![]() We mostly associate speed with moving objects than with scientific equations. ![]() However, when you take the time to think about it, you don't see the vectors and formulas from your textbook, but instead a cyclist, flying jet or speedometer needle. Currently, our understanding of these phenomenons are good enough to calculate theoreticaly the speed of light on an other planet if we know the composition and properties of that planet.We have all heard of speed, but how confident are you in your understanding of this concept? Speed is, by definition, purely related and connected to physics. On an other planet, the speed of light would depend on the refractive index of the medium composing the planet (a gas, liquid or solid, as long as its transparent). For example, n=1,33 for water, n=1,5 for glass and n=1,0003 for the air in the atmosphere (at standard pressure and temperature). Of course, the value of n depends on the material used (and is even affected by the wavelength of the light propagating), but values of n are well tabulated. In that case, the speed of light in the medium is c/n, and n is usualy measured experimentaly for various mediums. In the case of visible light and translucent mediums, for convenience we define the "refractive index" n of the medium as the square root of the relative dielectric constant (a value still adimensional and equal or greater than 1). So the new velocity is always smaller than c. One of these properties is the relative dielectric constant of a medium (an adimensional number of value always greater than 1), and often the speed of light in the medium is simply c divided by the square root of the relative dielectric constant. ![]() In that case, as for sound waves, the apparent speed of light will be changed depending on the medium physical properties, but in a different fashion than for sound waves. However, light interacts with matter, and this interaction results in the modification of the apparent speed of light on a macroscopical (human) scale. In the theory of special and general relativity, the speed of light is a universal constant c=300.000 km/s (with some rounding). For this reason, light can travel through space (i.e. Light, or electromagnetic waves, are oscillations of the electromagnetic field that exists independently of matter. On a microscopical level, sound waves are small vibrations of atoms/molecules that make the medium in which they propagate, for this reason sound cannot propagate in space. The reciprocal method has been used to determine the Earth's interior: by "listening" to sound waves (generated by nuclear explosions) from the surface of the Earth, it has been possible to calculate their velocities, and determine physical properties of the material composing Earth's interior. Hence, if the medium's properties are known, the speed of sound can be calculated theoreticaly. but not void), and the velocity of the wave will depend on some physical properties of the medium : its density, temperature, or other more complex quantities (such as adiabatic index for gases, and Young modulus for solids). Sound waves can only propagate in material medium (in gases, liquids, solids, plasmas etc. Light is an electromagnetic wave, and as for any wave its velocity is determined by the *medium* in which its propagating. Is it possible that the speed of light is different on other planets, like how on other planets, the speed of sound could be affected by the amount of particles in the atmosphere?
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