[HN Gopher] Entropy: A little understood concept in physics [video]
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Entropy: A little understood concept in physics [video]
Author : guptarohit
Score : 185 points
Date : 2023-07-02 10:23 UTC (12 hours ago)
(HTM) web link (www.youtube.com)
(TXT) w3m dump (www.youtube.com)
| bugs_bunny wrote:
| An interesting read related to this is the following article,
| which begins with von Neumann's comment that ``Nobody really
| knows what entropy really is."
|
| https://www.researchgate.net/publication/228935581_How_physi...
| floatrock wrote:
| The part that was new to me was the bit about how a space full of
| life tends toward more entropy faster than the same amount of
| space without life.
|
| Like the best ideas, it's simple and makes sense if you think
| about it, but it's still a really interesting framing that the
| complex machinery of life is really just the most efficient
| "entropy converter".
|
| If there's something about the arrow of time that speeds towards
| the heat death of the universe, we're just helping it go a tiny
| bit faster here on our floating speck of dust.
| Piezoid wrote:
| There is a video by PBS Space Time on that subject:
| https://youtu.be/GcfLZSL7YGw
| tgv wrote:
| Isn't there a theorem that says that the overall rate is
| constant in a closed system? If you look at the universe as a
| whole, it would be closed, and we wouldn't have any impact.
| However, I learnt about entropy decades ago, and never applied
| it, so don't take my word for it.
| Timon3 wrote:
| > Like the best ideas, it's simple and makes sense if you think
| about it, but it's still a really interesting framing that the
| complex machinery of life is really just the most efficient
| "entropy converter".
|
| Right? I think this could be a really interesting basis for a
| sci-fi novel - the first space-faring civilization in the
| entire universe trying to force every form of life to keep
| "entropy usage" to a minimum, so they can prolong their own
| life span.
| ineptech wrote:
| Not _exactly_ what you 're describing, but this is along
| those lines: https://slatestarcodex.com/2015/06/02/and-i-
| show-you-how-dee... It's pretty silly but the ending is both
| satisfying and relevant.
| hunter-gatherer wrote:
| Isaac Asmiov's short story "The Last Question" isn't super
| inline with your suggestion but is a short discourse on
| humanity's struggle against entropy.
| franky47 wrote:
| > a space full of life tends toward more entropy faster than
| the same amount of space without life.
|
| And a space full of children is exponentially faster at
| increasing entropy.
| quickthrower2 wrote:
| Does Einstein's model of time care about entropy. In other
| words if there are 2 regions, one where entropy is increasing
| at that time and one where it isn't as much, does it affect
| time?
| danq__ wrote:
| [dead]
| semi-extrinsic wrote:
| No. Entropy can be used to explain the direction of time, as
| a kind of symmetry breaking of all the microscopic laws that
| are symmetric in time. But it does not say anything about the
| "speed of time". Relativity does tell us the speed of time -
| it's the speed of light.
| Balgair wrote:
| To be suuuuuuper pedantic here: Relativity tells us that
| time is a dimension, one that is a bit unique. In that it
| has a constant attached to it. So, the 3 dimensions you're
| used to are just normal, they have no constants.
|
| (x,y,z)
|
| Meters of x are meters of z and meters of y. Relativity
| (and I'm really simplifying a lot by just saying
| 'relativity'), well relativity comes along as says that
| time is also a dimension, just with the constant of 'c'
| attached (the speed of light). That way you can convert
| seconds into meters.
|
| (x,y,z,ct) not just (x,y,z,t).
|
| So now the time dimension is much larger than the spatial
| dimensions. About 300,000,000 times larger, a third-ish of
| a billion. So a meter of x is ~1/3 of a billion meters of
| time.
|
| Now, there is a _lot_ more about relativity, like, just
| tons. And I skipped most of it. And trying to just say that
| time is a simple little conversion away from meters is just
| wrong. And how that all relates to entropy is a mess that
| we really haven 't figured out yet.
| cubefox wrote:
| The last sentence surprised me.
| kergonath wrote:
| And yet... If you want to get properly depressed, there
| is a lot we don't understand about gravity, either.
| klabb3 wrote:
| > Relativity does tell us the speed of time - it's the
| speed of light.
|
| I forget the name of the book, but it was trying to convey
| intuitions about relativity (first special, then general).
|
| It's very easy to make the mistake of trying to understand
| space and time first, concepts we think we intuit, but in
| relativity these are somewhat higher-level concepts.
| Instead, start with what the most fundamental part of the
| theory and go from there: _the speed of light is constant_.
| Accept that first. It's the comfort zone. You can always
| return safely to this point.
|
| So, when moving to space and time, the book explained it
| like this: _everything_ moves at the speed of light, at all
| times. It's just that instead of x,y,z - we add t, time, as
| well. So for an object that's still, all it's movement is
| through the time dimension. Conversely, an object that
| moves incredibly fast, like a photon, already "used" it's
| speed in the spatial dimensions, so it doesn't "age" in
| terms of time.
|
| This is just special relativity, but I liked this approach.
| It's basically embracing the theory first instead of trying
| to shoehorn it into the world we have so many
| misconceptions about.
| antimora wrote:
| Just recently I also watch a video by Sabine Hossenfelder called
| "I don't believe the 2nd law of thermodynamics":
| https://www.youtube.com/watch?v=89Mq6gmPo0s
|
| I recommend this video as well.
| lisper wrote:
| A pithier way to introduce this topic: the first law of
| thermodynamics, a.k.a. the law of conservation of energy, is that
| energy cannot be created nor destroyed, only transformed from one
| form to another. In light of this, how can there ever be a
| shortage of energy?
|
| [Note that this is intended to be a rhetorical question advanced
| for the purposes of pedagogy. If you find yourself wanting to
| post an answer, you have missed the point.]
