5 ms·

I was talking to an astrobiologist a couple years ago, wondering what she looks for as signs of life. We got into talking about all sorts of things, from prion

I was talking to an astrobiologist a couple years ago, wondering what she looks for as signs of life. We got into talking about all sorts of things, from prions to the LUCA (last common universal ancestor). At some point when had asked enough questions about what is and isn’t alive, and she basically said it’s not even clear what is and isn’t biology.

astrange 5y ago
Local decreases in entropy seems like some kind of definition of life, but presumably it has to get big enough to start changing the atmosphere for anyone to notice it from another planet.
- nsonha 5y ago
  hey stupid question, what is entropy, how do you measure it?
  - kergonath 5y ago
    It’s not a stupid question at all. Entropy is a made up number like energy. In a physical system it is related to how many arrangements of atoms/molecules/things there can be that would result in the system being in the state you can see. Energy tells you how difficult it is to put a bunch of particles (atoms, for example) in a given state. Entropy is how many of such states are possible. Ordered states (like crystals) have lower entropies than disordered states (like liquids), because there are fewer ways of arranging atoms and still get a crystal compared to the combination of possible positions for each atom in a liquid. In systems that are isolated (i.e. they cannot exchange energy with anything else), entropy cannot decrease on average. In non-isolated systems, anything goes and entropy can go up or down locally all the time. We don’t measure it directly. What we can do usually is measure how much something heats when we give it energy, and work from there to deduce any entropy change using equations from thermodynamics or statistical Physics. It is impossible to measure an absolute entropy in general (in the same way as it is impossible to measure an absolute energy in general). There are lots of caveats, asterisks, and cases that look like exceptions; that’s a quick and dirty description.
    - xaedes 5y ago
      > In a physical system it is related to how many arrangements of atoms/molecules/things there can be that would result in the system being in the state you can see. Mind if I ask follow-up question, what is a "state"? Doesn't the entropy change when I change my definition of the state? If I go to the extreme and there is only one kind of state, an actual arrangement of particles, fields etc., the entropy would be the same of each and everything (one possible arrangement per state). Does that make entropy an entirely subjective measure?
      - TchoBeer 5y ago
        iirc, a macrostate has a canonical definition (something like energy, the number of particles, and the volume)
      - kergonath 5y ago
        > Mind if I ask follow-up question, what is a "state"? Sure. Actually there are two main things we call states. The first one (macroscopic state, or macrostate) is what we think as characteristic properties of a bit of matter, e.g. "1 kg of liquid water at 300 K under atmospheric pressure" compared to "1 kg of solid water at 250 K under atmospheric pressure". The second one (microscopic states, or microstate) is the way the particles that constitute this bit of matter are arranged. The naming is a bit unfortunate and it can get technical quite quickly, but the distinction between macroscopic and microscopic states is crucial. So, a more precise version of my previous post would be something like that. In the liquid example, there are many, many ways of distributing the H2O molecules that would result in the same macroscopic description. This means that there are many microscopic states that are consistent with the