[jackson1372]

Consider an analogy to the 'reality' of right/left orientation. We stand on opposite sides of a table, and I say "There's a cup on the right" and you say "There's a cup on the left". Who's right?

Well, there's not really a deep disagreement because facts about left/right are observer-relative. Furthermore, we might reasonably conclude that left/right orientation isn't part of the fundamental structure of the world.

Now, if I looked at a timeline of events and said "Trump became president after Obama" and you (or any other observer from any other point in time) said "Trump became president before Obama", who is right?

Barbour is saying that this disagreement is like the left/right dispute. Maudlin is saying that it isn't.

I think it's fair to characterize this disagreement as being about whether time is really part of the fundamental structure of the world. And it's clear that there's a meaningful disagreement here.

[adrianratnapala]

I am going to leave Barbour aside here, because I don't fully understand what his stance is. But if Maudlin is simply arguing that there is a fundamental time asymmetry in physics, then he his hardly arguing about the reality of time.

But I understood him as disagreeing, not just with Barbour, but with a whole lot of physicists (including me) who are "B-theorists" which is a horrible term that means you consider the whole history of the universe as the fundamental reality and see change as a thing observed by beings within it.

But that doesn't mean I think physics, or the history of the universe are time symmetric. My own view is there is some fundamental (but not very well understood) time asymmetry in physics. Yet I am still a B-theorist.

But the alternative called "A-theory" (which I think Maudlin believes) is that time, and change are more fundamental than physics, so the fundamental reality is "the state of the universe _now_" and the laws of physics tell us how it has changed and will change.

A-theory is also perfectly cogent, but it is separate from time symmetry. The only link being: IF physics is time symmetric (which it isn't) THEN B-theory implies there is no fundamental difference between time directions, while A-theory still allows it.

[jackson1372]

I should disclose: I'm an A-theory-supporting philosopher. (Sigh. Why are philosophers and physicists always disagreeing?! We should be friends!)

My above analogy to the left/right issue is the example I use to introduce students to the A- vs. B-theory.

The A- vs. B-theory debate is, of course, complicated, and you point to one possible complication: can we separate the issue of temporal asymmetry from the issue of temporal change?

As I see it, the answer to this question is "No".

We can talk about how some physical object 'changes' its spatial properties as we move from one spatial slice to another. For instance, as we move from the pointy-end of a cone to its base, each circular slice of the cone 'changes': they keep getting bigger until you reach the base.

This use of 'change' is symmetrical. There's no preferred direction of 'change' here. It makes equal sense to speak of changes "from bigger circle-slices to smaller-circle slices" as it does the other way around.

But in the context of the temporal dimension, talk of 'change' is not symmetrical. It's fine to say "The President changed from Obama to Trump", but you can't say "The president changed from Trump to Obama".

The point of all this just is: the issue of whether (temporal) change is symmetrical is tied up with the issue of whether time itself is symmetrical. You can't have your cake and eat it too. There's either an objective temporal ordering of physical events from past to future (as A-theory posits) or there isn't (as B-theory posits). If you're a B-theorist, you have to say that "Trump came after Obama" is like saying "The cup is on the left".

This is not a refutation of B-theory. But it, I hope, makes clearer what the stakes are. And when those stakes are made clearer, B-theory, in my view, looks less attractive.

[adrianratnapala]

Ok, I think I accept your point about distinguishing a symmetrical concept of "change" applied to asymmetrical things from a truly asymmetrical concept of change. That clarifies what I was getting at in my last paragraph. But I am still surprised to see it as a characterisation of the A vs. B debate[1]

So please help me check I am understanding the terms of that debate correctly.

(1) Even if it is not the way you explain it to students, do you accept my claim that A-theory considers time and change as being more fundamental than physics?

(2) If I further said "... as opposed to considering time change to be properties of the history of the universe", then would you accept the implied dichotomy?

[1] And I say the A-B is in turn not truly a debate reality of time, which was just an unfortunate framing by McTaggart.

[jackson1372]

1. I wouldn't put it in terms of time being "more fundamental than physics". We can mean two things by 'physics': the thing that is the object of study in the Physics department or the theory that gets generated by that study. Time, I take it, is one aspect of the object of study, one aspect of the natural world. So the question is: does the natural world, at the most fundamental level, have asymmetric temporal order? Or, rather, is asymmetric temporal order mearly an 'illusion' that gets explained away once you have the most fundamental picture of the world?

That's likely still unclear, but that's to be expected. We typically can't ask questions about fundamental structure without invoking that fundamental structure itself. That's why I find the analogy to left/right so helpful. We have a pretty good idea of what it means for left/right orientation not to be built into the fundamental structure of nature. And so that can provide a way of testing various claims about the fundamentality of asymmetrical temporal ordering.

2. I'm not sure I have a grip on what this additional thing is supposed to add. But I'll note that you reference 'history' and one might reasonably think that that is a temporal notion. If you meant 'history' to mean something like "the universe's extension in the temporal direction", that doesn't quite seem to allow for the distinction I took you to be trying to make.

(And, yes, McTaggert's framing is highly unhelpful.)