5 ms·

Arxiv link to the actual paper: https://arxiv.org/abs/2008.02373 https://arxiv.org/abs/2008.02373 Looking at the actual graphs, what is presented appears at fi

Arxiv link to the actual paper: https://arxiv.org/abs/2008.02373 https://arxiv.org/abs/2008.02373 Looking at the actual graphs, what is presented appears at first blush to be a bistable system prepared in its higher-energy stable state. (Or at least, prepared in a way that several of its constituent subsystems "fall into" that state.) Transitions to the lower-energy stable state require overcoming a barrier, and jumping over that barrier is of course faster at higher temperatures. At lower temperatures you essentially have a slow internal self-heating from the periodical jumps of subsystems through this forbidden region by thermal excitations, not unlike phosphorescence having to tunnel through an energetically forbidden region; this self-heating prevents the thing from cooling down fast enough. Actual applicability to water is kind of harder to evaluate. I've long thought that you could indeed have an Mpemba effect caused by persistent induced convective currents in a fluid: so a hotter thing placed into a fridge might create a more dramatic internal flow as its outer boundary layer cools and changes in density, the convective current would certainly increase the slope of the hotter cooling system beyond the naive temperature scaling, and might then persist as a physical difference even after the two systems arrive at the same temperature, leading to a faster cooling speed of the convective system that "started out hotter". But I mean I have never really done experiments to confirm that sort of thing.

saeranv 6y ago
> Actual applicability to water is kind of harder to evaluate. I've long thought that you could indeed have an Mpemba effect caused by persistent induced convective currents in a fluid: so a hotter thing placed into a fridge might create a more dramatic internal flow as its outer boundary layer cools and changes in density, the convective current would certainly increase the slope of the hotter cooling system beyond the naive temperature scaling, and might then persist as a physical difference even after the two systems arrive at the same temperature, leading to a faster cooling speed of the convective system that "started out hotter". But I mean I have never really done experiments to confirm that sort of thing. This sounds like what I was thinking but I'm not sure. Do you mean the actual boundary layer in the hotter fluid would have a higher convective heat transfer, due to increased turbulence/thermal mixing at the surface of hot water (versus cold water)? Thus hot water will cool faster?
thewileyone 6y ago
You've put into scientific terms what I can't cause it's been too long since I've done any physics. In layman terms, the molecules in hot water are bouncing around more and thus easier to form crystal structures because of the higher-energy state. Molecules at room temperature have to be forced into crystal structures which takes longer.
- baddox 6y ago
  A little more explanation is required, since the boiling pot still needs to pass through room temperature on its way to freezing, and it’s not clear how at that point it has a head start on the other room temperature pot.
  - jhrmnn 6y ago
    Especially since there is no such thing as water memory.
    - iso947 6y ago
      I wonder how many kids watching frozen 2 will subconsciously accept “water has memory” as a fact and believe it in 20 years time, being unable to source why they think it, just something “they know”
      - thaumasiotes 6y ago
        That's something people have believed basically forever. The most modern example would be homeopathy, which is explicitly based on the idea that water has memory and significantly predates Frozen 2. So, the answer to your question is "a really large number", but it's not obvious that Frozen 2 would have influenced them.
        stan_rogers 6y ago
        It would be more correct to say that homeopathy became explicitly based on water memory after atomic theory and the size of atoms achieved mainstream acceptance. And while Dalton and Avogadro had certainly made convincing arguments based on chemistry in the early 19th century, and thermodynamicists like Boltzman had models of heat and heat transfer that seemed to depend on particles, it wasn't until after Einstein's paper on Brownian motion and Rutherford's interpretation of the Geiger-Marsden experiment that homeopathic dilution just being dilution became an unsustainable notion. It was always a silly notion, especially when numbers like 30C - a dilution of one part in 10^60, or one molecule of the target substance in 3 × 10^34kg of water, after you understand what molecules are - were being thrown around, but there was at least a plausible explanation before, say, WW1.
mNovak 6y ago
I'm reading this as hot water having more effective heat transfer deep into the container, potentially via some type of self-mixing due to turbulence from the sharp temperature gradient? This would seem very easy to measure, with and without internal barriers to prevent large scale convection.

saeranv 6y ago

> Actual applicability to water is kind of harder to evaluate. I've long thought that you could indeed have an Mpemba effect caused by persistent induced convective currents in a fluid: so a hotter thing placed into a fridge might create a more dramatic internal flow as its outer boundary layer cools and changes in density, the convective current would certainly increase the slope of the hotter cooling system beyond the naive temperature scaling, and might then persist as a physical difference even after the two systems arrive at the same temperature, leading to a faster cooling speed of the convective system that "started out hotter". But I mean I have never really done experiments to confirm that sort of thing. This sounds like what I was thinking but I'm not sure. Do you mean the actual boundary layer in the hotter fluid would have a higher convective heat transfer, due to increased turbulence/thermal mixing at the surface of hot water (versus cold water)? Thus hot water will cool faster?

thewileyone 6y ago

You've put into scientific terms what I can't cause it's been too long since I've done any physics. In layman terms, the molecules in hot water are bouncing around more and thus easier to form crystal structures because of the higher-energy state. Molecules at room temperature have to be forced into crystal structures which takes longer.

baddox 6y ago
A little more explanation is required, since the boiling pot still needs to pass through room temperature on its way to freezing, and it’s not clear how at that point it has a head start on the other room temperature pot.
- jhrmnn 6y ago
  Especially since there is no such thing as water memory.
  - iso947 6y ago
    I wonder how many kids watching frozen 2 will subconsciously accept “water has memory” as a fact and believe it in 20 years time, being unable to source why they think it, just something “they know”
    - thaumasiotes 6y ago
      That's something people have believed basically forever. The most modern example would be homeopathy, which is explicitly based on the idea that water has memory and significantly predates Frozen 2. So, the answer to your question is "a really large number", but it's not obvious that Frozen 2 would have influenced them.
      - stan_rogers 6y ago
        It would be more correct to say that homeopathy became explicitly based on water memory after atomic theory and the size of atoms achieved mainstream acceptance. And while Dalton and Avogadro had certainly made convincing arguments based on chemistry in the early 19th century, and thermodynamicists like Boltzman had models of heat and heat transfer that seemed to depend on particles, it wasn't until after Einstein's paper on Brownian motion and Rutherford's interpretation of the Geiger-Marsden experiment that homeopathic dilution just being dilution became an unsustainable notion. It was always a silly notion, especially when numbers like 30C - a dilution of one part in 10^60, or one molecule of the target substance in 3 × 10^34kg of water, after you understand what molecules are - were being thrown around, but there was at least a plausible explanation before, say, WW1.

mNovak 6y ago

I'm reading this as hot water having more effective heat transfer deep into the container, potentially via some type of self-mixing due to turbulence from the sharp temperature gradient? This would seem very easy to measure, with and without internal barriers to prevent large scale convection.