7 ms·
Atlas observes light scattering off light
- mmmBacon 8y agoLight 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-light/ https://physicsworld.com/a/light-is-seen-to-scatter-off-ligh... 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
- SiempreViernes 8y agoI think this mainly centres on just what should count, here[0] is a paper from 1996 about using astrophysical gamma ray sources to detect the effect of the intergalactic background light, which is also photon photon interactions. [0]: http://adsabs.harvard.edu/full/1996ApJ...456..124M http://adsabs.harvard.edu/full/1996ApJ...456..124M
- keyle 8y agoAny potentially practical use?
- not_kurt_godel 8y agoPurely speculating as a physics-interested layperson, but I imagine this probably either changes or confirms/enhances confidence of various cosmological observations where photon-photon interaction might play a role. For example, maybe certain parameters/characteristics of the CMB have uncertainties potentially related to this phenomenon which can now be tightened up, which could have ramifications for inflation theory, Higgs/particle theory, string theory, etc
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- rohan1024 8y agoNot a physics expert. Projecting in the air without a screen could be a potential use.
- sudhirj 8y agoThat still seems impractical, at least until we miniaturize a particle accelerator. The event is so improbable (two photons banging into each other) that you need a super dense cloud of them to get a measurable effect.
- db48x 8y agoLol, 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.
- YaxelPerez 8y agoI know a couple rouge nation-states that might be interested
- SomeHacker44 8y agoRouge? Communist? “Better dead than rouge?” Hm.
- edf13 8y agoThe lol isn’t needed
- lugg 8y agoNeither is the rouge but here we are.
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- Iwan-Zotow 8y agoSure making real Jedi saber
- igravious 8y agoThank you Would have been nice if that info was featured somewhere in the article. I don't like to say it but this am I correct in saying that this article gives off the impression that this is an experimental first in terms of directly observing this photonic interaction?
- Blackbeard_ 8y agoThey do try to mitigate this criticism in the first line of the article: "Light-by-light scattering is a very rare phenomenon in which two photons – particles of light – interact, producing again a pair of photons." This is the process that they observe (for the first time) whereas E144 observed the Breit-Wheeler process. You can argue that this is too narrow a definition of light-by-light scattering but they have at least tried to make it self consistent...
- cozzyd 8y agoLight 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).
- saagarjha 8y agoHow do we detect high-energy cosmic rays, which to my knowledge can exceed this "limit" by several orders of magnitude?
- marcosdumay 8y agoIf 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
- saagarjha 8y agoThat 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!
- adrianratnapala 8y agoThose rays are unlikely to be photons, so EM-self interaction is not likely to stop them. The articles linked in this sub-thread all speculate/assume that the cosmic rays were protons.
- cozzyd 8y agoThey're not gamma rays :) But a similar limit happens to protons (the GZK limit at 50 EeV, where protons and CMB photons can make a Delta+). For heavier nuclei, photodisintegration will happen at even higher energies.
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- pieterk 8y agoDumb question but is this unexpected because photons are bosons?
- orbifold 8y agoThis 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.
- gus_massa 8y agoI 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.
- jcims 8y agoIf 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?
- kijin 8y ago> what time is it when they interact? In which frame of reference?
- jcims 8y agoI 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.
- kijin 8y agoThe 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!
- plibither8 8y agoQuantum 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 :)