[HN Gopher] Opposing arrows of time can theoretically emerge fro...
___________________________________________________________________
Opposing arrows of time can theoretically emerge from certain
quantum systems
Author : jnord
Score : 90 points
Date : 2025-02-16 22:38 UTC (1 days ago)
(HTM) web link (www.surrey.ac.uk)
(TXT) w3m dump (www.surrey.ac.uk)
| chrisjj wrote:
| Clickbait title. The body reports no evidence.
| dang wrote:
| Ok, we've replaced it with a perhaps more representative phrase
| from the first paragraph.
| chrisjj wrote:
| Sorry to have been sufficiently specific. I am critical of
| the article, not the post.
|
| "Physicists uncover evidence of two arrows of time emerging
| from the quantum realm"
| luxuryballs wrote:
| "suggesting that time's arrow may not be as fixed as we
| experience it"
|
| this is what they mean by stretching spacetime right?
| andrewfromx wrote:
| So if you visualize milk spilling on a table in reverse that
| sense of that being fake or wrong or not possible is just your
| human point of view? Better to think of that like you are seeing
| a pendulum. Moving time backwards isn't impossible. It's just
| improbably?
| klysm wrote:
| That might be a bad example due to gravity
| someothherguyy wrote:
| https://www.nature.com/articles/s41598-025-87323-x
| MattPalmer1086 wrote:
| I have never understood why physicists get so hung up on the
| arrow of time and entropy.
|
| If you have a casual system, then statistically, things will tend
| to become more disordered over time, because there are just more
| ways to be disordered than ordered.
|
| Lots of clever people seem to agonise about this, but I don't see
| any problem. What am I missing?
| DiogenesKynikos wrote:
| That the laws of physics are symmetric under time reversal[0],
| so any statement you make about how things should evolve "over
| time" also holds if you go backwards in time.
|
| The apparent arrow of time is commonly thought to have to do
| with initial conditions. Entropy was low right after the Big
| Bang, creating an asymmetry between the two time directions.
|
| 0. Technically, the laws of physics are symmetric under
| stimultaneous inversion of time, flipping of all charges, and a
| mirror reflection of space.
| JohnKemeny wrote:
| What is time?
| GenerocUsername wrote:
| Our brains operate on the principle entropy, therefore we
| experience time as the macro evolution of the universe along
| the dimension of increasing entropy.
|
| Time itself does not exist, we merely experience it because
| it is how our consciousness experiences reality
| geysersam wrote:
| Do you mean to say that our experience of time depends on
| what we see happening around us? If I place you in a room
| with nothing but a video tape playing backwards, would you
| experience time going backwards? That doesn't sound
| plausible.
| garciasn wrote:
| On a universal scale, time is a distinctly human
| construct and it clearly warps based on location,
| distance from Earth, and is heavily influenced by our
| biology and location in the solar system (days, years,
| etc).
|
| It just doesn't matter on a universal level.
| nntwozz wrote:
| This is not consensus.
|
| Kurzgesagt - In a Nutshell has a very good video on it:
|
| Did The Future Already Happen? - The Paradox of Time
|
| https://youtu.be/wwSzpaTHyS8
|
| "To be honest, no one knows. What we've learned are two
| possibilities to describe time, but they're not the only
| ones. Some scientists think that the idea of "now" only
| makes sense near you, but not in the universe as a whole.
| Others think that time itself doesn't even exist - that
| the whole concept is an illusion of our human mind. And
| others think that time does exist, but that it's not a
| fundamental feature of the universe - rather, time may be
| something that emerges from a deeper level of reality,
| just like heat emerges from the motion of individual
| molecules or life emerges from the interactions of
| lifeless proteins."
|
| They touch on block universe: https://en.wikipedia.org/wi
| ki/Eternalism_(philosophy_of_time...
|
| ...and growing block universe: https://en.wikipedia.org/w
| iki/Eternalism_(philosophy_of_time...
|
| "The growing block universe, or the growing block view,
| is a theory of time arguing that the past and present
| both exist, and the future as yet does not. The present
| is an objective property, to be compared with a moving
| spotlight."
|
| Excellent video worth your time.
| GenerocUsername wrote:
| OP here, and no, that's not what I mean.
|
| What I mean is that since our brains store memories and
| generally process information utilizing entropic systems,
| we perceive time as always moving forward. There is no
| fundamental physical direction to time... But since our
| experience slides along an axis of increasing entropy, we
| perceive the whole universe sliding on that axis as well.
| MrMcCall wrote:
| > Time itself does not exist, we merely experience it
| because it is how our consciousness experiences reality
|
| That's backwards: Time exists to allow our consciousness to
| experience change over time. This is because time exists to
| allow change over time to the physical universe.
|
| That said, there is only the ever-present Now in a
| constantly evolving universe.
| Trasmatta wrote:
| Some of what you perceive as "getting hung up" may must be
| physicists trying to explain the oft misunderstood concept of
| entropy to people who have misconceptions about it.
|
| That being said, there are still plenty of unanswered questions
| around time in physics. Even just open questions about block vs
| growing block universe are unanswered, and tied into the
| ultimate nature of time.
|
| Also, we know that entropy was much lower in the past, but we
| don't know WHY.
| mitthrowaway2 wrote:
| Liouville's theorem implies that for a causal system, the
| volume of phase space is constant; therefore the total
| configuration space of the system does not grow over time.
