|Light exists within a spectrum of the larger EM field - of course that's only part of the story|
What is light? Is it matter, is it energy, is it a particle is it a wave or is it some unique physical construct that has no parallel? These were questions that inspired Newton, Descartes, Einstein, Maxwell, Lorentz and many others. In the 5th century BC the philosopher Empedocles postulated that light emanated as rays or beams from the human eye outwards (more of wave view). Five hundred years later, Lucretius described Light as atomic particles shooting here and there in straight lines. Renee Descartes proposed that light was more like a wave than atomic particle. Newton came back and said no - light really was particles. In the 1800's, a number of scientists working with electricity discredited the Newtonian view of light as particles and the Electromagnetic Wave theory became the dominant view. Then came Einstein, Relativity and Quantum physics - and once again the notion that light was made of particles came back - but this time as part of a "duality." The current view of what light is may sound a bit confusing then - according to modern to Physics; it is both a particle and a wave and they aren't necessarily related.
|In the photoelectric effect - more energetic light knocked electrons out of the metal while less energetic light didn't|
At this heart of this dispute is what appears to be conflicting evidence - some illustrating wave-like behavior for Light and other evidence showing particle-like behavior. The folks who believe it is a wave view light as radiation (e.g. radiated energy in wave form patterns); yet starting with Einstein and then with the Quantum school of Physics Light is viewed as packets (of particles) called Quanta (or Photons) that when combined exhibit wave like behavior or form waves. Here are the current definitions:
The "classical" view of light is as a wave. The wave involves perturbations in both the electric and magnetic fields as light travels through space at the speed of light (299800 km/s).
The quantum view of light is as a particle-like wave packet. Each wave packet is called a photon. Each photon of a certain wavelength has the same amplitude and shape, so that the energy of each photon is the same. The energy of a photon is given by Planck's equation: E = hν = hc/λ. Higher intensity of light corresponds to a greater number of photons passing by per unit time.
|The light waves and particles behave differently, how can they be the same?|
So is light an electromagnetic wave or is it Photons and Quanta? Is it energy or is it matter or is it both? This a tough question. Light can't be described in the same manner as any other elemental matter in Physics / Chemistry. It is not something that has a nucleus with electrons spinning around it. It also doesn't seem to be related to subatomic particles of matter either. Light also represents certain special characteristics as it 1 - can be used to transmit information and 2 - seems to be the fastest moving thing in Universe. This is where Light and Time begin to intersect as the speed of Light or C is the measure of both distance and time at the galactic and universal scale. Here is the current, commonly accepted definition of what an electromagnetic field is:
An electromagnetic field (also EMF or EM field) is a physical field produced by electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction. It is one of the four fundamental forces of nature (the others are gravitation, the weak interaction, and the strong interaction).
The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwell's equations and the Lorentz force law. From a classical perspective, the electromagnetic field can be regarded as a smooth, continuous field, propagated in a wavelike manner; whereas from the perspective of quantum field theory, the field is seen as quantized, being composed of individual particles.We could spend a lot more time trying to understand both the particle view and the wave view theories, but ultimately we end up in the same place. Our current understanding of what light is seems contradictory and incomplete.
At this point, I'm going to turn back to what I described as Intuitive Physics in our last post. Can an intuitive approach help us reconcile the two worldviews on Light? I think it can and does. There are times when in any field of endeavor we confuse the issue - this happens in IT, in politics and so on. In the case of light - the real issue, as I see it, is defining what it consists of, not how it behaves. We could use any number of analogies to highlight this distinction - for example - how does one explain the collective movement of fish in a large school or birds in a flock? What seems apparent when looking at nature is how these 'collective' forms express themselves over and over again. A flock of birds or a school of fish exhibit group behaviors which are wholly unique and different than the behaviors of individual birds or fish - in fact they are behaviors that simply aren't possible at the individual level.
|Schools of fish exhibit behaviors which seem to be reflective of a larger natural Geometry...|
The collective nature of the group of individuals transforms that group into something more than sum of its parts (or particles). When we study birds or fish however, there is never any question that a flock or a school is made up of individual birds or fish, because we can see and measure them directly. And of course, birds and fish are clearly constructed from known matter (elements) - we know this because all aspects of this analogy are subject to examination at our immediate scale of reference.
