Saw this as an answer to another question and can’t wrap my head around the logic. What? How? Why??
In: 56
The universe is causal in that an event at spot B just about always has some input from spot A. The rate at which this *cause* can move is capped at *c*.
The causes are just propagation of the fundamental forces and whatnot.
It’s the speed of light. Nothing travels faster than it. So nothing can effect something in a shorter time frame than it would take for light to traverse the distance.
So lets say you want to destroy an asteroid with your new death star. The asteroid is 1 light second away. You fire the laser. A person on the asteroid won’t have any idea that the laser was fired until 1 second later when the first photons reach it. It is impossible for anything from your death star to effect or “cause” anything on the asteroid until that period of time passes. Because that is the least amount of time an action performed on your death star can interact with anything on the asteroid.
So the speed of causality is just that concept taken as a whole. There is even an causal horizon. Basically there is a distance from you, that due to the accelerating expansion of the universe, means that anything beyond that point you will never interact with. Even if you were somehow able to travel at the speed of light, the object would always be traveling away from you and accelerating to a faster and faster speed.
The speed of causality is roughly 3×10^8 m/s, which is to say, the speed that light just happens to go at.
More conceptually, the speed of causality puts a limit on what events can be causally related.
For instance, the Moon is roughly 1 light second away from the Earth. No events happening on the Moon for the next second can in any way depend on events happening on *Earth* for the next second. Events happening on the Earth for the next second can only possibly effect events on the Moon that happen *more* than a second in the future. There is simply no avenue for information about an event on Earth to reach the Moon in less than a second.
Why does this concept exist? Well that’s a bit tricky. To some degree, the laws of physics simply exist, and we just have to accept them. We can sometimes say “this law of physics has to work in this way because these *other* laws of physics work in *that* way”, but then you’re just pushing the question back to those other laws of physics.
So for instance, we could look at Albert Einstein’s Special Theory of Relativity. One of the consequences of Special Relativity is that there is *no such thing* as an objective “present”. In one frame of reference Event A might happen prior to Event B, while in another frame of reference, Event B might happen prior to Event A. There might then be other frames of reference in which Events A and B occur simultaneously.
Now rather critically, the degree to which events can be moved around in relative time is dependent on how far apart the events are. If Event A takes place on Earth at time 0s, and Event B takes place on the Moon at time 0.5s (in the reference frame of the Earth), then there will exist other reference frames in which Event *B* takes place at time 0s, while Event *A* takes place at time 0.5s – in that light second of spacial distance, we get a second of “wiggle room” on the ordering of events.
If there weren’t a finite speed of causality, this could cause problems for us. For instance, if A could *cause* B, then we’re going to have reference frames in which the *effect* (B) happens *earlier in time* than the cause (A). Now maybe the universe would be perfectly fine with something like this happening, but at the very least, it’s not something that our limited human brains know how to deal with.
But because there *is* a finite speed of causality, we know for a fact that event A could *never* be a cause of event B, and therefore it doesn’t matter if we swap the ordering around.
But of course, this still really just pushes the question back. The questions then become, *why is special relativity true?* and *why is it important for causes to precede effects?* And yeah, maybe we can come up with answers to those questions, but said answers will just raise more questions of their own.
It’s the fastest speed that one event can cause another.
So if you do anything, wave at the Moon, send a radio signal to the Moon, connect a stick to the Moon from the Earth and then give it a push, etc, etc, etc the fastest that something could happen on the Moon from that would be that event travelling to the Moon at the speed of light. That’s the max. It could travel a lot slower such as in the case of a stick connected to the Moon from the Earth and then pushed.
This is because anything with 0 mass travels at the speed of light and anything with more than 0 mass travels slower than the speed of light.
As to why this is so the answer is because that’s how the universe works.
Causality sounds like a big scary word but it’s actually an incredibly simple concept that we all intuitively understand – it’s just cause and effect. Some event occurs and effects something else. You drop the plate you’re holding, and the plate hits the floor and breaks. Super easy – you dropped the plate (cause) and it broke (effect). That’s all causality is.
Now the universe has a maximum speed limit, which “happens” to be the speed light travels at in a vacuum (it’s not a coincidence, more on that in a moment) – just under 300,000 kilometers per second. Nothing can travel faster than this, and light travels at exactly this speed. In fact, all massless particles *have* to travel at exactly this speed. This is the speed of causality. It’s no coincidence that light and other massless particles travel at this speed, and the source of this speed limit comes not from the fact that light travels at the speed, but from the fact that it’s a the fundamental speed limit of the universe for *everything.* In other words, light obeys the speed limit of causality, not the other way around.
Because of that speed limit, it takes time for distant events to have a causal relationship. Let’s say you press a button (cause) and it makes bell ring (effect). The fastest that information can travel from the button to the bell is the speed of light, so if you’re 300,000k away from the bell (the distance light travels in one second),it can’t ring sooner than 1 second after you sent it. If the bell rings sooner than 1 second, it couldn’t have been you that caused it because it takes the bell a full second to “know” that you pushed the button. In other words, those 2 events are not causally related. There’s no way for the button to have an effect on the bell because not enough time has passed for the cause to reach the effect.
Let’s say I send you a letter telling you that there’s a new game coming out for the Switch. After I send the letter, but before you get it, you buy a switch.
In analyzing this interaction one may say “I told you about the game before you bought the switch, so that may have caused you to buy the switch.” In response, someone may point out that the letter hadn’t arrived at your house by the time you made your purchase, so there’s no way that my letter could have caused your decision: letters move at some finite speed.
