Do physicists need the idea of 'cause' or 'causation' to do physics?

Does it appear in physics, either in theory, answered here, or in experiment? In a way analogous to how names and mathematics do.

If not, then it seems like a folk concept which exists only to help us learn physics or be inspired by it.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – David Z
    Jan 28, 2017 at 2:27
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    $\begingroup$ The notion of "cause" doesn't serve much purpose. If you say "A is caused by B", all you have done is replace the question "what causes A?" by "what causes B?" Of course some philosophers used that to conclude there must be an ultimate cause of "everything" (a.k.a. "god") but again, that conclusion (even if it is true) doesn't have much scientific value. The essential feature of a "good" scientific theory is that a small number of assumptions explain many things - but "explanation" is not the same as "causation". $\endgroup$
    – alephzero
    Jan 28, 2017 at 4:47
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    $\begingroup$ I'm voting to close this question as off-topic because it is a question about metaphysics, or the philosophy of science, not physics. $\endgroup$ Jan 28, 2017 at 21:23
  • $\begingroup$ @sammygerbil i disagree, it isn't a metaphysical question, which would be e.g. asking what you think causation is. i ask whether theory and experiment involves 'causation', no different to any other term that might appear in them $\endgroup$ Jan 29, 2017 at 6:06

3 Answers 3


It turns out that this is a very, very deep question, and I don't think there is a final authoritative answer on this. What we can say with certainty is that, contrary to what some of the comments to the original question imply, physics as a whole is absolutely not "all about finding causes". Indeed, a strong argument can be made that mature physical theories do not need nor use the concept of causes.

For example, in one of his Messenger Lectures at Cornell on "The Character of Physical Law" (Lecture 2 on "The Relation of Mathematics to Physics", worth watching no matter what...) Feynman discusses the example of Newton's theory of gravitation, and how it can be formulated in radically different ways: In Newtonian Mechanics we have a somewhat mysterious "action at a distance", with forces causing the specific motion of planets, whereas in modern formulations of classical mechanics we end up with an integral variational principle that does not (easily) map on any concept of causation (and almost looks teleological). And finally in general relativity, the picture changes again.

For some food for thought I would recommend John Norton's paper on Causation as a Folk Science. I will state for the record that I do not necessarily agree with everything Norton says on that topic, but his thoughts are certainly well worth considering.

Other than that, this question clearly veers quite far into the area of (legitimate) philosophy. Not that it's not also a good question to ask in this forum, but philosophers may have something to offer on this question as well.

  • $\begingroup$ i accpted this answer cos a) it answers the question and b) it answers it well. thanks! $\endgroup$ Jan 28, 2017 at 2:33
  • $\begingroup$ Thanks for posting a very thoughtful answer that makes me question my assumptions. But I bet I can dig up another Feynman quote that does stress the word why a lot :), although perhaps they were aimed at a different audience. I have deleted my comments, because the question is at a deeper level. What I am slightly afraid of is that modern physics has lost its way a bit, because the easy questions are sorted and the hard ones may involve moving the goalposts as to the purpose of physics. $\endgroup$
    – user140606
    Jan 28, 2017 at 2:53
  • $\begingroup$ The anthropic principle might be considered an example, and in my opinion string theory needs to produce something really significant within the next 20 years. Please don't misunderstood me, I am absolutely not reading that into your answer, it simply is my own view that without the basic why question, coupled with a lack of experimental techniques to verify the current vast amount of speculation related to regimes we may never reach, physics inevitability will veer towards philosophy. Thanks again for posting an answer that made me think. $\endgroup$
    – user140606
    Jan 28, 2017 at 2:53
  • $\begingroup$ Coincidentally, the anthropic principle, platonism, and the copernican principle converge into an extreme form known as the multiverse of Max Tegmark. More extreme than set theory! $\endgroup$
    – user126422
    Jan 28, 2017 at 3:08
  • $\begingroup$ @AlbertAspect The more you appear on TV today, the higher price you pay in the respect of your peers :). I think Penrose has a math based reality as a belief, but I don't think he needs or wants the multiverse to (dramatize is too unkind :) it. But would someone, somewhere, please produce evidence to support their ideas :) ? $\endgroup$
    – user140606
    Jan 28, 2017 at 3:25

Well, I will try an answer. I will try without worrying too much about 'deep philosophy', but just something about how it is used in physics. And without saying that you have to go read something. I may misstate a couple things, but will try to be as accurate as I can.

But I will stay away from philosophy or pure interpretation.

I think this below is not too different than what Countto10 is saying and possibly also JMLCarter

First various examples on how causality is so deeply embedded in the physics we know, and the physics we are investigating. Actually the summ of all examples is the main point.

