8 ms·

A Defense of the Reality of Time

73 points by digital55 9y ago

pavement 9y ago
Put another way, at absolute zero, in the void of deep space, does time effectively stop for a quantum system that does not otherwise change. Does time exist for systems with no events? If a tree falls in the forest, and no one is around to hear it, does it make a sound?
- 21 9y ago
  By our current understanding the quantum fields still exist and have non-zero energy even in the void of deep space. https://en.wikipedia.org/wiki/Zero-point_energy https://en.wikipedia.org/wiki/Zero-point_energy Particles will still be created and annihilated there and will interact with your system, so you cannot have no events.
  - lisper 9y ago
    And the reason you cannot have no events is that these events are what create (or, of you prefer, define) time.
    - AnimalMuppet 9y ago
      That's... one view. Do you have any actual evidence for it?
      - lisper 9y ago
        http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
        AnimalMuppet 9y ago
        That's a bunch of handwaving and "physics woo", not evidence. OK, you can make an argument for it, within at least some interpretations of QM. That doesn't mean the position is correct, though, and certainly doesn't mean that it deserves the degree of certainty with which you stated it.
        lisper 9y ago
        I don't know what you would expect evidence for such a claim to look like. Did you follow the chain of references back to this paper? http://www.sciencedirect.com/science/article/pii/S0167278998000451 http://www.sciencedirect.com/science/article/pii/S0167278998... https://arxiv.org/abs/quant-ph/9605039 https://arxiv.org/abs/quant-ph/9605039
        AnimalMuppet 9y ago
        No, I didn't follow the chain of references back to there, nor, now that I'm looking for it, do I see any such references. The reference that I did see was to here: http://blog.rongarret.info/2014/09/are-parallel-universes-real.html http://blog.rongarret.info/2014/09/are-parallel-universes-re... And that page has what I would regard as an appropriate level of skepticism for it's own position. It says things like "IMO". It says what it thinks is correct, but it recognizes that it could be wrong. (Reasonable enough, when you're saying that the Copenhagen interpretation of QM is wrong. That may in fact be correct, but dogmatism is not warranted yet, and may never be.) The links you present show a paper that puts forward a theory. Again, I'm not saying it's wrong, and it seems to make some things easier to understand. But where's the experiment that tests a prediction of this paper, which other interpretations of QM can't explain? Without that, it's just a nice theory. There are lots of those.
        lisper 9y ago
        > where's the experiment that tests a prediction of this paper I am nonplussed by this question. Did you even bother to read the first post I referred you to? Because that is all about a (thought) experiment.
- j_m_b 9y ago
  >Put another way, an absolute zero, in the void of deep space, does time effectively stop for a quantum system that does not otherwise change. Even at absolute zero, there are still quantum fluctuations which give rise to phenomena such as quantum vibrational energy in molecules.
  - mowenz 9y ago
    In such a scenario, where would this energy come from? Or am I not understanding this properly?
    - pavement 9y ago
      The general premise is that electrons would remain trapped in their shells, bound to protons, even in the cold dark. At the subatomic level, the system is still in motion, even if the temperature of the atoms themselves is zero. Even if they are stable, crystalized common elements, in a static molecular arrangement, uadulterated by decaying isotopes. Within the nucleus and among the electrons, a system would still exchange energy, even if there were no low energy photon emissions. Photons would still be in play between the electrons and the nucleus. Within the individual subatomic particles themselves, there is also activity. As far as we know protons don't decay, but it doesn't mean they are without events, even when no one is looking. Mostly I'm just trying to poke holes in the concepts and analogies that get bandied about, because when one attempts to apply common-sense, folksy wisdom to these topics, understanding frequently goes wildly off the mark.
      - mowenz 9y ago
        Thanks for responding, however I'm still confused. That is--I have no formal training in physics, but to my hobbyist's mind: interactions between subatomic particles creating scenarios where energy is produced in perpetuity either sounds like (1) the equivalent to a perpetual motion machine--impossible, or (2) mass-energy exchanged back and forth without radiating energy (but I am not sure if that's basically what's happening).
        9y ago
- kowdermeister 9y ago
  > Does time exist for systems with no events? Yes, haven't you been to any pointless meetings? :) > If a tree falls in the forest, and no one is around to hear it, does it make a sound? Obviously yes, sound is just resonating air and if physics is not broken it does make a sound.
  - ThomPete 9y ago
    No it doesent make a sound. That requires something to convert it into that sound otherwise its just soundwaves.
    - kowdermeister 9y ago
      This problem is a human made one and relies on the definition of the word, sound. Sound does not require an observer. Sound-wave is just scientific / pedantic way of saying sound. It's not just English, it means the same in my language.
      - ThomPete 9y ago
        Sound does require an observer indeed. Soundwaves even require observers and definition of them as soundwaves. Otherwise there is nothing.
        kowdermeister 9y ago
        Why? If nobody is there air stops being compressed?
        ThomPete 9y ago
        Because there is not interpreter to make it a sound.
  - woodandsteel 9y ago
    Like many words in the English language, the term "sound" has more than one meaning. One meaning is waves of compression in a gas, the other is the subjective experience in someone's mind.
threeseed 9y ago
> In special relativity, the time directions are structurally different from the space directions. In the timelike directions, you have a further distinction into the future and the past, whereas any spacelike direction I can continuously rotate into any other spacelike direction. This is something I've never understood about including time as just another dimension.
- whatshisface 9y ago
