4 ms·

Tangential comment, but it’s crazy to think about how, when we look up at the stars in the sky, we’re seeing light in wildly varying degrees of age. For exampl

Tangential comment, but it’s crazy to think about how, when we look up at the stars in the sky, we’re seeing light in wildly varying degrees of age. For example, when we look at the sun, that’s 8-minutes-old light. When we look at Polaris (the North Star), that light is 447 years old. When we look at Andromeda? Yeah, that light is 2.5 million years old.

HardCodedBias 1y ago
Nit: I think that the light from the sun is about 100k years old. Wild.
- aplummer 1y ago
  How can that make sense, the photons are emitted and fly straight at us
  - siavosh 1y ago
    The photons were created a long time ago in the core. It takes thousand of years for it to reach the surface, and THEN it takes 8 minutes to get to us.
    - greenbit 1y ago
      The photons created in the core are some seriously energetic gama rays. Sure, gama rays are very penetrating, but the solar core is dense, and it's about half a million miles to the surface, so these mostly get absorbed right there in the core, making stupendous amounts of heat. At any given depth that means that matter is going to re-emit photons, but never any more energetic than the original ones that are absorbed, but that radiation will be reabsorbed as well. That process of emission and reabsorption means that energy travels to the surface a lot slower than light in a vacuum, and sure, it takes a long time for that energy to reach the surface, but the photons that reach the earth are only the ones created close enough to the 'surface' to escape into space.
  - stevenwoo 1y ago
    Photons are not created on the surface but in the core where the environment has the higher pressure needed for the physical creation of the photon and the photon takes about that long to work its way out.
    - Ekaros 1y ago
      What is the ratio between those and well heat due to nuclear reactions and well pressure. Hot stuff generates visible photons. Say like incandescent light bulb. So there must be a range in age. As some closer to still hot surface don't need to travel through parts of the sun.
      - stevenwoo 1y ago
        So I am not an expert and recalling what I have read in several books and articles, but the conditions of fusion necessary to create photons only exists in the core of the sun. It was a mystery to us and we did not know until scientists were able to use quantum mechanics was able to explain the mechanism, it requires enough temperature and gravitational pressure to force subatomic particles close enough to overcome the forces that ordinarily keep them apart, and this only happens to a small percentage of meeting nuclei with quantum tunneling explaining how they overcome the forces that want to keep the nuclei apart - there's just so many particles squeezed close together that a small percentage that meet (possibly easier to visualize as the quantum wave function describing the position) fuse. This is also why we cannot use this method of fusion on earth - it's impossible to do on earth barring some sci fi artificial gravity invention. If this were not true and fusion could take place anywhere on the sun, the sun could rapidly use up all of the fuel of hydrogen. I am simply repeating what I have read - each photon has to make its way to the surface of the sun after many collisions, since the direction is random and not always outwards, it is theorized they just ping pong back and forth for x years where x can be hundreds of thousands of years. Here's the clearest explanation I found though they only use one slide on the photon's drunken walk https://svs.gsfc.nasa.gov/11084 https://svs.gsfc.nasa.gov/11084
    - Rover222 1y ago
      Is this in any sense hydrogen being converted to photons? Photons are massless, but… the mass of the elements in the star are converted to pure energy?
- qnleigh 1y ago
  Nit of a nit; the energy might take that long, but the photons that reach us on Earth are not directly created by the nuclear fusion reactions in the sun's core. Fusion creates high-velocity nucliei and other particles, but not visible light. The resulting heat creates photons which are rapidly destroyed by absorption. Only photon emission from the outer most layers of the sun reach Earth.
  - greenbit 1y ago
    I.e., that bit they refer to as the photosphere, effectively the radiating 'surface' of the sun, is the source of the solar photons that strike us here. That trip takes about 8 minutes.
  - HardCodedBias 1y ago
    You are totally right! I knew HN would nit my nit, well done!
thisoneisreal 1y ago
That thought randomly hits me all the time when I'm taking out the trash or whatever and just happen to look up. That and the fact that the Bootes Void and Phoenix A* exist out there.
SoftTalker 1y ago
Light doesn’t age. From its perspective it hit your retina the moment it left the star.
- jb1991 1y ago
  You are saying, from the perspective of light, whether it travels 1 mile or a trillion miles, that journey takes the same amount of time?
  - sh-run 1y ago
    I'm not a physicist, but I believe that's the exact insight that led to special relativity. It goes something like: If your moving at 1,000kmh next to a jet moving at 1,100kmh then the jet is moving at 100kmh relative to you. Eventually people realized those wasn't the case with light. No matter how fast the observer is, light still moves at 299,792,458m/s. Einstein figured out that if the speed of light is fixed despite relative motion, then time must slow down as you move faster. So from the perspective of a photon no time has passed since its departure.
  - seabass-labrax 1y ago
    I'd really highly recommend the Uncle Albert series of novels by Russell Stannard: https://booksforkeeps.co.uk/article/visiting-uncle-albert/ https://booksforkeeps.co.uk/article/visiting-uncle-albert/ The intuition you can develop about special and general relativity from these books is pretty amazing!
  - oneshtein 1y ago
    Yep, this is what he saying, but this is not what photon does. Photon must perform different amount of wave cycles to reach 1 meter or 1 trillion metters. These cycles can be counted.
    - cryptonector 1y ago
      > These cycles can be counted. In a lab setting, yes, but across such distances, no. Photons don't have a cycle counter on them, so they don't keep a cycle count and can't reveal that cycle count. All we can do is measure frequency/wavelength (spectrum, really, since we're going to see lots of photons, not really onesie/twosies) and intensity, and we can use the astrophysical distance ladder to figure out roughly where the emitter must have been.

