
Carlo Rovelli on challenging our common-sense notion of time - Deinos
http://nautil.us/blog/forget-everything-you-think-you-know-about-time
======
jnurmine
I am not really skilled in physics and have never understood why time would be
a tangible dimension like, say, width or height, instead of a mathematical
construction to argue about change.

I get it that it is convenient to talk of change in something, like movement
from state a to state b, under the guise of wrapping it in "time", but is
there some physical argument in support of time in general? Honest question.

Living beings getting older is their cells becoming more and more inefficient
in division and cell repair. One could likely achieve the same effects with
chemicals, but doing so would not mean time has run faster.

The concept of time for humans seems to be all about observable change, which
needs an observer with a memory to compare the current state with previous
state to be able to say: time has passed.

A rock changes via erosion and such, but it has no memory, and cannot observe
anything. Does a rock feel time? Of course not. Does it exist "in time"? Does
it, without an observer that somehow measures the flow of time (via changes in
Cesium atoms or something)? Or is the rock just existing and under the whims
of all forces of nature that might impact it and change it into smaller pieces
and eventually to sand, and so on.

I guess my question is: what exactly is time, physically, and why should it
have to exist as some sort of a physical process in the first place.

~~~
mikekchar
I have a kind of followup question (which, if I understand correctly, was
touched on a bit in the article). If time is a tangible dimension, is it
possible that the "passage of time" is an illusion? I remember the past and
because of causality, the events are ordered. Time seems to flow from the past
to the future and it never flows from the future to the past. This seems
obvious to us, but I've always wondered why time doesn't flow backwards.

Just to take a silly example, what if all the "points in time" just exist (and
are ordered -- I don't propose to break causality)? They don't flow at all.
From my perspective, at every "point" along the time axis, I can recall the
past and it will be ordered as if it "happened", but each point could be
independent (though constant). If I could remember "forward" through time,
then this would be obvious, but since I can only remember "backward" through
time, at every point it appears as if I have progressed through time.

I suppose the interesting thing is that causality is uni-directional. Things
can only happen in a certain order in the past. But this is not true of the
future. Even if I have perfect knowledge of the present, there are some things
I can not predict about the future (quantum mechanics FTW). I wonder why that
is (because we are flowing through time? Ha ha!)

Sorry for the diversion, but if someone that is better educated than me could
shed some light on the matter, I'd be grateful.

~~~
aaaaaaaaaab
You’ve rediscovered eternalism:
[https://en.m.wikipedia.org/wiki/Eternalism_(philosophy_of_ti...](https://en.m.wikipedia.org/wiki/Eternalism_\(philosophy_of_time\))

~~~
akvadrako
If eternalism is real it's hardly fair to say he _re_ discovered it - all the
discoveries have always existed.

~~~
stan_rogers
Discovery is not invention, it's literally just noticing (or uncovering in a
metaphorical sense) something that was always there.

~~~
radisb
How do you know?

------
mathgenius
> Unlike general relativity, quantum mechanics, and particle physics,
> thermodynamics embeds a direction of time.

This is the bit that irks me. Quantum mechanics, the real QM that physicists
actually use, involves collapse of the wavefunction. This is absolutely time-
asymmetric. But in all these discussions of "why is time one-way?" this never
seems to be mentioned. Apparently QM is not a real theory, and just a
placeholder until we can work something out properly. Irk!

~~~
jblow
Many physicists do not believe that collapse of the wavefunction is a thing.

~~~
mathgenius
Ok, but why is there no discussion at all? I stand by my irk.

~~~
whatshisface
You _can_ get every conclusion you want out of QM without any wavefunction
collapses, so there's no need to discuss it (depending on your purpose) and if
you're an MW-er you'd argue that, further, there's no reason to believe that
it even happens. Since there are several self-consistent "piles of words" that
all talk about the same math but contradict each other, (Does the wavefunction
collapse? Does 1600s literature simulate the simulacrum?) not talking about it
seems completely justified.

~~~
mathgenius
> You can get every conclusion you want out of QM without any wavefunction
> collapses

I don't think so.

> "piles of words"

I definitely agree about the piles. Maybe ordinary language is just to feeble
for this stuff.

------
gtrubetskoy
Would it be fair to say that the second law of thermodynamics is only "a law"
given our human perspective of the direction of time? If time has no "arrow",
but a memory is only possible in non-decreasing entropy which is our
perspective, there can be another perspective in which the big bang is the
future and our future is the past, only we cannot comprehend it because a
memory is not possible in decreasing entropy?

