4 ms·

Don't you need to do that anyway, because of gravity? Wouldn't the causality speed limit just cause those gravity interactions to arrive with a time delay rath

Don't you need to do that anyway, because of gravity? Wouldn't the causality speed limit just cause those gravity interactions to arrive with a time delay rather than being instantaneous? Which means that to simulate the universe you essentially have to keep a history of how gravity is propagating, which requires keeping more information than if interactions were instantaneous? In a sense perhaps this applies to light too, because since it has a finite velocity now you have to keep track of how all the photons individually propagate through spacetime, whereas if light traveled instantly this would not be necessary? EDIT: The advantage I see in a speed limit is that you should be able to compute what happens in a point of spacetime based only on the information that is around that point (which still might have come from any or all other particles in the universe, mind you). For me, this emphasizes how important locality must be and it basically converts the popular "spooky action at a distance" claims into nonsense to me. I guess that's why I'm a fan of the Many Worlds Interpretation.

LadyCailin 3y ago
Tangential question: is the speed of causality coincidentally the same as the speed of light? Or are they the same because of some underlying principal that inherently links them?
- satvikpendem 3y ago
  Correct, the speed of light is actually the speed of causality, we just so named it after light because that was what we first discovered as going at c, but many other things in the universe also due because it's the same underlying principle. That is why gravitational waves also travel at c, ie if you removed the sun instantaneously from the solar system, the Earth will continue to orbit for 8 minutes, as that is how long light (and the gravitational force) takes to get from the sun to the Earth.
- someplaceguy 3y ago
  I'm not a physicist, but AFAIU the speed of travel/causality for light is only the maximum speed of causality because photons have a mass of literally zero. If photons had non-zero mass they could only travel slower than the maximum speed of causality (which would probably be called speed of gravity rather than speed of light, in this alternative universe).
- squeaky-clean 3y ago
  Not an expert, but can't resist chiming in anyways... One thing to think about it what exactly is causality? There'll be tons of different definitions, but they'll all have one thing in common, events that cause "later" events, and/or events that depend on "earlier" events. And in a physics sense what is an event? An interaction between two things, right? And since there doesn't exist any force that can interact instantaneously across distance, the speed limit of causality is equal to the speed of our fastest forces. If we discovered some scifi-esque Tachyon particle that traveled at 2C, we could no longer say the speed of light is the speed of causality.
  - someplaceguy 3y ago
    I'm not an expert either, but I like to think that the speed of causality is the speed at which a piece of information (e.g. a particle, a gravity wave, etc) is traveling through space. So the speed of light / gravity is essentially the maximum speed of causality, because nothing can travel faster than that. EDIT: Or you can think in terms of how information propagates through spacetime. In this point of view, the speed of causality is always the speed of light, for everything, including particles with mass.
    - squeaky-clean 3y ago
      Yeah that's what I was trying to work towards. Basically that causality is an abstraction, or at least a "higher level" idea. And if we look at the components of it, we can see that interaction between two things is a central part of it. And an interaction between two things in our universe has a maximum bound of the speed of light (and gravity and so on). The speed of causality is just the speed of the fastest thing.
    - anon291 3y ago
      It's the reverse. Light and gravity travel at c because they (at least light) are mediated by particles / systems that have no mass. Mass is what slows the speed of causality for certain particles. For example, while the photons I emit may travel at C, the massive particles that make up 'me' cannot.
  - 3y ago
- d1sxeyes 3y ago
  They are the same, because there’s no such thing as the “speed of light”. Theoretically as I understand it, everything moves at exactly the same speed through space-time, whether it is light, the Earth, etc. At non-relativistic speeds, this means moving along the time axis at approximately one second per second, with the rest of the movement in space. At relativistic speeds, higher proportions of the “speed” of an object are along the time axis. As the energy requirements for moving massive objects through space at relativistic speeds are huge, we can only really observe this phenomenon with light, which has no mass, and therefore does not need huge amounts of energy to move through space. As a result, we call 186k miles per second the “speed of light” when actually it is just the maximum speed anything can travel through space, and due to light being massless, it happens to be the speed that light travels through space too.
  - anon291 3y ago
    This is my understanding as well. All massless particles must travel at the speed of light (they cannot be slowed down) and moreover, they must travel at the speed of light in all reference frames. Whereas massive particles can remain at rest. There's no such thing as an unmoving photon.
    - d1sxeyes 3y ago
      My point is subtly different, I think. The point I’m trying to make is that if you’re “at rest” in space, all of your movement is through time. If you are “at rest” in time, all of your movement is through space. As far as I understand it, having mass is basically a result of moving more slowly through space.
BiteCode_dev 3y ago
Gravity is not instantaneous.
knome 3y ago
https://spaceplace.nasa.gov/gravitational-waves/en/ https://spaceplace.nasa.gov/gravitational-waves/en/ Gravity waves also travel at the speed of light.
- someplaceguy 3y ago
  Isn't that what I was saying? Since they have a speed limit you have to keep track of all gravity waves associated with all particles of the universe throughout all time and space. So all particles still interact with all other particles, all the time, it's just that they do it with a time delay. If there wasn't a speed limit it would be much simpler because all gravity interactions would be instantaneous and you wouldn't have to keep track of gravity waves.
  - depressedpanda 3y ago
    If light acted instantaneously you would have to calculate the effect of it's rays everywhere all at once, which I think is quite expensive given how vast space is. However, since the speed of light is miniscule compared to the size of the universe you can ignore all but the most local interactions, and just schedule a computation sometime in the future when you know that the light vector will interact with something. While instant calculations would perhaps make for a simpler system conceptually, the speed limit and locality principle ensures that less processing power is needed (at the cost of a lot of memory).
  - wheelerof4te 3y ago
    As someone already mentioned, you would have to account for all the interactions with light -> everywhere at once. With a effective speed limit to space-time, you can "localize" the computation to the spaces where light has reached. And who knows, maybe light can't travel forever, it might just disappear after crossing some distance we still haven't measured (how we'd do that, who knows). Giving yet another evidence to the "grand simulation" theory. "The universe" is just a group of simulated worlds connected by interacting photon particles (light).
    - 3y ago
FreshStart 3y ago
Gravity effects are causality bound too?

LadyCailin 3y ago

Tangential question: is the speed of causality coincidentally the same as the speed of light? Or are they the same because of some underlying principal that inherently links them?

satvikpendem 3y ago
Correct, the speed of light is actually the speed of causality, we just so named it after light because that was what we first discovered as going at c, but many other things in the universe also due because it's the same underlying principle. That is why gravitational waves also travel at c, ie if you removed the sun instantaneously from the solar system, the Earth will continue to orbit for 8 minutes, as that is how long light (and the gravitational force) takes to get from the sun to the Earth.
someplaceguy 3y ago
I'm not a physicist, but AFAIU the speed of travel/causality for light is only the maximum speed of causality because photons have a mass of literally zero. If photons had non-zero mass they could only travel slower than the maximum speed of causality (which would probably be called speed of gravity rather than speed of light, in this alternative universe).
squeaky-clean 3y ago
Not an expert, but can't resist chiming in anyways... One thing to think about it what exactly is causality? There'll be tons of different definitions, but they'll all have one thing in common, events that cause "later" events, and/or events that depend on "earlier" events. And in a physics sense what is an event? An interaction between two things, right? And since there doesn't exist any force that can interact instantaneously across distance, the speed limit of causality is equal to the speed of our fastest forces. If we discovered some scifi-esque Tachyon particle that traveled at 2C, we could no longer say the speed of light is the speed of causality.
- someplaceguy 3y ago
  I'm not an expert either, but I like to think that the speed of causality is the speed at which a piece of information (e.g. a particle, a gravity wave, etc) is traveling through space. So the speed of light / gravity is essentially the maximum speed of causality, because nothing can travel faster than that. EDIT: Or you can think in terms of how information propagates through spacetime. In this point of view, the speed of causality is always the speed of light, for everything, including particles with mass.
  - squeaky-clean 3y ago
    Yeah that's what I was trying to work towards. Basically that causality is an abstraction, or at least a "higher level" idea. And if we look at the components of it, we can see that interaction between two things is a central part of it. And an interaction between two things in our universe has a maximum bound of the speed of light (and gravity and so on). The speed of causality is just the speed of the fastest thing.
  - anon291 3y ago
    It's the reverse. Light and gravity travel at c because they (at least light) are mediated by particles / systems that have no mass. Mass is what slows the speed of causality for certain particles. For example, while the photons I emit may travel at C, the massive particles that make up 'me' cannot.
- 3y ago
d1sxeyes 3y ago
They are the same, because there’s no such thing as the “speed of light”. Theoretically as I understand it, everything moves at exactly the same speed through space-time, whether it is light, the Earth, etc. At non-relativistic speeds, this means moving along the time axis at approximately one second per second, with the rest of the movement in space. At relativistic speeds, higher proportions of the “speed” of an object are along the time axis. As the energy requirements for moving massive objects through space at relativistic speeds are huge, we can only really observe this phenomenon with light, which has no mass, and therefore does not need huge amounts of energy to move through space. As a result, we call 186k miles per second the “speed of light” when actually it is just the maximum speed anything can travel through space, and due to light being massless, it happens to be the speed that light travels through space too.
- anon291 3y ago
  This is my understanding as well. All massless particles must travel at the speed of light (they cannot be slowed down) and moreover, they must travel at the speed of light in all reference frames. Whereas massive particles can remain at rest. There's no such thing as an unmoving photon.
  - d1sxeyes 3y ago
    My point is subtly different, I think. The point I’m trying to make is that if you’re “at rest” in space, all of your movement is through time. If you are “at rest” in time, all of your movement is through space. As far as I understand it, having mass is basically a result of moving more slowly through space.

BiteCode_dev 3y ago

Gravity is not instantaneous.

knome 3y ago

https://spaceplace.nasa.gov/gravitational-waves/en/ https://spaceplace.nasa.gov/gravitational-waves/en/ Gravity waves also travel at the speed of light.

someplaceguy 3y ago
Isn't that what I was saying? Since they have a speed limit you have to keep track of all gravity waves associated with all particles of the universe throughout all time and space. So all particles still interact with all other particles, all the time, it's just that they do it with a time delay. If there wasn't a speed limit it would be much simpler because all gravity interactions would be instantaneous and you wouldn't have to keep track of gravity waves.
- depressedpanda 3y ago
  If light acted instantaneously you would have to calculate the effect of it's rays everywhere all at once, which I think is quite expensive given how vast space is. However, since the speed of light is miniscule compared to the size of the universe you can ignore all but the most local interactions, and just schedule a computation sometime in the future when you know that the light vector will interact with something. While instant calculations would perhaps make for a simpler system conceptually, the speed limit and locality principle ensures that less processing power is needed (at the cost of a lot of memory).
- wheelerof4te 3y ago
  As someone already mentioned, you would have to account for all the interactions with light -> everywhere at once. With a effective speed limit to space-time, you can "localize" the computation to the spaces where light has reached. And who knows, maybe light can't travel forever, it might just disappear after crossing some distance we still haven't measured (how we'd do that, who knows). Giving yet another evidence to the "grand simulation" theory. "The universe" is just a group of simulated worlds connected by interacting photon particles (light).
  - 3y ago

FreshStart 3y ago

Gravity effects are causality bound too?