7 ms·

Does light have an infinite lifetime?

126 points by robertn702 2y ago

urbandw311er 2y ago
Anybody care to attempt a TL;DR of this?
- Leftium 2y ago
  Kagi Summary: The passage discusses whether photons, the quantum particles of light, have a finite or infinite lifetime. It explains that while photons can be created and destroyed through various interactions, they cannot truly be "killed off" entirely. Even as the universe expands and photons redshift to lower and lower energies, new photons will continue to be produced, ensuring that photons will always exist in some form. The passage also dismisses the "tired light" hypothesis as a way for photons to lose energy, as it is not supported by observational evidence. Overall, the passage concludes that based on our current understanding of physics, photons appear to have an infinite lifetime and will continue to exist in the universe indefinitely.
gwern 2y ago
Interesting. I had no idea that the universe would eventually be basically just photons. I wonder if that is why Stephen Baxter had his 'photino birds' as the final form of life in his Xeeleverse?
- mensetmanusman 2y ago
  There is some debate whether this is knowable, some particles are not believed to decay into light at a rate faster than their spontaneous creation from light. (And humanity wont be around to check).
- barfbagginus 2y ago
  The photino birds want the universe to be populated with white dwarfs, because they can feed off their gravity wells without the risk of deadly supernovas and black holes, which can kill them. After the xelee and humans leave, the universe becomes a cold place dominated by photino birds living in the cold pinpricks of white dwarfs. Eventually matter evaporates into photons, and the photino birds die. However, it turns out photino birds can always just time travel to a time when the universe still had matter. So they're more or less indifferent to the eventual heat death . See: https://xeelee.fandom.com/wiki/Photino_Birds https://xeelee.fandom.com/wiki/Photino_Birds I hypothesize that there is may be only one photino bird. When it appears to die, it is just traveling to another time. When we see multiple photino birds, we're just looking at different segments of the same bird's world line. These are my own speculations, inspired by Wheeler's idea that the universe has only one electron, which travels back in time as a positron, and interacts with itself so many times that it creates the observable universe of matter: https://en.m.wikipedia.org/wiki/One-electron_universe https://en.m.wikipedia.org/wiki/One-electron_universe
  - codyd51 2y ago
    One potential hiccup with your one-photino-bird-universe theory (which is quite fun!): I believe I remember a scene in which the ‘birth’ of a photino bird was described. If I remember correctly, it was indeed described as a clone of its parent.
    - kloop 2y ago
      As long as we only see one birth, that's probably fine. It just implies a stable time loop.
  - pavel_lishin 2y ago
    > However, Photino Birds carry with them one unassailable advantage the Xeelee could not overcome; they are not bound by time. They can freely exist in and travel to any point in time. The creatures do not age nor die naturally, not even by the end of the universe; they just travel back to the beginning of time, including to points so soon after the Big Bang that baryonic matter (including the Xeelee) could not yet exist. Where is this described in the books? I don't remember the Photino Birds having this ability.
- cwillu 2y ago
  I believe proton decay is still considered hypothetical, and the electron is pretty confidently believed to be stable; https://profmattstrassler.com/articles-and-posts/particle-physics-basics/why-do-particles-decay/most-particles-decay-yet-some-dont/ https://profmattstrassler.com/articles-and-posts/particle-ph... is a good overview.
  - ars 2y ago
    If protons do decay, their decay product would include a positron that would in turn annihilate with the electron. Leaving a universe with just photons and neutrinos. If neutrinos are their own antiparticle then they could react with each other and produce ??? Not sure, because you would have to find out of the lepton number is conserved.
  - A_D_E_P_T 2y ago
    > I believe proton decay is still considered hypothetical Definitely. > [link] is a good overview That overview seems to overstate the likelihood of proton decay. In fact, it has it backwards. The default position is that protons are stable, per the standard model, not that they're susceptible to decay.
    - jessriedel 2y ago
