What Are Some Discoveries at the Frontiers of Light Physics?
Light seems straightforward from a classical perspective. It gives us the ability to see and eat, for light bounces off objects into our eyes and lifeforms use light to power themselves and support the food chain. But when we take light to new extremes, we find new surprises waiting for us there. Here we present but a sampling of these new places and the insights they offer us.
Not a Universal Constant?
To be clear, the speed of light isn’t constant everywhere but can fluctuate based on the material it travels through. But in the absence of matter, light traveling in the vacuum of space should be moving at about 3*108 m/s. However, this doesn’t take into account virtual particles that can form in the vacuum of space as a consequence of quantum mechanics. Normally this isn’t a big issue because they form in anti-pairs and therefore cancel out rather quickly. But – and this is the catch – there is a chance that a photon could hit one of these virtual particles and have its energy reduced, therefore reducing its speed. Turns out, the amount of time drag per square meter of vacuum should be only about 0.05 femtoseconds, or 10-15 s. Very small. It can possibly be measured using lasers bouncing back and forth between mirrors in a vacuum (Emspak).
How Long Do They Live?
No photon has expired via decay mechanisms, where particles break down into new ones. This requires a particle to have mass, however, since the products will have mass too and energy conversion happens as well. We think that photons don’t have mass, but current estimates show that the most one could weigh is 2*10-54 kilograms. Also very small. Using this value, a photon should have at least a lifetime of 1 quintillion years. If true, then some photons have decayed because the lifespan is merely an average value and decay processes involve quantum principles. And the products would have to travel faster than photons, exceeding the universal speed limit we know of. Bad, right? Maybe not, because these particles still have mass and only a massless particle has unlimited speed (Choi).
Scientists have pushed camera technology to new limits when they developed a camera that records at 100 billion frames a second. Yes, you did not misread that. The trick is using streak imaging as opposed to stroboscopic imaging or shutter imaging. In the latter, light falls onto a collector and a shutter cuts off the light, allowing the image to be saved. However, the shutter can itself cause images to become less focused as less and less light falls into our collector as time decreases between shutter closings. With stroboscopic imaging, you keep the collector open and repeat the event as light pulses hit it. One can then build up each frame if the event ends up repeating itself and so we stack the frames and build up a clearer image. However, not many useful things we want to study repeat in the exact same way. With streak imaging, only a column of pixels in the collector is exposed as the light pulses on it. Though this seems limited in terms of dimensionality, compressive sensing can let us build what we would consider a 2D picture from this data by a frequency breakdown of the waves involved in the image (Lee “The”).
Certain materials can bend and manipulate the paths of photons and therefore can lead to new and exciting properties. One of these is a photonic crystal and it operates in a similar fashion to most materials but treats photons like electrons. To best understand this, think about the mechanics of photon-molecule interactions. The wavelength of a photon can be long, in fact a great deal more than that of a molecule and so the effects on each other are indirect and lead to what is known as the refractive index in optics. For an electron, it most certainly does interact with the material it moves through and therefore cancels itself out via destructive interference. By placing holes roughly every nanometer in our photonic crystals, we ensure that photons will have the same issue and create a photonic gap where if the wavelength falls into will prevent the transmission of the photon. The catch? If we want to use the crystal to manipulate light we usually end up destroying the crystal because of the energies involved. To solve this, scientists have developed a way to build a photonic crystal out of…plasma. Ionized gas. How can that be a crystal? Using lasers, interference and constructive bands are formed which don’t last long but allows for regeneration as needed (Lee “Photonic”).
High energy electrons offer many applications to physics, but who knew that they also generate special photons. These vortex photons have a "helical wave front" as opposed to the flat, planar version we are used to. Researchers at IMS were able to confirm their existence after looking at a double slit result from high energy electrons emitting these vortex photons, and at any wavelength that is desired. Just get the electron to the energy level you want and the vortex photon will have a corresponding wavelength. Another interesting consequence is a varing angular momentum associated with these photons (Katoh).
Imagine a wave of light that passes by without being displaced, even if an obstacle is in its way. Instead of rippling, it just passes by with little to no resistance. This is a superfluid-state for light and as crazy as it sounds it is real, according to work from CNR NANOTEC of Lecce in Italy. Normally, a superfluid exists at near absolute zero but if we couple light with electrons we form polaritons that exhibit superfluid properties at room temperature. This was achieved using a stream of organic molecules between two highly reflective surfaces, and with light bouncing around a lot coupling was achieved (Touchette).
Choi, Charles. “Photons Last At Least One Quintillion Years, New Study Of Light Particles Suggests.” Huffintonpost.com. Huffington Post, 30 Jul. 2013. Web. 23 Aug. 2018.
Emspak, Jesse. “Speed of Light May Not Be Constant After All, Physicists Say.” Huffingtonpost.com. Huffington Post, 28 Apr. 2013. Web. 23 Aug. 2018.
Katoh, Masahiro. "Vortex photons from electrons in circular motion." innovations-report.com. innovations report, 21 Jul. 2017. Web. 01 Apr. 2019.
Lee, Chris. “Photonic crystal club will no longer admit only puny lasers.” Arstechnica.com. Conte Nast., 23 Jun. 2016. Web. 24 Aug. 2018.
---. “The 100 billion frames per second camera that can image light itself.” Arstechnica.com. Conte Nast., 07 Jan. 2015. Web. 24 Aug. 2018.
Touchette, Annie. "A stream of superfluid light." innovations-report.com. innovations report, 06 Jun. 2017. Web. 26 Apr. 2019.
© 2019 Leonard Kelley