8 ms·

> Galactic spectra, which JWST started to send back in earnest at the end of last year, are useful for two reasons. > First, they let astronomers nail down the

by whiw 3y ago

> Galactic spectra, which JWST started to send back in earnest at the end of last year, are useful for two reasons. > First, they let astronomers nail down the galaxy’s age. The infrared light JWST collects is reddened, or redshifted, meaning that as it traverses the cosmos, its wavelengths are stretched by the expansion of space. The extent of that redshift lets astronomers determine a galaxy’s distance, and therefore when it originally emitted its light. Won't a photon climbing out of a huge gravity well have a huge redshift, thus confounding estimates of distance from us and estimated age?

ben_w 3y ago
> Won't a photon climbing out of a huge gravity well have a huge redshift, Yes > thus confounding estimates of distance from us and estimated age? Not if you know, or can get a good estimate of, the potential well it's climbing out of. That said, Brian Cox does sometimes joke that astronomers round π to 1; while I wouldn't know about the reality, it's probably safe to infer at least some frustration on his part about the precision of things in this field.
- hutzlibu 3y ago
  I also don't know how accurate that description is, but this comic comes to mind: https://xkcd.com/2205/ https://xkcd.com/2205/ But I mean, space is really, really big and we are observing from one single spot with (on cosmological scale probably) primitive technology. So of course most of it is guessing and when you "guess" a lot of things, it maybe does not matter a lot, if you have 3.41 Pi or 1, when the data you have are rough estimates anyway. But sure, when you do sloppy math, when you could have precision - that would be just wrong and unscientific.
  - dylan604 3y ago
    What if you have 3.14Pi?
    - dotancohen 3y ago
      Then you're so close to 10 that you can just use 10. I actually did that on a physics exam once. Somehow I had a value with pi squared. I just replaced it with 10. I don't remember if I got that question right or wrong - I'm hard pressed to think of a situation where pi squared actually legitimately shows up.
      - nvrgngvup 3y ago
        [dead]
      - rmu09 3y ago
        Surface area of a Torus?
        dotancohen 3y ago
        Not that I remember, or really relevant to undergrad physics - I'm pretty sure it was physics. Fusion and accretion disks are above my pay grade!
    - olddustytrail 3y ago
      That's pretty much pi squared. Which, for weirdly good reason, is close to the value of G in meters per second squared.
      - dylan604 3y ago
        only on this particular rock
        olddustytrail 3y ago
        Nope, on all rocks.
- marcosdumay 3y ago
  The joke was way more true at the 20th century. There were many really important measurements where we got unprecedentedly precision, enough to say it's X, 100X, or something in between. Nowadays astronomy got a lot more precise. But there are disagreements on how much confidence to put on that extra precision.
  - dylan604 3y ago
    sometimes, i wonder if astronomy/physics were to only use unsigned numbers, if things would just make more sense. you'd get much more precision, and then you wouldn't have to worry about "but the math says it's possible" issues by taking everything by * -1.
    - labster 3y ago
      Jesse, what are you talking about?
      - dylan604 3y ago
        Well, Mr White, if you look at all of the "weirdness" in physics with theories coming out because "the math says it's possible", then you'd see that it would be much more simple if we were not allowed to have negative numbers because we're only using u64 integers. we'd have more digits for precision as well.
        nickpeterson 3y ago
        I find that weird outliers are usually evidence that the underlying model is flawed in some fundamental way, but sometimes I think people believe the exceptions are fundamental, and a lot of effort goes to the wrong place.
- quickthrower2 3y ago
  I was confused... the symbol π is actually Pi, but it renders as something that looks like a different alphabet entirely on HN.
- gpgn 3y ago
  These are somewhated dated but well worth watching; the professor does all kinds of approximations. Frontiers/Controversies in Astrophysics with Charles Bailyn: https://youtu.be/ZiK4lg3Tzfs?t=1628 https://youtu.be/ZiK4lg3Tzfs?t=1628
mkoubaa 3y ago
My understanding is that a particles, like photons, don't have wave shifts. That's an emergent property of many particles
- alecst 3y ago
  The conventional Doppler shift and the equivalence principle imply a gravitational redshift for a photon. See page 102: https://preposterousuniverse.com/wp-content/uploads/grnotes-four.pdf https://preposterousuniverse.com/wp-content/uploads/grnotes-...
- stopping 3y ago
  No, individual particles can indeed be redshifted. The particle's wavelength is a fundamental property.
- blueprint 3y ago
  Your comment is a bit strange. Light doesn't travel as photons. Photons exclusively exist at the site and instant of detection of the wave of probability of detection that light really travels as. When light is redshifted, it loses energy, therefore the wavelength becomes longer.
  - tsimionescu 3y ago
    That is not the current understanding of quantum mechanics, as far as I know. Wave/particle dualism says that different experiments can either view light as a wave or a particle (never both) and that speaking about the nature of light when an experiment is not being performed is non-scientific by definition. Importantly, light very much behaves like a conventional wave in many real experiments - the interferometer experiment being one of the oldest and most well known. It is not a probability wave in that case, but an actual physical wave (now known to be an oscillation in the electro magnetic field, but long assumed to be a mechanical wave in the luminiferous aether).
