LIGO and railway semaphore: what they have in common

1. Step 1. LIGO — has part — Photodiode. A photodetector of the interference.
2. Step 2. Photodiode — is part of — Pulse oximeter. A photodiode.
3. Step 3. Pulse oximeter — has part — LED. Red and infrared LEDs.
4. Step 4. LED — is part of — Traffic light. LEDs.
5. Step 5. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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