      > The default position is that protons are stable, per the standard model, not that they're susceptible to decay. It's only the "default" in the sense that the simplest model explaining data gathered to date (the standard model) predicts no decay. However, most physicist do not believe the standard model is the last word (and surely it cannot be when you go to the Planck scale), and many models post-SM models predict proton decay. I would guess if you surveyed high energy physicists, you'd find the majority expect the proton does in fact decay, so it's the "default" in that sense.
      - A_D_E_P_T 2y ago
        Apologies for the late reply -- I just saw your comment now. I don't think it's the case that many post-SM models predict proton decay. A few do, but even those are now tightly constrained by observations that place the proton's half life at more than 10^34 years. A lot of models that previously predicted proton decay have been empirically falsified on such grounds, and those that remain are looking pretty shaky. So I don't know. If you run that survey, I believe that the average physicist would come out against proton decay. It would be an interesting survey, in any case!!
- zem 2y ago
  a truly beautiful exploration of that idea is landis's "the melancholy of infinite space": http://www.geoffreylandis.com/infinite.htp http://www.geoffreylandis.com/infinite.htp
- prettydeep 2y ago
  Roger Penrose has a theory about a cyclical conformal universe, which requires that the universe eventually only contains photons. As photons travel at the speed of light, they do not experience the passage of time, so they also do not experience distance (space). So in the deep remote future, when only photons are left, the difference between the very large and miniscule disappears, and you get all photons in the universe existing at the same point at the same time, which causes a new big bang. At least, that is Penrose's theory. I believe they found some evidence in the CMB supporting it.
  - AlexAndScripts 2y ago
    https://en.m.wikipedia.org/wiki/Conformal_cyclic_cosmology https://en.m.wikipedia.org/wiki/Conformal_cyclic_cosmology It seems the paper made mistakes, using a non-standard model of the CMB that failed upon replication. It's an interesting theory thoughb.
    - cmsj 2y ago
      As a non-Physicist, I absolutely love the CCC hypothesis and I really hope it turns out to be true. It seems like such a beautiful alternative to the idea of the heat death of the universe just lasting forever and nothing ever happening again.
  - Kranar 2y ago
    This is some absolutely crazy idea. I mean I'm in no position to analyze it critically but that is pretty mind blowing.
colechristensen 2y ago
Huh? Light has zero lifetime, along with all massless particles. When you travel at c your clock doesn’t tick. From the viewpoint of a photon it is emitted and absorbed at the same instant. You cannot decay if you don’t experience time. I’m not sure exactly how index of refraction works with this.
- Nevermark 2y ago
  > You cannot decay if you don’t experience time. Interesting take! But if photons couldn't decay due to not experiencing time, they couldn't do anything else either. The reality is that a photons creation and destruction are not prohibited, but simply "experienced" as two events at different locations at the same time, with the photon being the "thing" that connects those events. Given that interpretation, it might be reasonable to assume that all photons have beginnings and ends, regardless of the duration we perceive between them, or they wouldn't exist. Time being no barrier at all for photons.
  - supportengineer 2y ago
    Would that seem like a fold in spacetime to the photon?
    - nick7376182 2y ago
      Probably more like a summer breeze
    - Nevermark 2y ago
      I can't top the sibling comment about a summer breeze! But it is an interesting question. Not only does the photon not experience any delay between its two end points, but it experiences its path between them as a simple shortest-distance straight line segment, even if the same path looks like a curve through gravitationally warped space-time to us. The photon does experience a form of distance, i.e. the number of wave lengths between its ends. But just the number of cycles, not the actual wave lengths which we would see varying as we experienced dark energy and space stretching the photon's wavelength from our viewpoint. So a photon "experiences" two spacially separated ends, and a number of wave cycles between them, and that's it? Perhaps.
barfbagginus 2y ago
