5 ms·

"made of particles called baryons rather than dark matter" That's clumsily worded, as it makes it sound like it's still something exotic. _We're_ made of bary

"made of particles called baryons rather than dark matter" That's clumsily worded, as it makes it sound like it's still something exotic. _We're_ made of baryons: this is just normal matter.

vanderZwan 9y ago
As a former physics student, I see your point but disagree. Apologies for the wall of text about just this one choice of wording, but discussing why science communication is a very difficult trade-off takes a bit of space. One reason to stick to "baryons" is that it is the term used by the researchers in their papers. So it is more true to the research that is being summarised. The start of the article (unlike its title...) mentions that this research is about normal matter quite clearly, so this should give enough context to avoid confusion: > This is the first detection of the roughly half of the normal matter in our universe – protons, neutrons and electrons – unaccounted for by previous observations of stars, galaxies and other bright objects in space. You have probably heard about the hunt for dark matter, a mysterious substance thought to permeate the universe, the effects of which we can see through its gravitational pull. But our models of the universe also say there should be about twice as much ordinary matter out there, compared with what we have observed so far. Baryons are subatomic particles[0][1], not atoms, which is what most people think of when discussing "matter". If we say "normal matter", some people will think we're talking about chunks of dirt floating between galaxies. I'm not joking, have you ever been to a public physics lecture? And mistakes like this are not the audience's fault - it's the expert's fault for assuming non-experts know what they know. For people with the right background (chemists, physicists, astronomers, etc) it is clear this is "just normal matter". For everyone else - so most people - "normal matter" means matter in earth-typical circumstances of pressure and temperature, in a solid/liquid/vapour phase. The papers describe intergalactic plasma filaments with a temperature between 10⁴ to 10⁵ K. That is pretty exotic by normal-people standards. And yes, fire is a plasma, but for most people fire is just very hot gas, and the article even builds on that: > Because the gas is so tenuous and not quite hot enough for X-ray telescopes to pick up, nobody had been able to see it before. So the other reason reason to stick to "baryons" is that this is exotic matter from the point of view of non-experts. When describing research you have to start from their model of the physical world, and build your way back to the research. I think the choice in wording appropriate here. The other paragraphs make it clear this is about normal matter, while this sentence makes it clear it's not normal matter in Earth-circumstances, which is how the average human thinks about it if not corrected. [0] https://en.wikipedia.org/wiki/Baryon https://en.wikipedia.org/wiki/Baryon [1] https://simple.wikipedia.org/wiki/Baryon https://simple.wikipedia.org/wiki/Baryon (the Simple English wiki is probably more useful to us non-cosmologists here)
- andrewaylett 9y ago
  It seems I know just enough to be dangerous. Thank you for your well-written explanation.
  - vanderZwan 9y ago
    You're very welcome, and don't be hard on yourself! It is easy to underestimate how difficult it is to give good explanations to non-experts, regardless of the topic at hand. I mean, just look at how how much I wrote about the choice of one word. When writing for non-experts, an expert must imagine what it is like to not know the right answer. Our brains seem to be terrible at that: most people are unable to "not see" the cow in that famous gestalt picture once spotted[0], and I think the same thing applies to many kinds of thinking. The only way out for an expert seems to be a lot of exposure to "dumb questions" (there is no such thing, of course) from non-experts, to figure out what logic the latter's wrong conclusions are. It doesn't help that there are many more ways to come to perfectly logical but wrong conclusions based on wrong premises, than there are ways to come to the right conclusion with the right premises. And even once you are aware of these mismatches, you still have to explain yourself in such a way that there is a path from the incorrect interpretation to the more correct interpretation, without getting lost in trying to explain everything. So no wonder that even professional educators and science writers, who are supposed to be skilled at this, tend to do a poor job! [0] http://icog.group.shef.ac.uk/what-can-you-see-some-questions-about-the-content-of-visual-experience/ http://icog.group.shef.ac.uk/what-can-you-see-some-questions...
- raattgift 9y ago
  Funnily enough, cosmology's use of "baryon" predates the Standard model (it even predates Politzer, Gross & Wilczek 1973) and leans heavily on the Greek root "barus", meaning heavy i.e. having a significant rest mass (and thus implying "nonrelativistic", more below). Since then we've developed the Standard Model and its baryons have many properties that are essentially irrelevant at cosmological scales after reionization. More technically (and all to the first order), in \Lambda-CDM the first Friedmann equation can be written as H(a) \equiv H_0 {\sqrt {(\Omega_c + \Omega_b) a^{-3} + (\Omega_{rad+hdm}) a^{-4} + ... }}, the \Omega_{...} being density parameters. Although in this form "c" stands for Cold Dark Matter (CDM) and "b" stands for baryons, the important common attribute here is that the rest mass is high enough that absent highly atypical kicks they move slowly compared to photons[1]. I explicitly added "+hdm" (hot dark matter) to the radiation term, where hdm is mainly relativistic neutrinos, which aren't heavy and thus usually move at speeds closely approaching that of photons. The relativsitic/nonrelativistic split is important in structure formation, roughly because the former do not stick around during gravitational collapse. One can consider here that \Omega_c is (still) likely to be mainly nonrelativistic heavy uncharged leptons and that if we lump nonrelativistic charged leptons separately into \Omega_{b+c+l} term they would be a tiny contribution to the density. Relativistic baryons and relativistic charged leptons could be lumped in with \Omega_{rad}; it wouldn't make much difference. Experimental values for \Omega_{rad} are very small (~ 10^-4 vs \Omega_{b+c} ~ 3 x 10^-1, i.e. about 1:3000) and are often dropped for ease of calculation. Bound states are either relativistic or not, such things would just take the place of relativistic neutrinos and photons from \Omega_{rad}. Doing any of this is just fiddling at the margins, however. Cold Dark Matter and nuclei simply dwarf any other matter component across most of the universe's history. - -- [1] In the FLRW model the components of the \Omega_{...} densities (i.e., matter in the large) are homogeneous, compressible fluids of constant density at rest under a constant spatially isotropic tension in an "equatorial" slicing of the RW spacetime, so we don't even have relativistic baryons (for example): everything is moving inertially as spontaneous symmetry breaking freezes them out of the hotter denser earlier phase closer to the big bang. This is a reasonable approximation at the largest scales.