| However it can become harder and harder to precisely describe.
| Nevermark wrote:
| > Lots of clever people seem to agonise about this, but I don't
| see any problem. What am I missing?
|
| I feel the same way. The past is just the direction where
| things were more ordered (to be glib about it.)
|
| Even though the large scale implications of the statistics are
| profound, star fusion, rivers, etc. the "cause" is statistics.
|
| The one place (that I am aware of) where the statistics -> time
| effect still needs explicit work is how the timal fabric
| dimension operates differently (but very similarly) to the 3
| spacial fabric dimensions of space-time in the very large
| scale. Often represented by a negative (for time) and three
| positive (for space) signs in equations. But I assume when we
| understand how space in the small works, the topology will
| still have a statistical explanation.
|
| --
|
| A very close topic, and the same forehead slap feeling for me,
| is all the contortions many physicists go through to avoid the
| many worlds quantum interpretation. Which is the parsimonious
| interpretation.
|
| (Should more accurately be called the "quantum field web"
| interpretation, according to ... me.)
|
| Again statistics: we just think there is a "collapse" in
| possibilities when super-positioned information becomes to
| mixed up for us to remix.
|
| The alternative: a mysterious collapse with yet unexplained
| cause, with deep problems regarding how this introduces another
| unique "time" effect, of _nearly infinite new information
| constantly being injected into every interaction everywhere and
| everywhen_ in the universe! And this information is supposed to
| be causeless or self-caused in some mystical (but don't ever
| say "mystical"), way.
|
| And don't point out the mockery this makes of "conservation of
| information" if 99.99...99% of information now wasn't there
| just one quantum event ago, for all quantum events.
|
| Again, statistics completely explains why us biggies experience
| fields making choices, based on the field equation defining
| field connection (we experience as event) point distributions,
| and ... statistics.
|
| Yet every day physicists who can't admit to themselves that we
| are ourselves constantly expanding webs of superpositions, take
| for granted that anything they can scale up in the lab will
| have superpositions, subject to experimental practicalities.
|
| Because anything else would violate the field equations.
| Trasmatta wrote:
| > I feel the same way. The past is just the direction where
| things were more ordered (to be glib about it.)
|
| The only problem is that this ignores the fact that we still
| don't know WHY entropy was lower in the past. It's an open
| question in physics.
| mitthrowaway2 wrote:
| It also sidesteps another important distinction: why the
| past is the direction we can form memories of.
|
| Due to time-reversal symmetry, you could easily do a
| t<-->-t substitution, flipping the time axes around. Now
| entropy _decreases_ over time, with the big bang being the
| point of minimum entropy, located far ahead in time. But
| regardless of the convention, our memories exist along the
| direction between us and the big bang, rather than the
| other direction.
|
| So if increasing entropy is to be the arrow of time, it
| needs to relate to why we form memories of the past but not
| of the future.
| Nevermark wrote:
| > It also sidesteps another important distinction: why
| the past is the direction we can form memories of.
|
| That much is very well understood.
|
| Order from the past is what we harness to make memories.
| That's why you can't have memories the other direction.
|
| Think of a computer. It takes ordered energy, organized
| as a stream of electrons, which end up in a less
| energetic form we can't use and heat we can't use.
|
| But in the eddies of that entropy, the computer
| calculates a few things. Uses up that order to store a
| few things.
|
| The net is massive loss of order. One electron can go
| either way on a wire. With (usually) or against (mostly)
| voltage. (Because we randomly decided electrons are
| negative.)
|
| But a trillion electrons statistically can only go one
| way.
|
| The order we build in our minds and computers is a
| fraction of the order from the past. Which is just
| saying, we get order from the direction there was more
| order.
|
| You could set up a tiny few particle experiment, shield
| it from interactions, and operations can randomly go
| either direction.
|
| But our neurons and brains are built out of countless
| particles. We can only do ordered things with them by
| consuming order from the direction there was order.
|
| A windmill can do work, but only if you let higher water
| push the wheel. If the river stops from above, no amount
| of water below is going to power it. Statistically to the
| level of certainty.
|
| The order in the universes origin, statistically cascades
| away in all directions. And all those directions,
| radiating from order, away from order, are what lets us
| think, store memories, and have an experience we call
| time.
|
| But with a constant total net loss of order. We are just
| those eddies that defy the rivers flow, but only
| fractionally, by harnessing the rivers flow.
|
| (For how there can be places where order "first" appears,
| see my other comment. Unlike this one, it is highly
| conjectural.)
| mitthrowaway2 wrote:
| Yes exactly! (Great answer!)
|
| And this is why when the universe has a temporal entropy
| gradient, the direction of decreasing entropy gets
| labeled "the past". It's not because the past is the
| direction of "-t", but because the past is the direction
| of "-dS/dt". Otherwise we wouldn't form memories of it.
|
| This dissolves the mystery of why the past has lower
| entropy than the future despite the laws of physics being
| time-reversible, and replaces it with the question "why
| does the universe have an entropy gradient at all?"
| Nevermark wrote:
| Thanks, you said it better. The derivative makes it
| clear! The direction of maximum entropy decrease in our
| 4D topology, gets labeled "-t". The direction of maximum
| increase "+t".
|
| Did you read my other sibling comment about ordered
| regions of reality being inevitable?
