9 ms·

Also consider the speed of light is also the speed of causality. If there was no such limit it means it would be possible for effects to precede causes which wo

by heed 2y ago

Also consider the speed of light is also the speed of causality. If there was no such limit it means it would be possible for effects to precede causes which would lead to a very different kind of universe!

wruza 2y ago
Wouldn’t it just happen instantly and settle on some fixed point?
Dylan16807 2y ago
There's a lot of ways to implement that and most of them aren't a problem. For example: If there isn't a speed of light, how fast does light go? If it's variable but not instant, then depending on the details causality violations could still be very rare or impossible. If it's instant, then how do we define instant for different observers? I feel like relativity-style calculations don't really work. If "instant" is agreed upon by all observers then we won't have causality issues.
- dcow 2y ago
  Could you even measure or experience variable speed causality? Or, it doesn’t matter what made up constant you assign the speed of causality. You’re just bits on a page and you only perceive anything as the clock cycles.
  - ben_w 2y ago
    I've heard it claimed that we can only measure the round-trip speed of light, not the one-way speed of light, because the maths says that reality would look identical if it was 0.5c in the x+ direction and ∞ in the x- direction. I find this hard to stomach, but I'm going to trust it also applies to e.g. magnetism being Lorenz transformed electric fields, because relativity violates "common sense" all over the place and reality doesn't care about my stomach. https://www.youtube.com/watch?app=desktop&v=pTn6Ewhb27k&themeRefresh=1 https://www.youtube.com/watch?app=desktop&v=pTn6Ewhb27k&them...
    - AnimalMuppet 2y ago
      I also have heard that, multiple times. I don't buy it. I think there are at least two experiments that could show the difference. First, you could time the travel of light from one place to another. To do that, you need synchronized clocks. The easy way to do that is to start with clocks synchronized at a central point, then very slowly move them from the central point to the endpoints. Why very slowly? Because you have to worry about time dilation with the clocks. For small v, the difference in the rate of time is approximately v^2/2c^2 (to first order). The amount of time you have to maintain it is t = d/v. The corresponding difference in clock time still approaches zero as v approaches zero, so in principle, the clocks can be arbitrarily close to each other in time if you just move them slowly enough. But what if c has different values in opposite directions? Well, then time dilates different amounts for the clocks going in opposite directions, but the amount of time dilation for each clock still approaches zero if the velocity is low enough. Second: If you have a cyclotron or synchrotron, with charged particles moving in a circle in a magnetic field, and those charged particles are moving a significant fraction of the speed of light, if the speed of light is not uniform, their motion should deviate from a circle. Why? Because the force on them due to the magnetic field should be the same, but the acceleration should be different depending on what fraction of the speed of light they're moving. (Due to increased mass, if you think of it that way. If you don't, well, the equation doesn't change.) I think that some experiments would fail to show a non-uniform speed of light, but I think experiments could be devised that would show it.
      - Dylan16807 2y ago
        Unfortunately, if the one-way speed of light is anisotropic, the correct time dilation factor becomes 1/(γ(1−κv/c)), with the anisotropy parameter κ between -1 and +1.[17] This introduces a new linear term, meaning time dilation can no longer be ignored at small velocities, and slow clock-transport will fail to detect this anisotropy. Thus it is equivalent to Einstein synchronization. https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light A lot of scientists have thought about this. Step one is checking their work.
        2y ago
- setopt 2y ago
  “Instant” (i.e. infinite speed of light) also permits causality. That’s the historical Galilean model. That is in fact the only other way to make a causal universe that satisfies a few common sense assumptions (“the laws of physics are the same in every location”, “the laws of physics are the same in every direction”, “the laws of physics are the same over time”). “One more derivation of the Lorentz transformation” by Lévy-Leblond is a very accessible derivation of this if you’re interested in reading more. It was suggested that perhaps relativity should be taught this way in high school, instead of the historical approach of “c appears to be constant in experiments, so how do we work around that with math”.
