Earlier quoted context omitted.
Invisible light sabers. Cool
Just make sure you have some sunscreen on
> Just make sure you have some sunscreen on
in the thickness equivalent to 1cm lead sheet for a gamma-ray-screen factor of 2
ATLAS sees first direct evidence of light-by-light scattering at high energy
21–30 of 82 posts
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#22There'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…
I doubt that is the case. As the article nores photon-photon scattering has been a predicted for a long time by QED. Even if it wasn't detected before it's impact on redshift could be calculated.
If you look at the abstract on the linked nature page the interaction has a cross-section of 70nb, which corresponds to a circle of radius 1.5e-9 nanometer[1]. It might occur a few times near a supernova or the swirl of a an accretion disk but I doubt it has a meaningful effect in deep-space.
A more exciting result would have been if they wouldn't have seen this effect. That would mean the theory was wrong and we had a new datapoint to look at.
[1]: these measurements are only for a specific energy range, they might vary dramatically with different energy levels but the key point is that this result agrees with theoretical predictions meaning that is should also be possible to calculate the contribution to redshift due to photon-photon scattering.
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#23The bit about the trigger was interesting. Is that software/firmware? Or is that a hardware trigger they are discussing.
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.
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.
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#24There'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…
> 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.
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#25Anyone 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?
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#26There'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…
do you mean interaction between high energy gamma rays from distant quasars? That could change background radiation density, right?
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#27There'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…
I doubt that is the case. As the article nores photon-photon scattering has been a predicted for a long time by QED. Even if it wasn't detected before it's impact on redshift could be calculated. If you look at the abstract on the linked nature page the interaction has a cross-section of 70nb, which corresponds to a circle of radius 1.5e-9 nanometer[1]. It might occur a few times near a supernova or the swirl of a an…
but I doubt it has a meaningful effect in deep-space.
Don't underestimate how often unlikely events can happen in a big enough space...
As to photon-photon scattering, I might be wrong, but I don't believe it's considered in any models for expansion - similarly, if you do build light momentum transfer into your model, then expansion goes away - but because we "know" expansion to be true, those results aren't considered or published.
I think our givens are wrong. It'd hardly be the first time. I think I might be wrong. That'd hardly be the first time either.
But, as a betting physicist, my money is on expansion (at the very least acceleration) being bunkum.
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#28There'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…
> 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…
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.
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#29Anyone 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?
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.
Re: ATLAS sees first direct evidence of light-by-light scattering at high energy
#30Can someone explain this in easier terms for a layman like myself?
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 fin…
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).