1) in the Large Scale Structure of Spacetime, a book by Hawking, he laid out the way General Relativity (GR) is causal. It is really about the causal structure of Spacetime. It's a hard topic because GR allows you to do physics in any coordinate system, and the time sequence of things are not obvious sometimes. Penrose and others found coordinate systems, for example for Black Holes and other geometries, where one could see the causal structure. It involved creating coordinate systems where light rays travel at 45 degree angles, like in Minkowski spacetime. Those then define light cones, and GR, as well as Special Relativity (SR), say that an event can be causally connected to another if it is in the future light cone of it. Doesn't mean it is the cause, but it could be. If it isn't inside the light cone they are causally disconnected. The book is https://www.amazon.com/Structure-Space-Time-Cambridge-Monographs-Mathematical/dp/0521099064, but there's plenty of the topic in Wikipedia and accessible articles. In this fashion we can get also event horizons, such as the Black Hole horizon, or the Universe's event horizon, where anything beyond those can't affect (cannot possibly be a cause for) anything that happens on this side (yes, watch out what side you're on) of the horizon.

2) Those kinds of causality constraints started with SR imposing, and over time having it be accepted in physics, that matter/energy cannot go faster than light, and by the way, also information as is now also accepted. And all experiments and observations done are consistent with that.

3) So in Quantum Field Theory (QFT) the same holds, but in the language of quantum theory. When QFT was developed having causality be embedded in it was an important criteria. For instance, the expected value of a propagator of a QF is 0 for two states at two spacetime points which are outside their respective light cones. So two states that have a space like separation have a zero transition probability (now, I do remember on Tongs QFT course that in some cases it is not zero but approaches zero exponentially quickly, so there some fine points to account for, due to quantum's probabilistic nature). But it does not change the main fact of causal QFT.

4) certainly Quantum Electrodynamics follows that, as you'd expect since photons travel at c. But the theories fo electroweak forces and strong forces were done also with those causality constraints imposed.

5) So do physicists worry about what causes what? We do expect everything we study and everything we see to have an explanation - if it didn't it'd be magic. Physics officially does not believe in magic per say, but it does believe it doesn't know everything, and likes to find and explain magical things.

6) An example of one of those is entanglement. It's been explained, it's cause, and why two particles separated by spacelike distances can still be correlated. But there's still some doubt and some thinking on some of the entanglement effects that have been observed. Still, physics keeps looking for the description and use of it. Going further is the who,e set of work being done in the AdS/CFT correspondence, where there is a holographic relationship that look non-local, and still obeys some kinds of laws. There is more investigations in those areas, for quantum gravity and for information theory. Maybe cause and effect is too Linear A relationship, but still the thinking is that there would be a physics law that relates them. I'd call that, in simplistic terms, cause and effect.

7) So finally, yes, all physics looks for explanations and have them explain new observations, and only when you get consistent enough on it is it a physical law. And it may change as we learn more, it m turn I it a more complex law that reduces to the earlier one in the right limits. Thus in QFT and really all physics there is what is called 'effective field theory', where as one goes to higher energy or smaller scales a new theory is needed, which reduces to the earlier one at lower energies.

So, is that an answer to cause and effect? It is where we are in physics, and it is close enough to some idea of cause effect that only philosophy can find better terms

  • $\begingroup$ Hi Bob, I deleted my comments as too naive. But I have given, my probably equally naive, opinions below Pirx answer. In my view, physics has come up with lots of exciting new verifications of (100 year old theories), the LIGO experiment and non locality ( 50 years since Bell's Theorem), but as the harder questions arise, we need to stick to the mission statement, Physics is about why and empirically derived evidence, no matter how hard to obtain, is the bedrock of it. Otherwise, we are back where the Greeks were, sorry to be so pessimistic. But any other way is not physics, IMO. $\endgroup$
    – user140606
    Jan 28, 2017 at 3:11
  • $\begingroup$ I don't disagree. But the why I think is not in conflict with cause and effect, nor in other other methods in physics. Why is why just try to go deeper, nothing wrong with that, physics always learned more that way, as long as it was grounded in facts also. I don't understand why pessimistic (except for spell checker, darn it, will fix). Yes, it's harder, but it always was harder. And we've been stuck in not basic advances since GR and QFT, but not as long as the Greeks had to wait for Newton. We just can't see the higher energy physics, how can we theorize well that much? $\endgroup$
    – Bob Bee
    Jan 28, 2017 at 3:32
  • $\begingroup$ You are right (and you know all this already), but thinking just now, take string theory. If it pans out to be true (and I don't know enough to judge), we may not need accelerators of the size currently calculated. If we find that very close in to any particle you name, it's charge does not follow the expected law and is lower in value, then possibly, there is your compacted dimension. So we find out indirectly. As I self study, I get genuine fun out of learning that calculating basic QFT scattering amps is not that scary. That's my kinda physics :) 4 am here, packing it in.... $\endgroup$
    – user140606
    Jan 28, 2017 at 3:56
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    $\begingroup$ Yes, it'd be cool if string theory was true. Still, only not super microscopic extra dimensions are needed to have any significant change in the power laws at shorter ranges. But you're right, there's all kinds of mystery breaking possibilities that might hold up. Go to sleep, only 11 here $\endgroup$
    – Bob Bee
    Jan 28, 2017 at 4:11
  • $\begingroup$ Yeah, but unfortunately, nobody knows if string theory makes any sense as a physical theory, let alone whether it's "true" (whatever that may mean...). Personally, I think string theory is in the set of theories that Wolfgang Pauli would have put in the class of theories that are "not even wrong". $\endgroup$
    – Pirx
    Jan 28, 2017 at 19:14

