8 ms·
The dark side of light: negative frequency photons
- grueful 14y agoHere's the paper (PDF): http://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.108.253901 http://physics.aps.org/featured-article-pdf/10.1103/PhysRevL... It's about observations & theory where you can get waves in Cherenkov radiation which propagate against the dominant direction.
- ChuckMcM 14y agoThe interesting bit is that solving the equations gives you a solution with a negative frequency. Generally those negative frequencies are ignored because for most people a negative light frequency doesn't make sense. But if you extend the analysis further you get a positive frequency light out of interactions with that negative frequency light. And the paper talks a bit about observing those second order effects: "Here, we have shown how the same process generates a second, so-far-unnoticed peak that corresponds to resonant transfer of energy to the negative-frequency branch of the dispersion relation. " which means the negative frequency light was something but what it means isn't clear. It could be the tip of a new way at looking at light, or it could be nothing. Some of the experiments it suggests with respect to gravity waves are interesting.
- beagle3 14y agoAt the turn of the 19th century, physicists were quite sure that they had a complete model of the universe, with just two minor unsolved problems: The aberration of light / michelson-morley experiment (which required the theory of relativity to explain), black body radiation (which was eventually explained by quantum mechanics). Actually solving these unsolved problems turned physics on its head. Negative frequency light might be just a weird artifact. Or it might turn physics on its head. I'd put money on the former, though I hope for the latter. edit: fix typo "native frequency" -> "negative frequency"
- cgs1019 14y agoThose examples were deviations of reality from prediction/model. This is sort of the opposite -- the model predicts negative frequency light but we ignore this because it doesn't match what we thought reality was doing. Now we see that there is some tangible real-ness to these solutions. It may have interesting applications, but it wouldn't require us to change the theory (since the theory already predicts it correctly).
- JonnieCache 14y agoThere's a lot of this kind of thing about. Black holes and the big bang arguably started life in this category, no?
- WiseWeasel 14y agoI still don't get what a negative frequency is. The frequency is the number of times something happens in a given amount of time. Would a negative frequency be the number of events that were expected to happen but didn't? If so, how could this be differentiated from an error in our expectations.
- grueful 14y agoPhysics often uses the sign of a frequency to indicate the direction of travel with regard to a reference frame. E.g. 2 Hz is a wave oscillating two times per second and travelling to the right, -2 Hz is the same travelling to the left. In some cases direction isn't really meaningful (standing waves, or the height or pressure of a medium as seen by a stationary observer). In others, the direction of propagation can be quite significant.
- scoith 14y agoThis is not true. A wave propagates both in time and space. You're mixing these two. A negative frequency is like a wave travelling backwards in time, not left/right spatial direction.
- davorak 14y agoI have seen a negative sign grouped with frequency. Which was used to distinguish directionality. Most of the time it is understood from context what is meant, if not the speakers fall back on to more something more formal to ensure understanding.
- 46Bit 14y agoNot quite. A wave going left forward in time is equivalent to a wave 'going right' but backwards in time. Since it is further right in the past than now, it is actually going left. There's a lot of similar effects in Physics, especially with time and/or antimatter.
- scoith 14y ago
- crusso 14y agoI love the way that math reveals ultimate truths. We consider math to be some abstract thing that we're applying to help explain reality. Instead, it often appears that the abstraction is somehow the True Reality(tm), and our reality is really just an imperfect view of Math(tm). We view math about like we view electromagnetic waves with our eyes. We see effects of it and reflections and complex interactions of only small parts of the em spectrum. It's interesting to wonder if we could develop a way to "see math" in its pure form. (Sorry, Friday afternoon speculation... not currently taking any drugs.)
- sheeps 14y agohttp://en.wikipedia.org/wiki/The_Unreasonable_Effectiveness_of_Mathematics_in_the_Natural_Sciences http://en.wikipedia.org/wiki/The_Unreasonable_Effectiveness_...
- kjhughes 14y agoMath is but a medium for modeling. Any model can have predictive power. The challenge is in determining which surprising aspects of the model have faithful correspondence to the domain and which are nonessential artifacts of the modeling medium.
- jblow 14y agoNo. The surprising thing is that the models generate predictions far beyond the domains they were designed for (and far beyond the original knowledge of the people making the models), and that the predictions are so mindbogglingly accurate that there seems to be Something Else going on. See the Unreasonable Effectiveness of Mathematics link below.
- kjhughes 14y agoNo? Surprising thing: Math's not the only modeling medium that can be Unreasonably Effective. Unreasonable Effectiveness of Mathematics: Know; saw; upvoted apropos reference already.
- phaedrus 14y agoThis reminds me of P and N silicon; the negative frequency light is like hole current. (N silicon has "extra" electrons. P silicon has "missing" electrons that leave "holes", and you can have current where it's the holes that are moving.)
- nsxwolf 14y agoOk, so, this doesn't mean I can have a light bulb that makes the room dark?
- aortega 14y agoYou don't need anything fancy like negative frequency light bulb for that, only a regular light bulb that emits light 180 degree phase shifted to the already existing light in the room. It's actually impossible to do with a light bulb because the light that emits is not coherent but you can do it with a laser if the incoming light is also from a laser, in fact negative and positive laser interference is the basis for several technologies such as holograms.
