The Arrow of Time from an Overlooked Physical Law


In this post, I’m going to disagree with the following statement by physicist Sean Carroll concerning the nature of time:

“The weird thing about the arrow of time is that it’s not to be found in the underlying laws of physics. It’s not there. So it’s a feature of the universe we see, but not a feature of the laws of the individual particles. So the arrow of time is built on top of whatever local laws of physics apply.”–Sean Carroll,

That is a common position, but it could very well be wrong. Specifically, what could be wrong with it is the claim that the arrow of time is “not to be found in the underlying laws of physics.” That claim comes from ignoring the possibility that there could be real, dynamical, irreversible collapse in quantum theory. If there is such collapse, that provides the missing link between physical theory and the phenomena we see that reflect the arrow of time.

First, it should be noted that collapse has been a formal part of standard quantum mechanics since the brilliant mathematician/physicist John von Neumann formalized the theory back in the 1920s. Von Neumann referred explicitly to collapse as a discontinuous, indeterministic process, and noted that it was irreversible. However, in recent decades, it has become fashionable to ignore collapse, which means to (explicitly or implicity) use an Everettian or “Many-Worlds” approach to quantum theory. The Everettian approach denies that collapse ever occurs, so in that interpretation, all the laws are time-reversible. This assumption underlies the usual negative conclusion (exemplified above by Carroll’s statement) about the existence of any physical law that could account for the irreversibility we see around us.

This evolution toward Everettianism has occurred for several reasons, probably chief among them the ad hoc nature of many of the specific models of collapse, which make changes to quantum theory. Alternatively, many physicists assume that collapse is just something that happens in our minds–that it corresponds to updating our own subjective information about the world as we advance through spacetime. But in that case, it is assumed that we somehow ‘move through’ the world, following an unexplained arrow of time. Clearly, if we are going to just help ourselves to an arrow of time in our ‘movement through the world,’ we are not explaining it.

Carroll’s assumption that the arrow of time has to be ‘built on top’ of laws that lack such an arrow involves appealing to notions of entropy increase– roughly, the idea that in a closed system, disorder always increases over time. But entropy increase, which is a time-asymmetric law, cannot itself be obtained from the allegedly underlying time-symmetric laws; that’s part of the ‘mystery’ of time’s arrow.  Moreover, trying to get time’s arrow from entropy considerations alone involves identifying the future solely with the direction of decreasing order in systems. This identification rules out identifying a future direction with processes of increasing order, which are commonplace (e.g., plant growth). Allowing exceptions for ordering process associated with living things on the basis that they are open systems doesn’t take into account that the universe as a whole is a closed system, and that such order-increasing processes take place, alongside other order-decreasing processes, within that closed system. Since entropy both increases and decreases all around us, and yet our experience is always future-directed, appealing to entropy increase is inadequate to the task of explaining time’s arrow.

Thus, the problem will not be properly solved unless physical laws really do have some irreversible component. But maybe they do: maybe we should not be neglecting collapse. And there is a model of collapse that does not involve changing the basic theory–it’s the Transactional Interpretation (TI). The transactional process corresponds precisely to Von Neumann’s intrinsically irreversible ‘measurement’ process. According to TI, ‘measurement’ is not about the consciousness of an observer (a very common misconception)–rather, it’s a real, physical process. That process is defined non-arbitrarily here,  here  and here.

Thus, we gain an irreversible step at a fundamental level of physical systems. For example, take a closed box of gas. With only time-symmetric (reversible) laws, it’s actually impossible to explain why entropy does not decrease in that box of gas. Appealing to ‘random thermal interactions’ doesn’t help, because the sort of ‘randomness’ one needs is time-asymmetric (this is explained very nicely by Price).

With collapse included, as in the transactional process, the thermal interactions between the gas molecules give rise to true randomness. Each such interaction consists of one or more photons being delivered from one gas molecule to another, in an irreversible process (the technical term is ‘non-unitary’). One molecule is the emitter and the other is the absorber, and the process of delivery of the photon(s) establishes the future direction. (For details on this account of spacetime emergence, see this paper.)

Interestingly, this picture also disagrees with the common assumption that ‘even in empty space, time and space still exist.’ (S. Carroll, same reference) However, Einstein himself also disagreed with that common assumption: he stated that ‘There is no such thing as an empty space, i.e. a space without field. Space-time does not claim existence on its own, but only as a structural quality of the field.’  (Einstein, Relativity and the Problem of Space.) The transactional account of spacetime emergence is completely consistent with Einstein’s observation. In that account, transactions establish, through exchanges of mass/energy,  the structure that we call ‘spacetime.’ Without those transfers of mass/energy, there is no spacetime, and therefore no arrow of time.

11 thoughts on “The Arrow of Time from an Overlooked Physical Law

  1. Reblogged this on Transactional Interpretation and commented:

    I’m reblogging this as a counterpoint to a recent book by John Gribbin, “The Time Illusion,” claiming that the ‘block world’ picture of spacetime is settled science. In fact, it is not. There is no real physical evidence for the ‘block world’ model, and there are counterexamples to the claim that relativity requires such a model. The inability of the block world to provide a complete explanation for the 2nd Law of Thermodynamics is another reason to keep an open mind regarding alternative, ‘growing universe’ models.

