
Physics laws cannot always turn back time - vo2maxer
https://www.astronomie.nl/physics-laws-cannot-always-turn-back-time-77
======
tom-thistime
As usual with physics links, the layperson's blurb sounds kind of silly and is
easy to criticize. But the actual paper is linked and you can look at it:
[https://academic.oup.com/mnras/article-
abstract/493/3/3932/5...](https://academic.oup.com/mnras/article-
abstract/493/3/3932/5736044?redirectedFrom=fulltext)

General HN comment: I think our discussion on physics topics will really be
improved if we comment on the physics papers (start with the abstracts)
themselves.

The (completely understandable) trend right now is to focus on the press
release or science journalism that accompanies the link to the physics.
Unfortunately that normally leads away from the basics of what's actually
happening in the physics.

(Metaphor here: If you're a programmer, imagine reading about some CS topic in
Wired magazine, then looking at 200 comments from people who agree or disagree
with the pop sci writeup.)

~~~
leetcrew
New garbage collection algorithm makes manual memory management obsolete!
Kernel devs hate it!

~~~
diegoperini
You deserve reddit gold!

~~~
leetcrew
thanks, I really appreciate the kind words!

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sarah180
This headline is (unsurprisingly) misleading. The argument is actually "the
rounding error of this calculation is smaller than the Planck length, and we
still get rounding errors that compound to large amounts over time."

This is really an argument about the ability to measure and simulate, not
about which of the laws of physics are invariant under time reversal.

~~~
nabla9
The content of the matches the headline.

What they do is to increase the accuracy so much that physical perturbations
at the quantum level dominate, not numerical accuracy. Irreversible up to the
Planck length means fundamentally unpredictable.

[https://arxiv.org/abs/2002.04029](https://arxiv.org/abs/2002.04029)

~~~
carapace
> The time symmetry is broken because disturbances the size of the Planck
> length have an exponential effect.

Yeah, that's the point right there up front: Butterfly effect at the sub-
Planck scale.

~~~
perl4ever
I guess I'm too dumb to understand what is novel about this? What does it tell
me beyond James Gleick's popularization of chaos theory circa 1987?

In some other thread, people told me I was wrong about reality having finite
resolution. So _is there_ a "sub-Planck scale"? How can simulations tell you
if there is or isn't?

~~~
lonelappde
There isn't a sub Planck scale. That's the point. You'd need a sub Planck
scale in order to reverse the computation.

Chaos theory is math. This is physics showing how the math applies to reality.

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fallingfrog
Mm, no I think what this shows that in just the same way as the future is
uncertain (you don’t know which path you will observe an electron to take),
the past is uncertain in the same way (you can’t know which path an electron
took). They’ve just expanded that principle to a cosmic size (there are
several past histories, diverging on the Planck scale, that lead to the same
present day observations). That doesn’t mean physics is not reversible, it
just means there isn’t a _unique_ way to reverse it. Which, we already knew.

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zelphirkalt
Perhaps this is a philosophical question in the end again.

There are open questions: Why would their accuracy be sufficient? Is there a
proof of it being sufficient to make such a claim? Then we have: Is their
simulation correct? Can we even make a correct simulation inside a computer,
about 3 black holes and in addition to that, make general statements about
laws of physics, based on the results of that simulation? Perhaps to make a
perfect simulation, to be really sure, we would need an apparatus of as much
matter as fell into those black holes.

So I remain sceptical.

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gus_massa
Wen you use this level of precision, you need to consider the quantum effects.
And [the current version] of Quantum Mechanics is time reversible. The problem
is that they mix some Classical Mechanics that is time reversible and Quantum
Mechanics that is time reversible, and the wrong mix is not time reversible.

Autonitpicking: There are some technical details about time reversal, but as
far as we know the CPT symmetry is not broken.

~~~
crimsonalucard
I'm no physicist but shouldn't QM be not time reversible or even not
repeatable?

Based purely on the fact that events happen randomly within some constraint
then if you go forward, backward or repeat nothing can ever be the same even
when initial conditions are exactly identical?

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akimball
The evolution of the wave function which determines the probability
distribution sampled by observations is deterministic and reversible in all of
the quantum theories I have personally studied (which is not many).