Most definitions of cause involve some form of "arrow" of time. By "arrow" I mean that time is assumed to be unidirectional: causes are in the past and consequences in the future (I am not talking about the thermodynamical arrow of time). For our universe, however, it seems that most fundamental physics theories have a time reversible interpretation, in which causes can become effects and effects can become causes. This kind of arrow of time also appear in formal systems: theorems follows from axioms. However, you can use aesthetics to decide which principle is an axiom and which one is a theorem (theorems can become axioms and vice versa). In the same way, in physics you can chose what is in the past and what is the future (by this I mean the direction of the "arrow of time"). Viewed in this way, causality in its standard meaning no longer exists.

Causality is not apparent from the reversible theory, but is still important implicitly. But this sense of causality comes after the facts. By this I mean that the "reversible=non-causal" theory appears first and then some problems (mostly paradoxes) appear which need to be explained using additional assumptions. One example is Norton's dome, which seems to show that Newtonian mechanics has solutions in which effects are non-causal (like the bigbang, they happen spontaneously). Another example is the grandfather paradox, which some proposed it is solved by adding the principle of "cosmic censorship". In both examples you can eliminate those paradoxes if you add some constraints that were not initially part of the theory.

There are other possibilities too. Some macroscopic theories (for instance classical thermodynamics), or physics of alternative universes (for instance some cellular automata) are not time reversible. This introduces a non ambiguous arrow of time, in the sense that in one direction the past predicts the future, but in the other direction it does not.

You could also ask, like in quantum mechanics, that your theory be only predictive probabilistically. So, the effect can happen by chance, among a set of possibilities. But I do not want to discuss quantum mechanics. It has so many issues!

  • $\begingroup$ I agree the arrow of time has relevance to causality, but really on the boundaries only. Once you have an arrow of time you have causality, or some form of it as I described. The question of the arrow of time is related and not solved, though most people think there is and we know there is, just don't know exactly how it affects the issue) a breaking of T symmetry in some weak a force effects (CP is broken, CPT holds, T is broken). None of that really affects causality, in that something affects something else, with time being ....... Continued in next comment $\endgroup$
    – Bob Bee
    Jan 28, 2017 at 20:19
  • $\begingroup$ direction, but cause and effect intertwined. If there were a break of causality, it's been clear in all phsycis discussions that then anything could come out of anywhere, the whole of physical law breaks down. And yes, time's arrow remains a mystery, but all we have observed has a past and future, so it's even more speculative than any of the paradoxes you mention but don't cite, which have been mostly debunked. No sense mixing paradoxical a discussions with the main physics goals of finding explanations and predicting them, i.e., finding cause and effect. $\endgroup$
    – Bob Bee
    Jan 28, 2017 at 20:25
  • $\begingroup$ @BobBee I think the original was very confusing, I added the examples you asked, and some clarifications. Also, I deleted the reference to your example as I am not completely sure if they belong to this category. And finally, by time reversible I do not mean time symmetric, I mean that the past still predict the future. $\endgroup$
    – user126422
    Jan 29, 2017 at 3:12
  • $\begingroup$ I like your posting better now. I understand now what you mean by after the fact constraints. I just don't see physics as math, where everything has to be math-proof. After all we know about Godel's theorems. There has to be some physical constraints that are not in theorems, and unless and until it leads to some real physical problem it doesn't bother me much. But I do recognize that sometimes (just not usually) it can point to something that really needs fixing in the theory. Thanks for clarifying and for not including QM...I don't feel like even writing Copenhagen (oops) $\endgroup$
    – Bob Bee
    Jan 29, 2017 at 6:17

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