Which moves faster light or sound?
Myth

Sound always travels faster than light.

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).[1][2]

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.

TIP: Learn more about the difference between sound waves and light waves.

How Fast Is The Speed Of Sound?

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).[3]

Speed of Sound | Mechanical waves and sound | Physics | Khan Academy

How to Make Sound Travel Faster Than Light

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).[4][5]

5 Phenomena faster than Light. See a BigThink article on what can travel faster than light. Keep in mind that entangled particles are “entangled” data isn’t traveling faster than light, this is instead explained by “spooky action at a distance.”

What Is The Speed of Dark? This video shows how “non-things” can travel faster than light (like a shadow, or… sound).

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).

Utilizing Group Velocity

Oddly, we can also make sound travel faster than light speed by using a setup similar to the one Tennessee State University classroom did in 2007.[6]

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.

Faster Than The Speed Of Light

FACT: “In space, no one can hear you scream,” because there is no medium for sound to travel through in space.



Conclusion

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.


Citations

  1. The speed of sound in air
  2. ‘Mach c’? Scientists observe sound traveling faster than the speed of light
  3. Sound Pulses Exceed Speed of Light
  4. Photonic crystal
  5. Physicists Slow Speed of Light
  6. Sound breaks the light barrier


"Sound Always Travels Slower than Light" is tagged with: Energy, Light, Sound


Vote Fact or Myth: "Sound Always Travels Slower than Light"

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Norika pizza lover on
Doesn't beleive this myth.

It depends

Thomas DeMichele
Thomas DeMichele on

Agree, the article agrees. Since light doesn’t always travel faster than sound, the idea that sound always travels slower (or that light always travels faster) is a misconception or myth.