4 ms·

Photons do not create particles in a vacuum - they need to interact with matter to do that - one of the reasons being in order to conserve momentum, some of whi

by ars 3y ago

Photons do not create particles in a vacuum - they need to interact with matter to do that - one of the reasons being in order to conserve momentum, some of which goes to the matter. So at that meeting spot they would go right through each other.

btilly 3y ago
No, they do. When a particle meets its antiparticle, you get 2 energetic photons. All such interactions are perfectly reversible. So when 2 energetic photons meet, you can get a particle and antiparticle. Both energy and momentum are conserved in this process.
- ars 3y ago
  When they meet they produce photons traveling in opposite directions, so there is no net momentum. If there was incoming momentum from the particle (they they move both forward and toward each other) then the two emitted photons will also be traveling forward (i.e. at an angle toward each other). But here we explicitly placed the photons to be traveling in the same direction, so there is some net momentum that can not be carried by the particles. i.e. particle anti-particle annihilation will never produce two photons that are both traveling straight forward. So the event is not reversible.
  - btilly 3y ago
    No, we explicitly placed the photons to be traveling in different directions so that they could meet. If they meet, and they meet with sufficient energy, they can create particle anti-particle pairs. You can read more about this at https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics.
    - ars 3y ago
      The angle they are traveling is one that's impossible to reverse - in annihilation it's impossible for the two photons to be emitted at such a close angle. I read about the two photon physics - and the two photons can scatter off of each other, but they can't produce particles except in very high electromagnetic fields, which isn't the case here.
      - codethief 3y ago
        I'm really not following you. If two photons meet at an angle – any angle –, there will be a rest frame in which they're meeting from exactly opposite (180°) directions. They can absolutely produce electron positron pairs if their energy is high enough. The process γ γ′ → e+ e− is a classic calculation in QED, and although it's never been observed directly for two single/isolated photons[0], there is no good reason to believe QED should be wrong in this particular case and, indeed, all evidence we have is pointing towards this process being perfectly possible[1]. [0]: https://en.m.wikipedia.org/wiki/Breit%E2%80%93Wheeler_process https://en.m.wikipedia.org/wiki/Breit%E2%80%93Wheeler_proces... [1]: https://www.energy.gov/science/np/articles/making-matter-collisions-light https://www.energy.gov/science/np/articles/making-matter-col...

by ars 3y ago

btilly 3y ago

No, they do. When a particle meets its antiparticle, you get 2 energetic photons. All such interactions are perfectly reversible. So when 2 energetic photons meet, you can get a particle and antiparticle. Both energy and momentum are conserved in this process.

ars 3y ago
When they meet they produce photons traveling in opposite directions, so there is no net momentum. If there was incoming momentum from the particle (they they move both forward and toward each other) then the two emitted photons will also be traveling forward (i.e. at an angle toward each other). But here we explicitly placed the photons to be traveling in the same direction, so there is some net momentum that can not be carried by the particles. i.e. particle anti-particle annihilation will never produce two photons that are both traveling straight forward. So the event is not reversible.
- btilly 3y ago
  No, we explicitly placed the photons to be traveling in different directions so that they could meet. If they meet, and they meet with sufficient energy, they can create particle anti-particle pairs. You can read more about this at https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics.
  - ars 3y ago
    The angle they are traveling is one that's impossible to reverse - in annihilation it's impossible for the two photons to be emitted at such a close angle. I read about the two photon physics - and the two photons can scatter off of each other, but they can't produce particles except in very high electromagnetic fields, which isn't the case here.
    - codethief 3y ago
      I'm really not following you. If two photons meet at an angle – any angle –, there will be a rest frame in which they're meeting from exactly opposite (180°) directions. They can absolutely produce electron positron pairs if their energy is high enough. The process γ γ′ → e+ e− is a classic calculation in QED, and although it's never been observed directly for two single/isolated photons[0], there is no good reason to believe QED should be wrong in this particular case and, indeed, all evidence we have is pointing towards this process being perfectly possible[1]. [0]: https://en.m.wikipedia.org/wiki/Breit%E2%80%93Wheeler_process https://en.m.wikipedia.org/wiki/Breit%E2%80%93Wheeler_proces... [1]: https://www.energy.gov/science/np/articles/making-matter-collisions-light https://www.energy.gov/science/np/articles/making-matter-col...