I don’t fully trust my knowledge in this domain, but this particular example seems questionable to me just because “current sources” are kind of weird. I’ll throw about a few ideas from my undergrad EE (I didn’t focused on the electromagnetism side, so I’m a bit weak here).
Mentally, I use the abstraction that voltage (differences in electrical potential) causes current flows.
This probably isn’t quite right, but “current sources” are in some sense a bit fictitious. The defining feature is that they maintain constant current regardless of the load placed across the terminals, but in practice, you can set up a device that behaves like that by supplying whatever voltage is necessary to maintain a fixed current. So you can model a “current-source” as “a device which adapts its voltage difference to produce constant current”, which is compatible with a “voltage causes current” paradigm.
All real current sources have a limited range they can operate over dependent on how much voltage they can supply. If you had a truly ideal current source, you’d have an infinite energy machine.
See the Wiki entry on current sources, particularly the implementations. I didn’t read through these, but a glance at several says they’re on the inside they involve configurations of voltage sources. Figure 3 is pretty clear demonstration of how “current source” is made by an adaptive voltage source.
Caption: In an op-amp voltage-controlled current source the op-amp compensates the voltage drop across the load by adding the same voltage to the exciting input voltage.
Now, notwithstanding that, there are still interesting questions about causality. (Again proceeding with pretty entry-level knowledge of the physics here– I’m hoping someone will show up and add certainty and clarity.) There might be some clarity from thinking about charge instead of voltage and current. We observe that if you have electric potential differences (more charge concentrated in a place than elsewhere) and a conductive path between them, then you get current flows. Of course, you get differences in charge concentrations by moving charges around in the first place, i.e. also current flows. [The “charge movement” picture gets more complicated by how things like moving magnetic fields create voltage differences/current flows, I’m really sure how to unify the two.]
Instructively, electric potential energy is similar in some ways to gravitational potential energy. At least, both are conservative forces obeying inverse square laws. I can get gravitational potential energy by moving two bits of mass apart. If I release them and there’s a pass, the potential energy gets turned into kinetic energy and they move together. Of course, to separate them I had to move mass around. The motion of rolling a boulder up a hill and the motion of letting it roll down a
Electric potentials seem the same (at least when thinking about electrostatics). Separating charge (current flow) creates potential differences which can be released and translate into motion (current flow).
In terms of the causality though, there seems to be something asymmetric. In some cases I’m putting energy into the system, causing motion, and building up potential energy (be it electric or gravitational). In other cases, I’m extracting energy from the system by letting it be used up to create motion.
Cases where you have a current source that’s giving you energy, it probably it is the case that the potential difference can be described as the cause of the flow (even if potential difference produced by a device is adaptive somehow to get fixed rate of motion/current). No one thinks that the motion of the car causes the combustion (use of potential chemical energy) rather than the other way round even if I built my engine to produce fixed speed no matter the mass the vehicle it’s in.
I would venture that any competent electrical engineer has a picture at least this detailed and definitely does not think of voltage sources and current sources as black boxes rather than high-level descriptions of underlying physics which lead to very concrete and different predictions.
One problem I’ve been chewing on is how to think about causality and abstraction in the presence of a feedback controller. An un-blackboxed current supply is one example—my understanding it that they’re typically implemented as a voltage supply with a feedback controller. Diving down into the low-level implementation details (charge, fields, etc) is certainly one way to get a valid causal picture. But I also think that abstract causal models can be “correct” in some substantive but not-as-yet-well-understood sense, even when they differ from the underlying physical causality.
An example with the same issues as a current supply, but is (hopefully) conceptually a bit simpler, is a thermostat. At the physical level, there’s a feedback loop: the thermostat measures temperature, compares it to the target temperature, and adjusts the fuel burn rate up/down accordingly. But at the abstract level, I turn a knob on the thermostat, and that causes the temperature to change. I think there is a meaningful sense in which that abstract model is correct. By contrast, an abstract model which says “the change in room temperature a few minutes from now causes me to turn the knob on the thermostat” would be incorrect, as would a causal model in which the two are unconnected.
So… yes, the example given clearly does not match the underlying physical causality for a current supply. On the other hand, the same can be said with the voltage supply; the macroscopic measured behavior results from back-and-forth causal arrows between the EM fields and the charges. And that’s all before we get down to quantum mechanics, at which point physical causality gets even more complicated. Point is: all of these models are operating at a pretty high level of abstraction, compared to the underlying physical reality. But it still seems like some abstract causal models are “right” and others are “wrong”.
