The Mystery of Time’s Arrow | Hacker News Reader

The Mystery of Time’s Arrow

nautil.us

94 pointsfurcyd53 comments

What if time's arrow can be reverted, and is being reverted all the time, but we don't notice it because, just like and omelet would become an egg, our brain state would revert to a previous one?

As far as we know time could be running in reverse, but the process in our brains would make us feel like it flowing forward. The book "Permutation City" explores this idea, albeit in simulated brains.

Also check out some of the work that Daniel Sheehan is doing (from USD, not the lawyer) they just raised the needed funds to start a test on Quantum Retrocausation. Daniel also has a conference on Retrocausation, it's pretty great.

https://experiment.com/projects/are-we-catching-photons-trav...

That is in fact exactly what quantum mechanics predicts.

http://blog.rongarret.info/2014/10/parallel-universes-and-ar...

1) Galilean relativity actually plays a role in deriving the Schrodinger equation (at least convincing yourself it’s correct. See the books by Commins and Yaglom).

2) QM without statistical mechanics/thermo seems an odd basis to make prognostications about time or reversibility. Time isn’t an operator (see the classic Mandelstam-Tamm paper)

> Time isn’t an operator

Yes, that's because time isn't actually a physical phenomenon. It's a kind of illusion, an approximation to the truth, in the same way that Newtonian mechanics is an approximation of GR in the weak-field limit. The wave function is a static, four-dimensional "thing" (it's not actually a thing, but I don't know of a more general noun). Our perception of a classical universe that evolves in "time" is not a reflection of a real underlying physical process, it's an emergent property of a sequence of subsets of the wave function in which entanglements have accumulated. Quantum information theory predicts the emergence of subsystems that are all in classical correlation with each other, i.e. a "classical universe". But none of this is actually real in a metaphysical sense.

For all you know you don’t even experience time going forwards. At any given instance, how do you know that you “experienced” the past second. What if your conscious experience is just an instance at that point of existence? You can’t point to recollection of the past second because that recollection happens through time as well. What if that’s just an illusion of the brain state at that time?

Did Descartes really think?

Also known as a Boltzmann Brain.

https://en.wikipedia.org/wiki/Boltzmann_brain

I am very grateful to be introduced to this concept. During--and occasionally since--a particularly traumatic psilocybin trip some years ago, I had the distinct understanding that my consciousness had only just been put together (or, rather, re-arranged from another, very alien form), memories and all. It is comforting to have such bizarre experiences reflected in others' thought experiments.

Classically, we understand our neurological arrangement to be a function of each last instant's without discomfort. It's another thing entirely to consider how we might have been arranged from wildly different previous states.

Although there's no compelling evidence to support it, neither is there a way to disprove it, I'm afraid.

A really strange thing about Boltzmann brains is that if you suppose their existence, then the question "Am I a Boltzmann brain?" becomes complicated. It makes sense to ask "Is there a human thinking the thoughts I am thinking?" and "Is there a Boltzmann brain thinking the thoughts I am thinking?", but if you suppose the answer to both is yes, then when you ask the question "Which one am I?", there's no pure logic you can follow within your mind to answer that question, because both the human and the Boltzmann brain are in the same mental state and any logic either of them follow will be followed by the other, and any logic that comes up with a definite answer must be flawed because it comes to a wrong answer for at least one of them.

Of course, it's not particularly useful to consider the Boltzmann brain version of yourself because a Boltzmann brain's decisions don't have any consequences. Time is better spent considering decisions by the consequences of the human taking them. Maybe this implies that you should choose to define "yourself" to refer to both your mental state combined with the ability to make decisions of consequence, and then you can safely disregard the strange kinship you may have with a Boltzmann brain. Though now this line of thought invites interesting questions about how to define what makes a decision consequential, and what exactly makes human decisions consequential...

I see consciousness as the threading together of alternate futures where the Boltzmann Brain doesn't collapse out of existence.

> Did Descartes really think?

For the purposes of his proof, yes:

Dubito ergo cogito ergo sum.

The witness of a doubt is the witness of a thought is the witness of a thing, hence a thing must exist.

Descartes at that point in his argument doesn’t make any assertion to the nature of reality, merely that each of us can be sure that something exists, even if our interpretation is completely wrong: the very act of doubting existence is witness to existence.

People project their own model of self into those terms, but it’s merely trying to make a broadening categorical argument and not a particular interpretation of what the objects normally represented by those terms are or how they behave (eg, times exists).

A static Descartes with the experience of currently doubting is sufficient witness for the proof, which Descartes would have himself experienced directly — the cogito is a witness to that experience of Descartes.

