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>Actually, there is no clear definition of causality in classical and quantum physics, simply because the elementary equations of motions are time reversible (i

>Actually, there is no clear definition of causality in classical and quantum physics, simply because the elementary equations of motions are time reversible (if you also reverse charge and parity). For every phenomenon that can occur, the same phenomenon can occur backwards in time (with corresponding anti-particles). This is super misguided. Causation doesn't need physical grounding. The usual definitions are grounded in counterfactuals and interventions. It's like defining the causal effect as the difference in potential lines of code I write today, depending on whether I do or don't eat breakfast this morning, but then arguing that under classical mechanics the universe is deterministic so I couldn't possibly take an intervention to not eat breakfast. Really, it's just another argument that counterfactuals are counter factual, and therefore don't have a clear definition. But yeah, that's in the name already. Regardless, physical equations are pretty much all causal, because they are stable under intervention. If I intervene to do whatever, they still hold. That lets me cause a change in friction be intervening and swapping out a material.

sjg007 7y ago
I thought causality was a function of time by definition?
- deleted 7y ago
  [deleted]
- 6gvONxR4sf7o 7y ago
  Not quite, especially since time is such a weird concept when you dig far enough into the math. There are a bunch of competing definitions of a causal change, but arguably the most conventional one is the difference between an outcome if you do one thing versus another. For example, looking at whether I write this comment now instead of getting back to work after lunch, you could imagine that I'm more inclined to slack off after a heavy meal. We have two hypotheticals. Scenario 1) I eat curry for lunch. Scenario 2) I eat salad for lunch. In scenario 1, I am a bit sleepy and log on to HN and write this. In scenario 2 (hypothetically) I just get right back to work and this comment never exists. The causal effect (hypothetically) is that having curry instead of salad for lunch made this comment exist. Compare that to the effect of the comment I wrote before lunch. We can still talk about the causal effect of lunch on what I wrote before lunch. Assuming that physics works the way I think it does, the causal effect is that nothing changes, but it's still a thing you can formalize. You could imagine an episode of star trek involving time travel, where my actions today do affect things yesterday. Starfleet statisticians could still run time-reversed randomized trials, they'd just have to be really careful about their experimental designs.
  - ssivark 7y ago
    I like your original comment further up, but disagree with the following bit: > We can still talk about the causal effect of lunch on what I wrote before lunch. To the extent that the situation occurs in the physical universe, that is meaningless. The future can never be a causal parent of the past. That says there is much more to the typical causality question than just the direction of time. The difference between the “time reversibility” in physics and the typical ML example is that once you’ve decided on something as the effect, the other thing must be the cause. So the problem is quite simple if you’re not trying to figure out the nature/direction of time, and especially not with microscopic physics. Anyone curious about the relation between time reversibility in physics and causal reasoning could look at Scholkopf+Janzing 2016 for an interesting line of thought. Somebody should tell Yann Lecun that a little knowledge is dangerous.
    - 6gvONxR4sf7o 7y ago
      >The future can never be a causal parent of the past. That's a testable statement. The reason it's testable is because we can define a hypothetical causal effect of the present on the past, and then do experiments and show that it's always zero. I only mean that we can talk about it, not that we can show it to be anything but a non-effect, so I'm not sure I see where we're in disagreement.
      - ssivark 7y ago
        Ah, guess I misinterpreted “can still talk about” in the context of response to its parent comment.

sjg007 7y ago

I thought causality was a function of time by definition?

deleted 7y ago
[deleted]
6gvONxR4sf7o 7y ago
Not quite, especially since time is such a weird concept when you dig far enough into the math. There are a bunch of competing definitions of a causal change, but arguably the most conventional one is the difference between an outcome if you do one thing versus another. For example, looking at whether I write this comment now instead of getting back to work after lunch, you could imagine that I'm more inclined to slack off after a heavy meal. We have two hypotheticals. Scenario 1) I eat curry for lunch. Scenario 2) I eat salad for lunch. In scenario 1, I am a bit sleepy and log on to HN and write this. In scenario 2 (hypothetically) I just get right back to work and this comment never exists. The causal effect (hypothetically) is that having curry instead of salad for lunch made this comment exist. Compare that to the effect of the comment I wrote before lunch. We can still talk about the causal effect of lunch on what I wrote before lunch. Assuming that physics works the way I think it does, the causal effect is that nothing changes, but it's still a thing you can formalize. You could imagine an episode of star trek involving time travel, where my actions today do affect things yesterday. Starfleet statisticians could still run time-reversed randomized trials, they'd just have to be really careful about their experimental designs.
- ssivark 7y ago
  I like your original comment further up, but disagree with the following bit: > We can still talk about the causal effect of lunch on what I wrote before lunch. To the extent that the situation occurs in the physical universe, that is meaningless. The future can never be a causal parent of the past. That says there is much more to the typical causality question than just the direction of time. The difference between the “time reversibility” in physics and the typical ML example is that once you’ve decided on something as the effect, the other thing must be the cause. So the problem is quite simple if you’re not trying to figure out the nature/direction of time, and especially not with microscopic physics. Anyone curious about the relation between time reversibility in physics and causal reasoning could look at Scholkopf+Janzing 2016 for an interesting line of thought. Somebody should tell Yann Lecun that a little knowledge is dangerous.
  - 6gvONxR4sf7o 7y ago
    >The future can never be a causal parent of the past. That's a testable statement. The reason it's testable is because we can define a hypothetical causal effect of the present on the past, and then do experiments and show that it's always zero. I only mean that we can talk about it, not that we can show it to be anything but a non-effect, so I'm not sure I see where we're in disagreement.
    - ssivark 7y ago
      Ah, guess I misinterpreted “can still talk about” in the context of response to its parent comment.