# Scientists: Spreading sound in solid hydrogen can reach speeds of up to 36 kilometers per second.

Middle school physics tells us that the speed of sound is not a definite number, but varies according to different media and conditions. The speed of sound in the air is approximately 340m/s at 1 standard atmospheric pressure and 15 degrees C. In solids, sound travels much faster than in air and liquid. For example, the speed of sound in an iron rod can reach about 5130m/s at 20 degrees C. That’s why when a train comes from a distance, we hear the vibration of the tracks first, and then we hear the train’s siren.

In general, the speed at which sound travels in solids is related to the stiffness and density of the medium. It can be said that the more “hard” the material, the faster the speed of sound.

So how fast can the speed of sound be? A paper published in Science Advances on October 9th gave a figure of 36,100m/s. That’s nearly twice the speed of sound in diamond, the hardest natural substance!

Sound waves are essentially a form of energy transfer in the medium. Einstein’s theory of special relativity set an absolute speed limit for the propagation of waves, the speed of vacuum light, about 300,000 kilometers per second. However, scientists had not previously known whether sound waves also had an upper velocity limit when they spread through solids.

In their paper, scientists from research institutions such as Queen Mary University of London and the University of Cambridge point out that the upper limit of the speed of sound depends on two basic constants without a strut: the fine structural constant and the ratio of proton electron mass.

The so-called no-volume number is a simple number, there is no such as length, time and other units. The number of non-measures we are more familiar with includes π, natural constant e, and so on. The fine structure constant α is such a measureless number, which refers to the ratio of the velocity of electrons to the speed of vacuum light in the first Bohr orbit. Proton electron mass ratio is more intuitive, i.e. proton and electron mass ratio.

These two massless numbers finely control the processes of nuclear synthesis, the origin of heavy elements, and proton decay inside a star, and together, the two numbers determine that stars and planets can only form molecular structures that support life’s activity within a narrow range. In the paper, the authors present a new perspective: these two numbers also affect materials science and condensed state physics. The combination of the two can lead to a new quantityless number: the ratio of the speed of sound in the condensed phase to the speed of vacuum light.

Thus, they put forward the upper limit of the speed of sound in the condensed phase, and the result was about 36100m/s. So, what kind of condensed material can we find this extreme speed of sound? Scientists have experimented and calculated on a range of materials, further developing more specific predictions: the speed of sound will slow down as the atomic mass of the material increases. This means that the limit speed of sound appears in the smallest atom, the hydrogen atom.

However, the hydrogen atoms we are familiar with are all in the form of hydrogen, to “press” them into solids, it takes at least 250 Gpa, more than 1 million times the atmospheric pressure. At such high pressure, hydrogen is transformed into a metal solid conductor, just like copper. The scientists also predicted that it would be a room temperature superconductivity.

Using quantum theory of computational simulation, the researchers found that the speed of sound in solid hydrogen was indeed close to the upper limit proposed earlier.