As a comment to an answer Conifold mentioned that "Causes may not require any energy to produce the effect".

After thinking about that, I couldn't come up with any such causes outside of metaphysical causes such as Plotinus' One or Platonic Forms. Perhaps fields could be viewed as such causes? I would be interested in causes that would be considered in a philosophy of science context.

Rather than raise the question in the comment section, I will ask it separately:

What are example of causes that do not require energy to produce an effect?

I suspect the answer is obvious, but I can't think of any example at the moment.

6 Answers 6


1) Problem with the energy of causes

Energy-neutral causation need not involve anything supernatural or extraordinary. A rock on the tracks may cause a train to derail, but it does not expend any energy to do so, it simply redirects it. So does a rail switch. Reflecting on these examples shows that there is a problem with associating energy with causes in the first place, it mixes two incompatible notions of causation. The Cartesian "folk" notion is modeled on a somebody/thing (the "agent") standing over and above the causal chains and altering them by exerting "energy" to push things along. Cognitive psychologists attest that this is how infants grasp the "metaphysical" concept of causation, see e.g. On the Birth and Growth of Concepts by Mandler:

"Michotte (1963) showed that the timing of the launching events in which one billiard ball hits another is crucial for our illusion that we see one ball ‘make’ the other move. We see a causal relation when a conflict exists between two types of continuity cues... Of course, there is more to an adult concept of cause than one object launching another. However, as Leslie showed, infants are attuned to contact versus no contact between moving objects."

This is associated this with the physical exertion and direct contact required to get the ball rolling, and the seed of the folk concept of (efficient) cause is planted.

2) Physical causation and energy

In contrast, the physical concept of causation is essentially Humean, there is no such thing in the ontology. It mimicks the folk notion by using counterfactual alterations of "initial" conditions: event X is a (partial) cause of Y, if when we alter the initial conditions to exclude X the probability of Y goes down. This is the gist of the prevailing Lewis's theory of causation, and it clearly relates to our probabilities, not to something out there. "What caused Y?" is a purely pragmatic question of singling out some X from the background (presumed fixed), but for which Y would not have happened. The distinction between "the cause" and "the background" has no ontological significance, they are simply of different importance to us.

When the background is taken into account the "causes", at best, can be said to transfer energy to the "effects", they do not stand over the stream of events to inject it. This is what energy conservation amounts to. Feynman famously exploited the idea by proposing reversible quantum computers that perform computations without expending much energy. Incidentally, this is in agreement with the Aristotelian scheme, where the only source of energy is the prime mover, everything else just passes it along (unless we take some vague passages in De Anima to mean that the "divine in all of us" agent intellect somehow partakes in the prime mover).

But we can explain what the intuition is getting at. Event X amounts to some object(s) forming a particular configuration favorable to Y occurring down the road. At first, it seems that to be causally efficacious it must pack some energy that contributes to effecting Y. But the example of a rail switch shows that this need not be so - it may instead act as a trigger, that redirects motion, or a catalyst, that facilitates a chemical reaction without getting spent. One may object (as PeterJ did in the comments) that it still takes energy to place the trigger/catalyst into a strategic position. That is true, but it is quite different from the energy that it contributes as a "cause" to its "effect". The placement energy is, in fact, the energy of agent cause, which must be contemplated if we take the counterfactual alterations as really effected. However, it turns out, as we shall see next, that even that energy can be zero in some circumstances.

3) Agent causation without energy

To speak of the energy of causes one needs to reify the counterfactuals, as the folk notion does. Somebody/thing must alter the initial conditions in re, i.e. we must admit agent causation. The "agent" need not be anthropomorphic, it can even be inanimate, but must be able to alter causal chains from the "outside" (naturally, this is linked to objective chance and free will). Can it do so without expending energy? The surprising answer is yes, in both classical and modern physics. Classical systems with non-Lipschitz forces admit initial states where the solution to the equations of motion is non-unique, the "agent" can then "pick" one, while complying with all the conservation laws. The well known example is the ball sitting at the top of the Norton Dome, which is free to "choose" the direction to roll down. The example was already known to Boussinesq in the 19th century, whom Maxwell praised for linking the idea to free will, see History of the study of indeterminism in classical mechanics:

"It may at any instant, at its own sweet will, without exerting any force or spending any energy, go off along that one of the particular paths which happens to coincide with the actual condition of the system at that instant. In most of the former methods... there was a certain small but finite amount of... trigger-work for the Will to do. Boussinesq has managed to reduce this to mathematical zero..."

