Light: one of the biggest mysteries in physics

Light is one of those few topics where physicists have to say, ‘We don’t yet know why it is the way it is, we just know that it is that way.’

Let’s start simple.

Question 1: What is light?

When we think of light, we automatically think of visible light- what we can see with our eyes. But that is only 0.0035% of the total light, or electromagnetic, spectrum. The rest of the spectrum includes non-visible light, such as infrared radiation (what we feel as heat), x-rays (what the medical field’s X-ray machine uses to capture images of bones) or ultraviolet radiation (what causes sunburn). 

Every kind of light is made up of photons. They are tiny little pockets of energy that travel across space at 3 x 10⁸ meters/second at different wavelengths and frequencies. Imagine someone tosses you a tennis ball, but instead of it travelling straight towards you, it oscillates up and down in a wave pattern as it travels. If you take a measurement from peak to peak, this distance is called a wavelength. The tennis ball can move up and down in the wave pattern at different speeds. This speed is called the frequency. Photons can travel at different wavelengths and different frequencies depending on where it originated. The unique wavelength and frequency pair of each photon determines what kind of light it is- where it falls on the electromagnetic spectrum. For example, photons with much shorter wavelengths and therefore much higher frequencies fall towards the right-hand side of the spectrum and are likely gamma-rays or x-rays. On the other hand, photons with much longer wavelengths and much lower frequencies are on the left-hand side, meaning the photons are probably radio waves or microwaves. 

So far, so good. All of this makes sense, and physicists are fairly confident in this information. So, what’s the problem?

Question 2: Why is light so problematic?

The trouble with light is its behaviour. Remember those little pockets of energy that move up and down in a wave pattern? Well, that’s not exactly what happens. Light has a property that physicists call ‘wave-particle duality’, which is a fancy term for meaning that sometimes light behaves like a particle (photons) and other times it behaves like a wave. 

When it behaves as a wave, we get the electromagnetic spectrum. As mentioned above, the wave can have different peak-to-peak lengths and travelling speeds that we read as different types of light across the spectrum. 

But when the photon behaves as a particle, we get this tiny pocket of energy rocketing across the cosmos. It is the fastest thing in the known universe.

To understand the difference a little bit better, imagine you put the tennis ball in one of those pitching machines used for baseball players to practice their swing. It shoots the ball straight out of the front in a direct line and incredibly fast. This is light acting like a photon particle. 

Now, imagine you and a friend have a rope and each of you are holding on to either end. Your friend starts swinging their end up and down creating waves that travel down the rope towards you. The faster your friend swings their end, the faster the waves travel and the smaller the peak-to-peak distances (wavelengths) of the waves get, and vice versa if your friend slowly swings their end. This is light acting like a wave. 

The tricky bit is that physicists don’t know why the same pocket of energy can act like a photon particle in one instance, yet like a wave in another! The famous Double-Slit Experiment performed by Thomas Young in 1801 demonstrated this behaviour. Since then, the physics sub-field of quantum mechanics has developed and physicists now think that this behaviour is because of what they call the ‘Observer Effect’, which means that particles behave differently depending on whether or not they are observed. How does the particle know when it is being observed? Well, that is still a mystery to all. 

Written by Amber Elinsky

Related articles: Laser Interferometric Gravitational-wave Observatory (LIGO) / Dark Energy Spectroscopic Instrument (DESI)

REFERENCES

Wavelength/Frequency Image ref: BYJU’s educational tech company

Electromagnetic Image ref: Space.com

Baclawski, Kenneth. (2018). The Observer Effect. 83-89. 10.1109/COGSIMA.2018.8423983.