  2. It is entirely reasonable that the material domain of reality (our known universe) emerges from a substrate domain of reality which is not directly observable by us. This deeper substrate would thus continuously materialize the material substrate accounting for motion and the apparent passage of time. It may be that this process, occurring out of view, is what ‘collapse’ alludes to. It is possible the wave function and superpositon concepts in quantum physics suggest the existence of patterns of information extent and transforming using evolving rules we capture in part in science as physical laws. And it may be that the retrocausality and perpetuality implied in the advanced solutions while not found in the material domain are, like entanglement and non-locality, uniquely found in the deeper more fundamental Information domain of reality.

  3. Please let me say at the very beginning, that your papers not only made ideas of TI clearer but also the basic ideas of Quantum Mechanics clearer to me. I fully agree that what is observable (by senses) and called macroscopic or classical may emerge from a reality (something that exists) but not observable by senses. A quantum particle is not directly/empirically observable by senses; it requires a ‘measurement process’ to ‘infer’ its properties (position, momentum, spin, etc.) by observing the measuring device, which is classical/emoirically-observable because it is a huge collection of quantum particles. Since a single quantum particle itself is not empirically observable, what happens inside the measurement process is also not empirically observable, in other words, it happens, but not in the spacetime of Relativity and other branches of science.

    One question to verify my understanding of your work: Since you seem to say that interactions/transactions of the quantum particle’s wavefunction with absorbers in the measuring device constitute von Neumann’s (VN’s) Process 1, is it correct to suggest that the combined system of the particle and the device never evolves unitarily into a superposition of states following the Schrodinger equation ? After all, the combined system is an open system because the device has an environment. Because of the unitary evolution assumption (by including the brain and the universe), VN had to further assume the involvement of observer’s consciousness in the occurrence of the ‘collapse’, is it not?

    1. Thanks very much. Yes you are correct that the particle + device does not evolve unitarily into a superposition following the VN transition. That is how TI solves the measurement problem.

      1. Thank you.
        In that case, when we enter the classical electromagnetic four potential into the Schrodinger equation, what does it mean? are we assuming that the em field although classical, does not collapse the particle into a definite position because TI allows for the possibility of no collapse? Or does the em field bring the particle into a definite position?

  4. To understand the relation of the classical em field to the quantum level of the em field, read Chapter 6 of my CUP book, or this paper, Section 4:

    The short answer to your question is that the classical field is generated by a special kind of quantum state–a coherent state. It has a definite phase but not a definite number of photons. It’s a very strange animal. When there is ‘collapse,’ which occurs by absorption at the receiver, it is into a state with a definite photon number.

  5. Hi, Ruth. Got a couple questions for you.
    I’ve always had the impression that you believed the past ‘still’ exists in the causal set emergence picture. This makes sense to me due to the fact that (though we can’t actually verify space-time) all matter/energy behaves AS IF space-time exists as a fabric that changes in response to matter/energy…and gravity in particular might be difficult to explain without referring to ripples, dilations and contractions in the fabric. So I was wondering–if the space-time fabric exists in the past (and I’m not forgetting that the term, ‘past’, applies only locally), is this due to past space-time having been established by previous transactions, which, in essence, constructed it? The second question is, how does your position in the above article relate to your position on vacuum energy? Because I was thinking, the idea that ’empty’ space is boiling with particle/antiparticle-pairs would seem to indicate that ’empty space’ exists. But I’m likely missing something here!! :)

    1. First question: Yes.
      Second question: There is no literal ’empty space’ in RTI. The term ’empty space’ is a holdover from the unexamined assumption that particle interactions occur ‘in the container of spacetime’ — which is just a metaphysical presupposition.
      According to RTI. spacetime is just the network of actualized transactions. Actually there is no ‘vacuum energy’ in RTI either, because in the direct-action picture, virtual quanta are not excited field states. They convey forces, but not real energy. So there’s a lot of virtual activity going on in Quantumland, but none of it involves the transfer of real energy so none of it rises to the level of ‘spacetime’.

  6. Thanks, Ruth. I was thinking you must not believe the virtual particles are actual. I should probably get around to reading your books (which I might have by now, if I weren’t always writing myself); then I wouldn’t have to pester you with questions like these. Incidentally, your answer makes it much clearer to me, why you see no need to truly ‘quantize’ gravity–because all quantum behavior of space-time is actually not space-time behavior, making space-time an emergent classical structure. I also found your critique of entropy-as-explanation-for-our experience quite compelling. Of course, I think that even if there were no entropy-reducing processes, so that entropy growth was always uniform, it would still be a poor explanation. Because there’s no ‘mechanistic’ explanation for our experience at all, let alone which direction it flows–so why should anyone assume that entropy would ‘direct’ it? We would first need to know more about what the experience IS to make that assumption.

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