macroscopic state we observe. And, looking at a glass of water, we cannot say where the molecules are. On the other hand, in a perfect ice crystal, the positions of all the atoms constituting al the H2O molecules are uniquely determined by the crystal structure. So, looking at a perfect ice cube we can say where every molecule is. It gets a bit more complicated in reality because no crystal is ever perfect. There are defects that introduce some disorder, so there are more than one microscopic state, but much fewer than in the liquid. Entropy is larger for things that have more microscopic states consistent with their macroscopic state. You can also see here a hint of the link with information entropy in CS, if you think about the number of microscopic states as our knowledge of the molecules' positions. > Doesn't the entropy change when I change my definition of the state? If I go to the extreme and there is only one kind of state, an actual arrangement of particles, fields etc., the entropy would be the same of each and everything (one possible arrangement per state). It does sound very subjective. The conventional naming is a bit unfortunate and a consequence of the historical roots of statistical Physics in 19th-century thermodynamics. After all, there is no clear boundary between macroscopic and microscopic. If this bothers you, you can say that "macroscopic" refers to the thing you are looking at as a whole, and "microscopic" to its constituents. This framework works as long as the stuff you are studying is made up of smaller things. We commonly use atoms and molecules in examples because it is somewhat intuitive, but you could consider an atom itself as a macroscopic system and its quarks as its constituents. Or the universe and its galaxies clusters. The caveat is that the mathematical formalism is exact in the limit where the number of constituents is infinite, but might break down if their number is too small. That's why the distinction between macroscopic and microscopic is helpful. > Does that make entropy an entirely subjective measure? In a way, a bit. You can define the constituents seemingly arbitrarily (like considering molecules, or atoms as separate entities or not, adding electrons, etc). Adding more details gives a more accurate answer, but at some point it becomes irrelevant. So it is actually less subjective than relative. To keep (ab)using the water example, a molecule is made up of 3 atoms, each one having a position (3 positions, each one being a 3-dimensional vector, so 9 parameters in total). But when grouped in a molecule these positions are not independent, and need to be consistent with the O-H bond length and the H-O-H angle. A molecule is characterised by a position (3-d vector), an orientation along the axes of the reference frame (3 other parameters), an angle and two bond lengths (again 9 parameters in total). So there is no more information if you describe the ice cube as a collection of atoms than as a collection of molecules, even though the choice seems arbitrary.
  - BlueTemplar 5y ago
    It's a somewhat obsolete term for "lack of information" : http://www.av8n.com/physics/thermo/entropy.html#sec-s-not-knowing http://www.av8n.com/physics/thermo/entropy.html#sec-s-not-kn... (Yes, it is indeed context-dependent.)
    - nsonha 5y ago
      thanks I've always thought it's "disorder"
- kergonath 5y ago
  Local decrease in entropy does not necessarily indicate life, merely an non-isolated, out-of-equilibrium system.
  - astrange 5y ago
    I was using a very loose definition of entropy there, more like organization, but for instance if the atmosphere starts displaying large amounts of oxygen or the surface is suddenly covered in metal then something is happening there.
- pishpash 5y ago
  Then hurricanes would be life.