I feel like this article could be condensed into a simple answer. I got tto annoyed looking for it to find more than the answer, "yes, if the standard model holds" So at the risk of venting unconstructively, I wish I had a way of screening physics writing that is not a physics paper. Articles like this are frustrating because they only have one or two interesting tidbits for me, but they hide them in a whole lot of highschool level hand waving. Honestly HN would be perfect if it only allowed physics papers - no pop-physics - and if it banned any blogs or news sites with paywalls or newsletter nagware (looking at you, medium, new york times). Alas. I'm wishing for something I will have to build if I really want it.
westurner 2y ago
[flagged]
cwoolfe 2y ago
"God is light." (1John 1:5)
- pc86 2y ago
  Do you have something constructive to add related to the content of the article or are we just quoting random bible verses on HN for no reason?
  - ClarityJones 2y ago
    It wasn't exactly random. The topic of the article is that light is eternal, and the commenter shared a quote from over a thousand years ago stating the same thing. So, the idea of light having an infinite lifetime is apparently not new... even if the mechanics of light are better understood these days.
    - xyzelement 2y ago
      This is in line for me with the moment of "creation." For most of the history of science as a thing, the scientific view held that the universe was infinitely old cosmos without a beginning or end. The greek model. It was only in 1900s that big bang was theorized (by a catholic priest) that science now views that there was a moment before which the universe didn't exist and after which it did. The fact that someone reading Genesis would have had a more accurate conception of the origin of the universe, prior to big bang becoming popularized very recently in the grand scheme of things is noteworthy.
      - bobbylarrybobby 2y ago
        Broken clocks...
  - cwoolfe 2y ago
    "The light shines in the darkness, and the darkness can never extinguish it. God sent a man, John the Baptist, to tell about the light so that everyone might believe because of his testimony. John himself was not the light; he was simply a witness to tell about the light. The one who is the true light, who gives light to everyone, was coming into the world. He came into the very world he created, but the world didn’t recognize him. He came to his own people, and even they rejected him. But to all who believed him and accepted him, he gave the right to become children of God."(John 1:5-12)
deleted 2y ago
[deleted]
akomtu 2y ago
Isn't it a meaningless question? Photons aren't things, but loosely localized areas of motion energy that's temporarily assumed the shape of a photon. Upon collision with other similar waves, it may change shape and become another partickey like an electron. But the light itself is a motion itself, which is a pure abstraction. At the end of the universe, photons probably will keep spreading over larger and larger areas, slowly turning into a uniform sea of that pure motion.
- x86x87 2y ago
  Yeah, this is like asking if energy has an infinite lifetime.
  - akomtu 2y ago
    My suspicion that everything is made of pure abstract motion can be backed with a thought experiment. A spaceship with a light sail uses kinetic energy of photons that hit the sail. We know that anything can be dissolved into photons, we even know the exact amount of those photons from the e=mc2 formula, and when all those photons hit the light sail, they vanish from existence and become the kinetic energy of the spaceship. And what is kinetic energy? It's not even real. And as we know from the relativity theory, even as an abstraction, kinetic energy can't be measured with an absolute number, it has to be measured with respect to some imaginary reference frame.
    - bobbylarrybobby 2y ago
      The light also doesn't have an absolute energy though; like the ship's kinetic energy, the light’s frequency (hence energy) also depends on your relative speed. None of this stuff is “real”, but boy does the bookkeeping seem to work out...
doodlebugging 2y ago