  - Dylan16807 2y ago
    Couldn't you have the laws of physics change based on your speed but without changing based on location, direction, or over time? Also infinite speed of causality doesn't have to imply infinite speed of light, does it?
    - setopt 2y ago
      > Couldn't you have the laws of physics change based on your speed but without changing based on location, direction, or over time? No you can’t, that’s basically what e.g. the Levy-Leblonde reference proves :). I encourage giving a read if you’re interested! The proof is just a few pages long, and doesn’t require more advanced mathematics than the average intro to special relativity. If you’re willing to give up either causality itself, or the invariances of physical laws we discussed above, then of course many other alternatives open up. > Also infinite speed of causality doesn't have to imply infinite speed of light, does it? That is correct! Without experimental data, we can just prove that there must be a “speed of causality” that is constant for every observer in a universe with the properties we discussed above. That there exist “photons” in this universe that manage to travel at this speed is an experimental result. The exact value of that upper “speed limit” is also an experimental result.
      - Dylan16807 2y ago
        > The principle of reality is first stated in general terms, leading to the idea of equivalent frames of reference connected through "inertial" transformations obeying a group law. [...] Only the Lorentz transformations and their degenerate Galilean limit obey these constraints. > I will take as a starting point the statement of the principle of relativity in a very general form: there exists an infinite continuous class of reference frames in space-time which are physically equivalent. [...] no physical effects can distinguish between them. Sounds like this entire paper is built on a foundation of assuming the laws of physics don't change based on speed. Am I misreading? In that case, the paper proves that the Lorenz transforms are the only way to have both relativity and those rules, but they don't show that those rules by themselves imply relativity.
deleted 2y ago
[deleted]
MattPalmer1086 2y ago
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). Of course, this doesn't fit with a universe described by general relativity, where time can be different for different observers. But you wouldn't have a universe described by general relativity without that constraint in the first place.
- alde 2y ago
  How would you compare two infinities? E.g. speed of light inside a moving train vs speed of light outside of it.
  - MattPalmer1086 2y ago
    An infinite speed implies instantaneous effect. So it wouldn't matter how you were moving. If two people launched something that travelled with infinite speed, one on the train travelling at 100mph, and one on the ground beside it, it would take zero time for both of them to reach their destination. At least, that's what I surmise. I'm not a physicist.
- gavmor 2y ago
  Here's a great video explaining how, due to relativity, FTL travel can cause grandfather paradoxes: https://youtu.be/an0M-wcHw5A?si=RYFGOmQOlaC2t0bM https://youtu.be/an0M-wcHw5A?si=RYFGOmQOlaC2t0bM Edit: in short, not all reference frames can agree on the order of events, and FTL events propogate "backwards" between some reference frames.
  - MattPalmer1086 2y ago
    Nice video, thanks for posting.
  - david-gpu 2y ago
    That doesn't mean that light (causality) couldn't be faster, right? You could increase the speed of light (causality) as much as you want and wouldn't run into any paradox.
m3kw9 2y ago
Speed of light may be dependent on other constants and it will have then unpredictable effects
Etheryte 2y ago
The possibility of delays being zero does not imply that negative delays are possible.
- cjfd 2y ago
  Actually, it does. Because of relativity events that occur at the same time in one frame of reference do not occur at the same time in another. A delay of zero between two different points implies that there is a reference frame where the delay is negative.
  - soulofmischief 2y ago
    Relativity was derived as a direct consequence of imposing an invariant speed of causality to the Lorentz transformations, therefore it cannot tautologically be used as justification for an invariant speed of causality.