astrange 5y ago

Local decreases in entropy seems like some kind of definition of life, but presumably it has to get big enough to start changing the atmosphere for anyone to notice it from another planet.

nsonha 5y ago
hey stupid question, what is entropy, how do you measure it?
- kergonath 5y ago
  It’s not a stupid question at all. Entropy is a made up number like energy. In a physical system it is related to how many arrangements of atoms/molecules/things there can be that would result in the system being in the state you can see. Energy tells you how difficult it is to put a bunch of particles (atoms, for example) in a given state. Entropy is how many of such states are possible. Ordered states (like crystals) have lower entropies than disordered states (like liquids), because there are fewer ways of arranging atoms and still get a crystal compared to the combination of possible positions for each atom in a liquid. In systems that are isolated (i.e. they cannot exchange energy with anything else), entropy cannot decrease on average. In non-isolated systems, anything goes and entropy can go up or down locally all the time. We don’t measure it directly. What we can do usually is measure how much something heats when we give it energy, and work from there to deduce any entropy change using equations from thermodynamics or statistical Physics. It is impossible to measure an absolute entropy in general (in the same way as it is impossible to measure an absolute energy in general). There are lots of caveats, asterisks, and cases that look like exceptions; that’s a quick and dirty description.
  - xaedes 5y ago
    > In a physical system it is related to how many arrangements of atoms/molecules/things there can be that would result in the system being in the state you can see. Mind if I ask follow-up question, what is a "state"? Doesn't the entropy change when I change my definition of the state? If I go to the extreme and there is only one kind of state, an actual arrangement of particles, fields etc., the entropy would be the same of each and everything (one possible arrangement per state). Does that make entropy an entirely subjective measure?
    - TchoBeer 5y ago
      iirc, a macrostate has a canonical definition (something like energy, the number of particles, and the volume)
    - kergonath 5y ago
      > Mind if I ask follow-up question, what is a "state"? Sure. Actually there are two main things we call states. The first one (macroscopic state, or macrostate) is what we think as characteristic properties of a bit of matter, e.g. "1 kg of liquid water at 300 K under atmospheric pressure" compared to "1 kg of solid water at 250 K under atmospheric pressure". The second one (microscopic states, or microstate) is the way the particles that constitute this bit of matter are arranged. The naming is a bit unfortunate and it can get technical quite quickly, but the distinction between macroscopic and microscopic states is crucial. So, a more precise version of my previous post would be something like that. In the liquid example, there are many, many ways of distributing the H2O molecules that would result in the same macroscopic description. This means that there are many microscopic states that are consistent with the macroscopic state we observe. And, looking at a glass of water, we cannot say where the molecules are. On the other hand, in a perfect ice crystal, the positions of all the atoms constituting al the H2O molecules are uniquely determined by the crystal structure. So, looking at a perfect ice cube we can say where every molecule is. It gets a bit more complicated in reality because no crystal is ever perfect. There are defects that introduce some disorder, so there are more than one microscopic state, but much fewer than in the liquid. Entropy is larger for things that have more microscopic states consistent with their macroscopic state. You can also see here a hint of the link with information entropy in CS, if you think about the number of microscopic states as our knowledge of the molecules' positions. > Doesn't the entropy change when I change my definition of the state? If I go to the extreme and there is only one kind of state, an actual arrangement of particles, fields etc., the entropy would be the same of each and everything (one possible arrangement per state). It does sound very subjective. The conventional naming is a bit unfortunate and a consequence of the historical roots of statistical Physics in 19th-century thermodynamics. After all, there is no clear boundary between macroscopic and microscopic. If this bothers you, you can say that "macroscopic" refers to the thing you are looking at as a whole, and "microscopic" to its constituents. This framework works as long as the stuff you are studying is made up of smaller things. We commonly use atoms and molecules in examples because it is somewhat intuitive, but you could consider an atom itself as a macroscopic system and its quarks as its constituents. Or the universe and its galaxies clusters. The caveat is that the mathematical formalism is exact in the limit where the number of constituents is infinite, but might break down if their number is too small. That's why the distinction between macroscopic and microscopic is helpful. > Does that make entropy an entirely subjective measure? In a way, a bit. You can define the constituents seemingly arbitrarily (like considering molecules, or atoms as separate entities or not, adding electrons, etc). Adding more details gives a more accurate answer, but at some point it becomes irrelevant. So it is actually less subjective than relative. To keep (ab)using the water example, a molecule is made up of 3 atoms, each one having a position (3 positions, each one being a 3-dimensional vector, so 9 parameters in total). But when grouped in a molecule these positions are not independent, and need to be consistent with the O-H bond length and the H-O-H angle. A molecule is characterised by a position (3-d vector), an orientation along the axes of the reference frame (3 other parameters), an angle and two bond lengths (again 9 parameters in total). So there is no more information if you describe the ice cube as a collection of atoms than as a collection of molecules, even though the choice seems arbitrary.
- BlueTemplar 5y ago
  It's a somewhat obsolete term for "lack of information" : http://www.av8n.com/physics/thermo/entropy.html#sec-s-not-knowing http://www.av8n.com/physics/thermo/entropy.html#sec-s-not-kn... (Yes, it is indeed context-dependent.)
  - nsonha 5y ago
    thanks I've always thought it's "disorder"
kergonath 5y ago
Local decrease in entropy does not necessarily indicate life, merely an non-isolated, out-of-equilibrium system.
- astrange 5y ago
  I was using a very loose definition of entropy there, more like organization, but for instance if the atmosphere starts displaying large amounts of oxygen or the surface is suddenly covered in metal then something is happening there.
pishpash 5y ago
Then hurricanes would be life.