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scepticaladventure · 7 years

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9 Light - Some Important Background 18Aug17

Introduction

We observe the Universe, and physics within the Universe, and we try to make sense of it. There is often tension between our natural impression of the physical world and what our models and mathematical logic tell us.

Consider the most important of our senses – sight. Our eyes detect photons of light and our brain composes this information into a visualization of the world around us. That becomes our subjective perceived reality.

Nearly all the information we receive about the Universe arrives in the form of electromagnetic radiation (which I will loosely refer to as ‘light’).

However, light takes time to travel between its source and our eyes (or other detectors such as cameras). Hence all the information we are receiving is already old. We see things not as they are, but as they were when the light was emitted. Which can be a considerable time ago. Which means that we are seeing the objects when they were much younger than they are “now”. In other words, we are seeing back in time to what they looked like then.

Light from the sun takes nearly ten minutes to reach us. Light from the nearest star about 4 years. Light from the nearest spiral galaxy (Andromeda) is about 2 million years old (but Andromeda is becoming closer at about 110 km/sec). Light from distant galaxies and quasars can be billions of years old. In fact our telescopes can see light (microwaves actually) that is so old it originated at the time the early universe became transparent enough for light to travel at all.

Imagine we are at the centre of concentric shells, rather like an infinite onion. At any one moment, we are receiving light from all these shells, but the bigger the shell from which the light originated, the older the information. So what we are seeing is a complete sample of history stretching back over billions of years.

It would be mind boggling exercise to try to reimagine our mental model of what the universe is really like “now” everywhere. The only way I can think to tackle this would be some sort of computerized animation.

Even then there are a range of other distortions to contend with. All the colors we see are affected by the relative speeds between us and the sources of the light. And light is bent by gravity, so some of what we see is not where we think it is. There are other distortions as well, including relativistic distortions. So, in short, what we see is only approximately true. Believing what we see works well for most purposes on everyday earth but it works less well on cosmological time and distance scales.

Light is vital to our Perception of Nature

Electromagnetic radiation is by far the main medium through which we receive information about the rest of the universe. We also receive some information from comets, meteorites, sub-atomic particles, neutrinos and possibly even some gravitational waves, but these sources pale into insignificance compared to the information received from light in all its forms (gamma rays, x-rays, visible light, microwaves, radio waves).

Since we rely so heavily on this form of information it is a concern that the nature of light has perplexed mankind for centuries, and is still causing trouble today.

Hundreds of humanity’s greatest minds have grappled with the nature of light. (Newton, Huygens, Fresnel, Fizeau, Young, Michelson, Einstein, Dirac … the list goes on).

At the same time the topic is still taught and described quite badly, perpetuating endless confusion. Conceptual errors are perpetuated with abandon. For example, radio ways are shown as a set of rings radiating out from the antenna like water ripples in a pond. If this were true then they would lose energy and hence change frequency with increasing distance from source.

Another example: It is widely taught that Einstein’s work on the photoelectric effect shows that light must exist as quantized packets of energy and that only certain energy levels are possible. I think the equation e = h x frequency (where h is Planck’s constant) does not say this at all. The frequency can be any integral number or any fraction in between. The confusion arises because photons are commonly created by electrons moving between quantized energy levels in atoms, and photons are commonly detected by physical systems which are also quantized. But if a photon arrives which does not have exactly one of these quantised levels of energy and is absorbed, the difference simply ends up in the kinetic energy of the detector. Or so it seems to me.

The Early Experimenters

Most of the progress in gathering evidence about light has been achieved since the middle of the 17th century. Galileo Galilei thought that light must have a finite speed of travel and tried to measure this speed. But he had no idea how enormously fast light travelled and did not have the means to cope with this.

Sir Isaac Newton was born in the year that Galileo died (1642 – which was also the year the English Civil War started and Abel Tasman discovered Tasmania). As well as co-inventing calculus, explaining gravity and the laws of motion, Newton conducted numerous experiments on light, taking advantages of progress in glass, lens and prism manufacturing techniques. I think Newton is still the greatest physicist ever.

