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Last updated on Tuesday 13th June 2023

The First Heresy: Michelson – Morley proves light is a wave

The two foundation stones of modern physics are, in theoretical physics, Maxwell’s equations of 1873 and, in experimental physics, the Michelson-Morley experiment of 1887.

Maxwell’s equations showed that light acted like a wave and that the same type of wave could come in many different wavelengths. Many of these were then found, radio waves, X-rays, ultra-violet rays and so on. In these equations there was a constant – Maxwell used the symbol ‘c’ for it – which was found to be the speed light moved (in a vacuum). The equations required that all these other waves also always moved at that speed.

This was very odd. How fast something is going varies according to the observer. When you overtake another car, you can do so quite slowly although you are both going fast along the highway, compared to the buildings and trees. So how can a speed be constant? This was so odd a finding that it took forty-two years, until Einstein’s 1905 paper on (what we now call) Special Relativity before it was properly dealt with[1].

The Michelson-Morley experiment of 1887 showed how, even given the technology of the day, amazingly small effects could be measured by clever experimentalists. If light were a wave, there must be something for the wave to travel through, something they gave the name ‘Aether’ or ‘Ether’ after a Greek word for air. It was reasoned that, if there is such a medium in space, then the Earth is travelling through it and, if Earth is moving through something, from Earth it will feel like a ‘wind’ coming from whatever direction we are travelling in – like the wind you get moving in an open car. If there is such a wind, light should go at a different speed if sent across this wind or if it was sent upwind/downwind. Michelson and Morley compared the speeds of a beams of light, split into two and shone at right-angles to each other. Wherever the wind was coming from, it would affect the speed of the two beams of light differently. For example, as the earth spins, the equipment used by Michelson and Morley one of the beams of light would be moving in line with the spin, the other across it. But, even more so, as the earth goes round the sun, its direction of movement reverses, so it goes one direction in autumn and the other in spring. Both these movements should produce the effect of the ‘wind’ as the earth spins and moves through the stationary background ‘aether’ whether the aether itself was moving or not would be irrelevant: the two beams of light would move at different speeds whatever.

The carefully set up experiment showed that light does go the same speed in all directions as required by Maxwell’s equations.

This result has been confirmed many times since: the speed of light is the same in all directions, regardless of the direction and speed of the light source. It was generally agreed that this showed that there is nothing in a vacuum. The complete absence of any effect of moving through a medium, showed that the vacuum is truly empty. There is no ‘field’ or ‘medium’ of the type that is required for waves to travel through. This is how it has been taught at schools ever since as an unquestionable fact and a necessary fact for the much-proved theory of Special Relativity to work.

But this lack of anything in a vacuum is a problem, as the vacuum has measurable electromagnetic properties, its ‘permittivity’ and ‘permeability’[2]. Worse, it has vacuum energy, which can generate a measurable force, known as the ‘Casimir effect’. Indeed, the vacuum energy seems to be quite turbulent, it has ‘vacuum fluctuations’. Not only does the ‘nothing there’ theory of the vacuum have these observed problems, but modern Quantum Field Theory depends on, erm, the existence of a field in which there are ‘excitations’, even in the vacuum. Perhaps most obviously, if there is nothing in a vacuum, how does a light-wave go through it?[3] Waves are variations in or excitations in something that is there already – a medium. So: no medium, no wave: light must be a kind of miniature ball that could be thrown across the vacuum, a particle. But a particle’s speed is affected by the speed of the emitter, like throwing an apple from a moving train, light cannot be a particle or it would vary in speed wildly.

But the conclusions drawn from the Michelson-Morley experiment were in error. In their experiment, it was not the background that was moving past the light-emitting object, but the light-emitting object that was moving through the background. Unsurprisingly, people saw this was an irrelevant distinction, as the two situations are obviously identical, as far as the effect on the light is concerned.

But they are not identical. We can see this clearly with sound in air. If the air is moving – it is windy – then the speed of the air’s movement is either added to or subtracted from the speed of the wave, depending on whether the wind is toward or away from the observer. So, the speed the waves travel varies in line with the wind speed. Depending on its direction, you add or subtract the windspeed from the speed of sound in air at that temperature and pressure.

However, if sound is emitted by a moving vehicle its wavelength is changed but not its speed. The reason is quite simple. If the moving source of sound or light emits waves at a constant rate – a single note of sound or color of light – each ‘trough’ or ‘peak’ will be emitted closer together from the point of view of someone in front of the moving emitter and further apart from the point of view of someone behind the moving emitter. Higher pitched in front, lower pitched behind – the Doppler effect. Once emitted, there is no reason for their speed to be any different to any other wave emitted by a stationary emitter: it is the fixed speed determined by the medium the wave is going through. If the speed of the emitter needed to be added to the speed of sound, you would hear a head-on car crash twice: first with the speeded-up sound of the car approaching you and second with the slowed-down sound of the car going away from you. This does not happen.

The two situations – moving emitter and moving background/medium have very different effects that were confused in considering the ‘negative result’ of the Michelson-Morley experiment. You can see why. We used the phrase ‘the two are obviously identical’ above in describing the thought behind the experiment and you probably didn’t blink. Yet it is unquestionable that they are not the same, once it is pointed out.

