Does the fact that the fundamental laws are symmetric with respect to direction of time show that causation does not exist? Since causality always requires the cause to precede the effect, but laws of physics do not care about which direction time flows

• Causality is time asymmetric • Laws of physics are time symmetric -> There is no (fundamental) notion of cause in physics

Is this correct? Could anyone suggest some articles/papers about this?

  • IIRC this is one of the reasons to favour Copenhagen interpretation, i.e. wave function collapse. But to be fair, I am not well-read in this kind of philosophy of science.
    – Philip Klöcking
    Jul 4, 2019 at 13:37
  • Even if we overlook subtle asymmetries in the fundamental laws, they do not determine the course of events alone, initial conditions also do. And our universe started in a state of very low entropy, which is why we have the highly time-asymmetric second law of thermodynamics, which is often associated with the arrow of time and causation.
    – Conifold
    Jul 5, 2019 at 11:17
  • @Josh Forkosh - While it's true that the most fundamental laws we know of are CPT-symmetric rather than just T-symmetric, your example isn't a good one since time reversal symmetry is generally defined in a way that makes classical electromagnetism time-symmetric, and if two positive charges have initial velocity vectors towards one another they will move closer and then farther apart, so the same will be true if you play the movie backwards (and their trajectories will appear to be consistent with the same laws of EM when you play it backwards)
    – Hypnosifl
    Jul 5, 2019 at 16:05

1 Answer 1


What's missing from the picture is entropy; the tendency toward greater entropy establishes an effective 'direction of time' that correlates with our perceived sense of time passing. We see eggs break rather than spontaneously forming from broken pieces. We can conceive of physical trajectories that would 'reform an egg' due to reversibility, but in practice, entropy means that these trajectories are too improbable to occur.

Partly this is a matter of how we perceive and categorize the world. We can't perceive all the microscopic details of most 'disordered outcomes', so we lump all those states together in our mind into a general sense: "the egg is broken". There are more total ways for an egg to be broken than for it to be whole, strongly favoring breakage once we overcome the energy barrier holding the egg in its metastable 'whole' state.

As for why perceptually we feel as if we're following the thread of causality along the path of greater entropy, well, that's an open question. Certainly we can talk about causality from within this framework that feels most natural to us, so it doesn't really make sense to say that it doesn't exist. It does certainly raise questions about whether this particular frame for causality is actually privileged, though, as it's not inconceivable that perception could operate in the other 'direction'.

  • But not all physics laws care about entropy. For example conservation of momentum or even the ideal gas law for an isentropic process.
    – Cell
    Jul 4, 2019 at 23:02
  • Time asymmetry due to increasing entropy is not thought to be built into the fundamental laws of physics, instead it's thought to be a consequence of the low-entropy conditions near the big bang, although not enough is understood about quantum gravity and quantum cosmology for it to be well-understood why those early conditions were so low-entropy...you can see an example of a proposal for a possible explanation at preposterousuniverse.blogspot.com/…
    – Hypnosifl
    Jul 5, 2019 at 13:49

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