HN Theater @HNTheaterMonth

As an aside, the PBS space time video The Speed of Light is NOT About Light : https://youtu.be/msVuCEs8Ydo

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 https://youtu.be/1YFrISfN7jo

PBS Space Time did a piece on that. https://youtu.be/msVuCEs8Ydo

⬐ morninglight

This could have been an interesting video, but PBS had some nutty production values. Why did they insist on filling half the screen with some guy waving his arms to distract you? I hope the producer moved on to his true calling - gasoline pump videos.

⬐ synu

They have toned that down a bit over the years since that video was recorded six years ago. The videos are really informative, and you can always close your eyes and listen if you like.

PBS Spacetime has done some fantastic episodes on this including this one https://www.youtube.com/watch?v=msVuCEs8Ydo

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] https://www.youtube.com/watch?v=msVuCEs8Ydo

[2] https://www.youtube.com/watch?v=YycAzdtUIko

[3] https://www.youtube.com/watch?v=94ed4v_T6YM

[4] https://www.youtube.com/watch?v=HUMGc8hEkpc

[5] https://www.youtube.com/watch?v=ldVDM-v5uz0

[6] https://www.youtube.com/watch?v=hqoo_4wSkdg

[7] https://www.youtube.com/watch?v=EzZGPCyrpSU

[8] https://www.pbs.org/show/pbs-space-time/

[9] https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g

⬐ mech422

I love these... Though the short length is both a blessing and a bane. Just enough to peak your curiosity, but not long enough to really dig into something.

You’re correct. The speed of light is not actually about light.

https://youtu.be/msVuCEs8Ydo has a great explanation from the always excellent PBS Space Time, for the curious.

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:

https://youtu.be/msVuCEs8Ydo

Here is a 12 video playlist on relativity:

https://www.youtube.com/playlist?list=PLsPUh22kYmNAmjsHke4pd...

Here is a six video series on measuring space and time:

https://www.youtube.com/playlist?list=PLsPUh22kYmNBkabv9M4fX...

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] https://www.youtube.com/watch?v=CiHN0ZWE5bk

[1] https://www.youtube.com/watch?v=msVuCEs8Ydo

⬐ austincheney

Space is a medium of variable density that impacts the speed of light and everything else. That is clear with regard to solar winds and termination shock.

The curvature of space is directly related to time travel. Strange how you completely ignored that much larger portion of my comment to punch a straw man.

> is the same as a vacuum if spacetime curvature is the same.

If that were true particle physics wouldn’t need dark matter/energy to balance conditions that are irregular upon observable matter alone.

https://en.m.wikipedia.org/wiki/Causality_(physics)

Causality, at least in physics, is an abstract notion and not an empirical notion. In causality effects occur in the same order as their respective causes which is substantive for logical consider, but is not necessarily measurably accurate or expected.

⬐ irjustin

While there are a lot of big words being used, there appears to be a gap in understanding the core principles related to the article.

> If that were true particle physics wouldn’t need dark matter/energy to balance conditions that are irregular upon observable matter alone.

You responded to my vacuum statement as if we agree the vacuum is perfectly empty. In relativity, the vacuum is not considered empty[0]. Maybe I should have used "patch of space", but more importantly I said "if the spacetime curvature is the same". That literally defines how causality moves through it so that would be inclusive whatever was there including dark matter/energy, which brings us to...

> Causality, at least in physics, is an abstract notion and not an empirical notion... but is not necessarily measurably accurate or expected.

No it's not... From the wiki you quoted: "In Einstein's theory of special relativity, causality means that an effect can not occur from a cause that is not in the back (past) light cone of that event. Similarly, a cause cannot have an effect outside its front (future) light cone."

We're talking about relativity here, not philosophy or another topic where causality isn't well defined. It is not abstract. It is explicitly defined as "c" in Einstein's equations. I'm not sure what you're trying to prove/disprove here.

You can say I attacked a straw man, but the original comment was edited. So now, I don't know.

I recommend PBS SpaceTime[1] - I still don't understand a lot of things in this space even after watching many videos multiple times, but it really helped put pieces together.

[0] https://en.wikipedia.org/wiki/Vacuum_state#:~:text=According....

[1] https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g

⬐ austincheney

There were no big words or any form of exotic vocabulary.

> No it's not

Causality states that the order of effects must match the order of events. The Wikipedia articles states that almost verbatim. The mention of a light cone binds the general use of the word to its application of physics without changing the definition. No where does the article extend that definition to anything vaguely measurable.