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crimsonalucard
I'm talking about the actual materialization of the particle. Not the
probability wave itself. Dunno if it's the right term "materialization" but
basically I'm saying when it's no longer a probability wave but it becomes a
particle it appears at random places within the probability wave. The location
of the particle after the materialization of the particle itself will be
different forward or backwards through time? Does someone know?

~~~
gus_massa
Wavefunction collapse is a huge problem, and taken at face value make QM not
reversible.

Nobody is sure, but probably when you consider a bigger system that includes
the experiment and the observer, the time reversion is restored.
[https://en.wikipedia.org/wiki/Quantum_decoherence](https://en.wikipedia.org/wiki/Quantum_decoherence)

(Note that the observer in QM doesn't need magical properties like
consciousness, in some experiments it is enough to place a brick in the right
position.)

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rland
I have a question for those more knowledgeable about physics:

Everything we know so far points to time invariance * , right? What would
happen if we discovered that some things truly aren't time invariant? Would it
crucially change the fundamental understanding, or would it just be a new
class of phenomena?

* except maybe TFA which isn't really right based on the other comments I'm reading.

~~~
Enginerrrd
Not at all a physicist, but I think I can take a stab at this. I think so far
we only see CPT (charge, parity & time reversal) symmetry as being a fully
unbroken symmetry of nature within the context of what you're asking. (I
believe the weak force breaks a simple time reversal symmetry.)

Time symmetry is typically synonymous with energy conservation in classical
mechanics. It get's more complicated when you introduce quantum mechanics, and
lorentz invariance from relativity. In essence though, there's probably an
analogous invariant to energy conservation for CPT symmetry. Maybe this is
just lorentz invariance for a quantum field theroy? Not quite sure.

Implications of seeing that symmetry broken would be huge since the CPT
theorem implies that a quantum field theory obeying lorentz invariance obeys a
CPT symmetry. (I'm guessing by an application of Noether's theorem?) SO a
violation of that would be either a violation of lorentz invariance (which
would be VERY interesting) or some type of violation of the assumptions of
quantum field theory. In essence, it would mean new physics!

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jng
Isn't this paper more about simulation techniques than about reality itself?
It looks like time reversibiliy (sub-planck length or otherwise) should be
studied in closed form or at least in non-lossy mathematical ways, not
simulated with imprecise digital computers. Montecarlo simulation to calculate
pi is just not a good way to know whether the digits of pi ever repeat or they
don't.

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phkahler
The guys should dig up an old copy of "Chaos" and read it. Their simulations
exhibit sensitive dependence on initial conditions. The predicted orbits will
diverge similarly in the forward-time direction if tiny perturbations are made
to the initial conditions. They're also subject to rounding errors.

Nothing to see here about time reversal, its just chaos.

~~~
sarah180
This is all correct—but you should always look at the actual paper and not the
summary article that some journalist wrote. The authors are pretty clear about
what they've shown and how it fits into ideas about chaos.

~~~
scottlocklin
I disagree: the paper wasn't linked -the dumb summary of it was. The paper
itself is a trivial result of chaotic dynamics rather than "muh mind bending
woo headline."

Newsflash: a simple double pendulum also requires Planck length precision in
initial conditions (or would if it weren't for, like, friction) for
reversibility if you sit around and watch it for long enough and excite it
properly.

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scythe
The quantum mechanics of black holes is still open, isn't it? As I recall even
Hawking entropy depends on putting the (non-unitary) Second Law of
Thermodynamics in as a prior. This turns out to be consistent with (unitary)
string theory and LQG, so far at least.

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saadalem
To travel through time, you’ll have to apply the laws of special relativity.

Time Travel Is Possible, But We’ll Need Lots Of Negative Energy

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codesnik
well, system of black holes would lose some energy via gravity waves, this
isn't something that could be reversed, yes?

~~~
Florin_Andrei
In a simulation you would simply flip a sign on that one and it should go
back. I've done plenty of simulations myself where waves radiate from a
source, and those are perfectly reversible.

The article is pointing at a more subtle issue related to a fundamental small-
scale fuzziness which affects chaotic systems and breaks reversibility.

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chrisshroba
Probably a naive question, but why is this a subtle issue? Isn't this
basically the well-known butterfly effect?

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Florin_Andrei
In a nutshell, yes, it kinda is a sort of butterfly effect.

But look, you go to school and they drill into you until the cows come home
that quantum mechanics is time-reversible, and gravity is time-reversible, and
you just assume this is always true.

But turns out, all that's needed is a bit of small-scale fuzziness due to QM
effects, and then particle ensembles of 3 or more are suddenly not time-
reversible anymore, though each of the basic laws still are.

For someone like me this was a small shock.

~~~
perl4ever
Ok, but if you have the idea that "Planck scale" means something, then how can
it be a shock then?

Often understanding something better makes it _less_ obvious, but at my level,
I don't understand what's non-obvious about this.

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canjobear
This only shows that time evolution on the observer‘s branch of the wave
function is not reversible. Considering the whole wave function, the time
evolution is still unitary.

~~~
danbruc
This was not a quantum mechanical simulation, the wave function is not
relevant for this.