Any explanation that denies everyday's experience might be fun but mostly useless. That we cannot give a clear explanation of our perception of time does not deny it. Electricy existed before humans could explain it.

Lack of denial doesn’t prove existence

Neither does lack of proof of existence prove implausibility of existence

There is no future. Only present

There is no present just a discrete plank scale time that you can be ignorant and call it arrow of time, or be present and try not to fall in the illusion of continous flow.

I don't think the discreet model by itself explains the arrow but it is imo a part of the puzzle.

As you are very likely familiar with the 2-slit experiment, I am of the view that 'information' [?] flows from both directions of what we perceive as 'arrow of time'. The act of choice constrains the possible pathways and that reduction is what we call 'state collapse'. So without measurement at the slits, all possible pathways are active and naturally we perceive them as an 'interference pattern'. As soon as we close the door on 'choice' flow from 'future' has no other possible path way other than the single open slit. This we note as the 'particle' modality of the experimental setup.

That answers the so-called 'paradox' of the experiment but still doesn't address why we perceive an arrow of time going forward. Now returning to the 'discreet' aspect, I posit the 'computation' of the 'virtual smooth construct of perceived reality' is what we are registering as a forward movement in an inherently timeless universe. It all seems to be inexplicably bound up with the phenomena of consciousness.

(Not expert) Does this have to do with Feynman's one electron universe (https://en.m.wikipedia.org/wiki/One-electron_universe) that goes back in time?

Precisely what set those thoughts in motion for me a few decades ago.

Also, check out physicist Julian Barbour's notion that the universe is entirely static, and that our mind 'constructs' the illusion of time's arrow by orderings its perception of a collection of configuration spaces he calls 'nows'

FWIW, he’s the author of the article.

Zoinks! that'll teach me not to comment before reading the article. Thanks for the heads up!

It's interesting how, in the OP, Barbour doesn't use much of the language he leaned on for "The End of Time' [1999, the source of the ideas I was referring to]. Maybe couching his ideas in more normal language is helping him reach a wider audience, which is great.

Side note : I once emailed Barbour with some question about his view of Machian dynamics, and he emailed me back a thoughtful and kind reply - helluva nice guy

p.s. his "other" book, 'The Discovery of Dynamics' aka 'Absolute or Relative Motion?,' is also a great read

FYI, this article is based on his paper: https://arxiv.org/abs/1310.5167

Interesting, but isn't perception still a dynamic process requiring time? Or, said differently, how can a completely static universe support an apparently "evolving" perception?

Think of it like a map/reduce over rows in a database.

> is being reverted all the time

Think for a second about what you're saying here, and if it makes sense. What does 'all the time' mean in this context?

Good point, I haven't thought of that.

I replied to GP, outlining a potential resolution. Thought you might like thinking about that kind of thing, so letting you know directly.

and what does "for a second" mean?

It's not entirely incoherent, since time is relative to the geometry of spacetime and your trajectory through it. OP's original statement could be referring to the time coordianet in two separate frames of reference, so we might say that time (for us) is being reverted all the time (from someone else's perspective).

It's even theoretically possible to have closed-timelike curves in general relativity, which means that time would literally loop for some set of set of observers. Any matter riding on these curves does have to have periodic dynamics, so in a sense it would be evolving and then reverting endlessly.

To be fair though, the existence of such solutions to Einstein's equation is seen as problematic, since they raise questions of time-travel paradoxes and such. Also, I have only a passing familiarity with the details of the physics in such spacetimes, so take what I say with a hefty grain of salt.

Ah Eternal Return, it brings comfort to know that we may simply be living our exact same life over and over again for all eternity. Really motivates me to live to the fullest, to have the most enjoyable eternity.

> You can make an egg into an omelet, but you can’t turn an omelet back into an egg.

The 2015 IgNobel was awarded for uncooking an egg!

http://www.news.uwa.edu.au/201509258005/awards-and-prizes/eg...

> You can make an egg into an omelet, but you can’t turn an omelet back into an egg.

It's missing an asterisk:

* without expenditure of energy

You also can’t make an egg into an omelet without energy expenditure.

In a physical simulation, you can't simply set the timestep to -1 to recover previous states for the sole reason that each step adds inaccuracy to the model by way of rounding error.

Like this saying "There is nothing in the form of the laws of nature at the fundamental microscopic level that distinguishes a direction of time." There is nothing in the simulation which prefers to step time by 1 or -1 , but the unavoidable rounding error means it can't step exactly backward. I guess its intimately attached to the fact it cant step exactly forward.

