Particles that exchange the electromagnetic force (including light)

What is a photon?

A photon is a particle. It represents a quantum (the tiniest countable bit) of light or other forms of electromagnetic radiation.

This is where most definitions would stop – but the photon 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.

Here’s the really weird part, we discovered that the photon is both a particle and a wave.

It took some of science’s top superheroes to resolve this issue – including Albert Einstein and Niels Bohr.

So what?

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, electrons are released (a phenomenon called the photoelectric effect). For this scenario, 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.

What else?

A photon has no mass, which is why it can travel at the speed of light.

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.

In the image above, imagine the Lego floor is the Higgs field. The other three figures interact with the field and gain mass. But Flash does not interact with the floor/field and can travel at the speed of light as he has no mass.

Want more?

Given the fact that E=mc2 and photons carry energy, shouldn’t photons have mass? No. And this video offers an excellent explanation (with some great maths) as to why this is the case.

Five Photons by James Geach

Five Photons relays five fascinating astrophysical processes through the journeys of light across space and time.

What shape are photons?

This experiment came close to capturing a photon on camera – but the photon looks unlikely to ever reveal its true shape.

Meet Einstein’s Nightmare

An excellent show from Jim Al-Khalili explaining how particles (including photons) have tormented the greatest scientists.

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