[jzl]

I’ll steal a line from a superb YouTube physics channel (Arvin Ash): it’s not the speed of light, it’s the speed of causality. And the universe must have a finite speed of causality. Without even getting into math and physics, you can intuitively understand how infinitely fast causality would prevent time, and therefore everything else we know, from being possible.

[cypherpunks01]

Yes, I was often very confused as to why the speed of light shows up everywhere, until it was reframed for me in this way. The fact that light travels at the same speed regardless of your frame of reference becomes a little less mystifying.

It feels more intuitive to me when thinking about it as causality always unfolding around you at the same speed, no matter your own frame.

The constant c was not named for causality, but it is a nice coincidence.

[kazinator]

But causalities that don't propagate at the speed of light, and are not based on light, such as ordinary objects moving around and jostling each other, do not appear the same in every reference frame.

[JdeBP]

It's not just Arvin Ash. That's actually fairly common terminology amongst physics educators nowadays. For starters: You'll find a lot of physics YouTube channels that say "speed of causality". It has even started to make its way into the astrophysics and physics textbooks in the last couple of years.

[Jsebast24]

Einstein was not talking about light in his SR and GR theories. He was talking about the "speed" of light. As simple as that is, took me a long time to get it.

[jzl]

Interesting thanks. I hadn’t seen it elsewhere myself but I could see how it’s taken off. OP’s article almost gets there, but never says that specifically. Rather it says “c is not a property of light, it’s a property of the universe.”

[griffzhowl]

Landau & Lifshitz, in their (classic) book on The Classical Theory of Fields, begin with a section called "Velocity of propagation of interaction".

[verzali]

The question seems to me not why there is a speed of causality, but why the speed has this particular number. And it's not clear we know why that is, any more than we know why the proton mass is about 1836 times greater than the electron mass.

[JdeBP]

It's difficult to say that it has any given number, since our measurement units for both time and space are derived from it (via a short detour to the size of Terra, in the 18th century). It can have any finite number you like, just adjust the metre and second to match.

* https://www.youtube.com/watch?v=ZbGxXyqlhbU (FloatHeadPhysics on this)

One physics convention just sets its value to 1. All of those Minkowsky diagrams that we see are measured in light seconds on the space axis, in order to make c have the value of 1 space unit per time unit; so all of the graphical sheep, spaceships, cats, people, torches and stuff that are placed upon them are very much not to scale. (-:

[jzl]

Search YouTube for “universe fine tuning.” Then come back here in a few years when you’ve gotten through everything. :)

[Jsebast24]

If the speed of causality were to suddenly change by whatever factor, would we notice it?

[deniscoady]

If you are willing to accept multiverses then isn't this solved by the Anthropic principle where the speed of causality (or mass of the proton) is what it is, simply because it can sustain the evolution of eventual observers?

I don't think the numbers independently are valuable, but together the constants of physics are tuned to support life. To be honest, I dislike the Anthropic principle as a generalizable cop-out, but it nonetheless works.

[jerb]

Thanks, I’ve never heard this and it’s quite profound. It’s always bothered me that there even is a top speed, and further that mass becomes infinite as it’s approached. But “speed of causality” makes these less strange.

[dyauspitr]

Why does speed of causality make it any better? It’s still an arbitrary limit that’s even more abstract and intangible than speed.

[takinola]

The explanation that unlocked the intuition for me was the postulate that all objects in the universe are moving at the same velocity. Some are moving faster through time and some faster through space. If you move faster in time, you move slower in space and vice versa but the vector sum of your speed through space-time is the same. Therefore a photon is moving really fast through space but does not experience any movement in time.

[kazinator]

But how much something appears to moves in each of these dimensions depends on the observer.

[linuxdude314]

This isn’t true though…

[shadowgovt]

> Without even getting into math and physics, you can intuitively understand how infinitely fast causality would prevent time, and therefore everything else we know, from being possible.

Can you unbox this a little? I think I may just have Friday brain, but I'm having some difficulty convincing myself in the moment that infinite-speed causality development would prevent time.

[woopsn]

A system in which information is communicated instantly will quickly reach equilibrium, after which there is nothing left for any part to communicate to another. Eg diffusion of heat eventually results in a temperature distribution in which there is no longer a flow of heat.

[jmyeet]

So this seems like a better definition until you run into a problem, which you do pretty quickly: "casuality" isn't the easiest thing to define.

The best definition I think I've seen is to view the universe as a partially ordered set of events, meaning that you can only order events (in time) if they're within each other's cones of causality. Outside of that you cannot say which happened first. That's the partially ordered part.

But even that is incomplete and arguably even self-referential. What's a "cone of causality" (without relying on causality)?

Also, there's the issue of what exactly time is and whether events are time-symmetric or not. Many physicists seem to view time as an emergent rather than fundamental property of our Universe.

[cypherpunks01]

Causality is *bangs hammer on bell*

Time is a relationship between clocks.. beyond that, yes, it's hard to say exactly.

Time seems to be what prevents everything from happening at once.

[tshaddox]

That’s not intuitive to me. Any old physics engine in a video game has infinite speed of causality and all the other classical physics stuff seems to work, including time. There must be some other unmentioned property of our Universe’s physics that is important and which requires finite speed of causality.

[kgc]

Simulators do have finite time steps. Without them they'd present the end of a simulation once initiated.

[Jyaif]

Interestingly, infinite speed of causality in games works, but does not scale.

If you want to simulate a giant world with millions of players, you either have to slow down the frequency at which you update the world to give the computer enough time to do the computation, or you have to introduce some sort of speed of causality in the game in order to be able to distribute the computation across multiple nodes.

[jprete]

And the other half of this is that brains are extremely complicated casual chains. Causality can traverse the diameter of the brain something like 10^8 times in the period it takes for light to be perceived.

[nick3443]

Makes you wonder if neutron flux in a bomb during supercriticality becomes self aware for a short moment.

[kazinator]

No. There being no upper bound on the speed of motion (like in Newtonian physics) is not the same as infinitely fast causality.

[bloopernova]

"Light travels at the speed of causality". Why does light have that behaviour?

[aezart]

Because it has no mass.