    - _0w8t 3y ago
      Experimentally one never observes waves. Light is detected based on its interaction with electrons and that is always by an electron absorbing a quanta of energy, not via some continuous process as would be the case with waves. Classically one can imagine that as if electron was hit by a particle. But then we have light diffraction and interference, which classically is described as a wave. So from a classical point of view light travels as a wave but interact as a particle. As of nature of the light, then consider that there is a reformulation of a classical electrodynamics that eliminates electromagnetic waves all together. There are only electrons that interacts with each other directly with no waves in between. Feynman spent quite some time trying to develop quantum electrodynamic based on that. He failed. Still the point stands that we never observe light directly but only through its effects on electrons and other charged particles. So it could be that what we call light is a theoretical artifact and there is no light in reality.
pdonis 3y ago
> Won't a photon climbing out of a huge gravity well have a huge redshift It depends; the key factor is not how "huge" the gravity well is (in terms of how massive the object is), but how close to the black hole horizon the light is emitted. The vast majority of the light JWST is seeing from black holes is not from very close to the horizon. It's from the accretion disk, which is much further from the horizon and so the gravitational redshift is much smaller.
ajross 3y ago
> Won't a photon climbing out of a huge gravity well have a huge redshift, thus confounding estimates of distance from us and estimated age? Of course it will. It will also have a redshift related to the expansion of the space it's been travelling through. Both of those corrections are absolutely part of the model. It's not nearly as simple as "Astronomers forgot about General Relativity!". But I guess it's true (to be clear: I'm just an amateur in this field) that the ΛCDM model for cosmological evolution that we've all been looking at for the past decade or two isn't holding up well at all. It looked like it was pretty much there and just needed some fine tuning. Then we got a bunch of new data and everything's a mess. That's kind of exciting all by itself, though it's also leading to a bunch of nattering from the existing iconoclasts (MOND nuts in particular) whose theories are also not working very well to explain JWST observations. New insight needed, basically. We're all watching for updates.
- xigency 3y ago
  > Both of those corrections are absolutely part of the model Is there a way to gain a better understanding of how these parameters are modeled and what the scientific evidence is for the various phenomena in astrophysics? It's somewhat perplexing to me as an outsider of the field to understand how things like mass and distance of stars and planetary bodies are determined when 1) the scales are so outside of conventional experience 2) observation is limited to 2D imaging of the night sky 3) the observations in general are not consistent with our knowledge of gravity and relativity without adding hidden parameters.
  - antognini 3y ago
    I know Terence Tao (yes, that Terence Tao) is working on a book about the cosmic distance ladder, but sadly it's not out yet. (I guess he probably has other projects he's working on.) But he does have some slides from talks he's given: https://terrytao.files.wordpress.com/2020/10/cosmic-distance-ladder.pdf https://terrytao.files.wordpress.com/2020/10/cosmic-distance...
antognini 3y ago
The photons that are getting observed are emitted from the accretion disk, which is not very close to the event horizon. You can estimate how close to the event horizon the disk is based on how broad the spectral lines are. The part of the disk that is coming towards you will be blueshifted and the part of the disk that is rotating away will be redshifted. From that (which is independent of the overall depth of the potential well) you can figure out how fast the disk is rotating. And from that you can figure out how far away the disk is from the black hole.
- quakeguy 3y ago
  Well, this is awesome.
  - Smoosh 3y ago
    Sometimes I despair for humanity [see advertising/social media/politics], but then I see something like this and think, we're doing pretty well for self-taught primates.
    - dylan604 3y ago
      and then we see all of the belief systems to mock, punish, or worse people that are trying to expand human knowledge. looking back at history, you have to wonder where we might be now if there wasn't this attempts at mass eradication of learning. there are times i wonder if we're heading in that direction again. this time, rather than burning the Library of Alexandria or locking everything in catacombs, if we're not just trying to get there on a low-n-slow approach.
      - hattmall 3y ago
        I mean it really depends on your personal experience to how your perspective or "where we are now" feels. It's not like everyone is even remotely concerned with or benefitting from this "advancement." A lot of the so called progress has done nothing but destroyed and distort an enjoyable and peaceful existence.
codethief 3y ago
> Won't a photon climbing out of a huge gravity well have a huge redshift Yes. Though see the sibling comments on why the (remaining) gravity well the emitted photon has to climb out of is not particularly huge here. > thus confounding estimates of distance from us and estimated age? Generally speaking: No, because people are not stupid. :) See the integrated Sachs-Wolfe effect for a very similar situation (CMB photons traveling through gravitational wells) -> https://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect https://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect

