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View Full Version : Time reversal of electron positron annihilation?



WaxRubiks
2018-Aug-19, 09:06 PM
Take two electrons and two positrons, in an empty universe. Two will annihilate each other before the second two do the same.

Once all have been turned into gamma radiation, is there any way for time's progress to be measured?

If time can't progress, could it be that all the gamma waves will reform into their original particles?

23503

Shaula
2018-Aug-20, 04:31 AM
I don't understand - why would there be no way to measure time's progress? Watch the distance travelled by the gamma rays relative to yourself, they are moving at c. Done.

It is tough for gamma photons to interact without matter around to help, but it does happen. See https://en.wikipedia.org/wiki/Two-photon_physics

Exposed
2018-Aug-20, 02:17 PM
Maybe this would belong in Q&A, but what exactly is behind the mechanism of a particle and antiparticle "annihilating" each other? What exactly happens? An electron and positron for example, why do they annihilate each other?

Hornblower
2018-Aug-20, 02:23 PM
Maybe this would belong in Q&A, but what exactly is behind the mechanism of a particle and antiparticle "annihilating" each other? What exactly happens? An electron and positron for example, why do they annihilate each other?

Their physical attributes are such that they transform into a pair of 511,000 electron volt gamma photons flying off in opposite directions at the speed of light. I defer to experts in subatomic particle physics for more details about those physical attributes.

Shaula
2018-Aug-20, 05:43 PM
Maybe this would belong in Q&A, but what exactly is behind the mechanism of a particle and antiparticle "annihilating" each other? What exactly happens? An electron and positron for example, why do they annihilate each other?
Annihilation is really just a special case of particle interaction. Because the particle / antiparticle have the same mass and have opposite charges (of all kinds) it is possible and often highly favourable for them to convert to photons (or 'just energy'). In most other interactions there are some spare mass / quantum number / charge components that means the end products include massive particles as well as photons, or may be energetically less likely to happen. So annihilation got a special term all of its own because it seemed like an easy to spark interaction that resulted in nothing but energy left behind.