
ATLAS sees first direct evidence of light-by-light scattering at high energy - explore
https://atlas.cern/updates/press-statement/atlas-sees-first-direct-evidence-light-light-scattering-high-energy
======
rrmm
Neat-o. A little bit more info at [https://en.wikipedia.org/wiki/Two-
photon_physics](https://en.wikipedia.org/wiki/Two-photon_physics) . Basically
two photons with enough energy to interact via virtual pairs created.

It will be interesting to see the cross-section rates and maybe hopefully some
new physics (as the wikipedia page weakly hints).

~~~
phkahler
>> Basically two photons with enough energy to interact via virtual pairs
created.

Or could it be that a photon bends space-time as though it had mass, and they
simply follow the local curvature and it looks like scattering? Just
speculating, because for some purposes photons act like a solid particle with
their energy equivalent mass traveling at speed c.

~~~
jessriedel
It is not gravitational. That effect would be far, far too weak.

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raverbashing
Just as a reminder, those photons are high energy ones (Gamma-ray range) so no
light sabers, holograms or anything involving visible light I am afraid

~~~
anfractuosity
Electrolasers kind of seem slightly light saber-esque. I'm assuming
miniaturisation of them would be tricky though, as i'm assuming ionization
with a laser requires a lot of power.

~~~
cr0sh
I was about to make a comment regarding using UV lasers and ionization - then
decided to take a second to google it, and found this interesting (though
short) discussion:

[https://www.physicsforums.com/threads/laser-to-ionize-air-
to...](https://www.physicsforums.com/threads/laser-to-ionize-air-to-run-
current-laser-power-per-foot.795093/)

In short, I think your conclusion is likely correct...

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staticelf
Can someone explain this in easier terms for a layman like myself?

~~~
nimish
Photons carry no charge, so normally they don't interact with each other.
However, at high enough energies they do. The explanation given by the best
available theory is that there is enough energy that a virtual electron and
virtual anti-electron form, and since those have charges, they interact, and
from the outside it appears that the photons interacted.

This is totally impossible in the classical view, but is fine in the quantum
electrodynamic (QED) view.

~~~
pif
High energies are not absolutely necessary. Couples of "virtual" particles can
be created at any energy, it's just less likely for smaller energies. I think
the real problem was the experimental set-up: particle accelerators use
electric field to transfer energy to particles and magnetic field to control
their trajectory and eventually to cause collisions, but all of this only
works with charged particles. And photons carry no charge, as you said! As the
article mentions, the observation was finally made possible by accelerating
bunches of lead ions, as they carry around a cloud of high energy photons (I
suppose due to bremsstrahlung caused by chaotic movements within the bunches).

~~~
baq
> Couples of "virtual" particles can be created at any energy

doesn't the energy need to be at least the mass of the virtual particle?

~~~
roywiggins
The vacuum itself has enough energy that there are particle pairs popping into
existence and annihilating each other constantly.

[https://en.wikipedia.org/wiki/Vacuum_energy](https://en.wikipedia.org/wiki/Vacuum_energy)

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SolarNet
The bit about the trigger was interesting. Is that software/firmware? Or is
that a hardware trigger they are discussing.

~~~
whyever
I think at the moment they use a level 1 hardware trigger and a level 2
software trigger. ALICE is moving to a pure software trigger.

~~~
pif
When I last participated to ATLAS, working in the Trigger & Data Acquisition
group, ATLAS had 3 levels of trigger: the first was hardware, the others were
software. Regarding the first level, software was not an option due to the
extremely high bunch-crossing frequency (40 million times per second). The
difference between the 2nd and 3rd levels consisted of data availability
(region-of-interest only for 2nd, full detector for 3rd) and algorithms
permitted (light algorithms for 2nd, full reconstruction and advanced analysis
for 3rd). The maximum allowed output were, IIRC, 100 kHz for 1st level, 3 kHz
for 2nd level and 100 Hz for 3rd level.

~~~
ephimetheus
They've since scrapped L2, now it's only L1 (HW) and the High Level Trigger
(HLT) running on a server farm

~~~
pif
Thanks for the info. I left ATLAS beginning 2009, many other things evolved, I
suppose.

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yohann305
Anyone else here is having a hard time seeing what could be applied usage of
light-by-light scattering? (ie new computers? new internet? new telescopes?)

Can someone here give us some insights?

~~~
ekimekim
Currently, very little. According to the article, you need crazy particle-
accelerator-levels of energy to have them happen at all.

However, by studying and understanding such events, we can test and improve
our theories of physics, which may lead to breakthroughs in technology we can
only dream of.

For example, general relativity would have been considered completely useless
knowledge for any practical purpose when it was first formulated in the early
20th century. It only has any measurable impact at extreme velocities and
gravities. In the late 20th century, it proved instrumental to creating
accurate enough models for measuring the time delay of signals from
satellites, creating what we now know as the GPS system.