In experiment #42 Newton separated white sunlight into a spectrum of colors. With the aid of a second prism he turned the spectrum back into white light. The precise paths of the beams in his experiments convinced him that light was “corpuscular” in nature. He argued that if light was a wave then it would tend to spread out more.

Other famous scientists of the day (e.g. Huygens) formed an opinion that light was more akin to a water wave. They based this opinion on many experiments with light that demonstrated various diffraction and refraction effects.

Newton’s view dominated due to his immense reputation, but as more and more refraction and diffraction experiments were conducted (e.g. by Fresnel, Brewster, Snell, Stokes, Hertz, Young, Rayleigh etc) light became to be thought of as an electromagnetic wave.

The Wave Model

The model that emerged was that light is a transverse sinusoidal electro-magnetic wave, with magnetic components orthogonal to the electric components. This accorded well with the electromagnetic field equations developed by James Clerk Maxwell.

Light demonstrates a full variety of polarization properties. A good way to model these properties is to imagine that light consists of two electromagnetic sine waves travelling together with a variable phase angle between them. If the phase angle is zero the light is plane polarized. If the phase angle is 90 degrees then the light exhibits circular polarization. And so on. The resultant wave is the vector sum of the two constituent waves.

Most people are familiar with the effect that if you place one linear polarizing filter at right angles to another, then no light passes through both sheets. But if you place a third sheet between the other two, angled at 45 degrees to both the other two filters, then quite a lot of light does get through. How can adding a third filter result in more light getting through?

The answer is that the light leaving the first filter has two components, each at 45 degrees to the first sheet’s plane of polarization. Hence a fair bit of light lines up reasonably well with the interspersed middle sheet. And the light leaving the middle sheet also has two components, each at 45 degrees to its plane of polarization. Hence a fair bit of the light leaving the interspersed sheet lines up reasonably well with the plane of polarization of the last sheet.

Interesting effects were discovered when light passes through crystals with different refractive indices in different planes (see birefringence). Also when light was reflected or refracted using materials with strong electric or magnetic fields across them (see Faraday effect and Kerr effect).

Young’s Double Slit Experiment

Experiments performed by Thomas Young around 1801 are of special interest. Light passing through one slit produces a diffraction pattern analogously to the pattern a water wave might produce. When passed through two parallel slits and then captured on a screen a classic interference pattern can be observed. This effect persists even if the light intensity is so low that it could be thought of as involving just one photon at a time. More on this later.

The Corpuscular Model Returns

At the start of the 20th century, Albert Einstein and others studied experiments that demonstrated that light could produce free electrons when it struck certain types of metal – the photoelectric effect. But only when the incident light was above a characteristic frequency. This experiment was consistent with light being a sort of particle. It helped to revive the corpuscular concept of light.

Arthur Compton showed that the scattering of light through a cloud of electrons was also consistent with light being corpuscular in nature. There were a lot of scattering experiments going on at the time because the atomic structure of atoms was being discovered largely through scattering experiments (refer e.g. Lord Rutherford).

The “light particle” was soon given a new name - the photon.

Wave Particle Duality

Quantum mechanics was being developed at the same time as the corpuscular theory of light re-emerged, and quantum theories and ideas were extended to light. The wave versus particle argument eventually turned into the view that light was both a wave and a particle, (see Complementarity Principle). What you observed depended on how you observed it.

Furthermore, you could never be exactly sure where a photon would turn up (see Heisenberg Uncertainty Principle, Schrodinger Wave equation and Superposition of States).

The wave equation description works well but certain aspects of the model perplexed scientists of the day and have perplexed students of physics ever since. In particular there were many version of Young’s double slit experiments with fast acting shutters covering one or both slits. It turns out that if an experimenter can tell which slit the photons have passed through, the interference pattern vanishes. If it is impossible to determine which slit the photons have passed through, the interference pattern reappears.

It does not matter if the decision to open one slit or the other is made after the photons have left their source – the results are still the same. And if pairs of photons are involved and one of them is forced into adopting a certain state at the point of detection, then the other photons have the equal and opposite states, even though they might be a very long distance away from where their pairs are being detected.

This all led to a variety of convoluted explanations, including the view that the observations were in fact causal factors determining reality. An even more bizarre view is that the different outcomes occur in different universes.