Where do the photons that hit my retina go when I finish with them? If they have infinite lifetimes then does each one carry a memory of it's creation event? When I burn a log in my fireplace and a cinder flares and pops creating a spark, does the photon exist after the spark energy flares out and the cinder is no longer illuminated enough to be detected by my eyes? If photons are infinite then harvesting light would be the first step in deciphering the complete history of the universe. All that would be left for us to do would be to derive the algorithms to unravel and categorize each photon into discrete groups based on their historical particle paths. If we wanted to harvest light to test whether photons have infinite lifetimes we would need to design a structure that forms a light trap using materials with different refractive indices so that photons entering are forced onto paths from which they perfectly reflect in a lossless manner. If photons are infinite then mirrors may have a memory if we can trap and monitor the photons that pass through them and force them to unravel their travel paths. Why can't I step in front of the mirror and have it replay every event that the mirror has seen? It would be a better replacement for photo or video mementos of lost loved ones if we could simply take the mirrors from their homes and spin back to watch their happiest moments forever by reconstructing the photon impingement history of the glass and mirror substrate.
- adonovan 2y ago
  Feynmann is very clear that the photons bouncing off a mirror are not the same ones that hit it. They are absorbed by electric oscillators (free electrons in the material) and destroyed; then new ones are created and emitted. He's also careful enough to point out that electrons don't really have an identity that would allow you to meaningfully define "not the same". :)
  - dylan604 2y ago
    Electrons? We were just told that there is only one single electron in yesterday's posts. It's almost like they don't know what an electron is
    - doodlebugging 2y ago
holoduke 2y ago
Since there is no time from the perspective of a photon, there is only one photon everywhere at anytime. Our perception of multiple photons is incorrect. Its just multiple timelines with the same photon.
henry2023 2y ago
From the perspective of a photon, there is no such thing as time. It's emitted, and might exist for hundreds of trillions of years, but for the photon, there's zero time elapsed between when it's emitted and when it's absorbed again. It doesn't experience distance either.
- dataflow 2y ago
  Something about this doesn't make sense. This means that, from its own perspective, every photon is emitted at time T at location x, then absorbed at time T at location y, meaning there is no time "in between" those... yet somehow, at a distinct point in between those events, an object can get in the way and block the photon from reaching point y. Meaning you have two points in time that have the same value but aren't identical? How does that work, mathematically? Do you need nonstandard calculus or something to make sense of it?
  - favflam 2y ago
    Think of time as defined by how many times a photon bounces between two mirrors. If the two mirrors move at the speed of light with the photon, the photon will never reach the mirror to bounce. Therefore, the photon experiences no time. There is no universal time T in space time. Time is relative to the observer. I suggest reading https://en.wikipedia.org/wiki/Minkowski_space https://en.wikipedia.org/wiki/Minkowski_space for the mathematics.
  - cmsj 2y ago
    I am not a Physicist, so please don't rely on this, but my understanding is that this comes down to the frames of reference in special relativity. If you're in a car on the highway, an observer standing still outside the car sees you moving at 50mph, but an observer inside the car with you, sees you not moving at all, but the world rushing past at 50mph. These are two different reference frames, and the second one is called a rest frame - any object can be considered to have a rest frame of reference where it isn't moving at all. Any object that is, which has mass. Massless objects are always observed to be moving at the speed of light by any observer in any frame of reference. This is one of the most mind-bending parts of relativity, but it ultimately connects to the part of special relativity where speed affects time - the faster you go, the slower your clock runs compared to things moving slower than you, and the closer you get to the speed of light, the closer your clock gets to zero. Massless particles only travel at the speed of light, and thus have no clock, therefore they experience no time. I hope I got that right, but even if I did, I still don't feel like I actually understand it at all!
westurner 2y ago
Here, a second time, I point to this measured exchange of gravitational waves for photonic waves. If photons come from gravity, can photons end as gravity; and then do they have an infinite lifetime? https://news.ycombinator.com/item?id=40090332 https://news.ycombinator.com/item?id=40090332
Borrible 2y ago
Infinite in both directions of time?

