The universal speed limit of any kind of wave — whether electromagnetic or gravitational travelling through a vacuum has been known since Albert Einstein developed his theory of special relativity in 1905. But the maximum speed of sound moving through a solid or a liquid has just been calculated for the first time. It is about 36 kilometre per second, more than 8,000 times lower than the speed of light in a vacuum.
To make this calculation, Kostya Trachenko at Queen Mary University of London and his colleagues started with two well-known physical constants: the ratio of proton mass to electron mass, and the fine structure constant, which characteristics the strength of interactions between charged particles.
Trachenko says we have a pretty good idea of these values, because if they were changed even a bit, the universe wouldn’t look at all like it does.” If you change these constants by a few percent, then the proton might not be stable anymore, and you might not even have the processes in stars resulting in the combination of heavy elements, so there would be no carbon, no life,” he says.
Sound is a wave that spreads by making neighbouring particles interact with one another, so its speed depends on the density of a material and how the atoms within it are bound together. Atoms can only move so quickly, and the speed of sound is limited by that movement.
“The common wisdom was that diamond has the highest speed of sound, because it is the hardest material, but we didn’t know whether there was a theoretical fundamental limit to it,” says Trachenko. The theoretical bound is about twice the speed of sound in a diamond.
The speed of sound is also dependent on the mass of the atoms in the material, so there searchers predicted that solid metallic hydrogen — a material that theoretically exists at the centre of giant planets, but for which laboratory evidence has been hotly questioned — should have the highest speed of sound. They calculated that it should be close to the theoretical limit. They also looked at experimental data for 133 materials and found that none of them broke the limit.
【小题1】How did people find out the speed of waves?| A．By measuring the speed of sounds. |
| B．By using modern technology and science. |
| C．By depending on a great scientist’s theory. |
| D．By comparing the theories about the universe. |
| A．Interactions. | B．Particles. | C．Values. | D．Constants. |
| A．Diamond has surely the highest speed of sound. |
| B．He believes in a theoretical fundamental limit. |
| C．Solid and liquid materials have the same speed limits. |
| D．Diamond has yet to be proven the hardest material. |
| A．It has a close theoretical fundamental limit of speed. |
| B．There would be no carbon,no life in the universe. |
| C．Some materials broke theoretical fundamental limit. |
| D．It will surely travel to other sections of the universe. |