- sponk 14y agoHow exactly would light at "negative green" frequency be perceived by an eye/camera, when mixed with an equal amount of regular green? Would it cancel out, would the negative light have no effect, or would the powers add constructively?
- rcthompson 14y agoThe article doesn't indicate what the physical manifestation of negative-frequency wave would be. It only reports the observation of a positive-frequency wave that seems to require the transient existence of a negative-frequency intermediate. This is similar to how particles like the Higgs boson are not directly detected in particle accelerators, but are inferred from the directly-observable particles that they produce. The difference is that physicists are (I think) pretty sure that the Higgs actually physically exists, if only for a short time, while the same is not clear for the negative-frequency intermediate wave described here.
- rcthompson 14y agoThis reminds me a lot of the observation that the double-slit experiment still produces interference patterns when you send particles instead of waves, and them even when you send those particles one-by-one in order to exclude the possibility of inter-particle interference.
- erpa1119 14y agodoes this open the door to possibly traveling back in time?
- gus_massa 14y agoI read the original article that is much clearer about the details ( http://news.ycombinator.com/item?id=4429424 http://news.ycombinator.com/item?id=4429424 ). My explanation of the effect is long a bit technical. I hope that it is intelligible. (To keep this simple, I will ignore the phases of the waves.) * * * Complex notation: First, the equation for the electric field of the light is E = A cos(kz-wt) It's more convenient to write it as a sum of complex exponentials E = A [exp(i(kz-wt)) + exp(i(-kz-(-w)t))] /2 E = Re( A [exp(i(kz-wt))]) By the linearity of the equations, the exp(i(kz-wt)) part and the exp(i(-kz-(-w)t)) have the same behavior, so you usually write simply "E" = A exp(i(kz-wt)) and solve everything as if it where a complex function, but just before writing the final version or going to the laboratory you must remember that the other part was there, and that the real physical object is the real part of the function. * * * Standard non linearity effects: If the media is linear but not uniform, there appear other waves that travel in other direction z' (reflection/refractions). All of them have the same w. "E_tot" = A exp(i(kz-wt)) + A' exp(i(kz'-wt)) Really all of them have two parts, one with w and the other with -w, E_tot = A/2 [exp(i(kz-wt))+ exp(i(-kz-(-w)t))] + A'/2 [exp(i(k'z-wt))+ exp(i(-kz-(-w)t))] but usually you simply ignore that details, and only put a +cc (complex conjugate) or Re at the last minute. If the media is no linear there can appear waves with a different frequency w'. "E_tot" = A exp(i(kz-wt)) + A' exp(i(k'z-w't)) (There can appear more than two exponentials.) Again they have two parts, and the real thing is the real part. It's more clear to choose w' as a positive number, because (-w') will appear in the hidden part of the equation. E_tot = Re("E_tot") * * * New non linearity effects in this article: In this article they use a very sharp pulse in a very non linear material. So, from the "E" = A exp(i(kz-wt)) part they get two new exponentials "E_tot" = A exp(i(kz-wt)) + A' exp(i(k'z-w't)) + A'' exp(i(k_n''z-w_n''t)) where k' and w' are positive numbers as expected. But k_n'' and w_n'' are negative numbers!! They get this numbers from the same equation that has k' and w' as a solution, so all of them appear from the same mathematical term. They call this negative solution "NRR". But it is important to remember that the original E has two exponentials, so you must repeat all the computations with the other part "E*" = A exp(i(-kz-(-w)t)) everything is equivalent, so after some recalculations you get "E*_tot" = A exp(i(-kz-(-w)t)) + A' exp(i(-k'z-(-w')t)) + A'' exp(i(-k_n''z-(-w_n'')t)) where every k and every w has an additional "-". They call this part "NRR* ". But now -k_n'' and -w_n'' are positive numbers. We can change the names, and call k'' = - k_n'' w'' = - w_n'' and now k'' and w'' are positive numbers. So the first part of the solution is now "E_tot" = A exp(i(kz-wt)) + A' exp(i(k'z-w't)) + A'' exp(i(-k''z-(-w'')t)) and the second part is "E*_tot" = A exp(i(-kz-(-w)t)) + A' exp(i(-k'z-(-w')t)) + A'' exp(i(k''z-w''t)) And the real physical object is E_tot = ("E_tot"+"E*_tot")/2 So we can regroup the term. We exchange the terms with k'' and w'' that have the wrong signs from on part to the other, because the sum doesn't change. "E_totx" = A exp(i(kz-wt)) + A' exp(i(k'z-w't)) + A'' exp(i(k''z-w''t)) and the second part is "E*_totx" = A exp(i(-kz-(-w)t)) + A' exp(i(-k'z-(-w')t)) + A'' exp(i(-k''z-(-w'')t)) and as before E_tot = ("E_totx"+"E*_totx")/2 Now the interpretation of "E_totx" is straightforward. From the original field "E" you get three waves, with frequencies w, w' and w'', all of them positive. And in "E* _totx" is the complex conjugate part, so the final result is real. And they can measure the three waves. * * * Notes: Usually, the A'' coefficient is so small that all this strange part can be ignored, but they were able to measure it in the laboratory. One important detail is that k/w, k'/w' and k_n''/w_n''= k''/w'' are all positive, so they represent waves that travel in the same direction.