  When evaluating the coordinate-invariant length of [x,y,z,t] (where time is just another number), we use the minkowski metric: xx+yy+zz - tt, where time is treated differently with the minus sign. (By xx, I mean x*x=x^2.) So, that's essentially where, "time is a dimension" comes from, and that's where it goes. Aside, if you're wondering what coordinate-invariant length really is: just think about the fact that looking at a house from different angles will not change the distance between the doors. This length is calculated with the Pythagorean theorem involving xx+yy+zz, and when you extend it to lengths which are also invariant under different choices of the time coordinate you arrive at xx+yy+zz-tt. To complete what has turned into a brief introduction to SR, note how differences in the most natural choice of the direction of time might arise: We want the path of a stationary object to involve no movement through space, only the required advance into the future (through time). So, we note that time is parallel to the path of a stationary object. However, people moving relative to each other will disagree about which objects are stationary. Therefore, they will end up thinking differently about which way time points. Returning to my earlier analogy, this disagreement about which direction time points can be compared to the different viewers of the house, who may disagree about which direction forwards or left points. It isn't too unnatural when you think about it, which is a good thing because it is, well, natural.
- renox 9y ago
  Don't forget the twins paradox, it shows quite nicely that time and space (as one twin accelerate through space he ages slower than his twin) are linked, however counter intuitive this may be..
- Ono-Sendai 9y ago
  Time is not quite treated as 'just' another dimension in special relativity (SR). As whatshisface mentioned, the sign of the time coordinate is different when computing the interval. What this means in practice is that you can't 'rotate' from a motion forwards in time to backwards in time. Instead you get what is basically a shearing transformation. The amount of rotation is limited by the speed of light (c). So SR, I think, doesn't really say that time is just another dimension like the spatial dimensions. But it does mix the two together with a shearing transformation. In fact, I would say that special relativity doesn't have much in particular to say about what time is. It merely specifies the mathematical transformations between reference frames, including between the local times in those frames. You have to go to metaphysics to say what this really means. In this sense it's a bit a like quantum mechanics.
lux 9y ago
With regards to gravity affecting time, by this interpretation is it then saying time is itself constant but the movement/perception of object A within a stronger gravitational field is slowed compared to object B within a weaker one?
nsparrow 9y ago
If you are interested in an opposing viewpoint, the British physicist Julian Barbour has been trying to work out the math for a timeless physics: https://en.m.wikipedia.org/wiki/Julian_Barbour https://en.m.wikipedia.org/wiki/Julian_Barbour He lays out his theory in his book, The End of Time.
- pyedpiper 9y ago
  side note: Wiki mobile view renders quite nicely on a desktop for reading
  - veli_joza 9y ago
    Agreed, I've been using it as default for some time now. It's clean with no noise and far less distracting than standard page.
  - woodandsteel 9y ago
    "The idea that the block universe is static drives me crazy. What is it to say that something is static? It’s to say that as time goes on, it doesn’t change. But it’s not that the block universe is in time; time is in it. " I like how this guy thinks.
empath75 9y ago
To be honest, this guy sounds like a bit of a crank. His 'solution' isn't to any particular problem with the physics, just a personal preference about how time should be.
Koshkin 9y ago
Given that we can measure time, how can we think that we measure something that may be not real?
- GavinMcG 9y ago
  We can't measure time, at least not in the way that we measure anything else: the measuring device necessarily moves in that dimension in direct relation to whatever is being measured. Analogously, we can use a yardstick to measure space in a "static" way, but don't have a timestick – we have to actually step off each unit.
  - contravariant 9y ago
    But we do have a timestick? Sort off anyway. The Caesium 133 atom turns out to oscillate at a very consistent frequency, effectively marking equidistant points in time, similar to how a yardstick marks equidistant points in space.
    - Koshkin 9y ago
      Well, the clock's precision plays a very little role here. Still, you are correct, a clock moving through time (while measuring it) is wholly analogous to having to move a yardstick through space in order to measure distance.
    - GavinMcG 9y ago
      You're right about the equidistance. But we can't measure that oscillation, or anything else that takes place in the dimension of time, without moving through it ourselves. That's what I was getting at with the yardstick analogy: we can plop a yardstick down and look at the number representing the length. We don't need to actually position ourselves at point 0 and move ourselves to point N. That's not the case with time – when we take a measurement, we must start at point 0 and actually move along the time dimension to the point at which the measurement stops. (As a teacher it's always so fun seeing how an example gets interpreted entirely reasonably in way other than you intended.)
- 9y ago
discreteevent 9y ago
See also Lee Smolin and Roberto Unger: The singular universe and the reality of time
pierrebai 9y ago
One of them claim made is that time is real and fundamental. I think SR and GR teaches us that the fundamental concept is causality.
- anigbrowl 9y ago
  But causality is inherently temporal. If we imagine a cosmic pause button, it is only because of the passage of time that there can be a causal relationship between my dropping this teacup and its smashing on the floor. If I hit pause half-way there we can talk about how much potential and kinetic energy it has and so on, but (since the rest of the universe is paused) nothing we day can affect anything so no causal relationships can exist until we let things run again.
effie 9y ago
> ...time is going on, and we know what it means to say that time is going on. I don’t know what it means to say that time really doesn’t pass and it’s only in virtue of entropy increasing that it seems to. I always thought the same and guess most practical scientists do as well. There is nothing in thermodynamic puzzles that indicates there is some problem or insight to be gained on the 'nature of time', whatever that is.
adrianratnapala 9y ago
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.

