By Maria Anna van Driel
It goes through walls with the speed of 299792458 metres per second (approximately 300000 km/s (186000 mi/s)), but slows to a standstill in ultra-cold gases. It carries electronic information for radios and TVs, but destroys genetic information in cells. It bends around buildings and squeezes through pinholes, but ricochets off tiny electrons. It is “light” and it is made of photons. Radio waves are made of photons. X-rays are, you got it… made of photons.
From low energy radio waves to high energy gamma rays, light zips around us, bounces off us, and sometimes goes through us. Because it is so many things, defining light is a bit of a philosophical quandary. And although scientists have calculated the amount of visible light released into the universe by stars since the universe’s origin equals 4 x 10^84 photons, or, if you prefer, 4,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 photons, we know it primarily as the opposite of darkness. Most of the light is not visible to our eyes. It is like you are viewing a single 60-watt light bulb in complete darkness from about 2.5 miles away. But that does not mean it cannot be captured.
What is light?
Light could be a number of different things, depending on the circumstances. It could be the glow from a light bulb in your bedroom or the warm shining rays that beat down from the sun. Light can be natural (the sun) or manmade (candles or light bulbs), but no matter how it is created, we utilize light every single day. But the light scientists are talking about is part of the electromagnetic spectrum, which ranges from radio waves to gamma rays. Electromagnetic radiation (EMR) waves, as their names suggest are fluctuations of electric and magnetic fields, which can transport energy from one location to another. EMR can also be described in terms of a stream of photons which are massless particles each travelling with wavelike properties at the speed of light. In a nutshell, light is a form of energy and it travels in waves, similar to waves of water in the ocean. Except with light, the things doing the waving are electric and magnetic fields. Literally, light is a self-contained little bundle of these two fields, intertwined. That is why we call light electromagnetic radiation.
If you are floating in the ocean, you’ll move up as a wave passes you, then back down, then back up again when the next wave rolls by. The distance between these crests in the wave is called the wavelength. Since light is a wave, it has a wavelength as well, and this may be its single most important feature. That is because the energy of light is tied to its wavelength.
To put it simply, light is a type of radiant energy that we are able to visually perceive with our eyes. Over millions of years, our eyes have evolved to detect the kind of light the Sun emits most strongly. Well, that makes sense; that makes it easier for us to see! We call this kind of light “visible light”. And that is just the narrowest sampling of all the different wavelengths light can have.
But what does the speed of light actually look like?
Even it might sound like a ridiculous question, optical researchers at the California Institute of Technology recently built the world’s fastest camera which makes it possible to actually see light speed. Scientists at CALTECH now can capture X Y images but at 100 billion frames per second, that is 1 billion is 10 to the 9th. In fact they have upgraded their system to 10 trillion frames per second and with this type of rate, even a 100 billion frames per second; they can see a light pulse propagating in space and capture the scene literally at the speed of light.
The “Sonic Boom” for light
Normally sonic booms are created when an object moves faster than the speed of sound. But you should not be able to create a ‘sonic boom’ for light when nothing can travel faster than light. Unless you are a bit more specific.
“What we did was, we created a tunnel where the speed of light in the tunnel is greater than the speed of light in a medium and so we propagate a very short light pulse in that medium, Bren Professor of Medical Engineering and Electrical Engineering at the California Institute of Technology Lihong Wang explains in his You Tube video interview. “We spray some scatters within the tunnel, he says, so we generate secondary light sources and a light source will be propagated in the background medium so the light source will propagate at a greater speed and that create a superluminal light source.”
Now, light moves slower through the plate material than it does in the tunnel. So, as light scatters into the plates, it cannot keep up with the light in the dry ice fog. When this happens a cone-shaped wave-front of light forms behind the laser pulse just like a sonic boom shockwave created by a supersonic aircraft.
“If I speak and I stand still I emit approximately a spherical wave going out, Professor Lihong Wang explains. “But if I walk and talk it will be distorted. If I walk at a speed of sound or passing the speed of sound, I will create a cone structure that is called a “Sonic Mach cone” that is the sonic version of Mach cone and I was wondering if there’s this photonic version that we can image.”
While this was not the first time a photonic boom had been created – it was the first time one had been captured in real time. That is thanks to a new superfast camera that can capture images at 100 billion frames per second. They call it the “streak camera” whereby the technique employs a complex contraption that uses cameras and mirrors to build these slow motion movies tracking the lights movement across a scene. This streak camera is so fast that researchers ultimately hope to use it to image more than just laser pulses.
“One of the biomedical applications we are after is to image action potential propagation in a neural network, Professor Wang says in his video interview. “ So essentially we want to see the live traffic within the brain and find out how the brain is wired and that would elucidate the mysteries of the brain.”
To watch Professor Lihong Wang’s full video interview, https://youtu.be/BRLiXvX7uRw