Speed of Light
Written by John Carl Villanueva
When taken from Einstein's Special Theory of Relativity, the speed of light is the fastest that any particle can ever have. It's like a speed limit for everything in the entire universe. This speed varies as light travels from one medium to another and is at its maximum value when light passes through vacuum.
Coincidentally, all other electromagnetic waves have this same value whenever they travel through vacuum. Thus, while red, yellow, blue, and all other colors of light (as well as radio waves, microwaves, and x-rays) have different speeds when passing through different media, they have the same speeds in vacuum.
This magic number is 299,792,458 m/s or, in its more popular form, 3.00 x 108 m/s. If we convert this to units that we are more familiar with, then the speed of light is equivalent to 1,079,252,848.8 kilometers/hour (kph) and 670,616,629.38 miles/hour (mph).
How fast is it? – comparisons to the speed of light
To give us an idea how fast this is, let's compare this value with the speed of some of the fastest things on Earth.
- SSC Ultimate Aero, the fastest production car in the world: fastest recorded speed = 413 kph or just 0.000038% of the speed of light
- JR-Maglev, the fastest train in the world: fastest recorded speed = 580.973 184 kph or just 0.000054% of the speed of light
- X-15 of the 1960's, holds the fastest record for a manned aircraft = 7,272.625 kph or just 0.000674% of the speed of light.
Modern Physics' ubiquitous 'c'
Expressing speeds as a fraction of the speed of light in vacuum is quite common in science books and typically represented by 'c'. Hence, c = 299,792,458 m/s. Thus, 200,000,000 m/s is 66.7% of c or 0.667c. Similarly, 150,000,000 m/s is 50.0% of c or 0.5c.
The speed of light, expressed in c, is a staple symbol in Modern Physics, and has been immortalized in Einstein's famous equation E=mc2.
When subatomic particles travel at speeds comparable to c, they are said to be traveling at 'relativistic speeds'. In such cases, the principles of the Special Theory of Relativity should be factored in when physical quantities such as mass, momentum, energy, and time have to be calculated.
Speed of light and vast empty space
Despite the seemingly infinite magnitude of the speed of light when used in the context of earth-bound scenarios, its limit is made more noticeable when used in astronomy where massive entities like planets and galaxies are separated by the vastness of empty space.
For example, it would take billions of years for light to travel from outlying galaxies to Earth. Since it is light that carries information from these galaxies, what we are therefore seeing now from them through even our most powerful telescopes are events that have happened billions of years ago.
You can read more about the speed of light here in Universe Today. Want to know how galaxies can recede faster than the speed of light? We've also written about how gravity moves at the speed of light.
There's more about it at NASA. Here are a couple of sources there:
Here are two episodes at Astronomy Cast that you might want to check out as well:
- Light Speed, Andromeda Galaxy, Dark Matter and Black Holes
- Running Out of Gravitons and Hitting the Brakes at Light Speed
Filed under: Astronomy
