5 ms·

It was Feynman’s excellent QED (basically the ELI5 for quantum physics) that introduced me to the concept that the photon I shine on my wall with a flashlight d

by User23 4y ago

It was Feynman’s excellent QED (basically the ELI5 for quantum physics) that introduced me to the concept that the photon I shine on my wall with a flashlight did a few orbits around Jupiter too, it just cancelled out. I believe the “dippy” process to do that is called renormalization. Feels like it’s time for a reread.

CyberDildonics 4y ago
You think that when you turn on there are actual photon particles that traveled 48 minutes to jupiter and back?
- evanb 4y ago
  One absolutely astounding aspect of the path integral is that it ascribes amplitudes to trajectories that don't obey the (classical) equations of motion. If you like: the classical paths are at turning points of the action (loss function) but all other paths have some associated imaginary loss. In the path formalism we sum every possible path, weighted by the imaginary loss. Since the classical paths are at turning points, small deviations around that path tend to sum coherently, while large deviations tend to have neighboring paths that contribute with opposite phases. If you write the amplitude for a photon going from your flashlight to your wall, the path integral tells you to sum up all possible paths, not just the classical ones. So yes, the path integral formalism really does say that a photon goes from the flashlight, around Jupiter, and to the wall. But not in 48 minutes! In time = (distance to wall)/c! It goes every which way without caring about the classical law. The "insane" ones cancel. Well, if all the insane ones cancel, what's the point of all this? Just forget about them, they're a huge surplus of nonsense in the theory! However, it's perfectly possible to construct experiments where it's clear that photons really do "smell out" alternatives. Diffraction gratings, the double-slit experiment, etc., all show this quantum weirdness.
  - CyberDildonics 4y ago
    Are there any other household appliances that emit photons that circle around any other planets?
    - evanb 4y ago
      “Emit” usually means as radiation. The photons that take every path are not on-shell. When your refrigerator is open the light you see contains an amplitude of the photon going around Uranus.
      - CyberDildonics 4y ago
        Flashlight -> Jupiter Refrigerator -> Uranus What planet does the oven radiation go around?
        raattgift 4y ago
        > What planet does the oven radiation go around? All of them, including planets outside our observable universe. Same with the flashlight and refrigerator. Each photon also goes into and through all the planets, and near each of their electrons in an infinite number of ways. They also go through all the stars, all the vacuum, etc. And with all the speeds, all the accelerations, ... Most of these infinite possibilities are a negligible contribution to the final probability that photon-detected-at-B was photon-emitted-at-A. The non-negligible contributions come from the (still infinite) trajectories allowed by relativity and mostly from those near the lightlike geodesic.
        CyberDildonics 4y ago
        Which part of the lightlike geodesic's infinite relativity trajectories does my toaster glow come from?
        raattgift 4y ago
        The whole glow? Treat it classically, it's mostly a solid-state physics problem. One individual photon, treated quantum mechanically? From a single inelastic interaction between a free electron and a metal cation. That origin marks one end of the photon's worldline. Repeating for large numbers of photons will make you quickly bored, so you'll probably want to look into the Stefan-Boltzmann law (which still includes the constant c and Planck's constant) and the concepts in Bohr's correspondence principle and effective theory. More at <https://en.wikipedia.org/wiki/Joule_heating https://en.wikipedia.org/wiki/Joule_heating> if you're curious.
  - raattgift 4y ago
    > a huge surplus of nonsense in the theory It gets even worse if one starts thinking about the flashlight's filament or semiconductor, the wall, stratified Jupiter with its optical depth, the flashlight-wielder's visual system, and everything in between all being (sets of) quantum objects. Maybe the thing to do in this example is to assume correspondence and think classically, and insist on using a fewer-quantum-number example if one wants to think quantumly. However I tend to admire detailed wrestling with everything-is-quantum in an example like this. Maybe start by remembering that the spot-on-wall-to-retina is relevant, not just the flashlight-to-wall.
dvt 4y ago
> the photon I shine on my wall with a flashlight did a few orbits around Jupiter too This is a very confusing way to essentially say that an integral of exp(−iS) is taken over all possible paths for the action S. Your flashlight's photons don't literally do a few orbits around Jupiter. In fact, photons are quantum objects (not billiard balls) and they are "smoothed out" over spacetime, meaning they don't really have paths to take in the first place.
froeb 4y ago
This is a pretty common misconception of path integrals from how Feynman explains them to a non-physicist audience. The path integral formulation cannot work from a particle-based approach, where you integrate over all possible trajectories of electrons, photons... . Instead, you are actually integrating over all possible configurations of fields, the electron field, the photon field... . As far as I know, there is no way to view the universe as being fundamentally made of particles that is compatible with our modern understanding of QFTs. Also, the path integral really shouldn't be taken too literally. It's not reality, it's just an equation.
- evanb 4y ago
  > The path integral really shouldn't be taken too literally. It's not reality, it's just an equation. While it's always important to point out that the current paradigm might be overthrown tomorrow, it is ALSO important to understand different ways of describing what the current paradigm says! The "bizarre" trajectories that contribute to the path integral are not optional; they're really there and you can arrange interference experiments to make their physical importance clear.