126 points by robertn702 2y ago

urbandw311er 2y ago

Anybody care to attempt a TL;DR of this?

Leftium 2y ago
Kagi Summary: The passage discusses whether photons, the quantum particles of light, have a finite or infinite lifetime. It explains that while photons can be created and destroyed through various interactions, they cannot truly be "killed off" entirely. Even as the universe expands and photons redshift to lower and lower energies, new photons will continue to be produced, ensuring that photons will always exist in some form. The passage also dismisses the "tired light" hypothesis as a way for photons to lose energy, as it is not supported by observational evidence. Overall, the passage concludes that based on our current understanding of physics, photons appear to have an infinite lifetime and will continue to exist in the universe indefinitely.

gwern 2y ago

Interesting. I had no idea that the universe would eventually be basically just photons. I wonder if that is why Stephen Baxter had his 'photino birds' as the final form of life in his Xeeleverse?

mensetmanusman 2y ago
There is some debate whether this is knowable, some particles are not believed to decay into light at a rate faster than their spontaneous creation from light. (And humanity wont be around to check).
barfbagginus 2y ago
The photino birds want the universe to be populated with white dwarfs, because they can feed off their gravity wells without the risk of deadly supernovas and black holes, which can kill them. After the xelee and humans leave, the universe becomes a cold place dominated by photino birds living in the cold pinpricks of white dwarfs. Eventually matter evaporates into photons, and the photino birds die. However, it turns out photino birds can always just time travel to a time when the universe still had matter. So they're more or less indifferent to the eventual heat death . See: https://xeelee.fandom.com/wiki/Photino_Birds https://xeelee.fandom.com/wiki/Photino_Birds I hypothesize that there is may be only one photino bird. When it appears to die, it is just traveling to another time. When we see multiple photino birds, we're just looking at different segments of the same bird's world line. These are my own speculations, inspired by Wheeler's idea that the universe has only one electron, which travels back in time as a positron, and interacts with itself so many times that it creates the observable universe of matter: https://en.m.wikipedia.org/wiki/One-electron_universe https://en.m.wikipedia.org/wiki/One-electron_universe
- codyd51 2y ago
  One potential hiccup with your one-photino-bird-universe theory (which is quite fun!): I believe I remember a scene in which the ‘birth’ of a photino bird was described. If I remember correctly, it was indeed described as a clone of its parent.
  - kloop 2y ago
    As long as we only see one birth, that's probably fine. It just implies a stable time loop.
- pavel_lishin 2y ago
  > However, Photino Birds carry with them one unassailable advantage the Xeelee could not overcome; they are not bound by time. They can freely exist in and travel to any point in time. The creatures do not age nor die naturally, not even by the end of the universe; they just travel back to the beginning of time, including to points so soon after the Big Bang that baryonic matter (including the Xeelee) could not yet exist. Where is this described in the books? I don't remember the Photino Birds having this ability.
cwillu 2y ago
I believe proton decay is still considered hypothetical, and the electron is pretty confidently believed to be stable; https://profmattstrassler.com/articles-and-posts/particle-physics-basics/why-do-particles-decay/most-particles-decay-yet-some-dont/ https://profmattstrassler.com/articles-and-posts/particle-ph... is a good overview.
- ars 2y ago
  If protons do decay, their decay product would include a positron that would in turn annihilate with the electron. Leaving a universe with just photons and neutrinos. If neutrinos are their own antiparticle then they could react with each other and produce ??? Not sure, because you would have to find out of the lepton number is conserved.
- A_D_E_P_T 2y ago
  > I believe proton decay is still considered hypothetical Definitely. > [link] is a good overview That overview seems to overstate the likelihood of proton decay. In fact, it has it backwards. The default position is that protons are stable, per the standard model, not that they're susceptible to decay.
  - jessriedel 2y ago
    > The default position is that protons are stable, per the standard model, not that they're susceptible to decay. It's only the "default" in the sense that the simplest model explaining data gathered to date (the standard model) predicts no decay. However, most physicist do not believe the standard model is the last word (and surely it cannot be when you go to the Planck scale), and many models post-SM models predict proton decay. I would guess if you surveyed high energy physicists, you'd find the majority expect the proton does in fact decay, so it's the "default" in that sense.
    - A_D_E_P_T 2y ago
      Apologies for the late reply -- I just saw your comment now. I don't think it's the case that many post-SM models predict proton decay. A few do, but even those are now tightly constrained by observations that place the proton's half life at more than 10^34 years. A lot of models that previously predicted proton decay have been empirically falsified on such grounds, and those that remain are looking pretty shaky. So I don't know. If you run that survey, I believe that the average physicist would come out against proton decay. It would be an interesting survey, in any case!!
zem 2y ago
a truly beautiful exploration of that idea is landis's "the melancholy of infinite space": http://www.geoffreylandis.com/infinite.htp http://www.geoffreylandis.com/infinite.htp
prettydeep 2y ago
Roger Penrose has a theory about a cyclical conformal universe, which requires that the universe eventually only contains photons. As photons travel at the speed of light, they do not experience the passage of time, so they also do not experience distance (space). So in the deep remote future, when only photons are left, the difference between the very large and miniscule disappears, and you get all photons in the universe existing at the same point at the same time, which causes a new big bang. At least, that is Penrose's theory. I believe they found some evidence in the CMB supporting it.
- AlexAndScripts 2y ago
  https://en.m.wikipedia.org/wiki/Conformal_cyclic_cosmology https://en.m.wikipedia.org/wiki/Conformal_cyclic_cosmology It seems the paper made mistakes, using a non-standard model of the CMB that failed upon replication. It's an interesting theory thoughb.
  - cmsj 2y ago
    As a non-Physicist, I absolutely love the CCC hypothesis and I really hope it turns out to be true. It seems like such a beautiful alternative to the idea of the heat death of the universe just lasting forever and nothing ever happening again.
- Kranar 2y ago
  This is some absolutely crazy idea. I mean I'm in no position to analyze it critically but that is pretty mind blowing.

colechristensen 2y ago

Huh? Light has zero lifetime, along with all massless particles. When you travel at c your clock doesn’t tick. From the viewpoint of a photon it is emitted and absorbed at the same instant. You cannot decay if you don’t experience time. I’m not sure exactly how index of refraction works with this.

