4 ms·

> photons are massless, but they can undergo interactions Not by themselves. They have to bump into other particles to interact. Photons in motion do not exper

by 6nf 5y ago

> photons are massless, but they can undergo interactions Not by themselves. They have to bump into other particles to interact. Photons in motion do not experience time and can not 'change flavour' or whatever without bumping into something else. Photon decay is not an observed phenomenon and can only happen if a photon has a non zero mass.

pdonis 5y ago
> Not by themselves. They have to bump into other particles to interact. Not necessarily. There is a very small, but nonzero, probability for photon scattering (due, heuristically, to the small but nonzero probability for photons to become virtual electron-positron pairs). The probability is small, but it is nonzero, and that is sufficient to invalidate your claim. > Photons in motion do not experience time This is not correct. A correct statement would be that the concept of "proper time" does not apply to massless objects (objects that move on null worldlines) at all (it's mathematically ill-defined). But that does not mean there are not distinct events on a photon's worldline and that things cannot happen at those events even if the photon is moving by itself.
- 6nf 5y ago
  Can you link me to a source stating that photons moving at C does or does not experience time? I'm not a layman, give it to me straight. I'm happy to work through the math or whatever is needed. I'm asking because I'm very sure that you are wrong here and I would like to be convinced otherwise if possible. The Minkowski metric ensures that photons moving at C can not experience local time.
  - pdonis 5y ago
    > I'm very sure that you are wrong here What actual textbooks on relativity have you studied? If you haven't studied any, I would strongly suggest doing so before being this confident about your beliefs about relativity. Many pop science sources (including, unfortunately, books by physicists who should know better but who can't help themselves when there are no other experts peer reviewing their work) will say the kinds of things you're saying about photons "not experiencing time", but you won't find a textbook or peer-reviewed paper that says them, because such misstatements are weeded out. Sean Carroll has a set of free online lecture notes [1] that make a good start (they're more focused on GR than SR but they have an introductory section that covers SR). Taylor & Wheeler's Spacetime Physics [2] is a good introductory textbook as well. The fundamental physical point here is that timelike objects (objects with positive rest mass that follow timelike worldlines) and lightlike objects (objects with zero rest mass that follow null worldlines) are different things, and the concept of "experienced time" or "proper time" (the latter is the correct technical term) only applies to timelike objects. The mathematical basis for this is that timelike worldlines can be parameterized by arc length, and arc length along a timelike worldline corresponds to elapsed time on a clock following that worldline. Null worldlines, however, cannot be parameterized by arc length at all, so the fact that arc length along them is zero does not mean lightlike objects "experience no time", it means that the whole concept of "experienced time" doesn't even work for them: it's mathematically invalid since it requires parameterizing the worldline by arc length. Another way of seeing the fundamental difference is to look at how Lorentz transformations act on timelike and null vectors. Lorentz transformations hyperbolically rotate timelike vectors: that means the transformation changes which way in spacetime the vector points, without changing its length. But Lorentz transformations do not rotate null vectors: they dilate them, meaning they increase or decrease all components of the vector by the same factor, without changing its direction in spacetime. This means that a null vector is not a "limiting case" of any set of timelike vectors as far as the Lorentz transformations are concerned; null and timelike vectors are simply two disconnected sets of vectors with respect to Lorentz transformations. Which in turn means that the common pop science image of objects "experiencing less time" as they move faster and faster, until photons moving at the speed of light "experience no time" in the limit, is not correct: it is not a valid description of what is actually happening in the math. Lorentz transformations don't change the length of timelike vectors at all, so they don't change the "experienced time" along them. The apparent "time dilation" of an object that is moving relative to you is due to the angle in spacetime between your worldline and the object's worldline, not to any property of the object's worldline itself. But the "angle" here is a hyperbolic angle, and the hyperbolic angle between your worldline (you being a timelike object) and any null worldline (i.e., any worldline of a light ray or photon) is infinite--in other words, mathematically ill-defined. In short, pop science authors who make claims like "photons don't experience time" are putting an interpretation on the fact that a photon's worldline has zero length in the Minkowski metric that is not justified by anything in the actual math. They do it, unfortunately, because they believe (quite possibly correctly) that saying things like that, even if they're wrong, will sell more books than trying to teach their readers the actual science. [1] https://arxiv.org/abs/gr-qc/9712019 https://arxiv.org/abs/gr-qc/9712019 [2] https://www.eftaylor.com/spacetimephysics/ https://www.eftaylor.com/spacetimephysics/
    - 6nf 5y ago
      You're picking nits, I'm saying 'objects moving at C do not experience local time' and you're saying the same thing in more formal / fancy language without explaining where exactly my statement differs from yours.
      - pdonis 5y ago
        > 'm saying 'objects moving at C do not experience local time' and you're saying the same thing No, I'm not. See below. > without explaining where exactly my statement differs from yours You evidently failed to grasp the point of my statement that the concept of "experienced time", or "proper time", is not even well-defined for lightlike worldlines. Your statement was that photons "do not experience time". And you drew from that the implication that photons cannot undergo any kind of change while propagating freely. That implication is only valid if "do not experience time" means that the concept of "experienced time" is well defined for photons, and the time that they experience is zero. However, as I explained, the concept of "experienced time" is not well defined for photons. That means you cannot draw any implications either way about whether or not photons can "change" as they propagate freely, based on the fact that the Minkowski length of their worldlines is zero. There simply is no logical implication about "change" for photons from the Minkowski length of their worldlines. To draw any conclusions about whether or not a photon can "change" as it propagates, you have to look at other things.
        6nf 5y ago
        > the concept of "experienced time" is not well defined for photons. how is that different from 'photons don't have experienced time'
        pdonis 5y ago
        I explained the issue in the last two paragraphs of my post (the GP to this one). You drew an implication from "photons don't experience time" that is not valid. I explained why it's not valid.

by 6nf 5y ago

pdonis 5y ago

> Not by themselves. They have to bump into other particles to interact. Not necessarily. There is a very small, but nonzero, probability for photon scattering (due, heuristically, to the small but nonzero probability for photons to become virtual electron-positron pairs). The probability is small, but it is nonzero, and that is sufficient to invalidate your claim. > Photons in motion do not experience time This is not correct. A correct statement would be that the concept of "proper time" does not apply to massless objects (objects that move on null worldlines) at all (it's mathematically ill-defined). But that does not mean there are not distinct events on a photon's worldline and that things cannot happen at those events even if the photon is moving by itself.

