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Why No One Has Measured The Speed Of Light

Veritasium · explainer · Watch on YouTube

Fundamental physics

What it is

Veritasium explains why the one-way speed of light has never been measured, only the round-trip speed, since measuring it requires synchronized clocks that assume the answer. It explores the implications for relativity.

Why Hacker News recommends it

Used as the standard reference in threads about light speed, clock synchronization and untestable assumptions in physics.

  • 6 people recommended it
  • 7 different months
  • 3 strong recommendations
  • first 2020, last 2024

What HN commenters said

  1. “I found the video very compelling even as a thought experiment.”

    Compelling explanation that only two-way light speed can be measured

    aemreunal, 2020-12 · Strong recommendation · in “Well, Actually – Physics Answers”

  2. “I love Veritasium. Derek has done a number of videos that are awesome like this.”

    Listed among favorite Veritasium videos

    travisgriggs, 2021-11 · Strong recommendation · in “How imaginary numbers were invented [video]”

  3. “his channel has actually blossomed since then”

    Channel blossomed; one-way light speed video was well-researched and correct

    jerf, 2023-09 · Strong recommendation · in “How the Most Useless Branch of Math Could Save Your Life”

  4. “There is a Veritasium video really fun to watch which exmplains with examples why you cannot measure one way speed of light”

    Fun examples of why one-way light speed can't be measured

    faeyanpiraat, 2021-01 · Recommendation · in “New superconductor microprocessor yields a substantial boost in efficiency”

  5. “his videos on special [1] and general relativity [2] were real eye-openers for me”

    Eye-opening Veritasium relativity videos, though commenter dislikes new clickbait titles

    fsh, 2022-03 · Recommendation · in “We're building computers wrong [video]”

  6. “There is a cool video by Veritasium about the specifics of how the speed of light is actually measured (1).”

    Reveals only two-way light speed has been measured

    baxtr, 2023-07 · Recommendation · in “How is the speed of light measured? (1997)”

Excerpts from Hacker News comments, linked to the originals. Summaries and labels were written by a language model from those comments.

Topics: speed of light, special relativity, clock synchronization, veritasium, physics

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