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The Speed of Light is NOT About Light

PBS Space Time · explainer · Watch on YouTube

Fundamental physics

What it is

PBS Space Time argues that the speed of light is better understood as the speed of causality, the universal limit on how fast cause and effect propagate. Light travels at that speed because it is massless, but the limit itself is a property of spacetime.

Why Hacker News recommends it

HN commenters send people to it whenever a thread confuses the speed of light with a light-specific limit, and several say it deepened their understanding of relativity.

  • 8 people recommended it
  • 8 different months
  • 2 strong recommendations
  • first 2017, last 2023

What HN commenters said

  1. “The video about “the speed of causality” is the one that caused me to level up”

    Speed-of-causality episode that deepened the commenter's understanding of relativity.

    willis936, 2020-10 · Strong recommendation · in “NIST Pair of Atomic Clocks Reveal Einstein's Relativity at a Personal Scale”

  2. “it is a great thing to watch when you've got a short wait for something”

    Starting episode in a short, varied physics series.

    tzs, 2020-11 · Strong recommendation · in “Warp Drive News”

  3. “These PBS Spacetime episodes should help”

    One of several PBS Space Time episodes explaining the topic.

    neom, 2017-08 · Recommendation · in “Rumours swell over new kind of gravitational-wave sighting”

  4. “Something to head off likely confusion: The Speed of Light is not about the Speed of Light.”

    Clarifies speed of light versus maximum speed of causality.

    stcredzero, 2018-02 · Recommendation · in “Speed of light drops to zero at 'exceptional points'”

  5. “PBS Space time has a good video on this”

    Explains c as universal speed limit of causality.

    rhinokungfu, 2019-08 · Recommendation · in “Milky Way’s Black Hole Just Flared, Growing 75 Times as Bright for a Few Hours”

  6. “Take a look at this video by PBS Space Time on the topic”

    Explains what speed of causality means.

    dkersten, 2019-09 · Recommendation · in “Water found on a potentially life-friendly alien planet”

  7. “has a great explanation from the always excellent PBS Space Time”

    Great explanation that c is not about light.

    cthalupa, 2020-10 · Recommendation · in “Faster Than Light in Our Model of Physics: Some Preliminary Thoughts”

  8. “You may also like The Geometry of Causality”

    Early PBS Space Time episode on the speed of light.

    shagie, 2022-08 · Recommendation · in ““I survived the TCP transition” (2013)”

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, causality, relativity, physics, spacetime

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