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I think our habit of discussing such arguments as being about the "reality" of time is confusing and silly. For the most parts people (except maybe Julian Barb

I think our habit of discussing such arguments as being about the "reality" of time is confusing and silly. For the most parts people (except maybe Julian Barbour) are arguing about whether the the concepts of "now" and "change" come from outside of physics, in such a way that the present is in some sense more real that the future or past. If so we can and must think of physics as how "real things now, change". The alternative is that time is part of physics, and that every part of the history of universe is equally real, and that "now" and "change" are perceptions of observers within that history. Both views accept the reality of time. Things don't become less real just because they are physical facts rather than metaphysical necessities.

jackson1372 9y ago
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 9y ago
  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 9y ago
    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 9y ago
      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.
smaddox 9y ago
I would argue that the question you raise boils down whether or not the fundamental laws are deterministic or non-deterministic. If they're deterministic, the future and past are as fixed as the present, and therefore are equally as "real". If non-deterministic, then there is truly something unique about "now". If/how non-determinism can be rectified with relativity without invoking uncountably infinite realities, which is ontologically hard to swallow, is unclear.
- adrianratnapala 9y ago
  Imagine a bitmap image look at one scan line through it. Now if our bitmap were determined by a sufficiently deterministic 1-D cellular-automaton then our one scan-line would determine the rest of the picture. But if the bitmap was actually a 2-d picture drawn by an artist who had some freedom of choice, but also was constrained by some geometrical rules, then our scanline would not determine the picture, though it would correlate with other pixels. Yet the entire picture is still just as real as the scan-line we picked.

jackson1372 9y ago

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 9y ago
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 9y ago
  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 9y ago
    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.

smaddox 9y ago

I would argue that the question you raise boils down whether or not the fundamental laws are deterministic or non-deterministic. If they're deterministic, the future and past are as fixed as the present, and therefore are equally as "real". If non-deterministic, then there is truly something unique about "now". If/how non-determinism can be rectified with relativity without invoking uncountably infinite realities, which is ontologically hard to swallow, is unclear.

adrianratnapala 9y ago
Imagine a bitmap image look at one scan line through it. Now if our bitmap were determined by a sufficiently deterministic 1-D cellular-automaton then our one scan-line would determine the rest of the picture. But if the bitmap was actually a 2-d picture drawn by an artist who had some freedom of choice, but also was constrained by some geometrical rules, then our scanline would not determine the picture, though it would correlate with other pixels. Yet the entire picture is still just as real as the scan-line we picked.