Light can be reversed not only in space but also in time – as researchers exploring such “temporal reflections” are discovering a range of strange and incredibly useful effects.
Can we reverse time?
Many plans exist for backward time travel, but they usually involve inconsistencies that cannot be reconciled and depend on strange theoretical constructs like wormholes (which may not actually exist). However, when it comes to simply reversing the clock – like stirring a raw egg and watching the yolk and whites separate again – a rich and growing subfield of wave physics shows that such “time reversal” is possible.
Temporal reflection
Peers have been looking into temporal reflections for decades, but they have proven difficult to achieve practically because changing the optical properties of materials quickly and sufficiently is no small feat. However, researchers at the City University of New York have made a breakthrough: creating light-based temporal reflections.
The anti-timing effect
To do this, physicist Andrea Alù and his colleagues developed a “metamaterial” with adjustable optical properties that they can tune in a fraction of a nanosecond to decrease or increase the speed of light passing through. The metamaterial determines its properties based on its structures; many consist of arrays of microscopic rods or rings that can be tuned to interact with and manipulate light in ways that natural materials cannot. Using their strengths in temporal reflections, Alù says they have uncovered some surprises. “We now realize that [temporal reflections] can be richer than we thought because of the way we implement them,” he adds.
Anti-timing effects
Temporal reflections come with a suite of anti-timing effects that have been theoretically predicted but never experimentally tested with light. For instance, what is at the start of the original signal will be at the end of the reflected signal – a situation akin to looking at yourself in a mirror and seeing behind your head. Moreover, while a standard reflection changes how light travels through space, a temporal reflection changes the temporal components of light – that is, its frequencies. As a result, in the temporally reflected view, the back of your head is a different color. Alù and his colleagues have observed these effects in the team’s device. Together, they hold promise for advancing progress in the fields of signal processing and communications – two vital areas for your smartphone’s function, which relies on effects like frequency conversion.
Unraveling physics
Readers well-versed in the laws of physics can rest assured that Alù’s device does not violate principles of thermodynamics. For instance, the device does not create a source or destroy energy but simply converts it efficiently from one form to another – the energy gained or lost by the waves comes from that added or subtracted to change the properties of the metamaterial. But what about increasing disorder – entropy – over time, as thermodynamics states? How is light’s time reversal not equivalent to un-scrambling an egg?
As John Pendry, a physicist specializing in metamaterials at Imperial College London, explains, reversing light, although it seems strange, is completely consistent with the strict principles of thermodynamics. He says that an increase in entropy is really a matter of information loss. For example, lining up schoolchildren alphabetically, someone would know exactly where each child can be found. But let them wander on the playground, and there are a huge number of different ways that the children could be arranged, which equates to an increase in entropy, losing the information you had to locate each child. “If [something] is time-reversible, that means you’re not generating entropy,” Pendry says, even if it seems like you are. For instance, although the children are still running around to play, they know which lines to form to return to class when the bell rings – thus, entropy is not produced. “You’re not losing information,” he says.
Other Optical Phenomena
Reflection is not the only optical phenomenon that receives treatment in the time domain. In April, Bendry and a team of researchers, including Ricardo Sapienza from Imperial College London, conducted a temporal similarity experiment based on an ancient classic experiment that ultimately played a key role in confirming the wave nature of light. The physicist Thomas Young conducted in 1801, called the “double-slit experiment,” provided undeniable evidence of the wave nature of light, even in the face of later evidence that light behaves like a particle; scientists could only deduce that both descriptions apply. When a wave is sent to a barrier with two openings, the waves emerging from one slit will interfere with those emanating from the other. With light, this constructive and destructive interference appears on a screen beyond the double slit as multiple bright lines or “stripes.” Sapienza, Bendry, and their colleagues used indium tin oxide (ITO), a material that reacts quickly and changes from transparent to opaque, to produce “temporal slits.” They demonstrated that the light beam interacting with the temporal slits produces a corresponding interference pattern in frequency, which was utilized as a temporal similarity – meaning there are bright light lines at different frequencies.
Unraveling Physics
According to Inguita, what drives the experiments that swap time and space in optical effects is “the exciting and novel features we can find in the physics of light-matter interaction.” And there are many. Bendry describes with a smile how his temporal explorations with transformable materials revealed “very strange things,” including what he calls a “photon compressor.” Bendry’s photon compressor is a transformable material patterned with areas of different optical properties that affect the speed of light propagation. The lines can be tuned, forming a type of “metallic grid,” and when this metallic grid moves through the transformable material alongside light, it can effectively trap and steer photons together, compressing them. Further investigations also revealed that this type of photon compressor shares some characteristics with black holes, providing a more manageable laboratory model for studying those extreme astronomical objects. After unveiling a completely new temporal dimension for transformable materials, the photon compressors of black holes are just one of the strange phenomena that can be explored, and the possibilities are limitless.
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Source: https://www.scientificamerican.com/article/light-can-travel-backward-in-time-sort-of/
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