
When light and light collide - sohkamyung
https://particlebites.com/?p=6227
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imglorp
Even if this is a rare or difficult event, wouldn't it be happening fairly
often inside stars because of the law of large numbers? And if so, would it be
visible to astronomers somehow?

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SiempreViernes
The problem with that idea is that the insides of stars is _full_ of other
non-transparent stuff that photons strongly interact with.

The chances to interact is proportional to the _cross section_ , which for an
electron is around 0.6 barn. In comparison, the cross section for photon-
photon interaction was measured as 70 _nano_ barn, so there's a factor of 10^8
difference in probability of interaction. Photons doesn't outnumber electrons
by such factors.

Further, there are plenty of atoms in the outer layers of a star, and these
have something like another 10^8 advantage over electrons.

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WhitneyLand
Did anyone else know of Richard Feynman's as a giant name in physics, so many
contributions and insights, and then find themselves surprised when first
hearing him speak?

One day I was watching this guy on YouTube whose job is walking around selling
peanuts at a baseball stadium in Chicago, blue collar local accent in full
tow. He starts to chatter about in a casual, simple, mundane way about how his
day was going.

Except, it wasn't a baseball peanut barker, it was a Nobel prize winning
particle physicist, it was Feynman. If he would have sold peanuts part time at
a baseball stadium for five years maybe no one would have noticed anything out
of place.

It's not a criticism, it only raised his rank on my list of admired
scientists. I found the humility, his passion for making hard things as easy
to understand as possible, his ability to just be a regular guy around people,
really a beautiful thing.

(relevance: he's cited in the paper)

~~~
jl2718
I don’t have direct experience with the man, but I shared an office with his
lab technician. Story: first day at work, went up on the roof to change the
oil in the telescope actuator motors, Richard Feynman up there, naked, reading
a newspaper: “Oh yeah, forgot to tell you, I’m a nudist; get over it!”

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jl2718
Surprising that the author of such a technical piece talks about “particles
and waves” in the expository paragraphs. Let’s be clear. We are talking about
bosons and fermions.

Now let’s consider what is going on. We have two photons in and two photons
out in different energies and directions although the total energy and
momentum must be conserved. Anything could be happening in between, so let’s
stick with known phenomena.

We know about photon -> particle/antiparticle, and the reverse. We also know
about particle/antiparticle -> dual photon, and the reverse, which must exist
by time symmetry. So it’s not hard to see that dual photon ->
particle/antiparticle -> dual photon interactions are very possible.

So how do we get scattering? First, if the input photons are opposite
momentum, then the output photons may be in any opposite directions. Actually,
any two incident photon momenta define a cone of output direction
possibilities, and the lifetime of the +/\- particle system translates that
cone along the total momentum vector. Thus, many observations are possible at
the detector.

~~~
SiempreViernes
I think the expository paragraphs are quite good, and their use of "particle"
and "wave" is clear, logical, and effectively conveys their point.

However, I don't see what clarity you get from bringing in quantum statistics
to the discussion, the blobs at the centre of the diagrams represent both
fermionic and bosonic particle exchanges:
[https://arxiv.org/pdf/1601.07001.pdf](https://arxiv.org/pdf/1601.07001.pdf)

------
Zenst
Somewhat related and relevant:
[https://phys.org/news/2018-03-underway.html](https://phys.org/news/2018-03-underway.html)