"made of particles called baryons rather than dark matter" That's clumsily worded, as it makes it sound like it's still something exotic. _We're_ made of baryons: this is just normal matter.

vanderZwan 9y ago

As a former physics student, I see your point but disagree. Apologies for the wall of text about just this one choice of wording, but discussing why science communication is a very difficult trade-off takes a bit of space. One reason to stick to "baryons" is that it is the term used by the researchers in their papers. So it is more true to the research that is being summarised. The start of the article (unlike its title...) mentions that this research is about normal matter quite clearly, so this should give enough context to avoid confusion: > This is the first detection of the roughly half of the normal matter in our universe – protons, neutrons and electrons – unaccounted for by previous observations of stars, galaxies and other bright objects in space. You have probably heard about the hunt for dark matter, a mysterious substance thought to permeate the universe, the effects of which we can see through its gravitational pull. But our models of the universe also say there should be about twice as much ordinary matter out there, compared with what we have observed so far. Baryons are subatomic particles[0][1], not atoms, which is what most people think of when discussing "matter". If we say "normal matter", some people will think we're talking about chunks of dirt floating between galaxies. I'm not joking, have you ever been to a public physics lecture? And mistakes like this are not the audience's fault - it's the expert's fault for assuming non-experts know what they know. For people with the right background (chemists, physicists, astronomers, etc) it is clear this is "just normal matter". For everyone else - so most people - "normal matter" means matter in earth-typical circumstances of pressure and temperature, in a solid/liquid/vapour phase. The papers describe intergalactic plasma filaments with a temperature between 10⁴ to 10⁵ K. That is pretty exotic by normal-people standards. And yes, fire is a plasma, but for most people fire is just very hot gas, and the article even builds on that: > Because the gas is so tenuous and not quite hot enough for X-ray telescopes to pick up, nobody had been able to see it before. So the other reason reason to stick to "baryons" is that this is exotic matter from the point of view of non-experts. When describing research you have to start from their model of the physical world, and build your way back to the research. I think the choice in wording appropriate here. The other paragraphs make it clear this is about normal matter, while this sentence makes it clear it's not normal matter in Earth-circumstances, which is how the average human thinks about it if not corrected. [0] https://en.wikipedia.org/wiki/Baryon https://en.wikipedia.org/wiki/Baryon [1] https://simple.wikipedia.org/wiki/Baryon https://simple.wikipedia.org/wiki/Baryon (the Simple English wiki is probably more useful to us non-cosmologists here)

