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> The physical models we have are causal ones. The physical laws are correlational, not causal. F=ma doesn't tell you if acceleration causes force or vice vers

> The physical models we have are causal ones. The physical laws are correlational, not causal. F=ma doesn't tell you if acceleration causes force or vice versa. Causation is a humanly imposed concept . That's why we need to invent things like the Chronology protection conjecture. https://en.wikipedia.org/wiki/Causality#Physics https://en.wikipedia.org/wiki/Causality#Physics

srean 7y ago
> The physical laws are correlational You mean if I extend the spring the weight stretching it will increase ?
- ChrisLomont 7y ago
  You can do it by adding more mass in a fixed inertial frame, or by leaving the mass along and changing the reference frame. You cannot decide which happens, which is an insight from relativity. So, which was the cause? You cannot tell. You simply can do the math and see that it matches the experiment. Only later do humans try to call it cause and effect, because the math and the experiment match so often and so well. But this all can change if we find an experiment breaking the models. Before relativity, most people thought time was constant. They were wrong. So the above poster is correct in what we usually call cause and effect to simply be stronger correlations that match (current) models. That can all be changed in future understanding of Nature.
  - 6gvONxR4sf7o 7y ago
    Causation: if I do X, what happens to Y? For any well understood physical situation describing X and Y, the laws of physics tell you what happens to Y when you do X. If I swap out a mass on a spring for a heavier mass, physics will predict how much lower it hangs. The cause is me swapping one mass for another. The effect is where it now hangs. That's the whole point. If it wasn't this way, we'd say our physical understanding is wrong.
    - ChrisLomont 7y ago
      Having worked on physics projects for decades, the laws of physics at the lowest level are most certainly not this simple or causal. >For any well understood physical situation describing X and Y, the laws of physics tell you what happens to Y when you do X. Not true. If I create two electrons from photon collision, and measure one, will it be spin up? No way to tell. Only aggregates about large enough systems give any reliable answer to such questions. If look for radioactive decay from 10 U235 atoms for the next 5 minutes, will I see one? Again, there is no yes/no answer, only probabilistic ones. There is no underlying causality - only purely random events with no detectable cause. These are the dice that Einstein didn't like. There's plenty of similar questions that don't have a simple answer - only answers about large aggregates. Here's [1] a recent result showing causality is no where the neat and tidy thing you think it is. These results are all over modern physics. For example, the most accurate physical theories, such as QED, get the right answer by summing forwards and backwards in time. In QED the "future" affects results today as strongly as the "past". Causality is a psychological interpretation, but is not in the math or the underlying theories. As far as I know, all quantum field theories (which underlie all physics at the moment) all have such ambiguity or uncertainty about causality. The laws are math models that give answers. But the lowest laws are time reversible or time agnostic for the most part, and the foundational theories require travelling forwards and backwards to get the correct experimental values. Similarly, the laws of physics at the lowest level are not causal, but probabilistic. Only when aggregated do some experiments seem causal. From another direction, there's a massive body of literature on what is causal, and can you detect it. Read, for example, Judea Pearl's