
Atlas observes light scattering off light - ars
https://atlas.cern/updates/physics-briefing/atlas-observes-light-scattering-light
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mmmBacon
Light by light scattering was first observed at SLAC in the mid 90s by
experiment E-144.

[https://physicsworld.com/a/light-is-seen-to-scatter-off-
ligh...](https://physicsworld.com/a/light-is-seen-to-scatter-off-light/)

Link to paper from 1997: Positron Production in Multiphoton Light-by-Light
Scattering

(published in Phys. Rev. Lett., Vol. 79, p. 1626 (1997).)
[http://www.slac.stanford.edu/exp/e144/ps/positron_slacpub.ps](http://www.slac.stanford.edu/exp/e144/ps/positron_slacpub.ps)

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keyle
Any potentially practical use?

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rohan1024
Not a physics expert.

Projecting in the air without a screen could be a potential use.

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db48x
Lol, no. Photon-photon scattering is very rare; you only expect it to happen
in very high-energy situations. To scatter enough photons to create a visible
image in the air you would need to pour so much energy into the air that it
would turn into a plasma. A deadly radioactive plasma that explodes outward
from your holographic display and kills everyone nearby. This would not be
considered practical.

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edf13
The lol isn’t needed

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db48x
I think an important part of our humanity is our ability to laugh. I literally
laughed when I read the question, because of the absurd impracticality of it.
That's an important part of the answer.

On the other hand, it's true that a purely text-based medium like a comment
doesn't have room for a lot of nuance. So for the sake of clarity, I will say
that it was a good question. We should always look for ways to use new
discoveries to our advantage. Let's just stick to things which are possible,
like Dyson spheres, and leave the impossible for science fiction.

See also [https://what-if.xkcd.com/1/](https://what-if.xkcd.com/1/).

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cozzyd
Light by light scattering is also why we can't see gamma rays above 80 TeV or
so from far away (the gamma rays would pair produce e+/e- pairs with CMB
photons).

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saagarjha
How do we detect high-energy cosmic rays, which to my knowledge can exceed
this "limit" by several orders of magnitude?

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marcosdumay
If you are talking about [1], I think nobody knows how they are possible.

[https://en.wikipedia.org/wiki/Oh-My-
God_particle](https://en.wikipedia.org/wiki/Oh-My-God_particle)

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saagarjha
That article links to a much better written one that appears to discuss this
exact issue:
[https://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit](https://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit).
Thanks!

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pieterk
Dumb question but is this unexpected because photons are bosons?

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orbifold
This is not unexpected, QED (quantum electrodynamics) predicts this.
Classically it does not happen because classical electromagnetic fields are in
linear superposition. QED has loop diagrams involving virtual fermions that
mediate scattering, this is also the reason why these events are rare.

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gus_massa
I agree. More details:

The main problem is that photons only can interact with charged particles, but
photons have no charge, so the photons don't interact (directly) with photons.

The gluons that are the bosons of the strong force can interact between them,
so it's not a problem of bosons vs fermions. In particular, gluons have color
that is the equivalent of charge for the strong force.

The idea is that the photons can sometimes create a virtual electron-positron
pair that is very short lived [1], and the other photon can colide with the
electron or positron before they annihilate. This is very rare, and you can
ignore it unless you have a really huge amount of photons flying around.

[1] Insert here a technical remark about "virtual" and "very short lived".
It's more complicated. Take this as a metaphor to hide a lot of math.

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plibither8
Quantum physics of this type has always intrigued me, but I've never been able
to find a good resource to properly start learning it.

Could anyone point me to some good starting material? Thanks :)

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jcims
If you have two photons that have been emitted at different times, and no time
passes at the frame of reference of the photon when they travel, what time is
it when they interact?

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kijin
> what time is it when they interact?

In which frame of reference?

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jcims
I think that's the detail underlying my question. Does the interaction
collapse the frames of reference into one? I guess it doesn't have to.

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kijin
The time of collision is not a problem, it happens at a different time
depending on which photon's frame of reference we're talking about.

Which frame of reference the resulting particles might "inherit", well, that's
a very interesting question that I've never thought about!

