Does QM and classical physics create the irreversibility of time?

 At long last I’ve found a YouTube video that pretty much describes quantum mechanics (QM) as I would. In particular, the narrator (Arvin Ash) expresses the possibility that the transition from QM to classical physics provides the irreversibility of time that we all experience in everyday life. In other words, QM describes the future and classical physics describes the past. The narrator cites Lee Smolin, who actually says that QM describes the ‘present’ and classical physics describes the past. Now, I’ve read Lee Smolin’s book, The Trouble with Physics, and, from memory, he made no mention of this, so maybe this is a new idea from him (I don’t know).

My knowledge of QM is rudimentary at best, so I’m hardly one who can judge, but I’ve been thinking this way since I wrote a post called What is now? in 2015. Back then, I didn’t know that Freeman Dyson had similar ideas. A contributor to Quora, Mark John Fernee, who clearly knows a lot more than me, made a similar point about QM to classical physics being irreversible in time, and whom I quoted in a not-so-recent post.

 

Ash also explains entanglement and decoherence without getting too esoteric about it, and seems to promote the view that entanglement, in principle, could involve the whole universe. Decoherence is often explained as the ‘leaking’ of information. The important point is that decoherence (or the wavefunction collapse) comes from the quantum phenomenon interacting with other particles, that one assumes already exist.

 

The narrator conjectures at the end that the multiverse interpretation is still possible, but I’m not so sure. The whole point of MWI (multiple worlds interpretation) is supposedly that decoherence never happens, but this variation means that it still would happen, only in other universes. Sabine Hossenfelder makes a similar point in a YouTube video of her own. 

 

The other problem with MWI, as I see it, is that entanglement would necessarily incorporate a multiverse. I suspect adherents to MWI (and there are a lot of them) wouldn’t have a problem with that, but I don’t really know. Some highly respected physicists, like Sean Carroll, are advocates of MWI. I really admire Sean Carroll and he readily admits that MWI is one of his personal prejudices. I recently saw a talk he gave on ‘time’ for New Scientist, but he didn’t mention any of this. Instead, he talked about the role of entropy, including its ramifications for the evolvement of the entire universe. I’m a heretic on entropy in that I think it’s a consequence of the arrow of time, not its cause. Having said that, the low entropy state of the Universe in the beginning is still a conundrum, though gravity plays a role in increasing complexity in the Universe, in spite of entropy.

 

In another video (by Closer to Truth), Lee Smolin articulates the possibility that time and 

space may be separate after all, which I’m beginning to wonder myself. Besides, if the Universe has a boundary (or edge) in time, but not in space, that would infer that they are separate. We know that time on a cosmic scale is finite, because we can estimate the age of the Universe.

 

I believe we all live on the edge of time (all of the time), which is contentious. All physicists, that I read and listen to, argue that there is no universal now, and I’m told that to think otherwise is naive, 19^th Century thinking. They argue that Einstein’s theories of relativity rule it out, because clocks run at different rates depending on where they are in the Universe and how fast they are travelling relative to other observers. Actually, it’s dependent on how fast they are travelling relative to an observer following a geodesic in a gravitational field (in free fall or in orbit).

 

I’m well aware that different observers, in different parts of the Universe, see a different ‘now’, because they all see stars 100s or 1000s of light years from them, which means they see them at different ages. And, of course, some of those observers could hypothetically see some of the same stars at different ages, which means they all see a different ‘now’. And then, if they are in motion with respect to each other, that distorts the differences even further. For example, if you have 2 super novae occurring in the field of view of 2 spatially separated observers, they may well see them happening in opposite sequences. Notice that this is true even without relativistic effects.

 

So one shouldn’t be surprised if Einstein’s special theory of relativity tells us that simultaneity appears subjective, both in space and time, because it can be, even without relative motion. But obviously, this isn’t the case where causality is involved. Causality insists on a sequence in time by definition, and it has to be objective, irrespective of what observers see.

 

I like to look at the famous twin paradox, because I think it contains almost everything we need to know about the special theory of relativity. I know it’s a thought experiment, but real experiments done with atomic clocks and aeroplanes and satellites tell us it’s true. The important point is the end result – when the spacefaring twin returns, they are younger than their Earthbound twin. The same effect could be made by the twin travelling near the event horizon of a black hole, which was the premise for Chris Nolan’s movie, Interstellar (which had Kip Thorne as a consultant).

 

But here’s the thing: both twins agree on what time it is in the 4-dimensional spacetime of the Universe. So, 2 observers travelling along different paths can measure different durations of time by whatever means they have, but when they reunite, they agree on where they are in time, in the same way they agree on where they are in space. I can’t see how this is possible if there isn’t a universal ‘clock’, which is arguably the edge of time for the whole universe.

Addendum 1: Mark John Fernee, whom I reference in the main text, and has a PhD in physics, proposes a similar, if not better, argument than I do. He also gives the same explanation of entropy as an 'emergent' property through probabilities that I do in other posts. However, I expect he may not agree with me that there is a 'universal now'. This is his answer to, How did entropy become associated with time?

 

 Addendum 2: Another post by Mark John Fernee (8Aug22) gives an excellent synoptic description of the relationship between classical physics and QM, which reflects my own point-of-view.