|
| (Not being snarky, just that was the question I have a
| conjectural answer too.)
| Nevermark wrote:
| Yes! "Time" comes down to that one question.
|
| My provisional answer would be that reality is a web of
| constraints, which most certainly is essentially
| (deterministic chaotic) noise almost everywhere. But like
| the Mandelbrot set, has pockets where the chaos gets
| expressed as order.
|
| Because no order at all would actually be impossibly
| ordered.
|
| The same effect as proofs that if you simply add random
| edges to a graph, ordered patterns will emerge. What
| patterns is still random! But that patterns emerge is a
| certainty.
|
| Except instead of random, reality just churns and spans so
| many variations that there has to be pockets of order and
| of arbitrarily large size.
| tsimionescu wrote:
| > Again statistics: we just think there is a "collapse" in
| possibilities when super-positioned information becomes to
| mixed up for us to remix.
|
| You're ignoring all of the major problems which require
| additional assumptions for MWI to appear so parsimonious. The
| biggest of which being the preferred basis problem: a quantum
| system isn't just in a superposition of states along one
| axis, it's in a superposition of states along _the axis you
| choose to measure_. If you and I both choose to measure the
| same system along different axis, we 'll get different (but
| correlated) kinds of results about the same particle pair
| (which is the root of the Bell inequalities).
|
| Take an entangled pair of polarized photons, such that they
| must have opposite polarization, and send one towards you and
| one towards me. You'll measure the polarization of your
| photon along some axis, and I'll pick an axis that is 30% off
| yours. If we repeat the experiment many times, we'll conclude
| that percent of photons was polarized along the axis we each
| chose, and some percent was perpendicular. The percentages
| will be different, of course, and if we compare measurement
| results for individual events, we'll find that they correlate
| perfectly. But what can't be done, even in principle, is
| decide along which axis the photons were "actually"
| polarized.
|
| So, the problem for MWI is: how come classical objects have
| real properties independent of measurement choice, and they
| all happen to have properties along the same axis, given that
| the quantum objects they're made of don't, and there is no
| collapse?
|
| And this can't be explained away by statistics or entropy,
| because the amplitude of the wave function in QM is the same
| regardless of the choice of measurement axis. Even
| decoherence doesn't solve this problem, because lack of self-
| interaction doesn't affect the choice of basis.
|
| The other problems with MWI have to do with the very notion
| of doing probability calculations, and then statistics based
| on them, in a perfectly deterministic world where every
| outcome of every experiment is fully realized. If you flip a
| coin and both sides come up, how can you tell if the coin is
| biased or not? Or, since you prefer the quantity of
| information concept, in the MWI it seems that there is 0
| information from any experimental result, because you know
| ahead of performing the experiment that all of the possible
| results will happen with probability 1 (in some "branch"). So
| yes, information is conserved: it's always 0.
| Lammy wrote:
| When the object enters the timestream, time begins to
| correct itself. Let me use this example: Imagine four balls
| on the edge of a cliff. Say a direct copy of the ball
| nearest the cliff is sent to the back of the line of balls
| and takes the place of the first ball. The formerly first
| ball becomes the second, the second becomes the third, and
| the fourth falls off the cliff. Time works the same way.
| Nevermark wrote:
| Quantum field equations capture entanglement.
|
| Measurement choice impacts the field equations, because you
| are field equations too.
|
| So none of those problems exist. Just interpret the field
| equations as they stand.
|
| Yes, new information is information is conserved, if
| everything is predictable. Which the field equations are.
|
| That would not be true if collapse was real, as that would
| impact the system with new, previously unknown, inherently
| unpredictable information.
|
| (The fact that we as big things end up experiencing what
| looks like a collapse is explained by the field equations
| and the statistics of how quickly information, not
| carefully controlled, gets dispersed.
|
| At this point in the history of physics, we should be able
| to discard extra explanations for our experiences that are
| already completely and exhaustively explained by the well
| tested laws.
|
| The fact that the explanation requires thinking about
| things differently than ordinary experience, isn't an
| excuse for wedging poorly characterized and completely
| unnecessary fudge effects as shims into our intuition gaps.
|
| I am, of course, not arguing that collapse isn't a useful
| concept, with many pervasive _practical_ uses when full
| systems are not being modeled.
| tsimionescu wrote:
| > Measurement choice impacts the field equations, because
| you are field equations too.
|
| This sounds nice, but doesn't actually work if you try to
| apply it in practice, because however many experiments
| you'll perform on classical objects, you'll only ever
| observe properties for one basis of measurement, whereas
| you'll get different results for different bases for
| quantum objects.
|
| In other words, how come when I observe a single atom, I
| can see it in a state like (1/sqrt(2))|spin up> +
| (1/sqrt(2))|spin down>, but I can never observe a basket
| ball in a similar state? If it were just a matter of my
| field equations happening to only match certain kinds of
| states, that should apply exactly as much to a single
| atom as it would to ~6,022*10^23 atoms.
| Etheryte wrote:
| If that's true, then where did all the order that is there come
| from? Lots of open questions in that regard, plenty to agonize
| over.
| Buttons840 wrote:
| Is entropy real, or is it a purely intellectual tool of
| thought?
| MrMcCall wrote:
| With the CMBR being so low-temp (4K IIRC), combined with the
| vast distances of empty space (I know it's not really empty,
| but practically so), the tendency over time is that
| temperature will settle down to lower and lower energy
| states, as energy radiates outward. (Because no energy is
| being added to the system.)