> You can say I attacked a straw man, but the original comment was edited.

It was most likely edited hours before your reply. When I completed the edit there were no replies. I am living on the otherside of the world from the US in a far away timezone.

⬐ irjustin

> No where does the article extend that definition to anything vaguely measurable.

Ah, you're right, I didn't realize you dropped the "speed of" in the "speed of causality" (because the "speed of causality" can be measured and expected... what does "expected" even mean in this context?!). So going 2 replies up you linked to Causality and made a statement about causality but ignored the "speed of causality". So nothing was said?

So you agree with me!

> It was most likely edited hours before your reply. When I completed the edit there were no replies.

Ah! Don't worry, thankfully we know that I replied to you 35 minutes later thanks to Hackernew's API[0][1] and that I'm half a world closer in SGT, and that I was originally replying to something that was edited away.

[0] https://hacker-news.firebaseio.com/v0/item/24328890.json?pri... [1] https://hacker-news.firebaseio.com/v0/item/24329038.json?pri...

⬐ austincheney

> Ah! Don't worry, thankfully we know that I replied to you 35 minutes later

I don't find that as reassuring. Instead of an editing/time conflict the problem is just poor reading comprehension.

⬐ scarygliders

Upvoted you and thanks. I was hoping someone would mention "the speed of causality", because to my layman's mind, I've always found thinking about "the speed of causality" makes vastly more sense of the universe to me and makes spacetime/relativity/and-all-that-jazz far easier concepts to comprehend.

For example, if a powerful laser beam was shot off in a vacuum towards an observer who is far away, it's not the "speed of light in a vacuum" which dictates the time at which the observer detects that laser light, it's the speed of causality in that volume of spacetime which determines when that light beam reaches them/is detected.

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 - https://www.youtube.com/watch?v=msVuCEs8Ydo

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 - https://www.youtube.com/watch?v=msVuCEs8Ydo

⬐ nuccy

It is also peculiar that the energy conservation law is not applicable on cosmological scales, the light is redshifted due to expansion of the Universe and the energy difference between emitted and received light is "wasted", i.e. it doesn't feed the expansion in any way.

⬐ cygx

I don't know if 'feeding expansion' is necessarily the right way to think about this, but by the first Friedmann equation, loss of photon energy will in fact be balanced by the increased expansion rate...

⬐ nuccy

Sure, though the first Friedmann equation, or any other equation, where terms are balanced, e.g. a + b = 0, doesn't tell you anything about causality, though redshifting of light is exactly caused by expansion, so it is not strange that both are "balanced".

@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:

https://www.youtube.com/watch?v=msVuCEs8Ydo

You may need to go down the rabbit hole and watch the earlier videos on relativity and related topics.

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 - https://www.youtube.com/watch?v=msVuCEs8Ydo&t=5s.

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).

⬐ stronglikedan

> can the reverse happen with space events where light reaches you then something else?

I read that as: Can something come along and snuff us out after we've observed the light wave component of the event, such as a shock wave of some sort?

⬐ oarabbus_

>I read that as: Can something come along and snuff us out after we've observed the light wave component of the event, such as a shock wave of some sort?

That is the only way it can happen.

⬐ abtinf

My (limited) understanding is that the limit is only applicable to movement through space. The expansion of the universe is not bound by the limit, as it is expansion of space itself. Thus it seems incorrect to talk about a “universal speed limit of causality”. Am I missing an essential concept?

⬐ dodobirdlord

> Thus it seems incorrect to talk about a “universal speed limit of causality”. Am I missing an essential concept?

Probably. Why would points becoming more distant due to expansion of space be a counterexample to the speed of light being a limit on the speed of causal propagation? Spatial expansion doesn't move anything, so it can't propagate information.

⬐ aeorgnoieang

> Spatial expansion doesn't move anything, so it can't propagate information.

It certainly moves matter-energy in different regions of space with respect to one another.

⬐ marcus_holmes

It doesn't move anything, but it does mean that things that are now far away used to be closer, and therefore had the ability to affect us, but now don't.

Not that it matters, because the speed of light is still the limit: if they could influence us when they were closer, that would have happened by now.

⬐ LifeLiverTransp

Everytime i here this- it sounds like a cheesy speed limit introduced by a post-grad into a late-night hacked together simulation for which s/h/it couldnt aquire enough server power for real parallel execution.. we should make it our job, to label this project as a "F" for all to see..

Ignore me.

⬐ andrewjrhill

TIL. Very interesting.

