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ATLAS sees first direct evidence of light-by-light scattering at high energy
- rrmm 9y agoNeat-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 9y ago>> 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 9y agoIt is not gravitational. That effect would be far, far too weak.
- Florin_Andrei 9y agoFor that to happen, the energy-equivalent of the photons would have to be close to the Planck mass. That's an unbelievably high value for photons. We're not going to see that reproduced experimentally any time soon.
- ajross 9y agoYeah. The article is a little confused with terminology. Photon/photon interaction is "illegal" in QED in the same way and for the same reasons that wave solutions to classical electromagnetism don't interact (well, except to propagate). What's happening here are interactions between the two photons and the vacuum. So the new science here isn't unexpected confirmation of new physics, it's our ability to probe our existing understanding of the vacuum with new interactions.
- rrmm 9y agoRight, the new physics the wikipedia article is alluding to is that in previous experiments indirectly measuring two photon interaction cross-sections, the cross-section was higher than what the standard model would predict (but those experiments didn't have tight enough error bounds to say anything definitive).
- SolarNet 9y agoThe bit about the trigger was interesting. Is that software/firmware? Or is that a hardware trigger they are discussing.
- whyever 9y agoI 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.
- dukwon 9y agoSo is LHCb. The two experiments may share a data centre on the Prevessin site.
- pif 9y agoWhen 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 9y agoThey've since scrapped L2, now it's only L1 (HW) and the High Level Trigger (HLT) running on a server farm
- pif 9y agoThanks for the info. I left ATLAS beginning 2009, many other things evolved, I suppose.
- etatoby 9y ago
- platz 9y agopost from november http://cerncourier.com/cws/article/cern/66878 http://cerncourier.com/cws/article/cern/66878
- dogma1138 9y agoOne step closer to light sabers.
- etatoby 9y agoOne step closer to holograms.
- chi17 9y agoCould 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 9y agoThis is closer: http://www.pocket-lint.com/news/131622-holograms-are-finally-here-plasma-lasers-used-to-create-images-in-mid-air http://www.pocket-lint.com/news/131622-holograms-are-finally... 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.
- Coincoin 9y agoA real hologram, like in your last link, make the photons actually come from the correct angles so that everything is not in focus in your eye. Stereo imaging we see in current VR and TV technologies do not simulate that effect, which makes it hard for some people to use as everything is focused at infinity or on the screen. What the lens (in your eye) is currently focused on is a very important depth cue for our brain and the conflicting information coming from the lens and stereo vision make some people unable to use stereo imaging or cause eye pain and headaches. Unfortunately, real holograms are currently restricted to static images meticulously constructed using very advanced equipment that produces seemingly random images on films that interfere with each other to produce the hologram. Doing the same thing with moving images in real time would require too much computing power to currently be possible. Another way could be to track the lens refraction properties and the pupil size in real time and fake some depth of field on the projected stereo images.
- raverbashing 9y agoJust 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
- ChrisRR 9y agoInvisible light sabers. Cool
- raverbashing 9y agoJust make sure you have some sunscreen on
- yread 9y ago> Just make sure you have some sunscreen on in the thickness equivalent to 1cm lead sheet for a gamma-ray-screen factor of 2
- hinkley 9y agoSo NIST is telling me that's about 16cm of titanium. I suppose it would be easier to make yourself clothing out of full lotion bottles than try to slather that much on.
- Taniwha 9y agogiven this is generated by accelerating lead atoms - your light sabre should be quite capable of covering you (and everything around you) with tasty lead
- anfractuosity 9y agoElectrolasers 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.
- 9y ago
- staticelf 9y agoCan someone explain this in easier terms for a layman like myself?
- stephengillie 9y agoThey think they got photons to bounce off each other. It's a really old prediction but really rare, so hard to prove. They think it happened 13 times across 4 billion events.
- nimish 9y agoPhotons 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 9y agoHigh 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 9y ago> 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?
- madaxe_again 9y agoThere'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.
- jpttsn 9y agoThe way I've always heard it, as a layman, is that red shift could be explained only by an expanding universe. I'm sure there's more rigor to it than that, but it always seem s like a pretty big conclusion. A mildly surprising claim explained by a wildly surprising theory.
