What does "Time Running in Reverse" mean?

A glance at the physics and the metaphysics.

Feb 21, 2026

Newton’s Absolute Time vs. Leibniz’ Relative Time

Newton held space and time to be inert, uniform, absolute backgrounds “on” which everything exists and happens. His rival, Leibniz, believed that space and time were not absolute, but relative: for Leibniz, space is configured by the distances between things, and time is configured by the intervals between events: their non-simultaneity. The maturation of physical science has vindicated Leibniz.1

The best argument I’ve seen for the claim that in the physical world, time cannot go backward—not even in principle—is a highly illuminating and original paper by my father, Eugene Hecht. In “The Physics of Time and the Arrow Thereof,”2 he argued:

that even though the fundamental laws of physics are time-symmetric, all real-world macroscopic processes are irreversible; that they comprise sequences of microscopic events that are accompanied by the irreversible emission of radiant energy (photons and/or gravitons); that massless entities are distinctive in that they only travel toward the future; that this is the physical cause of the arrow-of-time, not the increase in entropy; that entropy is a theoretical construct and cannot possess energy, [since] it is a descriptor of the state of a system, not a causative agency capable of effecting change; that time is a descriptor of the unfolding of events.

The central point is soon restated:

Radiant energy, the emission of photons and/or gravitons, which accompanies all macro-happenings, is discussed. The dispersal of these massless entities is the fundamental irreversible process, and it goes beyond classical physics. This concept—which does not seem to have been explicitly considered before—is central to understanding the progression of time.

On these novel grounds, the paper establishes that time-reversal cannot become physically real. Of course, we often imagine events suddenly flowing backward, commonly conceived with the metaphor of a film projector turning its reels in the inverse direction, showing the frames in the reverse order. Popular science documentaries about entropy often show a glass of water being shattered by a bullet, then converging again into the pristine object it was before, as the projectile withdraws along the exact path it traversed before.

As the quoted paper explains, the distinction must be made between the entropic aspect of a shattering glass, and the radiant energy emission aspect of the same event. While the entropy of the breakage makes the process reversible in principle but asymptotically unlikely, the necessitous release of photons is intrinsically irreversible. It completely forbids the undoing of what happened.

But it seems to me another distinction is involved, a philosophical one which is not mentioned in the physics paper (though I suppose it must surely have been discussed by someone, somewhere). As you imagine the fragments coming back together and uniting into the unbroken vessel, all its wildly spilt water now restored to placid equilibrium inside—you are not necessarily imagining the reversal of time itself.

You’re imagining the reversal of the process you saw: the sequence of events that ended in scattered wet shards on the floor is now followed by another sequence of events, one that ends with a situation matching the initial state of the first sequence, before the bullet hit. Everything outside the room has continued as normal, with people aging, etc. Time has not reversed, only the local events have.

Now suppose that everything everywhere that occured during that messy minute gets undone, in a “mirroring” sixty seconds of ubiquitous, uniform reversal. Is time itself reversed? First of all, regardless of our philosophy of temporality, it would be impossible to observe such a reversal, since all observers would be captive to it.

Now, if Newton is right, absolute Time ticks on undisturbed, even though all the clocks move counterclockwise. But if Leibniz is right, and time is the relational measure of events with respect to one another, must we say time goes backward in this admittedly impossible scenario? Perhaps we cannot, since we still have an ongoing continuum, populated by successive states (S). Call the bullet’s arrival S.1; call the state of things at the midpoint of the glass’ shattering S.2; and S.3, the settled stillness of every resulting bit of glass on the floor. We have this sequence:

S.1 at T.1… S.2 at T.2… S.3 at T.3… S.2 at T.4… S.1 at T.5…

The reversal occurs at T3, and it is the thing Hecht Sr.’s paper shows is impossible. But I’m not focused on the physics, just a question of philosophy. Suppose we imagine a “strong version” of Newtonian Time itself changing direction:

We have S.1 at T.1, S.2 at T.2, S.3 at T.3, followed by S.2 at a returned T.2. Then T.1 returns and behold, the state of affairs we call S.1 happens again: the bullet touches the unbroken glass. Even the radiated photons have returned to the atoms that emitted them (physically impossible, but easy to imagine).

All these events, these states of affairs, have happened before. First they happened in a forward sequence, and then they ran in reverse. The distinction between absolute Time and relative time—if it turns out to be coherent—would seem to forbid just this. It would seem that for Leibniz, time is the flow of events; therefore, if the flow of events reverses, so does time itself. By contrast, an absolute, Newtonian Time can only be said to “run backwards” if it not only erases the results of a process, it also causes it never to have occurred in the first place.

Yet this is inconceivable, since the backward direction of temporality consists in the reversal of a stream of events which has, by definition, already indeed transpired.

