7.1M views · Oct 7, 2015 · 12:46 · 8 HN recommenders · 15 mentions · discussed 2017–2023
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 points people here: 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.
“The video about “the speed of causality” is the one that caused me to level up”HN comment, 2020
“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.
PBS SpaceTime has a treatise on relativity. There are a dozen videos worth watching on this topic alone. The video about “the speed of causality” is the one that caused me to level up: this video Here is a 12 video playlist on relativity: link Here is a six video series on measuring space and time: link
“it is a great thing to watch when you've got a short wait for something”
Starting episode in a short, varied physics series.
There's an episode of PBS Space Time about this, "The Speed of Light is NOT About Light" [1], which comes pretty close to making the same points you are making. It might be useful to watch this earlier episode, "Are Space and Time an Illusion?" [2], first. There is a later episode (or maybe a couple of them) that go into how interactions with the Higgs field make it so things with mass travel slowed than the speed of causality. To tie back to the warp drive news, they have an episode that would be a good place to start on warp drives, "Is The Alcubierre Warp Drive Possible?" [3]. There's also "Superluminal Time Travel + Time Warp Challenge Answer" [4], and "Will Wormholes Allow Fast Interstellar Travel?" [5]. A non-warp unknown physics drive that comes up a lot also got an episode, "The EM Drive: Fact or Fantasy?" [6]. There is at least one episode covering interstellar travel without using any unknown physics, "5 REAL Possibilities for Interstellar Travel" [7]. The scope of the series is basically anything vaguely connected to astronomy, cosmology, and quantum mechanics, and the episodes are all pretty short (5-12 minutes), so it is a great thing to watch when you've got a short wait for something. Here's the series home page [8], and their YouTube channel [9]. Also on the PBS streaming app, for those who like to get their physics on their big screen TVs. [1] this video [2] link [3] link [4] link [5] link [6] link [7] link [8] link [9] link
“These PBS Spacetime episodes should help”
One of several PBS Space Time episodes explaining the topic.
These PBS Spacetime episodes should help: The Speed of Light is NOT About Light - this video Is Quantum Tunneling Faster than Light? - link The Quantum Experiment that Broke Reality - link Pilot Wave Theory and Quantum Realism - link The Future of Gravitational Waves - link
“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.
Something to head off likely confusion: The Speed of Light is not about the Speed of Light. this video Don't get these two things mixed up: - The speed of electromagnetic waves in the local medium - The maximum speed our reality allows for causal phenomena
“PBS Space time has a good video on this”
Explains c as universal speed limit of causality.
Speed of light is not really just speed of light, but universal speed limit of causality. Light (electromagnetic waves just happens to be one of the few things capable of hitting that limit. Even the effect of gravity is limited to speed of light. PBS Space time has a good video on this - this video . So to answer your question, no it is not possible for anything to affect us any faster than the universal speed limit of causality (that light happens to be able to hit).
“Take a look at this video by PBS Space Time on the topic”
Explains what speed of causality means.
@mesid asked in a now dead comment: > What does speed of causality mean? And how is light so close to that? Take a look at this video by PBS Space Time on the topic: this video You may need to go down the rabbit hole and watch the earlier videos on relativity and related topics.
“PBS space time has a good video on this”
Explains c as the causality limit and space expansion.
Some of the confusion is, I think from referring to speed of light as "speed of light". When it really is speed limit of causality between 2 points in space. And light (em waves) just happen to be able to hit that limit. Now since expansion of space is itself not going to allow one point in space to cause any effect on another, it (the expansion) can be faster than speed of causality. PBS space time has a good video on this - this video
“PBS space time has a good video on this”
Explains c as the causality limit and space expansion.
Posted this to a point above, but probably more pertinent here, so reposting - Some of the confusion is, I think from referring to speed of light as "speed of light". When it really is speed limit of causality between 2 points in space. And light (em waves) just happen to be able to hit that limit. Now since expansion of space is itself not going to allow one point in space to cause any effect on another, it (the expansion) can be faster than speed of causality. PBS space time has a good video on this - this video
“has a great explanation from the always excellent PBS Space Time”
Great explanation that c is not about light.
You’re correct. The speed of light is not actually about light. this video has a great explanation from the always excellent PBS Space Time, for the curious.
“You may also like The Geometry of Causality”
Early PBS Space Time episode on the speed of light.
As an aside, the PBS space time video The Speed of Light is NOT About Light : this video As an aside to the aside - as I rewatch it I quickly notice how young he looks (and then note the date is 2015 on there - one of the early ones and the production is less refined). You may also like The Geometry of Causality link
“Physics tells us that the "speed of light" is actually the absolute speed of causality for mass-energy.”
Explains the speed of light as the speed of causality.
humanity will eventually create a "perfect" version of ourselves, which with then exterminate us because we are inferior Human cultural evolution has already created self-proclaimed "perfect" cultures that went on to attempt exactly this. The Nazis were only the most recent notable example. There are many examples in history of groups who considered it their holy duty or moral duty to take over the world. Fortunately, for most of human history, this hasn't been feasible, but technology is changing this equation. then the AI-human race will create a perfect society and branch off to explore the universe. Physics tells us that the "speed of light" is actually the absolute speed of causality for mass-energy. this video So until consciousness can inhabit a substrate independent of mass-energy and based on space itself, and thus not subject to the light speed limit, any intelligent civilization which expands is doomed to create its own rivals. In Michael Swannick's Vacuum Flowers the Earth is taken over by a hegemonic super-intelligence, but it's forever trapped there, because when attempting to expand, the light-speed lag will cause a sub-part of it to bud off and immediately attack its greatest rival -- which is always the rest of itself. In any case, civilizations are clearly not stable on cosmic time scales. Who in their right mind would think that billions of interacting sentients with their own agendas would produce any stable organization? It's ridiculous on the face of it.
Points to PBS Space Time episode on the speed of light.
Great answer, but do you have more information on this? PBS SpaceTime. The Speed of Light is not about Light: this video
Cited for the meaning of c as the speed of causality.
downvoted because time traveling through a medium vs traveling through curved space are not anywhere near the same underlying principles[0]. The implication being suggested is - if I was trapped in a block of plastic or... underwater, I would experience time different compared to someone standing on land just because "c" is slower. Clearly not the case. To be specific, when "the speed of light" is discussed like this, it usually means the "speed of causality"[1] which is the underlying meaning of "c" in relativity. The speed of causality through a medium (i.e. water) is the same as a vacuum if spacetime curvature is the same. [0] link [1] this video
“c" isn't "the speed of light" so much as "the speed of causality".”
Supports claim that c is speed of causality.
"c" isn't "the speed of light" so much as "the speed of causality". Light just happens to be massless packets of energy that move at the speed of causality. For all intents and purposes this is happening in real time from our local frame of reference. this video
“But that video makes pretty much the same argument.”
Cited as already making the same argument as a critiquing video.
I don't understand why the video needs be so negative. Why dump on other videos? And unfairly so! > Science educators love to make videos about relativity, but surprisingly few of them seem to understand what this stuff really means. The two examples the video gives of science educators that don't "know what this stuff means" are Don Lincoln and Matt O'Dowd. Both have PhDs in physics. Lincoln was on the team that discovered the Higgs boson. O'Dowd is a professor at CUNY. They know what they're talking about. Then the video gives as an example of a bad explanation the PBS Space Time on the speed of light. But that video makes pretty much the same argument. That the speed of light/causality falls out logically even if you don't postulate it. The author clearly didn't watch that video carefully this video I would have subscribed to this channel, but this tone is a bad look.