| sghiassy wrote:
| Usable energy is different than total energy. If energy isn't
| concentrated (say something like gasoline) it's not usable
| uoaei wrote:
| Energy is actually a red herring -- what's relevant here is
| work.
|
| Useful work, aka information, is work that can be employed in
| dynamics vis a vis processing. Useless work, aka heat, is the
| devil's share of the energy expenditure which is lost as
| entropy when undergoing a process.
| dekken_ wrote:
| > how can there ever be a shortage of energy
|
| I think it's not so much a shortage of energy, but that there
| thermodynamic equilibrium and thus no available energy to do
| anything.
|
| I don't think this will ever happen tho, it's pretty clear to
| me that making energy more dense is a universal process.
| sghiassy wrote:
| Not really - look up "heat death" of the universe.
| credit_guy wrote:
| The "heat death" of the universe is a concept that deserves
| to die. The second principle of thermodynamics is true only
| if you ignore gravity. In the presence of gravity, systems
| tend to go towards lower entropy, just see how a planetary
| system can form out of a gas cloud.
| consilient wrote:
| > In the presence of gravity, systems tend to go towards
| lower entropy, just see how a planetary system can form
| out of a gas cloud.
|
| This isn't correct: the entropy (and energy) of the gas
| cloud goes decreases as it collapses, but the entropy of
| its surroundings increases faster as it radiates.
| credit_guy wrote:
| Is that a fact? Or just a hypothetical way that could
| save the second principle?
| consilient wrote:
| It's a fact. See for instance
| https://arxiv.org/pdf/0907.0659.pdf
| credit_guy wrote:
| It's more like an opinion. Of one particular guy who has
| a Ph.D. in Physics. But there are many, and there is no
| consensus overall.
|
| Here's the relevant quote from wikipedia [1]
| Recent work has cast some doubt on the heat death
| hypothesis and the applicability of any simple
| thermodynamic model to the universe in general. Although
| entropy does increase in the model of an expanding
| universe, the maximum possible entropy rises much more
| rapidly, moving the universe further from the heat death
| with time, not closer. This results in an "entropy gap"
| pushing the system further away from the posited heat
| death equilibrium. Other complicating factors, such as
| the energy density of the vacuum and macroscopic quantum
| effects, are difficult to reconcile with thermodynamical
| models, making any predictions of large-scale
| thermodynamics extremely difficult.
|
| [1] https://en.wikipedia.org/wiki/Entropy#Cosmology
| consilient wrote:
| That's discussing the effects of metric expansion, which
| are not relevant for gravitationally bound systems. It
| also doesn't claim the second law of thermodynamics
| fails. On the contrary,
|
| > entropy does increase in the model of an expanding
| universe
| kergonath wrote:
| This is quite different from what you were saying about
| gravity reducing entropy, though. And I know quite well
| that having a PhD in Physics does not make someone right,
| but then quoting Wikipedia in such an argument is really
| not great.
| dekken_ wrote:
| I think of it as a death cult. Thinking we know all that
| there is to know about the universe to the point where we
| can declare with 100% certainty that any particular thing
| will happen is not scientific.
| kergonath wrote:
| Really? We are far from really understanding gravity, but
| I can very confidently tell you that if I kick a ball its
| trajectory will be a parabola (roughly, as a first order
| approximation and ignoring things like friction, which we
| can also calculate to a decent approximation). We can say
| where it will fall and give some confidence interval
| depending on the conditions and such. There is nothing
| unscientific about it.
|
| Thermodynamics is not magic. In the same way that we can
| predict the evolution of climate without knowing where
| every single cloud will be, we can make statements about
| the evolution of large systems even though our knowledge
| of their state is imperfect. Again, nothing unscientific
| about it.
| mNovak wrote:
| Not my area of expertise, but in the video at least, they
| indicate that black holes have very high entropy. So if
| we imagine gravity eventually pulling all those planetary
| systems together into some number of black holes, isn't
| gravity indeed pulling the system towards a high entropy
| state?
|
| The video actually directly addresses the gas cloud
| question, saying basically that a gas cloud is actually a
| highly improbable distribution of matter, whereas the
| eventual planetary system is much more probable. The
| claim being, that trend towards expected state is entropy
| increasing.
| jjaken wrote:
| Heat death doesn't mean "no heat" or that energy has
| depleted. It just means that energy is fully dispersed. All
| kg the hear exists, it's just that no one place has any
| more than anywhere else, and so there is no longer any
| transfer of energy.
| A_D_E_P_T wrote:
| > so there is no longer any transfer of energy.
|
| Ah, but there is.
|
| The Second Law is a statistical law, not an absolute law.
| On long enough timescales, low-probability fluctuations
| in local entropy will allow for energy transfer. These
| fluctuations will also allow for the formation of
| structures. (Boltzmann himself was of the opinion that
| the low-entropy universe emerged from a higher-entropy
| background state. And indeed there's nothing in physics
| to rule out the emergence of Boltzmann Brains and even
| Boltzmann Galaxies from homogeneous and maximally
| entropic universes in "heat death." This is a
| philosophical problem of the highest order, because it
| implies that we're not necessarily going from "less
| likely to more likely states" as the video implies, but
| rather from a relatively deterministic state to a
| probabilistic state.)
| jjaken wrote:
| Oh interesting, I didn't know that
| dahart wrote:
| Why is this pithier than the video? I'm not entirely sure I see
| added pedagogical value. Asking the rhetorical question how can
| there be a shortage of energy sounds a little like someone
| sort-of intentionally misunderstanding what that phrase "energy
| shortage" means in any practical economic context. "Energy
| shortage" is an economics phrase, not a physics phrase. The
| first law of thermodynamics doesn't suggest there can't be
| energy shortages on earth, because the phrase "energy shortage"
| is not used to suggest a loss of energy to the universe, energy
| shortages are all about not having enough specific forms of
| energy in specific places at specific times [1], and it's no
| surprise that we can't capture dissipated heat, or that a local
| power system has a maximum limit at any given time, for
| example.
