Is it a bird? Is it a plane?
No. It's a photon.
A photon is a fundamental particle of light.
But, just like Superman, it's difficult to pin down exactly what the humble photon is.
This is actually the subject of one of the most important arguments in physics. For a long time, scientists could not decide if the photon was a wave or a particle.
And, here's the really weird part, they discovered that the photon is both a particle and a wave.
It's like Superman and his alter ego Clark Kent have merged into one entity. And it took the superheroes of science to resolve this issue - including Albert Einstein and Max Planck.
How can light be a particle and a wave?
Light has a split personality. It behaves like a wave and a particle.
The wave properties of light are easy to observe. Light reflects and refracts, for example.
But when you shine light onto a metallic surface and electrons are released (a phenomenon called the photoelectric effect) then light does not behave like a wave.
Building on Max Planck's black body radiation theory, Einstein explained the photoelectric effect by proposing that light is localised into small bundles - which were later called photons.
In other words, photons act as both a wave and a particle all of the time. This is known as the wave-particle duality.
If light is a particle, how can it travel at the speed of light?
In the simplest terms, because a photon has no mass.
Particles gain mass as they travel through something called the Higgs field. Different particles interact with the Higgs field with different strengths to acquire different masses.
A photon does not interact with the Higgs field, so it is massless and can travel at the speed of light.
It's also incredibly difficult to capture a photon on camera to see what it looks like. Some experiments have come close - but, whether it's a bird or a plane, the photon looks unlikely to ever reveal its true shape.
If you want to get your teeth into some serious science, this paper will tell you everything you need to know about photons. And here's a more in-depth look into the photoelectric effect.
And this video explains to wave-particle duality brilliantly:
What is Sunday Science?
Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand.
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