Researched by Thomas DeMichelePublished - July 5, 2016 Last Updated - September 29, 2016
What Travels Faster, Sound or Light?
Sound usually travels slower than light, but not always. Under normal conditions, light moves roughly a million times faster than sound, but under the right conditions sound can travel faster than light.
The reason sound typically travels slower than light is because light naturally travels (in a true vacuum) at the fastest possible speed information can travel (light speed). Light (AKA electromagnetic energy) and doesn’t require a medium to travel through, while sound must always travel through a medium (traveling at about 332 meters per second through air molecules). With that said, there are a few “cheats” to make the sound go faster than light and faster than light speed (c).
So while we can say light typically travels faster than sound, we can’t say sound always travels slower than light or that sound can’t travel faster than the speed of light. We explain below.
FACT: Sound travels faster in solids (about 6,000 meters per second), about half that speed in water, and then of course much slower in a gas like air (about 332 meters per second). The more tightly packed the molecules, the faster the molecules vibrate and the faster sound travels.
FACT: The standard metric for the speed of light is that of light traveling in a vacuum. This physical constant, denoted as “c,” is roughly 186,000 miles per second (or 299,792,458 meters per second). Light speed is roughly one million times the speed of sound in air. Light always moves near light speed (although typically slower as nature abhors a vacuum), but light can bounce off objects (slowing its linear movement at light speed) and can be slowed in specific situations (such as if it is trapped in a photonic crystal). See Physicists Slow Speed of Light from 1999, which explains how light can be slowed by a factor of 20 million (thus much slower than sound).
There are a few different ways to make sound travel faster than light, and faster than light speed.
Slowing Down Light
“No thing” travels faster than light, but sound isn’t a thing, it is a disruption of molecules, and light is a thing, it is electromagnetic energy. Due to this distinction, it is possible to cheat by slowing down light (in say a photonic crystal). If we slow down light, we can shoot some sound off at 340-ish meters per second and have sound technically travel faster than light (but not faster than light speed).
FACT: If a tree falls in the forest, it makes a sound. The laws of physics don’t stop working just because we aren’t around to verify them. Sound is a disruption of molecules and for every action there is an equal and opposite reaction. The tree must make a sound (unless it falls in true empty space).
In a normal dispersive medium, the velocity of a wave is proportional to its wavelength, resulting in a group velocity that is slower than the average velocity of its constituent waves. But in an “anomalously” dispersive medium — one that becomes highly absorbing or attenuating at certain frequencies — velocity is inversely proportional to wavelength, meaning that the group velocity can become much faster. – Sound breaks the light barrier
Faster than the speed of light. If you had asked me before I read the paper, I would have said, “of course sound travels slower than light,” but apparently there are a few loopholes. Still, in normal situations neither sound nor physical objects nor information can travel faster than the speed of light, and only pure massless energy can travel at the speed of light.
FACT: Both gravity and sound are “classical waves,” unlike light, which is a quantum wave. Classical waves don’t propagate instantly. Instead, they travel through a medium; thus we “hear” an exploding neutron star after we see it, just like we see a firework before we hear it.
The rule is “no thing” (anything comprised of electromagnetic energy, of which all matter is) can travel faster than light speed, but non-things (anything not comprised of electromagnetic energy, like a shadow or sound) can.
Thus, although sound will almost always take longer to propagate than light (and thus, we will always “see the flash” before we “hear the bang”) sound can actually break the light barrier under the right conditions. All this to say, the idea that sound always travels slower than light is oddly a myth.
Author: Thomas DeMichele
Thomas DeMichele is the content creator behind ObamaCareFacts.com, FactMyth.com, CryptocurrencyFacts.com, and other DogMediaSolutions.com and Massive Dog properties. He also contributes to MakerDAO and other cryptocurrency-based projects. Tom's focus in all...