|
| Something similar could perhaps be said for the video's
| approach; "what do we get from the sun?" is an ambiguous
| question, not necessarily a fair setup to ask a lay person when
| you have entropy in mind as the answer. We do get energy from
| the sun, that is a correct answer, and we use some of it before
| it goes away. But, there is the nice a-ha that all the energy
| from the sun eventually leaves the earth, right?
|
| [1] "An energy crisis or energy shortage is any significant
| bottleneck in the supply of energy resources to an economy."
| https://en.wikipedia.org/wiki/Energy_crisis
| JumpCrisscross wrote:
| It's pithy, but in the way of word play. Energy,
| colloquially, means useful energy. The question collides the
| conventional and technical definitions to create the illusion
| of profundity.
| lisper wrote:
| Pithy != profound. The intent was to get people to think
| about the fact that the word "energy" means different
| things in different contexts, and that the thing that
| actually has value is not energy but the _absense_ of
| entropy.
| pcwelder wrote:
| So the law of increasing entropy is not a fundamental law of the
| reality because it can be derived from other fundamental
| equations.
|
| Suppose I show you a snapshot of a random universe, would you be
| able to tell if the entropy of the universe is going to increase
| or decrease as the time progresses?
|
| Let's assume that universe's entropy would increase. Consider
| another universe exactly the same as current universe, but all
| the particles' velocities reversed. Then this universe's entropy
| would decrease.
|
| So you are equally like to select both the universe and hence the
| original assumption of increasing entropy is wrong.
|
| Discarding quantum properties of the particles, is it then fair
| to say that time's direction is unrelated to whether entropy
| increases or decreases?
| [deleted]
| [deleted]
| canjobear wrote:
| The expected increase in entropy can be derived from laws of
| mechanics plus the critical stipulation that, in the past,
| entropy was very low. Essentially, physical systems want to be
| in high-entropy states. So if you observe one to be in a very
| low-entropy state, then you can conclude that with high
| probability the future of that system will go to higher-entropy
| states.
|
| > Suppose I show you a snapshot of a random universe, would you
| be able to tell if the entropy of the universe is going to
| increase or decrease as the time progresses?
|
| Yes, if it has low entropy then entropy will probably increase;
| if it has high entropy then the entropy will probably fluctuate
| up and down statistically.
|
| > Let's assume that universe's entropy would increase. Consider
| another universe exactly the same as current universe, but all
| the particles' velocities reversed. Then this universe's
| entropy would decrease.
|
| The key is that you're exponentially unlikely to find yourself
| in a universe where all the particles' velocities are reversed.
| See this: https://en.wikipedia.org/wiki/Fluctuation_theorem
|
| The probability that a system randomly evolves in a way that
| reduces entropy is very very small.
| cubefox wrote:
| > > Suppose I show you a snapshot of a random universe, would
| you be able to tell if the entropy of the universe is going
| to increase or decrease as the time progresses?
|
| > Yes, if it has low entropy then entropy will probably
| increase
|
| The problem is that it probably increases in _both_ time
| directions, such that the state of minimum entropy is _now_.
| As you said, we have to stipulate that the entropy in the
| past is low, we can 't (yet?) infer it from observation.
| Which raises the question what justifies us making this
| assumption in the first place.
| rixed wrote:
| If by "random universe" you mean a universe in which all states
| of every particule are random, then my understanding is that we
| would probably conclude that entropy is neither increasing nor
| decreasing. Our universe is not random. We could spot local
| phenomenons where entropy is clearly going in one direction. We
| assume everywhere the entropy would go in the same direction
| (increasing), and deduce from this hypothesis that the universe
| started with a very low entropy.
| friend_and_foe wrote:
| The video talks about how the earth radiates away the same amount
| of energy as it gets from the sun, just red shifted. In light of
| this, let's talk about climate change, global warming and the
| greenhouse effect.
| szundi wrote:
| Just after Sabine's
| lll-o-lll wrote:
| Yes, and the Sabine video contained far more information (lower
| entropy?) and one genuinely interesting idea I'd never heard
| before. The idea that heat death may not be the end of
| intelligent life! The concept this relies on is the idea that
| macro states are actually just combinations of micro states all
| of which have the same probability. E.g. the sequence 1, 2, 3,
| 4, 5 and 3, 1, 4, 2, 5 are equally likely if you are selecting
| 5 random numbers (1-5), but the ordered sequence is an
| important state to _us_. The Big Bang - > heat death is just
| super unlikely state to super likely state, but these macro
| states are poorly defined. They matter to _us_. So perhaps the
| universe goes on with complex life harvesting neg-entropy from
| a heat death configuration of micro states as they slowly
| transition from "extremely unlikely" to "likely" in the context
| of something incomprehensible to humans. I feel like the idea
| would need more mathematical meat on the bones to go further,
| but still an intriguing thought!
| cubefox wrote:
| I suspect that "macro state that is more important to us"
| really is special in some objective way, not just
| subjectively.
| spuz wrote:
| I think the concept would be easier for me to understand if we
| talked about the inverse of entropy - i.e. some kind of
| measurement for the concentration of useful energy or "order". I
| think it would then be more intuitive to say that this
| measurement always decreases. Do we even have a word for the
| opposite of entropy?
| MaxRegret wrote:
| Negentropy? This is a concept in information theory, but maybe
| also in physics.
| winwang wrote:
| And it's closely related to the Gibbs free energy (available
| energy), which decreases with increasing entropy, all other
| things equal.
| vehicles2b wrote:
| I find it intuitive to think of such "ordered" distributions as
| having a higher compression ratio. (Ie compare the file sizes
| of zipping a file with just ones or just zeros vs zipping a
| file with a uniform, random mixture of ones and zeros.)