73 points by digital55 9y ago

Put another way, at absolute zero, in the void of deep space, does time effectively stop for a quantum system that does not otherwise change. Does time exist for systems with no events? If a tree falls in the forest, and no one is around to hear it, does it make a sound?

21 9y ago
By our current understanding the quantum fields still exist and have non-zero energy even in the void of deep space. https://en.wikipedia.org/wiki/Zero-point_energy https://en.wikipedia.org/wiki/Zero-point_energy Particles will still be created and annihilated there and will interact with your system, so you cannot have no events.
- lisper 9y ago
  And the reason you cannot have no events is that these events are what create (or, of you prefer, define) time.
  - AnimalMuppet 9y ago
    That's... one view. Do you have any actual evidence for it?
    - lisper 9y ago
      http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
      - AnimalMuppet 9y ago
        That's a bunch of handwaving and "physics woo", not evidence. OK, you can make an argument for it, within at least some interpretations of QM. That doesn't mean the position is correct, though, and certainly doesn't mean that it deserves the degree of certainty with which you stated it.
        lisper 9y ago
        I don't know what you would expect evidence for such a claim to look like. Did you follow the chain of references back to this paper? http://www.sciencedirect.com/science/article/pii/S0167278998000451 http://www.sciencedirect.com/science/article/pii/S0167278998... https://arxiv.org/abs/quant-ph/9605039 https://arxiv.org/abs/quant-ph/9605039
        AnimalMuppet 9y ago
        No, I didn't follow the chain of references back to there, nor, now that I'm looking for it, do I see any such references. The reference that I did see was to here: http://blog.rongarret.info/2014/09/are-parallel-universes-real.html http://blog.rongarret.info/2014/09/are-parallel-universes-re... And that page has what I would regard as an appropriate level of skepticism for it's own position. It says things like "IMO". It says what it thinks is correct, but it recognizes that it could be wrong. (Reasonable enough, when you're saying that the Copenhagen interpretation of QM is wrong. That may in fact be correct, but dogmatism is not warranted yet, and may never be.) The links you present show a paper that puts forward a theory. Again, I'm not saying it's wrong, and it seems to make some things easier to understand. But where's the experiment that tests a prediction of this paper, which other interpretations of QM can't explain? Without that, it's just a nice theory. There are lots of those.
        lisper 9y ago
        > where's the experiment that tests a prediction of this paper I am nonplussed by this question. Did you even bother to read the first post I referred you to? Because that is all about a (thought) experiment.
j_m_b 9y ago
>Put another way, an absolute zero, in the void of deep space, does time effectively stop for a quantum system that does not otherwise change. Even at absolute zero, there are still quantum fluctuations which give rise to phenomena such as quantum vibrational energy in molecules.
- mowenz 9y ago
  In such a scenario, where would this energy come from? Or am I not understanding this properly?
  - pavement 9y ago
    The general premise is that electrons would remain trapped in their shells, bound to protons, even in the cold dark. At the subatomic level, the system is still in motion, even if the temperature of the atoms themselves is zero. Even if they are stable, crystalized common elements, in a static molecular arrangement, uadulterated by decaying isotopes. Within the nucleus and among the electrons, a system would still exchange energy, even if there were no low energy photon emissions. Photons would still be in play between the electrons and the nucleus. Within the individual subatomic particles themselves, there is also activity. As far as we know protons don't decay, but it doesn't mean they are without events, even when no one is looking. Mostly I'm just trying to poke holes in the concepts and analogies that get bandied about, because when one attempts to apply common-sense, folksy wisdom to these topics, understanding frequently goes wildly off the mark.
    - mowenz 9y ago
      Thanks for responding, however I'm still confused. That is--I have no formal training in physics, but to my hobbyist's mind: interactions between subatomic particles creating scenarios where energy is produced in perpetuity either sounds like (1) the equivalent to a perpetual motion machine--impossible, or (2) mass-energy exchanged back and forth without radiating energy (but I am not sure if that's basically what's happening).
