Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Can you make a shock wave of light by breaking the light barrier just like supersonic airplanes break the sound barrier?

Category: Physics      Published: April 9, 2013

Yes and no. It depends on the material you are in. In order to keep things from traveling into the past, and thereby preserve local conservation of mass-energy, and thereby prohibit the universe from exploding in an instant, nothing can travel faster than the speed of light in vacuum. Supersonic airplanes break the sound barrier by flying faster than the speed of sound. This is possible because sound is just a traveling vibration of air molecules. As airplanes approach the speed of sound, their sound waves pile up into a wall of air pressure that shatters apart weak airplanes. Airplanes that are strong enough can poke through this wall of air pressure and create a shock wave that trails behind them. When this sonic shock wave passes ground observers, we hear it as a sonic boom. By analogy, if a space ship traveled faster than the speed of light, it would create a shock wave made entirely of light. The problem is that nothing can go faster than the speed of light in vacuum, so a space ship can never go fast enough to break the light barrier. It's not a question of engineering, but of fundamental physics. As an object approaches the speed of light, it takes an increasing amount of energy to accelerate. It would take literally an infinite amount of energy for a space ship to exactly reach the speed of light in vacuum. This fact is verified every day in particle accelerators.  Using the electrical power consumption of a small town, particle accelerators channel that energy into getting a handful of tiny particles like protons and electrons traveling very close to the speed of light in vacuum. Every year, particle accelerators are being improved to reach ever higher speeds. For instance, one decade the record was that the particles were traveling at 99.99% the speed of light in vacuum, and then the next decade an improved machine reached 99.999% the speed of light in vacuum, and the following decade saw a record of 99.9999% (these numbers are for illustration purposes only and are not exact). The current record is held by the LHC which has accelerated protons to 99.999997% the speed of light in vacuum. A ship in space can't ever reach the speed of light in vacuum, and therefore can't ever break the light barrier and create an optical shock wave.

Everything discussed so far applies only to vacuum. The situation gets interesting when you are not in vacuum. When traveling through a material such as water or glass, the speed of light in that material is significantly slower than the speed of light in vacuum. An object can go faster than the speed of light in a material without breaking any fundamental laws. And as you would expect, an object traveling through a material at a speed that is faster than the light in that material does indeed create an optical shock wave. This shock wave of light is known as Cherenkov radiation. In order for the effect to be noticeable to the naked eye, the object has to be traveling very quickly through a fairly dense material. For instance, nuclear reactors spit out electrons at very high speeds as by-products of the nuclear reaction. These high-speed electrons travel through the cooling water faster than the speed of light in the water, and therefore create shock waves of light. This Cerenkov radiation is observed as an eerie blue glow.

Even though air is very close to being a vacuum, it is not exactly a vacuum. The speed of light in air is slightly less than the speed of light in vacuum, so objects can actually break the light barrier in air without going faster than the fundamental limit of the speed of light in vacuum. It is just much harder to get Cherenkov radiation in air than in water because higher speeds are required. But it does happen. Supernovae shoot out particles at amazingly high speeds (but always less than the speed of light in vacuum). When these particles hit earth's atmosphere, they are known as cosmic rays. Some of these cosmic rays and their by-products are indeed fast enough to create optical shock waves as they travel through the air. Although the atmospheric Cherenkov radiation from cosmic rays is too faint to see with the naked eye, it is detected by cameras and serves as an important tool for scientists studying cosmic rays.

Topics: Cherenkov radiation, light, optical shock wave, optics, radiation, shock wave, sonic boom, speed of light