StringJunky Posted October 2 Share Posted October 2 Quote Abstract When a pulse of light traverses a material, it incurs a time delay referred to as the group delay. Should the group delay experienced by photons be attributed to the time they spend as atomic excitations? However reasonable this connection may seem, it appears problematic when the frequency of the light is close to the atomic resonance, as the group delay becomes negative in this regime. To address this question, we use the cross-Kerr effect to probe the degree of atomic excitation caused by a resonant transmitted photon, by measuring the phase shift on a separate beam that is weak and off-resonant. Our results, over a range of pulse durations and optical depths, are consistent with the recent theoretical prediction that the mean atomic excitation time caused by a transmitted photon (as measured via the time integral of the observed phase shift) equals the group delay experienced by the light. Specifically, we measure mean atomic excitation times ranging from (−0.82±0.31)τ0 for the most narrowband pulse to (0.54±0.28)τ0 for the most broadband pulse, where τ0 is the non-post-selected excitation time, given by the scattering (absorption) probability multiplied by the atomic lifetime τsp. These results suggest that negative values taken by times such as the group delay have more physical significance than has generally been appreciated. https://arxiv.org/abs/2409.03680 This caught my eye in my FB feed. Thought it might interest a few here. 2 Link to comment Share on other sites More sharing options...
studiot Posted October 2 Share Posted October 2 (edited) 5 minutes ago, StringJunky said: Thought it might interest a few here. Thank you it does, +1 Sounds like the (Ivor) Catt effect again. I will investigate further. 7 minutes ago, StringJunky said: Should the group delay experienced by photons be attributed to the time they spend as atomic excitations? However reasonable this connection may seem, it appears problematic when the frequency of the light is close to the atomic resonance, as the group delay becomes negative in this regime. Thinking about your quote, Phase delay comparison is only appropriate for continuous waves. It is inappropriate for one which is absorbed and re-emitted at some random phase. Perhaps the full text will explain it better. Edited October 2 by studiot 1 Link to comment Share on other sites More sharing options...
swansont Posted October 2 Share Posted October 2 Last time this came up it was because the pulse width of the light was ignored. I notice that the article I read said that “there appears” to be a negative time, not there actually is. Link to comment Share on other sites More sharing options...
StringJunky Posted October 2 Author Share Posted October 2 6 hours ago, swansont said: Last time this came up it was because the pulse width of the light was ignored. I notice that the article I read said that “there appears” to be a negative time, not there actually is. Would the link I gave be a progression from that conclusion, having eliminated the pulse width of light, or taken it into account, and the anomaly is still there, like it says at the end: Quote These results suggest that negative values taken by times such as the group delay have more physical significance than has generally been appreciated Link to comment Share on other sites More sharing options...
exchemist Posted October 3 Share Posted October 3 (edited) 22 hours ago, StringJunky said: This caught my eye in my FB feed. Thought it might interest a few here. I'm not great on the physics of this but my likely imperfect understanding is there is a change in phase velocity, when the frequency of the light is close to an absorption band of the medium. This arises due to an increasing degree of coupling of the atoms of the medium with the EM field, as the frequency approaches the absorption frequency more closely. The resulting phase velocity can be either <c or >c depending which side of the absorption frequency you are. But this seems to be talking about the group velocity, not the phase velocity. I hadn't realised this too could be slowed down by passage through the medium. Is that what they mean or am I misinterpreting it? It crossed my mind that this "-ve time" thing could perhaps be associated with reconciling a phase velocity >c with the fact that no actual information is transmitted >c. Can anyone shed light (haha) on this for me? Edited October 3 by exchemist Link to comment Share on other sites More sharing options...
StringJunky Posted October 3 Author Share Posted October 3 1 minute ago, exchemist said: I'm not great on the physics of this but my likely imperfect understanding is there is a change in phase velocity, when the frequency of the light is close to an absorption band of the medium. This arises due to an increasing degree of coupling of the atoms of the medium with the EM field, as the frequency approaches the absorption frequency more closely. The resulting phase velocity can be either <c or >c depending which side of the absorption frequency you are. But this seems to be talking about the group velocity, not the phase velocity. I hadn't realised this too could be slowed down by passage through the medium. Is that what they mean or am I misinterpreting it? It's way outside what I know. Link to comment Share on other sites More sharing options...
exchemist Posted October 3 Share Posted October 3 1 minute ago, StringJunky said: It's way outside what I know. OK let's see if someone else bites. (Hope not just Mordred, as he tends put up walls of Greek at the drop of a hat.😄) Link to comment Share on other sites More sharing options...
StringJunky Posted October 3 Author Share Posted October 3 4 hours ago, exchemist said: OK let's see if someone else bites. (Hope not just Mordred, as he tends put up walls of Greek at the drop of a hat.😄) Link to comment Share on other sites More sharing options...
Mordred Posted October 3 Share Posted October 3 6 hours ago, exchemist said: OK let's see if someone else bites. (Hope not just Mordred, as he tends put up walls of Greek at the drop of a hat.😄) Gee didn't know I was bilingual lmao. You can relax on this particular topic lol. Though I'm still studying the paper. I do agree with Swansont's assessment. 1 Link to comment Share on other sites More sharing options...
exchemist Posted October 3 Share Posted October 3 46 minutes ago, Mordred said: Gee didn't know I was bilingual lmao. You can relax on this particular topic lol. Though I'm still studying the paper. I do agree with Swansont's assessment. 👍 Link to comment Share on other sites More sharing options...
toucana Posted October 3 Share Posted October 3 7 hours ago, exchemist said: I'm not great on the physics of this but my likely imperfect understanding is there is a change in phase velocity, when the frequency of the light is close to an absorption band of the medium. This arises due to an increasing degree of coupling of the atoms of the medium with the EM field, as the frequency approaches the absorption frequency more closely. The resulting phase velocity can be either <c or >c depending which side of the absorption frequency you are. But this seems to be talking about the group velocity, not the phase velocity. I hadn't realised this too could be slowed down by passage through the medium. Is that what they mean or am I misinterpreting it? It crossed my mind that this "-ve time" thing could perhaps be associated with reconciling a phase velocity >c with the fact that no actual information is transmitted >c. Can anyone shed light (haha) on this for me? Sabine Hossenfelder has just posted a YT video about this paper within the last hour: https://www.youtube.com/watch?v=ErLHm-1c6I4 And yes, it is the group velocity of wave packets, not the phase velocity of individual packet elements that is under discussion here. The conclusion seems to be that no violation of causality is involved here - so you can stand down your time-machine construction projects 😉 2 Link to comment Share on other sites More sharing options...
Mordred Posted October 3 Share Posted October 3 1 hour ago, toucana said: Sabine Hossenfelder has just posted a YT video about this paper within the last hour: https://www.youtube.com/watch?v=ErLHm-1c6I4 And yes, it is the group velocity of wave packets, not the phase velocity of individual packet elements that is under discussion here. The conclusion seems to be that no violation of causality is involved here - so you can stand down your time-machine construction projects 😉 Thanks nice catch on a recent link +1 Link to comment Share on other sites More sharing options...
exchemist Posted October 3 Share Posted October 3 1 hour ago, toucana said: Sabine Hossenfelder has just posted a YT video about this paper within the last hour: https://www.youtube.com/watch?v=ErLHm-1c6I4 And yes, it is the group velocity of wave packets, not the phase velocity of individual packet elements that is under discussion here. The conclusion seems to be that no violation of causality is involved here - so you can stand down your time-machine construction projects 😉 Aha, that's quite a nice explanation. Link to comment Share on other sites More sharing options...
Recommended Posts