| Solvency wrote:
| Huh? I've never really understood this metaphor.
|
| Take any photograph in Photoshop. First, save one copy of it
| as a compressed JPG.
|
| Now, on the original, add a small densely repeating tiled
| pattern multiplied on top as a layer. Like a halftone effect,
| dot texture, whatever. Technically you're adding more order
| and less chaos. The resulting image won't compress as
| efficiently.
| dist-epoch wrote:
| The idea is to use the best compressor possible. So called
| Kolmogorov complexity.
| [deleted]
| knolan wrote:
| Exergy
| syntaxing wrote:
| Not trying to be cheeky but wouldn't the opposite of entropy be
| work?
| cjs_ac wrote:
| Ectropy has been suggested, but the term is not in common use.
| danq__ wrote:
| [dead]
| Timon3 wrote:
| I might be completely off kilter here (please tell me if that's
| the case!), but what makes sense to me is to think about "how
| much entropy is left", as in "how much can entropy still
| increase between the current state and the highest entropy
| state". That flips the meaning to describe what you're talking
| about, and feels very intuitive to me.
| spuz wrote:
| When you say "how much entropy is left?" you make it sound
| like a quantity that is decreasing, not increasing. That
| seems incorrect, no?
| Timon3 wrote:
| That's why I specified that it's meant as "how much can
| entropy still increase between the current state and the
| highest entropy state". If you can't read it in that sense,
| either ignore the shorter question or read it as "how much
| increase of entropy left". Or maybe "how much entropy left
| until max".
| nobody9999 wrote:
| >"how much entropy left until max".
|
| 10^106 years[0] worth or so.
|
| [0]
| https://en.wikipedia.org/wiki/Heat_death_of_the_universe
| kledru wrote:
| we do have a word -- "negentropy". Physicists also sometimes
| find it easier to talk about "negentropy" instead of entropy.
| bob1029 wrote:
| > Do we even have a word for the opposite of entropy?
|
| I've always referred to the inverse as "information" or
| "order".
| cshimmin wrote:
| "information" is not a good term for the opposite. Entropy is
| a well defined concept in information theory (and can be
| connected to the physical concept). More entropy means more
| information, not less.
| bob1029 wrote:
| I agree. Order is a better term to describe it.
| Predictability. More entropy = more total possible states
| the system can be in.
|
| I tend to use "information" to refer to a statistically-
| significant signal or data that the application/business
| can practically utilize. This is definitely not the same as
| the strict information theoretical definition.
| kgwgk wrote:
| > Entropy is a well defined concept in information theory
| (and can be connected to the physical concept). More
| entropy means more information, not less.
|
| More entropy means more missing information.
|
| https://iopscience.iop.org/book/mono/978-0-7503-3931-5/chap
| t...
| kikokikokiko wrote:
| As I understand it in information theory, "more entropy"
| equals "more information is necessary to fully describe
| this thing", but I may be wrong.
| kgwgk wrote:
| "more [additional] information is necessary to fully
| describe this thing" = "more missing information [in the
| incomplete description of the thing]"
| dangitnotagain wrote:
| Potential
| tnecniv wrote:
| Normally we care not about entropy but changes in entropy or
| entropy measured in comparison to a reference distribution, so
| you can just take the negative of that difference.
| danq__ wrote:
| The most intuitive explanation of entropy ever:
|
| entropy is a fancy way of explaining probability.
|
| Things with higher probability tend to occur over things of lower
| probability.
|
| Thus when certain aspects of the world like configurations of gas
| particles in a box are allowed to change configurations, they
| will move towards high probability configurations.
|
| High probability configurations tend to be disordered. Hence the
| reason why we associate entropy with things becoming increasingly
| disordered. For example... gas particles randomly and evenly
| filling up an entire box is more probable then all gas particles
| randomly gathering on one side of the box.
|
| If you understand what I just explained than you understand
| entropy better than the majority of people.
| guerrilla wrote:
| It's actually amazing how bad science popularizers are at
| explaining entropy. It's really not that difficult if they would
| just take some time and think before they speak (which I'm sure
| this video proves based on what people are saying about it.)
| martythemaniak wrote:
| Great video, but very wrong to cite Jeremy England. Ilya
| Prigogine came up with the concept of Dissipative Structures and
| won the Nobel Prize in Chemistry in 1977 for that work. He also
| has a couple of pop-sci books on he subject that I found super
| illuminating. They are a little bit challenging to read, but
| they're very thorough and Order out of Chaos in particular has a
| fantastic summary of 400 years of philosophy of science. Highly
| recommend reading the OG.
| ctafur wrote:
| According to entropy, and thermodynamics in general, I can't
| recommend enough the notes [0] of prof. Zhigang Suo of Harvard.
| It's a new way of presenting thermodynamics and I finally get
| it... contrary to when I took a thermo course at university.
|
| [0]:
| https://docs.google.com/document/d/10Vi8s-azYq9auysBSK3SFSWZ...
|
| Also prof. Suo puts entropy as the main character of the "play".
| The other concepts (temperature, etc.) are defined from entropy.
| manojlds wrote:
| When I was watching the video I was thinking this deserves to be
| posted on HN and yup, someone already has.
| hilbert42 wrote:
| That's an excellent overall summary as he covers almost every
| aspect of the subject albeit in brief. It would be good if he
| produced a second video dealing with the low entropy of incoming
| energy from the sun and the higher entropy of radiated energy
| from earth and relate that to global warming.
|
| In all the debate over global warming little is talked about why
| say CO2 and other greenhouse gasses increase the earth's
| temperature and how they shift the wavelength of the radiated
| energy from earth. In other words we need to explain in simple
| terms why the incoming and outgoing energy can remain the same
| yet the earth's temperature has increased.