~~~
soVeryTired
I've heard people make the argument about GR being essential to the GPS system
before, but the skeptic in me has doubts. I don't deny that it's useful to
have a correction derived from relativity, but is it really as _essential_ as
people make it out to be? If we hadn't known about GR when we built the GPS
system, wouldn't we have realised there was an unexpected drift in the clocks
and found a way to recalibrate them periodically?

~~~
mitchty
Yes its essential. Because the satellites are moving at orbital velocity,
their frame of time is ever so slightly different than ours. To keep accuracy
timewise without special and general relativity would be difficult if not
impossible without understanding relativity generally. Could we have done GPS
without relativity? Maybe, but we'd have to discover it with the satellites to
be able to use it.

A quick note from: (i'm too lazy to do the calculations right now)
[http://www.astronomy.ohio-
state.edu/~pogge/Ast162/Unit5/gps....](http://www.astronomy.ohio-
state.edu/~pogge/Ast162/Unit5/gps.html)

    
    
        If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time.

~~~
hinkley
I think I see what he's saying. Those satellites are moving at a fixed
velocity. So the time dialation effect on them relative to the ground should
be static.

It could reasonably be the case that we just adjusted the clocks on the
satellites by .05% or whatever the drift rate is by measuring them against
known points until we get it right. Good enough engineering.

Eventually someone would have asked some scientists to explain why it's
happening.

~~~
mitchty
> I think I see what he's saying. Those satellites are moving at a fixed
> velocity. So the time dialation effect on them relative to the ground should
> be static.

The issue you have is that the dilation effects aren't static, they're all
relative to each satellite and ground observer and are constantly shifting
based on the orbits. Basically the premise you have to accept to allow for
"just adjust the clocks" is too basic. This is why both general and special
relativity come into play in GPS. You might get away with adjusting for a
single observer, but not all observers.

Note that we already have the clocks purposefully skewed to account timewise
for their orbital speed and still require constant updates. I just don't see
that happening through "good enough engineering". If you're a nanosecond off
you're off by over 1 kilometer and things get worse from there.

Plausible as a hypothetical gedanken experiment? I suppose, plausible in
reality? I'm skeptical that you'd be able to do it. Would be akin to launching
a rocket to the moon without understanding how to fly.

------
platz
post from november
[http://cerncourier.com/cws/article/cern/66878](http://cerncourier.com/cws/article/cern/66878)

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awinter-py
light seeking light doth light of light beguile

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dogma1138
One step closer to light sabers.

~~~
FoeNyx
Not sure you'd want some high energy photons scattering on your face after the
collision of the light sabers. Except if you want to call it "the photon
menace".

~~~
dogma1138
Sounds just like watching the prequels.

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etatoby
One step closer to holograms.

~~~
chi17
Could you explain?

Also, do you not think that current implementations that use eye tracking are
not real holograms?

What's to say that holograms similar to those of Star Wars could not be
produced by inspecting the environment and automatically determining where
people's head and eyes are, e.g. via a combination of technologies similar to
the following:

Eye-tracking holographic table requiring 3D glasses:
[http://www.euclideonholographics.com/](http://www.euclideonholographics.com/)

3D TV not requiring special glasses for the 3D effect:
[http://www.ultra-d.com/](http://www.ultra-d.com/)

And what about things that have been called and accepted as holographs created
since the mid-20th century?

[https://en.wikipedia.org/wiki/Holography](https://en.wikipedia.org/wiki/Holography)

~~~
jerf
This is closer: [http://www.pocket-lint.com/news/131622-holograms-are-
finally...](http://www.pocket-lint.com/news/131622-holograms-are-finally-here-
plasma-lasers-used-to-create-images-in-mid-air)

Star Wars-style holograms implies that I don't need additional equipment or
special angles or anything, just a (basically) magical "hologram" projector.
As that link shows, it may not be entirely out of the question, but, well...
I'm not sure I want to be in the same room as one of those.

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madaxe_again
There's an interesting implication here for redshift observations and Hubble
expansion, insofar as it provides a mechanism for photons to lose energy while
in a highly sparse medium, populated largely only by other photons.

I'm keeping my money on expansion and dark matter being hooey brought about by
incomplete of incorrect understanding of light.

Only recently there was that rather interesting piece about simulated momentum
transfer from photons in media.

It's exciting - we'll potentially be lopping off a huge branch of dead wood
from the tree of science, from which new ideas can grow.

~~~
m1el
> I'm keeping my money on expansion and dark matter being hooey brought about
> by incomplete of incorrect understanding of light.

That's a very interesting hypothesis. Unfortunately, it's easy to verify that
photon-photon scattering doesn't explain the expansion of the universe.

1) photon-photon scattering is elastic, so no energy is lost, and no redshift
is occuring

2) if it isn't elastic, the scattering is a random process. So different
photons will lose a different amount of energy, which means that measured
spectra are going to be blurred.

3) rather than seeing redshift, due to photon-photon scattering, you'd see
fog, which gets cloudier and cloudier with distance.

~~~
madaxe_again
Those are all solid points, so I retract - but the momentum paradox bit
stands!

I wish I had someone else to chat physics with - isolation leads to screwy
notions which can easily be quashed by the right counterpoints.

Edit: a thought. Please (genuinely!) tell me where I'm wrong. As it's elastic,
could we not end up with groups of lower energy photons with the same vector
as an original high energy photon, which would similarly explain redshift?
Doesn't address your point re: fog, however, unless they're universally
tightly grouped.

~~~
triclops200
It looks like from what we know, 2 high energy (it's much less common with low
energy) photons collide and change direction: from what I understand, they
don't split into low energy photons. It'd be basically impossible for all the
high energy photons to scatter directly away from earth (why would the earth
be special?) leaving only low energy ones towards us.