andrewaylett 9y ago
It seems I know just enough to be dangerous. Thank you for your well-written explanation.
- vanderZwan 9y ago
  You're very welcome, and don't be hard on yourself! It is easy to underestimate how difficult it is to give good explanations to non-experts, regardless of the topic at hand. I mean, just look at how how much I wrote about the choice of one word. When writing for non-experts, an expert must imagine what it is like to not know the right answer. Our brains seem to be terrible at that: most people are unable to "not see" the cow in that famous gestalt picture once spotted[0], and I think the same thing applies to many kinds of thinking. The only way out for an expert seems to be a lot of exposure to "dumb questions" (there is no such thing, of course) from non-experts, to figure out what logic the latter's wrong conclusions are. It doesn't help that there are many more ways to come to perfectly logical but wrong conclusions based on wrong premises, than there are ways to come to the right conclusion with the right premises. And even once you are aware of these mismatches, you still have to explain yourself in such a way that there is a path from the incorrect interpretation to the more correct interpretation, without getting lost in trying to explain everything. So no wonder that even professional educators and science writers, who are supposed to be skilled at this, tend to do a poor job! [0] http://icog.group.shef.ac.uk/what-can-you-see-some-questions-about-the-content-of-visual-experience/ http://icog.group.shef.ac.uk/what-can-you-see-some-questions...
raattgift 9y ago
Funnily enough, cosmology's use of "baryon" predates the Standard model (it even predates Politzer, Gross & Wilczek 1973) and leans heavily on the Greek root "barus", meaning heavy i.e. having a significant rest mass (and thus implying "nonrelativistic", more below). Since then we've developed the Standard Model and its baryons have many properties that are essentially irrelevant at cosmological scales after reionization. More technically (and all to the first order), in \Lambda-CDM the first Friedmann equation can be written as H(a) \equiv H_0 {\sqrt {(\Omega_c + \Omega_b) a^{-3} + (\Omega_{rad+hdm}) a^{-4} + ... }}, the \Omega_{...} being density parameters. Although in this form "c" stands for Cold Dark Matter (CDM) and "b" stands for baryons, the important common attribute here is that the rest mass is high enough that absent highly atypical kicks they move slowly compared to photons[1]. I explicitly added "+hdm" (hot dark matter) to the radiation term, where hdm is mainly relativistic neutrinos, which aren't heavy and thus usually move at speeds closely approaching that of photons. The relativsitic/nonrelativistic split is important in structure formation, roughly because the former do not stick around during gravitational collapse. One can consider here that \Omega_c is (still) likely to be mainly nonrelativistic heavy uncharged leptons and that if we lump nonrelativistic charged leptons separately into \Omega_{b+c+l} term they would be a tiny contribution to the density. Relativistic baryons and relativistic charged leptons could be lumped in with \Omega_{rad}; it wouldn't make much difference. Experimental values for \Omega_{rad} are very small (~ 10^-4 vs \Omega_{b+c} ~ 3 x 10^-1, i.e. about 1:3000) and are often dropped for ease of calculation. Bound states are either relativistic or not, such things would just take the place of relativistic neutrinos and photons from \Omega_{rad}. Doing any of this is just fiddling at the margins, however. Cold Dark Matter and nuclei simply dwarf any other matter component across most of the universe's history. - -- [1] In the FLRW model the components of the \Omega_{...} densities (i.e., matter in the large) are homogeneous, compressible fluids of constant density at rest under a constant spatially isotropic tension in an "equatorial" slicing of the RW spacetime, so we don't even have relativistic baryons (for example): everything is moving inertially as spontaneous symmetry breaking freezes them out of the hotter denser earlier phase closer to the big bang. This is a reasonable approximation at the largest scales.