5 ms·

> things happening as a result of other things is pretty much the definition of "time moving forward" No, that's the definition of causality. Just because you

> things happening as a result of other things is pretty much the definition of "time moving forward" No, that's the definition of causality. Just because you are used to thinking of casual relationships in the context of forward-moving time, doesn't mean that time's arrow has anything to do with it. That's kind of a meta correlation/causation argument. In physics causality is more appropriately defined in terms of the propagation of forces/information. E.g. if you are farther than a distance c*t away from me, we cannot be causally connected because no signal moving at the speed of light could propagate (either forwards, or backwards in time) to connect our worldlines.

Udo 11y ago
> No, that's the definition of causality. [...] That's kind of a meta correlation/causation argument. It's more than that. The only way for us to actually tell which way the arrow of time points is by watching causal events. > In physics causality is more appropriately defined in terms of the propagation of forces/information. But that's exactly my point, I'm not sure where the miscommunication happened there. I object to the time-flowing-backwards statement in the article, because anyone who suggests a scenario where you're getting younger while being aware of it has left scientific reality behind. Moving on beyond the article: You can theoretically assert that the propagation of information could just as well happen backwards, but in the real world this becomes just exceedingly unlikely after just a couple of interactions following an event. So yes, this process has everything to do with the direction and the nature of time. Causality is not just another thing that mysteriously happens in one certain direction, it's a process that by its very nature can only happen in one direction. I have a feeling you're going to object to this, but in my opinion an information theoretical view of time is just as valid - if not more so - than to hang it on the direction of gravity. It's a construct that helps illustrate why time flowing backwards can never be a perceptible/measurable thing.
- ajuc 11y ago
  > It's more than that. The only way for us to actually tell which way the arrow of time points is by watching causal events. No. We can take arbitrarily spaced "snapshots" of a closed system, and determine which was taken earlier by measuring entropy. No need for casuality.
  - Udo 11y ago
    > No. We can take arbitrarily spaced "snapshots" of a closed system, and determine which was taken earlier by measuring entropy. That is not correct. A closed system may by convention still exchange energy with the outside (just not any matter). What you're really talking about is called an isolated system, which is a theoretical construct that does not occur in nature. Fundamentally, time, causality, and entropy are related, especially considering the idea of entropy as seen through a probabilistic lens with information causality in mind (see Pawlowski et al arXiv:0905.2292). However, I think causality overall has a stronger, more basic link to the direction of time, and here is why: Entropy can always be reduced locally in an open system without affecting the flow of time at all. Causality, on the other hand, is not subject to reversal under any known circumstances, as long as a reasonable amount of information and time is involved. If, by some yet unknown mechanism, causality were reversed then so would be the direction of time, which in turn means the whole thing could not possibly be measured - not even in principle. Hence the argument that this reversal may not be possible since it's literally something that cannot exist in our universe. No such link exists between entropy and time, respectively.

Udo 11y ago

> No, that's the definition of causality. [...] That's kind of a meta correlation/causation argument. It's more than that. The only way for us to actually tell which way the arrow of time points is by watching causal events. > In physics causality is more appropriately defined in terms of the propagation of forces/information. But that's exactly my point, I'm not sure where the miscommunication happened there. I object to the time-flowing-backwards statement in the article, because anyone who suggests a scenario where you're getting younger while being aware of it has left scientific reality behind. Moving on beyond the article: You can theoretically assert that the propagation of information could just as well happen backwards, but in the real world this becomes just exceedingly unlikely after just a couple of interactions following an event. So yes, this process has everything to do with the direction and the nature of time. Causality is not just another thing that mysteriously happens in one certain direction, it's a process that by its very nature can only happen in one direction. I have a feeling you're going to object to this, but in my opinion an information theoretical view of time is just as valid - if not more so - than to hang it on the direction of gravity. It's a construct that helps illustrate why time flowing backwards can never be a perceptible/measurable thing.

ajuc 11y ago
> It's more than that. The only way for us to actually tell which way the arrow of time points is by watching causal events. No. We can take arbitrarily spaced "snapshots" of a closed system, and determine which was taken earlier by measuring entropy. No need for casuality.
- Udo 11y ago
  > No. We can take arbitrarily spaced "snapshots" of a closed system, and determine which was taken earlier by measuring entropy. That is not correct. A closed system may by convention still exchange energy with the outside (just not any matter). What you're really talking about is called an isolated system, which is a theoretical construct that does not occur in nature. Fundamentally, time, causality, and entropy are related, especially considering the idea of entropy as seen through a probabilistic lens with information causality in mind (see Pawlowski et al arXiv:0905.2292). However, I think causality overall has a stronger, more basic link to the direction of time, and here is why: Entropy can always be reduced locally in an open system without affecting the flow of time at all. Causality, on the other hand, is not subject to reversal under any known circumstances, as long as a reasonable amount of information and time is involved. If, by some yet unknown mechanism, causality were reversed then so would be the direction of time, which in turn means the whole thing could not possibly be measured - not even in principle. Hence the argument that this reversal may not be possible since it's literally something that cannot exist in our universe. No such link exists between entropy and time, respectively.