    - cjfd 2y ago
      "Imposing an invariant speed of causality to the Lorentz transformations" does not sound quite right. I think it is more like assuming that the Lorentz transformations are the true symmetry of mechanics. If one wants to keep that causal, there cannot be information moving faster than the speed of light because there is a reference frame where said information would be moving backwards in time.
  - Etheryte 2y ago
    I'm not sure that holds when you take the speed of light to be infinite. Depending on which end you look at it, you'll either be dividing by zero or having infinite energy, so I don't think relativity the way we understand it would still make sense in any way.
    - cjfd 2y ago
      Well, it is quite well known what you get when taking the speed of light to infinity. It is classical mechanics.
choeger 2y ago
What even is the speed of causality? Is there any way to determine that causality has made it halfway from cause to effect? Or is this just a metaphysical way of saying that no particle can move faster than the speed of light, assuming that causality is just an abstraction of moving particles around?
- hughesjj 2y ago
  Network propagation delay ;-) Imagine the world without a speed of causality, where everything was updated instantaneously. No CAP theorm, no Byzantine generals. Being a programmer/information theorist would be so much easier lol
- crdrost 2y ago
  It kind of is that (a metaphysical restatement), but it's more precisely understood as a kind of half-statement of the theory. That is, if you assume relativity, then for anything which moves faster than speed c, there exists some reference frame where it appears to move backwards in time. (This needs to be slightly qualified because it's kind of like when you're looking in a mirror and you intuitively don't think it does what it actually does -- flip front to back -- but you mentally rotate and then think that it flips left-to-right. So to be clear, if someone on a hyperluminal rocket cracks an egg into a pan, there exists someone else whose best understanding of this situation is a rocket that is traveling "backwards" engine-first, onboard of which an egg is flying up from the pan into an eggshell. But you would mentally reorient to say that the rocket is traveling "forwards" and that "forwards" direction is backwards in time.) Now, this doesn't directly violate causality by itself, it depends on whether you can move faster than light according to an arbitrary observer. So if Carol goes faster than light according to Alice and then turns and goes faster than light according to Bob, and Bob is moving relative to Alice, only then can Carol potentially meet up with her "past self" according to Alice & Bob. The idea is that the first time she moves, Alice says she's moving very fast, but forward in time, and Bob says she's moving backward in time. Then the second time she moves, Bob says she's moving very fast, but forward in time, and Alice says she's moving backward in time. You combine these two to find that both agree that she has objectively moved backward in time. The way this manifests in the mathematics is that in relativity, after something happens, light kind of "announces" that it happened to the rest of the world, via an expanding bubble of photons traveling away from the event at speed c. This expanding bubble is formally known as a "light cone". There is another light cone as well: before the event happens you can understand a contracting bubble of photons traveling towards the event. And basically these partition the world into five regions: The contracting bubble is the "objective past" of the event, that bubble itself is the "null past" of the event, the spacetime between the bubbles is the "general present" of the event, the expanding bubble is the "null future" of the event, and the points inside of the bubble are the "objective future" of the event. Moving faster than light, is moving from the objective future of an event, into its general present. This is "general" because different reference frames regard these points as either before or after the event in time. You need a second trajectory to then go from the general present of the event, to its objective past.
K0balt 2y ago
Antimatter is often described as matter going the opposite direction in time, and this seems to hold for particle interactions and quantum physics. It breaks down for thermodynamics… but might this be because of the inherent time vector of the observer?