HardCodedBias 1y ago

Nit: I think that the light from the sun is about 100k years old. Wild.

aplummer 1y ago
How can that make sense, the photons are emitted and fly straight at us
- siavosh 1y ago
  The photons were created a long time ago in the core. It takes thousand of years for it to reach the surface, and THEN it takes 8 minutes to get to us.
  - greenbit 1y ago
    The photons created in the core are some seriously energetic gama rays. Sure, gama rays are very penetrating, but the solar core is dense, and it's about half a million miles to the surface, so these mostly get absorbed right there in the core, making stupendous amounts of heat. At any given depth that means that matter is going to re-emit photons, but never any more energetic than the original ones that are absorbed, but that radiation will be reabsorbed as well. That process of emission and reabsorption means that energy travels to the surface a lot slower than light in a vacuum, and sure, it takes a long time for that energy to reach the surface, but the photons that reach the earth are only the ones created close enough to the 'surface' to escape into space.
- stevenwoo 1y ago
  Photons are not created on the surface but in the core where the environment has the higher pressure needed for the physical creation of the photon and the photon takes about that long to work its way out.
  - Ekaros 1y ago
    What is the ratio between those and well heat due to nuclear reactions and well pressure. Hot stuff generates visible photons. Say like incandescent light bulb. So there must be a range in age. As some closer to still hot surface don't need to travel through parts of the sun.
    - stevenwoo 1y ago
      So I am not an expert and recalling what I have read in several books and articles, but the conditions of fusion necessary to create photons only exists in the core of the sun. It was a mystery to us and we did not know until scientists were able to use quantum mechanics was able to explain the mechanism, it requires enough temperature and gravitational pressure to force subatomic particles close enough to overcome the forces that ordinarily keep them apart, and this only happens to a small percentage of meeting nuclei with quantum tunneling explaining how they overcome the forces that want to keep the nuclei apart - there's just so many particles squeezed close together that a small percentage that meet (possibly easier to visualize as the quantum wave function describing the position) fuse. This is also why we cannot use this method of fusion on earth - it's impossible to do on earth barring some sci fi artificial gravity invention. If this were not true and fusion could take place anywhere on the sun, the sun could rapidly use up all of the fuel of hydrogen. I am simply repeating what I have read - each photon has to make its way to the surface of the sun after many collisions, since the direction is random and not always outwards, it is theorized they just ping pong back and forth for x years where x can be hundreds of thousands of years. Here's the clearest explanation I found though they only use one slide on the photon's drunken walk https://svs.gsfc.nasa.gov/11084 https://svs.gsfc.nasa.gov/11084
  - Rover222 1y ago
    Is this in any sense hydrogen being converted to photons? Photons are massless, but… the mass of the elements in the star are converted to pure energy?
qnleigh 1y ago
Nit of a nit; the energy might take that long, but the photons that reach us on Earth are not directly created by the nuclear fusion reactions in the sun's core. Fusion creates high-velocity nucliei and other particles, but not visible light. The resulting heat creates photons which are rapidly destroyed by absorption. Only photon emission from the outer most layers of the sun reach Earth.
- greenbit 1y ago
  I.e., that bit they refer to as the photosphere, effectively the radiating 'surface' of the sun, is the source of the solar photons that strike us here. That trip takes about 8 minutes.
- HardCodedBias 1y ago
  You are totally right! I knew HN would nit my nit, well done!

thisoneisreal 1y ago

That thought randomly hits me all the time when I'm taking out the trash or whatever and just happen to look up. That and the fact that the Bootes Void and Phoenix A* exist out there.

SoftTalker 1y ago

Light doesn’t age. From its perspective it hit your retina the moment it left the star.

jb1991 1y ago
You are saying, from the perspective of light, whether it travels 1 mile or a trillion miles, that journey takes the same amount of time?
- sh-run 1y ago
  I'm not a physicist, but I believe that's the exact insight that led to special relativity. It goes something like: If your moving at 1,000kmh next to a jet moving at 1,100kmh then the jet is moving at 100kmh relative to you. Eventually people realized those wasn't the case with light. No matter how fast the observer is, light still moves at 299,792,458m/s. Einstein figured out that if the speed of light is fixed despite relative motion, then time must slow down as you move faster. So from the perspective of a photon no time has passed since its departure.
- seabass-labrax 1y ago
  I'd really highly recommend the Uncle Albert series of novels by Russell Stannard: https://booksforkeeps.co.uk/article/visiting-uncle-albert/ https://booksforkeeps.co.uk/article/visiting-uncle-albert/ The intuition you can develop about special and general relativity from these books is pretty amazing!
- oneshtein 1y ago
  Yep, this is what he saying, but this is not what photon does. Photon must perform different amount of wave cycles to reach 1 meter or 1 trillion metters. These cycles can be counted.
  - cryptonector 1y ago
    > These cycles can be counted. In a lab setting, yes, but across such distances, no. Photons don't have a cycle counter on them, so they don't keep a cycle count and can't reveal that cycle count. All we can do is measure frequency/wavelength (spectrum, really, since we're going to see lots of photons, not really onesie/twosies) and intensity, and we can use the astrophysical distance ladder to figure out roughly where the emitter must have been.