- madaxe_again 9y agoAs a physics grad, red shift can only be explained by an expanding universe with our understanding of light physics in the mid 20th century. If you look at the world only through green glasses, you could be forgiven for thinking that everything was green. Your interpretation of observations is only as good as the theory you use to interpret. Think of epicycles, which for millenia seemed obvious and correct, until heliocentricity (for all bodies, not just earth) became the dominant model due to improved theory - but no new observations. If light behaves even slightly differently to how we currently believe, everything from galactic rotation to expansion goes out of the window. Here's the bit on momentum transfer - they even explicitly cite the implication for Hubble expansion. https://www.sciencedaily.com/releases/2017/06/170630085627.htm https://www.sciencedaily.com/releases/2017/06/170630085627.h...
- contact_fusion 9y agoThe article referenced in that piece is available here, from the arxiv: https://arxiv.org/pdf/1603.07224.pdf https://arxiv.org/pdf/1603.07224.pdf The article has been up for some time, but has just now been published by Physical Review A; it is common practice for physicists to post a preprint on arxiv prior to publication, which happens after peer-review. The momentum paradox you refer to is (possibly?) the Abraham-Minkowski controversy ( https://en.wikipedia.org/wiki/Abraham-Minkowski_controversy https://en.wikipedia.org/wiki/Abraham-Minkowski_controversy ) about electromagnetic momentum in dielectric media. I'm not an expert on this subject, but I would doubt that this new work definitively settles the controversy. Of course this is, no doubt, work towards settling the issue. My (limited) understanding is that the controversy is really about interpreting certain quantities that behave like momentum in certain contexts, and which contexts apply in certain experiments to measure them. I do not believe it constitutes a crisis in our understanding of light; this is a very technical detail. Of course, using the mechanism of momentum transfer to the transmitting medium as an explanation of redshift - and by doing so, refuting the expansion of the universe - is just yet another "tired light" explanation. (This refers to the idea of explaining redshift through a path-dependent loss of energy for photons traveling from great distances.) This is not a new notion, dating at least to Zwicky in the fifties. This article ( https://arxiv.org/abs/astro-ph/0106566 https://arxiv.org/abs/astro-ph/0106566 ) details efforts to demonstrate the reality of this expansion. These efforts do not assume anything about the exact mechanism responsible for tired light, merely the notion that light loses energy as it travels. They refute this to better than 10 sigma. >with our understanding of light physics in the mid 20th century. >If light behaves even slightly differently to how we currently believe, everything from galactic rotation to expansion goes out of the window. I... suppose I have to admit that if the current theory of light is wrong, then there may be changes to how we interpret these results. You should know though that this would be extremely unlikely. Generally revolutions in physics tend to subsume the effective results of the theories that are replaced. Quantum mechanics provides a good example: if you take the limit h -> 0 (making Planck's constant zero), you recover classical mechanics. Relativity provides another: if you take the limit c -> infinity in relativity, you also recover classical mechanics. Our understanding of light is quite good. Forgive me, but I'm detecting a little bit of an "international scientific conspiracy" vibe here. Unpopular work is published all the time, provided that it withstands scientific scrutiny. I know that sounds like I'm dodging the issue, but really, you can apply a cui bono here: what do scientists stand to gain by propping up "wrong" science? We don't get paid a lot, you know. >Think of epicycles, which for millenia seemed obvious and correct, until heliocentricity (for all bodies, not just earth) became the dominant model due to improved theory - but no new observations. I don't mean to pick nits, but it was improved observations (Kepler, Brahe, etc.) that drove the acceptance of the heliocentric model. Kepler was famous partly due to his unprecedentedly accurate measurements. Theory was not necessarily rigorous at the time, often referencing theological arguments; heliocentrism was hotly debated, but not novel. Later it was realized that epicycles form a basis set for any trajectory on the surface of a sphere; any trajectory can be reproduced using a sufficiently large number of them. This was a pitfall that astronomers of the time could never have known. Remember that they did not have Newton's insights yet. On a final note, its great to hear that ATLAS has some good evidence for gamma-gamma interaction! edit: fix link
- yohann305 9y agoAnyone 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 9y agoCurrently, 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 9y agoI'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?
- beojan 9y ago> 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? And hence have discovered GR in the process. Alternatively (and more likely) we would have decided the idea simply doesn't work.
- jerf 9y ago
- deleted 9y ago[deleted]
- awinter-py 9y agolight seeking light doth light of light beguile