Waves or Particles?
So… Light is sometimes a wave and sometimes a particle???
It has “wave-particle duality”?
Is it just that both descriptions are more like metaphors, and both are too partial?
It feels clunky and unsatisfying having to keep switching between models to adequately grasp what light actually is…
Isn’t there some model which encompasses and incorporates both perspectives in a unified way?
If there is, it’s proved pretty elusive so far!
Is there some way to imagine it, where both the wave-like and the particle-like behaviour arise naturally from the same model? Without such a vision, how will we ever understand what light is?
But that’s a surprisingly difficult question to answer (or find anyone offering descriptions of…).
Perhaps first we need to backtrack a bit and take stock.
So, what do we know about Light?
Well, it can get confusing because we talk about it as ‘energy’, ‘electromagnetic radiation’, rays, waves and photons (not to mention Bosons).
On top of this, according to ‘The Standard Model’ there are also ‘virtual photons’, which act as ‘force-carriers’ for the Electro-Magnetic Interaction and which are impossible to observe directly; only through their interactions.
Light itself is, rather ironically, invisible to us unless it is somehow diverted (scattered, reflected, refracted and so on) into our eyes (or by proxy, our cameras).
However, there are a few points that are nailed down…
We know that...
- In a vacuum, it travels at approximately 3 x 108 m/sec (3 followed by 8 zeroes) or 300,000 km/sec. It never travels faster (or slower) than that in a vacuum. This speed is referred to as c.
- Unlike other waves, but just like particles, it tends to travel in straight lines, with very little spreading out, as ‘rays’ or ‘beams’.
- Like both waves and particles it can bounce off a suitable surface at a predictable angle (reflect).
- Like a wave it has a regular oscillation, which has a frequency.
- It has a ‘wavelength’ equal to the speed c divided by the frequency.
- The oscillation is electrical in nature and has a side-to-side quality (it is a transverse wave), and that it also has a similar magnetic oscillation at right-angles to the electric one…
- Like other transverse waves, it can be polarised (meaning that all the electric oscillations are in the same plane).
- Like a wave, it changes direction (or refracts) when it moves from one material into another one at an angle.
- Like a wave, if it passes through a narrow opening (narrow in comparison to its wavelength), it spreads-out (or diffracts) on the far side.
- If we make a coherent beam pass through a suitable pair of narrow slits the diffracted light emerging from the slits will form an ‘interference pattern’ on a screen, even if the intensity of the beam is reduced to the point where only one photon at a time is passing through the slits, as if the photon was somehow going through both slits and interfering with itself… (This is not the same as saying that a single photon actually does this; only that the same interference pattern still gradually emerges, even if the individual photons are not passing through at the same time… Which is in itself a very interesting phenomenon.)
But what does it all mean?
We know lots about light and I dare say I’ll come back and add more to the list above; But I’d like to get to my point…
Everything above is a description of the behaviour of light. None of it explains anything, least of all why it can’t seem to ‘make up its mind’.
I’d like to find an explanation consistent with the evidence, which explains all of it… And perhaps opens new avenues to explore other sticking-points in modern Physics.
Answers that just beg more questions...
Surely there has to be some way to think about light which doesn’t require us to keep jumping between two models: “Now it’s behaving like a wave…” and “Now it’s behaving like a particle…”.
We need a model where this dichotomy doesn’t arise; a model with which both types of behaviour are consistent…
To be fair, part of the problem lies in the language we use.
Problems with Particles
Take the word particle for instance.
In common everyday English it means a tiny speck of something solid; something with a definite edge; something we might think of as a permanent object; something we would be able to point to and say “there it is”; and for things like electrons or protons, something we have got used to imagining to be a bit like miniature billiard balls; hard little spheres…
But when it comes to the fundamental particles of modern Physics, none of these things are true.
None of them!
And when we add in Louis de Broglie’s observation that particles (and indeed macroscopic objects too) can behave as waves, it seems like “game over” for the concept of particles.
And it is true; at a fundamental level, what we normally think of as particles simply do not exist (this is equally true for all fundamental particles; not just photons).
For this reason, some Physicists advocate the use of the word wavicle instead. It neatly combines the two concepts and it gets us away from picturing them as hard lumps of stuff.
But it still doesn’t tell us what light is; and there are problems with waves too.
What of Waves then?
The more concrete examples of wave that we have all have a definite medium, and the waves are some sort of ripple in that medium.
Sound travels through air, for instance, as “vibrations” or a repeating cycle of oscillations backwards and forwards, with ‘ripples’ of molecules being knocked forwards by impact from ‘behind’ and in turn knocking those molecules ‘in front’ and bouncing backwards, only to be knocked forwards again, repeating the cycle. It is a longitudinal wave, with the oscillations back-and-forth parallel to the direction the wave is moving forwards (‘propagating’). Waves tend to be like this when there is no surface involved, and they are moving through wide bodies of the medium.
