This implies either:

- The past is real and changeable (at a quantum level)
Or
- The particle knows the future; so the future is probably real (else just fully deterministic) — Devans99

A third alternative is that time is thermally emergent.

So the story there is that any localised thermal event - like a photon being emitted and absorbed - is timeless, or exists in its own present tense moment, in the sense that its future and past states are continuously connected.

So there is a change - how we define time passing. But the photon's story takes account of the whole of the path it must cross.

It is ruled by the least action principle and must take the most direct route - give and take the statistical fluctuations of its wavefunction. So the fact that spacetime expands and cools while it is moving - thus red-shifting it - and the fact that the environmental context might be changing in its future, as with quantum eraser experiments, all get factored into that overall wavefunction.

For the photon, there is no real past and future. Instead there is only the sum of some pathway which is like a single moment of action. There is a present that stretches like a strand to connect how things were before, and how things were after, because of an abrupt thermal change - the loss of energy at point A which became a matching gain of energy at point B. Or in fact an actual loss of energy at point B because the photon got red-shifted on the "journey".

Then orthogonal to that strand of action is the other thing of the global present which is the current average temperature of the expanding/cooling universe. Now we have a more concrete past and future in that the universe is loosing heat and red-shifting in a steady continuous fashion. It is not a punctate or quantum jump like a photon event. It is emergently a statistical and predictable curve of dissipation. The average energy density of yesterday was always higher. And tomorrow it will always be lower. So that creates a backdrop against which the temperature or energy level of any particle can be currently measured.

So time is a complex comparison of two opposing notions of "the present". You have the present of the individual quantum event. This is a jump from one energy level to another energy level. Quantum weirdness tells we have to take the non-locality of that seriously. The photon does get to see its whole journey all at once. It takes into account everything when responding to the constraints imposed by the least action principle. Its wavefunction is the retrospective account of everything that did in fact give the path some more complicated structure - like experimenters fiddling around with switchable paths.

Then out of a population of these basically statistical events, you get a universe that is expanding and cooling because they are happening. You get a global average that changes smoothly and predictably in a general direction. There is now no going backwards to the past because that would mean unscrambling the scrambled. It could happen theoretically, given absolute determinism - the ergodic hypothesis. But if you accept the reality of quantum collapse, as I'm doing, then the past does become irretrievable. It is real history. And also the future remains undefined because all things are still possible.

The future - from the global point of view that is the present tense for the universe as a whole - may be constrained by history. Yet it also contains all the further quantum fluctuations yet to happen. So the future may have "happened" for some photon winging its way toward some bit of experimental apparatus still lightyears in our collective future, but right now, for observers measuring things against the current thermal backdrop, the future remains full of surprises and unknowns. Time from the global perspective is split into the three emergent steps of past, present and future.

So you can imagine all this as global time - a present moment that is squashed flat like a cross-section view, sandwiched beween some wodge of past history and some wodge of future possibility. The present moment is defined by all the parts of the universe which share the same average energy density. Or more simply, has the same general background temperature.

Then particle or event time is a thin line that cuts across this temporal structure horizontally, at right angles to its "forward progressing present moment". The photon's journey looks to start in the past and end in the future. But from its point of view, there is only some quantum jump in terms of momentum and location. It didn't have to move through any kind of backdrop spacetime.

The photon simply connects two spacetime points in a thermally-constrained fashion. Its wavefunction took into account everything that existed "inbetween" and so affected its probabilities of being the way a thermal difference "eventually" got added to the bulk statistical structure of the expanding/cooling cosmos.

So in this experiment, a decision to raise a screen effects whether a particle goes through a double slit as a particle or a wave IN THE PAST. — Devans99

Did you actually read the article?

Most physicists think the answer is a resounding "no." No, we cannot kick back with retrocausality. Or, at the very least, the experiment, whether conducted across a lab or across galaxies, doesn't support the idea of time travel. — gizmodo.com

The article is written at a very basic layman level and with lots of journalistic flourish, but there are links to more in-depth explanations at the bottom.

Or it is just that the past has already happened and our senses haven't caught up.

Is there any valid interpretation of QM that asserts that the nature of the photon as it is emitted from the source is affected by the measurement that will eventually be taken of it? There might be. Experiments like before-before definitely show retrocausality under certain nonlocal interpretations.

Surely we can conclude time is real with 90% certainty? — Devans99

Proof of retrocausality would prove that time (an ontological distinction between past, present, and future) is not real, not the other way around. The experiment is no proof of retrocausality, just a demonstration of the nonlocality of some quantum interpretations.

Who is claiming that time isn't real?

Time: an ontological distinction between past present and future.

That is a wonderfully succinct definition of a rather illusive concept. Thank you for it's application.

M

The moderator claims my posts are of low quality, who knows why. So I can't answer this beautiful question without being struck down by lightening from the invisible realm.

PS

They omitted four of my best arguments, and won't return them!