|
| Order requires energy -- i.e. our highly thermodynamic body
| processes -- so as the temp lowers, disorder will tend to
| decrease.
|
| Maybe a physicist can correct me, but that seems like a
| simple yet accurate system-wide summation. Of course, local
| areas (such as Earth) can retain enough heat for a span of
| time to maintain complexly organized systems, but in the
| universe's time scale, it's going to keep winding down
| towards that CMBR, where ever little complexity will be
| possible.
| zmgsabst wrote:
| But steam has higher entropy than water has higher entropy
| than ice.
|
| I'm not sure I follow your description: as the universe
| empties, aren't we at some point left with less and less
| happening per cubic meter?
| lisper wrote:
| > What am I missing?
|
| Your theory:
|
| > If you have a casual system, then statistically, things will
| tend to become more disordered over time, because there are
| just more ways to be disordered than ordered.
|
| contains a tacit assumption that states are chosen at random.
| But assuming that is begging the question. _Why_ are states
| chosen at random? What is the source of the randomness?
| Newtonian mechanics doesn 't have any obvious source of
| randomness, and it's an open question whether quantum
| randomness is "really random". Bohmian mechanics is completely
| deterministic, and so is (obviously) superdeterminism.
|
| It's true that "there are more ways to be disordered than
| ordered", but in any time-reversible dynamic there are exactly
| as many states where entropy decreases as there are states
| where entropy increases because for any entropy-increasing
| state, the time-reversed state has decreasing entropy.
| MattPalmer1086 wrote:
| Even a completely deterministic causal system will tend
| towards greater disorder (if it wasn't already maximally
| disordered I guess). There is no need for randomness, just
| statistics.
|
| > but in any time-reversible dynamic
|
| Now that is making an assumption that the dynamics of an
| entire system could be thrown into reverse. Everything
| flipped to its opposite, and then proceeding onwards causally
| from that point.
|
| It also presumes that the system started in a state of order,
| got more disordered, and then you reversed it and then order
| "magically" appears. Your eggs unscramble themselves. If you
| merely take a disordered system and throw it into reverse,
| you will still just see a disordered system becoming more
| disordered, because there was never any surprising ordered
| state built in to be uncovered later.
| simiones wrote:
| > If you merely take a disordered system and throw it into
| reverse, you will still just see a disordered system
| becoming more disordered, because there was never any
| surprising ordered state built in to be uncovered later.
|
| This is circular. If the system is becoming _more_
| disordered, it follows that playing it in reverse will make
| it _less_ disordered. Sure, if the initial state wasn 't
| very ordered to begin with, this won't _look_ very
| different, but that 's entirely irrelevant: as long as we
| accept that "disorder" is a measurable objective property
| of a system, then there is a quantifiable difference
| between moving forward and backward in time. And this
| doesn't match either a deterministic time-reversal
| symmetric theory like classical mechanics, nor a
| deterministic CPT-reversal symmetric theory like QFT.
| lisper wrote:
| > that is making an assumption that the dynamics of an
| entire system could be thrown into reverse
|
| No, that's not an assumption, that's a mathematical feature
| of all known laws of physics. An it's not that "the
| dynamics could be thrown into reverse", it's that for every
| initial state, there is a corresponding initial state where
| the system runs in reverse, and hence, for every state from
| which entropy increases there is a corresponding state
| where it decreases. For a Newtonian system, it's a state
| where all the velocities have opposite sign. (For quantum
| systems it's a little trickier to describe.) So if you
| choose a state uniformly at random from among all possible
| states, the odds that you will end up with one where
| entropy is increasing is exactly 50%.
|
| And it gets even worse than that. In a universe that obeys
| certain conservation laws (which as far as we can tell ours
| does) a time-reversible dynamic is _unitary_ , which is to
| say, there is a one-to-one correspondence between an
| initial state and its successors. Therefore, for _any_
| initial state, the time evolution of that state must
| eventually loop back to its initial state [1], and so it
| must eventually enter an entropy-reducing state to get
| there.
|
| [1] https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_
| theor...
| cma wrote:
| ergodicity and Boltzmann brain parodoxes
| layer8 wrote:
| The question is why things become more disordered forward in
| time but not backwards in time, given that the physical laws
| have time-reversal symmetry. In other words, why does the
| macroscopic world seem to behave differently forward in time
| than backwards in time (e.g. biological life, or the evolution
| of stars), while the fundamental equations of physics don't.
| MrMcCall wrote:
| Well, we are a part of a physical system whose laws
| correspond to mathematical formulas, but _aren 't_ formulas,
| they're existing physical, energetic relationships maintained
| magically in realtime. And, yeah, it's magic, this universe
| we live in, my friend.
|
| So, while the math is reversable, time is a one-way street by
| virtue of the construction of the physical universe, which is
| not reversable. And it's that way because that's how this
| magnificent universe was instantiated.
|
| It may sound trite, but sometimes Occam is bang-on. As well,
| there are endless unknowns, but there are also unknowables.
|
| All matter is >99% empty, not counting field interactions. I
| don't know how much of it can be understood, but some of it
| is likely to be unknowable.