Waves on the surface of water, or along a string, however, involve an oscillation at right-angles (‘perpendicular’) to the direction the wave is propagating. These are transverse waves, the most visible and recognisable as waves in ordinary life.
This kind of wave can also be polarised (made so that all of the oscillations are in the same plane).
All of these examples involve oscillations in a medium; but light can travel through the vacuum of Space.
Can it be a wave if it doesn’t need a medium?
Doesn’t it suddenly sound much more like a particle again?
Or maybe it does need a medium, and that medium is somehow ubiquitous…
Perhaps it could be Spacetime itself; the very fabric of the Universe.
Or perhaps there is a universal electromagnetic medium which we haven’t yet identified as such.
A little History...
Late nineteenth-century Physics was diverted into a fruitless search for the light-bearing medium, or “Aether”, as all the indications at the time pointed to light being a wave, and a medium-less wave didn’t seem to make much sense. But nothing matching their expectations was found.
The fact that light can be refracted by a prism or a lens is circumstantial evidence that light has a wave-like nature, in that refraction is easy to explain for a wave if it slows down upon entry to a denser material or speeds up upon entry to a less dense one.
However, proponents of the ‘corpuscular’ (particle) nature of light, most notably Isaac Newton, also gave explanations which had held sway for around 150 years.
The final nail in the coffin of the idea of light as particles appeared to come in 1801 with William Young’s experiments shining a beam of light through a ‘double slit’, creating an interference pattern that could only be explained if light behaved as a wave being diffracted through the twin slits (more on this later…).
And so, for the next hundred years, the mainstream view of the Physics community was that light was a wave.
This was further bolstered by the work of James Clerk Maxwell, along with his famous equations which also showed that light was an electromagnetic phenomenon, and was part of a bigger spectrum of light-like Electromagnetic Waves, which we now know includes Gamma rays, X-rays and Ultraviolet at one end and Radio waves and Infra-Red at the other.
However, in addition to the ‘Aether’ problem, a second ‘fly in the ointment’ appeared in 1887 with Heinrich Hertz’s observation of the emission of electrons from the surface of certain materials when UV light is shone on them.
This ‘photo-electric effect’ was further-explored and elucidated by Max Planck in 1900, among others; but no-one could explain it. Strangely, cranking-up the light’s intensity (brightness) had no effect on how many electrons were emitted if the frequency of the ‘light’ was below a critical threshold value.
Then, in 1905, a young Albert Einstein suggested that the photoelectric effect could be explained if light energy arrived in discrete packets or ‘quanta’ which delivered energy proportional to the light’s frequency. This explained why brightness (how many or how quickly the packets arrived) had no effect on how many electrons were emitted, if the packets themselves weren’t big enough; the energy had to be delivered in packets that exceeded the amount of energy required to free the electrons.
In 1914, Robert A. Millikan experimentally tested Einstein’s theory and found it to be correct. This was behaviour of light which couldn’t be explained by thinking in terms of light as a wave.
In 1926 Gilbert N. Lewis coined the word Photon for Einstein’s quanta of light.
The idea of light as a particle simply refused to die…
So, what can more conventional particles tell us?
If we shift our focus from the rather slippery topic of light and turn instead to a more definite ‘particle’ such as an electron, things don’t get an awful lot better. Despite its particle-ness, it still isn’t a hard little ball, and we still can’t pin-down where exactly it is.
Famously Werner Heisenberg, in his eponymous exclusion principle, said that it was impossible to know both the velocity and the position of an electron at any moment in time. To know more you would have to interfere with it, and then it wouldn’t be true anymore.
And it’s not just that they move too fast for modern techniques to measure, and one day we’ll have better equipment… It’s more like they can be in more than one place at a time, distributed in a probabilistic way (sometimes described as a wave function, sometimes as a probability cloud).
Additionally, they don’t necessarily need to travel between points within those probability clouds; they may go by some path from A to B (or C or D or Z…), but they may instead disappear at A and re-appear at B (or C or D or X…) without going through any of the intervening space… This last behaviour is also known as ‘quantum tunnelling’ and can allow an electron to ‘get to’ any part of its probability cloud which has a non-zero value even if there is a barrier of some kind in the way…
In the 1920s, inspired by the observation that light could behave like a particle, Louis de Broglie wondered if any particles could behave like waves.
This idea was proven correct by Davisson & Germer with an ingenious twist on Young’s double-slit experiment, using a beam of high-energy electrons (so-called ‘cathode rays’, also the basis of old-fashioned CRT television screens and the electron microscope).
Just like light-waves the beam of electrons was diffracted through the twin slits and produced an interference pattern on a phosphorescent screen.
We come up against the same problem from the opposite direction…
So how do we move forwards from here?
Perhaps, having said something about what a fundamental particle isn’t, we should now try to say something about what it is – or at least might be some of the time.
This will be the main topic for my next what if…? post.
Stay tuned and let me know what you think…