Light is different - in fact all subatomic particles are different to some extent. They exist at scale that is difficult for us to comprehend and impossible to measure accurately. A quark is perhaps 10 to the -24 meters in length (which is a septillionth - 1000000000000000000000000 - of a meter). That's pretty small. Photons are much larger than that (light waves are between 300 to 500 nanometers, light particles however can't really be defined yet so there size seems to be more mysterious). This issue or question is perhaps the single most obvious example in Science of how the perspective applied to a problem can change theoretic outcomes or solutions related to it. In this case, our current perspective on light doesn't allow to reconcile our theories regarding it.
|The aftermath of the 2004 Tsunami|
Analogy 2 - The Tsunami
We'll use a real-world event to illustrate this next example. On Sunday, December 26, 2004 a massive underwater earthquake triggered one of the largest, most destructive waves recorded in human history. The wave spread across the entire Indian Ocean and somewhere around 230,000 people in 14 countries lost their lives. This event represents an extreme example of wave behavior. The Tsunami that was generated (they used to be called Tidal Waves but that term has fallen out of disfavor as the waves seem to have little to do with Tidal Forces) by what is estimated as a Magnitude 9.0 earthquake.
To give that perspective, we need to understand the measurement systems for Earthquakes; the Richter Scale and the Moment scale. The Richter Scale is a logarithmic scale meaning that each level is ten times greater than the previous one and involves an even higher multiple of energy. The Moment Scale does a better job of assessing the total energy released - in that scale the Earthquake that launched the Indonesian Tsunami was roughly equivalent in energy output to 616 million Hiroshima atom bombs.
I looked elsewhere and saw some radically smaller figures of energy release - I used this calculator to come up my number. (and some scientist content that the true size of the quake was 9.3 which leads to the following value - 1 billion 700 million Hiroshima bombs).
So how does any of this relate to the Particle v. Wave issue?
Let's break it down. Water exists as molecules composed of 2 Hydrogen and 1 Oxygen atom. Water combines to form droplets and then pools, then ever-larger bodies of collectively contiguous molecules. An ocean is nothing more than an extremely large collection of water molecules. Molecules are by nature particles. However, as we all know from personal experience, water when viewed collectively exhibits wave-like behavior. In fact, all other descriptions of wave behavior began from our initial experiences or observations of water. So, the intuitive physicist (e.g. you and me) would say something like this - water particles exhibit wave-like behaviors as energy is applied to them (the collective particles). The more energy applied within a specific frame of reference, the more powerful the waves become - however certain types of energy may affect the waves differently. The tidal motions of the Moon around the Earth express themselves in globally manifested yet relatively low energy waves. In other words, the daily tides are powerful enough to occur every day across the planet but end up producing low frequency - read small - waves. However a massive, abrupt release of energy such as that which occurred in the Indian Ocean on December 26, 2004 produced a relatively brief series of massively powerful high frequency waves (read large). The waves achieve their highest amplitude just as they reach the shore.
|Tsunami waves are different than all other ocean waveforms.|
This description actually parallels Einstein, Bohr's and Plank's discussion of the Photoelectric Effect closely in that the high frequency wave of Light displaced more electrons just as the high frequency Tsunami waves wave caused more destruction (more energy applied causes more energy released). The world of sub-atomic and atomic physic seems to correlate exactly to the realm of macroscopic physics in this case, doesn't it?
Let's take this a bit further though. The intuitive Physicist looking at the Tsunami waves never really questions the fact that water is made up of particles, because we already understand the water molecule and the elements it is made up of and can manage them at our scale of existence (from our immediate perspective). However the theoretical physicist has a much more daunting challenge with light. Light travels at almost impossible speeds, it is hard to capture and doesn't seem to have any real physical characteristics upon initial examination. So the theoretical physicists were forced to focus more on the behavior of light as opposed to the light itself as many of those behaviors and effects of those behaviors were measurable. They were left to speculate that light might be particles but it might be waves and both represented some sort of energy that simply hasn't been defined in the same manner the rest of the physical universe has been.
