4 ms·

> Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause. You mai

> Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause. You maintain incorrectly. All inertial frames are equally good in Special Relativity. If there is any inertial frame that observes events elsewhere in the universe where an effect precedes a cause, then causality has been violated. Furthermore, whenever Special Relativity is violated like that, there is always a way to get the violation to turn up in your own frame of reference, unless you constrain things so severely that what you are proposing isn't going to be useful for much.

Dylan16807 14y ago
All inertial frames are equally valid. All inertial frames see that the effect does not munge the cause, even if it happens out of order. I know that 'causality' is a bad term for me to use here; I'm open to suggestions. But my point is that cause and effect are still intact. Cause, with effect not inside its light cone in the past, sends information toward effect. Effect, with cause not in its light cone in the future, receives information from cause. I don't care what order those happen in, as long as the thread of causation is intact and one-way. Yes there are 'ways' to create proper violations, but you can avoid them if you intentionally choose to.
- nessus42 14y ago
  > I know that 'causality' is a bad term for me to use here; I'm open to suggestions. I think that you are looking for the term "closed causal curve" or "closed causal loop". If you use FTL to achieve this, this is often called a "closed spacelike curve". If you use GR without FTL to achieve this, it is often called a "closed timelike curve". I'm not convinced that other sorts of violations of causality are any better than closed causal loops, since you can typically use the other violations to create closed causal loops. And even if you do have closed causal loops, you can decide not to go back in time and kill your grandfather, so if the issue is just what you chose to do, then closed causal loops are no worse in this regard. In any case, whether or not you chose to go back in time and kill your grandfather, you won't succeed, since you did not succeed.
  - Dylan16807 14y ago
    Well the important part here is that you can pretty simply have a self-consistent universe that has mildly-limited FTL (and highly-limited time travel) if nobody makes a loop. And I'll try to use something like 'causal loop' in the future.
    - nessus42 14y ago
      And what happens if you try to make a loop? Do you think it sensible that the universe provide you with the ability to make causal loops and then somehow forbids you from actually making them, even though once you've been given the ability, they are very easy to generate? Additionally, if the universe is going to keep stepping in your way of what it has given you the ability to do, there's no reason for it to forbid you from making causal loops. All it has to do is forbid you from generating a paradox via such a loop. I.e., you can have a self-consistent universe that has unlimited FTL and unlimited time travel as long as everyone plays nice. In fact, you can have unlimited time travel even is everybody is trying to kill their grandfather. They just won't ever succeed. Never did I assert anywhere in this entire thread that a universe where causality is violated--or even one with closed causal loops--is impossible. They do have serious issues, however. And for anyone to propose FTL travel as a serious possibility and then ignore the causality issues as non-serious, either indicates some amount of either ignorance, disingenuousness, or rationalization. If, on the other hand, a proponent of FTL travel plainly states that they have no problem with a universe that always steps in somehow and prevents you from succeeding in killing your own grandfather, then at least they've made their position clear, and everyone can make up their own mind as to whether that's a likely way for the universe to be.
      - Dylan16807 14y ago
        There are other solutions, too. It might simply be infeasible to accelerate an FTL device to sufficiently relativistic speeds. Or what if there were fields coming off of FTL devices that interfered with the speed when they criss-crossed. That one would stop any sort of loop and it wouldn't require the universe to somehow specifically forbid paradoxes.

Dylan16807 14y ago

All inertial frames are equally valid. All inertial frames see that the effect does not munge the cause, even if it happens out of order. I know that 'causality' is a bad term for me to use here; I'm open to suggestions. But my point is that cause and effect are still intact. Cause, with effect not inside its light cone in the past, sends information toward effect. Effect, with cause not in its light cone in the future, receives information from cause. I don't care what order those happen in, as long as the thread of causation is intact and one-way. Yes there are 'ways' to create proper violations, but you can avoid them if you intentionally choose to.

nessus42 14y ago
> I know that 'causality' is a bad term for me to use here; I'm open to suggestions. I think that you are looking for the term "closed causal curve" or "closed causal loop". If you use FTL to achieve this, this is often called a "closed spacelike curve". If you use GR without FTL to achieve this, it is often called a "closed timelike curve". I'm not convinced that other sorts of violations of causality are any better than closed causal loops, since you can typically use the other violations to create closed causal loops. And even if you do have closed causal loops, you can decide not to go back in time and kill your grandfather, so if the issue is just what you chose to do, then closed causal loops are no worse in this regard. In any case, whether or not you chose to go back in time and kill your grandfather, you won't succeed, since you did not succeed.
- Dylan16807 14y ago
  Well the important part here is that you can pretty simply have a self-consistent universe that has mildly-limited FTL (and highly-limited time travel) if nobody makes a loop. And I'll try to use something like 'causal loop' in the future.
  - nessus42 14y ago
    And what happens if you try to make a loop? Do you think it sensible that the universe provide you with the ability to make causal loops and then somehow forbids you from actually making them, even though once you've been given the ability, they are very easy to generate? Additionally, if the universe is going to keep stepping in your way of what it has given you the ability to do, there's no reason for it to forbid you from making causal loops. All it has to do is forbid you from generating a paradox via such a loop. I.e., you can have a self-consistent universe that has unlimited FTL and unlimited time travel as long as everyone plays nice. In fact, you can have unlimited time travel even is everybody is trying to kill their grandfather. They just won't ever succeed. Never did I assert anywhere in this entire thread that a universe where causality is violated--or even one with closed causal loops--is impossible. They do have serious issues, however. And for anyone to propose FTL travel as a serious possibility and then ignore the causality issues as non-serious, either indicates some amount of either ignorance, disingenuousness, or rationalization. If, on the other hand, a proponent of FTL travel plainly states that they have no problem with a universe that always steps in somehow and prevents you from succeeding in killing your own grandfather, then at least they've made their position clear, and everyone can make up their own mind as to whether that's a likely way for the universe to be.
    - Dylan16807 14y ago
      There are other solutions, too. It might simply be infeasible to accelerate an FTL device to sufficiently relativistic speeds. Or what if there were fields coming off of FTL devices that interfered with the speed when they criss-crossed. That one would stop any sort of loop and it wouldn't require the universe to somehow specifically forbid paradoxes.