| s_m_t wrote:
| Let's do a thought experiment. Floating in an otherwise
| completely empty region of space is a bomb. It explodes. For
| quite some time after the explosion entropy will be
| decreasing in that region. I really don't see the mystery
| here.
| simiones wrote:
| The point is this: say we see a video of an otherwise
| completely empty region of space, with two masses orbiting
| each other for a while, that then fly off in different
| directions. We reverse the video and see two masses coming
| towards each other until they get into an orbit. Can we
| tell which of the two videos was the original and which was
| the reversed one? The answer is that we can't.
|
| However, say we receive a video of a billion billion such
| masses all starting in a single point, staying more or less
| still for a few seconds, and then moving out at high speed
| away from each other. It is obvious that this video is
| almost certainly playing forward in time, since the
| reverse, a billion billion balls all coming together to
| form a single object, is very very unlikely.
| rrmm wrote:
| Coming together to form a single object sounds like the
| work of a gravitational force.
| tasty_freeze wrote:
| I have a different take on this, only half joking.
|
| What if time is a free variable - it can move forward and
| backwards freely, perhaps like a drunkards walk? To a conscious
| being, any memory of having been at time t will be erased when
| t decreases. When t increases again, but you'll have no memory
| by which to detect the revisitation.
|
| I know that I just postulated that there is a time sequence by
| which the motion of time can be described. But that is only to
| make the idea graspable to our ordinary sense of time.
|
| One interpretation of what I described is the slab model of the
| universe where time is just another axis and the evolution
| along that line is fixed. Another interpretation would be that
| as time moves forward and backwards the state of the universe
| is free to change. In either case, our perception of our
| personal timelines would always appear to be unidirectional.
| ClumsyPilot wrote:
| > Another interpretation would be that as time moves forward
| and backwards the state of the universe is free to change
|
| What would be the measurable effect of this - would casualty
| actually be broken? Would we get Dejavu?
| n2e wrote:
| Anecdote, but this is how my observer experiences time.
| anodyne_one wrote:
| What do you mean by this?
| UltraSane wrote:
| The slab model of the universe is not compatible with quantum
| randomness.
| mitthrowaway2 wrote:
| Isn't it?
| rtpg wrote:
| > things will tend to become more disordered over time, because
| there are just more ways to be disordered than ordered.
|
| Isn't this a bit underspecified? Like a marble in a bowl has
| plenty of positions it can be in, but it tends to end up in the
| center due to the whole physical system doing its thing to roll
| a marble to the center.
|
| Or is there some deeper argument here?
| Crye wrote:
| You just added gravity, which is another form of entropy.
| jethkl wrote:
| Sharing a simple thought experiment that was shared with me
| years ago that explains (to me at least) why this is an
| interesting question. Imagine a billiard ball with nonzero
| velocity bouncing around an enclosed box. When the ball
| encounters a side of the box, it bounces off elastically. A
| replay of this ball's path over time is equally plausible if
| the replay were run forward or reversed. The preceding is also
| true if one imagines 2 or 3 balls, with the only difference
| being that the balls may also bounce off each other
| elastically. Even in this scenario, reversibility of playback
| holds no matter the configuration of the balls: they could all
| start clustered or be scattered and the replay would be
| plausible when played in either time direction. But this is no
| longer true when the box (now much larger) contains millions of
| billiard balls. If the balls start clustered together, they
| will scatter over time about the box and the replay of their
| paths has only one plausible time direction. This is because it
| is extremely unlikely that all the billiards will, simply by
| chance at some point in the future, collect together so they
| are contained within a very small volume. To summarize, in the
| "few scenario" we can plausibly reverse time but in the "many
| scenario", we cannot. The only difference between scenarios is
| the number of balls in the box, which suggests that time is an
| emergent property. layer8's answer elsewhere in this thread
| says the same, but more succinctly.
| aurareturn wrote:
| But this is no longer true when the box (now much larger)
| contains millions of billiard balls. If the balls start
| clustered together, they will scatter over time about the box
| and the replay of their paths has only one plausible time
| direction.
|
| I don't understand how increasing the number of balls means
| you can't reverse the playback.
| jethkl wrote:
| you can reverse the playback, all the physics of billiards
| bouncing around works equally well in either time
| direction.
|
| > If the balls start clustered together, they will scatter
| over time about the box and the replay of their paths has
| only one plausible time direction.
|
| It is extremely unlikely that all the billiards will,
| simply by chance at some point in the future, collect
| together so they are contained within a very small volume.
| this shows that there is asymmetry in which direction time
| flows.
| aurareturn wrote:
| How did the balls start clustered?
|
| I suppose we're now entering into the realm of what
| happened before the big bang?
| int_19h wrote:
| That's the thing - if the balls don't start clustered,
| then you "lose" the time direction again; now it doesn't
| matter which way you replay the trajectories, they are
| equally plausible in either direction. So if that's what
| you ultimately base the definition of time on, this
| implies that it is an emergent property.
| tsimionescu wrote:
| Actually no, this is a good question that hits at a
| tension between the second law of thermodynamics and the
| big bang model. The aftermath of the big bang and the
| inflation period are known to be times where the universe
| was extremely hot and dense. And yet, by the rule that
| entropy can't increase over time, it follows that they
| were the lowest entropy state of the universe: definitely
| lower entropy than what we have today. But how can an
| extremely hot plasma made up of all of the particles that
| today make up stars and planets and so on have been a
| lower entropy state than the galaxies of today?
