Sound waves require a medium to travel because they rely on the vibration and propagation of particles or molecules in that medium. In a vacuum, such as outer space or an airless environment, there is no medium present to transmit these vibrations, and as a result, sound cannot propagate.
When a sound is produced, it creates a series of compressions and rarefactions in the medium. For example, when you speak, the sound waves generated cause the air molecules to vibrate, creating areas of higher pressure (compressions) and lower pressure (rarefactions). These compressions and rarefactions then travel through the air, water, or any other medium, carrying the sound energy with them.
In a vacuum, there are no molecules or particles to vibrate and propagate the sound waves. Without a medium to transmit the vibrations, the energy of the sound cannot travel. Therefore, sound cannot be heard in a vacuum.
It's important to note that there are other forms of energy that can propagate in a vacuum, such as electromagnetic waves (including light), which do not require a medium to travel. But sound waves specifically rely on the physical interaction and movement of particles or molecules within a medium to propagate.