The question of what the universe sounds like is common, often fueled by science fiction. Sound is defined as a mechanical vibration, a pressure wave that requires a material medium, such as air or water, to travel. Since space is largely a vacuum, the vibrations we associate with sound cannot propagate between celestial bodies. However, we can translate the energy that does travel through space into a form we can hear, giving the universe an audible voice.
The Limits of Sound in Space
Sound waves are longitudinal waves that transmit energy through the movement of particles in a medium. A vibrating object pushes and pulls on surrounding molecules, creating alternating regions of high pressure (compression) and low pressure (rarefaction). This particle-to-particle interaction is the mechanism by which sound travels. In the near-total vacuum of space, the matter density is far too low to support this process effectively.
Interstellar space is an incredibly tenuous environment, typically containing only a few atoms per cubic centimeter. For comparison, the air at sea level on Earth is about ten billion billion times denser. The distance between particles in deep space means a vibration would stop before reaching the next atom. While space is not a perfect vacuum, the existing low-density gas and plasma cannot conduct audible sound across astronomical distances.
Capturing Cosmic Vibrations
Since mechanical sound waves cannot travel across the cosmos, astronomers rely on electromagnetic (EM) waves. Unlike sound, EM waves are composed of oscillating electric and magnetic fields that do not require any medium to propagate. These waves, which include visible light, radio waves, X-rays, and gamma rays, travel effortlessly through the vacuum of space at the speed of light.
Celestial objects emit energy across the entire electromagnetic spectrum, providing a wealth of data invisible to the human eye. Telescopes collect this energy, which is initially recorded as digital signals, or streams of ones and zeros. This raw data, whether representing the intensity of X-rays or the frequency of radio emissions, serves as the basis for creating an auditory experience. The challenge is translating these non-audible data points into a range the human ear can perceive and interpret.
The Process of Data Sonification
The technique used to convert non-audio data into sound is called sonification. This process is a deliberate mapping of data parameters to specific auditory properties, not a simple recording. The goal is to represent information hidden within complex datasets, making it accessible and revealing patterns not apparent in visual images.
A core technique involves mapping the data’s numerical values to the pitch of the sound. For instance, the vertical position of an object might be assigned to a specific musical note, with higher positions translating to higher pitches. Similarly, the brightness or intensity of the light detected is often mapped directly to the volume or loudness. Other data points, such as distance or color, can be mapped to elements like stereo position or to the timbre and instrument choice.
A significant challenge in sonification is the vast difference between the frequencies measured in space and the range of human hearing. The electromagnetic frequencies measured by telescopes are far too high for human perception and must be scaled down dramatically. This scaling involves shifting the original frequency down by many octaves to bring it into the audible range, which for humans is typically between 20 Hertz and 20,000 Hertz. This translation allows scientists and the public to “listen” to the universe’s energy signatures.
Examples of Cosmic Audio
One famous example of cosmic audio comes from the supermassive black hole at the center of the Perseus galaxy cluster. Pressure waves emitted by the black hole cause ripples in the surrounding hot gas, which are genuine sound waves. This actual sound has an unimaginably low frequency, calculated to be 57 octaves below middle C. To make this pressure wave audible, scientists had to resynthesize the signal by scaling it upward by 144 quadrillion to 288 quadrillion times its original frequency.
Another compelling sonification focuses on the Cosmic Microwave Background (CMB), the oldest light in the universe. The CMB contains minute temperature fluctuations, which are light echoes of the universe’s primordial sound waves from when it was a hot, dense plasma. Scientists map the mathematical components of these temperature variations, known as multipole moments, to frequency and amplitude. This process creates a complex harmonic chord, allowing listeners to perceive the acoustic patterns of the early universe.
Planetary radio emissions provide a distinct auditory experience, often captured by spacecraft like NASA’s Juno mission to Jupiter. Jupiter’s powerful magnetic field accelerates electrons toward its poles, generating intense decametric radio waves. The Juno Waves instrument detects these electromagnetic signals, which are processed by scaling the original high radio frequencies down into the human audible range. The resulting audio is a collection of whooshes, whistles, and crackles, representing the complex interaction between the planet’s magnetic field and its volcanic moon Io.