by whiw 3y ago

ben_w 3y ago

> Won't a photon climbing out of a huge gravity well have a huge redshift, Yes > thus confounding estimates of distance from us and estimated age? Not if you know, or can get a good estimate of, the potential well it's climbing out of. That said, Brian Cox does sometimes joke that astronomers round π to 1; while I wouldn't know about the reality, it's probably safe to infer at least some frustration on his part about the precision of things in this field.

hutzlibu 3y ago
I also don't know how accurate that description is, but this comic comes to mind: https://xkcd.com/2205/ https://xkcd.com/2205/ But I mean, space is really, really big and we are observing from one single spot with (on cosmological scale probably) primitive technology. So of course most of it is guessing and when you "guess" a lot of things, it maybe does not matter a lot, if you have 3.41 Pi or 1, when the data you have are rough estimates anyway. But sure, when you do sloppy math, when you could have precision - that would be just wrong and unscientific.
- dylan604 3y ago
  What if you have 3.14Pi?
  - dotancohen 3y ago
    Then you're so close to 10 that you can just use 10. I actually did that on a physics exam once. Somehow I had a value with pi squared. I just replaced it with 10. I don't remember if I got that question right or wrong - I'm hard pressed to think of a situation where pi squared actually legitimately shows up.
    - nvrgngvup 3y ago
      [dead]
    - rmu09 3y ago
      Surface area of a Torus?
      - dotancohen 3y ago
        Not that I remember, or really relevant to undergrad physics - I'm pretty sure it was physics. Fusion and accretion disks are above my pay grade!
  - olddustytrail 3y ago
    That's pretty much pi squared. Which, for weirdly good reason, is close to the value of G in meters per second squared.
    - dylan604 3y ago
      only on this particular rock
      - olddustytrail 3y ago
        Nope, on all rocks.
marcosdumay 3y ago
The joke was way more true at the 20th century. There were many really important measurements where we got unprecedentedly precision, enough to say it's X, 100X, or something in between. Nowadays astronomy got a lot more precise. But there are disagreements on how much confidence to put on that extra precision.
- dylan604 3y ago
  sometimes, i wonder if astronomy/physics were to only use unsigned numbers, if things would just make more sense. you'd get much more precision, and then you wouldn't have to worry about "but the math says it's possible" issues by taking everything by * -1.
  - labster 3y ago
    Jesse, what are you talking about?
    - dylan604 3y ago
      Well, Mr White, if you look at all of the "weirdness" in physics with theories coming out because "the math says it's possible", then you'd see that it would be much more simple if we were not allowed to have negative numbers because we're only using u64 integers. we'd have more digits for precision as well.
      - nickpeterson 3y ago
        I find that weird outliers are usually evidence that the underlying model is flawed in some fundamental way, but sometimes I think people believe the exceptions are fundamental, and a lot of effort goes to the wrong place.
quickthrower2 3y ago
I was confused... the symbol π is actually Pi, but it renders as something that looks like a different alphabet entirely on HN.
gpgn 3y ago
These are somewhated dated but well worth watching; the professor does all kinds of approximations. Frontiers/Controversies in Astrophysics with Charles Bailyn: https://youtu.be/ZiK4lg3Tzfs?t=1628 https://youtu.be/ZiK4lg3Tzfs?t=1628

mkoubaa 3y ago

My understanding is that a particles, like photons, don't have wave shifts. That's an emergent property of many particles

alecst 3y ago
The conventional Doppler shift and the equivalence principle imply a gravitational redshift for a photon. See page 102: https://preposterousuniverse.com/wp-content/uploads/grnotes-four.pdf https://preposterousuniverse.com/wp-content/uploads/grnotes-...
stopping 3y ago
No, individual particles can indeed be redshifted. The particle's wavelength is a fundamental property.
blueprint 3y ago
Your comment is a bit strange. Light doesn't travel as photons. Photons exclusively exist at the site and instant of detection of the wave of probability of detection that light really travels as. When light is redshifted, it loses energy, therefore the wavelength becomes longer.
- tsimionescu 3y ago
  That is not the current understanding of quantum mechanics, as far as I know. Wave/particle dualism says that different experiments can either view light as a wave or a particle (never both) and that speaking about the nature of light when an experiment is not being performed is non-scientific by definition. Importantly, light very much behaves like a conventional wave in many real experiments - the interferometer experiment being one of the oldest and most well known. It is not a probability wave in that case, but an actual physical wave (now known to be an oscillation in the electro magnetic field, but long assumed to be a mechanical wave in the luminiferous aether).
  - _0w8t 3y ago
    Experimentally one never observes waves. Light is detected based on its interaction with electrons and that is always by an electron absorbing a quanta of energy, not via some continuous process as would be the case with waves. Classically one can imagine that as if electron was hit by a particle. But then we have light diffraction and interference, which classically is described as a wave. So from a classical point of view light travels as a wave but interact as a particle. As of nature of the light, then consider that there is a reformulation of a classical electrodynamics that eliminates electromagnetic waves all together. There are only electrons that interacts with each other directly with no waves in between. Feynman spent quite some time trying to develop quantum electrodynamic based on that. He failed. Still the point stands that we never observe light directly but only through its effects on electrons and other charged particles. So it could be that what we call light is a theoretical artifact and there is no light in reality.