by heed 2y ago

wruza 2y ago

Wouldn’t it just happen instantly and settle on some fixed point?

Dylan16807 2y ago

There's a lot of ways to implement that and most of them aren't a problem. For example: If there isn't a speed of light, how fast does light go? If it's variable but not instant, then depending on the details causality violations could still be very rare or impossible. If it's instant, then how do we define instant for different observers? I feel like relativity-style calculations don't really work. If "instant" is agreed upon by all observers then we won't have causality issues.

dcow 2y ago
Could you even measure or experience variable speed causality? Or, it doesn’t matter what made up constant you assign the speed of causality. You’re just bits on a page and you only perceive anything as the clock cycles.
- ben_w 2y ago
  I've heard it claimed that we can only measure the round-trip speed of light, not the one-way speed of light, because the maths says that reality would look identical if it was 0.5c in the x+ direction and ∞ in the x- direction. I find this hard to stomach, but I'm going to trust it also applies to e.g. magnetism being Lorenz transformed electric fields, because relativity violates "common sense" all over the place and reality doesn't care about my stomach. https://www.youtube.com/watch?app=desktop&v=pTn6Ewhb27k&themeRefresh=1 https://www.youtube.com/watch?app=desktop&v=pTn6Ewhb27k&them...
  - AnimalMuppet 2y ago
    I also have heard that, multiple times. I don't buy it. I think there are at least two experiments that could show the difference. First, you could time the travel of light from one place to another. To do that, you need synchronized clocks. The easy way to do that is to start with clocks synchronized at a central point, then very slowly move them from the central point to the endpoints. Why very slowly? Because you have to worry about time dilation with the clocks. For small v, the difference in the rate of time is approximately v^2/2c^2 (to first order). The amount of time you have to maintain it is t = d/v. The corresponding difference in clock time still approaches zero as v approaches zero, so in principle, the clocks can be arbitrarily close to each other in time if you just move them slowly enough. But what if c has different values in opposite directions? Well, then time dilates different amounts for the clocks going in opposite directions, but the amount of time dilation for each clock still approaches zero if the velocity is low enough. Second: If you have a cyclotron or synchrotron, with charged particles moving in a circle in a magnetic field, and those charged particles are moving a significant fraction of the speed of light, if the speed of light is not uniform, their motion should deviate from a circle. Why? Because the force on them due to the magnetic field should be the same, but the acceleration should be different depending on what fraction of the speed of light they're moving. (Due to increased mass, if you think of it that way. If you don't, well, the equation doesn't change.) I think that some experiments would fail to show a non-uniform speed of light, but I think experiments could be devised that would show it.
    - Dylan16807 2y ago
      Unfortunately, if the one-way speed of light is anisotropic, the correct time dilation factor becomes 1/(γ(1−κv/c)), with the anisotropy parameter κ between -1 and +1.[17] This introduces a new linear term, meaning time dilation can no longer be ignored at small velocities, and slow clock-transport will fail to detect this anisotropy. Thus it is equivalent to Einstein synchronization. https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light A lot of scientists have thought about this. Step one is checking their work.
      - 2y ago
setopt 2y ago
“Instant” (i.e. infinite speed of light) also permits causality. That’s the historical Galilean model. That is in fact the only other way to make a causal universe that satisfies a few common sense assumptions (“the laws of physics are the same in every location”, “the laws of physics are the same in every direction”, “the laws of physics are the same over time”). “One more derivation of the Lorentz transformation” by Lévy-Leblond is a very accessible derivation of this if you’re interested in reading more. It was suggested that perhaps relativity should be taught this way in high school, instead of the historical approach of “c appears to be constant in experiments, so how do we work around that with math”.
- Dylan16807 2y ago
  Couldn't you have the laws of physics change based on your speed but without changing based on location, direction, or over time? Also infinite speed of causality doesn't have to imply infinite speed of light, does it?
  - setopt 2y ago
    > Couldn't you have the laws of physics change based on your speed but without changing based on location, direction, or over time? No you can’t, that’s basically what e.g. the Levy-Leblonde reference proves :). I encourage giving a read if you’re interested! The proof is just a few pages long, and doesn’t require more advanced mathematics than the average intro to special relativity. If you’re willing to give up either causality itself, or the invariances of physical laws we discussed above, then of course many other alternatives open up. > Also infinite speed of causality doesn't have to imply infinite speed of light, does it? That is correct! Without experimental data, we can just prove that there must be a “speed of causality” that is constant for every observer in a universe with the properties we discussed above. That there exist “photons” in this universe that manage to travel at this speed is an experimental result. The exact value of that upper “speed limit” is also an experimental result.
    - Dylan16807 2y ago
      > The principle of reality is first stated in general terms, leading to the idea of equivalent frames of reference connected through "inertial" transformations obeying a group law. [...] Only the Lorentz transformations and their degenerate Galilean limit obey these constraints. > I will take as a starting point the statement of the principle of relativity in a very general form: there exists an infinite continuous class of reference frames in space-time which are physically equivalent. [...] no physical effects can distinguish between them. Sounds like this entire paper is built on a foundation of assuming the laws of physics don't change based on speed. Am I misreading? In that case, the paper proves that the Lorenz transforms are the only way to have both relativity and those rules, but they don't show that those rules by themselves imply relativity.

deleted 2y ago

[deleted]

MattPalmer1086 2y ago

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). Of course, this doesn't fit with a universe described by general relativity, where time can be different for different observers. But you wouldn't have a universe described by general relativity without that constraint in the first place.

alde 2y ago
How would you compare two infinities? E.g. speed of light inside a moving train vs speed of light outside of it.
- MattPalmer1086 2y ago
  An infinite speed implies instantaneous effect. So it wouldn't matter how you were moving. If two people launched something that travelled with infinite speed, one on the train travelling at 100mph, and one on the ground beside it, it would take zero time for both of them to reach their destination. At least, that's what I surmise. I'm not a physicist.
gavmor 2y ago
Here's a great video explaining how, due to relativity, FTL travel can cause grandfather paradoxes: https://youtu.be/an0M-wcHw5A?si=RYFGOmQOlaC2t0bM https://youtu.be/an0M-wcHw5A?si=RYFGOmQOlaC2t0bM Edit: in short, not all reference frames can agree on the order of events, and FTL events propogate "backwards" between some reference frames.
- MattPalmer1086 2y ago
  Nice video, thanks for posting.
- david-gpu 2y ago
  That doesn't mean that light (causality) couldn't be faster, right? You could increase the speed of light (causality) as much as you want and wouldn't run into any paradox.