6nf 5y ago
Can you link me to a source stating that photons moving at C does or does not experience time? I'm not a layman, give it to me straight. I'm happy to work through the math or whatever is needed. I'm asking because I'm very sure that you are wrong here and I would like to be convinced otherwise if possible. The Minkowski metric ensures that photons moving at C can not experience local time.
- pdonis 5y ago
  > I'm very sure that you are wrong here What actual textbooks on relativity have you studied? If you haven't studied any, I would strongly suggest doing so before being this confident about your beliefs about relativity. Many pop science sources (including, unfortunately, books by physicists who should know better but who can't help themselves when there are no other experts peer reviewing their work) will say the kinds of things you're saying about photons "not experiencing time", but you won't find a textbook or peer-reviewed paper that says them, because such misstatements are weeded out. Sean Carroll has a set of free online lecture notes [1] that make a good start (they're more focused on GR than SR but they have an introductory section that covers SR). Taylor & Wheeler's Spacetime Physics [2] is a good introductory textbook as well. The fundamental physical point here is that timelike objects (objects with positive rest mass that follow timelike worldlines) and lightlike objects (objects with zero rest mass that follow null worldlines) are different things, and the concept of "experienced time" or "proper time" (the latter is the correct technical term) only applies to timelike objects. The mathematical basis for this is that timelike worldlines can be parameterized by arc length, and arc length along a timelike worldline corresponds to elapsed time on a clock following that worldline. Null worldlines, however, cannot be parameterized by arc length at all, so the fact that arc length along them is zero does not mean lightlike objects "experience no time", it means that the whole concept of "experienced time" doesn't even work for them: it's mathematically invalid since it requires parameterizing the worldline by arc length. Another way of seeing the fundamental difference is to look at how Lorentz transformations act on timelike and null vectors. Lorentz transformations hyperbolically rotate timelike vectors: that means the transformation changes which way in spacetime the vector points, without changing its length. But Lorentz transformations do not rotate null vectors: they dilate them, meaning they increase or decrease all components of the vector by the same factor, without changing its direction in spacetime. This means that a null vector is not a "limiting case" of any set of timelike vectors as far as the Lorentz transformations are concerned; null and timelike vectors are simply two disconnected sets of vectors with respect to Lorentz transformations. Which in turn means that the common pop science image of objects "experiencing less time" as they move faster and faster, until photons moving at the speed of light "experience no time" in the limit, is not correct: it is not a valid description of what is actually happening in the math. Lorentz transformations don't change the length of timelike vectors at all, so they don't change the "experienced time" along them. The apparent "time dilation" of an object that is moving relative to you is due to the angle in spacetime between your worldline and the object's worldline, not to any property of the object's worldline itself. But the "angle" here is a hyperbolic angle, and the hyperbolic angle between your worldline (you being a timelike object) and any null worldline (i.e., any worldline of a light ray or photon) is infinite--in other words, mathematically ill-defined. In short, pop science authors who make claims like "photons don't experience time" are putting an interpretation on the fact that a photon's worldline has zero length in the Minkowski metric that is not justified by anything in the actual math. They do it, unfortunately, because they believe (quite possibly correctly) that saying things like that, even if they're wrong, will sell more books than trying to teach their readers the actual science. [1] https://arxiv.org/abs/gr-qc/9712019 https://arxiv.org/abs/gr-qc/9712019 [2] https://www.eftaylor.com/spacetimephysics/ https://www.eftaylor.com/spacetimephysics/
  - 6nf 5y ago
    You're picking nits, I'm saying 'objects moving at C do not experience local time' and you're saying the same thing in more formal / fancy language without explaining where exactly my statement differs from yours.
    - pdonis 5y ago
      > 'm saying 'objects moving at C do not experience local time' and you're saying the same thing No, I'm not. See below. > without explaining where exactly my statement differs from yours You evidently failed to grasp the point of my statement that the concept of "experienced time", or "proper time", is not even well-defined for lightlike worldlines. Your statement was that photons "do not experience time". And you drew from that the implication that photons cannot undergo any kind of change while propagating freely. That implication is only valid if "do not experience time" means that the concept of "experienced time" is well defined for photons, and the time that they experience is zero. However, as I explained, the concept of "experienced time" is not well defined for photons. That means you cannot draw any implications either way about whether or not photons can "change" as they propagate freely, based on the fact that the Minkowski length of their worldlines is zero. There simply is no logical implication about "change" for photons from the Minkowski length of their worldlines. To draw any conclusions about whether or not a photon can "change" as it propagates, you have to look at other things.
      - 6nf 5y ago
        > the concept of "experienced time" is not well defined for photons. how is that different from 'photons don't have experienced time'
        pdonis 5y ago
        I explained the issue in the last two paragraphs of my post (the GP to this one). You drew an implication from "photons don't experience time" that is not valid. I explained why it's not valid.