| jjaken wrote:
| The important part to understand is timescales. In a day, the
| Earth does absorb some energy. Of course it does, plants
| collect it, solar panels collect, the ocean and land collect
| it. The amount Earth collects is a tiny fraction of what it
| releases. That collection isn't permanent though and is slowly
| released. Within a day, the Earth absorbs some energy, but over
| a long enough timescale, all of that energy is released again.
|
| The Earth is taking on energy every day from the sun. If we
| didn't release it all back, the earth would be warming much
| much faster. It only remains relatively cool because it
| releases almost as much as it receives.
|
| Another important note is that long term energy is not only
| stored as heat on earth. It's stored as potential energy in the
| atoms of cells in plants and animals. Think of how cold a
| gallon of gasoline is, yet how much energy it stores.
|
| For an example think of hot asphalt from a summer day. It gets
| real hot all day and slowly cools down at night. Sometimes it
| can be pretty warm to stand on the road even if it's a cool
| night.
|
| Within the human timescale, the Earth is retaining some (tiny
| fraction) of heat. That tiny fraction of heat is a very small
| window of heat that life can tolerate. It's not too much and
| not too little. If the earth were to retain just a tiny bit
| more, suddenly life can't tolerate it. On the scale of the
| universe, the difference between those realities is minuscule,
| even though it's enormous to us.
| gorhill wrote:
| My understanding:
|
| Global warming occurs because the previous equilibrium between
| incoming and outgoing energy has been broken by changes in the
| composition of the atmosphere.
|
| So until we reach a new equilibrium long after the atmosphere
| composition ceases to change, the outgoing energy will be less
| than the incoming energy.
| kergonath wrote:
| That's exactly it. Except that equilibrium will never be
| reached, it's more like tending towards a steady state.
| willis936 wrote:
| Which debate over global warming are you referring to? There
| are debates that involve atmospheric chemists that discuss
| Earth darkening cause and effects.
|
| https://en.m.wikipedia.org/wiki/Albedo
| hilbert42 wrote:
| Right, the average person hasn't a clue about albedo, nor do
| they know why say CO2 and CH4 increase global warming whereas
| others such as O2 are more benign.
|
| It may help lower the temperature of the debate if they did.
|
| _Edit, we 're pitching this discussion at the level he has--
| the lay public. Scientific argument over the minutiae is
| another matter altogether._
| jjaken wrote:
| Yeah debates in climate science are about phenomena
| laypeople don't even know exist. It's about how what we
| observe happens. No one is arguing about what we're
| observing, eg global warming.
| lvl102 wrote:
| I'd like to think of entropy in terms of randomness or rather
| uniqueness of elements/compounds within a system.
| 4ad wrote:
| I hate Veritasium's clickbait, and I think most of his videos are
| very poor, but this one is the exception. It's very well put
| together. The first ten minutes of the video is _exactly_ how I
| introduce entropy to people.
|
| Of course I can't give him a pass on how crass it was telling
| that women he has a PhD in physics (he does not). The video would
| have been so much better without that two seconds of footage...
| [deleted]
| [deleted]
| manojlds wrote:
| Most of his videos are GOOD, come on!
| kergonath wrote:
| The problem is that on a given specific subject you can never
| be sure whether he's exaggerating or misrepresenting things.
| Just this makes watching him a waste of time, because then
| you need to spend at least twice the time to fact check him.
| A bit like asking a question to ChatGPT. At least Wikipedia
| provides you links to proper sources.
| hilbert42 wrote:
| _" I think most of his videos are very poor,"_
|
| Why do you think so? (Most to me seem reasonable but one on
| speed of electricity stands out as badly done (he redid the
| video but it too could have been better).)
| pxeger1 wrote:
| He seems to exaggerate the importance of things when they
| make for a good story and sound interesting. This is a
| classic flaw in popular science but I think he's got a lot
| more egregious with it over the years.
|
| The worst example I remember, which is actually what drove me
| to unsubscribe, was when he said that the golden ratio was "a
| pretty five-y number" because it can be written as 0.5 + 0.5
| * (5^0.5). Anyone with a good mathematical background could
| tell you there's nothing five-y about 0.5 at all. I'll grant
| him, the golden ratio is still a little bit five-y because of
| the sqrt(5).
|
| The whole context and presentation seemed like it was
| designed to make the viewer feel like they'd learnt something
| even though nothing of substance was really delivered in
| those 20 seconds. He does that a lot.
| interestica wrote:
| > Anyone with a good mathematical background could tell you
| there's nothing five-y about 0.5 at all.
|
| Um, I disagree? Visually, the 5 is memorable here. "Fivey"
| just seems to mean "lots of the number 5"?
| pxeger1 wrote:
| I think the context was perhaps more important than I'd
| considered to explain the significance of "five-y". The
| implication was that the fiviness gives something of an
| intuitive explaination for why some shape had 5 sides.
| The presence of a [?]5 does (maybe) do this, but 0.5
| definitely doesn't. (Because as sibling comment points
| out, 0.5 = 1/2 and the 5 only appears due to our
| (arbitrary, from a mathematical point of view) choice of
| base ten.
| koningrobot wrote:
| I think the point is that 0.5 is 1/2, and the 5 digit
| only appears because of our base 10 presentation of
| numbers.
| AlexandrB wrote:
| I was quite disappointed by his video on self-driving cars.
| It presented a very one-sided view and felt like a puff piece
| (and, indeed, it was sponsored by Waymo). Tom Nicholas did a
| good job breaking down the problems with it: https://m.youtub
| e.com/watch?v=CM0aohBfUTc&pp=ygUPdG9tIHZlcml...
| hospitalJail wrote:
| His dandruff ad was pretty cringe IMO. But idk, it seems like
| his videos are really long for what they accomplish in
| general. He seems like the generic youtuber that milks every
| dollar of ad revenue and is shameless about it.