      - 9y ago
kowdermeister 9y ago
> Does time exist for systems with no events? Yes, haven't you been to any pointless meetings? :) > If a tree falls in the forest, and no one is around to hear it, does it make a sound? Obviously yes, sound is just resonating air and if physics is not broken it does make a sound.
- ThomPete 9y ago
  No it doesent make a sound. That requires something to convert it into that sound otherwise its just soundwaves.
  - kowdermeister 9y ago
    This problem is a human made one and relies on the definition of the word, sound. Sound does not require an observer. Sound-wave is just scientific / pedantic way of saying sound. It's not just English, it means the same in my language.
    - ThomPete 9y ago
      Sound does require an observer indeed. Soundwaves even require observers and definition of them as soundwaves. Otherwise there is nothing.
      - kowdermeister 9y ago
        Why? If nobody is there air stops being compressed?
        ThomPete 9y ago
        Because there is not interpreter to make it a sound.
- woodandsteel 9y ago
  Like many words in the English language, the term "sound" has more than one meaning. One meaning is waves of compression in a gas, the other is the subjective experience in someone's mind.

threeseed 9y ago

> In special relativity, the time directions are structurally different from the space directions. In the timelike directions, you have a further distinction into the future and the past, whereas any spacelike direction I can continuously rotate into any other spacelike direction. This is something I've never understood about including time as just another dimension.

whatshisface 9y ago
When evaluating the coordinate-invariant length of [x,y,z,t] (where time is just another number), we use the minkowski metric: xx+yy+zz - tt, where time is treated differently with the minus sign. (By xx, I mean x*x=x^2.) So, that's essentially where, "time is a dimension" comes from, and that's where it goes. Aside, if you're wondering what coordinate-invariant length really is: just think about the fact that looking at a house from different angles will not change the distance between the doors. This length is calculated with the Pythagorean theorem involving xx+yy+zz, and when you extend it to lengths which are also invariant under different choices of the time coordinate you arrive at xx+yy+zz-tt. To complete what has turned into a brief introduction to SR, note how differences in the most natural choice of the direction of time might arise: We want the path of a stationary object to involve no movement through space, only the required advance into the future (through time). So, we note that time is parallel to the path of a stationary object. However, people moving relative to each other will disagree about which objects are stationary. Therefore, they will end up thinking differently about which way time points. Returning to my earlier analogy, this disagreement about which direction time points can be compared to the different viewers of the house, who may disagree about which direction forwards or left points. It isn't too unnatural when you think about it, which is a good thing because it is, well, natural.
renox 9y ago
Don't forget the twins paradox, it shows quite nicely that time and space (as one twin accelerate through space he ages slower than his twin) are linked, however counter intuitive this may be..
Ono-Sendai 9y ago
Time is not quite treated as 'just' another dimension in special relativity (SR). As whatshisface mentioned, the sign of the time coordinate is different when computing the interval. What this means in practice is that you can't 'rotate' from a motion forwards in time to backwards in time. Instead you get what is basically a shearing transformation. The amount of rotation is limited by the speed of light (c). So SR, I think, doesn't really say that time is just another dimension like the spatial dimensions. But it does mix the two together with a shearing transformation. In fact, I would say that special relativity doesn't have much in particular to say about what time is. It merely specifies the mathematical transformations between reference frames, including between the local times in those frames. You have to go to metaphysics to say what this really means. In this sense it's a bit a like quantum mechanics.