This is why the shattered virtual object wont rearrange perfectly going backwards - it didnt smash perfectly when it was going forward.

I had an idea for making previous states recoverable by applying extra rounding between every timestep, but now I suspect this will not entirely work either, as some least significant imprecision occuring during the timestep will still occasionally trip the more significantly rounded 'interstep' state values, irrecoverably.

Rounding errors aside, you can still experience information loss when stepping time. Any observed state which could have resulted from more than one previous states (by the rules of your simulation) represents an irreversible transition.

This was an interesting point which I missed at the time. However this situation would only occur extremely rarely in the normal running where the timestep is set small enough to produce reasonably accurate/continuous behaviour. Basically a state is so intricate and detailed that a coincidence of that kind is difficult to envisage, among multiple bodies, holding space and velocity values. And since going forwards or backwards in time always produces a single solution, (rather than multiple solutions) the idea of multiple plausible inputs still seems reliant on rounding error to me or numerical oddities which get more unlikely the more precision and smaller timestep is employed.

Perhaps the simulation isn't time reversible, but that doesn't say anything about the reversibility of the laws of physics and the reversibility of the universe. I.e., reversing the entire universe would still run the simulation backwards, correctly, because it takes place at a lower level.

However, I think the point of the article is that given a deterministic universe, even though it doesn't "run" forwards or backwards but just exists throughout space-time, observers inside the universe can still observe an "arrow of time" in the direction of more stability.

Yes. Or restated, that it's almost only ever possible to form memories of a universe that has increased in entropy.

Im not sure. Simulated observers could also observe the direction of time progression; that orders in the past become irrecoverable to them is a mechanistic result of infinite precision not being preserved from one moment to the next.

If the simulation was quantized, say like using integers of a large enough size to encompass all things in your simulation, it would be fully reversible I believe. Of course I'm speaking from my limited understanding of physics engines in computers today like bullet or equivalent. Isn't floating point just like a compression for large numbers in limited bits, and the only reason it would not be reversible? The other potential non-reversibility is the fact that the physics engine uses some sort of integration like adding up forces at each time step unlike the universe which I believe is instant. (verlet integration? Google says that is reversible.)

I do see floating point as like automatically compressed integers, but unlike simulations such as, Conway's game of life, the operations in a physics simulation produce real number values even if we begin with integer state, due to square roots and divisions mainly. I think whatever strength of integer or floated precision used to store results, it will never be possible to tell whether a certain parameter was eg. 328480284374964325 or 328480284374964326. And we need to be able to tell exactly what the input state for any output state must be, in order to have reversibility.

Does anyone know how this work relates to Rovelli's thermal time hypothesis? (https://en.wikipedia.org/wiki/Thermal_time_hypothesis)

I thought Rovelli was into loop quantum gravity

It strikes me as analogous to the near and far field of a radio antenna? The complicated initial close-in chaos corresponds to the complicated near field of an antenna. As the EM wave propagates further away from the antenna it assumes a simpler "far field" structure. On other words, as the wave gets further away from the antenna the physical extent of the antenna becomes neligible compared to the distance propagated, so the field looks more and more like that from a simpler point source.

I always thought the mystery of why everything going the same way was a bit odd considering our perspective.

It's like having a particle system create a million particles at x=0 with random velocities then measuring a sample at 5000<x<5500 and wondering why everything that goes through that range has a positive x velocity.

e.g. you could have folks on the other end of the big bang, where the farther you go back from our perspective, it looks like going into their future instead of further into their past

That would be a fascinating idea for sci-fi! The inhabitants of two distant worlds would be able to see each other's future but not their own.

I think it's no mystery at all if you consider Heisenberg's uncertainty principle. To reverse the arrow of time would mean that all particles reversed their momentum. Observing that maybe in a limited area of space would tell us that the arrow of time has clearly been reversed since things move towards more organization rather than less. The observed entropy would decrease.

But since particles do not even have precise momentum it makes no sense that you could precisely reverse it. This imprecision is everywhere and as it interacts with its surroundings it can only grow larger. Therefore entropy can never decrease and time can never flow backwards.

Even if you could reverse the momentum of all particles, that wouldn't be enough because T-symmetry can be violated.

This is an interesting idea, and well presented.

I think the conventional explanation of the low entropy state of the early universe is a brief period of “inflation” that resulted in an extremely smooth and uniform density throughout the universe. Does the theory presented in this article replace inflation?

Wow, what is happening to funding of academic research? Why do we need a "Kickstarter" for scientific experiments?

https://experiment.com/projects/are-we-catching-photons-trav...

Is appealing to distributed individual interest worse than appealing to corporate interest?

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