The OP is about what might underlie that intuition—what “right” and “wrong” mean for abstract causal models.
Yeah, that all seems fair/right/good and I see what you’re getting at. I got nerdsniped by the current source example because it was familiar and I felt as phrased it got in the way of the core idea you were going for.
The person who properly introduced me to Pearl’s causality stuff had an example which seems good here and definitely erodes the notion of causality being uni-directional in time. It seems equivalent to the thermostat one, I think.
Suppose I’m a politician seeking election:
At time t0, I campaign on a platform which causes people to vote for me at time t1.
On one hand, my choice of campaign is seemingly the cause of people voting for me afterwards.
On another hand, I chose the platform I did because of an action which would occur afterwards, i.e. the voting. If I didn’t have a model that people would vote for a given platform, I wouldn’t have chosen that platform. My model/prediction is of a real-world thing. So it kinda seems a bit like the causality flows backwards in time. The voting causes the campaign choice same as the temperature changing in response to knob-turning causes the knob-turning.
I like the framing that the questions can be posed both for voltage supply and current supply, that seems more on track to me.
This and the parent comment were quite helpful for getting a more nuanced sense of what you’re up to.
Point is: all of these models are operating at a pretty high level of abstraction, compared to the underlying physical reality. But it still seems like some abstract causal models are “right” and others are “wrong”.
There would be an analogous example in hydraulics where positive displacement pumps are constant flow (~current) sources and centrifugal pumps are constant(ish*) pressure (~voltage) sources. The resistor would be a throttle.
In this case it is the underlying physical nature of the types of pumps which causes the effect rather than a feedback loop.
This probably isn’t quite right, but “current sources” are in some sense a bit fictitious. The defining feature is that they maintain constant current regardless of the load placed across the terminals, but in practice, you can set up a device that behaves like that by supplying whatever voltage is necessary to maintain a fixed current. So you can model a “current-source” as “a device which adapts its voltage difference to produce constant current”, which is compatible with a “voltage causes current” paradigm.
All real current sources have a limited range they can operate over dependent on how much voltage they can supply. If you had a truly ideal current source, you’d have an infinite energy machine.
This is all true, however, voltage sources are equally fictitious, and a truly ideal voltage source would also be an infinite energy machine. As you increase the load, a real-life voltage source will start to behave more like a current source (and eventually like a smoke generating machine).
Yes, I suppose that’s right too. A voltage source can’t supply infinite current, i.e. can’t maintain that voltage is the load’s resistance is too low, e.g. a perfectly conductive path.
I don’t fully trust my knowledge in this domain, but this particular example seems questionable to me just because “current sources” are kind of weird. I’ll throw about a few ideas from my undergrad EE (I didn’t focused on the electromagnetism side, so I’m a bit weak here).
Mentally, I use the abstraction that voltage (differences in electrical potential) causes current flows.
This probably isn’t quite right, but “current sources” are in some sense a bit fictitious. The defining feature is that they maintain constant current regardless of the load placed across the terminals, but in practice, you can set up a device that behaves like that by supplying whatever voltage is necessary to maintain a fixed current. So you can model a “current-source” as “a device which adapts its voltage difference to produce constant current”, which is compatible with a “voltage causes current” paradigm.
All real current sources have a limited range they can operate over dependent on how much voltage they can supply. If you had a truly ideal current source, you’d have an infinite energy machine.
See the Wiki entry on current sources, particularly the implementations. I didn’t read through these, but a glance at several says they’re on the inside they involve configurations of voltage sources. Figure 3 is pretty clear demonstration of how “current source” is made by an adaptive voltage source.
Caption: In an op-amp voltage-controlled current source the op-amp compensates the voltage drop across the load by adding the same voltage to the exciting input voltage.
Now, notwithstanding that, there are still interesting questions about causality. (Again proceeding with pretty entry-level knowledge of the physics here– I’m hoping someone will show up and add certainty and clarity.) There might be some clarity from thinking about charge instead of voltage and current. We observe that if you have electric potential differences (more charge concentrated in a place than elsewhere) and a conductive path between them, then you get current flows. Of course, you get differences in charge concentrations by moving charges around in the first place, i.e. also current flows. [The “charge movement” picture gets more complicated by how things like moving magnetic fields create voltage differences/current flows, I’m really sure how to unify the two.]