We could repeat this exercise with progressively faster methods of communication, up to the point of boring a hole through the planet, pulling a vacuum on it, and shining a laser through that hole. This information travels as fast as possible–the speed of light–but still at a finite speed. If, between the time when I send the information and the time when you receive it, you make a decision then we *know* that that information didn’t cause the decision. This actually happened with a stock exchange, where information was released in, if I recall correctly, Chicago, then trades executed in New York “in response” to that information being made public, but the time between the information being released and the trades being made was so short that it was proven that there was no way the information had traveled fast enough. It was *impossible* for that information to have caused those trades, which showed they had been carried out for some other reason (insider knowledge, in this case).
The reason we care about this interaction is because we expect–and all physics supports this expectation–that causes cannot come after their effects. If I tell you about the new Switch game tomorrow then that can’t cause you to buy a game today. The speed of causility just takes that one step further and notes that causes have to not just come before their effects, but have to come before their effects by long enough for the information to get from the cause to the effect.
That may seem like a pointless thing to go into all of the trouble of specifying, but it becomes more important when you start looking at scenarios in relativity where time and space are not the rigid constructs that we’re used to in classical physics. One of the outcomes of relativistic physics is that you could have two events, A and B, where A comes before B in one reference frame, but B comes before A in another, and they’re simultaneous in a third. However, if it is possible that A *caused* B then A will precede B in *every* reference frame. For it to be possible that A caused B we could start at the time and place where A occurs and travel towards the time and place where B occurs, moving at the speed of causality (light). If we get to the place where B occurs before B actually occurs then A could have caused B.
It’s basically the speed of light.
But it has been abstracted to the speed of information interchange.
Consider two electrons. They exchange photons at the speed of light.
Now consider them being very far a part. Space is expanding at a rate that they are moving away from eachother slightly over 49% of the speed of light. They are still friends and they can share information. Very red shifted but they still play photon ping-pong.
But then space expand so much that they go 51%. None of the electrons are even near faster than light, but their relative speed is now 102%.
So they can’t share anything. Not even a single photon as it will never catch up. That’s the speed of consequence. Where one particle simply can’t have any effect on the other.
There are entanglement and why not strings that muddies the water; but the idea is that once you get over the speed of light, you are not part of our universe anymore. As gravity is also bounded at this rate it is better to talk about speed of consequence than speed of light.
But everyone says speed of light and that’s OK.
It’s like the FPS of the universe. Nothing can be rendered faster than that.
Light happens to go at that speed limit. So does other things, like gravity. It’s the speed limit for all things, effects, and information.
So maybe you know that light from the Sun takes 8.3 minutes to get to Earth, right? Well so does the Sun’s gravity. That is to say, if the Sun stopped existing, we would continue to orbit *where it was* for 8.3 minutes, until both its light and its gravitational pull on us both stopped and we’d fly away in darkness in a straight line.
Okay… things do not happen instantly.
If I do something here, the “reality” of that change ripples out across space and time. Until that ripple, that “bubble” of change, moves past you, you have no idea what I’ve done.
If I turn on a flashlight and point it at you, until the reality bubble of that change ripples outwards and contacts you, you still think the light is off.
Hence, what we call the “speed of light” isn’t just the speed of light. It’s the speed of reality. The speed of you causing something to happen, and that change rippling out across the universe. The speed of “causality”.
Everything that happens, from an atomic level, to gravity, to light from the sun, etc. all happen at a given point. But the fact that they happened at that point cannot affect ANYTHING else in the universe until the reality-bubble of that change has caught up with those other things. If the sun were to disappear now, we’d not know for 8 minutes – that’s how long it would take for the “reality” of the sun disappearing to ripple across space until it reached the Earth. For those 8 minutes, it would still look like the sun was there, we’re still continuing being pulled in by the gravity of it, etc.
The fact is that in everyday life, the speed of light is SO FAST that we don’t see this effect ourselves. A beam of light can circle the earth 7 times in a second, so we never really see this effect ourselves.
But the reality of any change, force, influence, energy, effect, collision, anything whatsoever… it ripples outwards from that event at the speed of light, and until it reaches you it basically “hasn’t happened yet” from your point of view.
We notice this effect in astronomy, even in our satellites (they sent us beams of light or radio waves or whatever, but it takes a little while for them to reach the Earth, it is not instantaneous). When the distances are vast, this effect is noticeable.
If you are an observer sat in the middle of space, and someone somewhere does ANYTHING, the reality of their actions, and all the actions that causes, don’t reach you for a period of time. That reality is moving outwards from that event at the speed of reality / light / causality. Until it does reach you, it literally hasn’t happened yet as far as you’re concerned, or as far as anything is concerned.
And there’s no real way to cheat it. If someone created a black hole in a nearby part of space, we literally would be unable to detect it “faster” in any way whatsoever. If we put a probe right next to it, that probe could only send us a signal to tell us what it detected at – at best – the speed of light. So that signal of “Hey, watch out! Someone built a black hole!” would only reach us at the same time as, or after, the gravitational effect of the black hole itself.
The speed of light is poorly named. It’s the speed of reality catching up across the universe.
And in real terms, absolutely nothing happens “instantly”. Not even gravity, light, etc. The best they can do is the speed limit of the universe… the speed of light. The speed of reality.
Causality is actuality. Trigonometricolepticalatomically speaking, there are a methalontropy of neurons that fire against protons causing a proto-reverse-mitochondrial reaction birthing speed rates
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