⬐ lenkite

The speed of shadow can be faster than light though.

https://physics.stackexchange.com/questions/335537/can-a-sha...

⬐ teddyh

A shadow isn’t actually a thing. It’s an image, like a mouse pointer. If I had a sufficiently large screen, and I made the mouse pointer jump (by setting its position) to the other end of the screen, I could calculate its speed and make that number higher than the speed of light. But has anything actually moved? No it hasn’t, because a mouse pointer, like a shadow, is only an illusionary image of a thing, not an actual thing.

⬐ marcus_holmes

from the point of view of someone in the shadow, and subsequently not in the shadow, the speed of light is still the limit. If the light source is 1LY away, it will take 1LY for me to notice that I am no longer in shadow, regardless of how fast the shadow moves. There's no way of measuring the shadow movement that isn't limited by the speed of light.

⬐ close04

An absence of something can only be defined relative to that something and never taken just by itself. So a shadow only exists as a function of light (no light) or a consequence of the absence of light. So it would not travel faster than light in a way that can carry additional information.

Quantum entanglement works faster than light but cannot carry any information. As such the speed of causality (and implicitly of light) is still the real limit.

⬐ pmohun

Thanks for this clarification. I have been wondering about this for awhile and your explanation helped a lot.

⬐ Terr_

For an example of why this is wrong, consider someone flicking a laser-pointer beam across the moon.

The "laser dot" appears to move faster than light. However, it's not really an object, it's simply a pattern, the same as if you loaded a thousand railguns and fired a thousand hypervelocity bullets at the moon at almost the same moment.

Sure, the "smoke" from each hit would appear to "travel" across the surface of the moon, but in reality it's a thousand independent things moving much slower than light.

⬐ anonytrary

I think those answers are actually wrong. When the object casting the shadow moves, the shadow remains in the same place for an observer in it until the light from the source reaches the observer inside the shadow.

I know that's a weird explanation, so consider:

  t0: S~~~>O     U (shadow exists)

  t1: S~~~~~~>   U (shadow exists)
      
  t2: S~~~~~~~~~>U (shadow !exist)

Where "S" is a light source, "O" is an opaque object, "~" are photons traveling to the right, "U" is the observer, and "t0", "t1", and "t2" are times (increasing).

At time "t0", "U" thinks he's in the shadow.

At time "t1", "U" thinks he's in the shadow.

At time "t0", "U" thinks he isn't in the shadow, since the photons are now hitting him.

A similar calculation/thought-experiment can be done for shadows with "angular momentum", in case you think the tangential velocity of the shadow will exceed the speed of light.

⬐ lenkite

Thanks for that nice diagram! What happens when the light source moves and the shadow is far bigger than the object casting it ? Wouldn't the speed of the shadow on the surface be faster than the speed of light ?

⬐ TeMPOraL

I think it would be, but it still doesn't help with anything as the shadow only "updates" when new light hits the surface and reflects back to you, which is bound by the speed of light.

If you were standing in that shadow, I think you'd see it recede backwards at the speed of light.

⬐ anonytrary

For example, when a light source is super close to an object and the shadow gets super big really far away, and the light source moves?

  t0: SO          U1 (U1 in shadow)
                  
  t1: S~~~~~~~~~~>U1 (shadow leaves top first)
       O~~~~~~>   .
         ~>      .
                .
              .
             '
            U2

Dotted line is the path of a fixed point on the shadow during the time S moves.

I didn't do the precise math, but I'm pretty sure the tangential velocity of the shadow along the dotted line won't be greater than the speed of light. The curvature of the "wave front" formed by the tips of the arrows ">" above will be lesser than the dotted line curvature, so the photons near the top of the diagram hit the dotted edge before the ones towards the bottom. This is because the source, S, takes time to move away from O.

Note that the wavefront formed by the photons moves radially outward from S, but ascii art is limiting.

⬐ foobarrio

No information can be conveyed by the wavefront and so nothing is actually moving than the speed of light. What you diagrammed is called the Lighthouse Paradox:

* https://en.wikipedia.org/wiki/Lighthouse_paradox

There are similar things that appear to exceed the speed of light:

* https://en.wikipedia.org/wiki/Faster-than-light

See "group" and "phase" velocities for similar things to the lighthouse.

