid, we consider that ice is still composed of the chemical compound we call water. When water evaporates we call it water vapour, but although it is like a gas, we still consider it to be molecules of water in the air.
"In the first place, we see that what we just now called water, by condensation, I suppose, becomes stone and earth; and this same element, when melted and dispersed, passes into vapour and air. Air, again, when inflamed, becomes fire; and again fire, when condensed and extinguished, passes once more into the form of air; and once more, air, when collected and condensed, produces cloud and mist; and from these, when still more compressed, comes flowing water, and from water comes earth and stones once more; and thus generation appears to be transmitted from one to the other in a circle. "
("Timaeus" by Plato (427-347B.C.).)
So when the ancients spoke of "earth, water, air and fire"; they were thinking of what we would call: "solid, liquid, gas and radiant energy", and they recognised that each could transform into the others. So we could just as well call the photon by its ancient name: "fire".
"In the next place we have to consider that there are divers kinds of fire. There are, for example, first, flame; and secondly, those emanations of flame which do not burn but only give light to the eyes; thirdly, the remains of fire, which are seen in red-hot embers after the flame has been extinguished."
("Timaeus" by Plato.)
I hope I've managed to convey some of the magic and mystery of the thing we call "light", and maybe even some excited anticipation about what layers of reality nature might reveal to us next. "What was the point of explaining a straightforward law, derived directly from experiment, in terms of hypothetical entities?" I leave that for you dear reader. Perhaps we can merely call it "curiosity". We complete our discussion of matter in the article Atom.
Any comments welcome.
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