# Q & A: material speed limits

Q:
Is the speed of a wave through a material dependent only on the makeup of the material? If so, then each substance has its own "speed limit". What about the substance dictates this max speed? Each material only allows a wave of energy to go through the material at one speed; further energy simply adds height wavelength, following this logical progression. Then I think the maximum speed limit of the "material" of space is 186k miles per hour. Further energy put into an object's speed in space simply adds mass to the object. Is there a correlation here? In this way, I think thinking of space is like thinking of a universe filled with water. Please answer my questions!
- Luke S. (age 15)
Wauwatosa, Wisconsin, United States
A:
The speeds of various waves in materials and in space are variable, depending on the material and the type of wave as well as the frequency of the wave. There's a special case, things with no rest mass traveling in a vacuum, which has a fixed speed, the speed of light. This speed limit applies to materials too, although most waves in materials travel more slowly. There's no special speed limit for different materials. For example, neutrinos, which interact very weakly with materials, need very little energy to travel at almost the speed of light through the Earth, people, etc.

It's true that adding energy to something traveling near c increases its mass (in the E=mc2 sense, not the rest mass.) Of course since mass is equivalent to energy, just measured in units different by a factor of c2, we really aren't saying anything here beyond adding energy adds energy. The important thing is that the momentum is given by this m times the velocity, v. Thus giving the object an energetic push does increase its momentum by increasing m even though v hardly changes once it gets close to c.