6 ms·

Hard or impossible to visualize, but as the temporal dimension is so much larger, most of the deformity is in the time dimension. But yes, spacetime itself is

Hard or impossible to visualize, but as the temporal dimension is so much larger, most of the deformity is in the time dimension. But yes, spacetime itself is jiggly like Jell-O. As we can't visualize 4d spacetime, most analogies will be wrong. But as photons don't experience time themselves, thinking about the geodesic path is probably less error prone than thinking of it as squishing physical objects. Objects being pulled towards slower flows is the intuition that matches the math best for me.

sunshinerag 3y ago
what does "photos don't experience time themselves.." mean? why not?
- alfiopuglisi 3y ago
  Photons travel at the speed of light, and at that speed, any "subjective" time is zero. In Einstein's theory of special relativity, the faster you go, the slower your proper time appears to an external observer. At the speed of light, this effect reaches infinity.
  - euroderf 3y ago
    Okay, so what does a photon observe when it partakes of Cherenkov radiation ?
  - criddell 3y ago
    > the faster you go, the slower your proper time appears to an external observer From my perspective, it takes about 8 minutes for a photon from the sun to hit my eye. From the perspective of the photon, a little time has passed, no? Doesn't the atmosphere and passing through my glasses slow it down a wee bit? Can the photon "know" that its position has changed between emission and absorption? From the photons point of view, I must be very, very close to the sun, right?
    - ben_w 3y ago
      Light does slow in a medium, the statement presumes the light is in a vacuum. From the point of view of the photon, "forwards" is, like time, a null[0] dimension. [0] I may be using that word imprecisely, but I can't think of a better one.
      - adgjlsfhk1 3y ago
        This isn't quite true. At a macro level, light slows down. At a micro level, photons travel at the speed of light, get absorbed and re-emitted, and change directions. These effects average out to looking like photons traveling slower.
        User23 3y ago
        At the micro level there are infinitely many paths the photon follows and most of them involve it traveling considerably faster than the speed of light. While they cancel out, if you omit them then the calculations don’t give correct results, so they’re happening inasmuch as anything can be said to happen scientifically. If you want to object that that contradicts GR well there is a Nobel prize or ten waiting for whoever squares that circle.
        deleted 3y ago
        [deleted]
    - derefr 3y ago
      It takes time for a photon to move in your reference frame, but time within the photon's own reference frame is not advancing at all during that. Within the photon's reference frame, the photon exists instantaneously, simultaneously, at its emitter and absorber. Its whole existence "brings together" the spacetime it was emitted from and the spacetime it is absorbed into, at a single 4D pinch-point. It's like the whole universe is squished flat into two hyperplanes of "everything behind the photon at time of emission" and "everything ahead of the photon at time of absorption", and those two hyperplanes have no distance between them. > Doesn't the atmosphere and passing through my glasses slow it down a wee bit? When a photon is travelling through anything other than vacuum, it's not "slowed down." It's repeatedly being absorbed and re-emitted. (Or rather, it's being absorbed, and new photons that happen to be mostly equivalent are being emitted.) The refractive index of a material is effectively a measurement of the likelihood of absorption, times the average per-particle time-delay between absorption and re-emission.
- nyrikki 3y ago
  Nothing that travels at light speed experiences time. For a photon, emission and absorption is a single event.
- drbw 3y ago
  Try this from Fermilab's Dr Don Lincoln: https://www.youtube.com/watch?v=6Zspu7ziA8Y https://www.youtube.com/watch?v=6Zspu7ziA8Y
  - raattgift 3y ago
    I did, and enjoyed it as a good pop-sci outreach video, and do not fault it given that's what it is. The lim v->c analysis is standard and well-presented, but his conclusion about photons' time is liable to confuse people (including other physicists who aren't relativists). I refer you to my comment elsewhere in this thread <https://news.ycombinator.com/item?id=36537015 https://news.ycombinator.com/item?id=36537015> which has a paragraph about this video (your link having partially motivated my comment), and which explains that it is common for relativists to use affine time for massless particles.

sunshinerag 3y ago

what does "photos don't experience time themselves.." mean? why not?

alfiopuglisi 3y ago
Photons travel at the speed of light, and at that speed, any "subjective" time is zero. In Einstein's theory of special relativity, the faster you go, the slower your proper time appears to an external observer. At the speed of light, this effect reaches infinity.
- euroderf 3y ago
  Okay, so what does a photon observe when it partakes of Cherenkov radiation ?
- criddell 3y ago
  > the faster you go, the slower your proper time appears to an external observer From my perspective, it takes about 8 minutes for a photon from the sun to hit my eye. From the perspective of the photon, a little time has passed, no? Doesn't the atmosphere and passing through my glasses slow it down a wee bit? Can the photon "know" that its position has changed between emission and absorption? From the photons point of view, I must be very, very close to the sun, right?
  - ben_w 3y ago
    Light does slow in a medium, the statement presumes the light is in a vacuum. From the point of view of the photon, "forwards" is, like time, a null[0] dimension. [0] I may be using that word imprecisely, but I can't think of a better one.
    - adgjlsfhk1 3y ago
      This isn't quite true. At a macro level, light slows down. At a micro level, photons travel at the speed of light, get absorbed and re-emitted, and change directions. These effects average out to looking like photons traveling slower.
      - User23 3y ago
        At the micro level there are infinitely many paths the photon follows and most of them involve it traveling considerably faster than the speed of light. While they cancel out, if you omit them then the calculations don’t give correct results, so they’re happening inasmuch as anything can be said to happen scientifically. If you want to object that that contradicts GR well there is a Nobel prize or ten waiting for whoever squares that circle.
      - deleted 3y ago
        [deleted]
  - derefr 3y ago
    It takes time for a photon to move in your reference frame, but time within the photon's own reference frame is not advancing at all during that. Within the photon's reference frame, the photon exists instantaneously, simultaneously, at its emitter and absorber. Its whole existence "brings together" the spacetime it was emitted from and the spacetime it is absorbed into, at a single 4D pinch-point. It's like the whole universe is squished flat into two hyperplanes of "everything behind the photon at time of emission" and "everything ahead of the photon at time of absorption", and those two hyperplanes have no distance between them. > Doesn't the atmosphere and passing through my glasses slow it down a wee bit? When a photon is travelling through anything other than vacuum, it's not "slowed down." It's repeatedly being absorbed and re-emitted. (Or rather, it's being absorbed, and new photons that happen to be mostly equivalent are being emitted.) The refractive index of a material is effectively a measurement of the likelihood of absorption, times the average per-particle time-delay between absorption and re-emission.
nyrikki 3y ago
Nothing that travels at light speed experiences time. For a photon, emission and absorption is a single event.
drbw 3y ago
Try this from Fermilab's Dr Don Lincoln: https://www.youtube.com/watch?v=6Zspu7ziA8Y https://www.youtube.com/watch?v=6Zspu7ziA8Y
- raattgift 3y ago
  I did, and enjoyed it as a good pop-sci outreach video, and do not fault it given that's what it is. The lim v->c analysis is standard and well-presented, but his conclusion about photons' time is liable to confuse people (including other physicists who aren't relativists). I refer you to my comment elsewhere in this thread <https://news.ycombinator.com/item?id=36537015 https://news.ycombinator.com/item?id=36537015> which has a paragraph about this video (your link having partially motivated my comment), and which explains that it is common for relativists to use affine time for massless particles.