|
| Kind of sad that a expensive camera + clickbait
| thumbnail/title > Experts communicating clearly and
| accurately.
|
| I imagine he/his team is scouring youtube for the experts,
| and remaking their videos with more production value.
| willis936 wrote:
| His video on electromagnetism is still my gold standard for
| Lorentz's Law. It came out while I was taking emag. I liked
| it so much I showed it to my professor, who didn't name drop
| Lorentz all semester. The class was making sure no one got a
| BSEE without knowing Maxwell's Equations, which does warrant
| a semester. I guess it was more of a failing of the physics
| curriculum.
| andyjohnson0 wrote:
| > course I can't give him a pass on how crass it was telling
| that women he has a PhD in physics (he does not)
|
| To be clear, he does have a PhD but it is in physics education
| research, not physics.
| PrimeMcFly wrote:
| > I think most of his videos are very poor,
|
| This sounds quite bitter, as does griping about him mentioning
| his PhD.
|
| His videos have excellent production quality, and do a great
| job of communicating advanced STEM concepts to laypeople in an
| entertaining way.
|
| Maybe you don't _like_ them, but that doesn 't mean they are
| bad. Given their popularity, it would seem they are anything
| but.
| hospitalJail wrote:
| Last night me and my wife were deciding if we should watch The
| Witcher or this video.
|
| I decided I didn't have the brainpower/mental capacity to think
| about The Witcher and that this video on Entropy would be easier
| to digest.
| SanderNL wrote:
| Not sure if you are humble bragging, but I agree The Witcher is
| more demanding. Layers of meaning, emotions, allegory, subtext.
| It's no Shakespeare, but physics and math are simple in
| comparison especially in the wonderfully produced and easily
| digestible format of Veritasium.
| Balgair wrote:
| I mean, the author has a doctoral degree in science
| communication. It's his job and the point of the channel to
| try to make things easy to understand.
|
| The opposite is true with fiction. You're intentionally
| trying to have the audience make connections themselves, like
| a Sherlock story or the Great Gatsby. The point is in the
| discovery by the viewer.
| javajosh wrote:
| Entropy only made sense when I learned it from the perspective of
| statistical thermodynamics. It's a very programmerly
| understanding, IMHO, and it's quite intuitive. EXCEPT that the
| language used is ridiculous: _grand canonical ensemble_ indeed!
| Anyway, the idea that a system can be in some number of specific
| states, and that equilibrium is that unique situation where the
| number of possible specific states is at its maximum, really
| spoke to me.
| dekhn wrote:
| I took a stat thermo class and it was basically all about
| entropy, which was expresed as ln W- the log of the number of
| ways (permutations) that a system can be ordered, which gives a
| convenient denominator when calculating the probability of a
| specific permutation. Here's the professor's book, which was
| still only in latex form when we took the class:
| https://www.amazon.com/Molecular-Driving-Forces-Statistical-...
| javajosh wrote:
| yes, there are lots of quantitative details. I wanted to
| emphasize the key qualitative concept, from which the others
| can derive. In a similar way you can derive all of special
| relativity, and approach an intuition about the strangeness
| of spacetime, starting with only two ideas: the laws of
| physics are the same in all reference frames; the speed of
| light is constant. I prefer to start there and derive e.g.
| Lorentz factors than start with the mathy stuff.
| kergonath wrote:
| > starting with only two ideas: the laws of physics are the
| same in all reference frames; the speed of light is
| constant.
|
| Isn't this redundant, though? The constant velocity for
| light in a vacuum comes directly from the laws of
| (classical) electromagnetism in the form of Maxwell's
| equations. So "the laws of Physics are the same in all
| reference frames" implies "Maxwell's equations are valid in
| all reference frames", which in turn implies "the velocity
| of light in vacuum is the same in all reference frames".
| That's what I understood reading Einstein himself.
|
| I think it's much stronger that way. Otherwise we get to
| why light should be a special case, which is difficult to
| defend. The constant velocity of light (in vacuum) being an
| unavoidable consequence of the laws of Physics makes it
| much stronger.
|
| > I prefer to start there and derive e.g. Lorentz factors
| than start with the mathy stuff.
|
| That's how Einstein himself explained it (with trains and
| stuff, but still) and it makes a lot of sense to me. Much
| more than the professor who did start with the Lorentz
| transform and then lost everyone after 20 minutes of maths.
| guga42k wrote:
| If somebody needs to build an intuition about entropy he could
| think about simple problem.
|
| You are given insulated cylinder with a barrier in the middle.
| Left side of the cylinder filled with ideal gas A, and the
| right side filled with gas B. If given a particle one can
| distinguish A from B. The pressure and temperature on both
| sides are the same. Then you remove the barrier and gases mix.
| Question: how much work you need to do to revert the system
| into the original state? Hint: the work is equal to entropy
| difference between two states.
|
| More generally, if you have proper insulated system and leave
| it be for a while. All of sudden you will have to do some work
| to come back to the original state despite energy conservation
| law holds.
| kuchenbecker wrote:
| Enter Maxwell's demon as a completely valid solution to this
| problem showing you can decrease entropy within that system
| (but you need to exclude the demon from the system).
| ithkuil wrote:
| If you need to do work in order to revert to the previous
| state, does it imply you can extract work when going to the
| first to the second state?
|
| Given the scenario you just laid out it seems no work can be
| extracted just by letting mix two substances that are at the
| same temperature and pressure. But there is something about
| it that doesn't quite add up to my intuition of symmetry and
| conservation laws. Could you please elaborate more on that?
| guga42k wrote:
| >If you need to do work in order to revert to the previous
| state, does it imply you can extract work when going to the
| first to the second state?
|
| Nope. The work comes from the system coming from ordered
| state into unordered. Why the problem above is good for
| intuition because you can work out how to reverse the
| state. You invent semi-magical barrier which is fully
| transparent for particles A and reflects particles B, then
| you start to push such barrier from left to right up to the
| middle, compressing gas B (and making work!) and leave left
| part with gas A only, then repeat similar exercise on the
| right side.