At the same time as all this was going on, a different set of experiments was leading to a radical new approach to understanding the world of physics – Special Relativity. (See an earlier essay in this series.)

The Speed of Light

Waves (water waves, sound waves, waves on a string etc.) typically travel at well-defined speeds in the medium in which they occur. By analogy, it was postulated that light waves must be travelling in an invisible “lumiferous aether” and that this aether filled the whole galaxy (only one galaxy was known at the time) and that light travelled at a well defined speed relative to this aether.

Bradley, Eotvos, Roemer and others showed that telescopes had to lean a little bit one way and then a little bit the other way six months later in order to maintain a fixed image fixed of a distant star. This stellar aberration was interpreted as being caused by the earth moving through the lumiferous aether.

So this should produce a kind of “aether wind”. The speed of light should be faster when it travelling with the wind than if it travelling against the wind. The earth moves quite rapidly in its orbit around the sun. There is a 60 km/sec difference in the velocity of the earth with respect to the “fixed stars” over a six month period due to this movement alone. In addition the surface of the earth is moving quite quickly (about 10 km/sec) due to its own rotation.

In 1886 a famous experiment was carried out in Ohio by Michelson and Morley. They split a beam of light into two paths of equal length but at right angles to each other. The two beams were then recombined and the apparatus was set up to look for interference effects. Light travelling back and forth in a moving medium should take longer to travel if its path lines up with an aether wind than if its path goes across and back the aether wind. (See the swimmer-in-the-stream analogy in an earlier blog).

However, no matter which way the experiment was oriented, no interference effects could be detected. No aether wind or aether wind effects could be found. It became the most famous null experiment in history.

Fizeau measured the speed of light travelling in moving water around a more or less circular path. He sent beams in either direction and looked for small interference effects. He found a small difference in the time of travel (see Sagnac effect), but not nearly as much as if the speed of light was relative to an aether medium through which the earth was moving.

Other ingenious experiments were performed to measure the speed of light. Many of these involved bouncing light off rotating mirrors and suchlike and looking for interference effects. In essence the experimenters were investigating the speed of light over a two-way, back-and-forth path. Some other methods used astronomical approaches. But they all came up with the same answer – about 300 million meters/second (when in a vacuum.)

It did not matter if the source of light is stationary relative to the detection equipment, or whether the source of light is moving towards the detection equipment, or vice versa. The measured or inferred speed of light was always the same. This created an immediate problem – where were the predicted effects of the aether wind?

Some scientists speculated that the earth must drag the aether surrounding it along with it in its heavenly motions. But the evidence from the earlier stellar aberration experiments showed that this could not be the case either.

So the speed of light presented quite a problem.

It was not consistent with the usual behaviour of a wave. Waves ignore the speed of their source and travel at well defined speeds within their particular mediums. If the source is travelling towards the detector, all that happens is that the waves are compressed together. If the source is travelling away from the detector, all that happens is that the waves are stretched out (Doppler shifts).

But if the source is stationary in the medium and the detector is moving then the detected speed of the wave is simply the underlying speed in the medium plus the closing speed of the detector (or minus that speed if the detector is moving away).

The experimenters did not discover these effects for light. They always got the same answer.

Nor is the speed of light consistent with what happens when a particle is emitted. Consider a shell fired from a cannon on a warship. If the warship is approaching the detector, the warship’s speed adds to the speed of the shell. If the detector is approaching the warship then the detector’s speed adds to the measured impact speed of the shell. This sort of thing did not happen for light.

Lorentz, Poincaré and Fitzgerald were some of the famous scientists who struggled to explain the experimental results. Between 1892-1895 Hendrik Lorentz speculated that what was going on was that lengths contracted when the experimental equipment was pushed into an aether headwind. But this did not entirely account for the results. So he speculated that time must also slow down in such circumstances. He developed the notion of “local time”.

Quite clearly, the measurement of speed is intimately involved with the measurement of both distance and time duration. Lorentz imagined that when a measuring experiment was moving through the aether, lengths and times distorted in ways that conspired to always give the same result for the speed of light no matter what the orientation to the supposed aether wind.