⬐ internet_person

There is at least one known exception: neutrinos from a supernova. (Don't get too excited, this doesn't contradict Einstein, as we'll see.) In the hot, dense core of a supernova, photons will scatter around for a while before escaping. Neutrinos, which interact much less often than photons, and travel also more or less at the speed of light, get out basically right away. The difference can be as much as a few hours! Of course it's the high-energy photons that are more dangerous, so the neutrinos are really more like an early-warning system.

See e.g.: https://en.wikipedia.org/wiki/SN_1987A

⬐ oAlbe

That's still not an exception though. Neutrinos escape first by virtue of the fact that they don't interact with matter and don't bounce around, while light does. They aren't travelling faster than light, just travelling a shorter distance.

⬐ xmprt

The question wasn't about how fast neutrinos can travel but whether we can detect something before we see it.

⬐ chrischen

But we detect things with particles other than just light.

⬐ onetimemanytime

So x event happened 550,000 years ago and the radiation from it might kill us. You say that we may get a warning from neutrinos before the harmful stuff kills us?

⬐ throwaway2048

Its possible to exceed the local speed of light in many classes of material.

https://en.wikipedia.org/wiki/Cherenkov_radiation

⬐ saberdancer

This is very misleading. In fact, neutrinos are not going faster than speed of light (c), they are going faster than photons that are emitted from the supernova.

Not just that, but lights when it passes through a medium slows down depending on it's refractive index (speed of light in water is about 225 000km/h while in vacuum it's 300 000km/h). This is important distinction as recently we had an uproar when it looked like neutrinos are slightly faster than c (in the end, it was a measurement error).

You can observe effects of particles going faster than speed of light in a medium if you look at photographs of Cherenkov radiation.

⬐ Erlich_Bachman

That comment never wrote that neutrinos were faster than the speed of light?

⬐ pavs

No, but he did write "There is at least one known exception", the misleading part.

⬐ tromp

> in vacuum it's 300 000km/h

300 000 km per second

⬐ mcdevilkiller

Are you sure it's km/s? :D

⬐ phaemon

Yes, it is.

⬐ tromp

As https://en.wikipedia.org/wiki/Speed_of_light puts it:

"Its exact value is 299,792,458 metres per second (approximately 300,000 km/s"

⬐ Lukas_Skywalker

I'm sure it's a typo, but for others: speed of light is 300'000 m/s in vacuum, not km/h.

⬐ Cyberspy

It's not a typo - it's 299,792,458 m/s - approx 300,000,000 m/s or 300,000 km/s

⬐ lmilcin

The correct way to say it is that the speed of light is the speed of causality only in vacuum. In any medium the speed of light is slower but the causality or other fields (for example gravity field) that are not impeded by matter can and will propagate faster than the speed of light.

⬐ blueprint

Where does there exist a vacuum?

How does a particle or its environment measure the particle's traversal of a truly empty space?

Great answer, but do you have more information on this?

PBS SpaceTime. The Speed of Light is not about Light: https://www.youtube.com/watch?v=msVuCEs8Ydo

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

https://www.youtube.com/watch?v=msVuCEs8Ydo

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

These PBS Spacetime episodes should help:

The Speed of Light is NOT About Light - https://www.youtube.com/watch?v=msVuCEs8Ydo

Is Quantum Tunneling Faster than Light? - https://www.youtube.com/watch?v=-IfmgyXs7z8

The Quantum Experiment that Broke Reality - https://www.youtube.com/watch?v=RlXdsyctD50

Pilot Wave Theory and Quantum Realism - https://www.youtube.com/watch?v=RlXdsyctD50

The Future of Gravitational Waves - https://www.youtube.com/watch?v=eJ2RNBAFLj0

⬐ westurner

Thanks!

"Neutron star": https://en.wikipedia.org/wiki/Neutron_star

⬐ saganus

Awesome resources.

Thanks a lot!

I'll have to check them out and study up a bunch of stuff it seems.

⬐ programbreeding

Thank you for this. You linked to the youtube videos but I watch the PBS app on my Roku all the time and it's horrible at recommending what I should watch.

"Watched 'City in the Sky' [a 3-part series about airports and planes]? You'll love Downtown Abbey or The Great British Baking Show!"

I'm always looking for some actual good/educational shows on there, and there's so many great hidden gems, but they're all... hidden.

⬐ iaw

Setup your youtube account with liking/subscribing to the content you're interested in (even if it's from PBS) and you'll get some okay recommendations.

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.

https://www.youtube.com/watch?v=msVuCEs8Ydo

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.

I found this [1] PBS video useful.

[1] https://www.youtube.com/watch?v=msVuCEs8Ydo