Comments on this always include, somewhere, a point to the effect that ‘If this is so, how come 150 years of brilliant physicists have not noticed it?’ We don’t know and, for sure, some did challenge the absence of a vacuum medium – not least David Bohm, after he had written the textbook on Quantum Mechanics. But asking among the post-Doc. heretics provides some ideas why. It was an old experiment and where and why do you get such a view published – criticism of old thoughts doesn’t fit any journal? Professional physicists must be focused on their area of specialism – and are strongly taught not to go outside as silly errors are easily made and often harshly criticised. And this does not seem relevant to any particular area. It is taught at the earliest level – high-school – as an established fact. It does seem obvious that there cannot be an interstellar medium is the speed is unaffected by movement. To illustrate the difficulty, the Heretics came to question the conclusions drawn from the MM experiment results long after – literally years after – we had challenged other ideas and, even then, there were a lot of internal comments about the silliness of ‘checking that the earth is round’. Until we looked and saw how obviously MM proves light to be a longitudinal wave, as it is the only form that has a constant speed whatever the speed of the emitting source.

The Michelson-Morley experiment rules out the particle theory of light because a particle emitted from a moving object varies in speed by direction. Like an apple thrown from a train, you must add the speed of the emitter to the speed of the particle emitted, which they showed did not happen. Moreover, a particle must be emitted in a specific direction, which not only challenges the conservation of momentum but also begs the question as to how that direction is chosen – even a random emission requires a process.

Lateral waves are ruled out because they vary in speed at different wavelengths. Often light waves are illustrated as two lateral waves at right angles to each other, one electric, the other magnetic, waves. Regardless of the issues this raises in terms of what makes the waves reverse their direction at their peaks and troughs and the challenge presented to the conservation of momentum by emitting and absorbing vectored waves, it is ruled out by the wavelength/speed variation.

So, the only explanation for the invariant speed of light is that it is a longitudinal wave, as sound is. Longitudinal waves in a consistent medium move at a constant speed. The speed of sound (in air at constant temperature and pressure) is the best-known example. However fast a plane, say, is travelling (relative to an observer) the noise from it moves at a constant speed to that observer. Moreover, longitudinal waves are emitted (barring special circumstances) with a spherical wavefront, sorting out the conservation of momentum because it is propagating out equally in all directions.

Bizarrely, though, because particles, unlike waves, can cross an absolute void, and Michelson-Morley was misinterpreted as refuting the presence of a medium in the vacuum, it has often been seen as supportive of the particle theory of light. These ‘photon’ particles must move at the speed of light without accelerating and must accelerate when they move from a medium, like glass, where the speed of light is slower than it is in the vacuum, without needing energy. Certainly not a typical particle!

Alternatively, some suggest, the vacuum may be empty, but it is full of virtual particles, particles with only a probability of existence and light is a wave of existence-probability that flows through this. A complex way to try and have things both ways. It requires that probability can be objective and move in space a wavelike manner, (without needing a medium!), that particles can have zero size and a partial probability of existence that collapses to an actual particle at the exact moment that it is absorbed by, say, an electron. Fortunately, the (sum-over-paths) math of these virtual particles produces same results as the math of wavefronts, so little except conceptual clarity is lost by this picture.

Or you can mix these explanations in confusing ways.

But maybe life is simpler than these rather rococo constructions? If we run with the idea that light is a simple longitudinal wave with a (generally) spherical wavefront, just like a sound wave, we will see how much simpler so much of physics becomes in our next Heresies, while still fitting the same math.

But there appeared to be a crucial problem with this simple view of light as a wave.  When light is absorbed by an electron, the whole of its energy is absorbed more-or-less at one place – electrons are much smaller than light waves – and at one time. Waves have a size – the wavelength of a radio wave can be measured in metres and even kilometres. But even this pales in comparison with the possible size of the wavefront. In the case of a wave that has travelled from a star, the scale of its wavefront will be measured in light-years. Yet the energy of this whole wave is absorbed (close to) instantly, even though the speed required to make the connection all the way across such a wavefront is greater than the speed of light. This was thought completely impossible, justifying some of the baroque ideas we have discussed above which could provide some sort of solution to the riddle. But we now know that this instant change across the whole wave is indeed what happens. Since around 2015 experiments with ‘entanglement’ have been accepted by almost all experts, showing that such wave-linkages are not limited by the speed of light but are ‘non-local’ in the terminology[4]. This is explained in the Third Heresy.

So, the vacuum has a field that acts as a medium for light-waves and determines their speed. This avoids all the problems of an empty vacuum listed above. The only problem that remains is the perception that Special Relativity requires that there is no absolute point of reference: nowhere that is ‘still’ against which the speed of every object can be defined. But the presence of a field in the vacuum does not change the vacuum: it does not magically provide coordinates for locating the ‘centre’ or ‘still-point’ of space. Relativity is not affected.

Michelson-Morley proves that there is a field in the vacuum through which longitudinal waves travel. They are emitted with a spherical wave-front, so their momentum is equal in all directions. They cannot ‘break’, so when they are absorbed, they do so at one point and one time, in a way we will illustrate in the next heresy with a fish-tank.

The next Heresy explores the first consequences of this simplified picture of light.


[1] Although Lorentz (in 1892) and Fitzgerald (in 1889) had come up with a suggested solution to the problem. This was a bodge physically because their picture of what was happening was essentially wrong, but the math was fine and adopted into Special Relativity.

[2] Roughly speaking the vacuum’s ability to hold an electric charge and a magnetic force. Both are very small but larger than nothing-at-all’s ability of zero to hold these.

[3] David Bohm (1917-1992), one of the most influential of quantum theorists was called “The Quantum Heretic” because he showed how impossibilities arose from the idea of a completely empty, inert vacuum.

[4] In reality, the older models also required faster-than-light ‘collapse’ but this was concealed by the presentation that it was the ‘wave equation’, a mathematical artifact, that collapsed the large-scale virtual photon probabilities instantly into a single location the same instant it was absorbed.

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