|
| >Given the scenario you just laid out it seems no work can
| be extracted just by letting mix two substances that are at
| the same temperature and pressure. But there is something
| about it that doesn't quite add up to my intuition of
| symmetry and conservation laws. Could you please elaborate
| more on that?
|
| As far as I understand this asymmetry was the exact reason
| why entropy was introduced. Then later explained by
| Boltzmann via a measure of number of microscopic states.
|
| Naturally second law of thermodynamics forbids perpetual
| engines.
| Lichtso wrote:
| I think you can very well extract work from having a
| membrane and selectively let one substance mix into the
| other but not the other in the first [0]. It is called
| Osmosis [1].
|
| [0]: https://en.wikipedia.org/wiki/Semipermeable_membrane
| [1]: https://en.wikipedia.org/wiki/Osmosis
| ithkuil wrote:
| I guess what's confusing me in this scenario is that
| we're not saying that the two halves of the cylinder
| contain particles with different properties (e.g.
| different velocities) but only that we can "tell them
| apart" as if they were coloured differently, but
| otherwise behaving in exactly the same way.
|
| The former scenario is famously the setting for Maxwell's
| daemon. I was assuming this scenario is something else.
|
| I'm confused because on one hand I can see that it
| requires work to reorder the particles once they have
| been shuffled around. On the other hand I don't see how
| one could extract work while they get shuffled around if
| they all have the same momenta.
|
| Perhaps the answer is that we cannot have a system where
| the microscopic entities are at the same
| indistinguishable but also distinguishable. Perhaps if
| they had different "colours' it means they do interact
| differently with the environment? I'm still confused
| frankly
| passion__desire wrote:
| Entropy is mathematical force to be honest.
| canadianfella wrote:
| [dead]
| Solvency wrote:
| Huh? "Force"?
| esafak wrote:
| concept
| danq__ wrote:
| [dead]
| TexanFeller wrote:
| I recently enjoyed this presentation by Sean Carroll that touches
| on definitional and philosophical issues with entropy. The talk
| made me feel less stupid for not feeling entirely comfortable
| with how entropy was explained to me before. Turns out there are
| a few different ways to define and quantify entropy that are used
| in different contexts and they each have some unresolved
| philosophical issues.
|
| "Can you get time out of Quantum Mechanics?":
| https://youtu.be/nqQrGk7Vzd4
| kergonath wrote:
| I would recommend reading some Carlo Rovelli, it sounds like
| something you might like.
| cubefox wrote:
| Another surprising thing is that physicists have not yet
| succeeded in reducing the ordinary notion of cause and effect
| to fundamental physics. Carroll has also worked on this issue.
| m3affan wrote:
| I wonder how many concepts that are so complicated for our
| brain to formalize or even process.
| sourcecodeplz wrote:
| It was a great video
| [deleted]
| dist-epoch wrote:
| Sabine Hossenfelder also had a video recently on entropy:
|
| > I don't believe the 2nd law of thermodynamics.
|
| https://www.youtube.com/watch?v=89Mq6gmPo0s
| evouga wrote:
| What I really like about this explanation is that it highlights
| the fact that entropy is not a natural property of the physics
| system: entropy is only defined with respect to some coarse-
| graining operation applied by an imperfect observer of the
| system. So as Sabine points out it seems we should really be
| talking about multiple different entropies, each of which
| corresponds to a different mechanism for coarse-graining
| microstates into macrostates, with each different entropy
| changing at different rates depending on the coarse-graining
| mechanism and physical system. (And in particular, God
| observing the universe would not see entropy change at all;
| even if there were uncertainty in the initial conditions of the
| universe, God would see that uncertainty perfectly propagated
| with no loss of information, in a way made precise by
| Liouville's Theorem.)
|
| But even this is not the full story, because I can take a mass-
| spring network, and no matter how I choose to coarse-grain it,
| I will not see the entropy corresponding to that coarse-
| graining increase, because the trajectory of a mass-spring
| system is periodic. Entropy increase requires that the system
| is ergodic with respect to the chosen coarse-graining
| operation, i.e. that over long times the trajectory visits the
| coarse-grained states in a "random" and uniform way. It's not
| at all obvious to me why the dynamics of particles bouncing
| around in a box have this property, and particles attached in a
| mass-spring network do not; and neither the Sabine nor the
| Veritaserum videos address this or why we should expect all
| practical real-world physical systems to be ergotic with
| respect to practical coarse-graining mechanisms.
| dist-epoch wrote:
| > mass-spring system is periodic
|
| I don't pretend to understand this stuff, but wouldn't a real
| mass-spring system slowly stop, due to friction, air
| resistance, heat dissipation, ...? So a real system wouldn't
| be periodic.
| consilient wrote:
| Yes, but they're talking about an idealized harmonic
| oscillator, not a physical mass-spring system.
| sidlls wrote:
| Periodic doesn't mean perpetual, perfect, constant periodic
| motion (in general).
| thumbuddy wrote:
| In my opinion the most misunderstood concept from physics is
| probably any exponential relationship. I realize we could view
| entropy to be one of those if we flip the relationship around and
| equate for microstates making my statement the superset. But
| generally speaking, Ive seen both lay people and experts struggle
| to reason about them, especially with complex numbers.
| dangitnotagain wrote:
| Entropy should be redefined as "the distribution of potential
| over negative potential."
|
| Whether discussing what is over what may be, or thermal
| equilibrium, potential distribution describes it all!
| antimora wrote:
| The video did not explain why the sun is a low entropy source. I
| found this explaining what I am sharing with you:
|
| So, the sun is a low-entropy source of energy, and Earth (and
| everything on it) increases that entropy as it uses and then
| reradiates that energy. This process is entirely consistent with
| the second law of thermodynamics.