Nevermark 2y ago
> You cannot decay if you don’t experience time. Interesting take! But if photons couldn't decay due to not experiencing time, they couldn't do anything else either. The reality is that a photons creation and destruction are not prohibited, but simply "experienced" as two events at different locations at the same time, with the photon being the "thing" that connects those events. Given that interpretation, it might be reasonable to assume that all photons have beginnings and ends, regardless of the duration we perceive between them, or they wouldn't exist. Time being no barrier at all for photons.
- supportengineer 2y ago
  Would that seem like a fold in spacetime to the photon?
  - nick7376182 2y ago
    Probably more like a summer breeze
  - Nevermark 2y ago
    I can't top the sibling comment about a summer breeze! But it is an interesting question. Not only does the photon not experience any delay between its two end points, but it experiences its path between them as a simple shortest-distance straight line segment, even if the same path looks like a curve through gravitationally warped space-time to us. The photon does experience a form of distance, i.e. the number of wave lengths between its ends. But just the number of cycles, not the actual wave lengths which we would see varying as we experienced dark energy and space stretching the photon's wavelength from our viewpoint. So a photon "experiences" two spacially separated ends, and a number of wave cycles between them, and that's it? Perhaps.

barfbagginus 2y ago

I feel like this article could be condensed into a simple answer. I got tto annoyed looking for it to find more than the answer, "yes, if the standard model holds" So at the risk of venting unconstructively, I wish I had a way of screening physics writing that is not a physics paper. Articles like this are frustrating because they only have one or two interesting tidbits for me, but they hide them in a whole lot of highschool level hand waving. Honestly HN would be perfect if it only allowed physics papers - no pop-physics - and if it banned any blogs or news sites with paywalls or newsletter nagware (looking at you, medium, new york times). Alas. I'm wishing for something I will have to build if I really want it.

westurner 2y ago

[flagged]

cwoolfe 2y ago

"God is light." (1John 1:5)

pc86 2y ago
Do you have something constructive to add related to the content of the article or are we just quoting random bible verses on HN for no reason?
- ClarityJones 2y ago
  It wasn't exactly random. The topic of the article is that light is eternal, and the commenter shared a quote from over a thousand years ago stating the same thing. So, the idea of light having an infinite lifetime is apparently not new... even if the mechanics of light are better understood these days.
  - xyzelement 2y ago
    This is in line for me with the moment of "creation." For most of the history of science as a thing, the scientific view held that the universe was infinitely old cosmos without a beginning or end. The greek model. It was only in 1900s that big bang was theorized (by a catholic priest) that science now views that there was a moment before which the universe didn't exist and after which it did. The fact that someone reading Genesis would have had a more accurate conception of the origin of the universe, prior to big bang becoming popularized very recently in the grand scheme of things is noteworthy.
    - bobbylarrybobby 2y ago
      Broken clocks...
- cwoolfe 2y ago
  "The light shines in the darkness, and the darkness can never extinguish it. God sent a man, John the Baptist, to tell about the light so that everyone might believe because of his testimony. John himself was not the light; he was simply a witness to tell about the light. The one who is the true light, who gives light to everyone, was coming into the world. He came into the very world he created, but the world didn’t recognize him. He came to his own people, and even they rejected him. But to all who believed him and accepted him, he gave the right to become children of God."(John 1:5-12)

deleted 2y ago

[deleted]

akomtu 2y ago

Isn't it a meaningless question? Photons aren't things, but loosely localized areas of motion energy that's temporarily assumed the shape of a photon. Upon collision with other similar waves, it may change shape and become another partickey like an electron. But the light itself is a motion itself, which is a pure abstraction. At the end of the universe, photons probably will keep spreading over larger and larger areas, slowly turning into a uniform sea of that pure motion.

x86x87 2y ago
Yeah, this is like asking if energy has an infinite lifetime.
- akomtu 2y ago
  My suspicion that everything is made of pure abstract motion can be backed with a thought experiment. A spaceship with a light sail uses kinetic energy of photons that hit the sail. We know that anything can be dissolved into photons, we even know the exact amount of those photons from the e=mc2 formula, and when all those photons hit the light sail, they vanish from existence and become the kinetic energy of the spaceship. And what is kinetic energy? It's not even real. And as we know from the relativity theory, even as an abstraction, kinetic energy can't be measured with an absolute number, it has to be measured with respect to some imaginary reference frame.
  - bobbylarrybobby 2y ago
    The light also doesn't have an absolute energy though; like the ship's kinetic energy, the light’s frequency (hence energy) also depends on your relative speed. None of this stuff is “real”, but boy does the bookkeeping seem to work out...