Instructively, electric potential energy is similar in some ways to gravitational potential energy. At least, both are conservative forces obeying inverse square laws. I can get gravitational potential energy by moving two bits of mass apart. If I release them and there’s a pass, the potential energy gets turned into kinetic energy and they move together. Of course, to separate them I had to move mass around. The motion of rolling a boulder up a hill and the motion of letting it roll down a
Electric potentials seem the same (at least when thinking about electrostatics). Separating charge (current flow) creates potential differences which can be released and translate into motion (current flow).
In terms of the causality though, there seems to be something asymmetric. In some cases I’m putting energy into the system, causing motion, and building up potential energy (be it electric or gravitational). In other cases, I’m extracting energy from the system by letting it be used up to create motion.
Cases where you have a current source that’s giving you energy, it probably it is the case that the potential difference can be described as the cause of the flow (even if potential difference produced by a device is adaptive somehow to get fixed rate of motion/current). No one thinks that the motion of the car causes the combustion (use of potential chemical energy) rather than the other way round even if I built my engine to produce fixed speed no matter the mass the vehicle it’s in.
I would venture that any competent electrical engineer has a picture at least this detailed and definitely does not think of voltage sources and current sources as black boxes rather than high-level descriptions of underlying physics which lead to very concrete and different predictions.
One problem I’ve been chewing on is how to think about causality and abstraction in the presence of a feedback controller. An un-blackboxed current supply is one example—my understanding it that they’re typically implemented as a voltage supply with a feedback controller. Diving down into the low-level implementation details (charge, fields, etc) is certainly one way to get a valid causal picture. But I also think that abstract causal models can be “correct” in some substantive but not-as-yet-well-understood sense, even when they differ from the underlying physical causality.
An example with the same issues as a current supply, but is (hopefully) conceptually a bit simpler, is a thermostat. At the physical level, there’s a feedback loop: the thermostat measures temperature, compares it to the target temperature, and adjusts the fuel burn rate up/down accordingly. But at the abstract level, I turn a knob on the thermostat, and that causes the temperature to change. I think there is a meaningful sense in which that abstract model is correct. By contrast, an abstract model which says “the change in room temperature a few minutes from now causes me to turn the knob on the thermostat” would be incorrect, as would a causal model in which the two are unconnected.
So… yes, the example given clearly does not match the underlying physical causality for a current supply. On the other hand, the same can be said with the voltage supply; the macroscopic measured behavior results from back-and-forth causal arrows between the EM fields and the charges. And that’s all before we get down to quantum mechanics, at which point physical causality gets even more complicated. Point is: all of these models are operating at a pretty high level of abstraction, compared to the underlying physical reality. But it still seems like some abstract causal models are “right” and others are “wrong”.
The OP is about what might underlie that intuition—what “right” and “wrong” mean for abstract causal models.
Yeah, that all seems fair/right/good and I see what you’re getting at. I got nerdsniped by the current source example because it was familiar and I felt as phrased it got in the way of the core idea you were going for.
The person who properly introduced me to Pearl’s causality stuff had an example which seems good here and definitely erodes the notion of causality being uni-directional in time. It seems equivalent to the thermostat one, I think.
Suppose I’m a politician seeking election:
At time t0, I campaign on a platform which causes people to vote for me at time t1.
On one hand, my choice of campaign is seemingly the cause of people voting for me afterwards.
On another hand, I chose the platform I did because of an action which would occur afterwards, i.e. the voting. If I didn’t have a model that people would vote for a given platform, I wouldn’t have chosen that platform. My model/prediction is of a real-world thing. So it kinda seems a bit like the causality flows backwards in time. The voting causes the campaign choice same as the temperature changing in response to knob-turning causes the knob-turning.
I like the framing that the questions can be posed both for voltage supply and current supply, that seems more on track to me.
Positive reinforcement for noticing getting nerdsniped and mentioning it!
This and the parent comment were quite helpful for getting a more nuanced sense of what you’re up to.
Good summary.
There would be an analogous example in hydraulics where positive displacement pumps are constant flow (~current) sources and centrifugal pumps are constant(ish*) pressure (~voltage) sources. The resistor would be a throttle.
In this case it is the underlying physical nature of the types of pumps which causes the effect rather than a feedback loop.
*At least at lower flow rates.
Really nice example, thanks!
This is all true, however, voltage sources are equally fictitious, and a truly ideal voltage source would also be an infinite energy machine. As you increase the load, a real-life voltage source will start to behave more like a current source (and eventually like a smoke generating machine).
Yes, I suppose that’s right too. A voltage source can’t supply infinite current, i.e. can’t maintain that voltage is the load’s resistance is too low, e.g. a perfectly conductive path.