by User23 4y ago

CyberDildonics 4y ago

You think that when you turn on there are actual photon particles that traveled 48 minutes to jupiter and back?

evanb 4y ago
One absolutely astounding aspect of the path integral is that it ascribes amplitudes to trajectories that don't obey the (classical) equations of motion. If you like: the classical paths are at turning points of the action (loss function) but all other paths have some associated imaginary loss. In the path formalism we sum every possible path, weighted by the imaginary loss. Since the classical paths are at turning points, small deviations around that path tend to sum coherently, while large deviations tend to have neighboring paths that contribute with opposite phases. If you write the amplitude for a photon going from your flashlight to your wall, the path integral tells you to sum up all possible paths, not just the classical ones. So yes, the path integral formalism really does say that a photon goes from the flashlight, around Jupiter, and to the wall. But not in 48 minutes! In time = (distance to wall)/c! It goes every which way without caring about the classical law. The "insane" ones cancel. Well, if all the insane ones cancel, what's the point of all this? Just forget about them, they're a huge surplus of nonsense in the theory! However, it's perfectly possible to construct experiments where it's clear that photons really do "smell out" alternatives. Diffraction gratings, the double-slit experiment, etc., all show this quantum weirdness.
- CyberDildonics 4y ago
  Are there any other household appliances that emit photons that circle around any other planets?
  - evanb 4y ago
    “Emit” usually means as radiation. The photons that take every path are not on-shell. When your refrigerator is open the light you see contains an amplitude of the photon going around Uranus.
    - CyberDildonics 4y ago
      Flashlight -> Jupiter Refrigerator -> Uranus What planet does the oven radiation go around?
      - raattgift 4y ago
        > What planet does the oven radiation go around? All of them, including planets outside our observable universe. Same with the flashlight and refrigerator. Each photon also goes into and through all the planets, and near each of their electrons in an infinite number of ways. They also go through all the stars, all the vacuum, etc. And with all the speeds, all the accelerations, ... Most of these infinite possibilities are a negligible contribution to the final probability that photon-detected-at-B was photon-emitted-at-A. The non-negligible contributions come from the (still infinite) trajectories allowed by relativity and mostly from those near the lightlike geodesic.
        CyberDildonics 4y ago
        Which part of the lightlike geodesic's infinite relativity trajectories does my toaster glow come from?
        raattgift 4y ago
        The whole glow? Treat it classically, it's mostly a solid-state physics problem. One individual photon, treated quantum mechanically? From a single inelastic interaction between a free electron and a metal cation. That origin marks one end of the photon's worldline. Repeating for large numbers of photons will make you quickly bored, so you'll probably want to look into the Stefan-Boltzmann law (which still includes the constant c and Planck's constant) and the concepts in Bohr's correspondence principle and effective theory. More at <https://en.wikipedia.org/wiki/Joule_heating https://en.wikipedia.org/wiki/Joule_heating> if you're curious.
- raattgift 4y ago
  > a huge surplus of nonsense in the theory It gets even worse if one starts thinking about the flashlight's filament or semiconductor, the wall, stratified Jupiter with its optical depth, the flashlight-wielder's visual system, and everything in between all being (sets of) quantum objects. Maybe the thing to do in this example is to assume correspondence and think classically, and insist on using a fewer-quantum-number example if one wants to think quantumly. However I tend to admire detailed wrestling with everything-is-quantum in an example like this. Maybe start by remembering that the spot-on-wall-to-retina is relevant, not just the flashlight-to-wall.

dvt 4y ago

> the photon I shine on my wall with a flashlight did a few orbits around Jupiter too This is a very confusing way to essentially say that an integral of exp(−iS) is taken over all possible paths for the action S. Your flashlight's photons don't literally do a few orbits around Jupiter. In fact, photons are quantum objects (not billiard balls) and they are "smoothed out" over spacetime, meaning they don't really have paths to take in the first place.

froeb 4y ago

This is a pretty common misconception of path integrals from how Feynman explains them to a non-physicist audience. The path integral formulation cannot work from a particle-based approach, where you integrate over all possible trajectories of electrons, photons... . Instead, you are actually integrating over all possible configurations of fields, the electron field, the photon field... . As far as I know, there is no way to view the universe as being fundamentally made of particles that is compatible with our modern understanding of QFTs. Also, the path integral really shouldn't be taken too literally. It's not reality, it's just an equation.

evanb 4y ago
> The path integral really shouldn't be taken too literally. It's not reality, it's just an equation. While it's always important to point out that the current paradigm might be overthrown tomorrow, it is ALSO important to understand different ways of describing what the current paradigm says! The "bizarre" trajectories that contribute to the path integral are not optional; they're really there and you can arrange interference experiments to make their physical importance clear.