Mentally switching over to a Leibnizian conception does not avail, for on that view, what constitutes temporal progression is the separation of events, their non-simultaneity—not their sequence. For Leibnizian time to be neutralized, all processes must simply stop (and this, too, would be unobservable, not only because all observers would be included in the cessation, but also because observation is itself a process). In such a totally static universe, Newton’s time would tick on forever, despite the absence of change. Leibniz’ would not. But that’s time stopping, not time going backwards.

I’ll close with another quotation from that remarkable publication:

Contributing to the asymmetric progression of time is the fact that photons only travel toward the future, a property exclusive to massless entities. To appreciate what is meant by that assertion, consider any macroscopic occurrence, for example, an exploding firecracker. Imagine all the particles (down to the subatomic level) initially flying out from the blast. Now assume time could run backwards and the explosion become undone. The laws of physics allow all those material fragments rushing outward to somehow come to a stop (transferring energy from kinetic to potential), and thereafter be brought back to reform the firecracker. Yet this does not spontaneously happen. In fact, it cannot happen, because the flood of photons released cannot come to a stop, let alone be reversed in their trajectories by any existing physical agency.

The photons cannot “be reversed in their trajectories by any existing physical agency.” This is entirely germane to the physics under discussion in “The Physics of Time and the Arrow Thereof.” But metaphysical thought is not so circumscribed; the impossible but quite thinkable return of emitted photons requires no “existing physical agency” in our imagination. To make it not only imaginable but possible would require God. Yet even this would leave intact the question discussed above. Could God not only undo every aspect of an explosion, including the radiation of massless lightspeed bosons (i.e., photons), but also cause the event never to have occurred?

Here we are in the domain of theology that pertains to Divine omnipotence. We can reason with Alfred North Whitehead, Charles Hartshorne, and other Process Theologians that if God has made us genuinely free, the very meaning of that freedom entails His inability to foreknow our free actions and decisions (hence the title of Hartshorne’s strange, superb 1984 book, Omnipotence and Other Theological Mistakes).

Yet Augustine (354-430) famously declared: “If God foreknew my free will, it was nevertheless not nothing that he foreknew.” Maneuvers like that one can preserve Divine omnipotence, but the more of them we opt to make, the further away from normative Aristotelian logic we recede, away from the left brain hemisphere’s defensible stronghold, and into the right hemisphere’s wild regions; out from Horatio’s philosophy, and into the “more things in Heaven and Earth” than are dreamt of in it.

Even if God cannot do what is logically incoherent, this need not diminish His power. If the feat we deny to God is itself an illusion of grammar, it cost nothing to pare away what is nothing to begin with. Hobbes cites “contradictions in terms,” for example “an incorporeal body.” Later on, Kant mentions “wooden iron.”

Yet the groves of Paradise may count among their living trees an iron oak, whose inner life resembles that of its mortal cousins here below (where panpsychists, including some botanists, have attributed a modicum of quiet consciousness to trees). Whatever more ambitious definition today’s biology may afford, surely when Kant wrote “wood” it simply meant the flesh of trees that grow or grew before—wherever they are, and of whatever sort. As for the “incorporeal body” mocked by the good Thomas Hobbes as neither real nor unreal, but meaningless—he at his death may have had an experience like those reported by so many resuscitated cardiac arrest patients: they exit from their corporeal (literally, “bodily”) body, but find themselves still localized in space, still somehow shaped as human figures, yet invisible to their living, weeping kin, for lack of material flesh. Sometime a living relative can see the apparition of the soul, and try embracing it, but as when Achilles reached for his Patroklos in Iliad XXIII, his arms pass through the “incorporeal body.”

A glass of water shimmers on the windowsill. If a bullet smashes it, can time flow backward to restore it? We have seen several levels of impossibility, each demanding greater power for its overcoming:

To fetch every fragment, each speck of powdered glass, and glue it back together: extremely difficult and yet, for some obsessive tragic craftsman, quite possible.
To run the whole event in reverse: utterly unlikely, yet possible in principle—but only for a physics that neglects the crucial feature of real-world events: they emit radiation that’s intrinsically irreversible.
To run the whole event in reverse, including a physically unlawful reversal of the photons emitted in the course of the event’s atomic interactions: only God could do it, and only under panentheism: the view that God is ubiquitous in the physical world, but also infinitely beyond it in every sense, so that He is not bound by those physical laws He created.
To undo the event, not merely resequencing its moments into an inverted order, but to erase from actuality the very occurrence of what transpired, so that, because of this Divine action in the eternal present, the past event in question never happened: the God for whom this is feasible is plenipotentiary—a robustly omnipotent God.

That is quite enough about “time running in reverse.”

Jamey Hecht, Author

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