monograph "Causality" or some of his other stuff, or simply browse wikipedia starting with him. For example, when you drop a ball, it falls to earth. Quantum mechanically, there is a probability it simply quantum tunnels to another galaxy. That it most often falls to earth becomes a law, but it is imprecise and not completely correct. It's an approximation. So every law of physics is merely a strong correlation. And there's currently plenty of experiments trying to disentangle these issues with causality and locality. So sure, at the freshman physics level causality is a simple thing. But Nature does not follow almost any of those rules with certainty or unerring rigor. Those are approximations and simplifications. [1] https://physicsworld.com/a/quantum-mechanics-defies-causal-order-experiment-confirms/ https://physicsworld.com/a/quantum-mechanics-defies-causal-o...
- buboard 7y ago
  hooke's law correlates spring length with force, but doesnt prescribe an order of events. You can increase the weight or move the spring to a higher gravitational field or something else we dont' know yet. However, as humans we can't reason of a way to increase the length of the spring without changing the force, therefore we establish a causation model that "if increase-weight then extends". However if by some empirical process we could confirm that "if extends then increase-weight", hookes law would still be the same (we can do by e.g. putting it in a moving frame of reference)
6gvONxR4sf7o 7y ago
Causal effects are (typically) defined via potential outcomes[0]. If I do X, what happens to Y? The laws of physics can be used for that, which is what I mean when I say they're causal. If our physical laws couldn't predict what effect a given cause has, we'd say they're wrong. Like if I couldn't swap out one mass on a spring (in an known scenario with an ideal spring blah blah) for another mass (cause) and predict how much lower the new mass will hang (effect), we'd say we don't understand spring physics. There are unresolved foundational questions like those discussed in your link, but those aren't practically relevant, just like missing foundations of set theory wouldn't prevent us from balancing checkbooks. There's some notion of arithmetic I'm using for my checkbook, just like there's some notion of causation when I apply a 10N force (cause) to my 1kg mass and get 1 m/s of acceleration (effect). Foundationally formalizing it is neat, but unnecessary. [0] Pretty much all of the rest of the time, they're still compatible with this definition.
- buboard 7y ago
  i see what you mean but it 's kind of circular: the causal model is assumed, then observations are made and a law is formed which can be used to make predictions. The law itself is symmetric in time though and can make predictions in reverse, so the causal model is not baked in it. Regardless, this causal model works for physics which has well defined hypotheses and well defined deterministic systems. In biology , establishing causation (and thus explanability) is tricky because , even though hypotheses are well defined, the systems are not very deterministic. In ANNs OTOH, even though the systems can be very deterministic, there is very little to make in terms of hypotheses. An explanation of the sort "you have cancer because neurons 10, 18, and 19 fired" is not satisfactory enough to pass the human test. It may be that for some complicated problems, searching for patterns in the neurons in order to explain them may prove to be futile. Not that people should give up on that, but not everything has a neat closed form explanation. Lecun mentioned above that you may have recurrent relationships (which also occur in quantum systems), and these muddy the waters a lot, making it difficult to establish cause and effect. It is also a major pain in neuroscience, when real neurons are seen as an evolving dynamical system.
  - 7y ago

srean 7y ago

> The physical laws are correlational You mean if I extend the spring the weight stretching it will increase ?

ChrisLomont 7y ago
You can do it by adding more mass in a fixed inertial frame, or by leaving the mass along and changing the reference frame. You cannot decide which happens, which is an insight from relativity. So, which was the cause? You cannot tell. You simply can do the math and see that it matches the experiment. Only later do humans try to call it cause and effect, because the math and the experiment match so often and so well. But this all can change if we find an experiment breaking the models. Before relativity, most people thought time was constant. They were wrong. So the above poster is correct in what we usually call cause and effect to simply be stronger correlations that match (current) models. That can all be changed in future understanding of Nature.
- 6gvONxR4sf7o 7y ago
  Causation: if I do X, what happens to Y? For any well understood physical situation describing X and Y, the laws of physics tell you what happens to Y when you do X. If I swap out a mass on a spring for a heavier mass, physics will predict how much lower it hangs. The cause is me swapping one mass for another. The effect is where it now hangs. That's the whole point. If it wasn't this way, we'd say our physical understanding is wrong.
  - ChrisLomont 7y ago
    Having worked on physics projects for decades, the laws of physics at the lowest level are most certainly not this simple or causal. >For any well understood physical situation describing X and Y, the laws of physics tell you what happens to Y when you do X. Not true. If I create two electrons from photon collision, and measure one, will it be spin up? No way to tell. Only aggregates about large enough systems give any reliable answer to such questions. If look for radioactive decay from 10 U235 atoms for the next 5 minutes, will I see one? Again, there is no yes/no answer, only probabilistic ones. There is no underlying causality - only purely random events with no detectable cause. These are the dice that Einstein didn't like. There's plenty of similar questions that don't have a simple answer - only answers about large aggregates. Here's [1] a recent result showing causality is no where the neat and tidy thing you think it is. These results are all over modern physics. For example, the most accurate physical theories, such as QED, get the right answer by summing forwards and backwards in time. In QED the "future" affects results today as strongly as the "past". Causality is a psychological interpretation, but is not in the math or the underlying theories. As far as I know, all quantum field theories (which underlie all physics at the moment) all have such ambiguity or uncertainty about causality. The laws are math models that give answers. But the lowest laws are time reversible or time agnostic for the most part, and the foundational theories require travelling forwards and backwards to get the correct experimental values. Similarly, the laws of physics at the lowest level are not causal, but probabilistic. Only when aggregated do some experiments seem causal. From another direction, there's a massive body of literature on what is causal, and can you detect it. Read, for example, Judea Pearl's monograph "Causality" or some of his other stuff, or simply browse wikipedia starting with him. For example, when you drop a ball, it falls to earth. Quantum mechanically, there is a probability it simply quantum tunnels to another galaxy. That it most often falls to earth becomes a law, but it is imprecise and not completely correct. It's an approximation. So every law of physics is merely a strong correlation. And there's currently plenty of experiments trying to disentangle these issues with causality and locality. So sure, at the freshman physics level causality is a simple thing. But Nature does not follow almost any of those rules with certainty or unerring rigor. Those are approximations and simplifications. [1] https://physicsworld.com/a/quantum-mechanics-defies-causal-order-experiment-confirms/ https://physicsworld.com/a/quantum-mechanics-defies-causal-o...
buboard 7y ago
hooke's law correlates spring length with force, but doesnt prescribe an order of events. You can increase the weight or move the spring to a higher gravitational field or something else we dont' know yet. However, as humans we can't reason of a way to increase the length of the spring without changing the force, therefore we establish a causation model that "if increase-weight then extends". However if by some empirical process we could confirm that "if extends then increase-weight", hookes law would still be the same (we can do by e.g. putting it in a moving frame of reference)

6gvONxR4sf7o 7y ago

Causal effects are (typically) defined via potential outcomes[0]. If I do X, what happens to Y? The laws of physics can be used for that, which is what I mean when I say they're causal. If our physical laws couldn't predict what effect a given cause has, we'd say they're wrong. Like if I couldn't swap out one mass on a spring (in an known scenario with an ideal spring blah blah) for another mass (cause) and predict how much lower the new mass will hang (effect), we'd say we don't understand spring physics. There are unresolved foundational questions like those discussed in your link, but those aren't practically relevant, just like missing foundations of set theory wouldn't prevent us from balancing checkbooks. There's some notion of arithmetic I'm using for my checkbook, just like there's some notion of causation when I apply a 10N force (cause) to my 1kg mass and get 1 m/s of acceleration (effect). Foundationally formalizing it is neat, but unnecessary. [0] Pretty much all of the rest of the time, they're still compatible with this definition.

buboard 7y ago
i see what you mean but it 's kind of circular: the causal model is assumed, then observations are made and a law is formed which can be used to make predictions. The law itself is symmetric in time though and can make predictions in reverse, so the causal model is not baked in it. Regardless, this causal model works for physics which has well defined hypotheses and well defined deterministic systems. In biology , establishing causation (and thus explanability) is tricky because , even though hypotheses are well defined, the systems are not very deterministic. In ANNs OTOH, even though the systems can be very deterministic, there is very little to make in terms of hypotheses. An explanation of the sort "you have cancer because neurons 10, 18, and 19 fired" is not satisfactory enough to pass the human test. It may be that for some complicated problems, searching for patterns in the neurons in order to explain them may prove to be futile. Not that people should give up on that, but not everything has a neat closed form explanation. Lecun mentioned above that you may have recurrent relationships (which also occur in quantum systems), and these muddy the waters a lot, making it difficult to establish cause and effect. It is also a major pain in neuroscience, when real neurons are seen as an evolving dynamical system.
- 7y ago