~~~
danharaj
The second law of thermodynamics is about entropy, which is an observer
dependent quantity. It is the discrepancy between what one knows about a
system (e.g. its temperature) and its state (e.g. the position and momentum of
every particle in a classical gas). A being that knows the exact state of a
system does not observe entropy increasing or decreasing. At least
classically.

I don't understand the rest of your question.

~~~
SomeHacker44
Digging deep into my memory of classical mechanics, I think this is along the
right lines. However, I believe (seem to recall) entropy is an actual measure
of the number of ways a system can be ordered in its details and still produce
the same generalized outcome.

A crystalline solid has relatively low entropy because you can be reasonably
sure where each nucleus is, as they are highly ordered. A gas or plasma can
have the nuclei distributed nearly randomly. Both systems can be measured
generally (stochastically, or of their overall or average properties) but the
number of ways you can organize all the nuclei to get those results very
different between the two systems.

Feel free for someone more knowledgeable to correct or expand.

~~~
dodobirdlord
That's basically the gist of it. The entropy of a system is proportional (via
Boltzmann's constant) to the natural log of the number of available
microstates, where _microstates_ are configurations the system could be in,
and _available_ denotes that they have the same total value for all conserved
quantities as the aggregate system. Consequentially the system is free to
spontaneously transition between any of its available microstates.

------
kwhitefoot
If you found that interesting you might also enjoy Rovelli's defence of
Aristotle's physics:
[https://arxiv.org/abs/1312.4057](https://arxiv.org/abs/1312.4057)

------
eveningcoffee
We know that the universe went through various very fast state changes in its
very first fraction of seconds.

Given that the gravity must have been immerse then how should we understand
this time?

~~~
gregdunn
>Given that the gravity must have been immerse then how should we understand
this time?

We don't really know. There are speculative theories, but they don't all
agree.

It's very difficult because the temperature would have been so hot at the very
start that instead of four fundamental forces (of which gravity is one), we
would have had only one. There's also the Hartle-Hawking State, which is a
theory that proposes at the very "start" there simply was not time, only
space, and that time coming into existence was part of the process that
occurred during the planck epoch.

There's also the Grand unification epoch, inflationary epoch, electroweak
epoch, quark epoch, and hadron epoch that occurred in the universe before a
second had passed, all with very different implications for how physics could
have worked, so the answer would change multiple times during that period.

------
rbrbr
This guy uses rather confusing descriptions of his theories and ideas. I
respect his approach but other are much better in explaining these topics.

~~~
keithpeter
Rovelli mentions his paper from 2009 with the title 'Forget Time' in the
video. The paper can be downloaded from [1].

I enjoyed the presentation. Talking for an hour on this kind of material with
just a piece of string and a couple of watches is fairly impressive. The
thought experiment with the wooden box and green/red balls (in two size
categories unrelated to their colour) was thought provoking.

I'd like to see someone define/construct two sets of macrostates that give a
different time scales in a system with one set of microstates. For all I know
someone may have actually done that. The paper referenced does have a section
on the 'thermal time hypothesis'.

[1] [https://arxiv.org/abs/0903.3832](https://arxiv.org/abs/0903.3832)

------
interfixus
> _In fact, clocks tick slower when they are in a stronger gravitational
> field_

They shouldn't, if they are properly made. They should tick and measure at
their usual rate. Their local timeframe is out of whack with somewhere else,
but that's got nothing to do with the clocks.

~~~
gregdunn
It's unfortunate you're getting downvoted, because this is correct.

For anyone that is dubious: Black holes are one of the common ways this gets
discussed, because it's such an extreme example, and you can read more about
it at
[http://www1.phys.vt.edu/~jhs/faq/blackholes.html#q11](http://www1.phys.vt.edu/~jhs/faq/blackholes.html#q11)
\- this goes into a bit about photons and the event horizon - you can ignore
that and just consider it in regards to the time dilation effects of gravity.

~~~
interfixus
Thank you. It is faintly disturbing how many downvotes short, factually
incontrovertible comments may often garner on HN.