lux 9y ago

With regards to gravity affecting time, by this interpretation is it then saying time is itself constant but the movement/perception of object A within a stronger gravitational field is slowed compared to object B within a weaker one?

nsparrow 9y ago

If you are interested in an opposing viewpoint, the British physicist Julian Barbour has been trying to work out the math for a timeless physics: https://en.m.wikipedia.org/wiki/Julian_Barbour https://en.m.wikipedia.org/wiki/Julian_Barbour He lays out his theory in his book, The End of Time.

pyedpiper 9y ago
side note: Wiki mobile view renders quite nicely on a desktop for reading
- veli_joza 9y ago
  Agreed, I've been using it as default for some time now. It's clean with no noise and far less distracting than standard page.
- woodandsteel 9y ago
  "The idea that the block universe is static drives me crazy. What is it to say that something is static? It’s to say that as time goes on, it doesn’t change. But it’s not that the block universe is in time; time is in it. " I like how this guy thinks.

empath75 9y ago

To be honest, this guy sounds like a bit of a crank. His 'solution' isn't to any particular problem with the physics, just a personal preference about how time should be.

Koshkin 9y ago

Given that we can measure time, how can we think that we measure something that may be not real?

GavinMcG 9y ago
We can't measure time, at least not in the way that we measure anything else: the measuring device necessarily moves in that dimension in direct relation to whatever is being measured. Analogously, we can use a yardstick to measure space in a "static" way, but don't have a timestick – we have to actually step off each unit.
- contravariant 9y ago
  But we do have a timestick? Sort off anyway. The Caesium 133 atom turns out to oscillate at a very consistent frequency, effectively marking equidistant points in time, similar to how a yardstick marks equidistant points in space.
  - Koshkin 9y ago
    Well, the clock's precision plays a very little role here. Still, you are correct, a clock moving through time (while measuring it) is wholly analogous to having to move a yardstick through space in order to measure distance.
  - GavinMcG 9y ago
    You're right about the equidistance. But we can't measure that oscillation, or anything else that takes place in the dimension of time, without moving through it ourselves. That's what I was getting at with the yardstick analogy: we can plop a yardstick down and look at the number representing the length. We don't need to actually position ourselves at point 0 and move ourselves to point N. That's not the case with time – when we take a measurement, we must start at point 0 and actually move along the time dimension to the point at which the measurement stops. (As a teacher it's always so fun seeing how an example gets interpreted entirely reasonably in way other than you intended.)
9y ago

discreteevent 9y ago

See also Lee Smolin and Roberto Unger: The singular universe and the reality of time

pierrebai 9y ago

One of them claim made is that time is real and fundamental. I think SR and GR teaches us that the fundamental concept is causality.

anigbrowl 9y ago
But causality is inherently temporal. If we imagine a cosmic pause button, it is only because of the passage of time that there can be a causal relationship between my dropping this teacup and its smashing on the floor. If I hit pause half-way there we can talk about how much potential and kinetic energy it has and so on, but (since the rest of the universe is paused) nothing we day can affect anything so no causal relationships can exist until we let things run again.

effie 9y ago

> ...time is going on, and we know what it means to say that time is going on. I don’t know what it means to say that time really doesn’t pass and it’s only in virtue of entropy increasing that it seems to. I always thought the same and guess most practical scientists do as well. There is nothing in thermodynamic puzzles that indicates there is some problem or insight to be gained on the 'nature of time', whatever that is.

adrianratnapala 9y ago

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.