| metalman wrote:
| it does not need anything other than plausible start and
| stop conditions to point out how time reversal is
| implausable, the time reversal idea only works under the
| implausable condition of only happening in a pefect
| steady state, no begining no end, just threading a piece
| of film in backwards, after editing out the set dressers
| at work without answering the fundamental question of
| "how did we get here anyway" its not a theory, its slight
| of hand edit: if it was an educational, thought
| experiment and labeled as such fine, sure, but its
| nothing more than that, :) , but could be less
| zmgsabst wrote:
| If there's only finitely many states, they will
| eventually -- unless something prevents it.
|
| https://en.wikipedia.org/wiki/Poincare_recurrence_theorem
| cjfd wrote:
| Seems accurate enough.
| reedf1 wrote:
| The reason that physicists get excited about entropy is that it
| is an extremely useful tool.
|
| "things will tend to become more disordered over time, because
| there are just more ways to be disordered than ordered."
|
| You can derive all of classical thermodynamics from this one
| observation.
|
| At a high level, physics is just symmetries and laws, and
| efforts to poke holes in them. We are "hung up" because they
| form the base of the core lines of inquiry in the field, not
| because we don't understand them.
| LinuxAmbulance wrote:
| "...suggesting that time's arrow may not be as fixed as we
| experience it."
|
| That sentence is doing a lot of heavy lifting in this article.
| pizza wrote:
| The Minkowski metric is Ds = sqrt(-Dt^2 + Dx^2
| + Dy^2 + Dz^2)
|
| One aspect of this is that, if you sub `t -> -t'`, that's just as
| good a solution too. Which would suggest any solution with a
| positive time direction can have a negative time direction, just
| as easily. Is this widely assumed to be true, or at least
| physically meaningful?
|
| There's also Wick rotations, where you can sub `t -> it'`, and
| then Minkowskian spacetime becomes Euclidean but time becomes
| _complex-valued_. Groovy stuff.
|
| I'm not much of a physics buff but I loved reading Julian
| Barbour's _The Janus Point_ for a great treatment of the
| possibility of negative time.
|
| The craziest thing I've seen though is the suggestion that an
| accelerating charge, emitting radiation that interacts with the
| charge itself and imparts a backreacting force on the charge,
| supposedly has solutions whose interpretation would suggest that
| it would be sending signals _back in time_. [0]
|
| [0] https://en.wikipedia.org/wiki/Abraham-Lorentz_force
| echelon wrote:
| How much of this is just the mathematical model permitting such
| things?
|
| Has any of this been experimentally fit to reality?
|
| > sending signals back in time.
|
| First encode stock movements. Then technological discoveries.
| If it works like many-worlds, then even NP hard problems.
|
| With sufficient bandwidth and time delta, you'd also have to
| hope that the future isn't sending anything nefarious that
| could lead to your demise. A future adversary could final
| destination you pretty easily and use you as a pawn to enrich
| itself, and there's not much you could do except stop listening
| - but it'd know you planned that too. A future adversary would
| be the scariest adversary.
| vjerancrnjak wrote:
| People play with models like this and get to white holes.
| echelon wrote:
| Maybe the reason we don't see aliens is that the first ones
| to break physics restart the universe [1].
|
| Fragile universe hypothesis [2].
|
| [1] eg. nucleate vacuum collapse
|
| [2] cf. fragile world hypothesis
| morpheos137 wrote:
| Stop now.
| ashoeafoot wrote:
| Atomics? Climate Change? Technology advances? Ineffective
| appeals to the better angels of our nature, that clearly
| go extinct first the moment a resource window snaps shut
| ?
|
| Should have written queries into the data mountain of
| humanity,learn about whats possible and whats not
| possible. The time we could reuse from ineffective public
| appeals to work on solveable problems
| zmgsabst wrote:
| And then you get in a car accident when driving a time you
| otherwise wouldn't have been, wherein you maim the otherwise
| grandfather of a founder of the mega corporation temporally
| exploiting you -- thereby eliminating the whole event.
|
| Messing with your own history, given that most systems are
| chaotic, seems inherently risky. Even in warfare, you'd have
| a hard time predicting the outcome over more than a short
| interval.
| ta988 wrote:
| No, that's just life. If you are afraid of changing
| something and decide to do nothing, you are still changing
| things by not acting as a beeing with agency.
| MathMonkeyMan wrote:
| The Minkowski metric is time reversal symmetric. The bigger
| question in particle physics is "what are the symmetries of the
| action?"
|
| It's not time reversal symmetric, but it's probably [CPT][1]
| (charge-parity-time reversal) symmetric.
|
| If you forget about quantum field theory and consider classical
| physics in Minkowski space (or Cartesian space), then t -> -t
| indeed doesn't change the physical laws. You could tell the two
| apart, though, provided that the system is far from thermal
| equilibrium (e.g. "why is this egg uncracking spontaneously?").
|
| [1]: https://en.wikipedia.org/wiki/CPT_symmetry
| foundry27 wrote:
| Barbour is criminally underrated as a physics author. He's
| published a lot of interesting ideas regarding the role of
| time, or lack thereof, in modern theories! (The End of Time,
| and its treatment of Causality as a direct substitute for time
| in any future theory of everything, was very fun)
| Xmd5a wrote:
| Someone came up with a very similar theory (two arrows of
| time diverging from the same point, the big bang). They even
| gave their theory the same name: Janus.
