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.
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.
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.
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
He lays out his theory in his book, The End of Time.
> 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.
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.
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.
> This is something I've never understood about including time as just another dimension.
There's actually a trilogy by Greg Egan based on the premise that time is just like a spacial dimension (i.e., where the Minkowski metric is instead xx+yy+zz+tt). It radically changes physics and chemistry, and the books are set in this different universe.
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..
> ...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.
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.
Just speaking from Julian Barbour's The End of Time perspective:
When they say time isn't real, they mean something more like "not ontologically fundamental" -- i.e. you can produce a model with no explicit reference to time, and it can produce all the same predictions as a "timeful" model -- of course, you'd have to do some final transformation at the end to express the time values if you want it to compare to a timeful one.
Yes, you can measure it, but that measurement is wholly implied by all the other (ontologically fundamental) ones.
Compare to a word in which people used four dimensions to describe space. In that case, you could correctly argue that, "hey, space is only three dimensions. Yes, you're measuring a fourth one, but that's redundant with the first three; once I know the first three, I can tell you what your measurements of the fourth one will be."
Barbour's argument, then, is that you can derive the laws of physics in terms of a rule for "which universe configurations are possible", and make that your primary model; time then just becomes a label for a measure you apply between different configurations (the "Machian distinguished simplifier"), but is wholly implied by them.
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.
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.
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?
[+] [-] adrianratnapala|8 years ago|reply
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|8 years ago|reply
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.
[+] [-] smaddox|8 years ago|reply
If/how non-determinism can be rectified with relativity without invoking uncountably infinite realities, which is ontologically hard to swallow, is unclear.
[+] [-] nsparrow|8 years ago|reply
He lays out his theory in his book, The End of Time.
[+] [-] pyedpiper|8 years ago|reply
[+] [-] threeseed|8 years ago|reply
This is something I've never understood about including time as just another dimension.
[+] [-] whatshisface|8 years ago|reply
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.
[+] [-] Ono-Sendai|8 years ago|reply
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.
[+] [-] justinpombrio|8 years ago|reply
There's actually a trilogy by Greg Egan based on the premise that time is just like a spacial dimension (i.e., where the Minkowski metric is instead xx+yy+zz+tt). It radically changes physics and chemistry, and the books are set in this different universe.
http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html
[+] [-] renox|8 years ago|reply
[+] [-] pavement|8 years ago|reply
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|8 years ago|reply
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.
[+] [-] j_m_b|8 years ago|reply
Even at absolute zero, there are still quantum fluctuations which give rise to phenomena such as quantum vibrational energy in molecules.
[+] [-] kowdermeister|8 years ago|reply
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.
[+] [-] effie|8 years ago|reply
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.
[+] [-] empath75|8 years ago|reply
[+] [-] Koshkin|8 years ago|reply
[+] [-] SilasX|8 years ago|reply
When they say time isn't real, they mean something more like "not ontologically fundamental" -- i.e. you can produce a model with no explicit reference to time, and it can produce all the same predictions as a "timeful" model -- of course, you'd have to do some final transformation at the end to express the time values if you want it to compare to a timeful one.
Yes, you can measure it, but that measurement is wholly implied by all the other (ontologically fundamental) ones.
Compare to a word in which people used four dimensions to describe space. In that case, you could correctly argue that, "hey, space is only three dimensions. Yes, you're measuring a fourth one, but that's redundant with the first three; once I know the first three, I can tell you what your measurements of the fourth one will be."
Barbour's argument, then, is that you can derive the laws of physics in terms of a rule for "which universe configurations are possible", and make that your primary model; time then just becomes a label for a measure you apply between different configurations (the "Machian distinguished simplifier"), but is wholly implied by them.
[+] [-] GavinMcG|8 years ago|reply
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.
[+] [-] pierrebai|8 years ago|reply
[+] [-] anigbrowl|8 years ago|reply
[+] [-] lux|8 years ago|reply
[+] [-] discreteevent|8 years ago|reply