m3kw9 2y ago

Speed of light may be dependent on other constants and it will have then unpredictable effects

Etheryte 2y ago

The possibility of delays being zero does not imply that negative delays are possible.

cjfd 2y ago
Actually, it does. Because of relativity events that occur at the same time in one frame of reference do not occur at the same time in another. A delay of zero between two different points implies that there is a reference frame where the delay is negative.
- soulofmischief 2y ago
  Relativity was derived as a direct consequence of imposing an invariant speed of causality to the Lorentz transformations, therefore it cannot tautologically be used as justification for an invariant speed of causality.
  - cjfd 2y ago
    "Imposing an invariant speed of causality to the Lorentz transformations" does not sound quite right. I think it is more like assuming that the Lorentz transformations are the true symmetry of mechanics. If one wants to keep that causal, there cannot be information moving faster than the speed of light because there is a reference frame where said information would be moving backwards in time.
- Etheryte 2y ago
  I'm not sure that holds when you take the speed of light to be infinite. Depending on which end you look at it, you'll either be dividing by zero or having infinite energy, so I don't think relativity the way we understand it would still make sense in any way.
  - cjfd 2y ago
    Well, it is quite well known what you get when taking the speed of light to infinity. It is classical mechanics.

choeger 2y ago

What even is the speed of causality? Is there any way to determine that causality has made it halfway from cause to effect? Or is this just a metaphysical way of saying that no particle can move faster than the speed of light, assuming that causality is just an abstraction of moving particles around?

hughesjj 2y ago
Network propagation delay ;-) Imagine the world without a speed of causality, where everything was updated instantaneously. No CAP theorm, no Byzantine generals. Being a programmer/information theorist would be so much easier lol
crdrost 2y ago
It kind of is that (a metaphysical restatement), but it's more precisely understood as a kind of half-statement of the theory. That is, if you assume relativity, then for anything which moves faster than speed c, there exists some reference frame where it appears to move backwards in time. (This needs to be slightly qualified because it's kind of like when you're looking in a mirror and you intuitively don't think it does what it actually does -- flip front to back -- but you mentally rotate and then think that it flips left-to-right. So to be clear, if someone on a hyperluminal rocket cracks an egg into a pan, there exists someone else whose best understanding of this situation is a rocket that is traveling "backwards" engine-first, onboard of which an egg is flying up from the pan into an eggshell. But you would mentally reorient to say that the rocket is traveling "forwards" and that "forwards" direction is backwards in time.) Now, this doesn't directly violate causality by itself, it depends on whether you can move faster than light according to an arbitrary observer. So if Carol goes faster than light according to Alice and then turns and goes faster than light according to Bob, and Bob is moving relative to Alice, only then can Carol potentially meet up with her "past self" according to Alice & Bob. The idea is that the first time she moves, Alice says she's moving very fast, but forward in time, and Bob says she's moving backward in time. Then the second time she moves, Bob says she's moving very fast, but forward in time, and Alice says she's moving backward in time. You combine these two to find that both agree that she has objectively moved backward in time. The way this manifests in the mathematics is that in relativity, after something happens, light kind of "announces" that it happened to the rest of the world, via an expanding bubble of photons traveling away from the event at speed c. This expanding bubble is formally known as a "light cone". There is another light cone as well: before the event happens you can understand a contracting bubble of photons traveling towards the event. And basically these partition the world into five regions: The contracting bubble is the "objective past" of the event, that bubble itself is the "null past" of the event, the spacetime between the bubbles is the "general present" of the event, the expanding bubble is the "null future" of the event, and the points inside of the bubble are the "objective future" of the event. Moving faster than light, is moving from the objective future of an event, into its general present. This is "general" because different reference frames regard these points as either before or after the event in time. You need a second trajectory to then go from the general present of the event, to its objective past.

K0balt 2y ago

Antimatter is often described as matter going the opposite direction in time, and this seems to hold for particle interactions and quantum physics. It breaks down for thermodynamics… but might this be because of the inherent time vector of the observer?