|
| https://januscosmologicalmodel.com/januspoint
|
| There are other players concerned with similar ideas:
|
| - Negative mass, Farnes:
| https://en.wikipedia.org/wiki/Dark_fluid
|
| - Mirror-image universe going backwards in time from the big
| bang, Turok:
| https://www.newscientist.com/article/mg25734230-100-neil-
| tur...
| lmm wrote:
| > Which would suggest any solution with a positive time
| direction can have a negative time direction, just as easily.
| Is this widely assumed to be true, or at least physically
| meaningful?
|
| It's widely assumed to be true and not at all physically
| meaningful. If you sub x -> -x then that's just as good a
| solution as well, i.e. just as you can count x as running from
| west to east or east to west and the results will be the same,
| you can also count time as increasing away from the big bang or
| as decreasing away from the big bang and all your calculations
| will be the same.
| mvdtnz wrote:
| Greg Egan wrote a science fiction book in a world with two
| timelike dimensions - that is, two terms in the Minkowski
| metric are negative. It's a fascinating world to explore. The
| book is titled Dichronauts.
| alkonaut wrote:
| If I toss a ball at a 45 degree angle then the motion it
| describes is a quadratic parabola. That means the solutions for
| where it hits the ground is going to be either a positive t,
| and a positive x (It lands somewhere in front of me, after a
| second or so), but also a negative t and negative x (it lands
| right behind me, right before I threw it). But the equation
| having those solutions doesn't mean there is any physical
| meaning to that solution. Isn't this (possibly) the same thing?
| pizza wrote:
| I guess then, why is what that solution lacking physical
| meaning versus the normal one - in some sense that means what
| it corresponds to is something other than a part of physics,
| but then what is that?
| alkonaut wrote:
| The arc describes the full motion, and the solution we seek
| is when F(x)=0 which is when the ball hits the ground.
| There is no mathematical curve that starts at my hand at
| (x=0m, y=1m) and ends at the ground. We use the full
| quadratic curve just because its a suitable model for the
| motion, on the part of the motion we know the ball takes.
|
| The use of a quadratic to solve the throw is a mathematical
| model. We say that "the value x describing the when the
| ball lands must satisfy the quadratic equation F(x)=0" but
| that does NOT imply the opposite, which is "all x that
| satisfy F(x)=0 describe a valid motion of the ball."
|
| So when we get two answers, e.g. F(-1)=0 and F(15)=0 for
| the two points when the ball is at ground level, that means
| only this: if I had thrown the ball from ground level to
| follow the same curve land in the same place at x=15, then
| I would have stood 1m further back when I threw it. It does
| have physical meaning, but there is nothing curious about
| the physical meaning.
|
| This throw _is_ symmetrical in time though, in the sense
| that if I throw the ball with the same speed in the
| opposite direction starting at x=15 then it will land
| exactly in my hand. (But the equation here is y=F(x) and
| not parametrized on time).
| TeMPOraL wrote:
| I'd say 'jerf is correct here:
| https://news.ycombinator.com/item?id=43078998
|
| The equation we use to describe the motion does _not_
| contain a term for the ground being there. That
| assumption exists _outside_ of the model described by the
| equation, and you use that assumption _after the fact_ to
| reject a solution that would otherwise be valid, and
| describe the movement of the ball that 's pulled down by
| gravity.
|
| In this form, it doesn't really describe a proper orbit,
| just a trajectory of being pulled down by a constant
| force. I believe this would correspond to an infinitely-
| ish heavy object located infinitely far below. The proper
| equation that gives you an orbital curve has the force of
| gravity proportional to inverse-square distance _and_
| point at the center of the body, which is what makes it
| possible to describe a circular or elliptical motion this
| way. Parabolic orbits exist too, but they 're interpreted
| as failed orbital capture - "object is moving so fast
| that it'll curve around and fly away to infinity before
| turning around and coming back".
|
| And in all cases, the solutions make physical sense (+/-
| infinity), on the assumption the trajectory doesn't cross
| the ground, as there's no term for it there :). If you
| want, you can describe the ground as another equation (or
| inequality), and solve the resulting system - it'll then
| be clear what exactly is it that rejects some of the
| solutions.
| jerf wrote:
| The second solution doesn't have the ball _land_ behind you
| before you throw it... It has the ball _emerge from_ the
| ground, from the orbit it was in, on its way up to your hand.
|
| What both solutions correspond to is the completion of the
| orbit the ball is briefly in while it is in free fall, in
| both directions. Every body in free fall (ignoring air
| resistance) is for that time in orbit around the center of
| gravity of the Earth. It's just the the body can not complete
| that orbit due to the fact it impacts the ground, and in the
| time-reversed direction, couldn't have come from that orbit
| initially because of the ground.
|
| It's not mystery getting in the way of the equations, it's
| the physical ground.
|
| (There are many other deviations from the highly idealized
| "orbit the Earth as if it was a stationary body in perfect
| Newtonian physics" but compared to air resistance you will
| not be able to witness any of those effects with anything you
| can throw with your arm from the ground.)
| westurner wrote:
| /? "time-polarized photons"
| https://www.google.com/search?q=%22time-polarized+photons%22
|
| https://www.scribd.com/doc/287808282/Bearden-Articles-Mind-C...