Lorentz developed a set of equations (Lorentz transformations for 3 dimensional coordinates plus time, as corrected by Poincaré) so that a description of a physical system in one inertial reference frame could be translated to become a description of the same physical system in another inertial reference frame. The laws of physics and the outcome of experiments held true in both descriptions.

Einstein built on this work to develop his famous theory of Special Relativity. But he did not bother to question or explain why the speed of light seemed to be always the same – he just took it as a starting point assumption for his theory.

Many scientists clung to the aether theory. However, as it seemed that the aether was undetectable and Special Relativity became more and more successful and accepted, the aether theory was slowly and quietly abandoned.

Young’s Double Slit Experiment (again)

Reference Wikipedia:

“The modern double-slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles; moreover, it displays the fundamentally probabilistic nature of quantum mechanical phenomena.

A simpler form of the double-slit experiment was performed originally by Thomas Young in 1801 (well before quantum mechanics). He believed it demonstrated that the wave theory of light was correct. The experiment belongs to a general class of "double path" experiments, in which a wave is split into two separate waves that later combine into a single wave. Changes in the path lengths of both waves result in a phase shift, creating an interference pattern. Another version is the Mach–Zehnder interferometer, which splits the beam with a mirror.

In the basic version of this experiment, a coherent light source, such as a laser beam, shines on a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen, as a result that would not be expected if light consisted of classical particles.

However, the light is always found to be absorbed at the screen at discrete points, as individual particles (not waves), the interference pattern appearing via the varying density of these particle hits on the screen.

Furthermore, versions of the experiment that include detectors at the slits find that each detected photon passes through one slit (as would a classical particle), and not through both slits (as a wave would). Such experiments demonstrate that particles do not form the interference pattern if one detects which slit they pass through. These results demonstrate the principle of wave–particle duality. “

In this author’s view, there is so much amiss with this conventional interpretation of Young’s Double Slit Experiment experiment that it hard to know where to begin. I think the paradox is presented in an unhelpful way and then explained in an unsatisfactory way. It is presented as a clash between a wave theory of light and a particle theory of light, and it concludes by saying that light therefore has wave-particle duality.

Deciding that a photon has “wave-particle duality” seems to satisfy most people, but actually it is just enshrining the problem. Just giving the problem a name and saying “that is just the way it is” doesn’t really resolve the issue, it just sweeps it under the carpet.

In this author’s view, what the experimental evidence is telling us is that light is not a wave and that it is not a particle. Neither is it both at the same time (being careful about what that actually means), or one or the other on a whimsy. It is what it is.

Here is just one of the just one of this author’s complaints about the conventional explanation of the double slit experiment. In my opinion, if you place a detector at one slit or the other and you detect a photon then you have destroyed that photon. Photons can only be detected once. To detect a photon is to destroy it.

A detector screen tells you nothing about the path taken by a photon that manages to arrive at the final screen, other than it has arrived. You have to deduce the path by other means.

Wikipedia again: “The double-slit experiment (and its variations) has become a classic thought experiment for its clarity in expressing the central puzzles of quantum mechanics. Because it demonstrates the fundamental limitation of the ability of an observer to predict experimental results, (the famous physicist and educator) Richard Feynman called it "a phenomenon which is impossible […] to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the mystery [of quantum mechanics].” Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment.”

There is a class of experiments, known as delayed choice experiments, in which the mode of detection is changed only after the photons have begun their journey. (See Wheeler Delayed Choice Experiments, circa 1980’s – some of these are thought experiments). The results change depending on the method of detection and seem to produce a paradox.

Reference the Wikipedia article on Young’s slit experiment, quoting John Archibald Wheeler from the 1980’s: “Actually, quantum phenomena are neither waves nor particles but are intrinsically undefined until the moment they are measured. In a sense, the British philosopher Bishop Berkeley was right when he asserted two centuries ago "to be is to be perceived."

Wheeler went on to suggest that there is no reality until it is perceived, and that the method of perception must determine the phenomena that gave rise to that perception.

They say that fools rush in where angels fear to tread. So, being eminently qualified, the author proposes to have a go at explaining Young’s Double Slit Experiment. But first he would like to suggest a model for photons based on the evidence of the experiments, Einstein’s Special Relativity and some fresh thinking.

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