🔍 Photons Through Two Slits: What Really Happens?
Feynman’s approach begins with a simple question: What do photons actually do?
Imagine:
- Source S emits a photon
- A barrier with two slits (A and B)
- A detector at D registers the photon’s arrival
🔬 Experiment 1: No Detectors at the Slits
The photon travels from S to D. According to Feynman, it doesn’t choose just one path (A or B)—it simultaneously explores all possible paths. The result? Interference—a wave pattern at D showing the photon behaves as a wave.
🎯 Experiment 2: With Detectors at A and B
If we place detectors to track which slit the photon passes through, the situation changes dramatically:
- The detector shows the photon passed either through A or B
- No interference appears at D
- The photon now behaves as a particle
🧠 Feynman’s Explanation:
The key lies in how we observe. If we “watch” the photon’s path (measure it), we eliminate all other possibilities, and the photon “chooses” one path. If we don’t measure the path, the photon explores all paths simultaneously—and interference arises from summing all possibilities.
This isn’t just philosophy—it’s a mathematical fact. Feynman’s path integral sums the probabilities of all possible paths.
⚛️ Three Elementary Events: The Entire World of Light and Matter in Three Steps
Feynman claimed that all interactions between light and matter can be reduced to just three basic processes:
- A photon travels from point to point (propagation)
- An electron travels from point to point (charge movement)
- An electron absorbs or emits a photon (interaction)
🧱 How Do These Three Steps Create Everything?
- Reflection? An electron in a mirror absorbs a photon, then emits a new photon in the opposite direction.
- Refraction? Photons interact with electrons in glass, slow down, and change direction.
- Transmission? Photons pass through matter via successive absorption and re-emission.
All optical phenomena—from mirrors to lenses—arise from these simple rules. Feynman diagrams are visual representations of these very processes.
👻 Virtual Photons: Ghosts That Mediate Forces
The deepest idea of QED: Electromagnetic forces are mediated by virtual photons.
What are they?
- A virtual photon is not like an ordinary photon—it exists only during the exchange of momentum between particles
- It arises due to the Heisenberg uncertainty principle for energy and time:ΔE⋅Δt≥ℏ/2
- This allows energy to be “borrowed” from the vacuum briefly—long enough for a virtual photon to be born.
🌌 Measurable Evidence: The Lamb Shift and Casimir Effect
- Lamb Shift: A tiny shift in the hydrogen spectrum due to electrons interacting with virtual photons in the vacuum.
- Casimir Effect: Two metal plates in a vacuum attract each other due to differences in pressure from virtual photons between them and around them.
🎯 How Do Virtual Photons Transmit Force?
- Repulsion (two negative charges): Like throwing a ball to each other—exchanging a virtual photon “pushes” them apart.
- Attraction (positive and negative): Like catching the same boomerang—exchanging a virtual photon “pulls” them together.
This isn’t just an analogy—it’s an exact mathematical description.
🔭 Significance: Light as a Game of Probabilities and Possibilities
Feynman shows us that light is neither a wave nor a particle—but something deeper: a manifestation of probability and all possible paths.
QED isn’t just a theory—it’s a way of thinking. It revealed to us:
- That the vacuum is not empty—it teems with virtual particles
- That measurement changes reality
- That forces are mediated by particles
This may be the greatest triumph of the human mind—a theory that describes reality with incredible precision of 0.000000001%.
In the next post: How QED predicted the anomalous magnetic moment of the electron and why it’s the most accurate measurement in the history of science.
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