For the sake of ease of imagination, maybe it's good to use a Machian defintion of time, time is the relative configuration of all physical bodies+fields. I also want to be agnostic about the flow of time if possible (e.g. is it fundamental or emergent for entities like us).

I can imagine time doubling or halving as producing no discernable effects because if everything is relative how could one tell?

I can't imagine reversing the rate though. It does not seem enough to say one would just see eveything reverse. At face value, it seems like no matter what, all physicalists would say a signal traveling to the brain is necessary for an experience or sensation, and all signals travelling away can't produce any experience at all. You certaintly wouldn't see a glass unbreaking, photons leaving the eyes would impede that. But you wouldn't even conceive anything at all right, let alone see? Not just a zero value for experience, a NULL value, or maybe better that whole category doesn't exist. Observers simply don't exist with that constitution?

Do physicalists provide any intuition on this?


  • Your question just made me think of it, but reversing of time as you describe it can't happen for simple physical reasons. Going forward, light bounces on an object, reach your sensitive retina and provokes a nervous impulse. This can't happen backward (impulse -> light from the retina -> bounces back on the object) because your retina can't emit light in response to a nervous impulse. Like a sausage factory: putting a wiener at the end won't give you a live pig at the entrance.
    – armand
    Commented Sep 17, 2021 at 3:01
  • 1
    @armand It can happen backwards, as far as physics is concerned, QED is time-symmetric. Absorption of a photon puts atom into an excited state, with electron at a higher energy level, which can be followed by the electron dropping to the lower level with a photon emitted. Or, the energy can be channeled into an impulse. In reverse, the impulse puts some retina atoms into excited states and they emit photons.
    – Conifold
    Commented Sep 17, 2021 at 8:11
  • @conifold no. Eyes and solar panels don't glow when you put electricity in it. Unless you put enough amperes they heat and start glowing black body radiation, but that's another problem.
    – armand
    Commented Sep 17, 2021 at 9:00
  • 1
    @armand Second law broken, Nature, 2002:"Researchers have shown for the first time that, on the level of thousands of atoms and molecules, fleeting energy increases violate the second law of thermodynamics".
    – Conifold
    Commented Sep 17, 2021 at 11:46
  • 1
    @armand consider a rock falling off a cliff. It hits the ground and bounces, producing vibrations. All of this can be understood with Newtonian physics, and Newtonian physics is time-symmetric; it is theoretically possible for vibrations to converge on the rock on the ground, causing it to bounce up and bounce again and rise to the top of the cliff. It won't happen in the forward time direction because of entropy, but Newton's laws allow it.
    – causative
    Commented Sep 17, 2021 at 22:11

1 Answer 1


If all of time was reversed, we would not perceive any difference. We would remember the "future" (positive time direction) and try to predict the "past" (negative time direction). It's just a relabeling of the time axis. Nothing really changed.

Reversing time means that all the same universe states still happen, just that we've ordered them in reverse. This includes mental states; so no one is going to have a mental state of "surprised to see a window unbreaking," because that mental state didn't happen in the forward-time universe. Their mental states are going to be the same as they were in forward-time, so they will notice nothing strange.

  • So the direction of time has no physical/emperical effect?
    – J Kusin
    Commented Sep 17, 2021 at 1:49
  • This may very well be correct, but I am unable to grasp your answer as is (even though your answer is clear). If there is a direction at all, reversing it must have an ontological or physical effect. Are you in essence denying a direction? Just states laid out without arrows connecting them? Like a numberline with no direction. But isnt this denying causality as fundamental?
    – J Kusin
    Commented Sep 17, 2021 at 18:38
  • @JKusin The direction of time that matters to us is how different physical events relate to each other. The events are laid out with equations connecting them. If you just relabel the time axis so it goes backwards, without changing any events, you haven't changed anything physically meaningful. It's as if you had swapped all positive charges for negative charges and vice versa; it makes no physical difference since you're just relabeling + and -, but that's not denying charge itself.
    – causative
    Commented Sep 17, 2021 at 19:03
  • If I can "do something" without any effect, don't we want to eliminate that notion from our ontology? Wasn't absolute space so criticized for if a box in empty space is shifted left one foot there is no discernable effect? Why speak of a direction of time as fundamental at all then?
    – J Kusin
    Commented Sep 17, 2021 at 19:28

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