This is somewhat exotic, but quantum mechanics provides a more straightforward means of accomplishing energy-neutral causation, by altering the probabilities of the wave-function collapse. This is the basis of Eccles's model of the agent causation by the human brain:

"Without violation of the conservation laws quantum selection is the only possible way of producing different final states from identical initial conditions in identical dynamical situations, and thus with the same values of the conserved quantities... We put forward the hypothesis that mental intention becomes neurally effective by momentarily increasing the probabilities for exocytoses in a whole dendron and, in this way, couples the large number of probability amplitudes to produce coherent action."

Esfeld in Is Quantum Indeterminism Relevant to Free Will? gives a critical discussion of Eccles's (and other "quantum mind" Cartesian interactionism) proposals. The problem is that while this mechanism escapes violating the conservation laws it still requires violating physical laws, namely the probability distributions prescribed by quantum mechanics. And, at present at least, we have no empirical evidence that they are so violated:

"If intentions, as conceived in the framework of interactionism, are to exert a regular influence on physical events, the probability rules of the physical theory in question do not indicate the complete probabilities of these physical events... In short, instead of having to endorse an additional force for metaphysical reasons, we have to endorse a change to the probabilities that a physical theory indicates for metaphysical reasons."

Be it as it may, even agent causes can act effectively without any energy costs. What is required of such causes is injecting information, objectively conceived, into the stream of events, not energy.

  • I would have thought a rock on the tracks will have no effect unless energy is expended in the collision with the train. Mostly heat and a little noise. If not then we have fiound an energhy-free way to crush rocks.
    – user20253
    Commented Feb 15, 2019 at 14:26
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    @PeterJ Take it to simply deflect the train off the tracks, like a rail switch, or like a rigid obstacle deflects a ball off of its original path. Its energy remains the same. The heat and the noise come from the train's kinetic energy, not the rock's or switch's.
    – Conifold
    Commented Feb 15, 2019 at 14:29
  • Okay. But why doesn't the energy of the train count as part of the cause? If the train isn't moving then the rock will not cause an effect. Doesn't the the effect require energy to be expended? .
    – user20253
    Commented Feb 15, 2019 at 15:27
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    @PeterJ If you want to say that the mixing folk and physical ideas about "causing" leads to incoherence you'll get no argument from me. What you are saying makes sense if we imagine some agent doing those things to the rock, the train, etc., from some background state where they weren't already there. But that physical interactions involve energy exchanges does not mean that folk "causes" contribute the energy. The event is not "energy-free", but the rock as the "cause" is energy-neutral.
    – Conifold
    Commented Feb 15, 2019 at 15:49
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    @ManudeHanoi If it comes from the train then you agree that the rock does not expend it. And there is nothing in mechanics forbidding the Norton dome, so the impossibility is, at best, a conjecture about as yet unknown laws of nature. Maxwell did not think that triggering must contribute energy, for example, and even without Norton dome such energy can be reduced to arbitrarily small amounts. What then is wrong?
    – Conifold
    Commented Feb 18, 2019 at 4:39

The question would benefit from some clarification. Energy (or perhaps more precisely mass-energy) is a conserved quantity. So in this general sense nothing 'requires' energy; energy just gets transformed or transferred. A body accelerating under the influence of gravity is an example of gravitational potential energy being transformed into kinetic energy. An elastic collision between bodies is an example of a transfer of kinetic energy without any net transformation into a different kind. One might imagine a configuration of colliding bodies where the resulting speeds remain the same and only the directions change. There would then be no net transformation or transfer of energy, though one might describe the collision as the cause of the change in directions.