doodlebugging 2y ago

Where do the photons that hit my retina go when I finish with them? If they have infinite lifetimes then does each one carry a memory of it's creation event? When I burn a log in my fireplace and a cinder flares and pops creating a spark, does the photon exist after the spark energy flares out and the cinder is no longer illuminated enough to be detected by my eyes? If photons are infinite then harvesting light would be the first step in deciphering the complete history of the universe. All that would be left for us to do would be to derive the algorithms to unravel and categorize each photon into discrete groups based on their historical particle paths. If we wanted to harvest light to test whether photons have infinite lifetimes we would need to design a structure that forms a light trap using materials with different refractive indices so that photons entering are forced onto paths from which they perfectly reflect in a lossless manner. If photons are infinite then mirrors may have a memory if we can trap and monitor the photons that pass through them and force them to unravel their travel paths. Why can't I step in front of the mirror and have it replay every event that the mirror has seen? It would be a better replacement for photo or video mementos of lost loved ones if we could simply take the mirrors from their homes and spin back to watch their happiest moments forever by reconstructing the photon impingement history of the glass and mirror substrate.

adonovan 2y ago
Feynmann is very clear that the photons bouncing off a mirror are not the same ones that hit it. They are absorbed by electric oscillators (free electrons in the material) and destroyed; then new ones are created and emitted. He's also careful enough to point out that electrons don't really have an identity that would allow you to meaningfully define "not the same". :)
- dylan604 2y ago
  Electrons? We were just told that there is only one single electron in yesterday's posts. It's almost like they don't know what an electron is
  - doodlebugging 2y ago

holoduke 2y ago

Since there is no time from the perspective of a photon, there is only one photon everywhere at anytime. Our perception of multiple photons is incorrect. Its just multiple timelines with the same photon.

henry2023 2y ago

From the perspective of a photon, there is no such thing as time. It's emitted, and might exist for hundreds of trillions of years, but for the photon, there's zero time elapsed between when it's emitted and when it's absorbed again. It doesn't experience distance either.

dataflow 2y ago
Something about this doesn't make sense. This means that, from its own perspective, every photon is emitted at time T at location x, then absorbed at time T at location y, meaning there is no time "in between" those... yet somehow, at a distinct point in between those events, an object can get in the way and block the photon from reaching point y. Meaning you have two points in time that have the same value but aren't identical? How does that work, mathematically? Do you need nonstandard calculus or something to make sense of it?
- favflam 2y ago
  Think of time as defined by how many times a photon bounces between two mirrors. If the two mirrors move at the speed of light with the photon, the photon will never reach the mirror to bounce. Therefore, the photon experiences no time. There is no universal time T in space time. Time is relative to the observer. I suggest reading https://en.wikipedia.org/wiki/Minkowski_space https://en.wikipedia.org/wiki/Minkowski_space for the mathematics.
- cmsj 2y ago
  I am not a Physicist, so please don't rely on this, but my understanding is that this comes down to the frames of reference in special relativity. If you're in a car on the highway, an observer standing still outside the car sees you moving at 50mph, but an observer inside the car with you, sees you not moving at all, but the world rushing past at 50mph. These are two different reference frames, and the second one is called a rest frame - any object can be considered to have a rest frame of reference where it isn't moving at all. Any object that is, which has mass. Massless objects are always observed to be moving at the speed of light by any observer in any frame of reference. This is one of the most mind-bending parts of relativity, but it ultimately connects to the part of special relativity where speed affects time - the faster you go, the slower your clock runs compared to things moving slower than you, and the closer you get to the speed of light, the closer your clock gets to zero. Massless particles only travel at the speed of light, and thus have no clock, therefore they experience no time. I hope I got that right, but even if I did, I still don't feel like I actually understand it at all!

westurner 2y ago

Here, a second time, I point to this measured exchange of gravitational waves for photonic waves. If photons come from gravity, can photons end as gravity; and then do they have an infinite lifetime? https://news.ycombinator.com/item?id=40090332 https://news.ycombinator.com/item?id=40090332

Borrible 2y ago

Infinite in both directions of time?