| ...
| https://scholar.google.com/scholar?hl=en&as_sdt=0%2C43&q=%22...
| ... "p sychon ergetics" .. /? Torsion fields :
|
| - "Torsion fields generated by the quantum effects of macro-
| bodies" (2022) https://arxiv.org/abs/2210.16245 :
|
| > _We generalize Einstein 's General Relativity (GR) by
| assuming that all matter (including macro-objects) has quantum
| effects. An appropriate theory to fulfill this task is Gauge
| Theory Gravity (GTG) developed by the Cambridge group. GTG is a
| "spin-torsion" theory, according to which, gravitational
| effects are described by a pair of gauge fields defined over a
| flat Minkowski background spacetime. The matter content is
| completely described by the Dirac spinor field, and the quantum
| effects of matter are identified as the spin tensor derived
| from the spinor field. The existence of the spin of matter
| results in the torsion field defined over spacetime. Torsion
| field plays the role of Bohmian quantum potential which turns
| out to be a kind of repulsive force as opposed to the
| gravitational potential which is attractive_ [...]
| _Consequently, by virtue of the cosmological principle, we are
| led to a static universe model in which the Hubble redshifts
| arise from the torsion fields._
|
| Wikipedia says that torsion fields are pseudoscientific.
|
| Retrocausality is observed.
|
| From "Evidence of 'Negative Time' Found in Quantum Physics
| Experiment" https://news.ycombinator.com/item?id=41707116 :
|
| > _" Experimental evidence that a photon can spend a negative
| amount of time in an atom cloud" (2024)
| https://arxiv.org/abs/2409.03680_
|
| /?hnlog retrocausality (Ctrl-F "retrocausal", "causal")
| https://westurner.github.io/hnlog/ )
|
| From "Robust continuous time crystal in an electron-nuclear
| spin system" (2024)
| https://news.ycombinator.com/item?id=39291044 ;
|
| > [ Indefinite causal order, Admissible causal structures and
| correlations, Incandescent Temporal Metamaterials, ]
|
| From "What are time crystals and why are they in kids' toys?"
| https://bigthink.com/surprising-science/what-are-time-crysta...
| :
|
| > _Time crystals have been detected in an unexpected place:
| monoammonium phosphate, a compound found in fertilizer and
| 'grow your own crystal' kits._
|
| Ammonium dihydrogen phosphate:
| https://en.wikipedia.org/wiki/Ammonium_dihydrogen_phosphate :
|
| > Piezoelectric, birefringence (double refraction),
| _transducers_
|
| Retrocausality in photons, Retrocausality in piezoelectric time
| crystals which are birefringent (which cause photonic double-
| refraction)
|
| Is it gauge theory, though?
|
| From https://news.ycombinator.com/item?id=38839439 :
|
| > _If gauge symmetry breaks in superfluids (ie. Bose-Einstein
| condensates); and there are superfluids at black hole thermal
| ranges; do gauge symmetry constraints break in [black hole]
| superfluids?_
|
| Probably not gauge symmetry there, then.
| layer8 wrote:
| From the paper's conclusion:
|
| "Our findings are consistent with the second law of
| thermodynamics and emphasise the distinction between the concepts
| of irreversibility and time-reversal symmetry. Once the arrow of
| time and a particular low entropy initial condition at have been
| chosen, then the von Neumann entropy will increase forward in
| time from the temporal origin. However, a different choice of the
| arrow of time would have implied the same dynamics. The Markov
| approximation applied to the time-reversed evolution leads
| likewise to the same dissipation and entropy increase.
| Consequently any thermal equilibrium state for a forward-running
| trajectory is also an equilibrium thermal state for any time-
| reversed trajectory, and entropy increases in both directions:
| the system thermalises into both time directions. [...]
|
| "Furthermore, we speculate that these results may reflect on the
| cosmological arrow of time. In fact, the natural assumption that
| the universe was dissipative from time zero onwards would suggest
| that a model of it would rely on the Markov approximation
| performed at the moment of the Big Bang. If so, this would imply
| that two opposing arrows of time would have emerged from the Big
| Bang, which would account in turn for the maintenance of time-
| reversal symmetry despite the ensuing dissipative nature of the
| universe. We would happen to live in one of them, where
| dissipation and entropy increase are common experience, but
| unaware of the existence of the other alternative possibility."
| hoppp wrote:
| When can we send information back in time? I just want to send
| myself some stock market data. Im pretty sure I can encode all
| information I need in 1 qbit(buy or sell).hehe
| cjfd wrote:
| The standard answer is that apparently the initial state of our
| universe had a low entropy and that has been increasing ever
| since.
| throwaway314155 wrote:
| That's the standard answer to a question you're not responding
| to, as far as I can tell.
| pingou wrote:
| It seems to me that time could run backward and it would not make
| any difference to an observer, and the universe could alternate
| between running backward or forward and that would be the same to
| us, cause and effects would be reversed but it would only be
| visible to an outside observer, that has somehow time running in
| a different direction.
| pwr22 wrote:
| There's a Philip K Dick story that covers this type of
| thinking, "Your appointment will be yesterday".
| janpmz wrote:
| How long will it take for people to realize, they are looking at
| a contradiction?
| steveBK123 wrote:
| Better headline - physicists discover you could theoretically "Do
| Tenet".
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