Often when we think of some action or activity as 'requiring' energy, what we are referring to is free energy. The bulb in my flashlight requires a source of free energy from the battery to cause it to light up. The cells in my muscles require a source of chemical free energy in order to allow me to contract my muscles and cause things to move. Free energy is not conserved and can be lost by being dissipated as heat. So your question might be interpretable as: Can some physical cause and effect relation happen without loss (or expenditure) of free energy? The answer is: only if it is reversible, in the thermodynamic sense. At the macroscopic level of the world at which we operate, processes are all irreversible, so there is always some free energy required which will end up as heat. At the quantum level, particles may behave reversibly. The connection between the two is the concern of statistical mechanics.

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    Perhaps the original answer from which this is taken involves loss of free energy. I'm not yet sure. +1 Commented Feb 15, 2019 at 12:46

1-Energy is conserved, if it is spent from a form of energy, then in is earned in another form

2-Energy doesnt necessarily "belong" to an object or another, for example, gravity potential energy depends on 2 objects

3-Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. Therefore an object can move (more precisely, it can keep on moving) without energy transfer. So an object here now with a given speed (cause), can be there then (consequence), without energy transfer or expenditure.

4-Observing a phenomenon (a consequence) always requires energy transfer

  • Good point that an object continues in motion with the same velocity without requiring energy. It is not making any change that would require energy. From its perspective it is standing still. +1 Commented Feb 18, 2019 at 12:33

While energy is in movement, it is expressed as the product of an effort variable, a flow variable, and time (for example, voltage x current x time = watt-hours).

It is possible to define an energy flow in which either the effort variable or the flow variable is suppressed, in which case the flow is called a signal.

Signals propagate causation (that is, they transfer information) with zero energy flow: they can control other processes without themselves expending energy to do so.

If the flow variable is suppressed, the resulting effort signal is said to be a high-impedance signal. If the effort variable is suppressed, the resulting flow signal is said to be a low-impedance signal.

An example of a high-impedance signal is a scuba regulator, which senses ambient pressure (effort) and controls the flow rate of high-pressure air out of the air tank. An example of a low-impedance signal is a stage microphone, which senses pressure changes (sound waves) and produces a current (flow) signal as an output, which then controls the flow of power to a loudspeaker by way of an amplifier.

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    Good answer, but I'm dubious about the claim that a signal propagates causation with zero energy flow. For one thing, in real world applications, a signal's medium will always have some impedance. Also, for a signal to achieve some effect, it must interact with something and be processed, which implies driving a load, or performing a computation. Either of these will consume free energy if they are irreversible.
    – Bumble
    Commented Feb 19, 2019 at 9:13

A ball will move under the influence of gravity. No energy is required to "cause" this. In fact, the total energy of the ball remains a constant (where the total energy is given as potential plus kinetic energy).

  • The ball only moves when a support keeping it from moving has been removed which requires work.
    – Cell
    Commented Feb 15, 2019 at 2:23
  • This would be a field example, but isn't energy involved? +1 Commented Feb 15, 2019 at 2:25
  • @Frank Hubeny Energy is involved but I’m not quite sure if I would say it causes the motion. In fact, energy is involved in any physical event - what is your definition of “causes”?
    – Jbag1212
    Commented Feb 15, 2019 at 6:03
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    @Jbag1212 depending on the altitude, there is potential energy, which transforms into kinetic energy as the ball moves to the ground. Potential energy is energy because the only way to cancel it is to provide a floor that pushes up with a certain force.
    – SmootQ
    Commented Feb 15, 2019 at 8:43
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    There is a concern about what causes are, but I don't know enough about that either . I would accept a definition of cause that did not require the use of energy if an example was convincing enough. Indeed, I would consider metaphysical causes as not requiring energy. They may perhaps create the energy itself. But I am excluding them in the question restricting it to causes as used in the philosophy of science. Commented Feb 15, 2019 at 10:37

Radioactivity is a spontaneous decay that does not need any energy but sets some free.

As the others mentioned your question might need to be refined a bit - this post here answers the part of "a system that does something without having added energy to it right before".

Another possibility to argue against is that every system has been added energy or there was already energy when the definition of that system started.

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