[andsoitis]

> How could an effect precede a cause if there were no speed limit to causality?

> No matter how fast an effect propogates, it is always after the cause (with an infinite speed, I guess effects happen instantaneously, but not before).

If everything happens instantaneously then there is no real cause and effect, and the universe would be over before it really got started.

[amelius]

No speed limit does not mean that everything goes infinitely fast.

[MattPalmer1086]

Agreed, an infinite speed was just the most extreme edge case of having no limit.

[kazinator]

But that case can be explicitly ruled. There being no highest speed doesn't mean that there is infinite speed. No edge, no edge case.

[lazide]

If the speed limit is infinite, what else would you expect to happen?

[bryanrasmussen]

old grannies driving at 30mph on the freeway, me at infinity.

on edit: not everything travels at the speed limit, if the speed limit right now is the speed of light - then why doesn't everything travel at the speed of light?

People say if the speed limit was infinite that everything would happen instantaneously - but they still need to explain why everything should go at the speed limit in this other universe, when not everything goes at the speed limit in ours.

[lazide]

Almost everything (electrical fields, atomic radius, even speed of sound in materials) seems to derive in some way from the speed of light and related effects.

[bryanrasmussen]

perhaps this is an effect of having a speed limit in a universe, if a universe does not have any set speed limit (which is somewhat different than the phrasing speed limit is infinite) perhaps the discussed derivation of other speeds would not exist in the way it does in our universe.

[lazide]

Anything is possible if anything is possible.

In this universe, fundamental forces like electromagnetism directly contain things like c, so if c is infinite, everything is going to work very differently.

[amelius]

Light traveling at infinite speeds, atoms and such not.

[andsoitis]

If effects were instantaneous then atoms would not exist.

[amelius]

There can be many types of effects in a hypothetical universe.

Imagine a universe like Conway's way of life, where only neighboring cells can be affected in one timestep. Now add to it a rule that all blocks have a color, and the color of all blocks are changed when one block changes color. Now you have a universe with both immediate and non-immediate effects.

[jodrellblank]

But what is “One time step” in that universe? We have the idea of a light clock - light bouncing between two perfect mirrors in a vacuum - as an ultimate clock.

The distance between the mirrors is a number of meters. A meter is based on how far light travels in a second. How long it takes light to go between them is based on the speed of light. Speed, distance and time are connected.

If we untether the speed of light and it’s unlimited, then in some sense there is no way to say how long it takes light to bounce between the mirrors - it doesn’t take any time. And there is no way to say how far apart the mirrors are, if light passes between them instantly that implies there must be no gap to cross. If light crosses no distance in no time then it also bounces back covering no distance in no time, ahh does lots of bounces in no time. There goes the concept of a time step and any concept of “non immediate effects”.

If you try and add time as a separate thing, then you have some kind of Conway’s game simulation - but that gives you a way to track where light is (which simulation cell it’s in) and therefore a kind of distance (how far the mirrors are apart in simulation cells) and then you lock down how light moves in “simulation cells travelled per timestep” which brings you back to a fixed speed of light again.

[andsoitis]

what assumptions have to be true for such a universe to exist? did it just appear fully formed with N number of cells and defaulting to a color?

a hypothetical universe is mostly worth discussing seriously if there's a physics that is coherent, not just a mathematical landscape. At least it isn't that interesting in the discussion of universes, but might be in discussing mathematical ideas, but those do not necessarily mean there's a universe represented by it.

[lazide]

A ‘one level of cells in one timestep’ is a speed limit, and a very slow one actually.

[simonh]

Absence of a speed limit doesn't mean all speeds are infinite. It's just the picture everyone assumed before relativity.

[tremon]

You're saying that an atom's decay rate is a function of the speed of light? What proof do we have of this? Does Newton's law of momentum also erroneously leave out the c component?

[lazide]

If decay rate is related to the elements composition (seems to be true), and the forces holding elements together include Electromagnetism and the Lorentz Force which also seems to be true, then yes. [https://en.m.wikipedia.org/wiki/Classical_electromagnetism_a....]

See also [https://en.m.wikipedia.org/wiki/Electromagnetic_four-potenti...]

Notably, light is a form of electromagnetism, so this shouldn’t be as surprising as it is. c is an explicit part of many formulas, interestingly. And electromagnetism was the first thing tackled in special relativity.

[jncfhnb]

The speed limit being infinitely large does not intuitively imply that anything should be able to go at infinite speed.

[withinboredom]

I'm having trouble with this assertion. Light travels slower in water than in air, by your assertion that light is the limit of causality; then surely we can create a paradox with ftl right in a pool.

[Etherlord87]

Light moves with `c` speed regardless of medium. Whenever we say light moves slower in a medium, we simply mean it is measured to be slower, it is macroscopically slower, it's as if having a hypothetical vehicle that, when it moves, it always moves with a constant speed, but you measure it by taking the time of departure in place A, time of arrival in place B, measure the distance |AB| on a map, and from that calculate the speed of the vehicle. Your measurement will be affected by exact path shape (which isn't a straight line), as well as the number of times the driver decided to take a break to sleep in a motel, eat something, go to a toilet on a gas station etc.

[jncfhnb]

This example seems… bad to me. Are you simply saying that light moves slower from A to B through a medium like water because it takes a path that is less direct to navigate the medium?

