[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.

[waynesonfire]

if the sun were to disappear, how long would it take for you to notice?

[kazinator]

Trick question. Am I someone on Earth? (If so, 8 minutes). Or am I in a ship that is hurtling toward the Solar System at 0.999c, roughly in line with the Sun-Earth axis? If so, although I see speed of light as c, the Sun-Earth distance is foreshortened relativistically, so it will be less than 8 minutes between me perceiving the event that the Sun disappeared and perceiving the event that the Earth lost illumination.

[tedk-42]

This doesn't argue their point.

If a neutron disappeared, you'd NEVER noticed that causality.

[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. (-:

[lordfrito]

> One physics convention just sets its value to 1.

Interesting. So c² would also equal 1. Which (in those scaled units) implies E = m.

This not only greatly simplifies the relationship but actually makes much more sense that way.

[sroelants]

This convention of units is called "natural units", and it also sets a bunch of other units to be 1, depending on the flavor (planck's constant, boltzmann's constant, etc...)

Not only does this clarify the relationships, but in many ways, these "natural constants" are an artifact from our past ignorance. Boltzmann's constant, in a way, is nothing more than a "conversion factor" that we hold over from the time when we believed temperature and energy to be two separate concepts. In the same way, the speed of light is an artifact from a time when we considered time and space to be distinct concepts, measuring them in distinct units, and needing a conversion factor (in units of distance/time) to map between them.

It's as if we would all collectively agree that the "up" direction, from now on, would be measured in floops, and the slope of a hill would be measured in floops/meter.

From a philosophical point of view, it's not just saying "measure time in units such that c = 1". It's saying "let's consider time to be a distance, and measure it in the same units as we do the other ones".

[linuxdude314]

This is the normal implication with C constant. Nothing is different.

[jzl]

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

[tasty_freeze]

No thanks. Every time I hear the argument brought up, the person putting it forward says it like it is a mic drop moment and they never discuss the counter arguments. It is effective for people who haven't heard the argument before or haven't thought it through.

Here is an example. The charge of an electron is exactly the opposite of the charge of a proton, to within measurement error (like ten digits). This is simply something which has been measured, but physics has no explanation for why they are the same. Getting to the point, what is more likely: that a God created it that way in order to achieve His goals, or that there is some reason connecting the two charges such that they must be of exactly the same magnitude but we just haven't figured it out?

I'm putting my money on the latter. If there is an all-powerful creator, there is no reason to have fine tuning at all -- He could just force the desired behavior and outcome.

[GoblinSlayer]

Presumably charge is a property of electromagnetic field, so it's expected that they match.

[jzl]

Fine tuning is a pretty broad concept and is mostly orthogonal to religious beliefs.

[Jsebast24]

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

[hinkley]

You know how the OceanGate sub imploded so fast that their brains didn't even have time to register they were dead?

If the basic laws of the universe change, we'll be disincorporated before we have a chance to know it's happening, because it will happen at whatever the speed of light is in the new balance, and the chemical processes that make us tick will change to other chemical processes that don't actually work anymore.

In the new universe silicon based life might function. Or stars might not work anymore.

[BurningFrog]

It appears as "c" in the important equation E = mc².

So I expect it would change a lot.

[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.

[adastra22]

If the universe’s causal mechanisms were infinitely fast, the entire history of the universe would play out instantly in zero time, and we’d skip straight to the heat death of the universe.

The fact that time even exists is implied by / a result of causal actions having some finite propagation time.

[Terr_]

> causal actions having some finite propagation time.

I think I know what you're getting at, but somehow the phrasing bothers me, as if there is meta-time or as if cause and effect have time between them... for the photon at light speed, time isn't passing, it's emitted and then zero "time" later it hits something very far away.

It's more like we somehow need to think of cause and effect chains that have orderings without time.

I wonder if future generations will ever look back and casually quip something about "well they believed X existed, that was their problem, it all makes intuitive sense if you just..."

[mikhailfranco]

Yes, the basic idea is that photons do not 'experience' time. They 'experience' creation, all points along their path, and absorption 'simultaneously'.

However, you have to be careful with terminology. There is no inertial frame co-moving with the photon. All we can say is: as a massive particle gets faster relative to an observer's frame, the time it experiences relative to the observer's frame becomes shorter, and in the limit, as it approaches the speed of light (but never reaches c), the experienced relative time approaches zero (but never reaches 0).

This is well explained by Don Lincoln on the Fermilab YT channel:

Do photons experience time?

https://www.youtube.com/watch?v=6Zspu7ziA8Y

[linuxdude314]

This is not true. You can easily measure TOF of a photon using an APD in Geiger mode or other detectors easily purchasable in 2024.

So what exactly are you talking about?

[lijok]

> for the photon

he’s talking about the photons frame of reference

[BoiledCabbage]

That's really profound - thanks for sharing that insight. I've never looked at it that way before.

[jiggawatts]

That doesn’t work that way.

Imagine a universe simulated in a computer with “ticks” where the entire state is updated.

It would be different to ours, but it would work just fine.

[BoiledCabbage]

While what you say is true, it seems to me like what you're describing isn't quite the same definition of causality being infinitely fast.

The fact that causal effects happen in the next tick means some minimal time has passed. So in your definition causality can cover the entirety of a finite universe in an incredibly short amount of time (one tick). But it seems like that's not the same as covering the entirety of a finite universe in zero time. In that case, every result would happen within the same tick as its cause.

[jiggawatts]

There's more than one way to "implement a universe", but one hypothetical way to run a simulation is to alternate the simulation of forces and effects/causes.

A simulation could use particles with attributes like position, velocity, and also a "sum of forces". Then each update has the following steps:

1. Reset the sum of forces for all particles to zero.

2. For all particles, add the contribution to the forces on it from all other particles.

3. For all particles, update the next particle position based on the collected forces.

In the above, there's no intermediate state between updates, everything moves to the next position synchronously from the perspective of in-universe observers. (The external simulation can update particles one-at-a-time, but this is not an "observable" inside the simulation.)

[adastra22]

The cellular automata view of reality is entirely compatible with what I said.

[kingkongjaffa]

Well because its decoupled from light itself or its properties.

If there’s some universal limit for causality itself, then light just happens to be the fastest thing among all the other things subject to causality.

[thiagotomei]

But it is not a coincidence. Light — the EM field waves — propagates at causality speed because the EM field respects a particular property of the universe, the so-called gauge symmetry. That is intimately connected to the fact that the photobs has no mass.

Other similar particles, like the W and Z bosons, are manifestations of the weak field. Since that field breaks the symmetry, those particles have mass and move slower.

BTW, that symmetry breaking is the very same one that physicists talk about when we discuss the Higgs boson.

[linuxdude314]

First person in this thread to make any sense… The level of physics knowledge has declined on this site.

[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.

[takinola]

Correct. The experience of time and space is dependent on the observer.

[linuxdude314]

This isn’t true though…

[lijok]

Would love to understand more, care to elaborate?

[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.

[hinkley]

Without time does causality exist? Does anything happen at all? What is it to 'happen' except that something was one way and now is another?

I can't even describe 'happen' without using verb tenses, which represent time.

[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.

[tshaddox]

How does a finite speed of causality help simulation performance? You still end up computing the interaction of every event with every particle in the Universe. Despite being amortized, you still need to compute the same number of interactions per unit time.

[Jyaif]

Finite speed of causality helps scale the compute across multiple servers.

If you have an infinite speed of causality, a server may need to receive data from all the servers. In the worst case, all servers will need to receive data from all the servers.

With finite speed of causality, a server only needs to receive data from the servers that simulate parts of the world that are nearby.

[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.

[flawn]

Exactly - Light cannot be subject to causality (in the philosphical sense)