Owing to the wave nature of light there are many ways that such different waves can interact with each other, but also with materials. Everyone knows about reflecting light with a mirror, which is a property of materials like metals, but specific structures can cause the light to behave in a way that creates rather amazing results.
Examples of this are cases of iridescence in nature (like butterfly wings) and eye color, where the perceived colors are the result of environmental light interacting with these structures rather than pigmentation or dyes. An even more interesting interaction has now been demonstrated by reflecting multiple microwave radiation beams off each other, creating a time reflection.
The study by [Emanuele Galiffi] et al. (shared copy) was published in Nature Physics. By creating a metamaterial that allows for temporal coherent wave control (CWC) the electromagnetic radiation was controlled to where it allowed for this kind of unusual interaction. The key here being that there is no major constructive or destructive interaction between the two waves as with spatial CWC, rather the wave reflect off each other, or more specifically the time interface.
Although the popular reporting talks about ‘turning back time’ and ‘watching the back of your own head in a mirror’, the impact is far less dramatic: in the article conclusion the researchers mention unveiling new light-matter interactions in the microwave- and other parts of the spectrum, as well as new ways to control and shape light.
Top image: Temporal coherent wave control and photonic collisions enabled by time-interfaces. (Credit: Emanuele Galiffi et al., Nature Physics, 2023)
Cool, and somewhat weird. Not sure I understood this. Could this perhaps make giant space telescopes more practically realizeable? It would be so cool if we one day could directly image extrasolar planets in great detail…
I believe this allows us to now realize non-causal filters and “things” meaning, stuff that “goes back in time” that can infer information can be used instead. Also in terms of using items in the frequency domain we’re able to use the energy of both +/- frequency range now instead of one half of the spectrum.
Most filter theory assumes a Linear time invariant and causal outcome, because there was a notion that “You can never go back in time”. Likely this will allow for innovations in extremely efficient and lower power optiical and electrical materials. Someone can correct me if I’m wrong but if you can use the other half of the spectrum then you”re approaching 2x efficiency based on causal only filtering and methods.
In english -> we can manipulate signals (electrical, optical, etc) without the need to add more power to a system and use the waves and their interactions to perform the amplication/filtering using the physics of the wave and temporal properties gathered.
” Someone can correct me if I’m wrong but if you can use the other half of the spectrum”
You mean like negative Hertz?
(This is over my head)
Tldr: All filter theory assumes causal systems (t >0), this means anything that creates a mirror in time ( t <0 ) does wonky things and needs to be processed differentlt
More detailed:
So any “thing” that modifies signals in any way (filter, amplifier, attenuator, buffer) takes energy.
Useful things to look up: Fourier Transform
Two forms of analyzing a signal are time domain and frequency domain. Freqeuncy is done through Fourier analysis via the mathematical concept of the Fourier Transform. Thus you can map a time domain process (filtering, amplifying) to the frequency space. Meaning, positive time, maps to positive frequency. You can mathematically map signals this way when you analyze them as well.
The Fourier transform however has interesting properties namely that of symmetry. A pure cosine wave or cos(wt), where w = angular frequency, has a mirror image of peaks across the y axis, at positive and negative w, the angulsr frequency. To use this, we typically use another function to get the positive side and perform other operations on the signal to remove the negative component, not without mathemstical consequences. For things like energy the total energy of the wave can be derived by something called Parseval’s Theorem using a square of the freq spectrum, since it takes both components ands squares things, but for operations that don’t require that, you’re performing math to get a useable signal.
The difference here is that causal systems can only work in positive frequency and our math and design tools works in positive time, and we’re limited in how we can modify a signal since we’re always doing something to the positive side of the frequency graph.
In this papers case though, if you can create some sort of “filter” that creates a mirror in the time domain that does map to negative frequency and thus you can modify the negative side of the spectrum. This means we can achieve different more efficient filters with potentially less “information” or energy since we’re not cutting out half the graph.
As far as I remember, negative frequency just means that vector rotates “backward”. I can only guess that this is exactly what happens if you reverse time direction.
Thanks to both of you for explaining that to me, I’m beginning to catch on!
And [Ted], I believe you misspelled “mathemystical”!
B^)
Science Fiction author Wil McCarthy is a pioneer in metamaterials, holding several patents and authoring the nonfiction book Hacking Matter. He explored the extreme possibilities in a very fun book series “The Queendom of Sol”, starting with The Collapsium, The Wellstone (his name for the primary metamaterial), Lost in Transmission, and To Crush the Moon.
What an unfortunate abbreviation