[felbane]

I agree, that answer is misleading. The way I've always understood it: light is an EM wave, and it interacts with medium that it travels through. When traveling through a vacuum, the "beam" source is the origin, but when traveling through a medium the "beam" is a propagation of emissions from the matter absorbing, oscillating, and re-emitting a photon. These interactions take (an extremely small, but nonzero) amount of time, but the light being absorbed and emitted always travels at c.

[withinboredom]

No. It actually goes slower. https://youtu.be/uo3ds0FVpXs?si=b7sDxNuQkTuwAkcP gives a pretty good overview.

Edit: wrong one https://youtu.be/yP1kKN3ghOU?si=hsBj0RpzOb3JZWdS the one above is the "why."

[dcow]

No. It propagates slower.

https://youtu.be/KTzGBJPuJwM?si=Pab2F0EFsB4oGhUo

[Etherlord87]

The absorptions are the "breaks" in the analogy.

[MattPalmer1086]

Causality does not depend on the speed of light. Rather, light travelling in a vacuum happens to travel at the maximum speed of causality.

If light is slower in other mediums, that has no effect on how quickly causation can happen.

[coldtea]

>Light travels slower in water than in air, by your assertion that light is the limit of causality

The limit of causality is the light speed limit in vacuum, not "whatever happens to be the max speed of light in some medium".

Light (as in visible light) is also irrelevant to this, it's just an example of something moving at that speed.

[withinboredom]

I fail to see how a vacuum permits violations of casualty. Care to explain?

[coldtea]

Who said anything about a vacuum permitting violations of casualty?

You wrote: "I'm having trouble with this assertion. Light travels slower in water than in air, by your assertion that light is the limit of causality; then surely we can create a paradox with ftl right in a pool."

I answered that the "speed of casuality" is not "how fast light travels in a given medium": it's the maxiumum speed of light, which is the speed of light in a vacuum.

So that the light travels slower in a pool doesn't mean we can violate casuality - the overall casuality "speed limit" remains regardless (it's a maximum limit in the universe, not a regional one).

Btw, light doesn't really slow down in a medium like water. Photons always travel at the speed of light. The aggregate light appears to slow down in the water, as invidividual photons are converted to energy when interacting with the water particles and then the energy is emitted again as new photons.

The photons while they exist (i.e. before and after the conversion to/from energy) always run at the speed of light, even inside a medium like water or whatever else. Some of them will be converted to heat though, warming up the water - but in that case they're not light anymore.

[MattPalmer1086]

The vacuum has nothing to do with causality.

It's just that light (if there is nothing in its way, so in a vacuum) will travel at the max speed of causality.

Causality violation can happen in general relativity when something moves faster than the max speed of causality (which is the same speed as light in a vacuum).

[andsoitis]

> Causality violation can happen in general relativity

The only theoretical case I’m aware of are closed timelike curves (CTC) which are paths in spacetime that loop back on themselves, allowing an object to return to its own past. A famous example is the Gödel metric, a solution to Einstein’a field equations.

It should be noted, however, that these solutions are generally regarded as unphysical because they require conditions that don’t seem to exist in our universe (such as a globally rotating universe).

[marcus_holmes]

Yes. You can pretty easily think of an experiment where a waterborne person throws a switch that changes some distant object, only to see that object change "before" they threw the switch because the experiment actually communicated the switch change via an airborne method unavailable to the waterborne observer.

[OkGoDoIt]

I don’t think that actually works. In this case you’re talking about a round trip, with the switch’s outbound signal traveling fast (airborne/vacuum light speed) and the return signal of the object visually changing traveling slower (water light speed). The total round-trip where you see the effect of flipping the switch would take longer if either leg involved water, but it wouldn’t cause the perception of it to happen ahead of the act of flipping the switch.

[withinboredom]

Observer A and observer B are mermaids. A throws a switch that turns on a light on a light house. Relative to each observer, the can see the cause and effect. B invents a periscope that allows them to see faster than light. Now B will be able to see the light turn on before the switch is flipped.

Replace periscope with “wormhole” and you get a more traditional experiment. The question of can we use this to violate casualty is non-sensical, because we can’t violate casualty (even with faster than light travel). In the traditional experiment, if I see the light turn on, the cause has already happened; sending a message “back in time” won’t change that.

However, this is only because all frames of reference stay the same. If you could actually travel back in time, who knows what would happen. That’s largely why this whole conversation makes no sense. You can’t violate casualty with FTL, only with time machines and FTL isn’t a Time Machine.

[MattPalmer1086]

FTL is shorthand for "Faster than Light" but here it really means "Faster than Light in a Vacuum".

Light actually has nothing to do with it; it just happens to travel at the max speed allowed by the universe when there's nothing that impedes it's motion (i.e. in a vacuum).

So the acronym should really be "FTLIAV"!

[withinboredom]

There are many things that can go faster than light, most of which we don't know about yet. But one thing is for sure, quantum entanglement can be undone faster-than-light. It's just that nobody has yet figured out how to send information through that medium, and it may even be impossible. But clearly, causality isn't being violated here and it goes faster than light in a vacuum.

[MattPalmer1086]

In general relativity, FTL can be used as a time machine.

[withinboredom]

Then you should be able violate casualty with simple water, otherwise FTL cannot be a Time Machine because it is quite simple to go FTL.

[SoftTalker]

This is the experience of a photon.