|
| The relationship between light frequency and entropy comes from
| the fact that entropy is a measure of disorder or randomness.
| High-frequency light, such as ultraviolet or visible light, is
| more ordered and less random than lower-frequency light, such as
| infrared or microwave light.
|
| This is due to how light is structured. Light is made up of
| particles called photons, and each photon carries a certain
| amount of energy. The energy of a photon is directly proportional
| to its frequency: higher-frequency photons carry more energy than
| lower-frequency ones.
|
| So, if you have a fixed amount of energy to distribute among
| photons, you can do so in many more ways (i.e., with higher
| entropy) if you use low-energy, low-frequency photons. That's
| because you would need many more of them to carry the same total
| amount of energy.
|
| On the other hand, if you use high-energy, high-frequency
| photons, you would need fewer of them to carry the same total
| amount of energy. There are fewer ways to distribute the energy
| (i.e., lower entropy), so this arrangement is more ordered and
| less random.
|
| Therefore, high-frequency light is considered a lower-entropy
| form of energy compared to low-frequency light, because the
| energy is concentrated in fewer, more energetic photons.
| kgwxd wrote:
| > The video did not explain why the sun is a low entropy
| source.
|
| Laymen to the extreme but, didn't it? The thing about the low
| entropy of the universe near the big bang, gravity naturally
| bringing things together, and such?
| guga42k wrote:
| >The video did not explain why the sun is a low entropy source.
| I found this explaining what I am sharing with you:
|
| to my best understanding, to go from high entropy state to low
| entropy state you need work to do. The sun is a source of
| energy to do the work
| rssoconnor wrote:
| While this is a reasonable historical explanation of entropy, and
| explains that we don't gain net energy from the sun, it still
| misses the mark on what entropy is now known to be.
|
| Entropy isn't a property of an object, or a system or things in
| physics. Entropy is a property of our _description_ of systems.
| More precisely it is a measure of how poorly a given
| specification of a physical system is, i.e. given description of
| a systems, typically the pressure / volume / temperature of a gas
| or whatnot, how many different physical systems correspond to
| such a description.
|
| In particular, _thermodynamic entropy is Shannon entropy_.
|
| In the case where the description of state specifies a volume of
| phase space wherein a physical state lies within, then the
| entropy is the logarithm of the volume of this fragment of phase
| space. If we take this collection of states and see how they
| evolve in time, then Liouville's theorem says the volume of phase
| space will remain constant.
|
| If we want to build a reliable machine, i.e. an engine, that can
| operate in any initial state that is bounded by our description,
| and ends up win a final state bounded by some other description,
| well, in order for this machine to preform reliably, the volume
| of the final description needs to be greater than the volume of
| the description of the initial state. Otherwise, some possible
| initial states will fail to end up in the desired final state.
| This is the essence of the second law of thermodynamics.
|
| I want to emphasis this: entropy exists in our heads, not in the
| world.
|
| E.T. Jaynes illustrated this "5. The Gas Mixing Scenario
| Revisited" in
| https://www.damtp.cam.ac.uk/user/tong/statphys/jaynes.pdf where
| two imaginary variants of Argon gas are mixed together. If one
| engineer is ignorant of the different variants of Argon gas, it
| is impossible to extract work from the gas, but armed with
| knowledge of the difference (which must be exploitable otherwise
| they wouldn't actually be different) work can be extracted.
|
| Knowledge _is_ power.
|
| Taking an extreme example, suppose we have two volumes of gas at
| different volumes / pressures / temperature. We can compute how
| much work can be extracted from those gases.
|
| But, suppose someone else knows more than just the volume /
| pressure / temperature of these gases. This someone happens to
| know the precise position and velocity of every single molecule
| of gas (more practically they know the quantum state of the
| system). This someone now gets to play a the role of Maxwell's
| demon and separate all the high velocity and low velocity
| molecules of each chamber, opening and closing a gate using their
| perfect knowledge of where each particle is at each moment in
| time. From this they can now extract far more work than the
| ignorant person.
|
| In both cases the gas was identical. How much useful work one can
| extract depends on how precise one's knowledge of the state of
| that gas is.
| cubefox wrote:
| Hossenfelder says the same in her entropy video. A really
| interesting hypothesis.
| pcwelder wrote:
| This is eye opening. Thanks a lot for this comment and linking
| the pdf. I loved E.T. Jayne's Probability Theory book, so
| looking forward to reading this pdf too.
| kgwgk wrote:
| There are a few chapters of an unpublished book on
| thermodynamics here: https://bayes.wustl.edu/etj/thermo.html
|
| This article is also interesting: "THE EVOLUTION OF CARNOT'S
| PRINCIPLE" https://bayes.wustl.edu/etj/articles/ccarnot.pdf
|
| Building on these ideas, the first five chapters of this
| (draft of a) book from Ariel Caticha are quite readable:
| https://www.arielcaticha.com/my-book-entropic-physics
| ko27 wrote:
| Entropy is very much "real" and it exists outside of our mind.
| The resolution to Maxwell's demon is that knowledge of every
| particle's state is not free, you need to increase the system's
| entropy by obtaining knowledge more than you can ever eliminate
| by opening chamber doors.
|
| If it only existed in our minds and not in physical reality
| that would mean it would be possible to construct a device that
| decreases global entropy on average.
| [deleted]
| Lichtso wrote:
| I think the most unintuitive even unsettling aspect of entropy is
| that the entropy of black holes is proportional to their surface
| area, not their volume [0]. That is only briefly mentioned in the
| video and not discussed any further.
|
| [0]
| https://en.wikipedia.org/wiki/Holographic_principle#Black_ho...
| max_ wrote:
| Sad he didn't talk about Shannon Entropy
| danq__ wrote:
| [dead]
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