Let's extend that back out to our Tsunami example - so this behavioral approach would be like us focusing primarily on the wave propagation of the Tsunami and the resulting destructive impacts to coastline rather than looking at water dynamics at a molecular level or even trying to account for discrete / specific damage to the environment such as the injuries caused by the rushing water on the hundreds of thousands of victims. In any scientific endeavor, we make pragmatic trade-offs about what is feasible or reasonable to measure and what isn't. For a Tsunami which is occurring at macro scales, we can realistically measure everything from water dynamics at smaller scales to the global wave propagation emanating from the quake epicenter.
|Yin and Yan - wave or particle; or perhaps either / or is the wrong perspective|
So what are we trying to say? Here it is - the wave / particle duality is not a complicated or bizarre aspect of nature. It is due entirely to the perspective within which we view the problem. In both the water and light scenarios we are dealing with an entity or element that does exists as particles in its smallest form (this has been proven conclusively for light). For both light and water we are able to witness unique behaviors that arise when particles are grouped together and subjected to different types of energy. In both cases those behaviors manifest themselves in the form of waves - in other words when grouped light (quanta) or grouped water (just water) move together with certain energies, they will form frequencies - those frequencies are the distances between particle waves. The amount of energy or the nature of how the energy is applied will change the nature of those frequencies. High energy application will cause shorter more rapid waves - in electromagnetism this becomes x-rays in water it becomes waves that travel 600 miles an hour miles an hour underwater until reaching the coast where the energy not already spent is released (in slower, yet much higher waves).
In current scientific theory, we've made simple logical error when viewing the wave / particle duality question - we confused our initial trade-off decision for how to study light with the actual nature of light itself. In other words, have attributed to light's behavior a structure that in fact only exist as part of a collective behavior. So, just as a Tsunami cannot exist with trillions upon trillions of water molecules, neither can a light wave exist without trillions upon trillions of light particles (of an indeterminate nature). Experiments on light are able to discern both the structure of light as well as its collective behavior which gives rise to our current view that a duality exists. Yet that duality is no more complex than the duality between water molecules and Tsunamis. One fits neatly within the other (the particle within the wave). It's all a matter of perspective and our ability to view the problem.
This is precisely why de Broglie was able to extend Einstein's photoelectric effect to all matter and have it work out. This is known as the concept of "matter waves" or "de Broglie waves" in physics, which earned him the Noble prize in 1929.
In quantum mechanics, the concept of matter waves or de Broglie waves reflects the wave–particle duality of matter. The theory was proposed by Louis de Broglie in 1924 in his PhD thesis. The de Broglie relations show that the wavelength is inversely proportional to the momentum of a particle and is also called de Broglie wavelength. Also the frequency of matter waves, as deduced by de Broglie, is directly proportional to the total energy E (sum of its rest energy and the kinetic energy) of a particle.
|We know matter consists of particles - having wave-like behavior doesn't then make it a wave only.|
So light, like all matter of the physical universe can be described as individual particles that when grouped collectively exhibit uniform properties that are more profound than the sum of the parts (in the case of water and the Tsunami, water through its waves becomes an energy storage and translation medium. This wave behavior is a fundamental feature of the universe itself and is intimately connected to motion, gravity and all other field like characteristics defined by modern physics.
So back to our question and the matter of perspective. Is light a wave or a particle or both? It is both, but one is required in order to satisfy the other. In other words, light waves without particles cannot exist any more than a Tsunami can exist with water molecules. This is an important distinction because if we accept it, then we can bridge Classical Physics (electromagnetism) with Relativity and Quantum theory without any concerns of weird, unexplained dualities. Also, there is another critical consideration in regards to viewing light as a particle - it is not limited to Locality. In other words, particles are not constrained to the speed of light C that restricts (electromagnetic) wave movement.
This has some pretty interesting implications both for Quantum Computing and Time.
In our next post we begin to explore Relativity and introduce Mach's Principle.
copyright 2013, Stephen Lahanas