💡 Feynman Diagrams

Animated QED interactions — electrons, positrons, photons and virtual particles

Interaction

Playback

Møller Scattering Two electrons exchange a virtual photon (wavy line). Neither electron is identified after the interaction — they are identical particles. This is a t-channel process at leading order in QED.

Legend

Electron (e⁻)
Positron (e⁺)
Photon / virtual γ
Virtual particle
Vertex (interaction)

What This Simulation Shows

Feynman diagrams are pictorial representations of mathematical expressions describing the behaviour of subatomic particles. Each diagram represents a term in a perturbation series expansion of the interaction amplitude. The lines represent particle propagators, and the dots (vertices) represent interaction terms — in QED, each vertex contributes a factor of the coupling constant α ≈ 1/137.

How to Use

Did You Know?

Richard Feynman invented these diagrams in 1948 as a bookkeeping tool for calculating scattering amplitudes in quantum electrodynamics. Each additional vertex in the diagram adds a factor of α ≈ 1/137 ≈ 0.007, making higher-order diagrams progressively smaller contributions — this is why QED predictions match experiment to better than 1 part in 1010, the most precisely tested theory in all of science.

About this simulation

This tool animates six Feynman diagrams from quantum electrodynamics (QED): Møller scattering, Bhabha scattering, Compton scattering, pair production, electron-positron annihilation, and the self-energy loop correction. Each diagram is built from a fixed set of fermion lines, wavy photon lines and vertex points that are drawn progressively along a timeline, so you can watch particles travel from the initial state through each interaction vertex to the final state. The canvas axes are explicitly labelled time (horizontal) and space (vertical), reflecting how physicists actually read these diagrams.

🔬 What it shows

Six QED processes rendered as animated diagrams: Møller (e⁻e⁻→e⁻e⁻) and Bhabha (e⁻e⁺→e⁻e⁺) scattering via a virtual photon exchange, Compton scattering (e⁻γ→e⁻γ) via a virtual electron propagator, pair production (γ→e⁻e⁺) near a nucleus, electron-positron annihilation into two photons, and a self-energy loop where an electron emits and reabsorbs its own virtual photon. Electrons are blue solid lines with forward arrows, positrons are red lines with reversed arrows, and photons — real or virtual — are drawn as wavy lines, dashed when virtual.

🎮 How to use

Click a button in the Interaction panel to switch between the six diagrams; the info box below updates with a description of the selected process. Use Restart to replay the animation from t=0, Pause/Play to freeze or resume it, and the Speed slider (0.2× to 3×) to slow down or speed up how quickly the diagram is drawn. Each element (fermion line, photon, vertex) appears only once the animation's internal time reaches its assigned phase window, which is how the diagrams build up vertex by vertex.

💡 Did you know?

Richard Feynman introduced these diagrams in 1948 purely as a bookkeeping device for perturbation theory, yet they became the standard visual language of particle physics. Every extra vertex in a QED diagram contributes a factor of the fine-structure constant α ≈ 1/137 ≈ 0.007 to the amplitude, so higher-order diagrams with more loops and vertices shrink rapidly in importance — which is part of why QED's predictions match experiment to better than one part in 10 billion.

Frequently asked questions

What do the lines and dots in a Feynman diagram mean?

Straight lines with arrows represent fermions: an arrow pointing forward in time is an electron, and a reversed arrow (drawn as if moving backward in time) represents its antiparticle, the positron. Wavy lines represent photons, the force carriers of the electromagnetic interaction; a dashed wavy line marks a virtual photon that is exchanged between particles but never directly detected. The dots, called vertices, mark points where particles interact, and each vertex contributes a factor of the coupling constant to the interaction's probability amplitude.

What is the difference between Møller and Bhabha scattering in this simulation?

Møller scattering shows two identical electrons (e⁻ + e⁻ → e⁻ + e⁻) exchanging a single virtual photon in what's called a t-channel process, and because the two outgoing electrons are indistinguishable, there is no way to say which incoming electron became which outgoing one. Bhabha scattering instead involves an electron and a positron (e⁻ + e⁺ → e⁻ + e⁺); besides the same t-channel photon exchange, the pair can also momentarily annihilate into a virtual photon and re-form via an s-channel process, and the two contributions interfere quantum mechanically.

How does the simulation represent Compton scattering?

The animation shows an incoming electron and an incoming photon meeting at a vertex, after which the electron continues briefly as a virtual (dashed) propagator before a second vertex emits an outgoing photon and an outgoing electron. This matches the real physics of the Compton effect, in which a photon transfers some of its energy and momentum to an electron and emerges with a longer wavelength — historical evidence that light behaves as a particle as well as a wave.

What happens in the pair production and annihilation diagrams?

In pair production, an incoming photon (needing at least 1.022 MeV, twice the electron rest-mass energy) interacts near a heavy nucleus, which absorbs recoil momentum, and converts entirely into an electron-positron pair via E = mc². The annihilation diagram runs this process in reverse conceptually: an incoming electron and positron meet at a vertex and annihilate into a virtual photon, which itself decays into two real outgoing photons — two are required so that momentum is conserved in the centre-of-mass frame.

What does the self-energy loop diagram represent?

The self-energy diagram shows a single electron line entering, briefly emitting a virtual photon that it then reabsorbs further along its path (drawn here as an arc with a looped photon beneath it), and continuing on as the same electron. This loop is a higher-order quantum correction to the electron's propagator that effectively shifts its measured mass and charge; because such loops formally produce divergent quantities, physicists handle them through a procedure called renormalisation.