
A Defense of the Reality of Time - digital55
https://www.quantamagazine.org/a-defense-of-the-reality-of-time/
======
adrianratnapala
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
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.)

------
nsparrow
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
side note: Wiki mobile view renders quite nicely on a desktop for reading

~~~
woodandsteel
"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.

------
threeseed
> 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
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.

------
pavement
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?

~~~
kowdermeister
> 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
No it doesent make a sound. That requires something to convert it into that
sound otherwise its just soundwaves.

~~~
kowdermeister
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
Sound does require an observer indeed. Soundwaves even require observers and
definition of them as soundwaves. Otherwise there is nothing.

~~~
kowdermeister
Why? If nobody is there air stops being compressed?

~~~
ThomPete
Because there is not interpreter to make it a sound.

------
effie
> ...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.

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empath75
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.

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

~~~
SilasX
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.

~~~
Koshkin
Even if time is an emergent phenomenon, it does not mean that it is not real
or somehow "less real". I mean, yeah, we ourselves are "not ontologically
fundamental", since we are built from atoms, but I dare to say we are real
nonetheless.

------
pierrebai
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
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.

------
lux
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?

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