pdonis 3y ago

> Won't a photon climbing out of a huge gravity well have a huge redshift It depends; the key factor is not how "huge" the gravity well is (in terms of how massive the object is), but how close to the black hole horizon the light is emitted. The vast majority of the light JWST is seeing from black holes is not from very close to the horizon. It's from the accretion disk, which is much further from the horizon and so the gravitational redshift is much smaller.

ajross 3y ago

> Won't a photon climbing out of a huge gravity well have a huge redshift, thus confounding estimates of distance from us and estimated age? Of course it will. It will also have a redshift related to the expansion of the space it's been travelling through. Both of those corrections are absolutely part of the model. It's not nearly as simple as "Astronomers forgot about General Relativity!". But I guess it's true (to be clear: I'm just an amateur in this field) that the ΛCDM model for cosmological evolution that we've all been looking at for the past decade or two isn't holding up well at all. It looked like it was pretty much there and just needed some fine tuning. Then we got a bunch of new data and everything's a mess. That's kind of exciting all by itself, though it's also leading to a bunch of nattering from the existing iconoclasts (MOND nuts in particular) whose theories are also not working very well to explain JWST observations. New insight needed, basically. We're all watching for updates.

xigency 3y ago
> Both of those corrections are absolutely part of the model Is there a way to gain a better understanding of how these parameters are modeled and what the scientific evidence is for the various phenomena in astrophysics? It's somewhat perplexing to me as an outsider of the field to understand how things like mass and distance of stars and planetary bodies are determined when 1) the scales are so outside of conventional experience 2) observation is limited to 2D imaging of the night sky 3) the observations in general are not consistent with our knowledge of gravity and relativity without adding hidden parameters.
- antognini 3y ago
  I know Terence Tao (yes, that Terence Tao) is working on a book about the cosmic distance ladder, but sadly it's not out yet. (I guess he probably has other projects he's working on.) But he does have some slides from talks he's given: https://terrytao.files.wordpress.com/2020/10/cosmic-distance-ladder.pdf https://terrytao.files.wordpress.com/2020/10/cosmic-distance...

antognini 3y ago

The photons that are getting observed are emitted from the accretion disk, which is not very close to the event horizon. You can estimate how close to the event horizon the disk is based on how broad the spectral lines are. The part of the disk that is coming towards you will be blueshifted and the part of the disk that is rotating away will be redshifted. From that (which is independent of the overall depth of the potential well) you can figure out how fast the disk is rotating. And from that you can figure out how far away the disk is from the black hole.

quakeguy 3y ago
Well, this is awesome.
- Smoosh 3y ago
  Sometimes I despair for humanity [see advertising/social media/politics], but then I see something like this and think, we're doing pretty well for self-taught primates.
  - dylan604 3y ago
    and then we see all of the belief systems to mock, punish, or worse people that are trying to expand human knowledge. looking back at history, you have to wonder where we might be now if there wasn't this attempts at mass eradication of learning. there are times i wonder if we're heading in that direction again. this time, rather than burning the Library of Alexandria or locking everything in catacombs, if we're not just trying to get there on a low-n-slow approach.
    - hattmall 3y ago
      I mean it really depends on your personal experience to how your perspective or "where we are now" feels. It's not like everyone is even remotely concerned with or benefitting from this "advancement." A lot of the so called progress has done nothing but destroyed and distort an enjoyable and peaceful existence.

codethief 3y ago

> Won't a photon climbing out of a huge gravity well have a huge redshift Yes. Though see the sibling comments on why the (remaining) gravity well the emitted photon has to climb out of is not particularly huge here. > thus confounding estimates of distance from us and estimated age? Generally speaking: No, because people are not stupid. :) See the integrated Sachs-Wolfe effect for a very similar situation (CMB photons traveling through gravitational wells) -> https://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect https://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect