10 ms·
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 fini
by jzl 2y ago
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.
- JdeBP 2y agoIt'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 2y agoEinstein 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 2y agoInteresting 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 2y agoLandau & Lifshitz, in their (classic) book on The Classical Theory of Fields, begin with a section called "Velocity of propagation of interaction".
- jprete 2y agoAnd 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 2y agoMakes you wonder if neutron flux in a bomb during supercriticality becomes self aware for a short moment.
- verzali 2y agoThe 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 2y agoIt'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 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 2y ago> 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 2y agoThis 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".
- cypherpunks01 2y agoYes, 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 2y agoBut 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 2y agoif the sun were to disappear, how long would it take for you to notice?
- tedk-42 2y agoThis doesn't argue their point. If a neutron disappeared, you'd NEVER noticed that causality.
- deleted 2y ago[deleted]
- kazinator 2y agoTrick 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.
- jmyeet 2y agoSo 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 2y agoCausality 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 2y agoWithout 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.
- jerb 2y agoThanks, 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 2y agoWhy does speed of causality make it any better? It’s still an arbitrary limit that’s even more abstract and intangible than speed.
- adastra22 2y agoIf 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.
- jiggawatts 2y agoThat 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 2y agoWhile 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.
- 2y ago
- shadowgovt 2y ago> 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 2y agoA 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.
- bloopernova 2y ago"Light travels at the speed of causality". Why does light have that behaviour?
- tshaddox 2y agoThat’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 2y agoSimulators do have finite time steps. Without them they'd present the end of a simulation once initiated.
- Jyaif 2y agoInterestingly, 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 2y agoHow 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 2y agoFinite 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.
- takinola 2y agoThe 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 2y agoBut how much something appears to moves in each of these dimensions depends on the observer.
- takinola 2y agoCorrect. The experience of time and space is dependent on the observer.
- linuxdude314 2y agoThis isn’t true though…
- lijok 2y agoWould love to understand more, care to elaborate?
- kazinator 2y agoNo. There being no upper bound on the speed of motion (like in Newtonian physics) is not the same as infinitely fast causality.